High‐potassium volcanic rocks from the Okinawa Trough: Implications for a cryptic potassium‐rich and DUPAL‐like source

Abstract

Major element, trace element, and Sr–Nd–Pb isotopic compositions of high‐potassium calc‐alkaline rhyolites (the T6 rhyolites) collected from the western slope of the middle Okinawa Trough (MOT) are reported in this article. These high‐potassium rhyolites have similar major and trace elemental compositions as Type 2 rhyolites from the MOT, except that the former contain higher concentrations of K2O, Th, U, and Rb and lower concentrations of Sr and Eu. Experimental data indicate that the T6 rhyolites cannot be sourced from low‐ to mid‐potassium MOT rocks by either fractional crystallization or partial melting processes but are instead derived from a cryptic potassium‐rich source beneath the Okinawa Trough (OT). Considering that high‐potassium volcanic rocks are distributed throughout the East China Sea Shelf, the Ryukyu Arc, and Kyushu Island, the high‐potassium OT rocks could represent an important connection between the OT and its adjacent regions. Compared to volcanic rocks from the MOT axial zone, the T6 rhyolites have more depleted Sr–Nd isotopic compositions (87Sr/86Sr = 0.7035–0.7036, 143Nd/144Nd = 0.51291–0.51297) but much more enriched compositions of 207Pb (207Pb/204Pb = 15.60–15.63) and 208Pb (208Pb/204Pb = 38.45–38.57), thus recording a significant DUPAL‐like signature. These isotopic anomalies cannot be explained by the incorporation of subducting sediments or crustal contamination but instead imply the presence of a DUPAL‐like mantle source beneath the OT. The decrease of the DUPAL‐like anomaly from the T6 rhyolites to the MOT axial zone volcanic rocks is consistent with the injection of an normal mid‐ocean ridge basalt (N‐MORB) mantle source, such as asthenospheric material, into pre‐existing DUPAL‐like mantle during back‐arc extension.

Geochemistry of silicate melt inclusions in middle and southern Okinawa Trough rocks: Implications for petrogenesis and variable subducted sediment component injection

“…The boundaries of different series were drawn according to Roberts and Clemens (). High‐K field of samples drawn from the Northern Taiwan Volcanic Zone (NTVZ), middle Ryukyu, and middle Okinawa Trough (MOT) pumice are also shown as a point of comparison (Shinjo et al, ; Shinjo & Kato, ; Wang et al, ; Wang et al, ; Y. X. Zhang, Z. G. Zeng, et al, ) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…In terms of regional geological settings of the OT, K‐rich volcanic rocks have been found in the Ryukyu Arc (Shinjo, Hasenaka, & Fujimaki, ; Shinjo & Kato, ) and in the NTVZ (Wang et al, ; Wang et al, ) (Figure b). Recently, Y. X. Zhang, Z. G. Zeng et al () also noted that a K‐rich source may have been involved in the production of high‐K pumice in the MOT (Figure b). These observations allow us to speculate that a K‐rich mantle source is widespread in the OT but not yet widely recognized.…”

“…SMIs, as samples of primitive liquid, can represent snapshots of magmatic conditions and can probe the genesis and evolution of andesitic magma (Roedder, ). Basaltic magma in MOT is characterized by lower K content levels (Figure b) and melting experiments conducted on low‐ to medium‐K material as starting materials can only generate low‐ to medium‐K melts (Roberts & Clemens, ; Y. X. Zhang, Z. G. Zeng et al, ). Fractional crystallization experiments suggest that medium‐K basaltic magma can evolve into only medium‐K melts (Blatter, Sisson, & Hankins, ; Nandedkar, Ulmer, & Müntener, ).…”

Key parameters of the structure and evolution of the Okinawa Trough: Modelling results constrained by heat flow observations

“…To a certain extent, this process decreases the heat flow in the SOT. Meanwhile, intense magmatic activity has not occurred yet in the northern Okinawa Trough because it is at a less mature extensional stage (Zhang et al, ).…”

“…The Iheya Ridge, consisting of a series of parallel ridges oriented in the ENE direction and associated with linear magnetic anomalies, is characterized by dyke intrusion (Sibuet et al, 1987). The Okinawa Trough is presently undergoing rifting and is characterized by numerous normal faults, crustal extension, and magmatic intrusion (Lee et al, 1980;Zhang et al, 2017). The oldest volcanic activity along the Ryukyu Arc consists of two phases: Middle Miocene (17.7-13.1 Ma) and Late Miocene (6 Ma ;Shinjo, Ban, Saito, & Kato, 2008).…”