MAGMATISM IN THE SOUTH CHINA BASIN .1. ISOTOPIC AND TRACE-ELEMENT EVIDENCE FOR AN ENDOGENOUS DUPAL MANTLE COMPONENT

A widespread episode of intraplate volcanism followed the cessation of sea-floor spreading in the South China Basin approximately 32-17 Ma BP), affecting large parts of southern China and Indochina, and penetrating oceanic basement and stranded microcontinent fragments. Geochemical data for post-spreading seamount and island lavas define suites of quartz tholeiite, olivine tholeiite, alkali olivine basalt and nephelinite, characterised by OIB-type incompatible-element distributions. High-K alkalic lavas show extreme enrichment in large-ion lithophile and high-field-strength elements relative to N-MORB. Sr-87/Sr-86 and Nd-143/Nd-144 ratios are depleted relative to bulk Earth values and partially overlap with Central Indian Ridge MORB and associated OIB. In contrast, Pb-208/Pb-204 and Pb-207/Pb-204 ratios are variable and surprisingly radiogenic for given MORB-like Pb-206/Pb-204. The isotopic and trace-element systematics confirm source heterogeneity but appear to be decoupled, implying complex mantle enrichment histories. At least two heterogeneous source components are required: a depleted but "contaminated" Indian Ocean MORB type, and an EM-2 reservoir whose isotopic composition corresponds to continent-derived sediment. Dupal-like Pb isotopic compositions (DELTA-7/4Pb=2-13, DELTA-8/4Pb=45-73) are shared by intraplate basalts from Hainan Island, the Penghu Islands, northern Taiwan and post-collision arc basalts from the Philippines. It is proposed these reflect endogenous mantle processes related to disaggregation of the south China margin rather than a northward extension of the southern hemisphere Dupal anomaly.