Three types of amphibolites were found around a large serpentinite body in the basement of the Ohmachi Seamount in the Izu-Bonin-Mariana arc: epidote-albite amphibolite, epidote-garnet amphibolite, and garnet-zoisite amphibolite. The epidote-albite amphibolite shows a mid-ocean ridge basalt geochemical affinity based on whole-rock chemistry and Nd and Sr isotopes. The epidote-garnet amphibolite originated in an arc tectonic setting and contains relict glaucophane, showing blueschist-facies metamorphism (M D 1) prior to amphibolite-facies recrystallization (M D 2) that formed barroisite and paragonite. The Rb–Sr garnet-whole-rock isochron (289 ± 58 Ma) could represent the timing of blueschist-facies metamorphism. In contrast, the Rb-Sr barroisite-paragonite isochron of 67 ± 13 Ma is an errorchron or mixed age due to insufficient equilibrium with the whole-rock Sr isotope composition, with no geologic significance. The garnet-zoisite amphibolite formed in a mid-oceanic ridge setting and preserves relicts of eclogite-facies paragenesis (M R 2a), including zoisite, kyanite, omphacite, and magnesiohornblende, followed by amphibolite-facies metamorphism (M R 2b) represented by actinolite and then by a later M R 3 event that resulted in the formation of edenite and pargasite. The Rb–Sr mineral isochron (244 ± 28 Ma) defined by zoisite + magnesiohornblende/actinolite + edenite/pargasite represents the time at which the rock recrystallized in the amphibolite facies (M R 2b) or cooled (M R 3) through ~ 500 °C (the closure temperature of amphibole), indicating that eclogite-facies metamorphism occurred before ca. 244 Ma. The two Permo-Triassic isochron ages (289–244 Ma) indicate that the high-pressure metamorphism of the basement in the Ohmachi Seamount occurred by ancient subduction rather than along the modern subduction zone. The different protoliths and P–T conditions of amphibolites exposed in the serpentine mélange of the Ohmachi Seamount represent a mixture of rocks exhumed to the Earth’s surface from various depths in the subducted complex, which are interpreted as evidence for a fossil subduction channel preserved in a modern intra-oceanic arc.

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大町海山（伊豆-小宁-马里亚纳弧）角闪岩原岩和压力-温度条件的变化：现代洋内弧中化石俯冲通道的证据

在伊豆-薄宁-马里亚纳弧的大町海山基底的一个大型蛇纹岩体周围发现了三种类型的角闪岩：绿帘石-钠长石角闪石、绿帘石-石榴石角闪石和石榴石-黝帘石角闪石。基于全岩化学和 Nd 和 Sr 同位素，绿帘石-钠长石角闪岩显示出洋中脊玄武岩地球化学亲和性。绿帘石石榴石角闪岩起源于弧形构造环境，含有残余的蓝闪石，在角闪岩相重结晶 (MD 2) 之前显示出蓝片岩相变质作用 (MD 1)，形成了重晶石和帕罗长石。Rb-Sr 石榴石-全岩等时（289±58 Ma）可以代表蓝片岩相变质作用的时间。相比之下，67±13 Ma的Rb-Sr重文石-平行石等时线是由于与全岩Sr同位素组成的平衡不足而导致的错误年代或混合时代，没有地质意义。石榴石-黝帘石角闪岩形成于大洋中脊环境，保留榴辉岩-相共生的遗迹（MR 2a），包括黝帘石、蓝晶石、绿辉石和角闪石，其次是以阳起石和阳起石为代表的角闪岩相变质作用（MR 2b）然后是后来的 MR 3 事件，导致了伊甸岩和帕加石的形成。由 zoisite + magnesiohornblende /阳起石 + edenite / pargasite 定义的 Rb – Sr 矿物等时 (244 ± 28 Ma) 代表岩石在角闪岩相 (MR 2b) 中再结晶或冷却 (MR 3) 到 ~ 500 °C 的时间（闪石的闭合温度），表明榴辉岩相变质作用发生在大约 244 Ma 之前。两个二叠纪-三叠纪等时年龄（289-244 Ma）表明，大町海山基底的高压变质作用是由古代俯冲而不是沿着现代俯冲带发生的。大町海山蛇纹石混杂岩中暴露的角闪岩的不同原岩和 P-T 条件代表了从俯冲复合体中不同深度挖掘到地球表面的岩石混合物，这被解释为证据表明化石俯冲通道保存在现代洋内弧。两个二叠纪-三叠纪等时年龄（289-244 Ma）表明，大町海山基底的高压变质作用是由古代俯冲而不是沿着现代俯冲带发生的。大町海山蛇纹石混杂岩中暴露的角闪岩的不同原岩和 P-T 条件代表了从俯冲复合体中不同深度挖掘到地球表面的岩石混合物，这被解释为证据表明化石俯冲通道保存在现代洋内弧。两个二叠纪-三叠纪等时年龄（289-244 Ma）表明，大町海山基底的高压变质作用是由古代俯冲而不是沿着现代俯冲带发生的。大町海山蛇纹石混杂岩中暴露的角闪岩的不同原岩和 P-T 条件代表了从俯冲复合体中不同深度挖掘到地球表面的岩石混合物，这被解释为证据表明化石俯冲通道保存在现代洋内弧。