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

Abstract

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.

A Preliminary Framework for Magmatism in Modern Continental Back‐Arc Basins and Its Application to the Triassic‐Jurassic Tectonic Evolution of the Caucasus

“…The 41 samples from the Okinawa Trough are all subalkaline and consist of basalt and basaltic andesite (Figure 3; Guo, Zhai, Yu, Wang, et al., 2018; Guo, Zhai, Yu, Zeng, et al., 2018; Li et al., 2018; Shinjo & Kato, 2000; Shinjo et al., 1999; Shu et al., 2019; Zeng et al., 2010). These samples plot dominantly near the Th‐3Tb join on the Th‐3Tb‐2Ta diagram, exhibit slight to moderate enrichment in LREE and other highly incompatible elements, and show negative Nb‐Ta anomalies, generally rendering them geochemically indistinguishable from arc basalts.…”

“…Samples from the southern Okinawa Trough tend to have more pronounced negative Nb‐Ta anomalies and higher Th compared to rocks from the central Okinawa Trough, causing these samples to plot closer to the Th vertex and the Th‐3Tb join. The single basaltic andesite in the data compilation from the northern Okinawa Trough also exhibits LREE enrichment and a negative Nb‐Ta anomaly (Shu et al., 2019). Two basalt samples from the East China Sea shelf adjacent to the northern Okinawa Trough exhibit strong enrichment in LREE and other incompatible elements with no negative Nb‐Ta anomalies (Figures 1b, 3f, and 3g; Zeng et al., 2010).…”

Amphibole perspective to unravel the rhyolite as a potential fertile source for the Yonaguni Knoll IV hydrothermal system in the southwestern Okinawa Trough

“…Accordingly, the OT has extremely high heat flow values (Kong et al, ; Yamano, Uyeda, Foucher, & Sibuet, ; Zhang, Luan, Zeng, & Liu, ) and has active volcanism and hydrothermal activities. Many submarine volcanoes have been recognized in the OT (Sibuet et al, ), with volcanic eruptions ranging compositionally from tholeiitic basalt to calc‐alkaline rhyolite, which were generated from a depleted mantle source and affected by subduction components and/or upper crustal components (e.g., Chen et al, ; Guo et al, , ; Guo, Zhai, Yu, Cai, & Zhang, ; X. H. Li et al, , ; Shinjo & Kato, ; Shinjo, Chung, Kato, & Kimura, ; Shu et al, ; Shu, Zeng, Yin, Wang, & Chen, ; Yu, Zhai, Guo, Zhou, & Zong, ; Zhai et al, ; Zhang et al, ; Zhang, Zhai, Yu, Guo, & Wang, ; Zhao, Luo, Lai, Tian, & Xu, ). Numerous seafloor hydrothermal active sites and massive sulfide deposits have also been discovered in the OT (e.g., Glasby & Notsu, ; Halbach, Pracejus, & Marten, ; Ishibashi & Urabe, ; Kawagucci et al, ; Suzuki et al, ).…”