Magmatism at the onset of back-arc basin spreading in the Okinawa Trough

Abstract

The Okinawa Trough is a back-arc basin behind the Ryukyu Arc in the western Pacific. Near its southwestern end is Kueishantao located, an volcanic islet offshore northeastern Taiwan. Kueishantao is the last volcanic center at the southernmost end of the Okinawa Trough's spreading axis. We report the isotopic (Nd, Sr, O) and chemical compositions of fresh andesites from Kueishantao. These rocks reveal extremely low Nd isotopic values ( ϵ Nd = − 1.9 to − 5.2), very high Sr ratios ( 87Sr 86Sr > 0.705 ) and high δ 18O (between 7 and 8). Such a strong continental signature on Kueishantao can be explained by crustal contamination, most likely the magma resulting from a MORB-type magma assimilation with about 30% local continental crust materials (CC) and/or the thick overlying sediments during the onset of the “rifting” stage of the active back-arc basin. Basalts and dacites, which were dredged from the spreading axis in the central part of the Okinawa Trough (Iheya Deep and Aguni Deep, I&A), exhibit high ϵ Nd values (2.3–4.7) and low δ 18O values (< 6.6) (Honma et al., (1991)). This implies that less than 10% of continental crustal materials (CC) were involved in the magma generation. Based on the trace-element ratios ( Nb Zr vs. Ba Zr ) of I&A basaltic rocks, it suggests that the formation of I&A magma needs not only two components (MORB-type magma and CC) but also an island-arc basalt (IAB) component. Since the spreading of the Okinawa Trough is believed to have propagated southwestwards in the last 2 Ma, we suggest that the magma from the southern tip segment (Kueishantao) may result from the onset of the “rifting” stage, while the magma from the central segment (I&A region) may be generated in the more mature segment stepping in “drifting” stage in the back-arc basin.