Formation of the metamorphic sole of the Semail ophiolite: High-precision U-Pb dating of the preserved remnants of a subducted slab

蛇绿岩是洋壳和上地幔的碎片，它们已经侵位到大陆上。蛇绿岩的形成是板块构造的基本组成部分，可能与新俯冲带的形成有关。近年来高精度U-Pb测年技术的应用，加深了我们对世界上最大、研究最深入的蛇绿岩Smail蛇绿岩的形成和侵位的认识。该项目将在最近工作的基础上进行扩展，对蛇绿岩下持久存在的变质岩进行详细的高精度测年，蛇绿岩可能形成于俯冲带内超过35公里的深度。这些结果将解决关于蛇绿岩起源的长期争论，并为如何利用蛇绿岩来理解现代大洋中脊、俯冲带以及潜在的俯冲作用提供新的见解，俯冲是一个关键但鲜为人知的构造过程。这项研究将通过地球科学方面的研究生培训、阿曼学生的参与、公众宣传以及促进来自美国、英国、阿曼和阿拉伯联合酋长国的科学家之间的合作来促进预期的社会成果。作为逆冲板块的残留物，底面暴露可以更好地了解蛇绿岩的形成和逆冲作用。Smail蛇绿岩下两次大范围的变质作用取得了令人惊讶的结果，表明最早的海底变质作用与蛇绿岩地壳的形成同步或早于蛇绿岩地壳的形成，并且沿蛇绿岩长度方向的变质作用是历时的，跨度为1.3 Ma，与现有的模式相反。这些数据，结合现有的地球化学约束和估计的来自海底的变质压力，最好地解释了蛇绿岩在俯冲带背景下的形成，底部岩石的变质作用发生在俯冲板片中。为了更好地了解这些结果对蛇绿岩成因的影响，该项目的科学家们将在整个Smail蛇绿岩暴露的变质鞋底进行高精度的同位素稀释-热电离质谱(ID-TIMS)锆石测年。蛇绿岩底部的新数据集有可能为蛇绿岩的形成和侵位以及汇聚边缘的俯冲和折返过程提供变革性的洞察。

Ophiolites are pieces of oceanic crust and upper mantle that have been emplaced onto the continents. The formation of ophiolites is a fundamental component of plate tectonics, potentially related to the initiation of new subduction zones. Recent application of high-precision U-Pb dating has advanced our understanding of the formation and emplacement of the Semail (Oman-United Arab Emirates) ophiolite, the largest and best studied ophiolite in the world. This project will expand on the recent work, undertaking detailed high-precision dating of metamorphic rocks persevered below the ophiolite, which likely formed at depths greater than 35 km within a subduction zone. The results will address long-standing debates regarding the origin of ophiolites, and provide new insight into how ophiolites can be used to understand modern mid-ocean ridges, subduction zones, and potentially the initiation of subduction, a critical but poorly understood tectonic process. The research will advance desired societal outcomes by graduate student training in the earth sciences, involvement of Omani students, public outreach, and fostering collaborations between scientists from the United States, the United Kingdom, Oman, and the United Arab Emirates.A thin sole of amphibolite- to granulite-facies metamorphic rocks underlies the Semail and other Tethyan ophiolites. As preserved remnants of the underthrust plate, sole exposures can be used to better understand the formation and obduction of ophiolites. Recent data from two large exposures of the metamorphic sole below the Semail ophiolite yielded surprising results, indicating that that the earliest sole metamorphism was synchronous with or pre-dated formation of the ophiolite crust and that metamorphism was diachronous along the length of the ophiolite, spanning 1.3 Ma, contrary to current models. These data, combined with existing geochemical constraints and the estimated metamorphic pressures from the sole, are best explained by formation of the ophiolite in a supra-subduction zone setting, with metamorphism of the sole rocks occurring in a subducted slab. To better understand the implications of these results to the origin of ophiolites, the project scientists will carry out high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon dating at exposures of the metamorphic sole throughout the Semail ophiolite. The new dataset from the sole of the ophiolite has the potential to provide transformative insight into the formation and emplacement of ophiolites, and to processes of subduction and exhumation at convergent margins.