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

蛇绿岩是被放置在大陆上的海洋地壳和上地幔的碎片。蛇绿岩的形成是板块构造的基本组成部分，可能与新的俯冲带的形成有关。最近高精度U-Pb测年的应用，提高了我们对世界上最大、研究最充分的蛇绿岩Semail（阿曼-阿拉伯联合酋长国）蛇绿岩的形成和位置的认识。这个项目将扩展最近的工作，对蛇绿岩下面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深处超过35公里的地方。这些结果将解决长期以来关于蛇绿岩起源的争论，并为如何利用蛇绿岩来理解现代洋中脊、俯冲带以及潜在的俯冲起始（一个关键但知之甚少的构造过程）提供新的见解。这项研究将通过培养地球科学方面的研究生、阿曼学生的参与、公众宣传以及促进美国、英国、阿曼和阿拉伯联合酋长国科学家之间的合作，推动预期的社会成果。在塞梅尔和其他特提斯蛇绿岩之下有一薄底角闪岩-麻粒岩相变质岩。作为逆冲下板块保存下来的残余物，单独暴露可以更好地了解蛇绿岩的形成和逆冲。最近对Semail蛇绿岩下方的变质底进行的两次大规模暴露的数据得出了令人惊讶的结果，表明最早的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，并且变质作用沿着蛇绿岩的长度是历时的，跨越1.3 Ma，与目前的模型相反。这些数据，结合现有的地球化学限制和估计的鞋底变质压力，最好的解释是蛇绿岩是在超俯冲带背景下形成的，鞋底岩石的变质作用发生在俯冲板块中。为了更好地理解这些结果对蛇绿岩起源的影响，项目科学家将对整个Semail蛇绿岩的变质底进行高精度同位素稀释-热电离质谱（ID-TIMS） U-Pb锆石定年。来自蛇绿岩底部的新数据集有可能为蛇绿岩的形成和就位，以及收敛边缘的俯冲和挖掘过程提供革命性的见解。

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.