Abstracts No.28: Mafic Dykes and Emplacement Mechanisms, 1990, Adelaide SA

SECOND INTERNATIONAL DYKE CONFERENCE ADELAIDE, SOUTH AUSTRALIA 12-15 SEPTEMBER 1990

MAFIC DYKES AND EMPLACEMENT MECHANISMS

ABSTRACTS

Sponsored by:

Geological Society of Australia Inc and International Geological Correlation Program Project 257 (Precambrian Mafic Dyke Swarms) GEOLOGICAL SOCIETY OF AUSTRALIA ABSTRACTS No. 28

ACKNOWLEDGEMENTS The following organisations have provided substantial financial and logistical support for IDC-2: Ansett Australian Academy of Sciences, 25th IGC Fund Australian Committee for IGCP, Grant-in-Aid (Australian National Commission for UNESCO) Australian Tourist Commission Geological Society of Australia Inc International Geological Correlation Program Northern Territory Department of Mines and Energy Preview Resources Pty Ltd Qantas Rocklabs Limited South Australian Department of Mines and Energy University of New South Wales World Geoscience Corporation Limited

CONFERENCE SECRETARIATE: Australian Convention and Travel Services Pty Ltd, GPO Box 2200, Canberra, ACT 2601 Australia

ORGANIZING COMMITTEE: D r P . C . Rickwood (Chairman), University of New South Wales D r A . J . Parker (Secretary), S . A . Department of Mines & Energy Mrs M . Tucker (Treasurer), Adelaide D r D . H . Tucker, Preview Resources Pty Ltd International Representatives: Prof H . C . Halls, University of Toronto, Canada Prof D . L . Jones, University of Zimbabwe

PREFACE Abstracts of papers offered f o r presentation at the Second International Dyke Conference are set out in this Volume in a l p h a b e t i c a l o r d e r . At the time of going to p r e s s , s e v e r a l authors had neither registered n o r confirmed if t h e i r p a p e r was s t i l l to be p r e s e n t e d . H o w e v e r , since the collection of papers offered represents a substantial contribution to the understanding of mafic dyke swarms, their emplacement m e c h a n i s m s , p e t r o l o g y , g e o c h e m i s t r y , g e o p h y s i c s , paleomagnetism and tectonic significance, and since some authors were s t i l l expected to register, a l l abstracts that w e r e received are included in the Abstracts V o l u m e . The T e c h n i c a l P r o g r a m , however, comprises only those papers for w h i c h authors had registered at the time of going to press o r had indicated that they w o u l d be attending. M a n y abstracts and/or titles in this Volume d i f f e r from those in the Proceedings V o l u m e . The m a j o r i t y in the Abstracts Volume w e r e received p r i o r to those w i t h i n the Proceedings Volume but because the latter are generally m u c h s h o r t e r since they precede written t e x t s , the o r i g i n a l abstracts are included h e r e . Abstracts of papers w h i c h are also in the Proceedings Volume are indicated.

SOME EXAMPLES OF SM-ND DATING OF M A n C DYKES V. Amelin Institute of precambrim geology & geochronobgy of the USSR Acaderm of sciences Makarova embl, Leningrad 199034 USSR

Sm-Nd method has proved Its applicability for dating ol different mafic to ultramaflc rocks. Mineral composition of diabases provides wide enough range of Sm/Nd ratios for mineral Isochron dating. An Important advantage of the method Is that samples containing only common rock-forming minerals (PI + Aug +/- Hyp +/- Hbl) can be dated. Sm-Nd mineral systems are less sensitive to low-grade metamorphlsm or hydrothermal alteration than Rb-Sr and K-Ar systems. These processes can somethlmes cause excess scatter of analytical points resulting In ages that are Imprecise but lie within the error limits with more precise ones obtained for fresh rocks. Complete resetting of Sm-Nd mineral systems takes place only In high-grade metamorphlc conditions where minerals with contrasting REE distribution such as garnet are being formed and these cases can be easily detected. Variable contamination often observed In diabase dykes leads to geochronologlcally meaningless correlation lines on Sm-Nd Isochron plots for whole rock samples. Mineral separates of such samples yield parallel Isochrons with different The reliability of age determinations based on parallel Isochrons does not differ from that based on coinciding Isochrons. Thus Sm-Nd mineral Isochron approach yields relatively reliable geochronologlcal results for mafic dykes though less precise than the results of U-Pb zlrcon/baddeleylte dating. As examples the following Sm-Nd mineral Isochron ages are reported: A dyke from Southern Karelian swarm.. .1920+/-69 Ma, Two samples from Pudozhgor diabase dykes (coinciding Isochrons) 2000+/-55 Ma, .4 Two samples from Northern Karelian olivine gabbronorlte dykes (parallel Isochrons) 2470+/-65 Ma, .2 Two samples from Gashunnur dyke

2450+/-210 Ma.

4

complex, Mongolia (parallel Isochrons).320+/-36 Ma, £^^^=+7.6 319+/-79 Ma, eN(i=+6.1

THE ROZA MEMBER FEEDER DYKE SYSTEM, COLUMBIA RIVER BASAT T GROUP: COMPOSITIONAL VARL\TION AND EMPLACEI^NT SSAtkinson & RStJLambert University of Alberta, Edmonton, Canada

ISOTOPIC VARIATION ALONG THE ROZA DIKE , COLUMBIA RIVER BASALT, WASHINGTON AND OREGON C-OLUMBIA Atkinson S. and Lambert, R. st J., Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3? R o f r dVL^'IS!?^® ratios vary regularly along the 160 km long n^l^r^r. in eastern Washington and northeastern V. United States Pacific Northwest. This ^^^^^^^ crosscuts the tectonic suture mSrafran^^h .^orth American Precambrian cratonic rM?™^^^ oceanic Blue Mountains Province. As a Member of the Miocene Columbia River Basalt Group this young, well preserved and easily accessed feeder system is t t and isotopic study addressing Forty-two samples from along this NNW-SSE trending feeder system and its associated flows analyzed for major and trace elements and Sr, Nd, and Pb isotope ratios. In addition, twelve plagioclas4 phenocryst separates from samples along the feeder system their strontium isotope ratioL Along ' and neodymium and lead isotope variations have not been detected. However, the 87/86 I^n^^^i''® ratios of both whole rocks and plagioclase separates do vary in a systematic manner. The weighted mean plagioclase separates at the extreme southern dike exposure are .70515 and .70495 respectively. For both whole rock samples and ^^^^ ^^tio increases steadily At the most northern locality the strontium isotope ratios are .70540 and .70525 respectively. There is an approximately .00020 greater 87/86 strontium whole rocks above that of the mineral separates from the same localities all along the system. This type of isotopic variation has not been recorded in any previous work on the Columbia River Basalt Group, and may help establish the role of crustal contamination in the petrogenesis of this Miocene age flood basalt province. implications of this study include the possible existance of an isotopically zoned magma source and a unique opportunity to determine the vertical and horizontal propagation of the magma. Eventually isotopic work on the Roza Flow may also reveal a systematic variation.

MECHANISM OF DYKE INTRUSION INTO POORLY CEMENTED POROUS SEDIMENTARY ROCKS a Boer Geological Survey of Israel, 30 MaJkhe Yisrael St., Jerusalem, 95501, Israel Dike propagation mechanisms are studied from detailed examination of intrusion-related structures in the Ramon a r e a , southern Israel. About 200 dikes of basaltic and trachytic composition intruded Triassic and Jurassic rocks during the Early Cretaceous time. The dikes form a radial system which extends up to 20km from an unexposed central intrusion. Field indicators for propagation directions are segments and fingers; the predominant propagation direction is s u b h o r i z o n t a l , away from the central intrusion. In the pr^^sent study the mechanism of dike intrusion into poorly cemented porous sandstone is d i s c u s s e d . Most dikes are weathered and eroded; structures which are related to their intrusion are preserved along the erosion-resistant quartzitic host 'i'n inn structures Include 1-lOm wide s e g m e n t s , and V-lOcm wide and nL'JJJm T f " " displayed as grooves on the quartzitic host walls forming a "meandering" pattern and indicating the direction of the local, initial crack propagation. Opposing dlkp walls display exact mirror images of these fingers. The form and habit of the fingers suggest that dike propagation is associated with changes in pore-pressure due to penetration of dikf-related fluids. The following stages can be identified: first, the pomu.s sandstone is fluidized in front of the propagating d i k e . A viscous instability develops between the highly viscous fluidized sandstone and the less viscous magma fluids forming a zone of viscous fingering in front of the d i k e , ."^tteps and smooth patches of 0.1 to Im in a r e a , which appear side by side with the fingered zones on the dike walls indicate that r a p i d , brittle fracturing has also occurred during dike p r o p a g a t i o n . Intermittent with the slower viscous flow (whlcli is indicated by the fingered zones). F i n a l l y , the magma invades the nitial crack replaces the fluids which filled the fingers, and splits the fingers to both sides of the d i k e .

(w";

0

WEATHERED WALL

20

10

60 cm

Figure 1 North-south view of quartzitic sandstone at the contact with a d i k e . Note fingers, steps and smooth a r e a s .

A MECHANISM OF DYKE SEGMENTATION IN FRACTURED HOST ROCK GBaer & MBeyth Geological Survey of Israel, Jerusalem, Israel

Dikes are often segmented. Their segmentation has been attributed to changes in stress orientation ahead of the advancing dike or to host rock inhomogeneity. An additional segmentation mechanism, the slip along pre-existing fractures is suggested and exemplified by dikes from the Ramon and Timna areas, southern Israel. In the Ramon area, tens of horizontally propagating basaltic and trachytic dikes of Early Cretaceous age intruded a well-stratified sequence of sedimentary rocks; the dikes are all segmented at the vicinity of major lithological changes (Fig. 1). The mechanism for segment containment within distinct layers was examined by calculating the stress intensity factor at the dike tip approaching a mechanical interface. Interfaces separating layers with different stress magnitudes, different shear moduli, and no-friction interfaces were analyzed. It is apparent that dike segmentation and containment occurred mainly due to the different mechanical properties of adjacent layers and partly due to slip along bedding planes. In the Timna area, a vertically propagating doleritic dike of Late Precambrian age intruded a homogeneous granite. The dike is segmented along numerous planes of ancient dikes and joints, resulting in a complex zigzag wall pattern (Fig. 2). Detailed examination of the relations between the dike walls and adjacent fractures in the host granite enables making a clear distinction between fractures which predated the dike intrusion (some of which were utilized by the intruding dike), fractures which were generated by the intruding dike and fractures which are younger than the dike. The reconstruction of the dike walls to their pre-intrusion position indicates at least two separable stages of dike dilation oblique to the general trend of the dike (parallel to the older dikes) and a third, younger stage of shear parallel to the trend of the dike. The amount and trends of relative displacements across the dike vary along its course. The geometry of the dike walls and its segmentation pattern resemble those of some oceanic ridges and their associated transform faults and fracture zones.

SEPARATION OF EARLY PROTEROZOIC STRUCTURAL RELATIONSHIPS IN THE SCOURIE, SCOTLAND

MAHC DYKE SWARMS BY LEWISIAN COMPLEX, NEAR

BFBarooah Department of Geology and Mines, Assam, India &DJi£owes University of Glasgow, Scotland

Structural relationships permit the separation of different phases of injection of generally parallel nnafic dykes through polyphase defornned rocks of the Lewisian complex, a deep level terrane where dyke emplacement has been used as a time-marker for stratigraphic subdivision. Post-Archaean ages of intrusion are indicated by discordant relationships with structures of the early Proterozoic Inverian episode. The most prominently expressed of five petrographic types, for which petrographic and geochemical data are presented, has structural relationships and composition comparable to the member of the "Scourie dyke swarm" in the nearby Assynt district dated at 2.4 Ga. Other metadolerite dykes cut E-W-trending ductile shear zones that transect the NW-SEtrending Inverian linear belts: their emplacement was during the latter stages of the Laxfordian episode towards the end of early Proterozoic time. Accordingly with the time span of the mafic dyke injection being in the order of that for the whole of Phanerozoic time, the use of these dykes as stratigraphic time-markers must be treated with caution. Without their relationships to the crustal history recorded in the host terrane, or a multiplicitiy of radiometric dates, timing of emplacement is not firmly based. However both structural setting and precise geochronology do permit comparisons of early Proterozoic basic igneous activity and geotectonic environment in the Lewisian complex with that in the likely contiguous Baltic Shield.

PROTEROZOIC MARC DYKE SWARMS OF THE SAO FRANCISCO CRATON (SE BAHIA STATE, BRAZIL): PETROLOGY AND SR-ND ISOTOPES G. BeUiem (1), EM. PicciriUo (2), G. CcMizzim (3), R. Petrini (4), P. Comin Chiaramonti (5), L Cmtta (6), AJ. Me^ (7), MAF. Tamer de OUveim (8), S. Bertolo (1), & CM. Brito (7)

(1) Dept. of hineralogia Petrologia, PadDva, Italy; (£) Ivist. of Mineralogia I: Petrografia, Trieste (Italy); (3) Centro Studi Alpi Orientall, CNR, Padova (Italy); Inst, of Geocronologia Z, Geochimica Isotopica, CNR, Pisa (Italy); (CJ) Inst. of Mineralogia, Petrografia l-. Geochimica, Palermo (Italy); Dept. of Geofisica 2K VU 1 cano I og i a, Un. Napoli (Italy), (7) Inst. Astronomico t, Geofisico, Sao Paulo (Brazil); (8) Inst. de Geociencias, UFBA, Salvador (Brazil). Unmetamorphosed

late

Proterozoic mafic dykes of

SE

Bahia

State intrude Archean-early Proterozoic granulitic terrains.

The

dykes,

have

c. pependicular to the coastline, strike c. E-W and

reversed

(R) and normal (N) polarities. Both R-

(1.06-1.08

Ga)

and N- (1.01-1.06 Ga) dykes are characterized by tholeiites

with

low (<£•/•) and high o e x ) TiOg Both

reversed

differences

in

and

normal

and incompatible element contents. LTi and

HTi

dykes

major elements and incompatible

show

which

suggest

different parent melts. In general, LTi

dykes

reveal

similar isotopic

character!sties 1 AQ 1 UfA

LTi==0.70El and HTi=0.7019, and av. ('^Nd/^

important

element

(av.(

ratios and

HTi

y/ y6 Sr/ )qS

)o5 LTi=0.51116

and

HTi«0.511E7. R- and N-dykes do not show substantial chemical i SO tope di ffBrB\'\ces.

and

Basalt Rb/Sr

melts

may be related to mantle source regions

with

On

the

other hand, the high Rb/Sr ratios of both LTi and HTi dykes

(av.

0.05

ratios lower than that of the Bulk Earth (0.030).

and

enrichment

0.09, respectively) indicate an shortly

before

niagma

generation.

incompatible

element

GEOCHEMISTRY OF METABASIC DYKES. ALDAN SHIELD,

YAKUTIA

VJ. Beriozldn Yakutsk Institute of Geosciencee, Siberian Branch, Academy of Sciences, 39 Lenin pr., Yakutsk, U.S.S.R. Metabasite dykes and sills are considered which were emplaced in relation with Late Archean greenstone belts and Early Proterozoic basins •//hlch formed in activization zones of an Karly Archean basement. iletadiabase snd arcphibolite dykes and silla reach 100 m in thickness and aeversl km in lonchh. They often contain ophitic-textured relics and more rarely primary magmatic clinopyroxene and plagioclase. The greenahist to lower amphibolite facies metamorphism of the rocks v/as not always isochemical. Koat comnonly, there were increases in ll^O (up to 2-3^!), lla^o ( up to 3-6r'), Rb. Bu, tmd rarely Si02( up to 55%) and decreases in CaQ ( up to 5-6 , v.-hich was reflected in the appearance or increase of biotite, albite, and quartz. Ko changes are observed in other oxides, as well as in Cr, ]:i, Co, 7, .3c, r, and REE. The changes generally tc4k place in narrow, nenr-contact zones and shear zones. As a result, large amounts of subclcalic basitea are sometimes altered into paeudoalcalic basites. Considorlng tho above changes, -./ido i.:gO and FeO variationea in some of the bodies, abundances and patterns, and other features, the dykcD are primarily composed of tholeiitic magnia differentiates, In eitu differentiation is manifested weakly, only in thick bodies. Subr.lcalic and ^.lcalic banites noraetimes form composite dykes. V/ithln oep-rata tectonic domains of the Aldan Shield, up to 3-4 dyke conple::eG of differ .nt rges (2 to 3 ni.y.) occur -.vhich differ in geological, pctrographic and geochemical characteristics. Supposedly coeval dykes of various domoind also exhibit differences. Still there ic a tendency for incompatible elements depletion from older to younger basites. Preserved ophitic textures in the inotabasites suggest rapid crystallization of magma in tte surrounding cool rocks. Subsequent metarcorphism of the buriites, with insignificant chemical changes, took place in the upper crust.

DYKE SWARMS PROVINCES

DISTRIBUTION

PATTERNS

IN

THE

PRECAMBRIAN

AN. Berkovsky & AP. Platunova Institute of Precambrian Geology cmd Geochronology, nab makarova 2, 199034, Leningrad USSR

Mafic dyke swarms patterns in

the USSR, Australia, Canada, Africa,etc.

provide clues to the natxire of crustal evolution. For example, the dyke swarms of the granite-greenstone terrain (Karelia, Ukrainian shield, Zimbabwe) show variable dyke composition, may embrace the entire terrain and occur throughout the early history of the earth crust of a particular tectonic province. Older dykes ( ^ 1 . 2 Ga) are intimately related to the block structure to exhibit a specific dyke distribution pattern within a particular block suggesting its independent development at a definite stage. The older mafic dyke swarms appear to either absent or to be very scarce in the vast granulite-grade areas (Lapland block in the Baltic shield, Podolian bl6ck in the Ukrainian shield,; Arunta block i n Australia, Kapuskasing structural zone and Pikwitonei belt in Canada, Eastern Ghats in India).Unlike the older swarms, the later swarms ( ^ 1 . 2 Ga in age) are known to embrace several Precambrian blocks (Mackenzie, KolaOnegan swarms, some swarms in Siberia, Africa, India). Besides, the younger regional swetrms occur in the vast granulite-grade areas (Siberia, Australia, India). It appears that onward from ca 1.2 G a , a preexisting crustal pattern W g a n to be essentially changed to commence a platformicf

development of the amalgamated crustal blocks.

The available data suggest that the regional mafic dyke swarms may be useful in a global tectonic subdivision of the Precambrian provinces to be indicative of an independent development of different blocks in early Precambrian times.

METAMORPHOSED ULTRAMAFIC LAMPROPHYRE DYKES OF PROBABLE LATE ARCHAEAN AGE FROM THE SHAW BATHOLITH AREA, EAST PILBARA BLOCK, WESTERN AUSTRALL\ LFBettenay, NMSRock & PJMaiher Key Centre for Stategic Mineral Dqposits, Geology Dept., University of Western Australia, Nedkmds 6009 Australia

ABSTRACT: A suite of ultramafic lamprophyre (UML) dykes, each up to 7 km long and <2 m wide, cuts the Archaean (> 3.5-2.8 Ga) Shaw granitoid Batholith and intercalated greenstones. Field evidence suggests emplacement at 2.8-2.7 Ga. so that the dykes may belong to the oldest alkaline magmatism so far recognized anywhere. They are also the only known examples of metamorphosed UMLs. The dykes are locally disrupted to form trails of ultramafic pods; elsewhere they are sheared. The least deformed dykes consist of: abundant sodian titanian augite (Na20 and Ti02 both «1%); 10-20% of commonly euhedral. zoned, blue/green/brown primary amphibole (anophorite, arfvedsoniie, barroisite, katophorite), overgrown by metamorphic tremolite-actinolite; and variable quantities of quartz-chlorite-carbonate pseudomorphs interpreted to be after euhedral olivine phenocrysts. This mineralogy, together with their markedly porphyriiic textures, significandy higher whole-rock Ti02 (>1%), alkalis, CaO, Ca/Al and Fe/Mg, and lower Si02 than local komatiitic rocks, clearly identifies these dykes as distinct and lamprophyric. Metamorphosed equivalents are locally schistose ultramafic rocks, consisting essentially of colourless (tremolite-actinolite) amphibole and chlorite, but remain chemically distinctive. The dykes have close chemical analogues in 1.9 Ga rocks termed "ultramafic dykes of kimberlite affinity" from Kimberley (S.Africa). Both suites have retained relict mineralogy, but their original chemistries have been severely modified from igneous compositions, with loss of original enrichments in mobile and incompatible elements (K, Rb, Ba, Th, Zr, etc). The existence of this dyke-suite raises new possibilities of exploration for diamonds and/or carbonatites in the Pilbara Block,

THE DIFFICULTIES OF DATING MAFIC DYKES: AN ANTARCTIC EXAMPLE L.P, Black, JWSheraton Bureau of Mineral Resources, Canberra, Australia PD. Kinny & 5. Maxwell Research School of Earth Sciences, Australian National University, Canberra, Australia Dyke s w a r m s of the Vestfold Hills The Vestfold Hills are one of several Archaean cratons preserved in the East Antarctic Shield. They are chiefly composed of tectonically interlayered felsic onhogneisses, with subordinate m e t a s ^ m e n t s , emplaced during a narrow interval between 2526±6 Ma and 2486±6 Ma (Black et al., in press). The last significant deformation (D2) occurred at the end of this interval. These rocks are cut by a spectacular array of unfolded mafic dykes. At least five different generations have been identified on the basis of intrusive relationships, petrography and geochemistry. Except for a volumetrically minor alkaline suite, all are of tholeiitic composition. Three of these suites have yielded imprecise Rb-Sr wholerock isochron ages (Collerson and Sheraton, 1986) of - 2 4 0 0 Ma (high-Mg suite), 1791±62 Ma (Group I tholeiites) and 1374±125 Ma (Group II tholeiites). T h e a n a l y s e d d y k e a n d vein The subjects of this study are a late, roughly n o n h striking, 15m-thick tholeiite dyke from the Platcha area and a thin (1-2 cm wide) felsic vein which is entirely contained within the dyke, and normal to its margins. The mafic dyke is a marginally silicasaturated dolerite (50.6% SiOi) from the most common tholeiitic suite (Group II). It has a subophitic to intergranular texture and consists of sodic labradorite (54%), clinopyroxene (38%), opaque minerals (5%), dark reddish-brown biotite (2%), quartz (1%) and minor apatite. Some clinopyroxene has altered to pale-coloured amphibole. The felsic vein (61.7% Si02) consists of oligoclase-andesine (75%), quartz (15%) and pale to dark green amphibole (10%), with minor biotite, sphene, apatite and scapolite; it is best classified as a quartz diorite. The boundary between the vein and the dyke is abrupt, but plagioclase crystals locally protrude across the contact. Within a few millimetres of the vein the dolerite is extensively altered, with replacement of clinopyroxene by secondary amphibole, and sorne alteration of ilmenite to sphene. All available evidence suggests that the vein was a late magmatic / hydrothermal segregation localised in a cooling fracture within the dyke. Zircon U - P b geochronology Although not observed in thin section, zircon is present in both the mafic dyke and the felsic vein. These have been dated by the ion-microprobe U-Pb method (Compston et al., 1984). A large proportion of the zircons in both phases are relatively equant and anhedral. These grains yield an age of 2483±9 Ma; they are interpreted as xenocrysts incorporated from

country rocks near the site of emplacement. The remaining zircons in the dyke (about 50%) are elongated and multifacetted. They give a relatively uniform 207pb/206pb age of 1025±56 Ma. However, this age is believed to date not dyke emplacement, but an episode of metamorphic recrystallization peripheral to high-grade tectonism at that time in the adjoining mobile belt to the south. It is postulated that these zircons replaced pre-existing magmanc baddeleyite (Zr02). A small proportion (about 13%) of the zircons in the felsic vein are elongated, with simple prismatic and pyramidal faces. These high-U (28306760 [ig/g) and high-Th (3050-8350 |ig/g) grains grew at 1248±4 Ma, which we interpret as the time at which both the mafic dyke and felsic vein crystallized. The principal reasons for accepting these grains, and not the younger zircons in the mafic dyke as defining igneous crystallisation are, a) their simple igneoustype morphology, b) the improbability that zircon would have crystallized direcdy from the Si02-poor tholeiitic magma, c) the absence of other evidence of felsic rocks younger than 2477±5 Ma in the Vestfold Hills, from which these might otherwise have been inherited, d) the independent evidence that alteration of the dykes occurred during the --1000 Ma intense metamorphism and deformation documented in the Rauer Islands immediately to the south of the Vestfold Hills (Kinny and Black, 1990). Conclusions The analysed dyke contains zircons which both predate and post-date its emplacement age (as inferred from the felsic vein), but does not appear to contain any primary igneous zircons. Thus, it can be dangerous to use constituent zircons to derive, either directly or indirectly, the emplacement age of mafic dykes. More reliable dating methods would include U-Pb analysis of zircon from associated late-stage felsic segregations (paying due regard to possible inheritance), or of primary baddeleyite (if present) from the dyke itself. The inferred 1248±4 Ma emplacement age matches that of geochemically similar tholeiiuc dykes which transect the Archaean Napier Complex of Enderby Land, 1000 km funher to the west. References Black, L.P., P.D.Kinny, J.W.Sheraton & C.P.Delor 1990. Precambrian Research, in press. Collerson, K.D. & J.W.Sheraton 1986. Journal of Petrology 27, 853-886. Compston W., I.S.Williams & C.Meyer 1984. Journal of Geophysical Research 89, B525-534. Kinny, P.D. & L.P.Black 1990. Tenth Australian Geological Convention, Hobart, Abstracts, 251-2j2.

PETROLOGY OF PROTEROZOIC DYKE SWARMS FROM CENTRAL URUGUAY J. Bossi (1), N. Campd (1), L Cmtta (2), G. Demarchi (3), V. GirarcU (4), M. MazzuccheU (5), 0. RMend (5), AR. FragosoCesar (4), S. Simgd (3) & EM. Piccirillo (3)

(l)Faculty of Agronomy, Montevideo University (Uruguay>,(E) Napoli Univwreity Department of G^ofisica ^u 1 cano 1 og i a , (Italia), (3) I n s t i t u t e of f l i n e r a l o g i a L Petrografia, Trieste University (Italy),Institute of Geociencias, Sao Paulo University ( B r a z i l ) , <5) I n s t i t u t e of M i n e r a l o g i a L Petrologia, Modona University (Italy).

Unmetamorphosed dykes

middl^-late

( 0 - 6 to 3 0 m t h i c k ) o f F l o r i d a

(Uruguay)

strik©

N60-80E

and

granitoid

terrains.

tholeiitic

andeaj-basa1ts

plagioclasc?, a u g j t e quartz~feldspar

lo (S)

and

they

tholeiitic

- subcalcic

augite,

i ritergrowths occur

t r a c e e l e m e n t s , asi w e l l

two

dyke swarm

are

higher

relative

l e s s (Evolved

Zr/Ce

to

ratios

In g e n e r a l , compatible with

in t e r m s

iri p r o g r e s s .

grained

of

fractional

the d i f f e r e n t

opaques;

allow

southern of

the

lower

P/2r

respectively)

respectively). magma

crysta11ization

types

are

starting

from

C o m p o s i 11 o n s , S - a n d N - d y k e s w i l l

o f Sr a n d N d

of

dykes.

than those

3.8-^.6,

by

up

a5 p e t r o g r a p h y ,

(^0-77 and 3 . 2 - 3 . 0 ,

the generation yabbroic

p a r e n t m e l t s iMitli d i f f e r e n t discussed

andesites, made

0.55-0.-^0)

(i.e. 37-^8 and

N-dykes

gneissic-

t r e n d s . T h e d y k e s of the

(mg

Ga)

region

represented

(mg = 0 . 3 2 - 0 . 2 5 ) . S - d y k e s a l s o h a v e

t h o s e of

1.5

(pigeonite) and

and

(N) t r e n d

ar©

in c o a r s e r

Major

trend

(c-

- San Jos©

intrude early Proterozoic

general>

distingui^^h

northern and

In

Proterozoic - Durazno

isotope geochemistry,

be

currently

AUSTRALIAN MAGNETIC DYKES

DMBoyd Department of Geology and Geophysics, University of Adelaide, Adelaide, South Australia & DHTucker Preview Resources Pty Ltd, Adelaide, South Australia P application of image processing techniques to regional aeromagnetic data has led to K ^ ^ "kv u'^yJ'®! Australia that were not previously known. At least four major a J n f . J I T ^ , T^® established through the integration of the aeromagnetic data widi the available geological ^ t a . p e expansion and upgrading of the magnetic map of dykes of Australia has been Burea f Mi^ al R analogue data in newly generated digital form from the

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DYKE DETECTION AND ANALYSIS USING ENHANCED AEROMAGNETIC IMAGERY J. Broome Potential Fields Section, Continental Geosdence Division, Geological Survey of Canada, 1 Observatory Cres., Blg2 Ottawa, Ontario, Canada

Imagery produced from aeromagnetic data effectively displays magnetic anomalies associated with dykes. Experimentation with different signal processing and imaging methods has resulted in methods optimised to enhance the characteristic linear highwavenumber anomalies associated with dykes. The resulting images facilitate studies of regional variations in dyke orientation, composition and magnetization. Anomalies associated with small or weakly magnetized dykes that are invisible on conventional aeromagnetic contour maps and colour-intensity images are clearly displayed. Individual aftd composite shaded images allow directional enhancement and colour-coding of dyke-related anomalies. Regional images of the Canadian Shield generated from data interpolated onto a 812.8 m grid clearly display the radial pattern of the Mackenzie dike swarm. More detailed images of parts of the Abitibi Belt generated from;data gridded at 200 m illustrate the dyke detection capabilities of the imagery. Images of aeromagnetic data for other parts of the world, produced using the same methods, facilitates comparison of dyke-related anomalies in different areas.

ONTARIO, CANADA

UTHOPHILE AND PLATINUM-GROUP ELEMENTS IN SUPERIOR P R ^ C I S

G£. Bmgmam, AJ. Naldrett & H.C. Halls Department of Geology, University of Toronto. Ontario, Canada M5S 3B1

The most imponani platinum-group element (PGE) deposits are found in layered intrusions emplaced into the continental crust (e.g. Siillwaier and Bushveld Complexes) or are directly related to continental rift magmaiism (e.g. Duluth Complex. Noril'sk and Talnakh. U.S.S.R.). Mafic dyke swarms are generally believed to have formed in extensional continental environments, some may even represent feeders to continental flood basalts. Thus a systematic investigation of the PGE disuibution in these rocks should provide important information about the PGE concentraUons of continental basalts in general, as well evaluate their potendal to form PGE-rich deposits. Ten different dyke swarms from the Superior Province in Ontario, which range in age from 1100 to 2400 Mill years, have been systematically sampled and analyzed for lithophile elements and precious metals. The MgO content of the samples varies from 12 to less than 4 wt.%. All the dykes are of tholeiitic composition and follow subparallel Feenrichment trends. The mineralogy and the major element variauons indicate that plagioclase and pyroxene are die major crystallizing mineral phases. The trace element patterns of the parental magmas display market negative Nb-Ta anomalies and are enriched in highly incompatible elements relative to moderately incompatible elements. For example, light REE abundances vary between 20 and 200 times chondriuc, compared to heavy REE, which vaiy between 10-30 umes chondriuc. During fracuonal crystallization, individual dykes swarms maintain a rather constant LaAT) raUo. however, the ratio appears to increase systematically from about 2 in the oldest (Matachewan and Kenora-Kabetogama dyke swarms) to about 12 in the youngest Keweenawan dykes. This variation cannot be explained by crustal contamination or different degrees of partial melting in a common source. If crustal contamination causes significant changes in the LaA'b ratio it is clearly seen in the major element composition of the dyke. Assuming a degree of partial melting of 20-30% for parental magmas with LaA'b=2. then the degree of partial melting forming magmas with LaA'b ratio of 12 would only be 5-6%. This is unreasonably low. because it is not reflected in the major element composition. Therefore, the increasing LaAT) ratio should be a function of the source composition, which implies that the subcontinental mantle beneath the Superior Province became progressively enriched in incompatible elements from the late Aichean to the middle or late Proterozoic. The presently available data indicate a wide range in Cu and PGE contents (Cu: 10-260 ppm; Se: 0.2-1.8 ppm; <2-35 ppb; Pt: <1-20 ppb; Au: 0.1-5.4 ppb; Ir: <0.03- 14 ppb). These large PGE variations also occur within single dyke swarms (e.g.. the Kenora-Kabciogama dykes swarms), others, for example the Sudbury dykes, consistently have very low PGE concenuations. The distribution of the PGE is not correlated to variations of incompatible, lithophile elements, probably because of the segregation and accumulation of sulfide liquids. This is supported in some cases by a positive correlation of PGE and Se. although this is not observed in the whole sample set. The data suggest a complex history of sulfide saturation on the scale of individual dykes and single swamis. Tlius the consistenUy low PGE concentration in the Sudbury dykes could be due to the loss of PGE during a rather early event of sulfide saturation, maybe in a magma chamber which now contains a significant PGE deposit. The observation of relatively high, but variable PGE concentration in other dyke systems could indicate that sulfide saturation occured late within the dyke, probably caused by assimilation of crustal components or by cooling and fractional crystallization in the dyke.

PALAEOMAGNETISM OF MATACHEWAN DYKES IN VICINITY OF THE SOUTHERN PROVINCE OF CANADIAN SHIELD - TEST FOR FAULT BLOCK ROTATION KL. Buchan Geological Survey of Canada, 601 Booth SL, Ottawa, Ontario, KIA 0E8 Canada

Morris (1981) observed that paleomagnetic directions for 2200 Ma Nipissing diabase intrusions of the Cobalt and Blind River regions of the Southern Province in the Canadian Shield are discordant in declination. He proposed, therefore, that the two areas have undei-gone relative rotation about a pole within the Southern Province since emplacement of the Nipissing Diabase. To test this model. 2454 Ma Matachewan diabase dykes were sampled close to Nipissing intrusions in both regions. If relative rotation has occurred the magnetization of the Matachewan dykes, which predate the Nipissing intrusions, should show a similar discrepancy in magnetic declination between the two areas. Matachewan dykes are particularly well-suited for this test since their remanence directions are known to have very sha1 low inclinations.

PALAEOMAGNETISM OF PROTEROZOIC MAFIC DYKE SWARMS OF THE CANADIAN SHIELD

KLBuchan Geological Survey of Canada, 601 Booth St., Ottawa, & HCHaUs Department of Geology, University of Toronto, Erindale Campus, Mississauga, Ontario LSL 1C6

An important component of the p a l e o m a g n e t i c data base for North America is derived from mafic dyke s w a r m s of the C a n a d i a n S h i e l d . M a f i c dykes frequently carry a stable m a g n e t i c r e m a n e n c e whose primary or secondary n a t u r e can be tested using the b a k e d contact t e s t . In a d d i t i o n , the age of an i n c r e a s i n g number of these dyke swarms is p r e c i s e l y known from U - P b z i r c o n or baddeleyite dating. The paleomagnetic r e c o r d for P r e c a m b r i a n d y k e swarms of the Canadian Shield is r e v i e w e d . T h e r e l i a b i l i t y of paleopoles and baked contact tests from individual swarms is d i s c u s s e d . P a l e o p o l e s are correlated w i t h the m o s t r e c e n t radiometric a g e s . F i n a l l y , the uses and i m p o r t a n c e of dyke data in constructing apparent polar wander paths a n d in d e t e r m i n i n g p a l e o l a t i t u d e s for P r e c a m b r i a n North America are c o n s i d e r e d .

INTRUSION AND CRYSTALLISATION FEATURES IN PROTEROZOIC DYKE SWARMS A.Cadman, JIamey Dqxtrtmeru of Geology, Universuy of Leicester LEI 7RH, UK & RGFark ofGeobgy, University ofKeele, Stc^ STS 5 Bg, UK Department

In some deeply eroded Precambrian terranes which have extensive dyke swarms it is possible that we are observing the effects of flow of magma through dyke channels which may have been up to 10 - 20 km deep in the crust. This means that not only could substantial volumes of magma have flowed through these channels, but the country rocks may have been hot and the rate of cooling slow. As the dyke compositions range from quartz tholeiites to high-Mg picrites the magmas will have had variable physical properties (e.g. viscosity) and thus different flow behaviour. Moreover, whereas some are aphyric others contained substantial proportions of suspended crystals. The possibility arises that substantial thermal erosion of wall rocks could take place at depth. Some swarms seem to have been emplaced in response to essentially local extensional conditions, others in a more regional transtensional or even transpressional regirrie. In this contribution we describe intrusive features in a range of Proterozoic dykes from the North Atlantic craton. notably in Scotland and Labrador, and comment on the implications for intrusive processes at depth. Perhaps a majority of dykes are aphyric quartz dolerites which show little internal variation and may have been emplaced under simple laminar flow conditions; some textural variation may reflect multiple Intrusion of slightly different parental magmas. Other dolerite dykes however contain substantial proportions of plagioclase phenocrysts, the distribution of which emphasises processes such as flowage differentiation and multiple intrusion. It is the more magnesian noritic and picritic dykes however that display the greatest diversity of features, such as pyroxene-rich margins and significant cross-dyke petrological and chemical variations. These features can be assessed in terms of turbulent flow, thermal erosion and flowage differentiation with different assemblages of suspended crystals.

GEOCHRONOLOGY AND PALAEOMAGNETISM OF THE KULGERA DYKE SWARM, MUSGRAVE BLOCK, NORTHERN TERRITORY, AUSTRALIA

A. Cantacho, B. Simons Northern Territory Geobgicd Survey, PO Box 2655, Alice Springs NT. 0871 Australia & PW. Schmidt CSIRO. division of Expbratbn Geoscience. PO Box 136, North Ryde NSW 2133 Australia The Kulgera Dyke Swarm, located in the Eastern Musgrave Block, is part of the younger group of dolerite dykes defined by Wilson (1948) that occur on the SE margin of the Amadeus Basin, They intrude a gneissic granulite and granitic terrain of Late Proterozoic age, known as the Kulgera terrain (Camacho,1989). The dolerites are iinmetamorphosed and fresh, and form an east-west arcuate belt 90 km long by 15 km wide. Individual dykes are commonly 500 m long and 2 m thick, parallel and dip shallowly (5^-35^) to the south. However, larger intrusives attain dimensions of up to 6 km in length and 8 m wide. The Kulgera Dyke Swarm has been analysed for geochronology, rock magnedc properties and palaeomagnetism. The total rock and mineral constituents of a dolerite were analysed using the Rb/Sr method. The resulting isochron was model 1 and yielded an emplacement age of 1054 ± 14 Ma for the dolerites (Camacho & Gray in prep.). A muscovite and a biotite from the surrounding country rocks yielded ages of 1042 ± ? and 1060 ± 10 Ma respectively. The above ages indicate thermal resetting of the country rock minerals during dyke emplacement. Anisotropy of magnetic susceptibility (AMS) measurements indicate that the dolerite dykes have a magnetic fabric with a low anisotropy. This is consistent with a pristine igneous fabric on which there has been no tectonic overprinting. The palaeomagnetic results indicate a stable magnetic remanence direction coinciding with a magnetic pole at 16^S, 265^E. This pole is considered to be primary and is reasonably close to a 1120 ± 10 Ma Mount Isa pole (Duff and Embleton, 1976) and preliminary results from the Stuart Dyke Swarm north of the Amadeus Basin (Idnurm and Giddings, 1988). In addition to the stable NRM direction, an overprint component was present in many of the samples indicating palaeotemperatures of 200®C to 300®C. This direction indicates a pole position of SO'^S, 137°E, which coincides with an early Carboniferous Australian pole, suggesting that the overprinting has been caused by tiie Alice Springs Orogeny. This overprint direction is similar to the directions, previously thought to be primary, obtained from the Giles Complex and the Arunta Block, in addition to the Amadeus and Ngalia Basins. These results extend the known extent of tiie Alice Springs Orogeny into the Musgrave Block in the south. The lack of isotopic, AMS or palaeomagnetic evidence for a 600 Ma thermal event suggests tiiat the Petermann Orogeny has not effected the Proterozoic rocks of the Kulgera region, and that the Alice Springs Orogeny was a low temperature thermal event in this region.

LATE JURASSIC LAMPROPHYRE DYKES, BENDICX) GOLDFIELDS, VICTORIA B. Cameron Geology Dq>aitment, Monash University, Clayton, Victoria Australia & C£. Willman Victorian Geological Survey, PO Box 173, East Melbourne, Victoria. Australia

The Bendigo Goldfield is on« of the most productive turbidite-hosted gold fields in the world and is famous for its saddle reefs. The deformed Lower Ordovician sediments have been intruded by a swarm of lamprophyre dykes which typically occur in the hinge zones of anticlines and frequently follow pre-^existing faults. Datings by McDougal & Wiillman (1976), recalculated by Byrne (1985) to 149+5 Ma and 159+6 Ma, indicate a late Jurassic age. The dylces have obvious cross-cutting relationships with the earlier gold*-quartz veins and clearly postdate mineralization. Mineralogically, monchiquites are the most common variety with rarer camptonite and kersantite. Xenoliths composed of olivine and clino and orthopyroxene occur in some dykes. Along dyke margins, carbonate veinlets are common and some horizontal fibres suggest small strike-slip movements. At Kangaroo Gully a crudely stratified deposit consists of angular and waterworn Ordovician clasts, glacialy derived pebbles together with fragments of basic igneous rock within a glassy matrix. The fragments have a similar composition to the monchiquite dykes (Stillwell, 1911) and field relationships suggest the breccia has resulted from a diatreme emplacement associated with the intrusion of a monchiquite dyke. Another breccia occurs in the Central Deborah mine closely associated with a dyke which is rich in xenoliths. Breccia fragments are composed of reef quartz and country rock.

Byrne, D.R., 1985: Explanatory notes for Campaspe 1:10000/2.2 Geological map. Rep, geol. Surv. Vict,^ 1985/53. (Unpub/) McDougal, T. & Vellman, P., 1976 : Potassium-argon ages for some Australian Mesozoic igneous rocks. J. geol.. Soc_. Aust^j 23, pp l-iStillwell, F., 1911: Preliminary notes on the monchiquite dykes of the Bendigo goldfield. R. Soc.. Vict., 25^ pp 1-4.

INTERPRETATION OF TWO DYKE SWARMS IN MINNESOTA FROM HIGHRESOLUTION AEROMAGNETIC DATA VW. Chandler, BD. Schaap & DL. Southmck Minnesota Geological Survey, 2642 University Avenue, St. Paul. Minnesota 55114^1-57 USA High-resolution aeromagnetic data from a nearly completed survey of Minnesota provide one of the clearest regional perspectives available on Precambrian dyke swarms. In this study we use the aeromagnetic data to compare and contrast the Kenora-Kabetogama swarm (approx 2125 Ma), a radial swarm that is probably related to the evolution of the Penokean orogen, with the Carlton County swarm (approx 1100 Ma), a linear swarm that is oriented parallel to the strike of the Midcontinent Rift System and is clearly related to it. Both swarms are extensively covered by glacial till, and aeromagnetic data a.'^e essentia! to their study. Aeromagnetic data reveal that the Kenora-Kabetogama swarm is at least 300 km wide, 400 km long, and may contain thousands of dykes, making it the largest known dyke swarm in the United States. The dykes fan from trends of about 340^ at its eastern edge to about 300^ at its southwestern edge and, where exposed, are typically 20-70 m thick. The magnetic anomalies indicate that many dykes are straight and continuous for tens of kilometers and show no strongly preferred direction of branching. Prominent magnetic anomalies and minor gravity highs clustered near the center of the swarm delineate subswarms of large dykes, which model studies indicate are subvertical, 200-300 m thick, and 3-10 km deep. The great majority of Kenora-Kabetogama dykes are older than foredeep deposits associated with the Penokean orogen. and apparently do not cut rocks of the fold-and-thrust belt that is tectonically inboard of the foredeeps. However, a few dykes along the southwestern margin of the swarm do cut rocks of the Penokean orogen, and therefore may be part of a distinctly younger magmatic event. The rift-related Carlton County dike swarm differs significantly from the KenoraKabetogama swarm in structural style. The Carlton County dykes are restricted to within 75 km or less of the rift axis, and the along-strike curvature of many dykes gives the swarm a somewhat braided appearance. Although one exposed dyke is about 70 m thick, most are only 1 to 10 m thick. Aeromagnetic interpretation of these dykes requires caution: some negative anomalies that appear simple are actually the combined effect of several narrow, closely spaced dykes with reversed polarity, whereas some positive anomalies are actually side-lobe artifacts of reversed-polarity dykes. The profound differences between the Ksnora-Kabetogama and Carlton County dyke swarms have important tectonic implications, for they appear to reflect two radically different styles of lithospheric response to continental extension and magmatism. In the case of the Kenora-Kabetogama swarm, both extension and igneous activity were diffused over a broad geographic region and the continental crust underwent little disruption. Deep rifting did not occur and no record survives of associated volcanism. In the contrasting case of the Carlton County swarm, extension and igneous activity were focused along a much narrower zone, in which almost total failure of the crust occured, and extrusive sequences that are tens of kilometers thick are preserved in a deep linear rift. Whether the divergent styles reflect differences in mantle properties, rate of extension, ancestral structure, or some combination thereof, will require further investigation to resolve. This study was supported primarily by the Legislative Commission on Minnesota Resources. Additional data were acquired by the U.S. Geological Survey.

AEROMAGNEnC INTERPRETATION OF MAFIC DYKE SWARMS V. W. Chandler Ou Minnesota Geological 55114-1-57

Survey,

2642

Universily

Avenue,

St

Paul,

Minnesota

Aoroniagnetic data are useful for the study of mafic dyke swarms in Pr^cambrian shields, which typically involve large regions that are either poorly exposed or not mapped in adequate detail. In addition to the traditional use of aeromagnetic coniour maps, new computer-graphic techniques, such as high resolution color and shaded relief displays, now allow an overall view of the swarm, yet also show substantial detail of individual dyl^es. In nearly every Precambrlan shield area aeromaonetic data have played critical roles In dotormining the distribution of dyke swarms and their relationship to oiher tectonic features.. Through modeling, aeromagnetic data can also be used offeoilvely to estimate dyke parameters, including the thickness of sedimentary cover, the subsurface geometry of the dyke, and paieomagnetic directions. This paper demonstrates the usefulness of aeromagnetic data In dyke studios, but it emphasizes the limitations and pUfails of which the non-specialist shoukJ be aware. Aoromagnetk; interpretation of dyke swarms tends to oversimplify structure and underestimate the number o( dykes. Dykes will not produce magnetic signatures H iheir magnetizatk)n does not contrast significantly with that of the country rock, or if their magnetic oxfctes have been destroyed by metamorphism or deuterte alteration. Furthermore, dykes that are less than 5 meters In width are commonly not detected In aeromagnetic surveys flown 150 meters or more above magnetic basement. Dykes that are spaced closer than the flight elevation producQ an aeromagnetic anomaly that tuay be misinterpreted as reflecting a single dyke. The magnetic signature of a dike can be significantly affected by its orientation with respect to the earth's lield; for example, at low magnetic latitudes a nonh-south dyke In that is polarized along the earth's field will produce no detectable magnetk; anomaly. Cautton must also bo exercised in model studies of dykes, especially in the absence of geologk: or rock property data. Estimation of magnetization versus thickness for dykes that are Significantly nan-ower than the survey elevation becomes difficult, and only the product of the two parametors may be determined with any reliability. Failing to account for natural remanent magnetizatkjn (NRM), which in dykes can be several times stronger than the induced componeni, can lead to erroneous estimates of dip and magnetic properties. Magnetic properties within the dyke and adjacent country rock can bo extremely variable, and so average magnetiiations determined from simple model studies can be misleading. Finally, because the deeper parts of a dyke contribute so little to the observed anomaly, estimation of depth extent from aeromagnetic data is dlfficutt and error-prone. 1 he orientation of flight profiles relative to the dykes, the flight elevation, and the overall quality of data compilation all impact on the suitability of an aeromagnetic database for dyke Studies. A partteularly critical factor is having a ratto of elevation (above source) to line spacing that is low er»ough to assure that the narrow dyke anomalies can be traced confkJently across flight lines. For exanple, dyke swarms in the Canadian ShieW are dramatically portrayed in data that were acquired with an elevation/spacing railo of 2,64 (300nfve00m), whereas their signatures are virtually absent In data that were flown at a less favorable ratio of 5.28 (300m/1600m). Some methods of gridding and digitizing can obscure dyke signatures In machine-contoured maps These and other factors indk:ate that aeromagnetic coverage over many shield and shaltowbasement regtons will have to be Improved before the potenflal of the aeromagnetic method for dyke studies is fully realized This study has been supported in part by the Legislative Commission on Minnesota Resources.

UNMETAMORPHOSED NORTHERN AMAZON

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TROLElim ^ ^ j ^ M Tffl GUIANA -IN THE EVOLUIIUIN u r

BASALTIC MAGMATISM A. Choudhuri, UNICAMP, Campinas, Bmdl. AN.Sial UFPE, Rec^e, Brazil. & EP. OUveira University of Leicester, UK The northern Aiazon Craton has witnessed alioit continuous basaltic activity since the Lower Proterozoic. The earliest record oi basaltic aagiatisi is to be found in the northern Guiana Shield greenstone belt volcanics dated at 2.1 Ga.. (Rb-Sr, Ss-Nd and U-Pb ages are all very s i i i l a r ) . In the Hiddle Proterozoic, Urge voluies of basaltic lagia intruded the flat-lying Roraiia Group sandstones in thick s i l l s and associated dykes in Venezuela, Brazil, Guyana and Surinam. Unietaiorphosed tholeiite dykes in the age range 1.6-1.8 6a., and the s i l l s are referred to as the Avanavero Suite, and the Quarenta Ilhas volcanics in Branl are equivalent to these. In later periods during the Proterozoic, the basaltic lagiatisi diiinished considerably in volute, until there was another diking peak stretching between the Upper Proterozoic to the Mesozoic, the latter period of activity (the Apatoe Suite) representing the break-up of South Aienca and Africa. At this t i i e the Takutu Graben in Guyana and the contiguous part of Brazil was infilled by tholeutic lavas of the Apoteri Proterozoic dykes of the Avanavero Suite were considered to represent an abortive atte.pt at continental drift.. The che.ical co.position of these dykes indicates an appmiiate •differentiation trend' following the lines of a differentiated s i l l of the saie suite. Their low Ti, P and Zr coupled with hiQh LILE and LREE suggest their derivation fro. a depleted .antle wh ch was later jnnched n the latter eleaents. A regular and linear increase in TiQ, with lafic index .arks the differentiated nature, though their K,0/PA ratios re.ained fairly constant throughout. This .ight suggest Jhe tapping o a co.jon source or sources with si.ilar ratios. However, the later hesozoic Apatoe dykes o not show depletion to sa.e extent, possibly reflecting so.e change in the position and co.position of the " 9 " " Tes ec s (T /Zr, Tio'-Sio,. FeO-MgO-Al.O,) the Proterozoic dykes rese.ble Archaean basalts fro. t e Superior , an blsaltl and Greenland dykes. The age-related variation of Ti/Zr " ' P ' " basalts point to a single prior volcanic episode, the for.ation of the 2.1 Ga. ' ' probably depleted the .antle source of the dykes. The evolution of basic .a8.atis. with i . b eas ng olu.e and increasingly alkaline nature as recorded in Upper „ n in cichoeira Seca (1.04 Ga.) and Nova Floresta 0.96 Ga.) south of the A.azon, and indicates an increasing consolidation of the craton.

A COMPARISON OF THE MAGNETIC PROPERTIES OF DYKE SWARMS AND LAVAS IN THE BRITISH TERTIARY IGNEOUS PROVINCE P. Dagley, A£. Mussett & RR Skelhom Department of Earth Sciences, University of Liverpool UK

within the British Tertiary igneous province activity consisted of c e n t r a l intrusive complexes, lava p i l e s , dyke swarms and other minor intrusives. The dykes generally have a NW-SE trend and are normally found in local swarms w h i c h , like the lavas, are often but not always assciated w i t h a central complex. The lavas and dykes can be grouped by chemical composition but lavas and dykes for a given locality do not have the' same proportion of these groups. We have measured various magnetic properties (NRM, susceptibility. Curie temperature) of a large number of dykes and lavas and the results w i l l be p r e s e n t e d , particularly comparing the properties of lavas and dykes associated w i t h the-same complex.

PRECAMBRIAN MAFIC DYKES IN THE WESTERN ALDAN SHIELD GM. Drugova, VN. Verkhalo-Uzky, NL. Aleskeyev & N.G. Berezhnaya

A combination and

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at

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in t h e

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tfu3

enviroments.

MORB-RELATED DOLERITES ASSOCIATED WITH THE FINAL PHASES OF KAROO FLOOD BASALT VOLCANISM IN SOUTHERN AFRICA ARDwican, RAAnmtrong, AJJErkmk, Geochemistry Dqpartment, Universuy cfCape Town, Rondebosch 7700, Sou^ Africa JSMarsh Geobgy Department, Rhodes University, Grahamsiown 6140, South Africa & RTWatkins Geology Department, University of Cape Town, Rondebosch 7700, South j^nca The Karoo flood basalts of Southern Africa can be divided into two major groups. The Karoo basalts which covered the central and eastern portions of Southern Africa were erupted some 190 Ma ago and are best preserved in the highlands of Lesotho and along the Lebombo monoclihe which forms the eastern margin of the Kaapvaal craton. The Karoo basalts which are preserved in the Etendeka area of northwestern Namibia were erupted approximately 130 Ma ago and can be correlated with the Serra Geral basalts of the Parang Basin of Brazil. Associated with each of these, groups is a suite of dykes which are compositionally distinct from the basalts, having trace element and isotopic characteristics which suggest that they are MORB-related. The Rooi Rand dyke swarm crops out in the southern Lebombo between 26.5°-28®S. The swarm strikes almost north-south and it is continuous for a length of some 200km with a maximum width of about 20km. The central portion of the Rooi Rand dyke swarm could reasonably be described as a sheeted dyke complex since the proportion of wall-rock screens is only some 5-10% over a distance of 5km normal to strike and most dykes are composite. The precise age of the Rooi Rand dykes is not known. They intrude the Karoo basalts but not the overlying rhyolites (177 Ma) and are apparently tilted by the deformation of the Lebombo monocline which is late syn-volcanic. They probably predate the initial opening of the Indian Ocean (about 140 Ma). The Horingbaai dolerites form thin dykes and sills which intrude the base of the Etendeka lava pile along the coast of Namibia between 20.5°-22°S. They are believed to be immediately post-volcanic and probably coincide in age with the initial opening of the South Atlantic at this latitude (about 125 Ma). ^ Both the Rooi Rand and Horingbaai magma types are very similar in incompatible element abundance patterns to average MORB and contain significantly lower abundances of incompatible trace and minor elements than the Karoo basalts with which they are spatially associated. Both dolerite magma types show almost flat chondrite-normalised REE patterns. Both have depleted isotopic characteristics with negative Ssr and positive values in the same range as shown by MORB and some OIB. It is believed that at least the trace element and isotopic characteristics of the Karoo basalts are largely inherited from the sub-continental lithospheric mantle. This contrasts with the Rooi Rand and Horingbaai dolerites which could be derived from purely asthenospheric sources and which may be related to the initial stages of sea-floor spreading and the disruption of Gondwana. The two apparently unconnected magmatic events could be linked in the model recently proposed by White and McKenzie which relates continental flood basalts to decompression melting in the mushroom-like head of a newly initiated mantle plume. Initially, asthenospheric melts passing through thick subcontinental lithosphere could react with it to extract low-melting point fractions which give the Karoo basalts their apparent lithospheric signature. Later, as lithospheric thinning proceded, asthenospheric melts would pass through a shorter and more depleted lithospheric path and could be erupted with their asthenospheric compositional character intact.

EARLY PROTEROZOIC META-BASALTIC AMPHIBOLITIC DYKES IN THE SVECOFENNIAN ROCKS OF SW FINLAND

CEhlers & A. Undroos Immudomn for Geobgi och nuneralogi, Abo Akademi, 20500 Abo, Finland

Vast amounts of mantle-derived syntectonic tonalites and granodiorites intruded the slightly older volcanic rocks in SW Finland some L9 Ga ago in an extensive generation of mantle-derived continental crust. The newly formed crust was heterogeneously deformed, metamorphosed, and subsequently intruded by swarms of basaltic dykes which are later deformed in episodes of thrusting and shearing. The intrusive meta-basaltic dykes are overprinted by migmatites and migmatitic granites some 1830-1840 Ma in age. A well-exposed roughly E-W trending swarm of amphibolitic dykes can be seen in the migmatite-zone of southern Finland and in the neighbouring areas of Sweden. They intrude in part areas of strongly deformed granulite facies tonalitic slabs (of presently unknown age) within stacks of rock slabs with contrasting amounts of deformation and metamorphism, and in part areas of only slightly deformed (amphibolite facies) granodioritic intrusions ca 1890 Ma in age. Some new U-Pb zircon data concerning the strongly deformed tonalites and the mafic dykes will be presented. The E-W trending dyke swarm has rouhgly the same direction as the main tectonic transport direction of subsequent gently dipping or horizontal shears which accompany the wide spread migmatite formation and S-type granite intrusions in SW Finland. The intrusion of the dikes could be connected to a process of underplating of newly formed continental crust followed by intrusion of the later migmatite granites.

MAHC DYKES IN THE PRECAMBRIAN BASEMENT OF SOUTHEASTERN NIGERIA BNEkwueme Dqxjrtment ofGeobgy, University of Calabar, PMB. 1115 Calabar, Nigeria

The Nigerian Basement Complex c o n s i s t s c h i e f l y o f q u a r t z i t e complex and metavolcanosedimentarj s e r i e s

migmatite-gneiss-

ranging in age from

^ 2«5Ga to 0.5Ga. These were intruded mainly by Older Granites o f Pan A f r i c a n age ( 6 0 0 i l 5 0 M a ) *

Associated

with the basement rocks in a l l parts o f

are mafic dykes which are dominantly b a s a l t i c

to d o l e r i t i c

Nigeria

in composition*

Few gabbroic dykes a l s o o c c u r . The dyke rocks o c c u r as t a b u l a r bodies o f few metres to s e v e r a l k i l o m e t e r s l o n g . They c r o s s - c u t

the f o l i a t i o n o f

their

c o u n t r y rocks and in some c a s e s they have c h i l l e d margins. In s o u t h e a s t e r n N i g e r i a the heat of the i n t r u s i o n of t h e ^ e dykes superimposed a c o n t a c t metamorphism on the p r e v i o u s l y r e g i o n a l l y metamorphosed c o u n t r y r o c k s . As a r e s u l t ^ r o c k s around the malic dykes are ol ten h o r n f e l s i c

i n ' t e x t u r e and

c o n t a i n such minerals as c o r d i e r i t e and s i l l i m a n i t e . The emplacement o f mafic dykes in southeastern Nij2:eria was s t r u c t u r a l l y c o n t r o l l e d . They f o l l o w mostly NE-SW and ISNE-MSW d i r e c t i o n s

which are the

main f r a c t u r e trends in the basement. F i e l d evidence in parts o f

southeastern

N i g e r i a however, suggests that some of the mafic dykes were f o r c e f u l l y emplaced causing the f r a c t u r i n g of t h e i r host r o c k s . These are g e n e r a l l y f i n e - g r a i n e d r e f l e c t i n g t h e i r higher l e v e l o f emplacement. Two e p i s o d e s o f emplacement o f mafic dykes have been recognized in the s o u t h e a s t e r n N i g e r i a basement. The f i r s t and e a r l i e r phase o f dykes has s u f f e r e d d e f o m a t l o n and metamorphism and g i v e Rb-S r Isochron ages o f 7 8 4 ^ 3 1 Ma and 1313i:37 Ma. The second s e t of dykes i s more abundant and has n e i t h e r undergone d e f o m a t l o n nor metamorphism. Rb-iS r s y s t e m a t i c s on these rocks do not y i e l d

reliable

i s o c h r o n s due to t h e i r low c o n t e n t s of Rb and S r . K/Ar d a t i n g method has however y i e l d e d a whole rock age of 4781: 19 Ma f o r s i m i l a r unmetaaorphosed b a s a l t dykes in Ibadan southwestern N i g e r i a . This age has been i n t e r p r e t e d as an i n d i c a t i o n that the Pan A i r i c a n orogeny in Nigerian basement was terminated by the emplacement af these unmetamorphosed mafic d y k e s . Geochemical s t u d i e s show that both the metamorphosed and unmetamorphosed mafic dykes in southeastern Nigerian basement were d e r i v e d m o s t l y from tholeiltlc

b a s a l t i c magma. They sre however enriched in a l k a l i compared t o

most t h o l e l l t e s owing t o a l k a l i metasomatism which a f f e c t e d t h e s e d u r i n g the Pan A f r i c a n o r o g e n y . They f a l l

rocks

in the f i e l d of c o n t i n e n t a l

basalt.

M. Ernesto (2), MH. Furtado (1), JWP. Macedo (2) <&. LG. Pacca (1)

Instituto flstronomico e Seofisico, University of SSo P a u l o . Caixa P o s t a l 30627 - 01051 SSo Paulo - Brazil D e p a r t a m e n t o de Fisica Teorica e Experimental, University of K i o G r a n d e do N o r t e - C a m p u s U n i v e r s i t a r i o , 5 3 0 0 0 N a t a l (RN) - Brazi I .

In the n o r t h e a s t e r n m o s t region of B r a z i l a Mesozoic tholeiitic dyke swarm occurr which has been associated either with the S o u t h or C e n t r a l R t l a n t i c o p e n i n g . T h e K/flr d a t e s are concentrated between 170 to 130 M a , indicating a g e s r a n g i n g f r o m U p p e r J u r a s s i c to L o w e r C r e t a c e o u s . f composed by s u b - s w a rms " m o s 11 y trending E-W although different t r e n d s are also recorded They extend for m o r e than 2 0 0 km a c r o s s the R i o G r a n d e do Stlte e n t e r i n g the n e i g h b o u r i n g r e g i o n s of the C e a r i E a c h of

the s u b - s w a r m s

shows

a characteristic

remanent

I^nn! that they acquired their magnetwation during different times. However, the conlVAlnl ° characteristic magnetizations are consistent with Lower CreUceous or Upper Jurassic g e o m a g n e t i c f i e l d s . Hn e x c e p t i o n is m a d e for S n e of the subswarms (the second from n o r t h to south) wich possess a remanent magnetization comparable with a Triassic geomagnetic field. S i n c e the K / « r d a t a do not s u p p o r t this mfaopt believed that this anomalous direction of Da?^p n ^ ' reversing field. The elongated pattern of the remanence vectors also points to this dv P^teomagnetic inclinations of thSs^ s u g g e s t i n g that they w e r e not

brandP rin Norte cretaceous Parani

pa I eon,.,gne 11 c p o l e i n d i c a t e t h d l the R i o dyke swarm is older than the Lower Basin .nd the s s o c i a U d Pont

iUriLiH N^^^H ^

5 - t h P r n B r a z i l , which are r e U t e d opening. T h e r e f o r e the R i o Grande

Atlantic

oVllnZll

^^^^^^^

^^^

^

—

o do

^ ^ DIRECTIONS IN TWO MANC PROTEROZOIC DYKE SWARMS J R L J S ? CANADIAN SHIELD: AS ESTIMATED USING ANISOTROPY OF MAGNETIC SUSCEPTIBILITY DATA /^XIOUIKURI UR

R££mst 0^-C^ton Geosdence Centre and Dept cfGEohgy, University cfOttam, Ontano, Canada, KIN 6N5

direction of magma flow in small dyke swarms typically reveals a component of lateral flow away from coeval regarding the direction of magma flow within the great swarms that occur in the shield areas of the world. fonn^^^h^ ^^ Mackenzie (1.27 Ga) swarms are found throughout large areas of the Canadian Shield. Magma flow direction in these major swarms can be studied through measurement of the anisotropy of magnetic susceptibility (AMS). The AMS fabric (having the form of a triaxial ellipsoid) reflects the shape and orientation of magnetic grains which in a dyke may be aligned by magma ^ measurements were made on approximately 400 samples from both the largest dyke of the Abitibi swarm (the Great Abitibi dyke) and approximately 20 Mckenzie dykes. In dykes of both swarms, thl minimum axis of the AMS ellipsoid is typically aligned in a direction per^ndicular to the dyke-plane (and the maximum AMS axis is aligned within the dyke-plane), a feature indicating that the AMS fabric was primarily acquired during magma flow. Only a few Mackenzie dykes and Abitibi dyke, exhibit a different pattern in which the minimum axis of susceptibility is aligned parallel to the dyke-plane in a vertical direction; this pattern probably results from overprinting of the flow fabric by a stress-induced fabric during the latest stages of crystallization. ^ For those sites exhibiting a flow-derived AMS fabric, the orientation of the maximum AMS axis can be used to reveal the inclination of magma flow. In the Great Abitibi dyke the maximum AMS axis t^ically has shallow inclination suggesting horizontal flow away from the coeval Keweenawan rift system. In the case of the Mackenzie dykes, the AMS data indicate that the inclination of flow varies with K the focus of the swarm. Mackenzie dykes have a fanshaped distribution and converge towards an area marked by extensive coeval volcanic and plutonic rocks (the Coppermine volcanics and the Mi^kox Intrusion). Fifteen Mackenzie dykes located in the vicinity of the coeval volcanic/plutonic rocks exhibit steep maximum AMS axes consistent with a vertical direction of magma flow. Five other Mackenzie dykes located in two regions 500 and 1000 km from the volcanic/plutonic centre have AMS maximum axes which are shallow and are interpreted as indicating horizontal magma flow. J^e AMS data from dykes of both the hfackenzie and Abitibi swarms can be e^lained by magma flow upwards and outwards from coeval volcanic/plutonic centres.

REGIONAL DYKE SETS OF THE PECHENGA, KOLA PENINSULA (USSR) J A. Fedoiov Geological institute of die Kola Centre of the USSR, Academy of Sciences, Fersman 14, Apatity, USSR

Within the Pechenga region 5 mafic dyke sets of different age Imve been revealed (Fedotov, Fedotova, 1989). 1 . Metagab^onorites, 2. quartz metadolerites, 3 . composite set of Fe-enriched titaniferous olivine gabbro and metadolerite, picrodolerite, dolerite. These sets were distinguished based mainly on the dykes composition and intersections, most information being provided by abundances of SiO^i Ti02, FeO , MgO, N i , Cr, V , Zr in the rocks. Sets 1-4 are Early Proterozoic in age and show "svecolcarelian" ( 1.8Ga) metamorphism. The latter increases when approaching the Pechenga graben-syncline structure infilled by a thick volcanogenic-sedimentary pile. The dykes are well correlated as to their composition and age succession, with the reliably dated plutonic and volcanic rocks constituting the structure and, herein, their age fall into the 2,4-1.9 Ga range. The dykes of sets 1-2 show NE-trend, those of set.3 are NW - trending near the Pechenga syncline followed by NE-trend in the northern region when removing from this structure. The NW-trending set 3 includes large dykes of kersutite plagiodolerite and olivine gabbro ranging up to 26 long and 120-m thick known as the Nyassyuk swarm to extend for some 200 km along the south-western margin of the Kola platform. The set 4 is represented by a tract of intrusive bodies gently plunging southward. This zone extends in sublongitudinal direction along the Murmansk coast towards the border with Norway. The dolerite complex shows no metajnorphism and comprises 10 extended dykes defining a broadly NS-trending swarm about 50 km wide and 100 km long to cut across all the structures in the region. The dykes are arranged as microswarms (subsets) about 3-5 dykes in number and spaced hundreds of meters apart. A particular subset may include dykes of different age showing multiple mutual intersections. More than 30 dykes intersections have been recognized in the region. Their study reveals that an opening of a joint of a dyke emplacement occurred perpendicular to its strike, this being also supported by the turn of large xenoliths and the knee of dykes. At the same time, abundant shear zones are here spread to influence only some dykes. Besides, the dykes with bending of gneiss layers in their exocontact zones are found. This can be account from an alternation of compressional episodes to form shear zones and extensional episodes of dyke emplacement.

GALENA . SPHALERITE VEINS RELATED TO MAFIC DYKE SWARMS IN THE KOLA PENINSULA, USSR M.G. Fedotova Geological Institute of Kola Centre cf the USSR Academy of Sciences, Fersman 14, Apatity, USSR

Barite-calcite-quartz veins with pyrite-galenite-spt^alerite mineralization (galenate-sphaleritic veins) are widespread in the Murmansk coast of the Kola peninsula. They can be referred to as moderate-low-t® rocks genetically linked with a tectonic activation. These veins are known so far in terms of their attitude, mineral composition, vertical zoning, trace element distribution pattern, scheme of hypogene mineral formation in Caledonian times (Pedotova, 1978, 1980, 1989)• The comparative study of the veins within the Murmansk coast shows their predominant occurrence in the Pechenga region which is made up of highly migmatized gneisses of the Kola Group. The gneisses are cut by the N2-trending mafic dykes. The majority of the veins occur at the contact or inside the dykes. The dykes are represented by gabbro-norite, quartz metadolerite, olivine microgabbro, picrodolerite and range in age from 2.4 to 1.9^ Ga. The emplacement of the veins took place more than, 1.0 Ga later when compared with the enclosing rocks - dykes. The relation between the veins and dykes consists in their common confinement to the same weak tectonic zones. However, the enclosing dykes effect the vein composition. The galenite-sphaleritic veins are spatially confined to the coastal line and, likely to the long-lived deep-seated Karpinsky fault which defines the Murmansk (Barentz sea) line of the Kola peninsula. The veins exhibit a lateral zoning. A change of galenite-sphalerite-quartz assemblage via barite-quartz-calcitic with pyrite to sulfide-lacking quartz-calcitic assemblage occurs eastward. This compositional change of the veins throughout the Murmansk coast is consistent both with the stages of mineral formation and vertical zoning in the largest veins of the Pechenga region. This change seems to derive from the decrease in temperature of the process in the east direction;. The study of the dyke-vein paragenenis in the geological objects, showing no genetical dependence provides an additional information on the evolution of the structural zones where they are confined to.

EARLY PROTEROZOIC MAHC DYKE SWARMS GRANITE-GREENSTONE TERRAIN, BALTIC SHIELD

OF

THE

KARELIAN

SB. FeUtsyn, AN. Bericovsky, YuV. Awelin & VS. Semenov

(Institute of Precarabiian Geology & Geochronology, the U3S3R Academy of Sciences, 199034, nab. Makarova 2, Leningrad, USSR),

One of the largest wsarms on the Baltic shield is a regional dyke set of early Proterozoic dykes embracing the entire Karelian granitegreenstone terrain. The swarm is defined by gabbro-diabase dykes which are aeroraagnetically detected showing NN7/ and NW trends and having 10-15 km in length and up to 100 and more km in width. Petrochemical data on the dykes from the northern, central and southern Karelia alongside those from the Finnish Karelia suggest a similar composition of the dykes throughoht the Karelian swarm. Thick dykes ( ~ 100 m wide) exhibit compositional variations from the periphery to the dyke center. Sm-Nd ages of a dyke cutting the Burakov intrusion in the south-eastern Karelia, and central Karelian dykes are 1.9 and 2.0 Ga, respectively. So, both petrochemical and geochronological data support the recognition of the swarm as a single entity. A local longitudinal swarm in the northern Karelia is defined by gabbro-noritic dykes dated by Sm-Nd method at 2.45 and 2.47 Ga. Their age is consistent with that of the Olanga layered intrusions according to Finnish and Soviet data. Unlike regional swarms, the local swarms are shown to be often associated with coeval layered intrusions and volcanics to form the so-called triads.

fflGH - TIO2 (>3 WT%) CRETACEOUS DYKES OF NORTHEASTERN BRAZIL GEOCHEMISTRY AND PETROLOGY RV. Fodor, North CaroUm State Umersity, Raleigh, NC 27695, USA, AM. Sid Umersidade Federal de Pemarribuco, rec^e, PE Brazil & EH. McKee Survey, Menlo Park CA 94025 USA US Geological Mesozoic dikes associated with the opening of the Atlantic Ocean have TiO contents that range from less than 1 wt.% to over 4%. The greater occurrence of Atlantic dikes having low TiO has biased petrologic studies toward those with less than 2 wt.% TiO^. Our study addresses the less common variety, having > 3 wt.% TiO^, and we examine two provinces in northeastern Brazil about 700 km apart: Piaui state, within the Maranhao (MRH) basalt province, where high-Ti dikes have apparent K-Ar ages of 120 Ma, and Rio Grande do Norte (RGN) (northeast tip of Brazil), where one high-Ti dike is 89 Ma. RGN high-Ti dikes are associated with slightly older Cretaceous low-Ti dikes (<2 wt.% TiO ) (two K-Ar ages: 108 and 127 Ma), apparent Precarabrian low-Ti dikes (two dated at 555 and 836 Ma), and Tertiary basalt volcanic centers (20-30 Ma). Like high-Ti flood basalts (e.g., Parana basalt province, Brazil), the MRH and RGN high-Ti dikes are distinctive by high FeO* (>12 wt.%) and K 0 (>1%), and low MgO (<6%), and by evolved characteristics that do not represent fractionation from low-Ti primitive (MgO ^-8 wt.%) magmas. Results. Dike compositions: Both MRH and RGN dikes are'enriched in incompatible elements and LREE, where La. p'lOO. Differences in trace element contents between MRH and RGN high-Ti 8ikes are small but include lower Ti, P, Zr (225 ppm), Nb (20 ppm), K, and Y (36 ppm), and higher Sr (550 ppm) and Zr/Nb (12) in RGN than in MRH dikes for given MgO. Both dike provinces, however, have similar K/P and mantle-normalized Nb depletion. Preliminary data for initial Sr/ Sr include 0.70653-0.70762 for RGN, and 0.7057 for MRH; 6 0 for each is 6-7%o . Associated basalt: Precambrian and Cretaceous lowgTi dikes associated with RGN high-Ti dikes appear Nb-enriched, and have lower Sr/ Sr. ., 0.7027 ^^re-g^ and 0.70406-0.70618. The Tertiary centers are also Nb-enriched and have Sr/ Sr 0.7043. Precambrian dike 6 0 is 6.5-8.3%o, while Cretaceous low-Ti dikes have 6%,, and Tertiary basalt 7-8%« . Fractionation: Eleven analyzed MRH samples create a fractionated suite, where SiO^ is 49-56, MgO is 5.3-2.9 wt.%, and TiO is 4.2-2.6 wt.%. As Ti decreases, K, P, Rb, Ba, Zr, Nb, Y, and La increase, and V decreases. Major- and trace-element modeling shows that the compositional range represents about 56% crystallization of 4.6% pigeonite, 17% cpx, 25% pi, 5.5% rat, and 3.8% il from the most mafic representative (5.3 wt.% MgO). Conclusions. Isotope and trace-element compositions indicate that northeastern Brazil high-Ti dikes were not greatly affected by crust or alteration, and that their mantle-source compositions differed slightly from one another, representing some level of mantle heterogeniety -^'lOO Ma across '>'700 km east-west. More significantly, sources different from those for high-Ti dikes produced the low-Ti dikes and Tertiary centers in Rio Grande do Norte, and the Nb differences in northeastern Brazil dike sources may reflect influences of ancient subducted material in continental lithosphere. Until further isotope studies (in progress) better cha racterize dike sources, however, we cannot rule out some compositional differences across northern Brazil mantle as apparent due to different percentages of partial melting (e.g.. Nb residual after melting). High-Ti magmas with > 6 wt.% MgO that could have been parental to the high-Ti dike suites are not observed, but once crustal magma reservoirs with MgO'^5.3 wt.% became available, crystallization up through at least 50% produced a continuum of high-Ti basaltic compositions (Mg# 45 to 33).

FMFraser, AMJiq)good University cf St Andrews, Scotland &DRBowes University cf Glasgow, Scotland

before and between 6 d e f o r L ? ? n L i distinguished at intervals imprint on the "dykel" ^^^^ ^^^^ their mineral assemblages "^nd all P o s s e s f ^lana?" nH They can be grouped into ono of fSo^n ^ ^^ linear fabrics, mineralogy, chemistrv and 1 ° the basis of (to be p S s e n t I d f o ^ e a c h suite? compositional a s s o c i a t S n w i t f t i m e of consistent variation with age. There is nor systematic the contention that s u S intrut^.i evidence to support stable crust, nor can they be reaard/d ^"'Placed in a cool time interval separatiTa ^ representing a specific imply eniplacemeS'^ at \nterva\s durT"'"^ ^^^^^^ ^^^^ changing physical condiSi^ ^ protracted period of compression) in the course I extension and crustal evolution Hence be^oie'^T^J continental intrusions can be used a? i nH f . ^ composition of such specific times in t L cruftal h?^°' compositions at intrusions themselves must first o ? a n ' h ' ^^ ^^^ Of the events recorded in the host rocks ^^^^^^^^^ed in terms

LATE PROTEROZOIC RIFT-RELATED DYKES OF THE SOUTHERN AND CENTRAL APPALACHIANS. EASTERN U.S.A.

SA. Goldberg & JJi. Butler University cf North Carolina, Chapel Hill, North Carolina, USA.

temf^J''®

^ swarms in the southern and central S^^ates were motamorphosed and locally

In the higher-grade thrust sheets, there are few hydrous metamomhif. Z t l i - ' ^^ '^l^emical co-variation t r e n l T f C ^ ^ ^ f^nwiu cryat^hzation processes. Water-rock interactional L d c S c a l f f/^®®'']?, Chemical and isotope data from the least

S^sfeiSSSiSlii,^hough the dikes intrude Precambrian basement gneisses of contSenW

SSsS,"?™"" «•»—•1.1. TiO, SI.,"SFIKS" confirmed the role of ^anumf f^^ JSf. important constraiS

u^

dikes has the Paleozoic, an

^ l A L DYKE SWARMS AND RECONSTRUCTION OF THE PLEISTOCENF SUBMARINE VOLCANOES IN THE SHIRETOKO P E N m S U L H A S ^ ^

YGoto, Hokkaido University, Japan NGouchi Shiretoko Museum, Jqxm & TJtaya University of Science, Japan Okoyama

Late Neogene subaqueous volcanic rocks are widely distributed in the Shiretoko Peninsula, Northeast Hokkaido, Japan. They are composed of andesitic to basaltic hyaloclastites, which are associated with many oogenetic dykes and epiclastic volcanic breccias. The geology> petrography, major chemistry and K-Ar age determination of these volcanic rocks from the Cape Shiretoko area were examined to reconstruct the submarine volcanoes in the area. The hyaloclastites in the area have formed dome structure, and the oogenetic dykes have made up radial dyke-swarm in the dome. Existence of a volcanic neck in the center of the dome suggests that the dyke-swarm has made up a submarine strato volcano. The authers revealed that the two submarine strato volcanoes in the area, which have been formed by island arc type volcanism with largely variable compositions(Si02 = 49-64 wt.%) during Late Pliocene(1.7Ma).

EVDDENCE FOR LATERAL MAGMA INJECTION IN THE EARLY MESOZOIC DYKES OF EASTERN NORTH AMERICA JD.Greenough Dqxjrtment of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OWO &JFBodych Department of Earth Sciences, Memorial University of Ney^oundkmd, St. John's, NF, Canada, AlB 3X5.

Evidence from the geametry, ages, geochemistry, and petrofabrics of Early Mesozoic dykes in Eastern North America increasingly suggests that the swarm formed by lateral migration of magma for distances sometimes exceeding 2000 km. Maps of these dykes on Atlantic bordering continents show that prior to opening of the Atlantic they formed a large radiating swarm centered on present-day Florida with dyke density decreasi^ away from Florida. Analogy with smaller-scale radial dyke swarms emanating from a central conduit indicates that this is consistent with lateral injection. Despite wide distribution, most of the dykes were 'enplaced at 195+10 Ma and are geochemically very similar; observations indicative of a common petrogenesis. Geochemically more primitive dykes mostly occur in the south closer to the inferred injection centre. In Atlantic Canada the dyke-fed, 230 km long North Mountain Basalts show fractionation-related geochemical variations consistent with northeastward injection of magma. In the nearby Shelbume dyke centimetre-scale ranping structures cutting multiple chilled margins and truncated at magma-eroded chilled contacts indicate that magma had a subhorizontal northeastward trajectory. Qualitative evaluations of phenocryst orientations in another Atlantic Canada dyke suggest subhorizontal injection, a conclusion si^ported by the orientation of the magnetic anisotrc^y of the dyke. The data are not conclusive but they provide enou<^ si^port for the lateral injection hypothesis to warrant further determinations of magma trajectories.

DYKE EMPLACEMENT AT DIVERGENT PLATE BOUNDARffiS A.Gudmundsson Nordc Vokanohgicd Institute, University of Iceland, Reykjavik, Iceland. As the only subaerial part of the mid-ocean ridges, Iceland offers a unique opportunity to study dike emplacement at divergent plate boundaries Current nfting episodes and inferred dyke emplacement in the rift zone of Iceland can be compared with well-exposed Quaternary and Tertiary dyke swarms. Detailed field observations of some 2500 dykes and inclined sheets in Iceland, in combination with geodetic, seismic and chemical data on currently forming dykes in the rift zone, form the basis of a general model of dyke emplacement at divergent plate boundaries. The following are some conclusions of this model. The direction of flow of magma in dykes is normally either vertical or inclined. Purely lateral flow from crustal magma chambers, resulting in tens-of-kilometers long blade-like dykes, is probably uncommon. This conclusion is supported by (1) the apparent lack of crustal chambers in many dyke areas, especially at slow-spreading ridges. (2) comparison of the type of stress field necessary for generating blade-like dykes with the local stress fields of crustal chambers. (3) strike, dip and thickness distributions of dykes in the vicinity of, and distant from, crustal chambers. (4) the commonly large lateral offsets of dykes, (5) the theoretical volume relationship between dykes and their source chambers, and (6) the chemical difference between lavas proposed to be generated by a single blade-like feeder dyke. Lateral migration of earthquakes during rifting episodes may reflect lateral propagation, of a dyke fracture but flow of the associated magma may still be primarily vertical. Owing to tensile stress concentration, a particular rifting event is likely to start near the centre of the magma reservoir (located at the bottom of the crust) and to spread laterally (and vertically) along the reservoir's roof, thereby opening a pathway for vertical flow of magma from the reservoir. Flow of magma from a chamber fed by the reservoir would also start at approximately the same time and in some cases combine with the magma flow from the reservoir. A dyke formed in vertical flow of magma may. nevertheless, propagate laterally at a certain depth to avoid crustal layers of high compressive stress (stress barriers). Whether a dyke propagates laterally, changes into a sill, or stops propagating on meeting a stress barrier depends on the magmatic overpressure, but also on the anisotropy of the tensile strength of the crust. Low vertical tensile strength favours lateral dyke propagation beneath stress barriers, whereas low horizontal tensile strength favours sill formation. Field observations show that dykes are irregular in shape. Nevertheless, continuum-crack models where the dyke thickness varies as In a flat ellipse are good first-approximation geometric models for most dikes. Of the three basic continuum-crack geometries, i.e., the through crack, the part-through (semi-elliptical) crack and the penny-shaped (and elliptical) crack, the through crack is the most appropriate for feeder dykes but is also a good first approximation for many non-feeders exposed at shallow depths in extinct r i f t zones. Using this geometric model as a basis, and taking the Hooke's law as an adequate constitutive relation for the crust during rifting events, the model predicts that, beneath the uppermost 1-3 km of the crust, the number and lengths of dykes in any particular swarm should increase, but that the thickness should decrease, with depth in the crust. These predictions are supported by field observations.

GEOCHEMICAL AND MINERALOGICAL DYKES, WYOMING CRATON. USA

DIVERSITY

OF PRECAIVlBRIAN

RF. HaU, DJ. Hughes Department of Geobgy, Portsmouth Polytechnic, Portsnmah, UK & GL.Denver, Synder Colorado, USA uses, establish t J ^ ; objectives of studies of Precambrian mafic dyke swarms is to fo f r"^. correlate different types throughout the Jal L l ^ t L interpretations of the late stages of continennrL^ILf M f -^^^ntinental lithospheric mantle composition and melting llTtTr-l lkA I'? f throughout the Wyoming craton of northwestern USA. but unlike swarms in many parts of the world, they tend to have diverse orientations compositions and ages. Elsewhere, dyke swarms can often their consistent strike directions or particular chemical characteristics. In the least deformed parts of the Wyoming craton, where dyke swarms have not been brought into parallelism by subsequent episodes of tectonism, several different sets appear to be present, none of which is particularly more dense than another, or dominates on anything but a local scale, at most within an individual Laramide thrust block. Dykes of different composice^taiity cannot, therefore, be assigned to any one generation with

wohS

Two fundamentally different types of mafic dykes can be easily recognized by simple mineralogical and petrographic criteria. One type has plagioclase as the tirst precipitating phase. These dykes are most commonly ophitic or subophitic dolerites (plagioclase encased in calcic pyroxene), or in some instances plagioclase-phyric. Some are distinctive in that they carry plagioclase megacrysts in a doleritic mesostasis, and have come to be given the informal term "leopard rock" (something of a misnomer in view of their whitespotted appearance). The second broad group is noritic, and in these rocks plagioclase crystallised only as a groundmass or late oikocrystic phase, orthopyroxene and/or pigeonite were the liquidus phases, and calcic pyroxene also precipitated prior to plagioclase. While the compositional ranges of the plagioclases are very similar (An^^-An^j), the calcic pyroxenes are highly variable and differ from one dyke group to another. The pyroxenes in individual samples of doleritic dykes, for example, vary greatly in their Ca, Mg and Fe contents, partly in response to very localised Fe-enrichment of the tholeiitic liquid from which they crystallised, and because of the timing of plagioclase and Fe-Ti-oxide precipitation. They often fall into the field of metastable compositions. The pyroxenes in the rocks in which plagioclase crystallised only at a relatively late stage are more highly variable in Ca content, their more Fe-rich pyroxenes occurring only in the groundmass. The mineralogy of these rocks resembles that of komatiitic and modern boninitic lavas. The geochemistry of the different types of dykes is as distinctive as their mineralogy. The early-plagioclase rocks tend to have continental tholeiitic affinities while, in keeping with their mineralogy, those dykes which first crystallised Ca-poor pyroxenes appear to most closely resemble boninitic compositions. That such diverse compositional dyke types co-exist is highly significant in that different and mixed magmas of these compositions have been proposed as parental to the Stillwater Complex in the northern Beartooth Mountains. Noritic dykes are widespread, occuring not only marginal to this complex, but also in the Bighorn and Laramie Mountains. Disparities between the enrichment of ceratin elements indicate that high-level crustal contamination is unlikely to be solely responsible for their distinctive magnesian yet siliceous compositions.

PRECAMBRIAN MAFIC DYKES OF SOUTHERN GREENLAND RFHaU & DJBughes Portsmouth Polytechmc, UK. The Archaean and early Proterozoic gneisses of South-East Greenland are cut by abundant basic dykes. The recognition of different swarms in this region is hampered by the fact that most dykes were metamorphosed and deformed during the development of the Ammassalik early Proterozoic mobile belt. However, basic dykes in southern West Greenland comprise at least six different sets. Those within the Archaean craton remain as fresh, undeformed intrusions, apart from the effects of deuteric alteration and local faulting, and five dyke generations have been identified by their cross-cutting relationships and petrological characteristics. One of the oldest dyke swarms comprises N-S and ENE-trending magnesian norites (the "BN" dykes; MgO up to 23%), and appears to be densest towards the northern margin of the craton, which is bounded by the Nagssugtoqidian (early Proterozoic) mobile belt. The dykes occurring throughout the central and, more abundantly in the southern part of the craton comprise three generations of dolerites (the "MD" dykes) distinguished by their orientations and progressively evolved tholeiitic geochemistry. The earlier, magnesian dolerites (MgO = 10%) have low and unfractionated rare-earth element (REE) abundances (La = 15, La^/Lu^ = 1), while the REE in the later ones are moderately fractionated (La^^ = 50, La^Lu^ = 2.5), typical of continental tholeiites. The chemistry of these dykes contrasts with that of the northern norites which are strongly LREE-enriched (La^ = 50, La^/Lu^ = 8). A minor set of evolved, LREE- and Tirich, plagioclase-phyric dykes which occurs in the south of the craton appears to have derived from a separate and slightly different batch of magma to that which fed the MD dykes. The dense swarm of tholeiitic metadolerite dykes (the Kangamiut dykes) at the northern margin of the craton, is associated with the development of the Nagssugtoqidian mobile belt. However, there are few geochemical differences between these syn-tectonic dykes and those emplaced into the stable craton. Indeed, continental tholeiite dykes seem to be ubiquitous and to have changed little throughout the geological record. The mineralogy of the various dyke swarms within the Archaean craton of southern West Greenland also helps in distinguishing one type from another, and suggests that these dykes did not crystallise at any great depth. The norites are dominated by zoned orthopyroxene, olivine and later augite, all enclosed by plagioclase, whereas the dolerites are ophitic or subophitic, and contain highly complex, predominantly Ca-poor clinopyroxenes. Cr- and Fe-oxides crystallised early in the norites, but Fe-Ti-oxide is characteristically a late phase in the dolerites. Their complexity indicates that differentiation is as great on a millimetre scale as it is on the scale of the intrusion. The dykes in South-East Greenland could correspond to any of the six generations recognised at the west coast (or to none of them). Most are metadolerites whose chemistry resembles that of the MD and Kangamiut dykes. The dykes in the north comprise granulite or garnet-amphibolite facies assemblages, although many are totally undeformed. Those in the south, on the other hand, have complex primary pyroxenes resembling those encountered in the MD dykes. REE geochemistry confirms that norite dykes in South-East Greenland have the same distinctive compositions as their BN dyke couterparts at the west coast. However, their is little clear evidence of whether most of the dykes were emplaced syntectonically or simply deformed during the development of the mobile belt.

INTRUSION MECHANISMS AND POST-EMPLACEMENT DIFFERENTIATION OF BASIC DYKES RF. HaU & DJ. Hughes Portsmouth Pofytechnic, UK. One of the fundamental objectives in examining the structure and composition of basic dykes is to investigate the mechanisms of their emplacement as reflected by their geochemistry. For example, if a dyke crystallises from a vertical "wall" of magma, then perhaps it should be chemically zoned inwards from its (chilled) margins, upwards in response to crystal settling and the upward migration of progressively fractionated magma during sequential crystallisation, or possibly downwards because of more rapid crystallisation at higher levels in the intrusion (Fig. la). The pattern of chemical zoning may be further complicated if the magma emplacement has a significant horizontal component. Whether dyke magmas are emplaced predominantly vertically, horizontally along the dyke fissure, or fanning away from the source remains a matter of some debate. The reality of such geochemical zoning could only be ascertained by a rigorous vertical, lateral and longitudinal sampling programme. This simple scenario is, in any case, probably overshadoned by such factors as magma flow mechanisms (turbulent/laminar flow), rates of magma supply, cooling and crystallisation, as well as contamination, both in the magma chamber and at dyke - wallrock interfaces. Dyke margin-to-centre zoning can reflect either inward fractionation (basaltic margins to dioritic core), or apparently outward differentiation due to crystal (olivine, pyroxene) accumulation in the centre of the dyke or the continued injection of more primitive or ferromagnesian crystal-rich liquid, Despite the numerous and interactive petrogenetic influences, many dykes show little evidence of any significant zoning on a large scale; they tend to consist of monotonous, massive black rock. However, many do possess abundant evidence for considerable differentiation on a very local scale, and detailed mineral chemical analysis shows that minute samples of tholeiitic dolerite often appear to be a microcosm of differentiation within the dyke as a whole. This is as significant in horizontal as it is in vertical sheets. Horizontal sheets (usually sills) appear to be more efficient recorders of differentiation over the scale of the intrusion. They nearly all show layering to lesser or greater degrees, and in the simplest analysis differentiation can be considered in one direction, upwards (Fig lb). However, even strongly differentiated sills comprise small-scale assemblages which themselves record gross differentiation trends and paradoxically, can thus be interpreted to have crystallised as closed systems, contrary to the obvious evidence in the sill of bottom-to-top differentiation. Different types of extreme zoning trends encountered in basic dyke pyroxenes may be indicative not only of differences in magma type, but also of different magma flow mechanisms.

I V ^^ t [H N \>'\ \

(iil

4

"(il-

ia)

(b)

Figure 1. Possible chenical structure (dashed lines) and zoning trends (arrows) in (a) a vertical dyke, and (b) a horizontal sheet. The possible trends in (a) reflect (i) margin-tocentre zoning, (ii) variation with respect to height from the base (and from the top?) of the intrusion, and (iii) zoning with respect to distance away from the focus of intrusion. The chemical zoning in (b) is relatively simple layering.

THE EVOLUTION OF THE 2.45 GA MATACHEWAN DYKE SWARM, CANADA HCHaUs & MP Bates JT. Wilson Research Laboratories, Dept of Geobgy, University of Toronto, Erindale Campus, Mississauga, Ontario, Canada LSL 1C6

The Matachewan dyke swarm covers about 350.000 km^ and radiates northwards from a focal region centred approximately in Georgian Bay south of Sudbuiy. The average dyke width is about 20 m and the total magma volume is about 100,000 km\ Recent geochronological work suggests^ that the swarm developed over a period of about 5 Ma. The fan-shape of the Matachewan swarm IS relatively distorted, particularly where it crosses the Kapuskasing Structural Zone (KSZ) alone which d ^ p crust has been thrust to the surface about 2 Ga agol Detailed paleomagnetic work''^^ shows that the Matachewan swarm comprises dykes exhibiting opposite magnetic polarity and that a single reversal, from reversed to normal polarity, was captured during the magmatic episode. A peculiar feature of the swann is that a 50 km-wide zone of exclusively normal polarity dykes is found within the KSZ and predominantly reversed polarity dykes outside the zone along strLSce to the north and south. Either major transcurrent movements have offset Ae swarm several lO's of kilometres along the major KSZ boundaiy faults, or the polarity disconnnumes are related to differential upUfi along these faults, in which case the normal dykes represent a deeper and younger expression of the Matachewan swarm. Ongoing paleomagnetic studies are testing the transcurrent fault hypothesis but have yet to find convincing evidence of offset polarity zones, while the correlation of polarity with crustal depth finds support from petrographic differences between dykes within and outside the KSZ\ Since Matachewan dykes are absent from the highest grade granulite zones within the KSZ, a model of dyke swarm emplacement can be proposed whereby dykes were denved from magma chambers in the focal region and injected laterally in the upper part of <the crust as blade-like intrusions which migrated to greater depths in^ the later stages of swarm development. This model is supported by observations of lateral magma movement in Icelandic dykes of comparable width/length ratios^ and by field observations on Matachewan dykes which suggest a northward component of flow'. Also northward branching dykes outnumber those branching southwards by up to 2:l^ a possible consequence of magma derivation from the focal region. If the focd area was uplifted prior to magmatism (cf the 1.27 Ga Mackenzie dykes of northern Canada') radial cracks would form and bifurcate outwards as they propagated. Magma following these cracks would then generate the bias in dyke branching direction. In the general region of the Matachewan focus, volcanics, dated at 2.45 Ga form the basal unit of the Huronian\ Several gabbro-anorthosite bodies such as tiie East Bull Lake and Dunlop occur in the same region. These bodies yield U-Pb dates about 30 Ma older^ than tlie Matachewan dykes but they could represent the earliest (and shallowest) magma feeder chambers. Significantiy, for approximately 200 Ma before and after 2.45 Ma, they represent the only mafic igneous activity, other than the Matachewan dykes, to have occurred in the Superior Province and the coincidence of this activity with the focal region is strong evidence in favour of a genetic link with the Matachewan dykes. The foregoing observations bear a striking similarity to \hc Mackenzie swarm, in terms of the large size and magma volume of the swarm, the rapid 5 Ma duration of magmatism', tiie overall fanshape and the presence of volcanics and gabbroic plutons in the focal region. The Mackenzie dykes have recentiy been interpreted® as products of a hot mantie plume and a similar explanation may therefore hold for the Matachewan dykes. In both regions there is evidence for repeated mafic activity. In the Georgian Bayiake Superior region mafic activity occurred at 2.4510.5; 2.15±0.1; 1.85 and at 1.15ip.l Ga. In ihs Lake Superior region - 2.1 Ga dykes radiate from a region south of tiie lake tiiat includes the giant 1.6 Ga Wolf River granite batiiolitii. Recentiy tiie Keweenawan volcanics of the 1.1 Ga Mid>^ Continent Rift have also been interpreted^ as a mantie plume with a centre that is virtually identical with the earlier 2.1 Ga one. The Georgian Bay-Lake Superior region is one of three nodes of mafic magmatism tiiat border Archean cratons in Canada^® and may represent a long-lived site of episodic rDaiitle plume ascent. References: l:Heaman,1989; 2:rtrcival,1988; 3:Halls and Palmer,1989; 4:Bates and Halls,1989; 5:Sigurdsson,1987; 6:Halls.l982; 7:Krogh et al.,1984; 8:LeCheminant and Heaman,1989; 9:Hutchinson

a

mFahngJWr.

ISOTOPIC CONSTRAINTS ON THE PETROGENESIS DIABASE IN THE SOUTHWESTERN U.S.A.

OF

PROTEROZOIC

JGHammord Pasadena City College, Pasadena, CaUfbmia, USA. 91106 & JLWooden US. Geological Survey, Menlo Park, CaUfbmia, USA. 94025 In the s o u t h w e s t e r n U . S . A . , P r o t e r o z o i c d i a b a s e d i k e s and s i l l s e x t e n d from w i d e s p r e a d l o c a l i t i e s in c e n t r a l A r i z o n a to the western l i m i t of Precambrian c o n t i n e n t a l crust in the southern Death Valley region of C a l i f o r n i a . W i t h i n this area, dikes are most abundant in the c r y s t a l l i n e basement of the Colorado River trough r e g i o n . The d i a b a s e s a r e a b o u t 1.1 Ga o l d (based on reports for a few Arizona l o c a l i t e s ) , and this age is accepted for a l l of the samples because of their close geochemical kinship. The diabase is generally mildly a l k a l i n e , high in A I 2 O 3 , TiOo, and P205# low in CaO, and mildly LREE enriched. A l s o , two chemical groups have been recognized from P / Z r r a t i o s . The low P/Zr group is present throughout the r e g i o n , but the high P / Z r group is absent in the Death Valley region. New Pb and Sr isotopic data give further information on the mantle source and evolution of these rocks. Sm-Nd isotopic analyses are in progress. T h e o v e r a l l p a t t e r n o f t h e Pb i s o t o p i c d a t a indicates distinctive local mantle source characteristics. Despite contamination e f f e c t s obvious in a few samples, three groups are d i s t i n g u i s h e d on 2 0 6 P b / 2 0 4 P b versus 2 0 8 P b / 2 0 4 P b p l o t s . Central Arizona samples l i e along the Stacey-Kramers curve for average c r u s t (S-K) w i t h 2 0 6 P b / 2 0 4 P b = 1 7 . 3 - 1 8 . 5 . C o l o r a d o R i v e r trough samples l i e above and p a r a l l e l to S-K with 206Pb/204Pb =16.7-18.1. Death V a l l e y area samples also plot a b o v e S-K b u t with 206Pb/204Pb=17.8-19.0 and with samples from the Ibex H i l l s forming a lower 208Pb/204Pb subgroup. 206Pb/204Pb versus 2 0 7 P b / 2 0 4 P b shows the Death Valley samples to have the highest 2 0 7 P b / 2 0 4 P b . These d e f i n e a slope consistent with an age greater than 1.1 Ga, except the Ibex H i l l s subgroup d e f i n e s a tight group at s l i g h t l y higher 2 0 7 P b / 2 0 4 P b . Most of the Colorado River trough and central A r i z o n a samples d e f i n e a single s c a t t e r e d trend a l o n g a 1 . 1 Ga r e f e r e n c e i s o c h r o n w i t h l o w e r 2 0 7 P b / 2 0 4 P b , but anomalous data for the Turtle Mountains show high and scattered 2 0 7 P b / 2 0 4 P b and greatly lower 208Pb/204Pb values. Four c e n t r a l A r i z o n a and two C o l o r a d o R i v e r trough s a m p l e s d e f i n e a cluster with the lowest 2 0 7 P b / 2 0 4 P b values. These six are a l l high P/Zr rocks, and probably represent the most p r i m i t i v e magmas. C o n v e r s e l y , the h i g h 2 0 7 P b / 2 0 4 P b s a m p l e s of the Death Valley area are from the more evolved, low P/Zr group. The Sr isotopic data show c o n s i d e r a b l e s c a t t e r , and there is no relation between these data and geographic area or high and low P/Zr groups. However, initial 87Sr/86Sr ratios calculated assuming an age of 1.13 Ga c l e a r l y demonstrate that much of the diabase has retained its o r i g i n a l mantle source c h a r a c t e r i s t i c s . About h a l f of the s a m p l e s ( i n c l u d i n g four of the s i x s a m p l e s judged most p r i m i t i v e on the b a s i s of Pb isotopes) have i n i t i a l ratios of 0.699 to 0.704. It is assumed that the scatter in the Sr data is due p r i n c i p a l l y to c r u s t a l contamination. The samples with the highest ratios (0.708-0.717) were previously i d e n t i f i e d as h a v i n g a s i g n i f i c a n t c r u s t a l c o m p o n e n t for r e a s o n s such as abnormally high Si02 or K2O.

ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) IN BASALTIC DYKES RB. Hargraves, KV. Cashrmn & AE. Kneedler Department of Geobgicd and Geq)hysical Sciences, Princeton University, Princeton, NJ 08544, USA

D r e s u m S K l f i ? i.

MTT^ ^^^ around the ^^^ P^^fe^ed orlenlation of anlsotroplo ferrimagnetlo spinel cry^als Induced during flow of the magma or by growth during later cWallizatlon Two problems hindering the Intuitive understanding of AMS In these terms arei l)^^^^ Tl5Jt phenocrysts found In hypabyssal basalts are usually Irregularly s u K i r t o euheSral. but insistent departure from an effectively equant average shape be established No preferred orientation during flow would be expected; (2) l^ost spinels occur I n t S l a l l v and c S

tKlignmem?

^ a t oouid

^

AMS In cores from three basaltic dikes from swarms on the east coast of Brazil is conspiouous (K1/K3 from 1.03 to 1.14) and consistently oriented within site: K3 perpendicular to.. and Ki near horizontal and parallel lo - the strike of the dikes. Consistent with this. Indeperident field study of macroscopic features related to magma movement Indicates near horizontal ftow In these dikes to the W or NW parallel to strike (Correa Gomes, et al.. Workshop "Dlques Maficos Precambrlanos do Brasil,' 1989). From represenlaiive cores, thin sections were cut parallel to the Ki-Kj plane of their AMS ellipsoid. The Ti-Mt grains vary from subskeleial (especially In the thinner dikes) to ^regular subhedral. becoming progressively coarser toward the center of the dikes (see [ ®' volume). There are some microphenocrysts In the center of the thickest d»ce, but most of the spinels are interstitial and appear to have crystallized late. There Is no orientallon of these roughly equant grains, and their generally fine-graln and irregular habit discourages attempts at systematic orientation measurement. i'?'?".®'" O""®®'®' anisotropy. there is a marked orier^ ation of the plagiociase laths which Is symmetric with the AMS ellipsoid: long axis parallel to Ki. short axis parallel to Kj. The cores of these plagiociase crystals undoubtedly crystallized early, and their preferred orientation must be related to flow In the magma. The common symmetry with the AMS suggests that the preferred orientation (and/or distribution) of the late-crystallizing spinels is imposed by the preexisting feldspar fabric (Stacey, 1960)1 The anisotropic "shape" of the residual liquid volumes at the time the Interstitial spinels crystallized must favor the probability of any elongate growth (however slight) being parallel to the ambient feldspar template. If It proves true In general that AMS Is primarily a direct reflection of the preferred orientation of the early crystallizing silicate phases, then application of AMS to petrofabrlo and flow-dynamic studies in all Igneous rocks can proceed with better understanding and confidence. Ref.: Stacey, F. D. 1960. Magnetic anisotropy of Igneous rocks, J. Geophy. Res., 65, 2429.

A THREE-COMPONENT COMPOSITE DYKE AND INTRUSION, POINTE DU CRIARD, QUEBEC, CANADA

ITS

ASSOCIATED

MDHiggins Science de la terre, Universite du Quebec a Chicoutirni, Chicoutinu, Quebec, Canada, G7H 2B1

The unmetamorphosed, Cambrian, Pointe du Criade intrusion comprises a composite dyke about three metres wide which can be traced direqtly into a composite sill at least SO metres thick. The margins of the dyke arefine-graineddiabase, chilled at the contact TWs passes, over an interval of a few centimetres* into a leucogabbro* with crystals up to 2 cm lone. The contact is generally sinuous. The interior of the dyke is occupied by syenite, with a grain-size of about 1 cm. The sinuous contact with the leucogabbro is again gradational over a few centimetres and there are bulbous enclaves of leucogabbro within me syenite. This dyke can be traced without intemption into a sill. The lower contact of the sill has zones of diabase and leucogabbro about 1 metre wide similar to those in the dyke. The lowest part of the syenite zone of the sill is choked with xenoliths, mostly of diabase but with some of leucogabbro, which are normally size-sorted Many of these xenoliths are elongated, and can be up to two metres long, yet only ten cm wide. The bulk of the sill comprises syenite, with a very small proportion of mafic enclaves. The upper contact is not observed here. This sill can be traced over an area of 5 by 10 km, but the feeder dyke has not been observed elsewhere. In other parts of the sill the thickness of the different zones are very variable, and some may be repeated or omitted. There is also a fourth component, a monzonite, which was mixed with the syenite. Several conclusions can drawn from this intmsion: 1) The composition of a feeder dyke need have Uttlc in common with that of the intrusion that it feeds. Similarly, a single, well-defined intrusion can have extremely variable composition. 2) The number and variety of the components renders it unlikely that they were all derived from a single, stratified magma chamber: it is more likely that they were derived from several chambers and were mixed, and/or mingled in the dyke. 3) The bulk of the syenite in the sill displays no evidence of it's composite origin, except the presence of mafic xenoliths. 4) The initial fracture was opened up by a less viscous magma (diabase) which was Mowed by progressively more viscous magmas. The presence of mafic enclaves and/or mafic borders in some anorogenic felsic intrusions suggests that they may have been emplaced in this way. 5) Most of the diabase enclaves are too elongated to represent flakes of solid material. It is more likely that they are the result of magma mixing in the conduit

DYKE PROPOGATION RELATED DEFORMATION ASSOCIATED VOLATILE-RICH PHASE AT DYKE TIPS

PROCESSES

AND

JD.Hoek ^ Department of Stmctural Geology, Institute of EarOi Sciences, PO Box 80021, TA 3508 Utrecht, Netherlands

Microstructural investigation of dyke tips confirms the general presence of a process zone immediately ahead of the dyke. These process zones comprise the wallrock material undergoing plastic deformation due to dyke intrusion. The plastic deformation processes involved govern the actual dyke propagation mechanism. Elastic modelling predicts process zones in front of dyke tips, where elastically induced stresses increase beyond the tensile strength of the rock. Observed process zones are characterized by grainsize reduction and often show carbonate enrichment and/or replacement. The "tip" of the magmatic material is characterized by the presence of two distinct phases; a dark-colored phase and a light-colored phase. The darkcolored phase is the 'normal" magmatic rock comprising the bulk of the dyke. The light-colored phase is only present in the dyke tip, is rich in biotite, generally contains carbonate, and lacks opaque minerals. It further shows flow-interleaving with the dark-colored phase and contains similar phenocrysts. The light-colored phase is significantly rich in K, H2O and CO2, and poor in Si and S relative to the dark-colored phase. This indicates that the light-colored phase comprised a relatively volatile-rich fluid during intrusion. It is probably partly wallrock-derived. These observations suggest that dyke propagation takes place due to brittle tensile microfracturing in a small zone immediately ahead of the magma. This process may be significantly assisted by the presence in the dyke tip of a volatile rich fluid phase. The thus formed microbreccia is subsequently consumed in the magma. keywords: dyke propagation volatile phase

process zone brittle microfracturing

THE EMPLACEMENT MECHANISM OF CARBONATE BRECCIA DYKES J.D. Hoek Department of Structural Geology, Institute of Earth Sciences, PO Box 80.021 TA 3508 Utrecht, Netherlands

In Archaean basement of East Antarctica breccia dykes are emplaced in Late Proterozoic or possibly Phanerozoic time at relatively shallow cnistal levels. The breccia dykes are thin and occur preferentially on the borders of pre-existing lamprophyric dykes. The breccia consists of a matrix rich in carbonates, and contains angular fragments of wallrock (lamprophyric and gneissic material) as well as fragments of pseudotachylite. Interfaces between carbonate-rich breccia matrix and pseudotachylite fragments are lobate, giving the pseudotachylite inclusions an amoeboid appearance. Both phases include each other as globular vesicles. The pseudotachylite and the carbonate-rich material must therefore have been present as two separate viscous fluid phases during breccia emplacement.'The fluid nature of the breccia matrix during emplacement implies a (possibly shallow) magmatic origin for the carbonate-rich material. The pseudotachylite fragments are derived from the contacts between breccia and wallrock, where this material is often present as a thin coating. Where pseudotachylite occurs by itself it contains abundant carbonate- and quartz- filled vesicles indicating low confining pressures during pseudotachylite formation. Pseudotachylite fragments were incoiporated in the magma in a molten state. Geometrical relationships imply that pseudotachylite was formed due to the breccia dyke intrusion. The carbonate-rich magma intruded along preexisting dykes, generating fractures on their borders which were oriented oblique to the regional stress field. Pseudotachylite formed due to slip on these fractures ahead of the magma, where the opposing walls still exerted sufficient friction. Pseudotachylite formation implies seismic rates for fracture propagation. In fracture mechanics terms, the fractures were initially generated as mode III (anti plane shear) fractures, and subsequently dilated by the carbonate-rich magma. keywords:

breccia dyke mode III f r a c t u r e s

seismicity pseudotachylite

PALAEOMAGNETIC AND ^AR/^AR GE0CHR0N0LCX5ICAL STUDY OF A COOLING fflSTORY IN THE GRENVILLE PROVINCE USING A DYKE CONTACT H.Hyodo, Hiruzen Research Institiue, Okayama University of Science, Okayama 700 Japan DJ. Dunlop Laboratory, & D. York University of Toronto, Ontario M5S 1A7 Canada Geophysics

M a g n e t i c and a r g o n i s o t o p i c s y s t e m s a r e o f t e n c o m p a r e d as having mathematically s i m i l a r s t r u c t u r e of t h e i r thermal diffusion process. A paleomagnetic contact test and laser steph e a t i n g 40Ar/39Ar age analysis of a dyke contact near•Mattawa, O n t a r i o , C a n a d a in t h e s o u t h w e s t e r n p a r t o f t h e Grenville P r o v i n c e w e r e c a r r i e d o u t to s e e k m u t u a l c o n s i s t e n c y o f t h e t w o systems. T h e i s o c h r o n a g e of t h e d y k e w a s e s t i m a t e d ;to b e 5 7 0 ± 3 M a from a 0.5 m m - d i a m e t e r c h i l l e d m a r g i n s a m p l e . T h e p a l e o m a g n e t i c o v e r p r i n t i n g t e m p e r a t u r e a n a l y s i s g i v e s an e s t i m a t e of p a l e o a m b i e n t t e m p e r a t u r e o f 184 ± 4 0 ^C . F r o m t h i s v a l u e , t h e b u r i a l d e p t h o f t h e a r e a w a s c a l c u l a t e d t o b e 6 . 5 ± 1 , 7 k m , a s s u m i n g 16®C of s u r f a c e t e m p e r a t u r e a n d 26^C o f g e o t h e r m a l g r a d i e n t a t t h e t i m e of i n t r u s i o n . A l t h o u g h it w a s n o t p o s s i b l e to c o m p a r e t h e p a l e o - a m b i e n t t e m p e r a t u r e w i t h o n e o b t a i n e d from argon release p r o f i l e around the dyke contact, a hypothetical cooling curve obtained from 4 0 A r / 3 9 A r b l o c k i n g t e m p e r a t u r e a n a l y s i s of h o r n b l e n d e , biotite a n d K - r i c h f e l d s p a r g r a i n s is c o n s i s t e n t w i t h t h e p a l e o m a g n e t i c result. The estimated ambient t e m p e r a t u r e was compared w i t h the prev i o u s r e s u l t s in t h e C a n a d i a n P r e c a m b r i a n S h i e l d , a n d u p l i f t a n d c o o l i n g h i s t o r y o f the a r e a w i l l b e d i s c u s s e d .

MIDDLE PROTEROZOIC AND YOUNGER DYKES OF THE ADIRONDACK MOUNTAINS, NEW YORK, USA Y.V. Isachsen Geobgical Survey - New York State Museum, Albany, N.Y.

More than I3OO dikes, mainly vertical to subvertical, have been mapped in the Adirondack Mountain dome. Most are 1 m thick and are exposed over only a few meters, but several are exposed intermittently for distances up to I5 km, and have thicknesses ranging to lO m. Twenty four types are represented, ranging from ultramafic (pyroxenite,peridotite) to felsic (granite). All are restricted to the 1.1 b.y. basement, except for the lamprophyres which intrude the Paleozoic cover marginal to the dome. Mafic rocks clearly predominate. Their abundances, in decreasing .order, are as follows1 diabase and olivine diabase, unsubdivided "mafic dikes", basalt, gabbro, metagabbro, metadiabase, hypersthene metadiabase, and garnet metadiabase. Next most prominent are the lamprophyres (6 types) and granite pegmatites. Two unusual lamprophyre dikes Imve been found that contain 50 percent subangular to round xenoliths concentrated in the central part of the dike by flow differentiation. The xenoliths comprise a variety of metamorphic gneisses. Possible explanations arei 1) incorporation of an unexposed basal conglomerate in the Paleozoic sectioni 2) incorporation of a fault breccia followed by mechanical abrasion and possible resorption while entrained in the rising magma. Rose diagrams show the prominent strike directions to be as follows! 1) N3CE fanning to N70E for the basalt, diabase, olivine diabase, and "mafic dikes"', which corresponds to the dominant trends of faults and zero displacement crackle zones in the eastern Adirondacksi 2) WNW to EW for the lamprophyres; 3) NS to NNW for the metajnorphosed dikes. In an attempt to work out a paleostress history for the region 25 K/AT dates were obtained on mafic dikes. Howevei^ upon learning of an episodic argon loss at 180 Ma based on ^^ATP^AX release spectra, it became clear that the K/AT dates, ranging from 588 Ma to 261 Ma, are minimum ages, and do not date extensional events in the Phanerozoic. For the lamprophyre dikes, which post-date this 180 Ma event, however, K/Ar dates suggest an EW to WNW maximum compressive stress during Late Jurassic-Early Cretaceous time. Tectonically, the mafic dikes fall into three classesi 1) undeformed but statically metamorphosed to granulite facies in the Middle Proterozoic, with retained chill margin textures| 2) post-metamorphic swarms of Late Proterozoic age in the eastern Adirondacks that record the intracontinental rift-related volcanism preceding the opening of lapetus; 3) lamprophyres and associated dikes of Middle Jurassic to Middle Cretaceous age (1^6 Ma - 1?3 Ma), related to the thermal event that produced the Monteregion Hills of Quebec and the White Mountains Magma Series of New Hampshire.

SPATIAL ASSOCIATIONS BETWEEN POST-CRATONISATION DYKES AND GOLD DEPOSITS IN THE YILGARN BLOCK, WESTERN AUSTRALIA DJJsles & ACCooke World Geoscience Corporation, 17 Emerald Terrace, West Perth, Western Australia Although reliable dating evidence shows that the mafic/ultramafic dyke suiteCs) in the Yilgarn Block clearly post-date the main gold mineralising event(s). compelling spatial associations between the two geological phenomena are observed.

Often dismissed as being fortuitous, these associations are observed in seven of the ten major gold mining districts. A rigorous statistical assessment of the likelihood of dykes being present within the confines of a "gold camp" has not been attempted and would be difficult to formulate, given that much of the Yilgam gold is as yet undiscovered. Exposure in the Yilgam Block averages less than 10% and the dykes which are mainly unaltered, undeformed and of Proterozoic age, very rarely outcrop. By contrast, the dykes in contrast are well documented from both Government regional aeromagnetic data and high resolution non-exclusive aeromagnetics flown for mineral exploration. This paper aims to present evidence, through the aeromagnetic data, of the spatial associations and proposes that they can be explained by the dykes preferentially occupying planes of weakness established prior to gold mineralisation. The EW compressive event which caused brittle fracture of the Archaean Craton and largely passive intrusion of mafic material during the Proterozoic may have been a late stage variant of the stress regime which controlled the strike parallel shearing and created the dilatant zones in which much of the Yilgam gold was "deposited". Regardless of the validity of this interpretation, it is highly likely that the old shearing events set up a system of regional planes of weakness and equally likely that the post cratonisation, EW compression caused a ""reactivation" of these planes of weakness. The implication is that astute interpretation of the patterns of dyke distribution in relation to the ""strike parallel" tectonic events could potentially offer a means of identifying palaeo low stress zones which would have been very suitable sites for gold "deposition". The examples used to illuat ;he inferred associations are based on 1:1m ^ale, 1:250^000 resolution aeromagnetics and lead to .the coi aion that although geologically questionable the exploration problem at i inands that they be taken seriously and vigorously investigated.

PALAEOMAGNEnSM AND ITS RELEVANCE TO MAFIC DYKES

DL. Jones Departmmt of Physics, University of Zimbabwe, Harare, Zimbabwe

Palaeomagnetism involves the study and interpretation of the fossil magnetism in rocks. Sampling for palaeomagnetic purposes is done by either in situ drilling or blocking. Each method has advantages and disadvantages as do the various instruments used to measure remanent magnetization. A large number of results in the older literature are based on the blanket demagnetization technique; some of these are erroneous or incomplete but those* involving simple magnetic systems and history are valid and useful. However, palaeomagnetism, even of igneous rocks, is often a complex phenomenon involving a number of magnetic minerals, various types of magnetization, often with different times of acquisition, and overlapping coercivity and blocking temperature spectra. Thus if meaningful, complete results are to be obtained, careful detailed alternating field and thermal demagnetization of each sample is necessary and more recently developed analytical methods must be applied. These include principal component analysis and demagnetisation circle extrapolations. Where a number of magnetic components are identified it is extremely important to establish the relative and absolute ages of acquisition. Thus rock magnetic and radioisotopic work are necessary companions to palaeomagnetic studies• It has been established that mafic dykes accurately record ancient ambient magnetic fields. As a result palaeomagnetic studies on dykes are useful for a number of purposes. Early work established the Mashonaland and Umkondo dolerites as separate igneous episodes in the Zimbabwe craton and it is possible that palaeomagnetism may soon be useful in routinely mapping individual Mashonaland sills across the craton» Recently a number of dykes have been correlated with the Great Dyke of Zimbabwe. Palaeomagnetic work has been partly instrumental in establishing the vast extent of the Mackenzie swarm in Canada and the regional nature of the Umkondo igneous episode in southern Africa. Accurate palaeomagnetic and geochronological studies provide anchor points on apparent polar wander paths thereby facilitating inter-^continental comparisons. The magnetic record in a rock can reveal the tectonic history of an area. Such work has been done successfully in Zimbabwe where the Great Dyke and its satellites cross into the Limpopo and Zambeisi Belts. Studies of dykes, their baked contacts and beyond have been used to determine the primary nature of their magnetization• Canadian workers have shown that such studies can also provide information on the temperature and depth of intrusion of dykes and other related thermal parameters• Dykes can be used to study too geomagnetic field reversals and secular variation. It is clear that palaeomagnetism, which contributed so much in establishing the Theory of Plate Tectonics, is a powerful and versatile tool in the study of mafic dykes.

PETROLCXJY AND GEOCHEMISTRY OF THE UBENDIAN (EARLY PROTEROZOIC) MAFIC INTRUSIONS AND DOLERITE DYKE SWARMS OF THE MARUNGU PLATEAU (EASTERN ZAIRE) MXabengele, RTLubuIa, KTshimanga & DKapenda Department de Geobgie, Urdversite de Lubumbashi, B P, 1825 Lubumbashi, Zaire

Compared to most of the neibouring countries such as Zambia and Tanzania, the information about the mafic dyke swarms in Zaire is largely limited to regional mapping at a scale of 1:200 000 or more and much of the country has not been mapped in detail. So, geological information about dyke swarms are scarce; this situation is worsted by the lack of aeromagnetic background. The zairian working group has selected three areas of interest for the study of mafic dykes : (i) the eastern part (with possible correlation with Zambia, Tanzania, Rwanda, Burundi and Uganda); (ii) the central part; (iii) the western part (with possible correlation with northern Angola and Congo). This paper presents results gathered in the southeastern part of Zaire where early Precambrian mafic dykes are well exposed especially the central part of the Marungu plateau. They occur as NE-^SW, NW-SE and E-W trending dolerite dykes of variable width (10 - 100 m) and constitute a swarm about^ 50 km wide and 100 km long. The dolerite dyke strikes are concordant with the regional network. They intrude the calc-alkaline magmatic complexes of Ubendian age dated 1950 1700 Ma and thus are younger than 1700 Ma. From petrological point of view, the studied rocks correspond to olivine or quartz bearing gabbros and dolerites. On the whole, their mineralogy is fairly simple with plagioclase ± olivine ± bronzite + augite + subcalcic augite ± pigeonite ± Cr-free spinels. Chemical characteristics of these minerals and host rocks are similar to those of extensional regions magmas types series, especially continental tholeiites. However,they are enriched in LIL-elements (K, Rb, Ba) and have Th/Ta ratios ranging from 1.8 to 5.6. These values are intermediate between those of orogenic series (lAT and CAB) on the one hand, and MORB, continental tholeiites, alkaline and transitional basalts on the other hand. From geodynamic point of view, it is suggested that this tholeiitic magmatism could be related to the relaxation stage that followed the Ubendian compressive phases.

A DYKE SWARM OF CHHOTA UDEPUR AREA, GUJARAT STATE, INDIA RV. Karanth Department of Geology, Faculty of Science, M.S. University of Baioda, Vadodam - 390 002, India D. Sridhar Iyer G.O.D., NXO., Dona-Paula, Goa, 403 076, India S. Viswanalh C.SRR, LLT., Bombay, 400 076, India

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REGIONAL DYKE SWARMS RELATED TO THE DECCAN TRAP ALKALINE PROVINCE, INDIA SGKarkare & RKSrwastcm Department of Geology, Bancaras Hindu University, Varanasi - 221 005, India

Dyke s w a r m s r e l a t e d to the a l k a l i n e s u b p r o v i n c e w i t h i n the D e c c a n T r a p s can bee c l a s s i f i e d in to (a) linear s w a r m s and (b) radial swarms. Of t h e s e t h e f o r m e r are p a r a l l e l to two major l i n e a m e n t s viz. S a t p u r a s t r i k e a n d W e s t C o a s t fau 1 t / D h a r w a r s t r i k e . G r a b e n s and g r a v i t y h i g h s t r u c t u r e s are n o t e d p a r a l l e l to West C o a s t f a u l t and the N a r m a d a - S o n r i f t ( g r a b e n s ?) f o l l o w s an o l d line of w e a k n e s s dating back to P r e c a m b r i a n time. The radial s w a r m s are r e l a t e d to c e n t r e s of e r u p t i o n r e l a t e d to l i n e a m e n t s . Drainages also reveal the structural control such as (a) centrifugal and a n n u l a r in d o m i c a l parts r e l a t e d to c e n t r e s of e r u p t i o n , (b) m o d i f i e d d e n d r i t i c p a t t e r n a n d / o r trellis pattern o v e r d y k e s , and (c) a n g u l a t e or t r e l l i s p a t t e r n in s e d i m e n t a r i e s in the v i c i n i t y of the r i f t v a l l e y .

The lithology of the dykes is d o m i n a t e d by tholeiitic b a s a l t s and d o l e r i t e s a n d c o v e r s a w i d e v a r i e t y of o t h e r l i t h o t y p e s s u c h as C a r b o n a t i t e s , A l k a l i n e r o c k s , Pyroxenit^es, P i c r i t e s e t c . Several lithotypes occur in concentric fashion suggestive of r e s u l t a n t a r c u a t e t e a r i n g f r a c t u r e s ( cone s h e e t s ) and t h e s e are r e l a t e d to u p d o m e d p a r t s .

From a chronological sequence of e v e n t s in the various c e n t r e s of e r u p t i o n i n c l u d i n g n u m e r o u s ring c o m p l e x e s it m a y be c o c l u d e d t h a t dyke s w a r m s m a r k the t e n s i o n a l e p i s o d e of a t e c t o n i c c y c l e w h i c h is f o l l o w e d by a m a g m a t i c c y c l e . The b a s a l t i c r o c k s mark the m a j o r part of t h i s a c t i v i t y (CFB). The ring complexes represent the approaching end of the tectonic cycle and the C a r b o n a t i t e - N e p h e l i n i t i c p l u g s m a r k the d y i n g o u t of the m a g m a t i c cycle.

RELIABILITY OF PALAEOMAGNETIC DYKES OF THE UKRAINIAN SHIELD

INFORMATION ON

PRECAMBRL\N

AYa. Karzancm <$: NJP. MikhaUova Institute of Geqphysics, Ukrcunian Academy of Sciences, 252680 Kiev, Pdladin USSR

32,

A reliability of paleomagnetic information is a rather keen problem of the Precambrian terrains due to poor knowledge of the conditions of preservation of primary ferromagnetic minerals produced in a particular rock during its formation. In the study of Precambrian dykes from the Ukrainian shield, some rocks perspective for obtaining paleomagnetic information'are revealed to include: gabbro-dolerites, dolerites, lamprophyres, quartz porphyreies with one-component thermoremanent or multicomponent magnetization with stable characteristic components reliably derived from the latter. Magnetic mineralogical analysis of these rocks reveals a n u m b e r of magnetic-mineralogical informativity of dykes;: 1. the absence of superimposed metamorphism, e.g. amphibolization; the latter gives rise to the emergence of secondary magnetite at the expense of iron silicates transformation to result in a compilation of primary magnetization. However, the xistnece of relict TRI.i is likely to occur in some r o c k s , as evidenced by the Proterozoic dyke set in the Bazavluk a r e a , becajrse temperature of amphibolization does not exceed 500^0; 2. the existence of fine inclusions of ferromagnetic-carrier of onecomponent TRM in rock-forming silicates (pyroxenes, plagioclases); 3 . The presence of relicJs of primary ferromagnetic minerals in chilled margins; 4# the rocks with myrmekite-like cotectic fabrics where silicates are associated with titanomagnetite showing fine exeolutions. Curie point is here 570-^90^0.

ON THE USE OF QUANTITATIVE TEXTURAL INDICATORS OF DYKE-FLOW DYNAMICS

MEASUREMENTS

AS

AE. Kneedler, K.V. Cashman & R£. Margraves Department of Geological and Geopt^acd Sciences, Princeton University, Princeton, NJ 08544, USA

The prevalence and relative simplicity of thermal and mechanical models of dike injection and cooling have made dikes an attractive target for studies of crystallization in basaltic systems A paucity of good textural data, however, has left many numerical models untested, and major questions still remain concerning both specifics of flow dynamics during dike emplacement and the relationship of emplacement mechanism and cooling history to the resulting fabric of an individual sample - we beUeve that measured crystal size distributions and orientations of groundmass and phenocryst phases can be used to constrain both aspects of dike formation We have made quantitative textural measurements of oxides and plagioclase across a 30m diabase dike intruded into granulites in Salvador, Brazil. Plagioclase exists primarily as a ^oundmass phase and increases systematically in average size from the margin to the interior of the ( ^ e . Crystal size frequency distributions are approximately log-normal, with the frequency maxima skewed to the very small sizes; the size class of maximumfi-equencyincreases from the margin to the intenor. We suggest that plagioclase size distributions are a direct result of cooUng and emplacement history - plagioclase growth rates at the center of the dike can be estimated at -10 cm/sec at minimum for a simple conductive cooling model. Titaniferous magnetites are present as both groundmass and phenociyst phases. Crystal size distributions of these show a maximum in the smallest size class, with far fewer larger crystals - this pattern probably reflects two septate episodes of oxide nucleation. Average oxide sizes also increase from the margin to the i^tenor, but not in the simple manner predicted for diin dikes or sills (e.g. Spohn et al., 1988) We feel that tiiis discrepancy is due to redistribution of phenocryst phases during flow Additional evidence for this conclusion includes the variation of both major and compatible trace elements across the dike in a manner consistent with measured variations in modal abundance of phases; in contrast, incompatible trace elements for the same samples remain constant across the entire c ^ e width. These chemical data support the hypothesis of a single batch of magma with unevenly ^stributed phenociysts rather than multiple dike injections. In addition, magnetic susceptibihty vanes by a factor of 3 across the dike, mimicing measured changes in oxide modal abundance. Magnetic susceptibility also shows a pronounced anisotropy (AMS) with the minimum (K3) axis oriented peipendicular to the dike wall; such AMS fabrics are commonly interpreted to be indicative of flow alignment of magnetite crystals, and preliminary measurements suggest that they may also reflect alignment of silicate phases (Hargraves et al., this volume) Finally, crystal number densities of both phases decrease dramatically within 1 meter ot the dike margin, then remam constant across the interior of the dike. This pattem contrasts with the approximately linear decrease in crystal number densities measured across flashinjected (and relatively small) dikes (e.g. Gray, 1978), and probably reflects the achievement of equilibnum crystallization conditions at small undercoolings throughout most of the dike mtenor. From these observations we conclude that 1) a single batch of magma was involved in the tormation of this large dike, 2) significant flow concentration of phenocrysts modified the chemistry, modal abundance, and phenocryst size distributions - in this regard magnetic susceptibility measurements proved to be a reliable indicator of oxide abundance, and 3) measured crystal size distnbutions can be used to quantify the effect of such a process Plagioclase (groundmass) crystal sizes show a regular increase away from the margin and provide constraints on the crystallization kinetics as related solely to the rate of cooling of die dike. In contrast the more complex spatial distribution of average oxide sizes delineates zones of differing flow ' regimes dunng ^ke injection. Furthermore, we suggest that measurements of phenocr/st onentations used in conjunction with AMS data may further constrain the dynamics of dike

KIMBERLITE DYKES OF THE NORTHEASTERN SfflERIAN PLATFORM, YAKUTIA

VF. Komikm KN. Nikishov & OB. Oleinikov

Kimberlite dykes of the Yakutian province represent vertically or steeply disposed bodies varying from 0.I-0,5 to 2-3 m in thickness. They can be traced for several hundered meters to 500 and even 1200 m along the strike. The latter coincides with the direction of fractures in the sedimentgry cover. Some of the dykes are confined to the crests of miner anticlinal folds. In the southern fields of the Yakutian province the dykes are knovm to occur only in the deep portions of pipes. To the north, the proportion of dykes increases, reflecting a northward increase in erosion level. Kimberlite dykes are filled with massive, fine-porphyritic rock. They are free of ultrabasic and other rock xenoliths, as distinct from kimberlite breccias. The texture is due to the presence of olivine phenocrysts set in a groundmass consisting of various amounts of olivine, phlogopite, montic ellite, melilite, carbonate, serpentine, clinopyroxene, ore minerals, and some other minerals. Most common are dykes v/ith a pyroxene-micaceous micaceous-carbonate and, more rarely, melilite-phlogopite mesostasis. In contrast to kimberlite breccias, the rock has several peculiarities in composition of its rock-forming and and accessory minerals. Olivine is generally reacher in iron and higher in calcium and manganese. Matrix clinopyroxene is represented by titanium diopside, more rarely aegirine-augite. Micas in kimberlites with melilite-phlogopite and pyroxene-micaceous mesostases are chemically similar, but differ from those in kimberlites with a phlogopite-carbonate or phlogopite serpentine-carbonate matrix. The differences observed between matrix minerals of fissure kimberlites and kimberlite breccias suggest variations in melt composition during melting of source plagioclase.

PROBLEM OF MAGMA SOURCE OF A GIANT RADIATING MAHC DYKE SWARM IN A FAILED ARM SETTING SXwnarcpeU, KStSeymour, Department of Geobgy, Concordia university, Montreal, Quebec H4B 1R6 AFowler Department of Geobgy, University of Ottam, Ottam, Canada KIN 6N5 & HPintson Department de geologie, Universite de Montreal, CP. 6128, Succursale A, Montreal, Canada H3C 3J7 The approximately 700-km long, east trending diabase dyke swarm in the southeastern part of the Canadian Shield is a clear example of a giant radiating mafic dyke swarm injected into a failed arm setting. The failed arm is the Ottawa graben, a late Precambrian/early Cambrian, lapetan rift which extends into the continental interior from a prominent salient of the Appalachian fold belt. The salient itself seems to have been inherited from an lapetan RRR triple junction. The products of synrift volcanism at the triple junction are represented by a deformed and possibly thrusted volcanic massif of predominantly metabasaltic rocks. The dyke swarm shows a clear tendency to converge eastwards but it cannot be traced east of the Shield margin because of a Cambro-Ordovician cratonic cover. However, its projected focus is nearly coincident with the centre of the synrift volcanic massif. These geometric relations have led to the speculation that the dykes emanate from the same localized magma source which gave rise to the volcanic massif. However a comparative study of the chemistry of the dyke and volcanic rocks shows that they have been derived from distinctly different magma sources. Whereas the dyke rocks are continental tholeiites the volcanic rocks are "Within Plate" transitional basalts. Also the chemistry of the dykes does not show a compositional polarity. Despite these findings, the possibility that the dykes were injected laterally from a localized magma source cannot be ruled out. For example, a decreased depth of mantle melting during the initiation of seafloor spreading at the triple junction may have led to the generation of the tholeiitic magma which did not penetrate the already consolidated volcanic carapace but became laterally injected into the fault zones including those associated with the failed arm. Critical to the problem are high quality age data for the volcanic and dyke rocks. A reliable U-Pb zircon age of approximately 555 Ma is available for the volcanic rocks. U-Pb (zircon, baddelyite) age determi-nations now in progress should provide the much needed data for the dyke rocks.

MACKENZIE AND FRANKLIN IGNEOUS EVENTS, CANADA: MANTLE PLUME ORIGIN FOR FLOOD BASALTS AND RADIAL DYKE SWARMS AN. LeChenunarit, Geological Survey of Canada, 588 Booth St., Ottawa, KIA 0E4 Canada LM. Heaman Royal Ontario Museum, 100 Queen's Part, Toronto, M5S 2C6 Canada & RH. Rainbird University of Western Ontario, London, N6A 5B7 Canada

Abstract: The Mackenzie and Franklin igneous events, which were focussed in the same part of the northwestern Canadian Shield, produced voluminous flood basalts and giant diabase dyke swarms at about 1.27 Ga and 0.72 Ga, respectively. Precise U-Pb dates show that both of these large-scale magmatic outbursts were short-lived, each one spanning only a few million years. In both instances, magmatism was preceded by differential uplift and local subaerial exposure of sedimentary rocks deposited in epicontinental, marine basins. Sedimentologic-stratigraphic data from the Coppermine homocline and Elu basin indicate that uplift, followed by a brief marine transgression, occurred just before eruption of the 1.27 Ga Coppermine River basalts. Similarly, Shaler Group sedimentary rocks beneath the 0.72 Ga Natkusiak basalts record a marine to terrestrial transition, interrupted by a minor transgression and followed by local extension-related block faulting. Load features preserved along the basal contact of both basalt sequences indicate the underlying sediments were soft when volcanism began. The short duration, large volume and restricted focus of the Mackenzie and Franklin magmatism suggest that the igneous activity occurred above plume-generated hotspots. Pre-volcanic uplift is interpreted as thermal doming above the hot, buoyant mantle plumes. The sequence of uplift before magmatism suggests an active rather than a passive role for the upwelling asthenosphere. Brief transgressive intervals prior to volcanism may be due to subsidence in response to crustal extension and thinning prior to melt extraction. The magmatic events could have occurred due to arrival of a new mantle plume at the base of the lithosphere, with or without subsequent rifting, or could result from rift propagation across a plume-generated uplift. On-going U-Pb dating of dykes/sills and layered intrusions generated during the two magmatic episodes will provide further constraints to test models of plume initiation, rifting and ocean opening. The Mackenzie and Franklin events provide substantial evidence for the presence of vigorous mantle plumes in the Proterozoic and indicate an active role for plume-generated hotspots in disruption of the lithosphere.

MAHC DYKE SWARMS IN FINLAND - WITH SPECIAL REFERENCE TO HAME (1650 MA) AND SATAKUNTA (1250 MA) DYKE SWARMS H. Leino Geological Survey of Finland, 02150, Espoo, Finland Mafic dykes in Finland can be divided into several age groups; 570 - 650 Ma. 950 - 1150 Ma, 1220 - 1270 Ma (Posijoinian). 1500 - 1680 Ma (Subjoinian), 1750 - 1950 Ma and 2100 - 2200 Ma. The division is based on absolute age deierminaiions, cutting relations and the general apperance of the dykes. The Subjotnian Hame diabase dyke swarm in the Tampere - Heinola - Lappeenranta area in southern central Finland consists of more than 100 diabase dykes (Laitakari 1969). They have an age of ca 1650 Ma and sharply truncate the Svecofennian bedrock (ca 1900 Ma). The diabase dykes are in turn cut by nearly contemporaneous rapakivi granite and porphyry dykes related to rapakivi plutonism in SE Finland. The diabases are tholeiitic and unmetamorphosed. The widest dykes (about 1 km) occur between the Viborg rapakivi and its satellite plutons. Widths of over 100 meters can be observed about 70 km from the rapakivi plutons, while the widest dykes at a distance of 150 km from the plutons measure some 10 meters. The Ahvenisto gabbro anorthosite pluton is assumed to be the magma chamber for the diabase dykes due to the similarity of the autoliths and megacrysts in the diabases compare(^ to the rocks of the gabbro anorthosite pluton (Laitakari Leino 1989). This offers a unique possibility of studymg the relationship between a dyke swarm and the magma chamber feeding it laterally, both occurring at the same erosion level. According to radiometric datings the diabases intruded in, at least, two phases, with an age interval of about 20 Ma (Vaasjoki iS: Sakko 1989). The dyke rocks are thus assumed to represent two phases of magma taken from the same magm^i chamber. Vast masses of granitic magma developed in parts of the orogenic belt during the later phase of the Svecofennian orogeny. The large magma bodies rose gravitionally, causing radial rifting of the surrounding bedrock while smaller basaltic magma chambers in the neighborhood only locally disturbed the rising of the granitic magma. Basaltic magma, being more mobile, occupied most of the fissures in the fractured bedrock, whereas only a few fissures close to the rapakivi plutons were filled with granitic (rapakivi) magma, resulting in the formation of porphyry dykes. Postjotnian diabases, being about 400 Ma younger than the H^me diabases, are concentrated in western Finland, where they form voluminous sills and dykes in the Satakunta area. The diabases form the eastern part of the large Fennoscandian diabase complex and cut through Svecofennian bedrock (1900 Ma), the Laitila rapakivi pluton (ca 1550 Ma) and the Jotnian sandstone (1300-1400 Ma). The diabases are tholeiitic and unmetamorphosed. The sills occur in a NW SE trending rift valley, which is supposed fo have been formed contemporaneously with the intrusion of the diabases (Laitakari 1983). References: Laitakari, 1. (1969) On the set of olivine diabase dykes in HSme, Finland. Bull. Comm. e^ol. Finlande 241, 65 p. - " -

(1983) The Jotnian (Upper-Proterozoic) sandstone of Satakunta. Geol. Surv Finland Guide 11, pp 135 - 140.

- " -

^ H. (1989) A new model for the emplacement of the Hame diabase Dyke Swarm, central Finland. Geol. Surv. Finland, Spec. Paper 10, pp 7 - 8.

Vaasjoki, M. (1977) Rapakivi granites and other postorogenic rocks in Finland: Their age and the lead isotopic composition of certain associated galena mineralizations. Geol Surv Finland Bull. 294, 64 p. ' ' - " -

& Sakko, M. (1989) The radiometric age of the Virmaila diabase dyke: Evidence for 20 Ma of continental rifting. Geol. Surv. Finland, Spec. Paper 10, pp 43 - 44.

THE RELATION BETWEEN PROTEROZOIC SUBJOTNIAN DIABASES AND RAPAKIVI GRANITES IN SOUTHWESTERN FINLAND B. Undberg & 0. Eldund Abo Akademi University, Dept of Geology and Mineralogy, Domkyrkotorget i SF ' 20500 Abo Finland

In the Baltic shield major mafic (diabase) dyke swarms are spatially and temporally associated with rapakivi intrusions. In southwestern Finland the NESW trending Aland-Aboland diabase swarm (1600 Ma) is situated in an area between rapakivi intrusions which have intruded along the same direction. The steeply dipping dykes in the swarm mostly intruded prior to the rapakivi intrusions and show no interaction with the country rock. Close to the SW margin of the Aland rapakivi batholith there is a minor E-W trending diabase swarm with gently N dipping dykes associated with anorthosites. The mafic rocks are closely related to coeval intrusions of quartz porphyries of rapakivi type giving rise to magma mingling and composite intrusions. Gently dipping sills of diabase in this arfca have also, in places, caused melting of the overlying country rock producing hybrid and quartz porphyry like rocks which shows that the diabases are able to form acid (rapakivi) magmas. The close relationship between the diabases and the rapakivi granites in general in southwestern Finland indicate the beginning of continental rifting which started with the intrusion of the SW-NE trending Aland-Aboland diabase swarm into cold and brittle crust. Increasing temperature with accompanying melting of the crust led to small intrusions of acid magmas (quartz porphyries) locally mingled with basic (diabase) magma and intrusion of composite diabasequartz porphyry dykes. Either because of a higher level of intrusion or because the basic magma could not ascend through the partially molten crust the formation of horizontal basic intrusions

was favoured which in turn increased

the heat in the crust and accelerated the crustal melting thus giving rise to the rapakivi granite intrusions.

FLUID-MECHANICAL MODELS OF DYKE PROPOGATION AND MAGMA TRANSPORT JRLister Research School of Earth Sciences, Australian National University, Australia Now at: Institute of Theoretical Geophysics, University of Cambridge, England & R.CKerr Research School of Earth Sciences, Australian National University, Australia

ABSTRACT: Magma-driven fracture is an important mechanism both for vertical ascent through the Hthosphere and for lateral emplacement of dykes and sills. A scale analysis of the various driving and resisting pressures during dyke propagation shows that elastic stresses play a secondary role except near the dyke-tip and that the host-rock fracture resistance is negligible. The dominant presure balance is between local buoyancy forces and viscous pressure drop, neither of which, therefore, can be neglected in models. It follows also that the local density difference controls the height of magma ascent rather than the total hydrostatic head. Hence, magma tends to be arrested at, or near, its level of neutral buoyancy (LNB). Subsequently, magma is intruded laterally in dykes and sills directed along the LNB by buoyancy forces. A simple laboratory experiment demonstrates this physical principle of buoyancy-controlled propagation. Mathematical models of lithospheric ascent and lateral emplacement are described and solved. Though the models are much simplified, the solutions incorporate the correct dynamical interaction between the elastic and fluid-mechanical phenomenal. The calculated predictions are in reasonable agreement with geological observations.

MAHC DYKES AND THEIR TECTONIC SETTING IN THE SOUTHERN ADELAIDE FOLDBELT, SOUTH AUSTRALIA SFUu & PDFlenmg Dept Of Geology, La Trobe University, Bundoora, Victoria, Australia 3083 Numerous tholeiitic dykes and plugs intmded the Cambrian Kanmantoo Group metasediments and some high metamorphic grade Precambrian metasediments just west of the Kanmantoo Group in the the southern Adelaide Foldbelt, South Australia. They range in width from a few centimeters to 20-30 meters and in length from a few tens of centimeters to 1-2 kilometers. The dykes intruded at various stages of the deformation history of the Delamerian Orogeny (at about 500 Ma). Some of them intruded at the pre- to syn-DI stage. This indicates that the mantle had an important influence on the early (pre- to syn- D1) thermal activity in the southem Adelaide Foldbelt by supplying heat from the upwelling mantle and from the early intrusion of mafic magmas (Liu & Fleming 1989a). The geochemistry of these mafic dykes resembles those of oceanic basalts in many aspects (Liu & Fleming 1989b) including their low contents of LIL elements, flat REE patterns, low initial 87sr/86sr ratios and low-pressure type crystal fractionation. The timing and geochemistry of these mafic dykes and plugs in the southern Adelaide Foldbelt provides key information regarding the tectonic setting of the Kanmantoo Group (Liu & Fleming 1989a & b). The new data on the mafic dykes, with consideration of other available geological and geophysical data (especially the magnetic patterns illustrated by Brown et al. 1988), albw a reassessment of the Cambrian and early Ordovician tectonic development in southeast Australia. A general model involving extension and continental thinning and/or rifting to create new ocean in the region currently occupied by the Kanmantoo Group and its continuation under the Murray Basin is favoured (Liu & Fleming 1990). More specific models perhaps involving a back-arc basin setting, or possibly some other variety of marginal sea are examined after consideration of earlier models of von der Borch (1980), Scheibner (1985) and Brown et al. (1988).

THE MID-PROTEROZOIC DYKE SWARM OF MICA LAMPROPHYRES AND MICROSHONKINITES FROM ELCHURU, INDIA

VMadhcmn, TFSrinivasan, MSrinivas & KDcmd Department of Geobgy, Osmama University, Hyderabad'SOOOiJ/, India J. Mallikharjuna Rao, Naibnal Gec^l^sical Research InsiMe, Hyderabad-SOOOOf/, India An impressive network of biotite lamprophyre and microsnonkinite dyke swarm, represented as late stage intrusives, has invaded the mid-Proterozoic alkaline pluton at Elchura in Andhra Pradesh, south India.

The oblong alkaline

pluton, found emplaced within the Dharwar Craton

and

located northeast of the composite intracratonic Cuddapah Basin, is a symbolic manifestation of shoshonitic alkaline magmatism from this part of the Indian Fteninsular shield. Miaskitic nepheline syenites and their mafic-rich variant, malignite dominate the pluton followed by shonkinite in volumetric abundance.

The malignite and shonkinite are the

mesocratic and melanocratic varieties respectively and of the two, the shonkinite is found as autoliths, frequently brecciated and net-veined by nepheline syenite.

The Rb-Sr

isochron age for the Elchuru rocks (including dykes) is 1242 + 33 ma and their initial

ratio of

0.70434 ± 13 (2 77) indicate a mantle source for the parent magma. In the filchuru alkaline pluton, shoshonitic alkaline magmatism is recorded as a recurring phenomenon, starting with the earliest crystallization of shonkinite followed by the differentiation of a miaskitic liquid and its subsequent crystallization as nepheline syenites.

The end of this

magmatic process was marked by the regeneration of shoshonitic alkaline magma that profusely invaded the pluton as biotite lamprophyre (minette) and microshonkinite dyke swarm.

Thus

the same shoshonitic alkaline magma at Elchuru crystallized into three different but distinct textural varieties corresponding to two stages of crystallization: 1) the shonkinite (an earliest crystallized coarse grained plutonic rock), 2) the microshonkinite

(a uniformly fine grained

dyke rock) and 3) the biotite lamprophyre (a porphyritlc dyke with the mineralogical assemblage of shonkinite).

MARC DYKES, MAFIC ENCLAVES, AND HOST GRANITOIDS IN THE BERNASCONI PLUTON, PENINSULA RANGES BATHOUTH, SOUTHERN CALIFORNIA DRMason Amdel Limited, Adelaide, South Ausoxdia &LH.Cohen Dq)t of Eardi Sciences, University of Caltfomia Riverside, USA The Bernasconi Granite Is a small ("2 km?) granitoid pluton, one of rnany in the northernmost part of the southern California Batholith, California. It is distinguished by the presence of abundant mafic enclaves and minor but significant mafic dylces, all of which are veil-exposed in three dimensions. Ttie pluton is composed priricipally of biotite granite, but mafic enclaves are locally abundant and are entrained vithin a veil-defined magmatic flov foliation that is not recorded in the granitoid rocks themselves, liafic dykes, similar in macroscopic appearance to the enclaves, are restricted in distribution, some occurring as synplutonic dykes partly boudinaged vithin the tlov foliation, and some occurring as discordant postplutomc dykes. All of the dykes are relatively narrow (centimetres to tens of centimetres vide), and may be traced discontinuously for tens of metres. Tt^ie mafic dykes and enclaves share similar mineralogical and textural features. They are commonly granoblastic to lepidoblastic in texture, being composed of plagioclase, tornblende, and biotite, with variable amounts of quartz, minor ilmenite and sphene, and rare K-feldspar and augite. Many enclaves contain relict plagioclase phenocrysts of subhedral prismatic form, vith cores rangi ng An5Q_ gQ mantled by clear ri ms rangi ng An4o_ 20 Interaction betveen the mafic enclaves and dykes, and their granitoid host, is evident macroscopically. Alttough niost enclaves display stiarp contacts vith host granitoid, some unstrained enclaves display ghosting vith increasing abundance of felsic phases. This phenonienon is also observed in places along unstrained margins of synplutonic dykes. It is evident that some inclusions vere physically disaggregated by chemical interaction vith host granitoid. Variation in ferromagnesian mineral chemistry also indicates variable interaction betveen mafic material and host granitoid. Amphiboles display vide variations in Ca+Na+K, but limited variation in ^r^, within individual enclaves and dykes. The field, petrographic, and mineral ctiemical data are best explained by a petrogenetic model based on mingling of silicic and mafic magmas in the plutonic environment. Silicic magma, generated in tte lower crust, rose as a coherent pluton into middle to upper levels of the crust. Mafic magma injected t^le pluton during its uprise, forming aligned swarms of dissagregated enclaves and synplutonic mafic dykes. Late injections of mafic magma, that may have been emplaced after the pluton had ceased rising, formed discordant postplutonic dykes.

GEOCHEMICAL C3iARACTERISATION OF NORTHEAST SYDNEY BASIN. AUSTRALIA

DYKES

INTRimiMr INTRUDING

tmh THE

SMaxweU Department of Applied Geology, University of New South Wales, PO Box 1 Kensington, NSW 2033 "^.ruisaxi,

ABSTRACT: Investigation of dykes intruding the NewcasUe - Central Coast area of the Sydney Basin has distinguished two suites by means of trace element geochemical panems. Dykes from the main L i t e have been from NNvTfTI'' TH ' ' V ^ ^ ^°"tinentai alkali basalts o f T v k . I i w i . h MP^Q J ^ ^ originated within relatively undepleted upper mantle. The minor suite o dykes ( w ^ NE-SW stnJce) are derived from a more depleted mantle source, and postdate the matn suite, b a s ^ on f.eld observations. Trace element data indicate these dykes are continental tholeiites, and may be • from s ^ ' J n M c P f ^ Australian continental margin, which took place from 82-60Ma. Future work is suggested to clarify the temporal and genetic relationships between the Two dyke

PALAEOMAGNETISM OF THE EARLY PROTEROZOIC LAYERED INTRUSIONS AND ASSOCIATED DYKES, NORTHERN FENNOSCANDL\ SMertcmen & LJPesonen Geological Survey of Finland, Espoo, Finland

The p a l e o m a g n e t i s m of six E a r l y P r o t e r o z o i c (-2440 Ma) layered i n t r u s i o n s and a s s o c i a t e d d i a b a s e dykes in n o r t h e r n F i n l a n d (Fennoscandian Shield) was i n v e s t i g a t e d . Four^ r e m a n e n c e c o m p o n e n t s w e r e i s o l a t e d b o t h in the i n t r u s i o n s and in the d y k e s . In the dykes the m o s t common m a g n e t i z a t i o n A , w i t h the m e a n d i r e c t i o n D = 3 4 5 % I = 38° (k = 22, 95 = 1 2 % N = 5 d y k e s ) , is an o v e r p r i n t a c q u i r e d d u r i n g the p e a k of the S v e c o k a r e l i a n orogeny about 1880 Ma a g o . M a g n e t i z a t i o n B , w i t h the m e a n d i r e c t i o n D = 3 1 % I = 51® (k = 5 3 , 95 = 9 % N = 6 dykes) , records e i t h e r the t e r m i n a t i o n of the S v e c o k a r e l i a n orogeny about 1750 M a ago or a m u c h y o u n g e r g e o l o g i c a l e v e n t . C o m p o n e n t D has the m e a n d i r e c t i o n D = 117®, I = 55® (k = 12, 95 = 20®, N = 6 d y k e s ) . T e n t a t i v e tilt t e s t s for the l a y e r e d i n t r u s i o n s s u g g e s t t h a t c o m p o n e n t D was a c q u i r e d d u r i n g late stages of the p r i m a r y c o o l i n g of the i n t r u s i o n s about 2400 M a ago at the same time as t h e i n t r u s i o n s u n d e r w e n t t e c t o n i c t i l t i n g . P a l e o m a g n e t i c data suggests that the dykes have i n t r u d e d the l a y e r e d i n t r u s i o n s just b e f o r e the t i l t i n g took p l a c e and t h a t the t h e dykes have b e e n formed in the same m a g m a t i c p r o c e s s e s as the i n t r u s i o n s . The fourth c o m p o n e n t , E , h a s the m e a n d i r e c t i o n D = 259®, I = 49® (N = 2 d y k e s ) . C o m p o n e n t E is i n t e r p r e t e d as a t h e r m o c h e m i c a l o v e r p r i n t a c q u i r e d d u r i n g the J a t u l i a n (2100 Ma) r i f t i n g and f a u l t i n g p e r i o d . A n e w A p p a r e n t P o l a r W a n d e r Path (APWP) suggests t h a t F e n n o s c a n d i a h a s u n d e r g o n e c o n s i d e r a b l e d r i f t i n g and r o t a t i o n s d u r i n g 2750-1750 M a .

PALAEOMAGNETISM OF PRECAMBRIAN NORTHWESTERN KOLA PENINSULA.

MAHC

DYKES

OF

NF. MikhaUova, NN. Shatalov, IL. Goncharov ImtiMe of Geophysics, AS USSR, Kiev. & J A. Fedotov Geological Institute of Kola Center, Apatity 5 Precambrian dyke sets ranging in age from 2.5 to 1.0 Ga have been studied within the region: 1. gabbro-norites, 2. quartz metadolerites, olivine microgabbro, 4, picrodolerites, 5. dolerites. With the exception of the dolerites all the dykes display to a certain extent, the "Karelian" metamorphism whose grade increases when appr^ching the Pechenga, synclinorium. Among the dykes examined, distinguished are highly magnetic (x > lOOO.lO"'^ s i ) , weakly magneric (x <100.l0"5si) and magnetic with x falling into the 100-1000""^SI range. 3 types of contact zones in the mafic dykes have been distinguished based on the patterns of magnetic stability of NRIi,i: (1) samples from the contacts, irrelative to the dyke composition and those from the dyke "centre" show identical stability modulus and direction of NRft/i; (2) samples from the contact commonly show a complicated magnetization, however after cleaning distinguish NSM coiip6nent of the same direction as in the dyke center; (3) samples from the contacts are characterized by multicomponent magnetization with the predominant unstable magnetization. Characteristic magnetization of the mafic dykes in the region is distinguished as a result of A.P. cleaning and heating. It is considered to be thermoremanent in origin, fine dispersed inclusions of ferromagnetic in plagioclase being TRM-carriers. Virtual geomagnetic poles were calculated based on these TRM. The great majority of the poles are concentrated in the western margin of th^ North American craton, within a domain bounded by 225~255^E and 30-60^N. Their comparison with the paleopoles for the Baltic Shield (Palaeomgnetology, 1982, Mikkailova, Kravchenko, 198?, Pesonen et al., 1989) suggests that magnetization of 1-4 dyke derives from a geological event ("karelian" metamorphism?) in the 1990-1750 Ma range. The poles of the dolerite dykes fall into the curve portion with the ages: 1100-950 and 500-400 Ma. This daes not contradict ;%ingle isotopic determinations available by Rb-Sr method. The results obrained reveal an essential effect of regional "karelian" metamorphism on the mafic dykes, which vieled an original magnetization and constrained the application of palaeomagnet ic method in a chronological record of hypabyssal magmatism in the Pechenga region of the Kola peninsula.

THE

ASSOCIATION OF BASIC DYKES AND LAMPROPHYRES IN JETTY PENINSULA, MAC-ROBERTSON LAND, EAST ANTARCTICA (PETROGENESIS AND GEOCHEMISTRY) EV.

Mikh^ North Branch for Marine Geologic Exploration "Sevmorgeologia" Maklina 1, Leningrad 190121, USSB A sublongitudinal basic dyke swarm was mapped in detail in

Jetty Peninsula. 7he dykes, late Paleozoic in age, intersect the Archean crystalline basement. At least two events of dyke eiq;>lacement were recognized. The oldest dykes consist of subalkaline dolerites and augite->caii^)tonites including all intermediate rock varieties, marked by mineralogical, textural and petrochemical features. The second dyke generation is con^osed of ferrosyenodiorites which show some features of subalkaline biotite essezites and lao^rophyres. There are also rare dykes of camptovogesites and odinites. The rocks form a subalkaline series, however, CIPW composition and alkali-calcium parameter suggest alkaline affinities. The series shows a Bowen-like evolution trend with the end phases rich in Sr, Zr, Ba, T and poor in Co, »i, Sc, Or, Total enrichment in Ba and Zr is typical of all rock types. The evolution trend is caused by olivine and clinopyroxene fractionation. At the same time, many petrographic and petrochemical affinities are due to the influence of high concentrations of volatiles and to pneumatolytic differentiation. The formation of the dykes predated the development of the Lambert Glacier rift system. In geochemistry, the rocks are similar to other basic series of continental rift zones. The origin of the lamprophyres is believed to have been related to the interaction of the alkaline (subalkaline) magma and fluid flux.

PETROLOGY OF LATE PROTEROZOIC ALKALINE LAMPROPHYRES VESTFOLD BLOCK, EAST ANTARCTICA

OF

E.V. Mikhalsky & AV. Andronikov liorth Branch f o r Marine Geologic Exploration Maklina 1, Leningrad 190121,

"SETOORGEOLOGlA*^

USSR

iMumeroua M E dykes of alkaline basitea known in Vestfold iiills v/ere studied in 1 9 3 8 / 8 9 .

Dyke cuttings suggest that the al-

k a l i n e basites are older than 1 , 3 0 0 Ma old dolerites*

The alkal-

i n e dykes form some separate sv/arms. The rocks are dominated by caraptonites and raonchiquites v/ith intermediate v a r i e t i e s betv/een them. Field data suggest a discrete character o±" magmatic pro cess • At least tv/o phases of magma intrusion can be recognized.

The augite camptonite i s relatively o l d e r

as compared to the monchiquite. Petrochemically,

the former are

basic alkaline and the l a t t e r ultrabasic alkaline rocks. The monchiquite dykes are complicated by rare small lenticular and isometric bodies of similar composition. tain abundant small deep-seated Iherzolite

The bodies con-

nodules.

The alkaline lamprophyres may have been partially melted out from a deep source the depth of v/hich progressively increased and the rate of melting decreased. The nodules are believed to be of upper majitle o r i g i n .

The alkaline basites appear to have been

generated during the i n i t i a l

pre-rifting stage of crustal evolu-

t i o n . Many alkaline occurrences throughout central East

Antarctica

(from Bunger H i l l s to Enderby Land) may have marked an extensive ailkaline province v/hich appeared in the Late Proterozoic and continued into the Recent. REE and isotopic study i s under v/ay, i t i s hoped to be completed by May 1990.

MESOZOIC DYKE SWARM OF THE SAO SEBASTIAO ISLAND (SE BRAZIL): PALAEOMAGNETISM AND GEOCHEMISTRY CR. Montes-Lauar (1), I.G. Pacca (1), AJ. Me^ (1). EM. Picdrillo (2). G. Bellieni (3)&A.DeMin (2) (1) I n s t i t u t o R s I r o n o m i c o e Geofisico, U n i v e r s i d a d a de S8o Paulo (Brazil) (2) I s t i t u t o di MineraLogia e Petrografia, University di Trieste (Italy) (3) D i p a r t i m e n t o di M i n e r a L o g i a e P e t r o L o g i a , U n i v e r s i t y di Padova (Italy) T h e r e are many mafic dykes in southeastern Brazil w h i c h are apparently related to the Parana Basin lower cretaceous volcanism (5erra C e r a l Formation). These dykes are very f r e q u e n t in the P o n t a G r o s s a flrch r e g i o n w h e r e they form s w a r m s with a NW-5E directional trend and a l o n g the S a n t o s and Rio de J a n e i r o c o a s t l i n e , w h e r e their d i r e c t i o n s f o l l o w the B r a s i l i a n o (500 Ma) structures with a NE-SW trend . 0 g e o c h e m i c a l , p e t r o l o g i c a l and p a l e o m a g n e t i c a n a l y s i s is p r e s e n t e d for d y k e s along the southeastern coast of Brazil especially those o u t c r o p i n g at '.the SSo SebastiSo Island (53o P a u l o ) and results are compared with those o b t a i n e d for the P a r a n a Basin volcanics (Serra Q e r a l lava f l o w s and P o n t a G r o s s a flrch d y k e s ) . The island of SSo SebastiSo (SSo Paulo State) is mainly constituted by u p p e r cretaceous alkaline complexes whose emplacement o c c u r r e d after an i m p o r t a n t d y k e a c t i v i t y of lower c r e t a c e o u s a g e . The 5 2 o S e b a s t i S o ( 5 5 ) d y k e s (N4060E) intrude precambrian gneissic terrains and parallel those of s i m i l a r age outcropping along the S a n t o s - R i o de J a n e i r o ( 5 R J ) c o a s t l i n e . 55 - d y k e s are r e p r e s e n t e d by highTiOa ( ) 3 % ) t h o l e i i t e s (as those of 5 R J - d y k e s ) and acid rocktypes high in i n c o m p a t i b l e e l e m e n t s ( C h a p e c o t y p e ) t h e r e f o r e they can be a s s o c i a t e d w i t h f l o o d v o l c a n i c s of the n o r t h e r n P a r a n a B a s i n . N o t a b l y , C h a p e c o a c i d d y k e s are p r e s e n t in the lower c r e t a c e o u s N W - 5 E d y k e s w a r m s of the P o n t a G r o s s a flrch. Oriented blocks for paleomagnetic analysis were s a m p l e d f r o m e i g h t acid and b a s i c d y k e s at the S S o S e b a s t i S o I s l a n d , fl p a l e o m a g n e t i c p o l e was c a l c u l a t e d for them u s i n g also r e s u l t s o b t a i n e d f r o m four s i m i l a r d y k e s i n t r u d i n g the adjacent continental area. This paleomagnetic pole at not q u i t e a g r e e w i t h lower 3 1 4 . 0 " E 6 5 . 4 " 5 fl»e. = 4 . S*- d o e s cretaceous poles o b t a i n e d for Serra Qeral and the Ponta G r o s s a flrch.

PALAEOMAGNETRIC RESULTS FROM THE UMVIMEELA DYKE, ZIMBABWE MF. Mushayandebm & DL. Jones Department cf Physics, University cf Zimbabwe, Harare, Zimbabwe Th« Umvimeftla Dyke is one of the main satellite dykes of the 2.46 Ga northerly-trending Great Dyke. It lies to the w ^ t of the CrtLt l>yt and sub-parallelB the latter over most of its 500 km length. The elrTy IrT" 'f substantially within thfA-ohalan -early Protero^oio Limpopo Dclt at the southorn end of the Great Hn^i^ have shown that within the Belt the Umvimeela Dyke was remaiTnetised completely during a thermal event some 1.9 Ga ago. At its northern end the Great Dyke lies within the Pan - African Zambezi Belt. In this region the Great Dyke has, at some time or times, auffered considerable deformation and the Umvimeela Dyke parallels this deformation exoe pt in the extreme north where the Umvimeela, has not been located, New palaeomagnetic results ffom sites in the central und northern sect ions of the Umvimeela Dyke will be reported. These, together wi th previously reported pulaeoraagnetic results, are used inter alia to help understand the mode of emplacement of the Umvimeela and tile tectonic evolution of the Zimbabwe craton. Jones, D. L., P. L. MoFadden and I. D. M. Robertson. A palaeomagnetic study of Preoambrian dyke swarms associated with the Great Dyke of Rhodesia. Trans. Geol. Soo. S. Afr., 78, 57 - 65, (1975) Table 2.

Magnetic susceptibilities of UD sites.

Site

Distance (km)

Susceptibility (X 10-3)

Standard error (3C 10-» )

L M N 0 P Q

21 37 89 231 214 490

4.3 10.9 10. 8 10.7 9.0 5.4

0.1 0.7 1.6 0.5 0.4 0.3

DE DP DC

451 434 536

0.3

0,5 0.9 0.5 1.3 0.5 0.2

0.04 0.04 0.06

DH DI DK DM DN DO DP DQ DR

548 317 343 187 137 67 78 557 561

7.4 10.5 15.6 1.8 5.3 18.6 8.3 9.0 4.6 5.5 8.6 5.3 5.8

DA DS DT DU

572 582 575 577

0, 70 0.79 0.76 0.66

1.0 1.6 0.4 0.2 2.3

0.6

0.02

Notes: Distances are measured from the south end of the UD The first SIX sites are those of Jones et al., (1975)

IMPLICATIONS OF SPATIAL AND TEMPORAL COMPOSITIONAL VARIATIONS OF MATACHEWAN MAGMAS, SUPERIOR PROVINCE OF CANADA DD. Nelson & W.C. Phinney NASA Johnson Space Center, Houston Texas. USA Dikes of the large-scale (>250,000 km^) northwest-trending 2.45 Ga Matachewan dike swarm were emplaced in compositionally diverse terrains of the Superior Province of Canada. Nelson et al. (1987; 1988; 1989) have shown that the observed variation in dike composition was the product of a two-stage evolution: (1) AFC in the lower crust, assimilating tonalitic partial melts of precursor mafic crust, and (2) combined replenishment and fractional crystallization (RFC) at shallow crustal levels with contemporaneous dike emplacement. To test this model, and to evaluate spatial and temporal variations in dike composition, we analyzed dikes from four small-scale swarms along a 200 km east to west traverse.

These

dikes were emplaced over a period of time encompassing at least two paleomagnetic reversals (Halls and Palmer, 1989). Although overlap occurs, a systematic variation in dike composition is observed from east to west,

particularly in terms of incompatible element ratios:

varies from 1.31-1.65 in the east to 1.58 to 3.01 in the west.

(La/Sm)pj

Further, the western dikes trend

to more depleted levels of high-field-strength-elements: Ta/Ta" = 0.13-0.30 compared to 0.24-0.41.

Intradike variation is locally significant and reflects multiple pulses of magma

entering the dike. In most cases, intradike variations suggest progressive tapping of magma evolving in a subjacent chamber. In other cases, however, unrelated magmas have entered the same dike.

Importantly, although all magmas, east and west, are evolved (Mg# = 35-57), many

western dikes are as primitive as those in the east, having high MgO, CaO and Cr, and low La and Th, despite the regional differences in La/Sm and Ta/Ta*.

For all dikes. La correlates

negatively with MgO, Ta/Ta* and La/Sm. This observation, coupled with the low Mg # and the intradike variation, suggests that the compositional characteristics observed reflect processes of magma evolution rather than magma generation. AFC models, using tonalitic assimilants, indicate that the differences in incompatible element ratios of the eastern and western dikes could have been produced from similar depleted parental magmas at approximate Mg/ M^ (mass of assimilant/mass of cumulate) of 0.3 and 0.5, respectively. Required F values range to 0.8 in the west and to 0.7 in the east. Subsequently element abundances, but not incompatible element ratios, were modified by RFC at shallow levels. Considering both the results of the AFC models, suggesting higher rates of AFC in the western dikes, and the temporal implications of the paleomagnetic data, we suggest that the Superior Province lithosphere was moving relative to a thermal anomaly in the Late Archean mantle. Parental magmas delivered to the base of the crust may have been similar throughout this time period, however, the western magmas must have encountered a portion of the crust that had experienced the heat flux for a longer period of time. Basaltic magmas emplaced at or near the base of this warmer crust were able to assimilate the crust at higher rates.

^ ^ A MAGMATIC EVENT IN WEST GREENLAND

MAJOR

1645 MA

ALKALINE ^v/^xc

TFDNielsen Geological Survey of Greenland, Copenhagen, Denmark A well defined major 1650 Ma old swarm of fresh trachybasalt dykes is exposed along the west coast of Greenland. Individual dykes are up to 400 m wide and >400 km long. The swarm is >1200 km long and >150 km wide. At 70°N the dykes disappear to the SE under the Greenland ice cap. At Thule, 780n. the swarm may continue across the Nares Strait in the Alexander Fjord arel Ellesmere Isl. where dykes with the appropriate orientations are observed. The swarm intrudes Archaean terrains (c. 2800 Ma) in the Disko Bugt area (70 - 710N) and the Proterozoic terrains of the Rinkian-Foxe belt (c. 1850 Ma) in the Uummannaq Fjord - Melville Bugt region. At Thule (78°N) the dykes are disturbed by 1200 Ma old block faulting. The 1650 Ma Rb-Sr age and K-Ar ages from some of the dykes separate the swarm from common 800-650 Ma dykes in the Melville Bay area and Franklinian dykes on Baffin Island. Many of the dykes are slightly ne-normative and all have high K20/Na20 and low Ti02. The dykes can be singled out chemically by: 47 % > Si02 > 50 % A % > MgO > 7 %. 9 % > CaO > 6 % . 2.6 % > Ti02 > 1.2 and 3 % > K2O > 1 %.' The swarm thus represents a major Proterozoic structural and alkaline event. The age and distribution of dykes indicates that:

a: The distinct chemistry and the extent of the dyke swarm suggests that the magmas travelled hundreds of km without chemical modification, b: The Proterozoic Rinkian-Foxe belt cooled over a relative short periode from the peak of metamorphism at 1850 Ma and the closure of biotite at 1700-1740 Ma to allow brittle fracturing and dyke emplacement at 1650 M a . c: The dykes are easily identified, and when located on Ellesmere Isl. they can be used for estimates of the postulated Tertiary drift of Greenland relative to Ellesmere Isl. and North America.

LATE PRECAMBRIAN DYKE SWARMS OF THE ANABAR MASSIF, SIBERIAN PLATFORM

AVOkrugin, BYDleirukov, VTScm^inov & MDTomshin Yakutsk Institute of Geosciences, Siberian Branch, Academy of Sciences, 39 Unin Avenue, Yakutsk 677891, USSR Widespread in the Anabar maeeif are late Precambrian mafic dyke swarms, v/hich extend for several hundered km. The,dykes are generally several meters to several ten meters thick and rarely exceed several km in length. The rocks have thcleiitic-basalt and subalcaline compositions (gabbro-dolerites, quartz gabbro-dolerites, gabbre diorites, and monzodiorites)• The thicker dykes have a complex structure, due to nielt differentiation in extensive magma columnB. The presence of SBiall dykes filled with separate differentiates from basic melt is indicative of long-lived, deep-seated magma chambers. During the early Riphean f?tc?ge of igneous activity, the sublatitudinally-striking dyke sv^arias were formed. They are' located within deeply metamorphosed Archean rocks and consist of bodies of tholeiitic-basalt and subalcaline compositions. K-Ar ages for most of tte rocks vary within I800-I300 m.y. Deep ;differentiates of tholeiitic-basalt melts produced NV;-trending swarm located in the eastern part of the Anabar masr.if. There, the dykes are primarily composed of leucocratic gabbro-diorites. In the middle Riphean, there was re-emplacement of dykes of tholeiitic-basalt and subalcaline compositions v/ithin the same magma-feeding zones. They occasionally crosscut early Riphean basic dykes and intrude lower Riphean sediments. Their K-Ar ages vary from 1200 to 1400 m.y. The end of the middle uiphcc-n - beginninc of the l^te Riphean witnessed formation of the JT^^-striking swarms which intruded lower and middle Riphean sediments. These include subalcaline dykes in the eastern slope of the Anabar massif and thleiitic and subalcaline dykes (and sills) in its v;ostern 3lope. Their K-Ar ages fall within 1200-I900 m.y. This stage of magmatism ended in the emplacement of potassic, alcaline basaltoid magmatites.

MIDDLE PALAEOZOIC BASIC DYKE BELTS OF THE SIBEIUAN PLATFORM YAKUTL\ BV.Oleimkov Y^k Institute of Geosciences, Siberian Branch, Acaderm of Sciences. 39 Uninpr., Yakutia 677891. USSR Known in the eastern Siberian platform are three extensive dyke belts (Olenek, Vilyuy-Markha. Chara-Sinsk) and several dyke mvarius which formed In r e l a t i o n w i t h Middle Paleozoic r i f t i n g . The belts reach 800 km in length and 15-20 to 250 km in width. The density of the fioeulre intruaives is not unifotn throughout a belt. Areas of high density of the intrusivea where individual dykes are only a few ten meters apart alternate ( especially in a Chara-Sinsk belt) with areas, up to 30 km wide, where dykes are absent. The dykes intrude Silurian rocks and are overlain by middle Carboniferous sediments. Their K-Ar ages fall within 380-320 m.y. The dykes are formed by aubalcaline. tholeiitic-basalt melt with high Ti. P, K contents. Individual bodies are several km to 200 km long and several meters to 200 m wide. The predominating groups of bodies in the three belts differ in chemistry. There is strong evidence for lateral compositional zoning of the dykes across the strike of a belt, which reflects a kinetic environment of basalt melt evolution under tension and compression regimes and various mechanisms of basalt differentiation under progressively or stepwise changing physicochemical conditions during melt existence.

GEOCHEMISTRY OF THE MESOZOIC AMAPA AND JARI DYKE SWARMS, NORTHERN BRAZIL: PLUME-RELATED MAGMATISM DURING THE OPENING OF THE CENTRAL ATLANTIC EPDliveira, JTamey Dqxirtment ofGeobgy, University of Leicester LEI 7RH &XJJoao CPRM, 40 000 Bekm, Brazil

The Amapa and Jari dyke swarms from northern Brazil are spatially and temporally related to the opening of the Central Atlantic ocean. In pre-drift reconstructions of S. America and Africa the Amapa dykes are parallel to those of similar age in Liberia. It is possible then to make comparisons between Brazilian and Liberian swarms and also with those in the southeastern USA which may all be related to the development of the Cape Verde hotspot. Both dyke swarms comprise mainly olivine tholeiites and minor quartz tholeiites, but can be divided into two main groups on trace element grounds. Group I from each swarm has lower levels of incompatible elements and less fractionated REE patterns than Group II. A few dykes diverge compositionally from these two groups, some possibly as a consequence of crustal contamination, others perhaps reflecting a more direct plume influence in their source regions. Abundances of the strongly, lithophile elements K, Rb and Ba are more variable, a consequence of local high temperature hydrothemal alteration. Within-group dyke compositions can be accounted for fairly readily in terms of crystal fractionation of plagioclase, olivine and clinopyroxene, or by processes such as dynamic partial melting. However, whereas modelling of REE and trace element abundances suggests that both Jari basalt groups could be derived from a T-type MORB mantle source by different degrees of melting, the two Amapa groups require two separate mantle sources. Both swarms share many compositional features with continental flood basalts such as Deccan, Parana, and with tholeiitic dykes from Morocco; Group I has similarities with dykes from South Carolina, but Groups I and II both seem to have counterparts in Liberia. We examine the extent to which the range of dyke compositions can be accounted for in terms of Cape Verde plume - subcontinental lithosphere interactions and likely thermal influence of the Cape Verde hotspot, and make comparisons with other regions of plume-lithosphere interaction.

AMPHIBOLE CHEMISTRY OF DIABASE D Y K E S AS A N INDICATOR OF THE TIMING OF UPLIFT OF LOWER CRUST H.C. Palmer & RL. Bamett Department of Geophysics, University of Western Ontario, London, Ontario, Canada N6A 5B7

The continuity of east-west-trending sub-provinces of the central Superior province of the Canadian Shield is interrupted by a north-east-trending zone of granulite grade rocks of the Kapuskasing structural zone (KSZ) The eastern margin of the KSZ is a zone of cataclasis, the Ivanhoe Lake cataclastic zone, which juxtaposes the high grade rocks against greenschist facies rocks of the Abitibi sub-proyince. An older limit can be placed on the timing of ^fferential uplift from the amphibole chemistry of diabase dikes that cut the country rocks. Matachewan dikes form the oldest and most areally extensive of the dike swarms in the Precambrian of Ontario Kapuskasing dikes of ENE trend have a much more Umlted distribution being confined to the Chapleau block of the KbZ and the Wawa Domal Gneiss terrain. Matachewan dikes are apparently absent from the highest grade part of the Chapleau block of the KSZ but to the- west Kapuskasing dikes are seen to cut Matachewan dikes establishing the age relationship between them. Kapuskasing dikes, like Matachewan dikes, contain amphibole except at one locality on the footwall side of the Ivanhoe Lake Cataclastic Zone (LCZ). Amphiboles from Matachewan dikes in the low grade country rocks of the Abitibi Subprovince are ferro-edenites carrying 4 - 5 % ^ 2 0 3 ; this fact, together with the presence of well developed crystal faces of amphibole against plagioclase suggests that the amphiboles are igneous products. To the northwest of the KSZ AI2O3 contents are -5%. Beginning in t^he Wawa Gneiss Terrain, AI2O3 content in amphibole from both Matachewan and Kapuskasing dikes rises from these regional values to '10% in proximity to the ILCZ. The A1 contents indicate that both dike swarms were emplaced at a deeper crustal level in the KSZ and by implication the ifferential uplift along the ILCZ postdates Kapuskasing dike emplacement.

TECTONIC SIGNIFICANCE OF MAFIC DYKES IN SOUTHERN AUSTRALIA

AJ. Parker South Australian DepartmefU of Mines and Energy, Adelaide, South Australia

Mafic dyke swarms are common in many Precambrian and Palaeozoic provinces of Australia. They often form long linear zones of consistent orientation across entire provinces and, as such, contain vital information not only on the tectonic evolution of that province but also on relationships with adjoining provinces. Within southern Australia, each of the principal Precambrian crustal blocks has specific dyke signatures which record the post-orogenic extensional tectonic history of the blocks and also act as "finger prints" identifying each block. The large and very long Widgiemooltha Suite dykes of the Yilgam Block are a prime example because they transgress the entire block from east to west and are apparentiy unique to that block. No other Precambrian^ blocks of southern Australia contain dykes of similar size and orientation but magnetic anomalies on the northern margin of the Coompana Block may represent dykes of similar magnitude but different orientation. If the latter could be demonstrated to be of the same age and similar chemistry to the Widgiemooltha Suite then the Coompana Block could be considered as a fragment of the Yilgam Block albeit dislodged and rotated. Most of the Archaean reeks of the Gawler Craton were extensively deforaied during the ca 1850-1700 Ma Kinban Orogeny. Therefore, there is little chance Of major ca 2420 Ma dykes being preserved as long linear province-wide features. Nevertheless, there are no known deforced dykes of similar size and continuity to the Widgiemooltha Suite so the Archaean of the Gawler Craton may not have been contiguous with the Yilgam Block. Similar analysis and conparison of younger dyke swarms can be undertaken between the various Precambrian provinces of southern Australia. This suggests that much of southwestern Australia has been relatively stable and coherent since the Middle Proterozoic. With improved aeromignetic data supported by extensive drilling, mapping, geochemistry and geochronology, mafic dyke swarms offer a valuable means of reconstructing crustal plaies not only between continents (Australia and Antarctica) but also within continents.

MAHC DYKE SWARMS IN RELATION TO EXTENSIONAL TECTONICS: A PROTEROZOIC MODEL

A.Jdin Parker South Australian Department of Mines and Energy, PO Box 151, Eastwood, South AustraUa 5063

In recent years there have been a number of models proposing various forms of extension during the Precambrain. Some of these are based on modern extensional tectonic environments such as the Red Sea or East African rift system where mafica volcanism and intrusion play an important role not only in filling rift basins but also in accommodating at least some of the extension within the upper and lower plates (eg Voggenreiter et alw 1988). Mafic dykes are common in many Precambrian provinces (eg Mount Isa, Gawler Craton, Yilgarn Block: Parker et al.. 1987) so the question becomes, into what tectonic setting were such dyke swarms emplaced, are they related to Precambrian extension in the same way as they are to modern extension and can they tell us something about Precambrian crust/mantle processes? This paper will address these questions by looking at Early Proterozoic mafic dyke swarms on Eyre Peninsul;a in the southern Gawler Craton. Late Archaean and Early Proterozoic metasedimentary sequences of the southern Gawler Craton are intruded by at least three suites of mafic dykes : a suite of multiply-deformed amphibolite layers in layered orthogneisses and orthopyroxene-bearing granitoids; a suite of relatively-undeformed, north-soutk oriented, igneoustextured dykes (Mortimer et al., 1988); and a suite of undeformed, northwest-oriented dykes (Parker et al., 1987), All dyke swarms occur within intraplate environments and can be linked in very broad terms with other magmatic features related to rifting or particular stages of crustal extension and/or deformation. The multiply-deformed amphibolite intrusives were emplaced during the period 1845 - 1750 Ma (probably earlier rather than laterK the north-south dykes were emplaced shortly thereafter, but the northwest-trending dykes, the Gairdner Dyke Swarm, were emplaced ca 1100 Ma at about the same tine as rifting and formation of an extensive basic volcanic suite iimnediately adjacent to the Adelaide Geosyncline. Considering only the first two swarms directly related to formation of the Early Proterozoic, there are several features which are considered to be important: although there were multiple intrufilons according to structural f^terics^ all dykes h^ve similar geochemlcal signatures indicating a similar or related source;

dykes were emplaced mainly within an I-type orthopyroxenebearing granitoid complex, with similar geochemical signatures, east of a major mylonite zone on southeastern Eyre Peninsula; dykes of the southeastern region are geochemically similar to conformable amphibolites in the adjoining supracrustal sequence west of the main mylonite zone; dykes are sequence;

rare

or

non-existant

in

the

supracrustal

deformation of the dykes closely followed emplacement suggesting rapid transition from an extensional to compressional environment; and although deformed dykes may have been rotated, they are generally subparallel to or within 30° of .the gneissic foliation within host lithologies. It is concluded that the various dyke swarms all formed in a narrow zone of extension above a linear convection cell. The convection cell was the driving mechanism behind the extension but achieved this by pushing the plates apart rather than just dragging them. Therefore as extension proceeded, zones of initial extension rapidly moved into zones of compression, compression that was oriented essentially parallel to the original extensional direction. This occurs mainly in the lower plate of modern extensional models and accounts for many of the observed features not only in the Early Proterozoic development of the Gawler Craton but also in many other Early Proterozoic provinces.

References Mortimer, G.E., Cooper, J.A. and Oliver, R.L., 1988. Proterozoic mafic dykes near Port Lincoln. South Australia: Composition, age and origin. Australian Journal of Earth Sciences, 35: 93-110. t Parker, A.J., Rickwood, P.C., Baillie, P.W., Boyd. D.M., Freeman, M.J., McClenaghan, M.P., Murray, C.G.. Myers, J.S., and Pietsch, B.A., 1987. Mafic dykes swarms of Australia. In Mafic dyke swarms. Editors, Halls, H.C. and Fahrig. W.F.. Geological Association of Canada Special Paper 34 : 401-417. Voggenreiter, W. .. Hotzl, H. and Mechie, J., 1988. Low-angle detachment origin for the Red Sea Rift System? In: E. Bonatti (Editor), Zabargad Island and the Red Sea Rift. Tectonophysics, 150: 51-75.

EAST-WEST DYKE SWARM IN THE SOUTHERN EXTREMITY OF THE DELHI PRECAMBRIAN FOLD BELT, INDL\: ITS RELEVANCE TO GONDWANA PLATE TECTONICS PF. Patel, SPH. Sychantfmong, SJ. Desd & SS. Merh Department ofGeobgy. Faculty cf Science, MS. University of Baroda. Baroda 390 002, India East-West dyke 3wan» In North a u j « m t tu.a South Rajaetha/i. Mesten, India though r c o r d e d

. o.ntury back, wa, deBcribea f o r tho f i r c t t l . e by Coulson

(1933) ond lloron <«hI cUod. (1930) -ho referred t h e , nr. baelc dyVcee. Patol ( ; 9 7 l ) "ho

. . p p . d t h i . dyke

5«oXoeiea r.etti„e, Petrography

t . „ f i r o t ,«xicer to d o . o r t b . t h e i r ohe.„l«tr.y. He rcoorde''d more thnn V)0 d.7VeB

«nd on the b « i e o f t h e i r t c « u r e

c a l l e d t h e . as t r a o h y . . d e o U e L

p^r l „ v e ^ i g . t t o n e on the overall geological = e t t i n . of t h i o d ^ e ewai.

Iden-

t i c a l o c o u r « „ o e a in the neighbourln« areae, have eetabU.hed that those dyke, we, e.plaoed during the P^

folding. . c r i t i c a l apnral.al of c e o o h e . l s t r y of the

r aloni rcoke p o i n t , to t h e i r b . i n .

c r . g e „ l o voUunlc . o r i o e , and the oho^lcal

•data c f theee dykes indicate a d l e t U c t a f f i n i t y >rtth the Malanl voloanloa. The variation a i a « r . . (Ka^O . K^O

va

3iO,) f o l l o w , a trend euCabUshed f o r M a l a n l -

rook . . i t e ( s r i v a , t a v a . 1988). Con^iaerln, u . l . strong geoohe.lcal

ai™iiarlty

Olao the precenoe o f aUoat identical trachyandealte. In the Kal.^1 suite, the (.a. weet dyke ewaz. 1» c c r r o l a t a b l e with the Hala..l ig„„ouo oo„.plo, co^pri.infi a f^:ll — d

1970, 1973, s.-iikar, 1968).

750

to

(crawfo

The Maa«i i « n . o u » complex haa an average value of tho initl'al Bt,x3ntlu« i sot o p l c ratios considerably l o . (O.705), Cra..rord (I970). This low value pointa to the original

to have developed by p a r t i a ...eUin, o f ix>c.s of a non-contlnen

b a e e . . n t . Nonoally. continental baee.eut« have .uch higher I n i t i a l ctr^ntiu. i . o t o valu . , ,,,, ^^^^^^^^ ^^ ^^^^^^^ ^ ^ ^ ^^^^^^^ ^^ ^^^^^^ . e l t i n , or X.O.S in the upper . a n t l e ( Rin^v^od, 1975). According to t h i s p-rthor oro.enio v o l c ^ i o

fonned by partial . e l t i n . or ^ a r t . e o l o . i t c a and pyn^xe-

nlteo that represent p.^ducta or tranofo.nation or the abducted occanio oiMst alo the Beniorr zone. 3uch . a . . , . ,

while r i « i n . aorose the ..ntXo .^d^e to the overlytn

oniat. would tend to dirrer^ntiata at dirrerent depthc to f o i ^ a f i n a l p^duct of orosenio volcanic e « r i « e ooT,pri®inc b a e a l t c - andeeiteo - daoitoa -

rhyolit.e

(Ringwood, 1974, 19751 Nicholla, 1974| Nichollo and Ringwood, 1973). To explain occurrence o f an i d e n t i c a l volowiic aefiociation fixi-n North OuJ^^raj and South Rajaffthwi, a plate t e c t o n i c rnechanion involving crustal defoxviation, cratonio i;ix>v/th, and volcanic eruption aiid emplacemQnt of plutonic bodies during Late Proterozoic .:md Karly Cambriai, time hae been invoked. Earlier,

Syohanthavon«

(1978), Sychanthavone and Merh ( I 9 8 I . I985) have put forth a plate t e c t o n i c model errectively explaining varioua euiaodea or «tructural, motamorphic and i ^ e o u s evolution or the Delhi Preca/nbrian rold b e l t . They have c l a e ^ i f i o d the orocenic type« or igneous ^ictivitiea into throe a^e ^.roup.s corroepondia^ to three euccessi^ Delhi doldincn and re/jional meta/norphiana. The rinst two 00-axial Delhi f o l d i n e s (P^ azid F^) are l o l a t e d to the progressive subduction or the oceanic pl.ite, wheretJ the laat deromational event i s attributed to the c o l l i s i o n of the continental landHiaaaea comprising Raat Qondwana wid West Oondwana. during which the ea8t-wo«t foldo (p^) were euperimpoaed. The ajaematiano related to t h i s event ia ra;xnlfeBted in the Maani volcanic miite ai,d the eaert-weat dyke swan., point to t h e i r eoplace^enl along axial pl?\ne fractures or the F^ roldo.

GEOCHEMICAL ARGUMENTS AGAINST MATACHEWAN DYKES, ONTARIO

LATERAL

INTRUSION

OF

W.C Phinney & D.O. Nelson NASA Johnson Space Center, Houston Texas. USA

If dikes intrude laterally, there should be geochemical signatures that can be related along the length of the dikes. Kalsbeek and Taylor found identical geochemical and isotopic signatures throughout the entire length of a single dike that could be traced for 400km in Greenland. They were unable to convince themselves of exactly what mechanism provided the homogeneity but lateral intrusion was one of the options that was seriously considered. In the case of the Matachewan dike-swarm no individual dikes can be traced either in outcrops or by magnetic anomalies for more than a few kilometers along strike. However, the overall pattern of the dike swarm allows broad segments of the dikes to be roughly correlated along the strike direction. Thus, samples of many dikes taken along several parallel traverses perpendicular to the strike direction, each separated by tens of kilometers along the strike direction, should allow a test of the geochemical relations of groups of dikes along the general direction of strike,' Four, subparallel, across-strike traverses, each tens of kilometers in length, along a total strike distance of 300km, have provided samples for major and trace element analyses of many Matachewan dikes. Preliminary interpretations of over 100 analyses indicate that the trace elements in the dikes display different clusters or trends along different traverses with very little overlap between the different traverses. For example, two traverses separated by about 150 to 200 kilometers along strike, display Ta/Zr and Ti/Zr ratios that differ significantly and Ta vs. Zr trends that are distinctly separated. The different clusters and trends are not easily related by either fractional crystallization or multiple injections and suggest either different sources or assimilation of different materials. Because assimilation of the country rocks into which the dikes are emplaced can be demonstrated as negligible, one must call upon the assimilation to have taken place in magma chambers at lower crustal depths where temperatures of the country rock are high enough to invoke partial melting of the country rock and prevent chilling of the intrusive margins. The occurrence of large calcic megacrysts in most of the dikes requires that the melts spent some time in shallow crustal magma chambers where assimilation would be very minor compared to that in the lower crust. Thus, most of the dikes cannot have intruded directly from the lower crustal chambers where the bulk of any assimilation took place. If the differing chemical signatures require either different sources or different lower crustal assimilants for each traverse, then the need for differing sources or lower crustal assimilants at different distances along the strike direction of the swarm argues rather strongly against extensive lateral intrusion of the Matachewan dikes.

DYKE SWARM FROM PONTA GROSSA ARCH (SE BRAZIL): PETROLCX}Y AND PETRCXjENETIC ASPECTS EM. PicciriUo (1), G. BelUeni (2), G. Ccmizzim (S), P. Comm-Chiaramorid (4), R. Petrini (5), AJ. Melfi (6), JPP. Pinese (6), P. Zcmtedeschi (2), S. Bemm (1) & A DeMin (1).

(l)Istituto di Mineralogia-P^trogfafia, University of Trieste ( I t a l y ) } ( S ) D i p a r t i m e n t o dl M i n e r a I p g i a - P e t r o 1 o g i a , U n i v e r s i t y of Padova (Italy); (3)Centro Studi Alpi Orientali, C N R , University of Padova (Italy);(^)1stituia di Mineraiogia-PetrografiaGeochimica, University of Palermo ( I t a l y ) ; ( 5 ) I fitituto di Geocronologia e Geochimica Isotopica, CNR, Pisa ( Italy ) ; (6) Insti tuto A s t r o n o m i c o q Geofisi'co, U n i v e r s i t y of Sao. Paulo (Brazil). The Arch

Lower

C r e t a c e o u s m a f i c d y k e s w a r m of the

(PGA) is a s s o c i a t e d w i t h

Ponta

theiTift P a r a n i b a s i n . H U A - d y k 0 S a r e f o r m e d b y

the

tholelites

Grossa

the f l o o d b a s a l t s and r h y o l i t e s

two-pyroxene

and r a r e r h y o l i t e s . T h o l e l l t e s a r e r e p r e s e n t e d

dominant

h i g h T i O ^ (>ey.5HTi) group., and a s u b o r d i n a t e

<<2y.|LTl)

group

c h a r ac ter i zed

by

high

element contents, r e s p e c t i v e l y . Gr-Nd PGA-dykes plot

in the m a n t l e a r r a y

and do n o t s h o w c h e m i c a l

and

of

low

by

low

a

FiUg

incompatible

i s o t o p e s s h o w that m o s t

of

(€(Sr)»« c . 4-17, €(Nd)= c .

-3)

evidence supporting

appreciable

crustal

The important

chemical

1B contamination. Pyroxene av. S differences in

t e r m s of f r a c t i o n a l

c r y s t a l l i z a t i o n , but a r e c o n s i s t e n t

d i f f e r e n t m e l t i n g d e g r e e s of LTi=20

per

mantle, than

garnet peridotites

cent m e l t i n g ) , probably

Paleomagnetic

most

0=6.0±0.a.

b e t w e e n L T i and H T i bafsalt d y k e s c a n n o t a c c o u n t

data

belonging

and

to

PGA-dykes

are

y o ur n g e r

were

probably

towards the

continental

later e r o d e d . D y k e empla.cement o c c u r r e d d u r i n g

p h a s e s of r if t ing/f lexur ing

and

lithospheric

indicate that P G A - d y k e s

of P a r A n ^ f l o o d v o l c a n i c a -

f e e d e r s of the s t r a t o i d v o l c a n i c s e r u p t e d margin

(e.g- HTi»-9

for with

as in t.thiopia, Y e m e n and

early

Lebombo.

MAHC DYKE SWARMS IN THE USSR: A REVffiW AF. Platunova, AN. Berkovsky, D.V. Rwidqvist, LK. Levsky, VN. VerkhahVzky, JA Fedotov, AS. Yein, BR. Shpount & A.V. Okrugin Institute of Precambrian Geology and Geochronology, nab makarova 2, 199034, Leningrad USSR Insitute cf Geology, USSR Academy of Sciences, Karelian Branch, Petrozawdsk 185610 USSR Geological institute of the Kola Centre of the USSR, Academy of Sciences, Fersman 14, Apatity, USSR Yakutsk Institute of Geosciences, Siberian Branch, Academy of Sciences, 39 Lenin Avenue, Yakutsk 677891, USSR The mafic dyke swarms in the USSR (Fig.l) can be assigned to ^ categories, reflecting their dimensions: local, regional, transcontinental and intercontinental sets. Combining aero- and paleomagnetic and geological data supported by rare isotopic ages allowed the dyke swarms ranging from early x^ecambrian to Mesozoic and exhbiting different distribution patterns to be identified within th^ East European and Siberian cratons: in the Baltic, Ukrainian, Anabar and Aldan shields as well as in the Cis-Yenisei region. The Baltic shield (Fig.2) includes over 1(5 Precambrian dyke swarms varying in size from minor sets up to 700 km long and 6(30 km wide. They are confined to the different order blocks, show mainly tholeiitic composition, dominant HS-trend and intermittent emplacement throushout-^a.O Ga time-span;from ca 2.6 to (5.6. Ga. A liJ-trending Devonian swarm on the eastern xiussian platform is a giant extentional structure about 800 km long and 2S0 km wide (Fig.1). I'he Ukrainian shield is dominated by 6 regional and several local sv/arms which are 250 to 10 km long and 15O-IO km wide. The swarms are mainly NW- and

nearly E-V/-

trending, show compositional variations of the dykes from ultramafic to alkaline and acid varieties and fall into the 2.7-0.5 Ga range (Fig.5). 5 regional dyke swarms which are IJ.;- and NNW-trending and defined by dykes of tholeiite basaltic and subalkaline composition emplaced during ^ episodes within the 1.3-0.9 Ga range

have been

so far recognized in the /i^nabar shield (Fig.4). The Aldan shield is characterized by ^ trans- or, probably, intercontinental sv;arms of Ni^ trend (600 x IOO km) and several similar swarms of

Nv/ trend de-

fi.ied by dykes of tholeiites, calc-alkaline and trachybasaltic rocks dated at ca 1.7-0.85 Ga (Fig.5). fhe Gis-Xenisei massif in the south -western oiberian platform (Fig.6) e:diibits several regional and local dyke sets defined by tholeiitic to trachybasaltic dykes and showing

over 3OG km in length and jO km in width

to be dated in the

range from 1.7 to C.65 Ga. The study of the above swarms, which is in progress, may greatly contribute to understanding the tectonic divisibility and endogenic re^

»

gimes of dyke emplacement, composition and changes of source magma, in time and space, etc.

A GRAVITY AND MAGNETIC STUDY OF THE UMVIMEELA AND EAST DYKES, ZIMBABWE F. Podmore & MF. Mushc^andebfvu Department of Physics, University of Zimbabwe, Harare, Zimbabwe

The quartz-gabbro Umvimeela (to the west) and East Dykes are approximately parallel to the Great Dyke for much of its 500 km length as it bisects the granitic Zimbabwe craton. Preliminary gravity traverses suggest a non-vertical attitude for the Umvimeela and East Dykes. Some of the Bouguer anomalv profiles over the Great Dyke interpretted by Podmore (1985) are asymmetrical and indicate a dipping feeder dyke. Recent gravity and ground magnetic data collected on about 10 traverses over the Umvimela and East Dykes at points where there IS palaeomagnetic data to control the magnetic' interpretation is being analysed together with aeroraagnetic data to determine whether the Dykes are vertical or inclined. Evidence for any longitudinal variation in dyke width and dip will be cn:)jnpared with the gradual northerly convergence evident from geological mapping. The impJications of these findings for the overalJ history of the Zimbabwe craton will be discussed.

tectonic

PHANEROZOIC MAHC DYKE INTRUSIONS FROM THE fflGH GRADE TERRAIN OF SOUTHWESTERN INDIA: K-AR ISOTOPE AND GEOCHEMICAL IMPLICATIONS TJiadhakrishm, MJosq)h, PKThampi Centre for Earth Science Studies, PB. 7250, Trivandrum-WS 031, India & WMitcheU School cf Physics, University of Newcastle Upon Tyne, NEl 7RU, UK Mafic dyke intrusions manifest the Phanerozoic basic magmatism in the high grade region of the southwestern India. They are conspicuously smaller in size and variable in trend, age and mineralogical and chemical characteristics. K-Ar results are compatible with ages of t 6 Ma for the NE-SW dolerite dyke suite, 81 * 3 Ma for the NNW-SSE leucogabbro dyke suite, 61 t 9 Ma for the NW-SE dolerite dyke suite and >05 t 2 Ma for the ENE-WSW isolated basaltic dykes. Doierites are olivine or quartz normative with affinity towards MORB or CTB, while leucogabbros are chiefly quartz normative with mild alkaline character. NNW leucogabbros and ENE doJorite dykes are enriched in incompatible trace elements relative to the NE dolerite dykes but the NW doierites arc relatively depleted. Relative REE abundances range from flat pattern in the NW doierites to highly fractionated patterns in the NNW leucogabbros while the NE doierites show both enriched and depleted LREE patterns. Fractionation of minor amounts of mafic phases is evident; however, the incompatible element chemistry is not significantly affected by fractional crystallisation or crJstal contam'ination. The dyke suites exhibit marked variation of incompatible and partially compatible trace elements either within or in between the dyke swarms that would require heterogeneous mantle sources. Tectonicaliy, the Phanerozoic dykes are closely related to the rifting and break up of Gondwanaland and the region along the western continental margin was Continuously under extensional tectonic regime that may have developed various aseismic ridges/plateaus in the northwestern Indian Ocean. (CJ AGALI DOLERITES

( a ) TftlVANORUM D O L E R I T E S

Lo

Ce

Pf

Nd

Sm

Eu

( d ) KOTTAYAM LEUCOGAeBROS

(b) KOTTAYAM DOLERITES

OTRamo Depanment of Geology, University of Helsinki, P.O. Box 115 SF-W171 Helsinki, Finland In southern Finland, there are two Proterozoic cratonic magmatio episodes involvinq the emp acement of tholeiitic diabase dykes. The older Subjotnian (ca. 1.6 Ga) andThe Z n ^ ^ Jotnian (1.26 Ga) episodes are distinct in terms of cheniioal and i s o t o p i r c o m ^ ^ ^ dykes and they exhibit also different iithologic associations. The o^er e S e is n l i r

association) and has resulted In the emplacement of silicic-basic composite

frnmirnf T r ^ m a S c Ltivrtr^^

as a prominent feature. The younger episode comprises ^

0' coeval silicic

Geocliemically. the older Subjotnian diabases are high Ti basalts ranging from olivine tholeiite o c^uartz tholeiite. They are strongly enriched in incompatible trace elements showing low ^^^Sm/'^^Nd ratios (0.12 to 0.13) and high concentrations of Pb (6 to 20 p p m r cluster around CHUR ( +1.6 to -1.1) and correlate element concentrations and fractionation parameters. In the ^ P b / ^ V b vs. Pb/ Pb diagram the dykes plot close to the evolution of average crustal Pb and fall along a secondary isochron with an apparent age of 1854+ -65 Ma. which is about 200 l^a in excess of their crystallization age. The younger Jotnian diabases are high Ti olivine 147c' dyl^es. they are less enriched in incompatible elements L

° and show significantly lower concentrations of SB'"'" ^Ndf^) values are positive ranging from +1.6 to +3.2. In the ^^^Pb/^o^Pb vs. Pb/ Pb diagram the dykes plot below the evolution line of ai'erage crustal Pb closer to model mantle evolution. S)7

Both the older and the younger diabase dykes intruded the juvenile Svecofennian (l 9 to 1 8 Ga) orogenic crust of southern Finland. The mantle source tapped by the younger dykes was one with a distinct LREE-depleted character, as is demonstrated by the positive e (T) values. Also the Pb isotopes of these dykes show mantle type composition. In contrasMhe older dykes exhibit smaller, partly negative. values and were thus either derived from a different mantle source or were affected by the continental crust during their passage through it. Isotope-compositional correlations and crust-type Pb isotopic composition are in favour of the latter hypothesis; the age of the secondary isochron matches well with the age of the Svecofennian orogenic crust. What reason is there for these two cratonic magmatic episodes to yield compositionally and lithologically such different associations? One important controlling factor may have been the tectonic environments of the two magmatic episodes. The older episode may have happened in an active rift with large-scale mantle upwelling causing major thermal anomalies at the base of the continental crust and extensive partial melting in the crust itself, giving rise to the contemporaneous silicic magmas. In such an environment, mantle-derived mafic magmas would be most susceptible to contamination with crustal components. The younger magmatic episode may have been a passive rift that formed simply as a result of two-dimensional plate motions, rather than by mantle upwelling, and without any major partial melting of the crust. The tectonic framework could have been a collisional rift associated with the onset of the Sveconorwegian-Grenvillian orogeny 1.25 Ga ago. Another important factor controlling silicic magmatism associated with these diabase dyke swarms could have been the overall post orogenic heat budget of the lithosphere regime that the dykes penetrated. After the early Proterozoic orogeny the lithosphere cooled slowly and may have been, at 1.6 Ga. hot enough to aid considerably in triggering the partial melting to yield the silicic rapakivi magmatism. By Jotnian time, the lithospheric regime had cooled considerably, and thus did not promote partial melting in the continental crust.

EMPLACEMENT TECTONICS AND THEIR CONTROL ON DIFFERENTL\TION IN A LATE PRECAMBRL\N MAFIC DYKE SWARM, LOWER ORANGE RIVER, SOUTHERN AFRICA LORD. Ramome & DL. Reid Department of Geochemistry, University of Cape Town, South Africa

A m a j o r swarm of mafic dykes occurs in the lower Orange R i v e r region of southern A f r i c a , and w a s emplaced during the early stages of the PanA f r i c a n orogenic episode (700-500 M a ) . The dykes represent crustal extension during the rifting phase of the Gariep mobile b e l t , the latter representing the southerly continuation of the Pan-African orogens, of w h i c h tha Damara belt is the best known. Much of the swarm preserved today occurs in the adjacent pre-Gariep basement, but some of the dykes intrude into strata of b a s a l Gariepian sediments. Dyke density increases from E to W as the Gariepian rocks are approached. Late Gariepian deformation and metamorphism of the dyke swarm also increases towards the Gariep b e l t . Younger cover obscures the true extent of the dyke swarm but a conservative estimate is 400 km along strike and 150 km a c r o s s . Dykes intruding pre-Damaran basement in central and northern Namibia m a y also represent similar age swarms, and would extend the limits to o v e r 1000 km along strike. The biggest dyke in the swarm is followed by the Gannakouriep River (after which the swarm is named) in the north-eastern Richtersveld and can be followed for o v e r 100 km northwards into Namibia. At its widest point the Gannakouriep Dyke is 2 kilometres across and largely gabbroic. Zones of highly differentiated syenitoid rock types occur in places. While the Gannakouriep Dyke itself is exceptionally w i d e , the average width throughout the entire swarm is about 50 metres, indicating that there are few dykes in the 1-2 metre range, and serves to distinguish the Gannakouriep swarm from many others, particularly the late Mesozoic Karoo age dyke swarms that also occur along the west coast of Southern Africa. Biotite separated from fresh unmertamorphosed gabbro within the Gannakouriep Dyke yields a Rb-Sr age of -720 M a , while ^^Ar/^^Ar whole rock spectra yield a plateau age of about 540 M a . Whole rock RbS r data do not yield an isochron because of syn-emplacement contamination from the granitoid country rock. The youngest country rock are granites d a t e d at 920 Ma by U-Pb zircon, and the dykes are cut by 520 Ma old granites a n d syenites. At present no confirmation of the R b - S r biotite age is available, but it is probable that the 540 Ma K - A r age patterns represent the effects of later thermal events which include both late Pan-African metamorphism and granitoid intrusion. A r e g i o n a l g e o c h e m i c a l survey was preceeded by a detailed study of dyke profiles in three dimensions (transverse, along strike and through v e r t i c a l relief in deep canyons), in order to establish the scale of intra-dyke v a r i a t i o n . Wide fluctuations in a single dyke would negate any significance placed on limited sampling that necessarily accompanies a r e g i o n a l study of hundreds of dykes. As a result of such studies, it was possible to establish different groups of geochemical parameters that could be utilised to separate the effects of different petrogenetic p r o c e s s e s . It has b e e n possible to recognise and distinguish effects of such processes, and even contour their relative importance throughout the s w a r m .

^ Y ™ SWARMS IN SOME PARTS OF EVALUATION IN RELATION TO STRUCTU^ A ] ^ TECTONICS D. Atdum Rao National Geophysicd Research Institute. Hydrabad 500 007 India

In India it is believed that different magmatic episodes have been responsible for the emplacements of dyke swarms and these have been divided into two broad age groups, viz. (i) Cenozoic to late Palaeozoic and (ii) Pre-Canbrian. Aeromagnetic maps over dyke swarms in (1) Bundelkhand granite/gneiss around the Protorozoic Vindhyan Basin, (2) Crystallines (granite and gneisses), west of Cuddapah Basin, and (3) in the Narmada-Son lineament (NSL) belt, have been studied. The magnetic expression of the dykes in Bundelkhand granite/gneiss terrain and those in the western parts of Cuddapah Basin have wide variations in pattern and intensity and are mostly related to the possible structures associated with their emplacement. Whereas the dyke swarms in the NSL belt hfwe brought out a distinct linear and strong anomalies, within the Deccan lava environment. These observations and their significance in understanding the structural and tectonic setting vis-a-vis the age of emplacement of these dykes are examined. The limitations of detection of basic dykes through aeromagnetics are pointed out and discussed.

GEOCHEMICAL AND PALAEOMAGNETIC STUDIES ON PROTEROZOIC KARIMNAGAR MAFIC DYKE SWARM, INDIA

THE

MIDDLE

JMaUikharjum Rao, G.VSPoomachandra Rao & SKPatU Natbnal Geophysical Research Institute, Hyderabad 500 OCfJ, India M a f i c dyke swarms are c o m m o n in the P r e c a m b r i a n Indian shield where several Proterozoic sedimentary basins are d e v e l o p e d . The m i d d l e P r o t e r o z o i c d y k e s w a r m of K a r i m n a g a r is in the v i c i n i t y of the P a k h a l b a s i n and e x t e n d s in N E - S W direction for a b o u t 125 k m . M a n y of the s e g m e n t s s h o w local v a r i a t i o n f r o m N E - S W , E N E - W S W , E - W a n d r a r e l y N-S d i r e c t i o n s and t h e y a r e e i t h e r linear or c u r v e d . S i n g l e d y k e s v a r y in w i d t h from few m e t e r s to 1 0 - 2 0 m and run for d i f f e r e n t l e n g t h s . The d y k e s are either d o l e r i t i c or g a b b r o i c t y p e . P e t r o g r a p h i c a l l y a t l e a s t three g r o u p s a r e i d e n t i f i e d i e . , a) f r e s h d o l e r i t e d y k e s w i t h o p h i t i c / s u b o p h i t i c t e x t u r e , b) totally altered with relict igneous textures and c) c o a r s e grained gabbroic type. Most of these d y k e s contain orthopyroxene, titaniferous clinopyroxene, micropegmatite and opaques with sphene rims. Chemically all the d o l e r i t e s show tholeiitic composition, quartz normative and exhibit moderate iron enrichment in m a g m a t i c c o m p o s i t i o n . R a r e e a r t h element pattern show enrichment of light rare earth elements (60-70 times normalised value) and h e a v y rare earth elements (5-10 times normalised value) show flat p a t t e r n w i t h o u t a n y e n r i c h m e n t or depletion. Europium show a strong negative anomaly. 53 O r i e n t e d block s a m p l e s w e r e c o l l e c t e d from 15 different sites on d y k e s t r e n d i n g m o s t l y N E - S W and a l s b N - S , E - W , ENE-WSW for p a l a e o m a g n e t i c i n v e s t i g a t i o n s a l o n g the e n t i r e s t r i k e length. Several specimens were subjected to detailed laboratory d e m a g n e t i z a t i o n by the a l t e r n a t i n g m a g n e t i c field in s t e p s u p t o a peak field of 100 mT to find the c h a r a c t e r i s t i c m a g n e t i c v e c t o r in these d y k e s . A stable magnetic vector of D m = 5 2 . 5 , Im = - 2 4 . 5 (K = 2 1 . 8 1 , c X 3 5 = 8 . 6 5 , N = 12 s i t e s ) has b e e n i s o l a t e d f r o m t h e s e dykes along with two o t h e r r e m a n e n t v e c t o r s in the E S E a n d S q u a d r a n t s with i n t e r m e d i a t e - s t e e p u p w a r d and d o w n w a r d i n c l i n a t i o n s r e s p e c t i v e l y . These results c o n f i r m the e a r l i e r palaeomagnetic f i n d i n g s over a limited e x t e n t a p a r t from b r i n g i n g in to l i g h t the p r o m i n e n t d i r e c t i o n of m a g n e t i z a t i o n of the s w a r m . Age of these d y k e s c o u l d be y o u n g e r than the Proterozoic Pakhal s e d i m e n t s . Some of these d y k e s a r e seen intruding the P a k h a l s e d i m e n t s . T r e n d of the d y k e s w a r m is p e r p e n d i c u l a r to the Godavari r i f t (NW-SE) and t h e r e f o r e , s u g g e s t that the fractures developed d u r i n g r i f t m a d e p a t h to e m p l a c e the d y k e s a l o n g these fractures. C o m p o s i t i o n a l v a r i a t i o n s and p a l a e o m a g n e t i c e v i d e n c e s s u g g e s t that m u l t i p l e i n t r u s i o n s of the m a g m a . P r a b a b l e p e r i o d of e m p l a c e m e n t has been s u g g e s t e d b y the p a l a e o m a g n e t i c s i g n a t u r e of these Karimnagar d y k e s .

T m TONTA GROSSA DYKE SWARM AND ITS RELATIONSHIP WITH THE (SOUTHERN BRAZIL), BASED ON MJ£. Raposo, M. Ernesto & LA. Diogo InstUuto Astronomico e Geofisko, University of Sao Paulo. Caixa Postal 30JS27 01051 Sao Pauh-Sp, Brazil '

rhni Sou h^a BasL basin, dykes

Grossa r

'

dyke swarm corresponds, to hundreds Cretaceous age. trending NW-SE fro^

of tie

:Lre" ' t h ^ i n t e r i L o f the P a r a n a where they reach the n o r t h e r n part of the basin The however r a r e l y cut the P a r a n a f l o o d v o l c a n i c s .

Ihe younger

available than the

pa 1 e o m a g n e t i c d a t a Parana flood

Pa ^arana central region and the northeastern region where numerous

s h o w that volcanics

magmatic silf-type

the but

dykes 'can

are be

volcanis^L the a c t i v i t y • in the i n t r u s i ^ ^ ^ a^e a l s ^

of t u l ' ^ l o l t T i r o l T " ! and P r / r . U and P r e c a m b r i a n rocks cnat

these

dykes

c o n c e n t r a t e d in the n o r t h e r n p a r t ^ t e c t o n i c s w e l l that e x p o s e s PaleoLic which are all cut by -.the dykes ?he -gneti.ation dlrectton^'tndica c o o l e d d o w n at the s a m e r a t e

beino r e v e r s e d p o l a r i t i e s a r e r e c o r d e d , the f o r m e r be ng n,ore f r e q u e n t . M e a n m a g n e t i z a t i o n d i r e c t i o n s fo^ d y k e s J r o m d f f e r e n t r e g i o n s of the A r c h s e e m to c o r r e l a t e b e t t e r i ^ ^ L the ripes delimited by the NW-SE tectonic and/or L g L 5c l i n e a m e n t s that c h a r a c t e r i z e the P o n t a G r o s s a A r c h "•^gn'^tic

THE CAPE PENINSULA DOLERITE DYKE SWARM, SOUTH AFRICA DLReid Geochemistry Department, University of Cape Town, Ronsdebosch 7700 Soldi Africa

Dolerites of estimated Mesozoic age cutting early Paleozoic granites and overlying sedimentary strata in the vicinity of Cape Town have been described briefly on several occasions over the past 17 5 years. Outcrops were however confined to sea shore exposures and a few road cuttings and building excavations. That the dolerite dykes were part of a major swarm of regional extent was only recently recognised after a comprehensive aero- and marine magnetic survey of the south-western Cape was completed. The Cape Peninsula dyke swarm trends NW-SE roughly parallel to the continental margin in this region, as determined from off-shore bathymetry. Using the linear positive magnetic anomaly patterns, the swarm follows the continental margin from Cape Agulhas in the SE to beyond Saldanha Bay in the NW. Beyond these limits the continental margin changes trend and the swarm can not be detected. Detailed K-Ar dating of the best preserved dykes is still in progress, but preliminary palaeomagnetic data indicate a pole position intermediate between the 90 Ma old southern African kimberlites and the 180 Ma old Karoo dolerites and basalts, suggesting that the dyke swarm may be younger than the main period of Karoo magmatism in South Africa. Similar age relations have been established for other prominent dolerite dyke swarms of Karoo age, such as the Rooi Rand (Lebombo Monocline) and Horingbaai (northern Namibia). The chemical composition of the Cape Peninsula dolerites is similar to that already established for much of the Karoo dolerites and basalts, referred to locally as the Central Karoo magma type. One distinguishing feature is the prominence of highly fractionated ferrotholeiites (Mg Number -30, Total Fe as Fe203 -- 16%, Ni < 5ppm) , and the swarm displays a degree of differentiation not observed in any Karoo intrusive (except for some of the plutonic complexes). Results of detailed modelling of emplacement mechanism, petrogenesis and source region based on magnetic field image analysis, K-Ar dating, REE and radiogenic analyses, which are currently in progress, will be presented and discussed.

THE TECTONIC IMPLICATIONS OF THE MIDDLE PROTEROZOIC MARC DYKES IN THE SAO FRANCISCO CRATON OF BRAZIL PR Renne, TC. Onstott Dept of Geological and G&^t^icd Sciences, Princeton University, Princeton, NJ 08544, USA & W. Teixeifu InstiMo de Geosdences Universidade de Sao Paulo, CP. 20899 Sao Paulo, S.P. BrazU 05541 In the Sao Francisco Craton, numerous middle Proterozoic mafic dike swarms intrude early Proterozoic granulites and gneisses. Well-exposed along the eastern coast of Brazii are two of these mafic swarms, the east-west striking swarm at llheus and Olivenca and the north-south striking swarm at Salvador, 400 kms north of llheus. Biotite from the baked contacts at llheus and Olivenca yield 1011 ±24 Ma and 1078±18 Ma dates, respectively. The former result is concordant with a 1012±24 Ma ^^Ar/^^Ar date on plagioclase separated from an llheus dike. Biotite from the baked contacts at Salvador yield an 1021 ±8 Ma '^^Ar/^^Ar date, concordant with a 1003±33 Ma date on sericitized plagioclase from the same Salvador dike. The dates suggest that the two swarms are contemporaneous and that the episode of emplacement spanned several tens of millions of years. Macroscopic structures in the dikes indicate that magma flowed from south to north for the Salvador dikes and from east to west for the llheus and Olivenca dikes. If a common magma source supplied both dike swarms then the eruptive center would have resided off the east coast of Brazil. When South America is restored to a pre-drift position with respect to Africa, the Sao Francisco and Congo cratons become contiguous. The hypothetical center of eruption for the Brazilian dikes coincides with the Mayumbian basin of the Congo where a 4-5 km th ck sequence of basalts and rhyolites occurs. Oogenetic hypabyssal granites intrusive into the Mayumbian yield an age of 1027±56 Ma. Major and trace element analyses of the Sao Franciso dikes and the Mayumbian basalts are very similar to each other and to continental tholeiites in general. In the pre-drift reconstruction, the east-west strike of the llheus and Olivenca dike swarms is reoriented to north northeast by west southwest, perpendicular to the long axis of the Mayumbian basin. It is also parallel to the strike of 1.0 Ga mafic dike swarms which occur on the opposite side of the Mayumbian basin where they Intrude the FraHcevillean Group in Gabon. On the basis of this evidence, we propose ' iho Sao Francisco dikes and iM£^yumbian basin represent an rft basin that was active between f 060 ancf WtO Mat.

THE ANATOMY OF A DYKE AND THE DETERMINATION OF PROPAGATION AND MAGMA FLOW DIRECTIONS PCRickwood Department of Applied Geology, University of New South Wales, Kensington, NSW. 2033, Australia. The terminology of size and morphological features of Individual dykes Is reviewed and rationalised. terms.

Labelled diagrams rapidly convey the meaning of most

Reports of use of various dyke features to ascertain the direction of

propagation or of magma flow have been collated and their reliability of is indicated.

KEY WORDS: Dyke, dike, terminology, size, morphology, propagation direction, flow direction.

THE WrraiN-SWARM COMPOSITIONAL VARIATION OF PROTEROZOIC SODERMAN LAND DOLERITE DYKES, EAST CENTRAL SWEDEN H. Risku - Norja Department Mineral. Petrol. University of Lund, Solvegatan 13, 22362 Lund Sweden The Sodermanland dolerite complex comprises several swarms of post-Svecofennian mafic dykes in east central Sweden to the south, west and southwest of Stockholm. The available age data confirm the occurrence in the area of two sets of dykes, the about 1530 Ma old Group-! dykes, traditionally known as the Breven-Hallefors swarm, and the about 1000 Ma old Group-!! dykes which belong to the well known BDD-swarm (Blekinge Dalarna Dolerites). Two more swarms have been recognized on the basis of the occurrences in the field, the mineralogical characteristics and the chemical compositions. Thus at least four different swarms of post-Svecofennian dolerites can be distinguished: the W/WNW trending Group-! (Breven-Hallefors swarm), the N trending Group-ll (BDD swarm), the NW/N trending Group-Ill and the WNW/NNW trending GroupIV. The ages of Group III and Group IV are not known. Two of the swarms -Group ! and Group !!!- are examined more closely. Despite overall tholeiitic major element compositions each swarm has a specific geochemical signature and displays distinct trends in various diagrams suggesting, that the processes controlling the formation and evolution of the magmas have been substantially different. The compositional variability within Group ! cannot be explained by a simple evolutionary sequence from a common parent. The data suggest that a range of primary magma compositions has been created by varying degrees of partial melting at the source, that may even have been heterogeneous in small scale. The subsequent evolution of the individual dykes has followed broadly similar paths, although the extent of fractional crystallization and crustal contamination may have varied from dyke to dyke. The dykes of the swarm are linked with each other via common type of source; thus they are oogenetic, but not necessarily comagmatic. The composition of Group !l! is largely controlled by fractional crystallization accompanied by assimilation of crustal material within a magma reservoir. The intragroup variability is consistent with the evolution from a single parent magma. The parent is not a primary, mantle-derived melt but has a clear crustal imprint evident in the high Sr contents and the enhanced contents of Si02. There is a possibility of open-system evolution. The more enriched nature of Group I relative to Group III is evident i in the higher contents of K2O, Rb, P2O5, MnO, Rb, Y and Zr. However, the ratios of the incompatible elements and the patterns on the trace element spidergrams suggest that the Group-Ill dykes are derived from a more enriched source than Gr!. It is proposed that the apparent dichotomy is related to distinct styles of contamination and reflects the effects of magma chamber processes (Group III) and contamination en route (Group I).

LAMPROPHYRIC DYKE SWARMS AND PIPES IN WESTERN AUSTRALIA: AN UPDATED SUMMARY NMS. Rock Key Centre for Strategic Mineral Deposits, Geology Dept., Umersity of Western Australia, Nedlands, 6009, Australia

ABSTRACT: About 20 lamprophyric rock-suites are now known in Western Australia, with an age-range of Archaean-Miocene. Most suites are Precannbrian and dominated by dykes, though pipes are dominant in one or two suites. All but two suites cluster in or around the three major Archaean cratons (Kimberley, Pilbara and Yilgam). Hitherto undescribed discoveries include kimberlitic pipes of uncertain age in the Phillips Range (Kimberley). ultramafic lamprophyre dykes and pipes around Melrose (Yilgam), and minor swarms of ?early Proterozoic metamorphosed ultramafic lamprophyres in the Shaw Batholith (Pilbara). In addition, calc-alkaline lamprophyres previously reported from scattered localities in the Pilbara and Yilgam have been found to belong to major swarms of regional extent, comprising hundreds or thousands of individual dykes, sills and rare pipes over thousands of square kilometres. Kimberlites and lamproites have not yet been confirmed in either the Pilbara or Yilgam. The increasing recognition of widespread, deep-seated magmatism in all three cratons now makes them appear more similar to other better-documented cratons (e.g. Canadian Shield, Greenland, Kaapvaal), which are already known to be peppered with intrusions representing repeated and persistent lamprophyric magmatism over long periods of geological time. Broadly speaking, the magmatism of the Yilgam and Pilbara appears more similar to that of the Canadian or S.Greenland cratons than the S.African craton.

GEOCHEMISTRY AND TECTONIC SIGNMCANCE OF DYKES OF THE AVALON TERRANE, MASSACHUSETTS

PRECAMBRIANf?)

mmrn^mm^ M£jRoss Northeastern University, Boston, USA

during the l a t e PrecambrLn with Boston t e r r e n e occured the Paleo3oic ( B a ^ ^ ^ ^ ^ ) - - f i r m i n g Avalon block adjacent th<=. f extension within the of mafic magmas t n S n t i l y formed o^ / t h e rise Lesser episodes of l e f t - l a t e r ^ r l f NW-trending f i s s u r e s , along the thrust L l f ' e i r o S l S s i r w M f ^ ' " i T ^^^^ f o r the l e s s e r NE- to ' magmai^srwal^'sL'^Lit'ld' w'iu' c^'ntin"' Precambrian mafic (Ross. 1983. 1985, in p r e s s f ? ^ F N ^ i n i t i a t e d during the l a ^ e s ? P r . I l k extension r e l a t L ^^ ^ ^ e " ' g h ' ^ a t e ^ a t " r e s u l t e d from to the west according to B a f o s h n U t ) interbedded with l a t e Preoambrian tn r ; k a l t e r e d lavas the Boston Basin p e U o c x S w n " Cambrian metasediments within altered dikes d e s S i b ^ f Z ^ t l t - T ^ ''f are a v a i l a b l e . I t i s p o s s i b L t L f.^n ^ analyses of the lavas d o l e r i t e s described ^bove aid w L i alkalio Basin, represent arLte;i"ii ^ n

The NE- to ENE-trendin^ ^w^vm

trending swarm was intruded.

EMPLACEMENT OF DYKE SWARMS IN THE LOWER NARMADA VALLEY WESTERN INDIA DASant & RVKaranth Department of Geology, MS. University ofBaroda, Vadodara 390 009 India

High concentration of dykes of various dimensions is a conspicious feature

observed

in the Lower Narmada

the region have two main trends,

valley.

viz.

to N-S and t)elong to two distinct suites, carbonatite suite. Of the two groups,

Western India.

Dykes of

B-w to ENB-WSW and 2. NVV-SE namely basic suite and alkaline-

basic dykes occur all over the area

while the latter has a limited distribution. Alkallne-carbonatite suite of rocks are encountered mostly in the northern part of the Narmada River, between Panwad-Phenaimata

and ^ Ambadongar.

where

EN&-WSW

and

NW-SE fracture

systems intersect.

separate

Temporally,

the

magma

periods.

The Basic

appears

to have'

magma appears

been

emplaced

in

two

to have been emplaced soon

after the out pour of thick pile of Deccan basalt, in form of feeder dykes. EXirtng

same

time

(Paleocene),

basic

magma

has

also

filled

the

newly

developed fractures, resulted by the formation of Cambay basin in the western part and a regional uplift In the eastern part. The alkallne-carbonatite magma is

likely

accounts and

to have

moderate

basin.

been

for a strong

emplaced tectonic

transgression

and

at the close

of Eocene Period,

which

activity expressed by regional uplift inland strong

regression

in the adjacent Cambay

AN ATTEMPT TO DETERMINE UPLIFT OF THE SYDNEY BASIN, NEW SOUTH WALES, FROM THE PALAEOMAGNETIC SIGNATURES OF DYKE CONTACTS FROM lOAMA PWSchmidt CSIRO, Division of Expbrcabn Geoscience, PO Box 136, North Ryde NSW 2113

A dyke swarm invades the Late Permian lavas of the southern Sydney Basin, at Kiama. K - A r analyses of biotite and kaersutite (from a xenolith in a dyke) yield ages of 198±3Ma and 200±3Ma respectively, suggesting that intrusion occurred in the Early Jurassic. The dykes are w e l l exposed in a quarry at Bombo, north of Kiama. Seven of the dykes and their contacts have been studied palaeomagnetically to investigate the originality of the magnetisation (for a contact t e s t ) , any overprint magnetisations that may be recorded and the amount of unroofing since intrusion. While the characteristic remanence isolated after AF and thermal cleaning confirms an Early Mesozoic ^ge, there are complications with the contact magnetisations. Kiama is the namesake of the renowned Reversed Kiaman Interval, now called the Late Palaeozoic Reversed C h r o n , because it was from these Permian lavas that a reversely magnetised Palaeozoic rock was first identified. The lavas originally cooled when the geomagnetic field was reversed, and incidentally, when Australia was near the south pole. The characteristic magnetisation of the lavas is therefore directed downward, almost vertically. The dykes are normally magnetised. While both these components have been identified in the dyke contacts, their relative stabilities to thermal demagnetisation are not simply related. Often the overprint magnetisation is the most stable component present indicating chemical alteration of the magnetic m i n e r a l s . This has also been observed in dyke contacts elsewhere and offers an explanation as to why positive contact tests in palaeomagnetism are a rarity. The overprinting is not restricted to the dyke contacts but is also seen in samples a great distant from any known d y k e . This may reflect a general increased heat flux during dyke intrusion that has increased the temperature of the country rock remote from dykes. The identification of a second g e n e r a l overprinting event just prior to rifting of the Tasman Sea at about lOOMa may also be evident in the magnetic record.

DYKES OF CRATON

RARE

METAL

ONGONITES

FROM

THE

EAST

EUROPEAN

NN.Shatalov Institute of Geopt^sics, the Ukrainian Academy of Sciences, Kiev, USSR Dykes of Pbanerozoic rare metal ongonites representing subvolcanic analogs of Li-F rare metal granites have been earlier discovered in Central Mongolia (Kovalenko, 1976). The rock of similar composition, fabrics and age have been later found in the Baikal region, Kazakhstan, Tien Shan, East Sayan, North-East Tuva, Germany, USA and elsewhere in the world. The Precambrian ongonite dykes are so far known solely within the East European craton, in particular, in the Baltic and Ukrainian shields. They are established in Finland (Haapala, 1977) and North Ladoga region (Sviridenko et al., 198^0 on the Baltic shield, and within the Volyn and Cis-Azov geoblocks on the Ukrainian Shield (Zinchenko, 1982; Shatalov, 1989). Her^, the ongonites are structurally and genetically linked with rapakivi granites and younger minor intrusions of rare metal granites of a specific Kamennomogilsk type (Cis-Azov region). Two generations of the ongonites can be distinguished based on structural-geological and isotopic data. The dykes are usually NS- and N E - trending and range up to 20 m thick. The ongonites are composed of albite, K-feldspar and quartz, and topaz, fluorite alongside lithium and other micas being secondary minerals. The rocks often exhibit porphyritic texture. Phenocrysts (up to 15%) are represented by quartz, K-feldspar, albite, locally magnetite. Accessory mineral include zircon, sphene, monazite, anatase, pyrochlore, columbite, tantalite, cassiterite, orthite, garnet, rutile, ilmenite, magnetite, pyrite. Index geochemical elements in the ongonites are P , Li, Rb, Gs, Ta, Be, Sn, Hf, Nb. The Precambrian ongonites are similar to Phanerozoic ones in mode of occurrence, chemical and mineral composition, however, they differ from the latter in a higher degree of crystallization as a result of specific geologic-structural and geodynamic regimes of their formation. The ongonite dykes provide a direct sigh to discovery of Li-P-bearing granites. The findings of the ongonites within the East European craton suggest that they may occur in a wider age range of Precambrian subvolcanic rocks and serve an index factor to identify analogous rocks in the Precambrian terrains elsewhere in the world, firstly, in the structurai-genetical link with rapakivi granites.

MAHC DYKES IN THE VESTFOLD HDLLS - EAST ANTARCTICA SHIELD

M. Seitz Geology Department, University of Tasmania, Hobart, Tasmania, 7001 Australia The predominantly granulitic Archaean terrain of the Vestfold Hills, has been intruded over a period of ca. 1300 Ma at mid- to lower crustal levels by mafic dykes of various compositions. The aim of this study is to use the complex sequences of dyke emplacement, to constrain the pressure, temperature, and time evolution of the Vestfold Hills Area. Previous and on-going work has demonstrated that this is a "classical" area for the events which have affected the crust of a stable continental shield over a period of 2.4 billion years. Dykes were emplaced during three magmatic events in the early to middle Proterozoic:, aproximately at 2400, 1800 and 1300 Ma. Based on field relationships at least 7 major dyke suites can be distinguished, each exhibiting a well defined orientation. Significant geochemical variation also exists within a single dyke of a suite. During the first magmatic event mafic dykes were emplaced at ca. 2400 Ma and can be divided into two major groups, a Fe-rich tholeiitic and a high-Mg tholeiitic suite. The latter exhibits a more complex genetic and emplacement history with dykes of this suite associated with small, 1-5 m wide feeder systems associated with a ring complex. The ring complex of three distinctive lithologies, a homogeneous-, flacky- and rubbly norite. the norites were probably intruded successively. Base metal mineralization is associated with the rubbly norite, together with abubnant enstatite xenocrysts.The homogeneous variety displays small scale layering. The second magmatic event produced doleritic dykes which were emplaced about 1800 Ma ago. Thelast magmatic event produced abundant dykes of mainly Fe-rich tholeiitic character of -- 1300 Ma. Subordinate to these tholeiites, but relatively widespread are alkaline dykes which were emplaced before the tholeiites. Associated with the alkaline dykes are mantle xenolith which occur as ellipsoidal intercallations within the dykes. Emplacement temperatures of 1000-1250°C are calculated from orthopyroxene and clinopyroxene pairs for the various dykes. Calculated emplacement pressures, based on the aluminium exchange between clinopyroxene and plagioclase, for the oldest dykes indicate pressures of 3-4 kb, much lower than estimates from previous studies (7-8 kb). This suggests that the Vestfold Hill block must have undergone a much faster uplift history in the early Proterozoic, being lifted from deeper to mid- to lower crustal levels. This also may imply that the later metamorphic events have not exceeded pressures of 3-4 kb.

PRECAMBRIAN LINEAR DYKE SETS TECTONIC AND GEODYNAMIC ASPECTS

OF

THE

UKRAINIAN

SHIELD:

NN.Shaalov Institute of Geqphysics, the Uknurdm Academy of Sciences, Kiev, USSR

20 major d^ke sets including dykes of different composition (from ultramafic to felsic and alkaline), age (in the 2700-200 Ma range), and orientation are so far knowm in the Ukrainian shield. Associated with the dyke sets are various veins of pegmatites, aplites, quartz and metasomatic bodies bearing diverse mineralization. The dyke and vein bodies are confined to numerous and ratherrcomplicated fracture networks cutting across the gneiss-migmatites, granitoids and other rocks forming the heterogeneous Precambrian basement. The dyke sets are sporadic dykes and veins are controlled by deep faults which are fragments of the global and regional networks of tectonic divisibility* of the lithosphere. The dyke sets localized within the Volyn, Kirovograd, Cis-Dniep' rovian and Cis-Azov geoblocks exhibit predominantly NW-trend, rarely they are sublongitudinal, submeridional or north-easterly trending. The emplacement of individual dykes, veins andd' dyke sets is linked, in terms of geodynamics, with the environments of crushing and dominant lateral extension of a protocontinental crust in the region. Taking into account the erosional level (up to 3-5 km), the dyke sets are likely to be root feeding zones of volcanic fissure eruptions. Detailed study of the internal structure of these dykes enabled us tectonpphysic and geodynamic environments of dyke emplacement to be restored and elso to nplculate a value of extension of individual crustal domains in the shieldin a particular time-span of its development. The emplacement of thick dykes and veins is established to occur under maximal crustal extension and, hence, of a significant opening of the tectonic fraortures. Plowever, the same dyke sets exerted faulting and shearing during other tectonic episodes of dyke emplacement. Different concentration of dykes and dyke sets within individual geoblocks suggests inhomogeneous structure and different endogenic regime in the . evolution of the shield. The oldest mafic dyke sets are confined largely to the blocks of ancient consolidation whereas younger and ranging in composition to regenerated sialic blocks. So, the sets are indirect indicati•jons of lateral and vertical inhomogeneity of the crust, extents of its destrucion and mature.

AGE AND ORIGIN OF A COMPOSITIONAIJLY VARIED MAHC DYKE SWARM IN THE HUNGER HILLS, EAST ANTARCTICA, AND A COMMENT ON REGIONAL RELATIONSHIPS J.W. Sheraton, LP. Black, BMR, Paikes, Canberra ACT 2600 Australia MT. McCuUoch, Research School of Earth Sciences, ANU, Canberra 2600 Australia RJL Oliver Department of Geology and Geophysics, Adelaide University, Adelaide, 5000 South AustraUa

Five compositionally distinct suites of mafic dykes were emplaced in the Bunger Hills at about 1140 Ma ago. shortly after high grade metamorphism of ~1190 Ma and intrusion of mafic to felsic plutonic rocks (1150-1170 Ma). The dykes range from quartz and olivine tholeiites to slightly alkaline dolerites and picrite-ankaramites. Less abundant alkali basalts and trachybasalts were emplaced at 500 Ma. The geochemistry of most of these suites is complex, in spite of their restricted geographic distnbution, and implies mechanisms such as dynamic partial melting, RTF magma chambers, and various depths of segregation. Some degree of crustal contamination (by a siliceous. LILE and 87sr-rich partial melt or fluid) in AFC magma chambers also IS possible, but isotopic and geochemical data are equally consistent with, and indeed require, vanable LILE and LREE metasomatism of the mantle source regions. Apart from major differences in incompatible element (LILE, LREE, HFSE) ratios between suites, which imply derivation from at least six distinct mantle source reservoirs, there are significant intra-suite variations which also imply source heterogeneity (both lateral and vertical). At least three source components were involved in generation of the dykes, viz. depleted (asthenospheric?) mantle, Nb-poor enriched (lithospheric) mantle (probably including subducted sedimentary material), and a Nb-rich OIB-type component. Metasomatism of the source of the alkali basalts (as well as the plutonic rocks) dunng the late Archaean or early Proterozoic may have been contemporaneous with continental crust formation in the area.

The most abundant Bunger Hills dykes (groups 3 and 4, viz. tholeiitic to slightly alkaline dolentes) appear to have been derived from a heterogeneous, variably ennched (Sn 0.7027-0.7053), source similar to that of slightly older (1248 ± 4 Ma: UPb age 2) the Archaean Napier Complex and Vestfold Hills, >1000 kms to the west However, group 1 tholeiites and group 2 high-Mg tholeiites of the Bunger Hills are chemically quite different from any of the dyke suites in the Napier Complex or Vestfold Hills. Looking eastward, there are mafic dykes in the Windmill Islands of possibly much younger age and. at Commonwealth Bay, mafic dykes, now metamorphosed to amphibolites at possibly -1600 Mai are, presumably, much older. The dykes at neither of these two latter localities are compositionally very similar to those of the Bunger Hills. Mafic dykes of broadly similar age to those in the Bunger Hills, which crop out in the Yilgarn Block of Western Australia, are of interest because of the likely Gondwana juxtaposition of these regions, but, at present, there are insufficient published chemical and isotopic data concerning the Western Australian dykes to permit meaningful comparison. 1 Sheraton, J. W., Oliver. R. L. and Stuwe. K., 1989. Geochemistry of Proterozoic amphibohte dykes of Commonwealth Bay, Antarctica, and possible correlations with mafic dyke swarms elsewhere in Gondwanaland. Precamb. Res. 44, 353-361. 2 Black, L. P., Sheraton, J. W., Kinny, P. D. and Maxwell, S, 1990. The difficulties of dating mafic dykes; an Antarctic example.

PALAEOMAGNETIC STUDIES OF PRECAMBRIAN EL-SHADLI GEOSYNCLINAL METAVOLCANICS AND MESOZOIC DYKES OF KAHFA RING COMPLEX, SOUTH EASTERN DESERT, EGYPT R. Shereef

Dq)t of Geobgy, Faculty of Science, Assku University, Aswan, Egypt & EAbd El'AU Geomagnetism Dqxjrtment, Natbnal Institute for stronomical and Geophysical Research, Cairo, Egypt

This work io concerned with the paleomagnetia Btudiea ao well aa the determination of acme pole poaitiona for the precambrian of El^Shadli geoaynolinal metavolcanica and Heaozoio dykea of the kahfa ring complex in addition to the determination of apparent pole wandering parth (APhfP) of Africa. ... From fifteen aitea hloclosd, amaple were collected of El'Shadli geoaynclinal metavolcanica from the central part of South Eaatem Deaert (24.3^ N^ 34.5^ E). After careful magnetic cleaningtwo ultterily groupa were dentified. The mean direction of the firat group ia Ih^SSS^^ and ita paleopole poaition found at 47^1!^ 2S6^B conoiding with pole poaition of Hbozi complex^- Tanzania having nearly the aame age^ The mean direction of magnetization of the aecond group ia 1^24^ and the paleopole poaition found to be at ^S^N^' 324^8 conoiding with that determined for Dokhan voloanica of Egypt (Dainea et al., 1980). The authora attempted to determine the apparent pole wandering path for Africa during late precambrina and Early paleozoic age. The nat~ural remanent magnetisation (NRM) of aome 100 core aamplea repreaenting 10 aitea from El-kahfa ring complex dykeo are meaaured. paleomagnaLie aiudy of theae oampela ahowa the mean direction of CAim ia D ^ '64lo , I ^ G^ withoC^^ 17.1^. thio yielda a paleopole at 2S0^E 13.9^ and K ^ 14.6. Thia palopole poaition agi*eea with other meaozoic pole poaition fx^om Africa.

GEODYNAMIC REGIMES AND COMPOSITIONAL EVOLUTION OF DYKE SWARMS DURING NEOGAEA, SIBERIAN PLATFORM, YAKUTIA BR. Shpom & BV. Oleinikov Yakutsk Institute of Geosciences, Siberian Branch, Academy of Sciences, 39 Lenin pr., Yakutia 67789L USSR Study of the composition, ages and spatial distribution of the dyke swarms in the Siberian platform permitted reconstruction of geodynamic regimes for their formation in late Precambrian and Phanerosoic time» The Riphean, Vendian, middle and late Paleozoic, and early Mesozoic witnessed repeated cycles of rifting accompanied by basic igneous activity. Each cycle lasted 160-180 m.y. There was a gradual decrease in the degree of branching and areal distribution of the paleorifts and associated dyke sv/erms from Riphean to Cenozoic, On the other hand, there v/as a steady increase in the intensity of igneous processes reflected in the amount of igneous rocks erupted per unit geologic time (about 10 m.y.) and per unit paleorift area during the Neogaea, In paleorift structures, the dyk kes form extensive (thousands of km) s^vvarms in the shoulders of the depressions^ Some dykes and small sv/arms are restricted to centroclinal closures of the systems. In aborted paleorifts of middle Paleozoic, late Paleozoic and early Kesozoic age, the swarms can be traced along the axes of tension zones that are often associated with deep-seated faults, Petrochemically, the rocks of the dyke sv/arms are differentiates of alcalic-basic, subalcalic and normal tholeiitic-basalt melts, Alcalic-basic and subalcalic basites typically have a potassic character. The latter resulted from introduction of potassium and associated incoherent elements by mantle fluids into the source areas and intermediate-depth magma chambers. Subsequent differentiation of alcalized basalt melts producad a variety of basic and al-. calic-basic to alcalic (trachytic) and acid (rhjrolitic) rocks,Thus, the alcalinity and silica content variationas of basic complexes in paleorift systems were determined by the rhythm and intensity of the "fluid breath" of tie mantle. The rate of opening of the magmafeeding zones perhaps reflected in titanium variationes. Linear, ramp structural zones, which border on the periphery of riftogenic areas, were dominated by compression with short tension interwals. Due to hampered uprise of basalt, alcalic-ultrabasic (kimberlite), and alcalic (lamproitoid) melts to upper crustal levels, solitary plagiodolerite dykes and small plagiodolerite swarms were formed. They differ from those in the shoulders of the paleorifts by higher alumina and potassium contents and fall into the fields of subalcalic and alcalic basites on a silica-alcalies diagram© Standing apart among basites of tte Siberian platform are magmat ic associations of trap syneclises of late Permian to early Triassic age. This geological age for Siberian platform traps fails to corrBlate with that of trap magmatism on the earth's other ancient platforms, indicating fortuitous injection of voluminous, shallow basalt magmas. Judging by the structure of the deep-seated dyke swarms, the feeding zones of the trap syneclises on the Siberian platform have cellular appearance, each cell being approximately 30-50 km across. Such a peculiar morphostructure evidences that the trap syneclises resulted from diffuse spreading accompanied by high heat flow,Trap magmatites are represented by thleiitebasalt and picrite-basalt series that differ much from the paleorift associations.

SOME ASPECTS OF DYKE EMPLACEMENT AND CHARACTERISTICS IN THE TOWNSVILLE-INGHAM DISTRICT, NORTH QUEENSLAND PJStephemon James Cook University, TownvWe, Queenland 4811, Australia

ABSTRACT: Details of some locally intense dyke swamis in the Townsville-Ingham district are described. Three age determinations indicate that these dykes are Pennian, but some members may be older. All the dykes show varying degrees of low grade metamorphic alteration. The dykes are most commonly steeply-dipping, trending close to NW. Some statistics obtained by measuring every available dyke in particular areas indicate a relatively tight consistency in trends. Similar examination of mafic dyke widths show that the most common are less than 1 metre wide. With increasing widths, the dykes are progressively fewer, so that the widest dykes are least abundant. Outcrops are too incomplete to examine individual dyke persistence. There are local areas with high dyke frequencies, alternating with areas containing few dykes. The local zones with higher frequencies do not appear to form persistent regional features. Some local structural features (eg. faults) could have influenced local frequencies near them, but in general it is difficult to ascribe locally high frequencies to such circumstances. Dykes are generally simple, near-vertical tabular intrusions. Jointing in the country rocks causes, steps, vein off-shoots, and small local direction changes, but overall trends ignore these. Dykes were intruded along, and dilated, fractures and it can be concluded they were emplaced under an extension regime which was relatively uniform in its regional patterns. This simplistic geometry is modified by some details suggesting a sense for an apparent lateral component of movement when some dykes were emplaced. However, more critical indicators such as corresponding vertical markers in the country rock cut across by the dykes (such as aplite veins) accord better with simple dilation. Dyke compositions range from basic to acid. Geochemically, the mafic rocks have major element compositions which are tholeiitic to calc-alkaline. Trace element data suggests they have calc-alkaline to withinplate characteristics. There is a good representation of composite dykes, apparently throughout the region, and these have details consistent with classic examples described elsewhere, involving near-simultaneous magma emplacement. In many localities granitic plutons intrude country rocks which contain members of the NW swarm, and these can be seen to be older, cut off by, and locally metamorphosed by the granites. The granites are in turn intruded by dykes (seemingly similar to the older mafic ones) with similar trends. It would seem the dykes were generated from deeper sources and that their emplacement was interrupted by with granitic intrusion. The pregranitic and post-granitic dykes could have appreciably different ages, but there is no available radiometric information to test intervals. Even so, it is noteworthy that the same trend was maintained. It is surmised the dyke swarms span a period of more than 50 Ma.

PALAEOMAGNETISM AND GEOCHEMISTRY OF DYKES FROM WESTERN INDIA . ITS IMPLICATIONS TO THE EVOLUTION OF DECCAN FLOOD BASALT PROVINCE. KV. Subbarao Department of Earth Sciences, Indian Institute of Technology, Povm, Bombay 400 076. India & PR Hooper Dqxartment of Geology, Washington State University, Pullman, Washington 99164 USA.

Paleomagnetic, rock magnetic and geochemical (major, trace and REE) investigations have been undertaken on more than 100 dykes (about 250 samples) from parts of the northern (Lat. 23-21°N, Long. 76-74°E) and western (Lat. 20-16°N, Long. 75-72°E) Deccan Flood Basalt Province. Dykes in the Deccan are grouped into (a) a NNE-trending set parallel to the Narmada structure in the north; (b) a NNW- to N-trending set parallel to the west coast fault; and (c) a large set with a dominant NNE preferred orientation but represented by dykes of every orientation between Nasik and Mahabaleshwar (western Deccan). Extensive paleomagnetic measurements in different parts of the Deccan suggest that the whole tholeiitic sequence was erupted within three magnetic polarity epochs (N-R-N; D:335, 152, Presence of normal and and reversely + 52; 356, -50) . magnetised dykes strongly suggest the possibility of these being feeders to the flows of the region in which they occur. Further, most dykes are uniquely identifiable chemically and match fairly well with the compositions of the overlying lava flows in terms of the recently established stratigraphy for the western Deccan. Petrogenetically, all the feeder dykes follow similar low-pressure crystal fractionation of olivine gabbro assemblages from the same magma source (s) as the lava sequences. Thus there can be little doubt that these dykes represent at least some of the feeders to the flows. Based on low-field susceptibility and high-field hysteresis investigations on more than 500 specimens, the compositions and domain nature of the magnetic grains of the dykes broadly fall into three types: I. Cation deficient (CD) magnetite; II. Multidomain (MD) magnetite, and III. Members of titanomagnetite (TM) series of grains varying between TM 56 and TM 30 (the number indicating the atomic percentage of Ti in Fe3-x Tix04) . Most of the specimens show more than one Curie Point (450°K and 840°K) due to the mixed compositions of the magnetic grains. Recent stratigraphic mapping in the western Ghats provides a comprehensive view on the development of a large migrating

Deccan shield volcano centred around Nasik and Kalsubai but forming a volcanic ridge running south. This structure is characterised by the presence of randomly-oriented feeder dykes suggesting that the principal horizontal stress axes were equal in all directions. In other words, the crust was not undergoing regional extension during the main eruption. The structural features of the west coast, including the Panvel flexure (flows with steep westerly dips) and N-S trending faults, are accompanied by N-S oriented vertical dykes of lamprophyric, intermediate and silicic compositions similar to the youngest flows in the Bombay area. Some of these lamprophyric dykes aswell as coastal flows show normal polarity. The flexure, normal faults, dykes and flows clearly postdate the main tholeiitic flood basalt eruption of the Deccan. Thus, this represents a change from a uniform horizontal stress field during the main tholeiitic Deccan eruption to a non-uniform stress field which caused E-W crustal extension associated with the separation of the Indian and Seychelles plates. It is unlikely' that the massive Deccan Flood Basalt eruption was due to a plume mechanism advocated by White and MacKenzie (1988) .

NASIK West

(tj CO

DAHANU

?

|^.5 5'N

Jowhor Fm

P-POUOPUR B _ BUSHE or

COMPOSiriON

COMPOSITION

K_KHANOALA

COMPOSITION

Bh .BHIMASHANKAR South ©

of f

Allbag t

IGATPURI

COMPOSITION

J ^ JAWHAR

COMPOSITION

U-

UNCLASSIFIED DYKE COMP. NORMAL

Polo d pur Fm U

U

COMPOSITION

T ^ THAKURVAOl COMPOSITION

j

REVERSED

DYKES

Profiles across the Western Ghats scarp at 19*.55'N and 18*-20'N latitudes

EqsI

^ ^ GEOCHEMISTRY OF DYKE SWARMS WITH XENOUTHS OF FELSIC ROCK IN ZHONGTIAO MOUNTAINS, SHANXI PROVINCE, CHINA Sm Dazhong, Zhao Fengqing Gan Xiaochun Tianjin institute of Geology and Mineral Resources. No4, 8th Road. Dazhigu, Tianjin 300170 China

'' ^ ® exposed in the Precambr.lan metamorphic terrian of Zhong.iao Mountains, m which presents a special type of basic, intermedil^rand intermedxate-acid dykes containing considerable xeJoliths of sic rocks iSe general str e of the dykes is E-W or NE. direction and with dip a n g j f betwle.^ 45) - W . Tne ou.crop pac.erns of the dyke are monodyke, sometimes apophyses. The xenoliths of felsic rocks are mainly coarse-srained granitic porphvre ...... .excu.e even g.-anitic .jeginati'ce. To some extent, the quartz vein also can be seen in the dykes. Most of xenoliths are rectangular and have sharp contacts with tne .lost rock. The xenoliths with incompetent oriental arrangement are mainly distributes in the centre of the dykes, which show same trend -le aykes. The content of the xcnol th changes largely, from Sfi^ in the rim CO 9i% in the centre. The dykes without xenoliths are basic diabase, whereas those convaininxenol.tns are mainly andesite or dacite, which reflect the contamination of the felsic rocks to the dykes. The rock with ophitic texture are composed mainly of clinopyroxene, horblende, plagioclase, quartz, epidote and opaques. The dykes with (Na20+K20)/Al203 O.85 indicated their calcalkaline characteristics. .n the AFM diagram, most plots of the dykes located in calcalkal^ne areas As the content of Si02 increased, the content of the Na20.K20 increased and the content of the CaO, MgO, £ FeO decreased. The dykes with similar REE patterns ffpp^ fractional pattern ((La/Lu)n..4.78-18.8.3) with high 2 REE content l2.RLi:,-101.97-131.20ppm) and negative Eu anomalies (Eu-- =0.59-0.86). As the content of Si02 increased, the negative Eu anomalies increased and £HP.EE decreased, whereas t h e Z L R E E content slighly increased. The former reflected the crystal iraction of the magma, however, the later refered to the different degree of the crustal contamination . In Pearce's NMORB normalized geochem^cal patterns show enrichment of Sr, K, Rb, Da, Th, Ce, Zr, llf, Sm, and depletion of Ti, Y, Yb, Sc, Cr, as well as the negative anomalies of Nb, Ta. The REE and Pearce's geochemical pattern are similar to those of the basic and intermediate volcanics of Xiyan.nie Oroup, which give an evidence for that the dyKes formed in rift setting Within the episoded of the Xiyanghe volcanism {l.84Ca). The xenoliths of granitic porphyre contain high Si02 and Na20 content as well as low CaO, MgO and FeO content. The REE pattern is characterized by highly fi-actional pattern, low REE content and positive Eu anomalies. In the field, the xenoliths of felsic rocks are intruded or wraped up by the dykes, which indicated the xenoliths were formed more early and brought up by dyke from the granitic basement.

SECULAR CHANGES IN THE COMPOSITION OF PROTEROZOIC DYKE SWARMS AND THE GROWTH AND DEVELOPMENT OF THE SUBCONTINENTAL LITHOSPHERE J. Tamey, EP. OMira, T. Ahmad & A. Cadman Department of Geoahgy, wwersuy of Leicester, UK

The compositions of Proterozoic dyke swarms in general are closely similar to those of Phanerozoic continental flood basalts, but this type of magma is rare In oceanic regions. Consequently (although the effects of crustal contamination cannot always be ruled out) it is becoming increasingly

accepted that the source of these magmas lies in the

sub-continental lithosphere. On the one hand it is remarkable how early Proterozoic dykes or mafic rocks from different widely-dispersed cratons have essentially identical trace element compositions. On the other, dykes of different petrological type may have very different trace element compositions, which precludes them being derived from the same source. Further, there appear to be systematic changes in the compositions of dykes with time, in any one region, which are difficult to account for other than as some continual modification of the lithosphere source. We review some of these compositional variations, both -locally, and in space and time, with reference to dyke swarms and contemporaneous mafic rocks from the N. Atlantic craton, the Brazilian craton and N. India, and assess the possible implications for lithosphere evolution. The general picture that emerges is that as growth of the lithosphere goes hand-in-hand with that of associated continental crust, the lithosphere is initially contaminated

by these

crustal

components,

or through sediment

subduction.

The

lithosphere itself comprises refractory harzburgitic material as well as normal asthenospheric material which has frozen onto the base of the developing lithosphere; both become contaminated and thus have a "crustal" trace element signature. Subsequently, small degree

melts from the asthenosphere

continually

migrate

into the

sub-continental

lithosphere, and subtly change its trace element and isotopic composition. More substantial changes take place where mantle plume hot-spots interact with the lithosphere, and cause the more extensive melting which in many cases may be responsible for the dyke swarms themselves. In such cases, the dyke compositions may represent mixtures between plume and lithosphere mantle components. Because dyke swarms may be the only significant events on some cratons, following stabilisation, they may serve to monitor the further development of the lithosphere.

THE GOLD-SHOSHONITIC LAMPROPHYRE ASSOCIATION: NEW GEOCHEMICAL DATA AND THE TECTONIC CONTEXT OF ARCHAEAN LAMPROPHYRES FROM THE YILGARN BLOCK, WESTERN AUSTRALIA WH. Taylor, NMS. Rock DJ. Groves Key Centre for Mineral Deposits, University of Western Australia, Nedkmds, Western Australia 6009 Mafic to felsic dyke swarms, which range in composition from shoshonitic (or caio-aikaline) lamprophyres to quartz-feldspar porphyries, are spatially and temporally associated with Archaean mesothermal gold deposits. World-wide probably the most extensive emplacement of lamprophyres in the Precambrian occurred during the lata Archaean at 2.7-2.6 Qa e.g. in the Superior Province of Canada, the Yilgarn block of Western Australia and the Limpopo Belt of southern Africa. In both the Canadian and Australian provinces this age-range brackets the age of mesothermal gold deposition. fwlany shoshonitic lamprophyres have primitive characteristics (high Ni. Or and mg numbers) combined with unusually high abundances of incompatible elements (Ba, Sr. Rb, LREE) suggesting that they are near primary partial melts of metasomatized upper mantle perkJotlte. In view of their spatial and temporal links with Au deposits it has been suggested (Rock and Groves, 1988a, 1988b) that shoshonitic a^rophyres may also be enriched in Au and platinum group elements (PQEs) relative to other rock types, it has been funher suggested that lamprophyres could be one source for Au and/or mantle-derived 002 in mesothermal gold deposits. A compilation of literature data (Rock and Groves, l9S8a) has provided support tor the view that lamprophyres are Intrinsically Au-enriched (the median Au content of 88 samples IS 10 ppb and the mean is 87 ppb compared with average igneous rock abundances of ppb). This is also consistent with some Palaeozoic to Recent shoshonitic suites which have high Au + PGE abundances and are closely associated with gold and PGE mineralization (e.g. central Lachlan Fold Belt N.S.W., Wybom and Cameron, 1990; and the Uhir gold deposit, Papua New Guinea. Plimer et al., 1988).' The above results are. however, in contrast to recent studies of shoshonitte lamprophyres from the Superior Province of Canada (Wyman and Kerrich, 1988) and preliminary data from the Leonora area of the Yilgarn block (Jaques et al.. 1990) which show that lamprophyre Au contents are tow except in mineralized samples. In order to better understand variations in background abundances of Au and PGE in Archaean and Palaeozoic lamprophyres we have determined Au and PQE to ppb levels, together with a range of other trace elements, from suites of lamprophyres from the Norseman-Wiluna belt of the Yilgarn block and the Southern Uplands of Scotland. We selected more than 60 samples to span a range of alteration states (petrographically pristine to strongly altered) and proximities to known Au mineralization (i.e. remote from Au mineralization, associated with sporadic Au mineralization, associated with small, medium or large Au deposits l-IOt, I0-50t and >60t, respectively). Some samples had previously been analysed for Au. Although analytical accuracy and reproducibility are problematical at ppb levels due to grain-size and sample Inhomogenelty effects, our results, obtained by two different techniques, show that fresh to weakly altered lamprophyres have Au and PGE levels not significantly above normal background values for igneous rocks. For lamprophyres with Au above 8 ppb, the most significant factor that correlates with Au abundance is the degree of alteration suggesting that high-Au lamprophyres were mineralized by gold-bearing solutions. Au-enrichments are associated with an increase in Na20, sulphur and chlorine and depletion of the rock In Ca, Sr. and Ba reflecting albitization of feWspar and pyritization accompanying Au introduction. An extnnsic origin for Au is also consistent with the poor reproducibility of samples previously retorted to have high Au contents (>100 ppb). since grain-size ("nugget") effects will predominate in mineralized samples compared with intrinsically enriched Au, No correlation was found between lamprophyre Au content and proximity to known Au deposits. Au enrichment of lamprophyres and hence a probably unimportant role for lamprophyres as an Au source, the close association between lamprophyre dykes swarms and Au iT^'"® ® feature of many deposits (Rock and Groves. 1988a). The amprophyres and Au-mineralization may therefore reflect utilization of the same S A ^ K'® mantle-derived magmas and Au-mlneralizing fluids. Geochemically. fresh Archaean shoshonitic lamprophyres have closely similiar major and trace element abundances to Phanerozoic lamprophyres and shoshonitic volcanics. They are characterized by enrichments in Rb. K. ^rp abundances of Ta, Nb. and Ti. in modern tectonic envlnjnments these features ^o o S S i f r P'a«e-margin magmatism. Phanerozoic shoshonltte magmatism occurs in SanoSrtart^ <ate-cuMu«ion oottinga in mature continental and island arcs in response to cnanges m tectonic regime involving back-arc thrusting, rifting and/or transcurrent faulting (e.g. Indonesia.

Japan. Mexico, Fiji); and (2) in post-subductlon settings within colllsional twits wiiere transcurrem tectonics dominate and mantle sources have been previously suWuction-modifled (e.g. Papua New Guinea, northern Italy. Scotland). It seems clear that analogous subduction-related tectonic environments were present during the late Archaean (Barley et al.. 1989; Wyman and Kerrich, 1989). In the Norseman-Wlluna belt the timing of deformation, metamorphism and synkinematic magmatism (Barley et al.,1989) is such that setting (2) above Is considered more likely. Rfiterenpgi? Barley, M. et al. (1989) Geology. 17, 826-829. Jaques. A.L. (1990) BMP Research Newsletter, (In press). Perring. C.S. et al. (1989) Precambrlan Heaearch. 43.215-237. Plimer, I, et. ai. (1988) Bicentennial Gold 88, Extended Abstracts, 22,139-143. i^ock, N.M.S. & Groves, D.I. {1988a) Nature, 332,253-255. Rock, N.M.S. & Groves. D.I. (1988b) Geology, 16.538-541. Wyborn, D. & Cameron, W. (1990) 10th AGC Abstracts, 25,12e-127. Wyman D .A. & Kerrich, R. (1988) Econ. Geo!.. 83,454-461. Wyman D.A. & Kerrich. R. (1989) Jour. Geophys. Res.. 94,4687-4696,

™

PROTEROZOIC MAHC DYKE SWARMS AND ALKALINE INTRUSIONS IN ^ ^ T O N . SOUTH AMERICA. AND TO^tcSIC EVOLUTION BASED ON RB-SR. K-AR AND -AR--AR GEOOmONOL^Y WTeixeim Geosciences Institute, Umersity cfSao Paulo, P.O. Box 20B99

U n m e l a m o r p h o s e d b.sic dikeswcTMs i.nd 1 k <. 1 i ne r o c k s are f o u n d culling the prec^rtbrian basewent complexes and sediwentary covers, w.th.n the A . a . o n i . n Craton (AC). The basic HJeJCs f^n^r t'y clikes (niQsily), silU, stocks and f ows while the i.lki,line one constitutes ring, oval and el I I P 6 0 I dii 1 b o d i e s . More than 100 radiometric K-Ar, Rb-Sr and ''OAr-'^Ar determ.nations .,re available for the basic and alkaline y i t r u s i o n s in the i n v e s t i g a t e d a r e a . The age p a t t e r n s for the basic rocks d e f i n e f o u r p e a k s of a c t i v i t y , d u r i n g the g e o l o g i c a l ha; 1 ,500-1 ,300 H A ; 1 ,250-1 ,050 MA a n d 1 , 0 5 0 u o O H A . The a l k a l i n e magwatifcrt in t u r n was s y n c h r o n o u s with the b a s i c o n e , s p e c i a l l y for the 1 , 5 0 0 - 1 , 1 5 0 MA p e r i o d . The I d e n t i f i e d age g r o u p i n g are i n t e r p r e t e d as r e c o r d i n g the « a i n t i « e for e m p l a c e m e n t of d i k e s w a r m s a n d a l k a l i n e r o c k s . The a g e s ^re tectonically a s s o c i a t e d with the r e c o g n i s e d E a r l y a n d d i d d l e P r o t e r o s o i c s e o c h r o n o 1 o g i ca 1 p r o v i n c e s of the AC which are a result of the e v o l u t i o n of m o b i l e b e l t s , m a r g i n a l l y to a s t a b l e , older central do.nain. This model for such m a g m a t i c r o c k s is a l s o s u p p o r t e d by their c o h e r e n t age g e o g r a p h i c distribution within e a c h one y o u n g e r prov.n.:e, t o w a r d the s o u t h w e s t of the AC. The studied mafic d i k e s a n d a l k a l i n e r o c k s t h e r e f o r e may be r e g a r d e d AS i n t r a p l a t e igneous activity associated with the tectonic e v o l u t i o n of tlie p r o v i n c e s , and w h i c h e m p l a c e m e n t w a s c o n t r o l l e d by a c t i v e and r e a c l i v e d s t r u c t u r a l s y s t e m s ( N E - S U , N N E - S S W , N U - S E trends), within progressively stable geographic domains of the c r a 10 n .

DYKE SWARMS AS STRESS INDICATORS: TWO CONSTRAINTS AKTokarski Polish Academy (f Sciences, Krakow, Poland

Dyke swarms are commonly used as stress indicators. They are thought to Intrude perpendicularly to a j direction. However, this criterion can not Qlways be employed if dykes fill pre-existing fractures. For example, in numerous localities at Antarctic Peninsula magmatic dykelets form two complementary sets filling conjugate joint systems. Map-scale equivalents of these structures occur in Eglab massif (West African craton). There, magmatic dykes form complementary systems. The dykes fill pre-existing complementary fault systems.

Another constraint for use of a single set of dykes as stress indicator stems from geological relations observed at Cape Legoupil (Antarctic Peninsula). There, folded flysch rocks were intruded by two sets of magmatic sheet intrusions which are perpendicular one to another. The intrusions of the first set are parallel to bedding and cleavage surfaces. The intrusions of the second set are parallel to regional set of joints. Sheet intrusions of both sets pass laterally one into another which shows their contemporaneity. In the present paper this system of sheet intrusions is interpreted as emplaced during o period of multidirectional extension.

COMPOSITE DYKES OF THE VILYUISK PALAEORIFT SYSTEM, SIBERIAN PLATFORM, YAKUTIA MDTomlm & OVKoroleva Yakutsk Institute of Geosciences, Siberian Branch, Academy of Sciences, 39 Lenin Avenue, Yakutsk 677891, USSR The formation of the Vllyuisk paleorift system was accompanied by basic mugmatism. The Chara-Siiisk swarm of composite dykes was emplaced near its eoutheastem margin in the Middle Paleozoic (D^- C j ) . The swarm extends 250 km from north to east. It consists of numerous fissure bodies, both composite, with the following rock sequencej gabbro-dolei^ites — s u b a l c a l i c gabbrodolerites —t- quartz subalcalic gabbro-dolerites zonite porphyries

quartz mon-

quartz syenite-porphyries, and simple,

i.e.

filled with only one of the above differentiates. Composite dykes were formed in covoral stages. The ed urce basic magma had subalcalic, tholeiitic-basalt character^ Its deep-seated evolution, which started with crystallization differentiation at T=I250-I350°C and p v i o kbar, produced the following rock series: gabbro-dolerites

quartz ga.bbro-dolerites

-•^subalcalic quartz gabbro-dolerite.Each of the differentiates formed separate fissure intrusives. The next evolutionary stage of the pre-transformed, subalcal i c , tholeiitic-basalt melt proceeded at moderate pressures (P-^ 5-8 kbar) in a deep-seated, intermediate-level reservoir. A combination of crystallization-gravitational

differentiation

of fi^agrca and its mass exchange with the components of an inflowing, potassium-rich, transmagmatic fluid ( with prsdominantly fluid-magma interaction) provided a stepwise increase in the potassium content and an increase in the silica content of the melt. Pulo.-tory opsning of the intermediate-level reservoir led to successive separation of portions of still evolving magma. Its discrete arrivals into a hypobyssal reservoir produced first monzonite-porphyries and then syenite-porphyi-ies which formed both composite and simple dykes, A genetic relation of basic and silicic rocks is supported by Sr isotopic composition. Crustal contamination had no effect on the formation of the monzonitoides. Such an evolution of thleiitic-basalt melt is refferred to as a monzonitoid trend of differentiation.

NEOHELIKIAN GIANT DYKES, SOUTH GREENLAND B.GJ. Upton, SM. Becker, S.C. Mingard iSc AN. HalUday

Conspicuously

large Neohelikian dykes (ca. 1150 Ma) transect older

Proterozoic terrains in S. Greenland. Trending WSW, they lie to the S. of, and sub-parallel to, the Archaean craton margin. Occupying a zone ca. 70km broad, these late Gardar giant dykes are divisible into a western (Nunarssuit-lsortoq) group and an eastern

(Tugtutoq-nunataq)

group. The dykes, typically 200-500m wide but ranging up to 800m, are mainly

of

troctolitic

gabbros. More

differentiated

facies

(syeno-

gabbros and syenites) occur as continuous, and discontinuous, axial portions. gabbroic

Synformally rocks

and

layered

in

the

cumulates

developed, both

differentiates.

in

the

Generally • well-chilled

margins indicate that the dykes were emplaced in cool, brittle crust, as transitional alkali basalt/hawaiite magmas with notably high Al/Ca ratios. Low Mg numbers, Ni and Cr contents indicate that the initial magmas were differentiated: have

involved

extensive

pre-emplacement

sub-crustal

stages are inferred

and/or

deep

crustal

to

crystal

fractionation and concomitant underplating. Trace element, Sr and Nd isotopic data suggest that the magmas originated within, or interacted with the lithospheric mantle. Magmas of the northern giant dykes were somewhat more primitive than those

of

the

concentrations inferred

to

southern and La/Yb be

derived

group

and

had

ratios. Those from

lower of

lithospheric

the

incompatible

element

southern group

mantle

are

metasomatically

enriched in LREE, K, Rb, Ba and P. Both zones of giant dykes appear to be related to the widespread intra-plate rifitng events involving (for example) the N. An^ican Mid-Continent Rift. Evolution of the southern giant dyke system is analogous to that of modern volcanic lineaments in Ethiopia and Arabia, allied to Red Sea opening. It is suggested that the Gardar giant dyke groups represent a similar unsuccessful rift system.

ON THE MAGNETIC DETECTABILITY OF MESOZOIC PONTA GROSSA ARCH DYKE SWARM: AN INTEGRATED GROUND/AIRBORNE MAGNETOMETERY AND ROCK MAGNETISM STUDY N. Ussarm (1), FJF. Ferreira (2), A. Kofysnik (1), MJB. Raposo (1). EC. MoUm (1)& M. Ernesto (1)

Depto. Geofisica, InsCiCuto Astronomico e Geofisico, U n i v e r s i d a d e de S a o P a u l o , S a o P a u l o , B r a z i l . *Instituto Paulo,

de

Pesquisas

Tecnologicas

de

Sao

Paulo,

Sao

Brazil.

Mesozoic Ponta Grossa Arch Dyke Swarms occur between the e a s t e r n b o r d e r o f the P a r a n a b a s i n a n d the s o u t h e a s t e r n Brazilian continental margin. It c o m p r i s e s o f h u n d r e d s of 2 0 to 50 m e t e r s thick tholeiitic basalt dykes which intrude the Precambrian b a s e m e n t and Paleozoic sediments along the NW-SE direction. Preliminary analysis of a e r o m a g n e t i c data suggested that these dykes may extend northwesterly t o w a r d s the c e n t e r o f t h e P a r a n a b a s i n , under b a s a l t i c flows and p o s t - v o 1 canic sediments. In o r d e r to i n v e s t i g a t e t h e d e t e c t a b i 1 ity of these nonexposed dykes by a i r b o r n e m a g n e t o m e try, a combined study of ground magnetic s u r v e y a n d d y k e m a g n e t i s m w a s C a r r i e d o u t in o n e region (Guapiara Lineament) where d y k e s are exposed. A total component magnetic p r o f i l e of 12 k m w a s set u p w i t h an a v e r a g e s p a c e of 12.5 to 25 m e t e r s between readings. T h e r e is a c l e a r c o rre s pondence between exposed dykes and short-wave1ength magnetic anomalies. Many unexposed dykes were also detected from observed magnetic a n o m a l i e s and an a v e r a g e f r e q u e n c y of 2 d y k e s per k i l o m e t e r has been d e t e r m i n e d for this r e g i o n . The ground magnetic data w a s u p w a r d - c o n t i n u e d to c o i n c i d e w i t h the a l t i t u d e of one l i n e o f the a e r o m a g n e t i c s u r v e y f l o w n at 4 5 0 m e t e r s a b o v e ground level (a.g.l.). This process revealed t h a t m o s t of d y k e magnetic anomalies are h i g h l y a t t e n u a t e d at a l t i t u d e as l o w as 100 m e t e r s a . g . l . . At 4 5 0 m e t e r s , o n l y a n o m a l i e s a s s o c i a t e d w i t h a group of c l o s e d s p a c e d d y k e s or h i g h a m p l i t u d e ( a b o v e 4 0 0 0 n T on t h e ground) magnetic a n o m a l y of a single dyke remain. These results were i n t e g r a t e d w i t h d a t a of m a g n e t i c s u s c e p t i b i l i t y and natural remanent magnetization of dyke samples in order to establish a quantitative criteria of dykes d e t e c t a b 1111y by conventional airborne m a g n e t o m e t r y . For this p a r t i c u l a r r e g i o n it is f o u n d t h a t m o s t of the d y k e s c a n o n l y be d e t e c t e d by g r o u n d m a g n e t i c s u r v e y d u e to low m a g n e t i z a t i o n and s m a l l t h i c k n e s s of the d y k e s .

PETROLOGY OF THE EARLY PROIEROZDIC MAFIC DYKES IN NORTH KAREUA, EASTERN FINLAND XL VuoUo, T A Piirainen and P A Tuukki Departmmt of Geology, Umva:sity of Oulu, linnanmaa, SF-90570 Oulu, Fiidand

Mafic dykes, domfnantly in NW directions, are <x)mmon in tiie Archaean greenstone-granitoid terrain and Proterozoic metasediment cover in North Karelia, Eastern Finland. They have been divided into three groups, which are karjaiitic, Fe-tholeiitic and tholeiitic d ^ e s . The tentative ages of these groups are 2.2 Ga, 2 J Ga and L97 Ga, respectively. The karjalitic dykes are laid parallel with Proterozoic formations in the Archaean granitoids near

the unconformity and cut

the

lower

qiiartzites.. They have all the features of a layered intrusion containing a

succession

of

magnetite

uitramafic sequence

mafic

of

wehrlite and

gabbro

and

granophyre

cUnopyrozenite.

above

The

parent

magma determined from the chilled mRrgin was rich in Fe and USEE and poor in A1 as compared with a tholeiitic magma. The

Fe-tholeiitic

dykes

are

most

common

and

occur

as

swarms

throughout the whole area, whereas the tholeiitic ones have been found only in

connection

with

the

Proterozoic

quartzites.

The

primary

mineralogy of the Fe-tholeiitic dykes is, in general^ destroyed^ while the

tholeiitic

ones

are

only

feintly

altered

and

contain

pyroxenes and plagioclase. Chemically Fe-tholeiitic dykes

olivine^

are enriched

in Fe, Ti, Zr and Y and depleted in Mg, Cr and Ni as compared with the tholeiitic ones. The REE level of

Fe-tholeiites is higher

tholeiites, and (La/Sm)ir is 2.5 and 1,6, respectively.

than

The average

geochemical composition of the tholeiitic group is intermediate between those of modem oceanic basalts and island a r c tholeiites.

EVIDENCE FROM THE MATACHEWAN - HEARST TECTOMC EVOLUTION OF THE KAPUSKASING ONTARIO, CANADA

G^pSs

S V ^ RE ^ STRUCTURAL 2»NE,

of Physics, University of Toronto. Canada. M5S - 1A7 Canada

& RE. Ernst _ . OttamCarbon Geoscience Centre, Ottawa. KIN - 6N5 Canada

The Kapuskasina Structural Zone (KSZ) is a 500 km long NW trending band of complexly deformed high grade metamorphic rocks apparently emplaced by an intracratonic upthrusting. from NW to SE of one part of the Archean Superior Province crust upon another which took place sometime in the interval 1800-2r,50ria, A SE traverse across it is interpreted as a cross-section through Arrhean crust from current average exposure levels to about 20 km greater depth. Rocks of the KS? are cut by many N to NW trending diabase dykes of the well known 2 4 6 0 Ma MatachewanHearst swarm which has an unusual and complex form; it can be divided into three subswarms of indistinguishable age and. although the dykes are not straight, they appear to radiate from a centre which lies beyond the swarm to the south By digital image processing of Federal-Provincial aeromagnetic survey data, we have obtained a clearer picture of the dyke swarm's present extent and structure. The two easterly subswarms clearly are truncaled by the KSZ's eastern boundary faults The western subswarm does not reveal any major fault offset but does show a marked reverse S bend as it crosses the KSZ. On the assumption that the swarm was originally intruded radially, the horizontal strain suffered by the KSZ since 2 4 6 0 M a appears to be a dextral transcurrent deformation wherein the strain is continuous in the SW but narrows through a series of horsetail faults to a sharp offset of about 80 km in the NE Thus if the current view of the KSZ's evolution is correct, this deformation is a secondary reactivation and the primary period of thrust uplift must havepredated dyke injection

SHEETED DYKE-SILL SWARM IN OPfflOLITE OF THE TEKTURMAS ZONE CENTRAL KAZAKHSTAN AS. Yakubchuk Geological Department, University of Moscow, Moscow, USSR Sheeted dyke complexes were identified in ophlolites of the Central Kazakhstan recently. These are the sheeted sill complejcas in many cases• In the Tekturmae zone the dyke-sill swarm of the middle ordovician Bazarbaian ophiolite was studied. The dykesill swarm occurs between volcanic and layered complexes. The sills were Intruded after the "dyke In dyke" complex has f o r m e d . The connections between these two types of diabase bodies m a y be observed very rarely because of unsaLlsfied is well known that dyke swaniis

outcrop style. It

proves ^ the sea-spreading mo-

del of ophiolite's origin. It seems that ••sill in sill*? swarm may be interpreted by the same model, but under condition of synchronical tectonic layering of the oceanic-type crust. The sills have Intruded in the zones of the subhorisontal derangements on the boundaries between the complexes with different phisico-mechanical properties: into the dyke complex as maximally inhomogenous medium in comparison with other eubhorizontally stratified complexes of ophiolite; on the layei^ed complex-metaperidotlte boundary. The detacliinents might formed because of movement of the plates formed In ophiollte with different r a t e s . Such mechanism might realised on the periphery of the apreading centre and on axis traditional dyke in dyke swarm h a d to f o r m . In literature sill swarms are described also in the CanyonMountain (Oregon) and Betts-Cow ophlolites, in the ophlolites of the Altay-Sayan region (Siberia) and in other ophiolltic zones of the Central Kazakhstan.

VARIATIONS IN THE PETROCHEMICAL COMPOSITION OF BASIC IGNEOUS DYKES IN THE BALTIC SHIELD AS. Vein Imitute of Geology, 185610 USSR

USSR

Academy

of

Sciences,

Karelian

Branch,

Petrozavodsk

The petrochemical composition of basic igneous dykes from the Baltic Shield is discussed by means of a generalized

basaltic

tetrahedron on the basis of numerous data available in the literature• Its rock composition points form variation lines which reflect changes in the s i l i c a t e portion of the rocks• The position of the lines relative to standard mineral composition points indicates a variety of basic igneous dykes differing in both composition and the degree of differentiation. The greatest similarity is observed in the drected pattern i f the variation lines because the differentiation of this type of basic magma is largely due to the crystallization of pyroxene and plagioclase. Many variation lines run from the tetrahedron Ol-Opx-CpxPI to the adjacent tetrahedron Opx-Cpx-Pl-Q, thereby crossing the orthopyroxene plane. The composition of each dyke swarm is characterized by a definite point at which the Opx-Cpx-Pl plane is intersected by a variation line ( P i g . I ) .

One of the

ways of locating the point is to construct variation lines on the projection diagrams resulting from the parallel projeting of 01-(Cpx+Pl)-0px and 01-(0px-fCpx)-PI and to determine orthopyroxene and plagioclase content when crossing the lines (Cpx+Pl) - Opx and (Cpx-t-Opx) - P I ,

respectively•

The majority of the diabase dykes studied in Karelia,

USSR,

(Nos. 1-8, 10-14) strike NW and constitute an Early Proterozoic (2,2-1.9 Ga) NW dyke belt 300 km in width and 600 km in length. The variation lines have close points of intersection with the orthopyroxene plane. In this case the dyke swarms, which occur close to each other ( P i g . I , map), also are similar in composition on the diagram

(Pig.la).

A certain correlation in the composition and age of Karelian (I-I8) and Finnish (19-31) dyke swarms is observed on the diagram. Petrochemical data on Swedish and Norwegian dyke swarms (Pig# Ic) confirm a relationship between the age of a dyke swarm and the position of the variation line within the basaltic tetrahedron characterized by the point at which the

v a r i a t i o n l i n e and the orthopyroxene plane are

intersected.

The r e l a t i o n s h i p found between the p e t r o c h e m i c a l

composi-

t i o n and age and p o s i t i o n of a dyke swarm on the Baltic S h i e l d i s p o s s i b l y a s s o c i a t e d with changes i n the depth o f magma formation because a r i s e i n pressure s h i f t s

clinopyroxene-

p l a g i o c l a s e c o t e c t i c s towards p l a g i o c l a s e . P i g . I . P o i n t s at which the v a r i a t i o n l i n e s o f the c o m p o s i t i o n of the b a s i c igneous dykes of the B a l t i c S h i e l d s e c t the orthopyroxene plane and the schematic

inter-

distribution

o f the dyke swarms analysed. The numbers on the diagrams and the map a g r e e . Dyke swarms: I - Kundozero; 2 - P i a o z e r o - T i k s h e o z e r o ;

3 -

White Sea m e t a p o r p h y r i t e s ; 4 - P i a o z e r o ; 5 - V o i n i t s a ; 6 P i s t a j a r v i ; 7 - verkhneye K u i t o ; 8 - Lake Kamemoye; 9 - L o bash; 10 - P a i o z e r o ; I I - T u l o s ; 12 - Sukkozero; 13 z e r o ; 14 - S e g o z e r o ; 15 - Vetreny Poyas; l 6 -

Yango-

Petrozavodsk;

17 - S o r t a v a l a ; 18 - Valaam; 19 - L a a n i l a ; 20 - Hyrynsalmi; 21 - Pohjamaa; 22 - N i l s i a ; 23 - P e t a j a r v i ; 24 -

Petolahty;

25 - P o r i ; 26 - Satakunta; 27 - O r i v e s i ; 28 - Hame-Tampere; 29 - Hame; 30 - L o v a s j ^ v i ; 31 - Aland; 32 - B l e k i n g e ; 33

-

Kongsberg; 34 - Egersund; 35 - S u n n f j o r d ; 36 - R e s f j e l l ; 37 - O t t f j a l l e t ; 38 - Varanger.

Age

(Ga)

O 0.276-0.270 D 0.395-03^5 O 1.25-0.5 •1.65 A 1.90-lM 0 2.2-2.1

MULTIPLE DYKE EMPLACEMENT IN THE WONOMINTA BLOCK, WESTERN NEW SOUTH WALES, AND ITS TECTONIC SIGNIFICANCE IN RELATION TO THE TASMAN LINE

BZhou & KJMiUs Department ofGeobgy and Geq)hysics, The University of Sydney, NSW 2006, Australia Wonominta Block is located in far western New South Wales, Australia, about 200 km north-east of Broken Hill. The foundation of this block, known as Wonominta Beds, is composed of composite sequences of low to medium grade metamorphic rocks and has been recently correlated to Willyama Complex, Adelaidean and Kanmantoo sequences respectively (cf. Zhou & Mills, 1990). Detailed field mapping work has revealed multiple emplacements of mafic dykes in all these sequences. Early emplacements (both Willyama and Adelaidean equivalent) occur in the central part of metamorphosed volcanic sequences as feeding channels; while those in Kanmantoo equivalent sequences are observed crossing sedimentary sequences, implying post depositional emplacement. Petrographically, these dyke rocks are of two kinds, dioritic(amphibolite) and doleritic (metadolerite), and in the latter case, clinopyroxene appears as the only residue of magmatic phases. There are also some small lamprophyre and nundorite (aegerine-bearing rock) dykes occurring cross the major sequences, these are the products of metasomatism relating to post-orogenic magmatism. Electron probe study on pyroxene and geochemical work have revealed distinct chemical features of these dykes in different sequences. In summary, those in the old sequence are theoleiitic, while those in Adelaidean are alkaline; and dyke rocks in Kanmantoo groups are of typical intraplate magmatism. On the whole, the geochemistry of these dyke rocks support the geological correlation, which provide crucial evidence to tectonic modelling of Wonominta Block. Tasman Line was originally proposed as a stratigraphic boundary between Cambrian and Precambrian sequences in eastern Australia, but in recent literature, it is often used as a major tectonic boundary between Australian craton and eastern Phanerozoic fold belts. While in some cases, this may be correct, in others it has caused remarkable controversy. Wonominta Block is a typical example. Recently, it has been assigned alternately to part of the craton (i.e., on the western side of the Tasman Line, Powell et al. 1990), or part of the so-called Kanmantoo Fold Belt (on the eastern side; Scheibner, 1987). The present study has suggested, if there is a general stratigraphic boundary between Cambrian and Precambrian, this boundary can not be used as a tectonic boundary without modifications, thus, a concept of tasman transitional zone has been proposed to explain the discrepancy. The concept emphasizes transitional feature of tectonics between craton and Phanerozoic fold belts, and the study of dyke rocks in Wonominta Block has helped to characterize magmatic features in this transitional zone at different stages of evolution.