Evolution Processes of Ordovician-Devonian Arc System in the South-Kitakami Massif and its Relevance to the Ordovician Ophiolite Pulse
Evolution Processes of Ordovician-Devonian Arc System in the South-Kitakami Massif and its Relevance to the Ordovician Ophiolite Pulse
复制标题
北上市南地块奥陶系-泥盆系弧系演化过程及其与奥陶系蛇绿岩脉的关系
作者：
Ozawa;K.;Maekawa;H.;Shibata;K.;Asahara;Y.;Yoshikawa;M.
南北上地块是日本最古老的地质区域之一，志留纪地层为酸性火山碎屑岩，基底为奥陶纪-志留纪花岗闪长岩-英云闪长岩，表明它在志留纪已成熟到足以发育酸性火山作用。在南北上地块的北方和西缘，分布着奥陶纪弧蛇绿岩（Hayachine-Miyamori蛇绿岩）和在奥陶纪-泥盆纪期间某个时候出露的高压低温变质岩（Motai变质岩）。本文综述了南北上地块早古生代早古生代早陆-宫森蛇绿岩、莫泰变质岩等地质单元的年代学、地质学和岩石化学研究，并补充了新资料，以重建奥陶纪-泥盆纪南北上岛弧系。重建表明，在晚泥盆世之前的某个时候，会聚极性从东向西倾斜的俯冲发生了变化。早根-宫盛蛇绿岩是在东倾俯冲作用之上经过三个不同阶段形成的。两个独立阶段的覆盖板伸展诱导减压熔融与板片衍生流体的轻微参与之前和之后发生的熔融阶段下的板片衍生流体的强烈影响。第一次覆盖板块伸展发生在弧后一侧，形成弧后盆地。第二次发生在蛇绿岩折返之前，靠近弧前区域。我们假设，第二次减压熔融是由板片断裂触发的，在此之前，板片回滚引起沟槽平行楔形地幔流和非稳态流体和热输运，留下异常含水的残留地幔。早-宫蛇绿岩的形成历史表明，较弱的板块耦合作用可能为高含水地幔的折返提供了有利条件。除寒武-奥陶纪外，尚未发现与弧岩浆作用有关的高含水橄榄岩，这表明其对全球地球动力学，如地幔热状态和水循环的影响。
The South Kitakami Massif is one of the oldest geological domains in Japan having Silurian strata with acidic pyroclastic rocks and Ordovician–Silurian granodiorite–tonalite basement, suggesting that it was matured enough to develop acidic volcanisms in the Silurian period. On the northern and western margin of the South Kitakami Massif, an Ordovician arc ophiolite (Hayachine–Miyamori Ophiolite) and high‐pressure and low‐temperature metamorphic rocks (Motai metamorphic rocks) exhumed sometime in the Ordovician–Devonian periods are distributed. Chronological, geological, and petrochemical studies on the Hayachine–Miyamori Ophiolite, Motai metamorphic rocks, and other early Paleozoic geological units of the South Kitakami Massif are reviewed for reconstruction of the South Kitakami arc system during Ordovician to Devonian times with supplementary new data. The reconstruction suggests a change in the convergence polarity from eastward‐ to westward‐dipping subduction sometime before the Late Devonian period. The Hayachine–Miyamori Ophiolite was developed above the eastward‐dipping subduction through three distinctive stages. Two separate stages of overriding plate extension inducing decompressional melting with minor involvement of slab‐derived fluid occurred before and after a stage of melting under strong influence of slab‐derived fluids. The first overriding plate extension took place in the back‐arc side forming a back‐arc basin. The second one took place immediately before the ophiolite exhumation and near the fore‐arc region. We postulate that the second decompressional melting was triggered by slab breakoff, which was preceded by slab rollback inducing trench‐parallel wedge mantle flow and non‐steady fluid and heat transport leaving exceptionally hydrous residual mantle. The formation history of the Hayachine–Miyamori Ophiolite implies that weaker plate coupling may provide preferential conditions for exhumation of very hydrous mantle. Very hydrous peridotites involved in arc magmatism have not yet been discovered except for in the Cambrian–Ordovician periods, suggesting its implications for global geodynamics, such as the thermal state and water circulation in the mantle.