Geochemical and Sr―Nd isotopic constraints on the origin of volcanic rocks from the northern Okinawa Trough

Okinawa Trough is an active back-arc basin behind the Ryukyu Arc in the western Pacific. We report the geochemical and isotopic (Sr, Nd) compositions of volcanic rocks, which are recovered as lavas and pumices, dredged from northern Okinawa Trough. The volcanic rocks are composed of basaltic andesite, andesite, dacite, and trachydacite. Both intermediate and felsic rocks are characterized by enrichments in large-ion lithophile elements (LILEs) and depletion in high-field strength elements (HFSEs). Three groups of volcanic rocks have been identified. Andesite D5-1 and trachydacite D10 display identical Sr and Nd isotopic ratios, together with strongly enriched in light-REE and incompatible elements, compared to previously published basalts from middle Okinawa Trough (MOT). Andesite D5-1 and trachydacite D10 are derived from basaltic magma modified by fractional crystallization. Andesites D5-2, D5-3, and D5-4 exhibit lower ⁸⁷Sr/⁸⁶Sr and higher ¹⁴³Nd/¹⁴⁴Nd ratios than MOT basalts, transitional REE patterns and trace element ratios between adakite rocks and MOT basalts. Features of these samples can be interpreted by further mixing of the interaction magma of mantle wedge and subducted-slab-melt, with evolved basaltic magma. Dacite D22, occurring as pumice, shows undistinguishable trace element, including REE, and Sr and Nd isotopic compositions with rhyolites from the southern Ryukyu volcanic front, which indicates that this pumice was not emplaced in situ and most likely drifted from southern Ryukyu volcanic front by Kuroshio Current.