Back‐arc magma processes in the Okinawa Trough: new insights from textural and compositional variations of plagioclase in basalts

Abstract

The evolution of magma during ascent through the crust in the region of the Okinawa Trough is recorded by its mineralogy. In this study, we utilized the characteristics of plagioclase minerals in basaltic magma to deduce magma generation processes, by examining (via back‐scattered electron images) the compositional features of plagioclase phenocrysts in basalts dredged from the middle and southern portions of the Okinawa Trough, which provided a detailed account of the evolution and crystallization of the magma. The crystallization and evolutionary histories of magmas from the middle Okinawa Trough are distinct from those in the southern Okinawa Trough. In the middle Okinawa Trough, we identified seven textural varieties of plagioclase: homogeneous‐core, fine oscillatory‐zoned, core‐sieved, mantle‐sieved, patchy‐core, glomerocrystic and microlitic. Anorthite (An) contents in the cores of plagioclase crystals are 80–87%, while concentrations of Fe, Mg and Sr vary more strongly, especially in patchy‐core plagioclase. The homogeneous‐core, fine oscillatory‐zoned and glomerocrystic plagioclase crystallized in deep‐seated chambers. In contrast, the core‐sieved, mantle‐sieved and patchy‐core textures developed when magmas ascended slowly and were injected into shallow chambers, during which plagioclase was resorbed and dissolved due to decompression. In the southern Okinawa Trough, homogeneous‐core plagioclase crystallized when magma ascended rapidly through deep conduits and was injected into a shallow chamber; under these conditions, only homogeneous‐core and core‐sieved plagioclase phenocrysts crystallized in the shallow chamber. The core of homogeneous‐core plagioclase (An86–An93) was partially resorbed and contaminated by crustal material. Degassing and undercooling promoted the rapid crystallization of microlitic plagioclase, especially in the outermost rims, during and after the extraction of magma from shallow chambers, in both the middle and southern Okinawa Trough. Copyright © 2016 John Wiley & Sons, Ltd.

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

“…In addition, the orthopyroxene‐calculated P‐T results show there exists a magma chamber in the MOT, approximately 10 km. Combining with the characteristics of bi‐modal magmatism in the MOT, the implications of basalt‐hosted plagioclase (Lai et al, ), the implications of helium isotopes in volcanic rocks (Yu et al, ) and the implications of the influence of subduction components on magma composition (Zhao et al, ); we infer a two‐layered magma chamber may also exist in the MOT, and the depth of 10 km is shallow magma chamber.…”

“…Hot basaltic magma recharge likely occurred during the crystallization of the orthopyroxenes. This phenomenon was observed in plagioclase phenocrysts in acidic pumice with corroded cores (Zhai, ) and in plagioclase phenocrysts in basalts from the middle Okinawa Trough (Lai et al, ).…”

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

“…X. Zhang, Zeng, Qi, et al, 2017), which are generally considered to represent the products of relatively vigorous eruptions of evolved volatile-rich magma (Fiske, Naka, Iizasa, Yuasa, & Klaus, 2001;Kano, 2003;Klug, Cashman, & Bacon, 2002). These features imply that abundant subvolcanic magma chambers, which provide space for the storage and evolution of magma (e.g., Lai et al, 2016;Shinjo & Kato, 2000) and/or heat sources for hydrothermal systems (e.g., Hou, Ai, Qu, Zhang, & Tang, 1999;Tsuji et al, 2012), probably exist in the upper crust of the OT. 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.…”

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

“…Volcanic rocks in the OT can be used to understand the dynamic processes of the early stages of back‐arc spreading (Zhao, Zhai, Wang, & Yu, ). To date, many studies of OT volcanism have been conducted (e.g., Chung, Wang, Shinjo, Lee, & Chen, ; Honma et al, ; Ishizuka, Kawanobe, & Sakai, ; Z. Q. Lai et al, ; Li, Yang, & Wang, ; Liao, Huang, & Hu, ; Ma, Wang, & Jin, ; Meng, Du, Wu, & Long, ; Shinjo, Chung, Kato, & Kimura, ; Yu, Zhai, Guo, Zhou, & Zong, ; Zeng et al, ; Zhai & Gan, ), enhancing our understanding of the magmatic origin and evolution of the back‐arc basin. The OT volcanic rocks mostly fall within the low‐ to mid‐potassium series and exhibit bimodal distribution, in which basalts and rhyolites are dominant (Shinjo & Kato, ; Yan & Shi, ).…”