RE-OS ISOTOPIC COMPOSITIONS OF LATE MESOZOIC MAFIC ROCKS FROM SOUTHEASTERN COAST OF CHINA

Abstract

The representative basalts and hornblende gabbros samples have been selected from early Cretaceous "bimodal 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 γOs (130Ma) of two hornblende gabbros are -83.7 and -47.1 with unreasonable low initial (187Os/188Os) ratios of 0.0667 and 0.0205. The extremely low initial (187Os/188Os) ratios indicated that some geological processes have disturbed their Re-Os system and it is likely that a recent Re gain after emplacement caused their higher 187Re/188Os and lower(187 Os/188Os)i and γOs(t). In contrast, the basalts yielded reasonable γOs (130Ma) values from 21.4 to 267.8, with initial (187Os/188Os) ratios of 0.1531 - 0.4639. Combined Re-Os, Sm-Nd isotopic and trace element geochemical studies indicate that the early Cretaceous 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 southeastern coast of China should be depleted asthenospheric rather than enriched lithospheric mantle.