A complex magmatic system beneath the middle and northern Okinawa Trough: evidence from pyroxene characteristics

Abstract

The Okinawa Trough is an initial marginal back‐arc basin. In this paper, we present a comparative study of pyroxenes in white and black pumice from the Okinawa Trough to explore the magmatic evolution of the region and the characteristics of the magmatic system. The results show two species of pyroxenes in the white pumice (clinopyroxene and orthopyroxene) but only one species in the black pumice (orthopyroxene). Many isomorphic replacements among the cations occurred during clinopyroxene crystallization. Clinopyroxene‐melt thermobarometry yielded a temperature of 847–988°C and a pressure of 2.73–7.42 kbar, which vary considerably and correspond to a maximum depth of approximately 24 km. All of the orthopyroxenes in both the white and black pumice crystallized at similar pressures ranging from 0.67 to 4.59 kbar, which correspond to a maximum depth of approximately 15 km. The clinopyroxenes crystallized mainly in the lower crust, whereas the orthopyroxenes formed at the boundary depth between the upper crust and the lower crust. The similarities in the pyroxene chemical compositions and the whole‐rock characteristics between the two types of pumice indicate that both types of pumice are linked to the same magmatic system at depth. The pyroxenes crystallized from genetically related melts within a magmatic system that had undergone various degrees of differentiation. The physicochemical characteristics of the pyroxenes show that the magma chamber module changed during the crystallization process from clinopyroxenes to orthopyroxenes. This change most likely resulted from local characteristics of the near‐surface structure. These observations support the model of a ‘two‐layered magma chamber’ in the northern Okinawa Trough. Copyright © 2016 John Wiley & Sons, Ltd.

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

“…This result agrees with the hypothesis that a shallow magma chamber exists approximately 7-15 km beneath the seafloor Lai et al, 2016). Therefore, to a certain extent, this thermal structure supports the viewpoint of a two-layer magma chamber (trapezoid with red dashed lines in Figure 13; Liao et al, 2016;Zhai et al, 1997). The deep-seated magma chamber is located at a depth of 24-30 km, and the shallower one is~10 km.…”

“…At a depth of 10 km under the place of intrusion, the isothermal curves reach 890°C, which is consistent with the result of clinopyroxene‐melt thermobarometry. The temperature of clinopyroxene crystallization ranges from 800 to 881°C, and the depth ranges from 6 to 12 km (Liao, Huang, & Hu, ). This result agrees with the hypothesis that a shallow magma chamber exists approximately 7–15 km beneath the seafloor (Guo et al, ; Lai et al, ).…”

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

“…Solid arrows indicate plate motion vectors of the Philippine Sea Plate relative to the Eurasian Plate (Seno, Stein, & Gripp, 1993) [Colour figure can be viewed at wileyonlinelibrary.com] The OT has experienced strong levels of magmatic activity, which have generated abundant volcanic rocks (Gao et al, 2009). The NOT is dominated by rhyolites (e.g., Huang et al, 2006;Liao, Huang, & Hu, 2016;Zeng et al, 2010), and the SOT is dominated by basaltic rocks (e.g., Ishizuka et al, 1990;Li et al, 1997;Shinjo, Chung, Kato, & Kimura, 1999). In comparison, the compositions of the MOT volcanic rocks show bimodal characteristics, forming a basaltic-rhyolitic suite with a relatively small volume of intermediate rocks (e.g., Guo, Zhai, Yu, Cai, & Zhang, 2016;Huang et al, 2006;Li et al, 1997;Shinjo & Kato, 2000).…”

Petrology and mineralogy of pumice from the Iheya North Knoll, Okinawa Trough: Implications for the differentiation of crystal‐poor and volatile‐rich melts in the magma chamber

“…Over the past several decades, many investigations have attempted to reveal the petrogenesis of the OT rhyolites (e.g., Guo, Zhai, Yu, Cai, & Zhang, 2016;Honma et al, 1991;Huang et al, 2006;Shinjo & Kato, 2000;Zhai et al, 2017;, but only a few have focused on the magmatic evolution processes in the magma chamber beneath the OT. Some researchers have proposed a doublelayered magma chamber model, in which two magma chambers exist in the lower crust and upper crust of the OT, respectively (Liao, Huang, & Hu, 2016;Shinjo & Kato, 2000;Zhai, Chen, Wang, & Gan, 1997).…”