MAGMATISM IN THE SOUTH CHINA BASIN .2. POST-SPREADING QUATERNARY BASALTS FROM HAINAN ISLAND, SOUTH CHINA

Cenozoic magmatism on northern Hainan Island was part of a regional episode post-dating opening of the South China Basin (SCB). Tholeiite lava flows emanated from WSW-ENE-trending extensional fissures and are interlayered with mid-Miocene to Quaternary sediments, forming a 200-1000-m-thick sequence. Subsidiary alkali basalts were erupted from central volcanoes aligned on NW-SE-trending strike-slip faults, and in some cases contain ultramafic xenoliths, and megacrysts of augite, sapphire (corundum), zircon and anorthoclase. Cumulative thickness of the lavas rarely exceeds 200 m although sills of unknown age intrude early Oligocene to Cretaceous sediments at depths up to 3000 m. Lava compositions include quartz tholeiite, olivine tholeiite, alkali olivine basalt and basanite, and are subdivided into chemical types on the basis of temporal-spatial association. With time the basalts change from relatively thin intercalated quartz tholeiite and alkali olivine basalt flows to more massive olivine tholeiites. In common with other SCB eruptives, Hainan basalts resemble Dupal-type oceanic island basalt and reflect mixing of EM2 and N-MORB mantle reservoirs. Decoupled systematics of Sm/Nd and Nd-143/Nd-144 are at least partly due to differentiation during partial melting, possibly by decompression melting of plagioclase/spinel- and spinel-lherzolite. It is proposed that EM2 derives from sub-continental lithosphere or asthenosphere reservoirs, enriched by subduction prior to crustal extension.