Variability of protoliths and pressure-temperature conditions of amphibolites from the Ohmachi Seamount (Izu-Bonin-Mariana arc): Evidence of a fossil subduction channel in modern intra-oceanic arcs.
Variability of protoliths and pressure-temperature conditions of amphibolites from the Ohmachi Seamount (Izu-Bonin-Mariana arc): Evidence of a fossil subduction channel in modern intra-oceanic arcs.
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大町海山（伊豆-小笠原-马里亚纳弧）原岩和角闪岩压力-温度条件的变化：现代洋内弧中化石俯冲通道的证据。
DOI：
10.1007/s00710-020-00705-z
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发表时间：
2020
影响因子：
1.8
通讯作者：
T.
中科院分区：
文献类型：
--
作者：
Imayama*;T.;Ueda;H.;Usuki;T.;Minami;M.;Asahara;Y.;and Nagahashi;T.
在伊豆-小笠原-马里亚纳岛弧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斜长角闪岩：绿帘石-钠长石斜长角闪岩、绿帘石-石榴石斜长角闪岩和石榴石-绿闪长角闪岩。全岩化学和Nd、Sr同位素特征表明，绿帘石钠长角闪岩具有洋中脊玄武岩的地球化学特征。绿帘石-石榴角闪岩形成于岛弧构造环境，含残馀蓝闪石，在角闪岩相重结晶（MD 2）之前，曾经历蓝片岩相变质（MD 1），形成钠长石和钠长石。石榴子石Rb-Sr全岩等时线（289 ± 58 Ma）可代表蓝片岩相变质作用的时间。而Rb-Sr-Barroisite-paragonite等时线（67 ± 13 Ma）则是与全岩Sr同位素组成不平衡的错误或混合年龄，没有地质意义。石榴子石-绿辉石角闪岩形成于大洋中脊环境，并保留了榴辉岩相共生（MR 2a）的残余物，包括绿辉石、蓝晶石、绿辉石和镁角闪石，随后经历了以阳起石为代表的角闪岩相变质（MR 2b），随后经历了导致形成埃丹石和闪长角闪岩的MR 3事件。Rb-Sr矿物等时线（244 ± 28 Ma）由锆石+镁角闪石/阳起石+浅成钙长石/角闪石定义，代表岩石在角闪岩相（MR 2b）重结晶或冷却至~ 500 °C（角闪岩的闭合温度）（MR 3）的时间，表明榴辉岩相变质作用发生在约500 °C之前。244 Ma.二叠-三叠纪等时线年龄（289-244 Ma）表明，大町海山基底的高压变质作用发生在古俯冲带，而不是沿着现代俯冲带。大町海山的蛇纹混杂岩中出露的斜长角闪岩的不同原岩和P-T条件代表了从俯冲杂岩的不同深度挖掘到地球表面的岩石的混合物，这被解释为保存在现代洋内弧中的化石俯冲通道的证据。
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 (MD1) prior to amphibolite-facies recrystallization (MD2) 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 (MR2a), including zoisite, kyanite, omphacite, and magnesiohornblende, followed by amphibolite-facies metamorphism (MR2b) represented by actinolite and then by a later MR3 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 (MR2b) or cooled (MR3) 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.