The present study examines the petrology and geochemistry of the Early Paleozoic Motai serpentinites, the South Kitakami Belt, northeast Japan, to reveal the subduction processes and tectonics in the convergent margin of the Early Paleozoic proto‐East Asian continent. Protoliths of the serpentinites are estimated to be harzburgite to dunite based on the observed amounts of bastite (orthopyroxene pseudomorph). Relic chromian spinel Cr# [=Cr/(Cr + Al)] increases with decreasing amount of bastite. The compositional range of chromian spinel is similar to that found in the Mariana forearc serpentinites. This fact suggests that the protoliths of the serpentinites are depleted mantle peridotites developed beneath the forearc regions of a subduction zone. The Motai serpentinites are divided into two types, namely, Types 1 and 2 serpentinites; the former are characterized by fine‐grained antigorite and lack of olivine, and the latter have coarse‐grained antigorite and inclusion‐rich olivine. Ca‐amphibole occurs as isolated crystals or vein‐like aggregates in the Type 1 serpentinites and as needle‐shaped minerals in the Type 2 serpentinites. Ca‐amphibole of the Type 1 serpentinites is more enriched in LILEs and LREEs, suggesting the influence of hydrous fluids derived from slabs. By contrast, the mineral assemblage, mineral chemistry, and field distribution of the Type 2 serpentinites reflect the thermal effect of contact metamorphism by Cretaceous granite. The Ca‐amphibole of the Type 1 serpentinites is different from that of the Hayachine–Miyamori Ophiolite in terms of origin; the latter was formed by the infiltration of melts produced in an Early Paleozoic arc–backarc system. Chemical characteristics of the Ca‐amphibole in the ultramafic rocks in the South Kitakami Belt reflect the tectonics of an Early Paleozoic mantle wedge, and the formation of the Motai metamorphic rocks in the forearc region of the Hayachine–Miyamori subduction zone system, which occurred at the Early Paleozoic proto‐East Asian continental margin.

中文翻译：

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从日本东北南北上带的Motai蛇纹岩推导的早古生代前臂地幔中的板状流体交代作用

本研究研究了日本东北部的南北上带的早古生代Motai蛇纹岩的岩石学和地球化学，以揭示早古生代原始东亚大洲会聚边缘的俯冲过程和构造。蛇纹岩的原生石根据观察到的钟乳石（邻位or假晶）的量估计为Harzburgite到Dunnite。铬铁矿尖晶石Cr＃[= Cr /（Cr + Al）]随着铁矿的减少而增加。铬尖晶石的组成范围与马里亚纳前臂蛇纹岩中的组成范围相似。这一事实表明，蛇纹岩的原石是俯冲带前缘区域下方发育的贫化地幔橄榄岩。摩泰蛇纹岩分为两种，即1型和2型蛇纹岩。前者的特征是细粒状的蛇纹石和缺乏橄榄石，而后者的特征是粗粒状的蛇纹石和富含包裹体的橄榄石。Ca闪石在1型蛇纹岩中以孤立的晶体或脉状聚集体形式存在，在2型蛇纹岩中以针状矿物形式存在。1型蛇纹岩的Ca-amphibole在LILE和LREE中含量更高，这表明从平板中提取的含水流体的影响。相比之下，2型蛇纹岩的矿物组合，矿物化学和场分布反映了白垩纪花岗岩接触变质作用的热效应。1型蛇纹岩的钙闪石在来源方面与Hayachine-Miyamori蛇绿岩的钙闪石不同。后者是由早期古生代弧-弧后系统中产生的熔体的渗透形成的。