Subduction-modified subcontinental mantle in South China: Trace element and isotope evidence in basalts from Hainan Island

Abstract

Cenozoic lavas from Hainan Island, South China, comprise quartz tholeiite, olivine tholeiite, alkali basalt, and basanite and form a continuous, tholeiite-dominated, compositional spectrum. Highly incompatible elements and their relationships with isotopes in these lavas are shown to be useful in evaluating mantle-source composition, whereas modeling suggests that ratios of elements with bulk partition coefficients significantly larger than those of Nb and Ta may be sensitive to partial melting. Th/Ta and La/Nb ratios of alkali basalts are lower than those of tholeiites, and they are all lower than those of the primitive mantle. These ratios correlate positively with207Pb/204Pb and87Sr/86Sr ratios. Such relationships can be explained by mixing of depleted and enriched source components. A depleted component is indicated by alkali basalt compositions and is similar to some depleted OIB (PREMA). The enriched component, similar to sediment compositions, is indicated by tholeiites with high LILE/HFSE,207Pb/204Pb, and87Sr/86Sr ratios.