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

Abstract

Mineral chemistry, major- and trace-elements, and Sr–Nd isotopic compositions of high magnesian andesites and basalt from the central and southern Ryukyu arc, Japan, provide insight into the nature of the mantle wedge and reflect the magmatic evolution of the Okinawa Trough back-arc basin. The high-Mg andesites and basalts (∼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 87 Sr/ 86 Sr (0.7033–0.7035) and high 143 Nd/ 144 Nd (0.5129–0.5130) relative to the contemporaneously erupted calc-alkaline rocks in the region and the Okinawa Trough basalts. High-Mg andesites from Central Ryukyu are similar to adakitic high-Mg andesites, except for their low La/Yb (∼6) compared to that of adakites (>20). The combination of passive asthenospheric upwelling, induced by mid to late Miocene rifting of the Okinawa Trough, and a thermal rebound due to a ∼10–6 Ma subduction hiatus of the Philippine Sea Plate, probably resulted in a high geothermal gradient beneath the arc that eventually caused partial melting of the subducted slab. Subsequent interaction of the slab-melts with the overlying mantle wedge produced adakite-like high-Mg andesites at shallow level (<15 kb). The high-Mg basalt actually has transitional chemical characteristics between the high-Mg andesites and other calc-alkaline rocks. This basaltic magma may be generated by further interactions between slab-melts and the mantle wedge or, more likely, by mixing of high-Mg andesitic magma and wedge-derived primitive magma. In contrast, high-Mg basaltic andesites (∼13 Ma) from the southern Ryukyu arc are clearly different from the arc lavas and most high-Mg andesites. These rocks bear strong geochemical and isotopic similarities to those of Ocean–Island Basalts (OIB) with a subtle arc signature, although their occurrence is currently on the fore-arc rather than on the back-arc. This magmatism can be linked to Miocene intraplate volcanism that occurred around the Fujian–Taiwan region. This strongly suggests that the western part of southern Ryukyus was not a subduction zone in the middle Miocene and that an arc–trench system was established after collision of the Luzon Arc with the Asian continent near Taiwan, accompanied by NW subduction of the Philippine Sea Plate.