Petrogenesis of highly fractionated rhyolites in the southwestern Okinawa Trough: Constraints from whole-rock geochemistry data and Sr–Nd–Pb–O isotopes

In the southwestern Okinawa Trough, a cluster of approximate 70 submarine volcanoes is located at the cross back-arc volcanic trail (CBVT), with the eruptive products dominated by rhyolites. New whole-rock major and trace element and Sr–Nd–Pb–O isotopic data and previously available geochemistry data of volcanic rocks from this region are presented to determine the petrogenesis of these rhyolites. The rhyolites have high SiO ₂ (>70.01 wt.%), K ₂ O + Na ₂ O (6.71–7.16 wt.%) contents and differentiation index (DI) values (DI > 93), and are metaluminous to weakly peraluminous (A/CNK = 0.99–1.11) and medium-K calc-alkaline in composition. The relatively low 10 ⁴ *Ga/Al (2.06–2.17) and FeO*/MgO ratios (4.50–6.62), as well as Zr (91–174 ppm) and Zr + Ce + Nb + Y (175–271) contents suggest that the rhyolites, which contain hornblende, are generally similar to those of typical highly fractionated I-type granitoids. The rhyolites show arc-related features such as enrichment in LILEs (Rb, Ba, K), Th, U, Pb, and LREEs ((La/Yb) _N  = 6.21–6.74) and strong depletion in HFSEs (Nb, Ta, P, Ti) with both nearly identical and more enriched isotopic compositions, including εNd (from −3.2 to −3.5), ⁸⁷ Sr/ ⁸⁶ Sr ratios (0.705552 to 0.705713), δ ¹⁸ O values (+8.1‰ to +8.6‰), and Pb isotopic compositions ( ²⁰⁶ Pb/ ²⁰⁴ Pb = 18.568–18.589, ²⁰⁷ Pb/ ²⁰⁴ Pb = 15.641–15.656, and ²⁰⁸ Pb/ ²⁰⁴ Pb = 38.848–38.904). Based on these characteristics, we propose that subduction-related basalt magma and crust-derived magma mixed and homogenized within a shallow magma chamber with a homogeneous “base-level” isotopic signature of >0·7055 ( ⁸⁷ Sr/ ⁸⁶ Sr) and > −3 (εNd). This magma would have then experienced the extensive fractional crystallization of minerals, including plagioclase, hornblende, apatite, sphene, and Fe–Ti oxides, to form the CBVT rhyolites. Our geochemical interpretation of the existence of a long-lived, convecting magma chamber beneath the CBVT is also consistent with seismic tomography results.