Magmatism in the South China Basin: 1. Isotopic and trace-element evidence for an endogenous Dupal mantle component

Abstract

A widespread episode of intraplate volcanism followed the cessation of sea-floor spreading in the South China Basin (≈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. 87 Sr 86 Sr and 143 Nd 144 Nd ratios are depleted relative to bulk Earth values and partially overlap with Central Indian Ridge MORB and associated OIB. In contrast, 208 Pb 204 Pb and 207 Pb 204 Pb ratios are variable and surprisingly radiogenic for given MORB-like 206 Pb 204 Pb . 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 ( Δ7 4 Pb = 2−13 , Δ8 4 Pb = 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.