Petrology and geochemistry of igneous basement rocks, IODP Expedition 349, South China Sea

Abstract:

Expedition 349 is the first drilling expedition in the new International Ocean Discovery Program (IODP) and successfully drilled 5 sites in the South China Sea (SCS), a marginal basin whose origin is largely debated. An important objective of the expedition is to obtain oceanic crust samples to better determine the timing of the beginning and the end of spreading. Three of the Expedition 349 drill sites, U1431, U1433, and U1434, located near fossil spreading centers, recovered oceanic crust basalts. The recovered oceanic crust at Site U1431E consists of massive flows up to 26 m thick with thin, rubbly, aphyric to olivine-phyric flow tops. The massive flows are fine- to coarse- grained, with a mineral assemblage of olivine, plagioclase, and clinopyroxene, typical of mid-ocean ridge basalts (MORB). Plagioclases forming variolitic texture are common, indicating rapid initial cooling while coarsening grain size toward the center suggests slower cooling in the interior of these massive flows. The igneous basement at Sites U1433B and U1434 consists mainly of thinner, pillowed flows associated with hyaloclastite breccias. At Site U1433B, the upper section is composed of 38 m-thick pillowed flow units followed by 23 m-thick massive flow units. Recovery at Site U1434 was poor, with only ~3 m of mostly aphyric basalt with glassy to microcrystalline texture. The pillow basalt flows at Site U1433B are dominantly plagioclase-phyric, with minor olivine microphenocrysts, and mostly plagioclase and clinopyroxene as groundmass minerals, resembling MORB mineralogy. Chilled margins are common, with occasional remnants of fresh glass in them. Plagioclases in these thinner flows often form variolitic textures and exhibit zoning, with melt inclusions along cracks and cleavage planes, suggesting fast crystallization growth within compositionally evolving magma. The massive flow units at Site U1433B are similar to those at Site U1431E. Initial onboard major and minor elements data indicate that the igneous basement at Sites U1431E and U1433B are tholeiitic basalts similar to Pacific and Indian MORBs and different in composition from the alkalic seamount basalts of the SCS. Acquisition of geochemical data to test the origin of the oceanic crust of the SCS is ongoing and preliminary results will be presented.