Cenozoc lavas from Hainan Island, bouth China, comprise quartz tholeiite. olivinethoeiite, alkali basalt, and basanite and form a continuous, thoeiite-dominated, comopitionalspectrum. Highly incompatible elements and their relationships with isotope in these lavas areshown to be useful in evaluating mantle-source composition, whereas modeling suggests that ra-tios of elements with bulk partition coefficients significantly larger than those of Nb and Ta maybe sensitive to patial melting. Th/Ta and La/Nb ratios of alkali basalts are lower than those ofthoeiites, and they are all fower than tbe of the primitive mantle. These ratios correlate posi-tively with 207Pb/204Pb and 87Sr/86Sr ratios. Such relatioships can be explained by mixing ofdepleted and enriched source componentS. A depleted component is indicated by alkali basaltcompositions and is similar to some depleted OIB (PREMA). The enriched component, similarto sediment compositios, is indicated by tholeiites with high LILE/HFSE, 207Pb/204Pb, and87/86Sr ratios.In general, basalts from Hainan and the South China Basin (SCB) share common geo-chemical charcters, e.g. high Rb/Sr, Th/Ta, 207Pb/206Pb, and low Ba/Th ratios. Such ageochemical trend is comparable to that of EMII-type OIB and best explained as the result ofsubduction. Occurrence of these charcteristics in both continennd Hainan basalts and SCBseamount basalts indicates the presence of a South China geochemical domain that exists in themantle region below the lithosphere.
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