ALLOCHTHONOUS OCEANIC BASALTS WITHIN THE MESOZOIC ACCRETIONARY COMPLEX OF ALEXANDER ISLAND, ANTARCTICA - REMNANTS OF PROTO-PACIFIC OCEANIC-CRUST

The Mesozoic LeMay Group accretionary complex of Alexander Island, Ant arctica, contains thrust-bound slices of accreted ocean floor, ocean i slands and seamounts. They represent fragments of proto-Pacific oceani c crust, of which only a tiny remnant (the Phoenix plate) remains off northern Antarctic Peninsula. They therefore provide an excellent oppo rtunity to sample the ancient oceanic crust that formerly occupied the southern Pacific Ocean. All the basalts experienced sea-floor and sub duction/accretion metamorphism ranging from zeolite to transitional bl ueschist facies. On the basis of rare-earth and other immobile trace e lement characteristics, the basalts are divided into depleted MORE, N- MORB, E-MORB, and tholeiitic and alkaline GIB. Oceanic basalts occur w ithin two rock associations on Alexander Island, basalt-volcaniclastit e-chert and basalt-volcaniclastite-tuff. The basalt-volcaniclastite-ch ert rock association is dominated by pillow lavas which have light REE -depleted N-MORB geochemical characteristics, and is interpreted as re presenting ocean floor formed at spreading centres. Locally, sills of tholeiitic OIB intrude the sequence. The basalt-volcaniclastite-tuff r ock association exposed in the Lully Foothills was formed in shallow w ater during the Early Jurassic. It is geochemically varied, consisting of basalts with N-MORB, E-MORB and tholeiitic OIB characteristics. Th e association is interpreted to have been formed on a large seamount o r ocean island.