Re-Os isotopic compositions of late Mesozoic mafic rocks from southeastern coast of China

The representative basalts and hornblende gabbros samples have been selected from early Cretaceous "biniodal volcanics" and "composite plutons" from eastern Zhejiang and southeastern Fujian provinces of southeastern coast of China. We separated the Fe-Ti oxide concentrates in them and analyzed their Re-Os isotopic compositions. The gamma(0s)(130Ma) of two hornblende gabbros are -83.7 and -47.1 Nvith onreasonable loiv initial (Os-187/Os-188) ratios of 0.0667 and 0.0205. The extreniely low initial (Os-187/Os-187) ratios indicated that sonic geological processes have disturbed their Re-Os system and it is likely that a recent Re gain after emplacement caused their higher Re-187/Os-188 and lower (Os-187/Os-188), and gamma(0s)0(t) In contrast, the basalts yielded reasonable (gamma 0s)(130Ma) values from 21.4 to 267.8, with initial ((OS)-O-187/Os-188) ratios of 0.1531 similar to 0.4639. Combined Re-Os, Sm-Nd isotopic and trace element geochemical studies indicate that the early Cretaccous basaltic magmas might be derived from depleted rather than enriched mantle source, and may have been contaminated by crustal rocks and melts. During late Mesozoic, the sub-arc mantle from sontheastern coast of China should be depleted asthenospheric rather than enriched lithospheric mantle.