Geochemistry of high Mg andesites and the tectonic evolution of the Okinawa Trough Ryukyu arc system

Mineral chemistry, major- and trace-elements, and Sr-Nd isotopic compositio ns of high magnesian andesites and basalt from the central and southern Ryu kyu are, Japan, provide insight into the nature of the mantle wedge and ref lect the magmatic evolution of the Okinawa Trough back-are basin. The high- Mg andesites and basalts (similar to 6 Ma) from the central Ryukyu islands exhibit enrichment of large-ion-lithophile elements (LILE) relative to high -field-strength elements (HFSE), typical of subduction-related lavas. These rocks are distinct in having low Sr-87/Sr-86 (0.7033-0.7035) and high Nd-1 43/Nd-144 (0.5129-0.5130) relative to the contemporaneously erupted calc-al kaline rocks in the region and the Okinawa Trough basalts. High-Mg andesite s from Central Ryukyu are similar to adakitic high-Mg andesites, except for their low La/Yb (similar to 6) compared to that of adakites (> 20). The co mbination of passive asthenospheric upwelling, induced by mid to late Mioce ne rifting of the Okinawa Trough, and a thermal rebound due to a similar to 10-6 Ma subduction hiatus of the Philippine Sea Plate, probably resulted i n a high geothermal gradient beneath the are that eventually caused partial melting of the subducted slab. Subsequent interaction of the slab-melts wi th the overlying mantle wedge produced adakite-like high-Mg andesites at sh allow level (<15 kb). The high-Mg basalt actually has transitional chemical characteristics between the high-Mg andesites and other calc-alkaline rock s. This basaltic magma may be generated by further interactions between sla b-melts and the mantle wedge or, more likely, by mixing of high-Mg andesiti c magma and wedge-derived primitive magma. In contrast, high-Mg basaltic an desites(similar to 13 Ma) from the southern Ryukyu are are clearly differen t from the are lavas and most high-Mg andesites. These rocks bear strong ge ochemical and isotopic similarities to those of Ocean-Island Basalts (OIB) with a subtle are signature, although their occurrence is currently on the fore-are rather than on the back-are. This magmatism can be linked to Mioce ne intraplate volcanism that occurred around the Fujian-Taiwan region. This strongly suggests that the western part of southern Ryukyus was not a subd uction zone in the middle Miocene and that an are-trench system was establi shed after collision of the Luzon Are with the Asian continent near Taiwan, accompanied by NW subduction of the Philippine Sea Plate. (C) 1999 Elsevie r Science B.V. All rights reserved.