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 crustderived 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.
