Subduction erosion revealed by exhumed lower arc crustal rocks in an accretionary complex, northeastern China

Geology Pub Date : 2024-02-28 DOI:10.1130/g51803.1
Mengyu Xu, W. Xiao, Kai Liu, B. Wan, Ross N. Mitchell, G. Rosenbaum, Hao Wang
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Abstract

Subduction erosion at convergent margins is a leading mechanism for the destruction (recycling and reworking) of continental crust. But because of the lack of direct evidence, it is not straightforward to identify erosive events and their intensities in fossil subduction zones. The Heilongjiang accretionary complex in northeastern China was formed during the early Mesozoic subduction of the Paleo-Pacific Ocean. We investigated amphibolites from this accretionary complex, whose protoliths (based on whole-rock trace elements and Sr-Nd-Hf isotopes) were mafic continental arc magmatic rocks (255−249 Ma; zircon core U-Pb ages) from the upper plate. Phase equilibria modeling constrained by mineral geochemistry indicates that the amphibolites and their wall rocks were first heated to low granulite facies (750−800 °C, ∼7 kbar) at 251−244 Ma (zircon rim U-Pb ages) and then cooled to ∼700 °C with increasing pressure (8−9 kbar) before 213−187 Ma (titanite and apatite U-Pb ages). To explain the occurrence of the lower arc crustal lithologies in the accretionary complex and their metamorphic history, we propose that the subducting plate strongly eroded the forearc crust, allowing the plate interface to advance landward and scrape the amphibolites and wall rocks formed under the old arc, which finally were exhumed along the subduction channel and became components of the complex. The case study exemplifies direct petrological evidence of strong subduction erosion occurring in an ancient orogen, thus implying that consumption of the entire forearc crust could occur within only ∼50 m.y.
中国东北部一增生复合体中出露的下弧地壳岩石揭示的俯冲侵蚀作用
汇聚边的俯冲侵蚀是大陆地壳破坏(再循环和再加工)的主要机制。但由于缺乏直接证据,要确定化石俯冲带的侵蚀事件及其强度并非易事。中国东北部的黑龙江吸积复合体形成于中生代早期古太平洋俯冲时期。我们对该增生复合体中的闪长岩进行了研究,其原岩(基于全岩痕量元素和Sr-Nd-Hf同位素)是来自板块上部的黑云母大陆弧岩浆岩(255-249 Ma;锆石岩芯U-Pb年龄)。矿物地球化学制约的相平衡模型表明,闪长岩及其壁岩在251-244Ma(锆石边缘U-Pb年龄)首先被加热到低花岗岩面(750-800 °C,∼7千巴),然后在213-187Ma(榍石和磷灰石U-Pb年龄)之前随着压力的增加(8-9千巴)冷却到∼700 °C。为了解释增生复合体中出现的下弧壳岩性及其变质历史,我们提出俯冲板块强烈侵蚀前弧地壳,使板块界面向陆地推进,刮擦了旧弧下形成的闪长岩和壁岩,这些岩性最终沿俯冲通道出露,成为复合体的组成部分。该案例研究提供了古造山发生强烈俯冲侵蚀的直接岩石学证据,从而暗示了整个前弧地壳的消耗可能发生在短短的 50 米年之内。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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