THE ORIGINS OF PYROXENITES IN THE SOLOMON ISLANDS (SANTA ISABEL AND SAN JORGE). Thomas Berly Jorg Hermann^ and Richard J. Arculus ^. ^Geology Department, and ^Research School of Earth Sciences, Australian National University, Canberra. The Solomon Islands are a NW to SE-trending double chain of islands, the older basement of which was formed by S W-directed subduction of the Pacific Plate beneath the Indo-Australian Plate, between the Eocene and Early Miocene. At 10 Ma, the Ontong Java Plateau (OJP) collided with the Solomon arc. In response to this collision, polarity of subduction was reversed; NE-directed subduction beneath the Solomon arc began. Thin fault slices of peridotites, pyroxenites, gabbros, and basalts are now exposed in SE Santa Isabel and neighbouring San Jorge. Some pillow lavas have been identified as OJP basalts but the origins of the ultramafic rocks remain uncertain. The pyroxenites are associated in the field with harzburgites, dunites and sometimes incorporated into serpentine massifs (NW San Jorge). These coarse-grained rocks contain mostly (but in variable proportions) orthopyroxene (opx) (70