Geochemical and lead isotope compositions of olivine-hosted melt inclusions from the Yaeyama Graben in the southern Okinawa Trough: Implications for slab subduction and magmatic processes

Abstract

Chemical compositions were measured in olivine-hosted melt inclusions in basaltic volcanic rocks from the Yaeyama Graben in the southern Okinawa Trough to investigate the magmatic processes of erupted lavas in an incipient epicontinental back-arc basin in the western Pacific. Melt inclusions have widely variable element and Pb isotope compositions, while erupted host whole-rock compositions are homogeneous, suggesting that melt inclusions preserve more information about magmatic processes than their host whole rocks. Two groups of melt inclusions can be distinguished by their trace elements and Pb isotopes. Group 1 melt inclusions have geochemical compositions similar to those of their host whole rocks, which have low 207Pb/206Pb ratios (<0.850), and may be attributed to the contributions of recycled continental crust and subducted sediments to magma sources during Philippine Sea Plate subduction. Group 2 melt inclusions have higher 207Pb/206Pb ratios (>0.850) and Li, Rb, and U contents and lower Cu, Zn, and Pb contents than their host whole rocks. High Li, Rb, and U contents may be induced by specific melts generated by possible breakdown reactions of some accessory minerals, such as phengite and cordierite, while low Cu, Zn, and Pb contents are attributed to sulfide precipitation, metal flushing and/or metallic element fractionation during magma evolution. The Pb isotopes of the group 2 melt inclusions are similar to those of the Kuanhsi-Chutung basalts, which are correlated with some intraplate volcanism in the Fujian-Taiwan region. We infer that the group 2 melts were preexisting small volumes of magma that were emplaced in the middle-lower continental crust (~11 km), experienced extensive magma evolution and were entrained by upwelling magma. Our melt inclusion data suggest that magma sources are compositionally heterogeneous and that the mixing of magmas from different sources may be widespread beneath the southern Okinawa Trough because of Philippine Sea Plate subduction and preexisting intraplate magmatism.