GEOCHEMISTRY OF PALEOZOIC-MESOZOIC PACIFIC RIM OROGENIC MAGMATISM, THURSTON ISLAND AREA, WEST ANTARCTICA

Thurston Island, and the adjacent Eights Coast and Jones Mountains, re cord Pacific margin magmatism from Carboniferous to Late Cretaceous ti mes, The igneous rocks form a uniformly calc-alkaline, high-alumina, d ominantly metaluminous suite; some relatively fractionated granitoids are mildly peraluminous. The magmas were hydrous, a result of subducti on. Gabbros have compositions outside the range of mafic volcanic and hypabyssal rocks, as a result of cumulate processes. Trace element com positions of the mafic magmas range from a low La/Yb, Th/Ta end-member close to E-MORB in composition, perhaps contaminated by crust, to a h igh La/Yb, Th/Ta end-member, close to shoshonite, with strong magmatic arc trace element character. This variation may be a result of mixing of tholeiitic and shoshonitic end-members. Most silicic rocks could h ave been generated batch-wise from mafic magmas by fractional crystall ization of a phenocryst assemblage dominated by plagioclase, pyroxene/-amphibole, as seen in the cumulates. Cessation of magmatism at about 90 Ma approximately coincided with collison of a spreading centre bet ween the Phoenix and Pacific oceanic plates with the continent margin subduction zone. The rifting of New Zealand from West Antarctica and a ssociated extension probably was responsible for emplacement of a coas t-parallel Cretaceous dyke swarm.