MAJOR AND TRACE-ELEMENT, AND SR-ND ISOTOPE CONSTRAINTS ON THE ORIGIN OF PALEOGENE VOLCANISM IN SOUTH CHINA PRIOR TO THE SOUTH CHINA SEA OPENING

Paleogene volcanic rocks crop out in three sedimentary basins, namely, Sanshui, Heyuan and Lienping, in the attenuated continental margin of south China. Lavas from the Sanshui basin which erupted during 64-43 Ma are bimodal, consisting of intraplate tholeiitic basalt and trachyt e/rhyolite associations. Similar to Cretaceous A-type granites from th e nearby region, the felsic member shows peralkaline nature [Na2O + K2 O approximate to 10-12%; (Na + K)/Al approximate to 0.98-1.08], genera l enrichment in the incompatible trace elements and significant deplet ion in Ba, Sr, Eu, P and Ti. Although both types of the Sanshui lavas have rather uniform Nd isotope compositions [epsilon(Nd)(T) approximat e to +6 to +4] that are comparable to Late Cenozoic basalts around the South China Sea, the felsic rocks possess apparently higher initial S r isotope ratios (I-Sr up to similar to 0.713) and form a horizontal a rray to the right in the Nd vs. Sr isotope plot. Closed system differe ntiation of mantle-derived magmas in a 'double diffusive' magma chambe r is considered for the bimodal volcanism, in which the trachytes and rhyolites represent A-type melts after extensive crystal fractionation in the upper portion of the chamber. Such A-type melts were later con taminated by small amounts (1-3%) of upper crustal materials during as cent. On the other hand, composition of lavas in the other two basins varies from tholeiitic basalt to andesite. Their Sr and Nd isotope rat ios [I-Sr approximate to 0.705 to 0.711; epsilon(Nd)(T) approximate to +1 to -5] and generally correlative Nb-Ta depletions suggest a distin ct magma chamber process involving fractional crystallization concomit ant with assimilation of the country rock. We conclude that these Pale ogene volcanic activities resulted from the lithospheric extension in south China that migrated southwards and eventually led to opening of the South China Sea during similar to 30-16 Ma. (C) 1997 Elsevier Scie nce B.V.