The Iheya Graben is a back-arc spreading centre in the middle part of the Okinawa Trough. It is also located in the centre of an anomalous volcanic zone (volcanic arc migration phenomenon, or VAMP) and is characterized by bimodal volcanism, unusually high heat flow and active hydrothermal circulation. The subvolcanic magma plumbing system and the magmatic processes related to the formation of rare erupted intermediate lavas in this area remain uncertain. In this study, we conducted systematic mineralogical analyses (in situ major element, trace element and Sr isotopes) and whole rock geochemical analyses (major element, trace element and Sr–Nd isotopes) on an andesite (T5-2; type C andesite) and a rhyolite (C11; type 2 rhyolite), and present evidence for magma mixing in the origins of these lavas. Andesite T5-2 contains a mafic mineral assemblage and a silicic mineral assemblage, which are derived from a basaltic melt and a type 2 rhyolitic melt, respectively. A 4:6 mixture of basalt and type 2 rhyolite from the Iheya Graben reproduces the whole-rock major element, trace element, and Sr–Nd isotope compositions of T5-2. Rhyolite C11 contains a group of disequilibrium minerals that crystallized from a less evolved rhyolitic melt with relatively more enriched Sr–Nd isotope compositions, suggesting mixing of this melt with a more evolved and isotopically more depleted rhyolitic melt. This mixing process could produce a series of rhyolitic melts with a negative correlation between SiO₂ concentrations and ⁸⁷Sr/⁸⁶Sr ratios (or a positive correlation for ¹⁴³Nd/¹⁴⁴Nd ratios), which are recorded by the whole group of type 2 rhyolites. The results from mineral-based thermobarometers suggest that the premixing storage temperatures of the basaltic and rhyolitic melts are ∼1100 °C and 870–900 °C, respectively. The hybrid andesitic melt has temperatures of ∼950 to ∼980 °C. The magma storage pressures are not well constrained, ranging from ∼400 MPa to ∼100 MPa. We show that magma mixing plays a significant role in the origins of diverse volcanism in the middle Okinawa Trough; more specifically, two of the three types of andesites (types B and C) and one of the two types of rhyolites (type 2) are associated with magma mixing. We thus propose a complex magma plumbing system with multichamber magma storage and frequent magma mixing beneath the Iheya Graben.

中文翻译：

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冲绳海槽活跃的反弧扩散中心伊希亚格拉本（Iheya Graben）下岩浆混合的矿物学约束

Iheya Graben是冲绳海槽中部的弧后扩散中心。它也位于异常火山带（火山弧迁移现象或VAMP）的中心，其特征是双峰火山作用，异常高的热流和活跃的热液循环。该地区的火山下岩浆管道系统和与形成罕见的爆发性中间熔岩有关的岩浆过程仍然不确定。在这项研究中，我们进行了系统的矿物学分析（原位在安山岩（T5-2； C型安山岩）和流纹岩（C11； 2型流纹岩）上的主要元素，微量元素和Sr同位素）以及整个岩石地球化学分析（主要元素，微量元素和Sr-Nd同位素），以及目前在这些熔岩的起源中存在岩浆混合的证据。安山岩T5-2包含镁铁质矿物组合和硅质矿物组合，它们分别来自玄武质熔体和2型流纹岩熔体。来自Iheya Graben的玄武岩和2型流纹岩的4：6混合物可再现T5-2的全岩石主要元素，微量元素和Sr-Nd同位素组成。流纹岩C11包含一组不平衡矿物，这些矿物从较少演化的流纹岩熔体中结晶，且具有相对更丰富的Sr-Nd同位素组成，这表明该熔体与更演化且同位素更贫的流纹岩熔体混合。这种混合过程可能会产生一系列SiO之间呈负相关的流纹熔体_2个浓度和⁸⁷ Sr / ⁸⁶ Sr比（或¹⁴³ Nd / ¹⁴⁴呈正相关Nd比率），由整个2型流纹岩组记录。基于矿物的温度晴雨表的结果表明，玄武岩和流纹岩熔体的预混合储存温度分别为〜1100°C和870–900°C。杂化的安山岩熔体的温度约为950至980°C。岩浆的储藏压力没有得到很好的约束，范围从〜400 MPa到〜100 MPa。我们显示出岩浆混合在冲绳海槽中部的多种火山活动中起着重要作用；更具体地说，三种类型的安山岩（B型和C型）中的两种和流纹岩的两种类型中的一种（2型）与岩浆混合有关。因此，我们提出了一个复杂的岩浆管道系统，该岩浆管道具有多室岩浆储存能力，并且在Iheya Graben下方经常发生岩浆混合。