New insights into the origin of the felsic volcanic rocks in the middle Okinawa Trough

The Okinawa Trough (OT) is a nascent back arc basin developing in the eastern margin of the Eurasian Plate. The middle segment of the OT (MOT) is featured by bimodal volcanism with dominant basalts and rhyolites and scarce intermediate volcanic rocks. The MOT rhyolites are divided into two types: type 1 rhyolites have lower heavy rare earth element (HREE) contents while type 2 rhyolites feature higher HREE contents and have slightly more enriched Sr-Nd isotopic ratios than type 1 rhyolites. The MOT rhyolites share similar Sr-Nd isotopic compositions with the MOT basalts and thus they have been typically considered as products of crystallization differentiation of basaltic magma. However, the evidence is insufficient. This paper compared chemical trends of volcanic rocks from the MOT with fractional crystallization simulation models and experimental results and utilized trace element modeling (e.g., Th vs. Rb, Rb/Nd vs. Rb, Rb vs. Rb/V, 1/V vs. Rb/V) combined with quantitative calculations to re-examine the magmatic processes in generating the rhyolites. The trace element discriminant models show that the andesites indicate a fractional crystallization trend, type 1 rhyolites present a partial melting trend, and type 2 rhyolites define a mixing trend. Further, by establishing Rayleigh fractionation and batch melting models, we found that the andesites originated by fractional crystallization of basaltic magma while the rhyolites were the products of remelting of andesites. Type 2 rhyolites have higher Dy/Yb and Nb/Ta ratios than type 1 rhyolites, implying that the former might be influenced by a greater contribution of amphibole. Quantitative models suggest that type 1 rhyolites could have formed via the remelting of andesites when amphiboles remain in the source, whereas type 2 rhyolites could have formed without residual amphiboles. Quasi-linear correlations between some major oxides, trace element ratios and Sr-Nd isotopic ratios for type 2 rhyolites suggest that these rhyolites might have been contaminated by crustal rocks or melts with compositions similar to those of the Kueishantao (KST) andesites or the Middle Miocene upper crustal rocks of SW Japan. Therefore, isotopically enriched crustal materials likely remain in the OT crust.