Issuu

Geological Society of Australia

ABSTRACTS Number 7

Symposium on

Volcanism in Eastern Australia with case histories

from

New South Wales The Australian Museum 15 November 1982

SYMPOSIUM ON VOLCANISM

IN EASTERN

WITH CASE HISTORIES FROM

AUSTRALIA

N.S.W.

The Australian Museum, Sydney, November

ABSTRACTS

1982

VOLUME

Edited by F.L. Sutherland and J.R. Hardie

Geological Society of Australia Abstracts No. 7

THE GEOLOGICAL RESOURCES OF THE A U S T R A L I A N MUSEUM

MjU-jjbidl^

i telitVc ac'ti've^-'V (rv. Volcflir>.tc

SYMPOSIUM ON VOLCANISM IN EASTERN AUSTRALIA WITH CASE H I S T O R I E S FROM N.S.W. Convener. Dr F.L. Sutherland, N.S.W. Representative Specialist Group in Geochemistry, Mineralogy and Petrology, Geological Society of Australia. Held on Monday 15 November 1982 in the Hallstrom Theatre, The Australian Museum, 6-8 College St. Sydney. Sponsored jointly by the Australian Museum and the Geological Society of Australia. PROGRAMME 8.30 a.m.

Registration.

9.00 a.m.

Introduction. Dr H.C. Cogger, Deputy Director, The Australian Museum, and Dr F.L. Sutherland, Convenor,

F i r s t Session.

P a l a e o z o i c Examples. Chairman P r o f e s s o r Alan Voisey

9.10 a.m.

R. H. Rood ^ S.E. Shaw & G.H. Riley The velationshi-p between the Bindook Volcanic Complex and the Mavulan Batholith: Sr isotope evidence.

9.40 a.m.

E. Scheibner^, L.M. Barron & D.W. Suppel. The Mount Hope volcanic complex and comagmatic intrusions.

10.10 a.m.

S.E. Shaw \ R.H. Flood & R.H. Vernon, Permian Volcanism associated with the New England Batholith.

10.40 a.m.

P.W. Carr ^ Late Permian Shoshonitic Sydney Basin*

11.10 a.m.

Volcanism

Emeritus

in the Southern

Morning Tea

Second S e s s i o n .

Mesozoic - T e r t i a r y Dr R . A . B i n n s

Examples.

Chairman

11.30 a.m.

B. E m b l e t o n P . W . Schmidt and D.A. Clark. Geochronology and Palaeomagnetism of some Igneous Rock Deposits of the Sydney Basin.

12.00 noon

D.J. Martin ^ A layered intrusion from the Scone area. Hunter

12.30 p.m.

1.00 p.m.

D.H. French ^ Major Element Chemistry Complex.

of the Alkaline

Lunch, Rooftop Cafeteria.

Gunnedah

Valley.

Third Session.

Tertiary

Examples:

Chairman - Dr S . Y . Wass

2.00 p.m.

D.R. Mason^ Pyvoxenes of Ankavamitia and Related Alkali Basaltio Lavas of the Bawingtcn Tops Volaania Field, N.S.W.

2.30 p.m.

R.W. Schon® Petrology of Volaania Roaks from the Livevvool Rxnge, N.S.W.

3.00 p.m.

E. Middlemost^ Mioaene Shield Volaanoes of N.S.W.

3.30

Afternoon Tea

Last Session. 4.00 p.m. 4.30 p.m.

5.00 p.m.

Regional Carter

Examples:

Chairman - Dr A.N.

F.L. Sutherland^ Some regional controls on N.S.W.

volaanism.

J. D. Hollis^ Volcanism and the Mantle - Lower Oust Australia.

Jigsaw, S.E.

C.D. Oilier® Geomorphology and Volcanism.

5.30 p.m.

General Discussion on Overall Programme. Summing Up - Professor J.F.G. Wilkinson'^.

Speakers' Institutions. 1. 2. 3. 4.

School of Earth Sciences, Macquarie University Geological Survey of N.S.W., Sydney Dept. of Geology, University of Wollongong Division of Mineral Physics, C.S.I.R.O., North Ryde.

5. 6. 7. 8. 9.

Dept. of Geology, University of Newcastle Dept. of Geology & Geophysics, University of Sydney Dept. of Mineralogy & Petrology The Australian Museum Dept. of Geography, University of New England. Dept. of Geology, University of New England.

- 3 -

FOREWORD TO THE OPENING OP THE SYMPOSIUM BY DR H. J. COGGER, DEPUTY DIRECTOR OP THE AUSTRALIAN MUSEUM

The Symposium is a response by the Australian Museum to an approach by the NSW Division of the Geological Society for joint activities with Sydney geological institutions. It aims to blend fresh work of students with the wider outlook of experienced geologists on a topic of general interest to the NSW geological commtinity.

The theme on

Volcanism in Eastern Australia reflects current research at the Australian Museum and also provides a suitable focus for the Specialist Group in

Geochemistry, Mineralogy and Petrology.

No volcanoes were recently active in New South Wales, but there are many active researchers here on volcanic rocks. Most of the geological institutions in NSW are represented amongst the speakers and chairpeople.

They come from

university Geology and Geography Departments, the State Geological Survey and Department of Mineral Resources, CSIRO Mineralogical Divisions and not least,Museums in Sydney. The talk by Dr Brian Embleton of the CSIRO in the morning sessions on ages and magnetism of volcanic rocks from the Sydney Basin is interesting to note as some of the samples for the study came from the coffers of the Museum. The Museum collections have in fact become a resource for a number of recent volcanic studies by outside v/orkers from Australian and overseas institutions. The Museum a few years ago presented a Symposium dealing

- if . with Lord Howe Island, the eroded oceanic volcano of 7 million years age possessed by NSW.

However, this

Symposium incorporated biological and other aspects besides geology.

The present Symposium is the first purely geological

Symposium held at the Museum. Geology has become a very sophisticated, quickly developing and specialised science and this Symposium should enable attending researchers to keep up with recent local field, laboratory and Museum work.

When the Museum was

founded in 1827, the study of volcanic rocks in NSW was in its infancy.

Now they are mapped and studied in detail, but the

causes of volcanism remain a matter of hot debate. This One Day Symposium will not solve the volcanic problems of the world, but will foster a more detailed awareness of NSW

examples.

The first chairman for the sessions, Dr Alan

Voisey, most appropriately has many links with the represented institutions, having held professc^rial chairs in geology at the University of New England and Macquarie University and in being a former Chairman of the NSW Division of the Geological Society and a retired member of the Australian Museum Trust. The organising conmiittee would like to thank The Australian Museum Society for their help with the catering arrangements•

THE RELATIONSHIP BETWEEN THE BINDOOK VOLCANIC COMPLEX AND THE MARULAN BATHOLITH: Sr ISOTOPE EVIDENCE R. H. Flood, S. E. Shaw and G. H. Riley Lower Devonian I-type magma t i c a c t i v i t y in the north-eastern part of the Lachlan Fold Belt has resulted in the formation of large granitoid plutons, caldera collapse structures f i l l e d with s i l i c i c pyroclastic material and mass flow emplaced volcaniclastic sediments. Rb/Sr isotope data on the Marulan Batholith, the Banshea Adamellite at Mt. Werong, the main caldera f i l l ignimbrites of the Bindook volcanic complex, granitoid clasts within these ignimbrites and the Merrions Tuff are presented to evaluate their possible genetic relationships. Biotite Rb/Sr ages from the Marulan Batholith (average 400 Ma) are similar to the published K/Ar data suggesting a simple cooling history and l i t t l e i f any resetting effect by the Carboniferous deformation/metamorphism event that has affected most rocks in the north-eastern part of the Lachlan Fold Belt including the S-type Wologorong Batholith j u s t to the west. As the volcanic rocks do not contain unaltered primary b i o t i t e , comparison of their age and i n i t i a l 87sr/86sr ratios with the granitoids has of necessity been made using bulk rock Rb/Sr techniques in spite of the low Rb/Sr ratios of both I-type granitoids and volcanics. Although the resulting isochrons are poorly constrained, the calculated age and i n i t i a l 87sr/86sr ratio for a 12 point isochron of the granitoids C419 ± 33 Ma and 0.7062 ± 0.0008) i s v i r t u a l l y indistinguishable from the 19 point isochron defined by the Bindook volcanics and the Merrions Tuff (423 ± 19 Ma and 0.7061 ± 0.0002). The data are consistent with the granitoids and the volcanics being co-magmatic. There is a disparity in ages of the Marulan Batholith using b i o t i t e bulk rock compared with bulk rock methods, the biotite-bulk rock age being invariably younger, and this trend i s similar to that shown by other batholiths in the Lachlan and New England Fold Belts. The indicated i n i t i a l 87sr/86sr ratios of this group of plutonic and volcanic rocks (average0.7061 ) i s higher than the adjacent but younger Bathurst Batholith (0.7047) and higher than the Moruya Batholith of similar age further to the south. In common with other matched plutonic/volcanic associations in the Lachlan Fold Belt, the units are s p a t i a l l y related and are elongated in a general N-S direction with the volcanics largely occurring along one side of the Batholith. In the case of the Marulan/Bindook association the volcanics l i e generally to the west. The p o s s i b i l i t y that the Marulan/Bindook area has been t i l t e d to the west so as to expose the lower parts of the association to the east must be considered.

Flood and Shaw Macquarie University, North Ryde, N.S.W. 2113 Riley of the Supervising Geologist, Bondi, N.S.W. 2026 Office

- 6 -

THE MOUNT HOPE VOLCANIC COMPLEX AND COMAGMATIC INTRUSIONS* by E. Scheibner, L. M. Barron and D. W. Suppel Geological Survey of New South Wales, Sydney The Mount Hope Group (volcanic complex) and comagmatic intrusions originated in the southern part of the Cobar Trough during the latest Silurian to Early Devonian. The characteristic mainly explosive felsic volcanics and the typical bimodal volcanism (however, only weakly expressed) lead to the interpretation that at its initiation the southern Cobar Trough had a volcanic rift or volcano-tectonic depression character. Based on results of recent geological mapping it is suggested that this volcanic rift had the form of a composite resurgent submarine cauldron in the Mount Hope area. Evidence for this Mount Hope cauldron is: resurgence in the volcanic activity; the block structure which resulted in differences in individual block stratigraphy and development; and subsidence of the area after the initial volcanism and especially after the waning of volcanism which could be equated with cauldron subsidence. The present remains of the cauldron measure about 50 km width and 70 km length. The cauldron had a central stable horst which was formed by the Walters Range Block. The Mount Hope Group accumulated in the western part of the composite resurgent cauldron, while the Ural Volcanics formed in the eastern part. Based on geochemical and geological data it is speculated that during initial rifting heat was transferred not only by convection but also by basic magmas rising into the crust which had been subjected to an earlier, Silurian episode of partial melting and granite extraction. This reheating and introduction of volatiles (F, CI.) resulted in anhydrous melting leading to the formation of A-type granites (Boolahbone Granite) and associated volcanics. Following this event, higher crustal rocks (previously uneffected) became subject to partial melting, and S-type volcanics (rest of the Mount Hope Group) and associated granites (Mount Allen and Coan Granites) were formed.

Published with permission of the Secretary, New South Wales Department of Mineral Resources.

PERMIAN VOLCANISM ASSOCIATED WITH THE NEW ENGLAND BATHOLITH S. E. Shaw, R. H. Flood and R. H. Vernon Spatially associated with the New England Batholith are sequences of s i l i c i c calc-alkaline volcanic rocks. Geochemical, mineralogical and age characteristics of these volcanics suggest that in at least two areas the volcanics are the co-magmatic equivalents of the adjacent granitoids. The extensive Emmaville volcanic province, which includes both the Emmaville volcanics and the Dundee Rhyodacite, has chemical characteristics in common with the Moonbi Plutonic Suite, while the volcanic province around Ural la and Kentucky has characteristics similar to the Ural la Plutonic Suite. Although the Drake volcanic province has not been studied in detail, i t is probable that i t i s closely related to the Clarence River Plutonic Suite. No unambiguous volcanic equivalents of the Bundarra or Hi 11 grove Plutonic Suites have yet been found. The eroded remnants of the Emmaville volcanics form extensive deposits which encompass an area in excess of 7,000 km2 and consist of a complex series of r h y o l i t i c ash f a l l and ash flow (ignimbrite) units. Strati graphically overlying the Emmaville volcanics but locally deposited directly on metasedimentary basement i s a homogeneous sequence of ash flow deposits that form the Dundee Rhyodacite. Near the base of the rhyodacite there are e p i c l a s t i c deposits between individual ash flow units and although i t i s suspected from f i e l d outcrop patterns that the Dundee mass of the Dundee Rhyodacite i s composed of multiple ash flow units similar to those found in the underlying Emmaville volcanics, c r i t i c a l evidence i s lacking because of poor exposures. A foliation in these ignimbrites defined by both a phenocryst and a prominent lenticle alignment shows that these volcanics occupy a structural basin with sub-horizontal ash flow units in the centre of the basin and inwardly inclined dips rarely exceeding 30^ at the edge of the basin. Cross sections across the basin indicate a thickness of about 1.5 km. The volcanic rocks of the Ural la-Kentucky area consist of a series of faulted and gently folded l a t i t e lavas, breccias, volcanic sandstones and r h y o l i t i c pyroclastics with a combined thickness of about 1.2 km. These volcanics are truncated to the east, south and west by an arcuate ring of intrusive porphyritic latite with a subvertical mineral f o l i a t i o n . The intrusive porphyritic latite commonly shows fragmented plagioclase phenocrysts but no lenticle f o l i a t i o n , indicating a non-ash flow origin. This arcuate structure i s i t s e l f surrounded by an intrusive body of porphyritic quartz monzodiorite. The extremely fine-grained groundmass and phenocryst fragmentation of the intrusive porphyritic l a t i t e suggest that this unit could represent the ring dyke feeder for a sequence of pyroclastic rocks that are no longer preserved. The older extrusive l a t i t e lava flows are envisaged to occupy a collapse structure within the ring dyke. Chemically the intrusive l a t i t e dyke and the porphyritic quartz monzodiorite are indistinguishable. The high potash to soda ratio and high normative hypersthene to diopside ratio are characteristics of all of these rocks and of the nearby Ural la pluton.

The variation in composition from rhyolite to rhyodacite in the Emmavilie-Dundee area is typical of most caldera-forming pyroclastic eruptions whereby the lower members represent the top of a fractionated magma chamber and the upper more mafic eruptive units represent the lower parts of the magma chamber. At Kentucky i t is inferred that the mafic latite lava flows preceded the more s i l i c i c intrusive l a t i t e and that they may have formed a basement to the pyroclastic caldera f i l l since removed by erosion. Large caldera--forming s i l i c i c eruptions in many areas are preceded by mafic and intermediate lava flows, e.g. Jemez Mountains, Mew Mexico, and i t i s possible that the sequence at Kentucky may represent the lower parts of such a caldera collapse structure.

Macquarie University, North Ryde, N.S.W, 2113

LATE PERMIAN SHOSHONITIC VOLCANISM IN THE SOUTHERN SYDNEY BASIN

PAUL F . CARR* The Late Permian petrographic province of the southern Sydney Basin extends

along

the

south coastal region of New South Wales for 140km

between Durras and Wollongong and may extend offshore to the northeast for another

200km.

Outcrops

of volcanic

rocks

are confined to the

Nowra-Wollongong area and comprise a series of nine flows which occur in the upper part of the Shoalhaven Group and the lower part of the overlying Illawarra C o a l Measures, Each of the flows has the group name latite incorporated into the formal stratigraphic name but in terms of Si02-K20 relations the flows are classified as shoshonitic basalts (5), shoshonitic basaltic andesites

(3) and a shoshonitic andesite.

with

medium-

coarse-grained

A l l rock types are porphyritic

phenocrysts

plagioclase,

augite, Fe-Ti oxides and pseudomorphs after olivine.

calcic

The maximum An

content of phenocrystic plagioclase decreases from basic to intermediate rock types and has a compositional range from calcic andesine to sodic bytownite.

Groundmass plagioclase is generally richer in Ab than the

coexisting phenocrysts, whereas groundmass and phenocrystic clinopyroxene

grains

have

Wo37En45Fs^g.

similar

compositions

the

range

Wo43En42Fsi 5 to

Textural and chemical data indicate that the phenocrystic

phases are in equilibrium with the groundmass phases and are not xenocrystic in origin. groundmass

also

In addition to plagioclase and clinopyroxene, the

contains

fine-grained

chlorite and accessory apatite.

alkali feldspar, Fe-Ti oxides,

One shoshonitic basalt contains minor

interstitial analcite. A l l three rocks types are characterized by high contents of AI2O3 and large ion lithophile elements and by low contents of Ti02 and compatible elements.

REE patterns show moderate to strong enrichment of

the light elements.

M-values of the shoshonitic basalts are low with a

range of 30.5 to 47.0. unlikely

that

any of

These geochemical data suggest that it is very the

lavas

represents

a primary, mantle-derived

melt. Least

squares

mixing

calculations

indicate

that

relationships

between the three rock types can be explained by fractionation of one or more of the phenocrystic phases plagioclase, clinopyroxene, Fe-Ti oxides and olivine. * Department of Geology, University of Wollongong.

GEOCHRONOLOGY OF

SOME

IGNEOUS

B.J.J.

ROCK

Embleton,

AND

PALAEOMAGNETISM

DEPOSITS

P.W.

THE

SYDNEY

Schmidt,

D.A.

Clark.

BASIN

Palaeomagnetic data from igneous rock deposits in the Sydney Basin have provided a basis for defining Australian drift history during Mesozoic and Cenozoic time.

Radiometric dating of some deposits

for example, from localities at High Range, 152 m.y.; Mogo Hill, 58 m.y.;

Bondi, 151 m.y.;

Woy Woy, 47 m.y. and Peat's Ridge, 49 m.y. together

with other published information help calibrate the apparent polar wander track.

Several rock deposits have proved unsuitable for radiometric

dating purposes, for example, Dundas Breccia, Hornsby Breccia, St Marys Breccia, though resolution of their complex magnetic signatures may be used to date them indirectly by comparing the results with the data base. An essential component of this study has been the development of new analytical techniques to aid the resolution of sub-parallel primary magnetic directions and overprint magnetisations.

A LAYERED INTRUSION FROM THE SCONE AREA, HUNTER VALLEY D J MARTIN" A medium-size tholeiitic intrusion near Scone is emplaced at shallow level just west of the Hunter Thrust system.

Within chilled mar-

gins, olivine tholeiite is overlain by a rhythmically layered series, in which a sequence of microstructures repeats six times. of cumulate processes was found.

No evidence

The olivine tholeiite varies little

in microstructure or composition except for grainsize layer pairs 1 to 2 m thick, or 20 cm at some levels.

Orthopyroxene rims on olivine

evidence silica enrichment of the evolving liquid.

Each sequence (a

'unit') of the layered series consists of a homogeneous mafic layer A, and a felsic member alternating between andesine with augite dendrites (layer B), and comb-layered oligoclase (layer C).

Dendritic growth

patterns indicate crystallization upwards in units 1 to 3 and lower unit 4, and down in units 6, 5 and upper unit 4.

Fe-rich augite and

amphibole occur in the sodic upper margin of layer C of units 1 to 4. Mafic layer A compositions show little variation, geochemical trends being developed in the felsic layers. Unlike many differentiated intrusions which vary geochemically from the base upward, the Scone tholeiite has increasing amounts of Fe, Si, Na and K in the felsic layers from unit 5 downward.

A mechanism is

proposed, in which felsic fluid residual from the coarse-grained tholeiite accumulates in a boundary layer above the solidification front. Absorption of latent heat of crystallization enhances its density difference relative to the magma.

Gravitational instabilities develop,

causing blobs of fluid (thermals) to rise through the slowly circulating magma. size.

The resulting thinner boundary layer promotes finer grain-

Rapid temperature fluctuations (20 cm layers) precede breakup

and ascent of the whole boundary layer, with turbulence developing in the magma.

Some magma is entrained by the rising cloud of thermals.

This fluid separates at the top of the chamber by a double diffusion process into the mafic and felsic layers of one unit.

Six repetitions

during crystallization of the olivine tholeiite produce the normal geochemical trends, but the products thus acciomulate in inverse order. After solidification of the olivine tholeiite is complete, the reduction in heat flux allows the fluid layers to crystallise inwards, with dendrites and comb-layers forming under supercooled conditions. The most Fe-enriched level is in unit 4, inferred from microstructural details to be the last portion to solidify.

^ 'Macquarie University

MAJOR ELEMENT CHEMISTRY OF THE

ALKALINE GUNNEDAH COMPLEX

D.H. French-

The Gunnedah Complex is an alkali-olivine basalt intrusive complex consisting of at least five major sills (informally named the Black Jack, Hidden, Ivanhoe, Sylvandale and Swindle Sills), several dykes and minor sills. The dominant rock types of the sills are teschenite, analcime dolerite and olivine-analcime dolerite with minor analcime syenite, whereas the major rock types in the dykes are analcime basalt and camptonitic basalt. With one exception all of the rocks are nepheline normative and the basic rocks are basanitoids. Generally, the rocks of the complex are potassic and belong to the differentiation series alkaliolivine basalt->trachybasalt->-trachyandesite->tristanite->trachyte. Slight differences exist between each sill in the degree of undersaturation and potassium enrichment. The Black Jack Sill displays a greater degree of undersaturation than the Hidden and Ivanhoe Sills and the Ivanhoe Sill is less potassic and not as strongly undersaturated as the Black Jack and Hidden Sills. These differences reflect compositional differences in the parent magmas. The generalised differentiation trend is one of an initial enrichment in magnesium followed by a steady decrease. Concomitant with the decrease in magnesium is an increase in the alkalies, particularly potassium. The upper and lower chilled margins of the Black Jack and Hidden Sills are compositionally different and several sudden changes in chemistry occur in both intrusions, suggesting that multiple injection has taken place. Crystal fractionation of olivine, plagioclase, magnetite and subsequently clinopyroxene is considered to have been the dominant fractionation mechanism although gravitational settling of olivine and alkali diffusion were of minor importance.

'^he Department of Geology The University of Newcastle

- 13 -

PYROXENES OF ANKARAMITIC AND RELATED ALKALI BASALTIC LAVAS OF THE BARRINGTON TOPS VOLCANIC FIELD, N.S.W. D.R. Mason# Ankaramites constitute a significant proportion of the dominantly alkalic lava pile of the Barrington Tops Tertiary volcanic field. Crystallization differences between ankaramites and olivine-pyroxene basalts have been investigated by electron microprobe analysis of constituent clinopyroxenes . Large (up to 1 cm) clinopyroxenes of the ankaramites are composed mostly of Al-augite displaying modest range in composition (lOOMg/Mg+Fe* = 81.3-76.0; Ca46.8-49.7Mg43.2-38.3Fe9.9-12.1). Ratios of Al^^Al^"" (2.6) are consistent with cognate crystallization at moderate pressures (<10kb), presumably within crustal magma chambers. Rare cores are relatively Cr-rich ^^^2^3 ^ 0.9%). Titan-pyroxenes of low pressure (quench) origin are present as thin rims on megacrysts and small grains in the groundmass, and possess consistently high AI^VAI^^ (>6.0). In comparison, a variety of pyroxene populations is present in olivine-pyroxene basaltic rocks. These include: i) megacrysts comparable with those of the ankaramites, ii) megacrysts, probably also cognate, but with significantly different compositions (including Fe-rich types), and iii) xenocrystic Cr-diopside. The apparent lack of these other megacrystic and xenocrystic pyroxenes in the ankaramites may be due to their initial absence or, more likely, to settling out of earlier xenocrystic and megacrystic phases during crustal residence.

# Department of Geology, The University of Newcastle, Shortland, N.S.W. 2308.

- 14 -

PETROLOGY OF VOLCANIC ROCKS FROM THE LIVERPOOL RANGE, N.S.W. Richard W. Schon'^ The Liverpool Range (LR) is located in the southern part o£ the Oxley Basin, 200 km NW of Sydney. Within the Range, Eocene - Oligocene basic volcanics, up to 800 m thick, cover a 100 km x 60 km area. The LR adjoins the early Eocene Mt. Royal Range to the E, and the mid-Miocene Warrumbungle Range to the W, thus forming part of a westerly-younging E-W segment of the otherwise N-S - trending Great Dividing Range. K-Ar ages of LR rocks can be separated into two groups: a N-S line divides late Eocene lavas (38 - 40 m.y.), here assigned to an Eastern Volcanic Centre, from the early Oligocene (32 - 35 m.y.) rocks of a proposed Western Centre. LANDSAT imagery reveals physiographic contrasts between these two areas. The volcanics are all basic ( 47 ±3 % Si02 ). Alkali olivine basalts (AOBs) are abundant; hawaiites and mugearites rare. Two series can be recognized on the basis of major element chemistry: an alkalic, undersaturated series common to both Volcanic Centres, and a transitional series confined to the younger Western Centre. Basanitic rocks from the first series contain up to 17 % normative nepheline (Ne), the dominant AOBs have > 5 % N e , and the hawaiites and mugearites average 7% Ne. The second series has a more restricted range in composition from transitional AOB (with up to 7 %Hy) to hawaiite. Olivine-phyric rocks predominate in both series, although plagioclasephyric rocks and ankaramites are also present. As no aphanitic rocks have been found, the best candidate for a primitive composition is a basanite (7% Ne, 100 Mg/(Mg+Fe^"^ = 69, 250 ppm Ni ) with spinel Iherzolite inclusions, from the westernmost part of the Range. However, as this rock contains phenocrysts of olivine (Fosif) and calcic labradorite, its bulk chemistry cannot be taken as that of a primary liquid. Immediately to the west of the LR, basanites from Mount Pottinger and Round Mountain contain abundant mantle and lower crustal xenoliths. These basanites (with up to 13%Ne, 100 Mg/(Mg+Fe^^ =69 , and 370 ppm Ni) represent another possible primitive melt, although their age relation to the LR basalts is not certain. Trace element analyses of the more primitive compositions show that they are enriched in incompatible elements; e.g. La|sj/Ybjq ^ 10 . This enrichment is preserved in the more evolved rocks, and is present in both the alkalic and transitional series, indicating that they were probably formed by small degrees of partial melting of enriched garnet Iherzolite. Crystal fractionation modelling suggests that subsequent evolution was controlled by the removal of olivine and lesser amounts of plagioclase and clinopyroxene.

"Department of Geology § Geophysics, University of Sydney.

- 15 MIOCENE

SHIELD NEW

VOLCANOES

SOUTH

WALES

The Miocene shield volcanoes of New South Wales display remarkable geochemical, petrological and volcanological similarities; and the concept that they are all related to a discrete upwelling of sublithospheric heat and ' *magma'* that remained essentially stationary as the Australian plate moved northwards, is examined and elaborated upon.

It is hypothesized that

at the time when each of these shield volcanoes became extinct they were broadly similar in form and petrography.

By combining the geochemical,

petrographic and structural data available on the Tweed Volcano and the successively younger Nandewar, Warrumbungle and Canobolas volcanoes, one can construct a complete compound model, that explains both the surficial and subvolcanic characteristics of these volcanoes.

Such a model imposes strong

constraints on the processes that can be invoked to account for the origin and evolution of the rocks associated with these volcanoes. It is proposed that the partly layered gabbroic, monzogabbroic, monzonitic and syenitic rocks, presently found in the Mount Warning area, crystallized in a magma chamber beneath the main eruptive centre of the Tweed Volcano. At the present level of erosion the surficial edifice of the Nandewar Volcano contains large volumes of mugearite and benmoreite.

The less eroded Warrum-

bungle Volcano contains large volumes of basaltic rocks, and a complex central core that contains a wide variety of different types of rocks that have issued from separate local vents.

Canobolas Volcano is well preserved and it consists

of a central elevated core that contains many conical and domical bodies of trachyte and comendite and a wide variety of glassy and pyroclastic rocks. Lavas of more basic composition radiate out from this central core, A quantitative petrogenetic model is presented; and the essential processes are considered to be fractional crystallization and volatile-transfer.

These

processes operated in large compositionally zoned bodies of magma that evolved at shallow depths beneath these volcanoes.

Eric A.K. Middlemost Department of Geology § Geophysics University of Sydney, NSW. 2006.

REGIONAL CONTROLS ON NSW VOLCANISM F.L. Sutherland * Tertiary volcanic rocks in NSW cannot be considered in isolation from the whole E, Australian belt-

NSW has central volcanoes and basaltic

lava fields like other areas, but differs in having a greater share of olivine leucitites and no young volcanics (<ClOMa). Individual central volcanoes that erupted over 1-5 Myrs are part of a migratory chain extending from N. Queensland to Victoria.

Projections

of the chains, using new constants for K-Ar ages and a revised spreading history for the Southern Ocean (Cande & Mutter, 1982) plot over the Coral Sea-Louisiade-Tasman Sea triple point system at the time of active spreading (56-63 Ma).

New dates on the olivine leucitites and some

Queensland basalts also match this migration (Sutherland & Wellman, in prep.) The younger Lord Howe basaltic sea mount chain, Tasman Sea ,is also probably migratory and projects towards an Oligocene spreading system. Smith (1982) also suggested migration of Australia over the CoralTasman spreading ridges, but his mechanism requires delays of volcanism and uplift of up to 60 Ma.

This conflicts with the revised Southern

Ocean history, and some recent geomorphic studies. correlate volcanism with uplift.

Jones & Veevers (1982)

Pilager (1982) compared the Australian

central volcano trace to that predicted from the Hawaiian 'hot spot' chain. He concluded Australia was different and correlated the chain with a change in stress field causing cessation of volcanism. Newer Australian dates refute this. The exact cause of E. Australian volcanism and its relationships to uplift is under debate.

The roles of Australia's migration and involvement

of the Coral Sea-Tasman Sea spreading zones are still to be resolved. Further basalt dates from NSW will be critical to evaluate precise mechanisms.

^ Department of Mineralogy & Petrology The Australian Museum Sydney, N.SoW. 2000

- 17 VOLCANISM AND THE MANTLE-LOWER CRUST JIGSAW, S.E. AUSTR^IA Julian D. Hollis

Alkali volcanism in S.E. Australia over the past 200 m.a. has extensively sampled rocks from the deep lithosphere.

Composite inclusions

enable reconstructions to be made of various centimetric-scale structures of the upper mantle and lower crust.

Larger heterogeneities are

suggested by extrapolation, notably intrusions and metasomatised, vertically moved regions, some corresponding to diapirs.

There are five

main associations of inclusion types: 1. LHERZOLITIC: ubiquitous upper mantle peridotites with pyroxenite intrusives and cumulates.

Complex textures portray multiple partial

melting,metasomatic and remobilisation episodes.

Ca/Ca+Mg+ EFe of garnets

show a narrow range from ssilS in gnt Iherzolites to cx 14 in gnt pyroxenites. 2. ECOLOGITIC: upper mantle-lower crustal intrusives and cumulates of jadeitic cpx, Ca-rich pyrope-almandine, kyanite, Cr-corundum , apatite, scapolite and diamond + coesite inclusions (Copeton).

Xenoliths only

from kimberlitic sources, eg. Kayrunnera and ?Gloucester. 3. ALFAMIC: (Alkali Feldspar + Amphibole + Mica) inclusions represent hydrous, Ti, Fe-rich intrusives and cumulates, forming veins and bodies of Iherzite, diorite and anorthosite.

Apatite and sphene are frequent

accessories and garnets have Ca/Ca+Mg+ SlFe near 14.5 indicating basaltic affinities.

Responsible for metasomatism of surrounding rocks (eg. Bombo).

4. GRANULO-GABBROIC: deep crustal granulites and gabbros with diverse mineralogy and textures.

Some may represent regions underplated by

repeated basaltic intrusives.

Hydrous minerals largely represent metasomatic

additions from alfamic activity. 5. ZIRCOSPILIC: (ZIRcon + COrundum +SPinel + ILmenite) host rocks are poorly known but are represented by widespread xenocrysts.

Rare xenoliths

of zircon anorthoclasite (Rileys Peak, Moss Vale) and corundum anorthoclasite (Mt Leura, Qld) occur. between petrologic types are tabled

The principal composite relationships (next page).

These composite inclusions record multiple heating events that generated partial melting of mantle rocks with vertical movements of heat, metasomatising elements and magmas into the crust.

Department of Mineralogy and Petrology The Australian Museum, Sydney, N.S.W. 2000

Lherzolitic Association: Spinel Iherzolite xenoliths in J or intruded by, pyroxenite and cumulate peridotite. Metapyroxenite xenoliths and schlieren in, or intruded by, spinel Iherzolite and pyroxenite. Pyroxenite xenoliths in cumulate peridotite.

Lherzolitic and Alfamic Associations: Metapyroxenite, cumulate peridotite and spinel Iherzolite xenoliths and schlieren in, or intruded by, alfamics.

Alfamic Association: Mafic alfamic in contact with felsic alfamic. Course alfamic intruding , or with xenoliths of, fine alfamic.

Alfamic and Granulo-gabbroic Associations: Granulite intruded by, or as xenoliths and schlieren in alfamics.

^•

^ranulo-gabbroic Association: Mafic granulite schlieren in gabbro.

Granulo-gabbroic and Lherzolitic Associations: Mafic granulite with xenoliths and schlieren of spinel Iherzolite.

- 19 -

GEOMORPHOLOGY AND VOLCANISM C- D. OLLIER Geomorphically, eastern Australia Is an upwarped plain with a crest around the Great Divide, and an eastern edge marked by a Great Escarpment. Several alternative hypotheses have been proposed for the nature and timing of the uplift, Including: continued uplift over about 90 million years; uplift associated with the opening of the Tasman and Coral Seas, starting about 80 million years ago; the highlands were In existence In the Cretaceous and have undergone several cycles of erosion and renewed uplift. The resolution of these conflicting hypotheses must await further Information, some of which will come from geomorphology. A similar morphotectonIc situation Is found on other continents with "trailing edges", but while In Australia the uplift appears to have some causal connection with volcanism, Cenozolc volcanoes are almost absent in the uplifted rims of other continents. What Is so different about the morphotecton1cs of Australia that allows volcanlclty when all other trailing edges are devoid of volcanoes? Geomorphic studies in the eastern highlands are concerned with the relative dating of the formation of the old surface (palaeoplaIn), tectonic uplift, backward erosion of the scarp, and of course the eruption of the volcanlcs. These studies must also be related to sedimentation in neighbouring sedimentary basins. Drainage pattern analysis provides local detail. There are many examples of single lava flows following valleys, and eventually giving rise to deep leads and twin lateral streams. In Victoria there are many examples of repeated diversion in the Western Plains. Some major volcanoes disrupt earlier rivers which are forced to flow around the volcano in a new course. The Warrumbungles provide an example. Volcanoes have simple radial drainage, which may be superimposed on underlying bedrock and be preserved even when much of the original volcano is destroyed. Examples are the Ebor Volcano, Barrlngton Tops, The Box Hill, Mt Curcudgy and the Comboyne Plateau. As more examples come to light (but still await verification) It becomes Increasingly probable that the old concept of vast lava plains will give way to a pattern of distinct volcanic centres. This does not affect the hypothesis that the volcanlcs of eastern Australia Include major central volcanoes, which have a clear age-latitude relationship, and other eruptions associated with lava fields that have no such relationship. The nature of erosion of volcanoes is also being studied. In some Instances basalt acts as a tough caprock, protecting softer underlying bedrock from erosion. More commonly the basalt Is stripped off a tougher bedrock, creating benches that are exhumed fragments of the pre-basalt ground surface. By mapping more of this surface, and reconstructing more of the pre-basalt drainage patterns. It should be possible to decypher the palaeogeography of eastern Australia, which In turn will enable testing of tectonic hypotheses outlined In the first paragraph. 'University of New England