Collaborative Research: Record of UHP Terrain Exhumation Preserved in Shear Zones of the Western Gneiss Region (Norway)

根据板块构造理论，大陆可以相互碰撞，或潜入另一个板块(俯冲)到超过100公里的深度。地球表面岩石的存在表明，大陆俯冲是一种常见的过程，这些岩石含有只能在地球内部很深的地方形成的矿物。能够将岩石从相当深的地方带回地表的地质过程，也就是所谓的身体挖掘，仍然是个谜。这个项目的重点是研究世界上最大和最容易暴露的地区之一(挪威西部片麻岩地区)，这些所谓的超高压岩石(UHP)就是在这里暴露出来的。这项研究将考察这种挖掘遗骸的过程在过去是如何发生的。由于北美尚未发现大块超高压岩石，它们通常不容易在美国学院和大学的教学收藏中获得。该项目的教育活动之一将是创建一个以超高压岩石为中心的教学模块，其中包括岩石学和相关课程中使用的岩石和薄片集。将与地球科学教育专家合作，向15所学校分发收集资料和教学单元，以优化影响和多样性，并帮助管理教学效果和评估。该项目还包括一个展示和活动，利用来自西部片麻岩地区的现场照片和岩石样本，向参观内华达大学雷诺分校地球科学和矿物工程博物馆的K-12年级学生解释板块构造和俯冲带。此外，研究生和本科生将接受研究和外联活动方面的培训。由于超高压矿物组合的保存是零星的，而且这也是地壳折返反应程度的函数，因此超高压地体的折返速率和机制仍然鲜为人知。超高压地体的折返有两种端元模式：1)凝聚的低反应性岩石圈板块的教育；2)沿俯冲通道的高活性超高压岩体体的返回流动。挪威西部片麻岩地区是研究这些过程的理想地体，因为它被许多10-100米尺度的剪切带所横切，根据露头规模的横切关系和镁铁质(榴辉岩/角闪岩)岩体和寄主石英长石片麻岩中的变质叠加，提供了清晰的渐进折返历史。这些基本上未被研究的剪切带保存了从地幔到地壳深处折返过程中压力-温度-时间-变形(P-T-t-D)条件的最佳记录，因此是评估所提出的折返模型的主要候选者。研究剪切带内岩石学和地球化学记录的方法包括：(1)详细的野外填图，从露头到区域，结合整个西部片麻岩地区剪切带内的微结构分析，以确定矿物，包括可测定日期的矿物的递进变形-重结晶；(2)石英中钛温压计，追踪上升和减压过程中的条件；以及(3)基于利用锆石和独居石测年的高温和中温年代学，对路径的(U)HP部分和路径的高压至角闪岩相部分的金红石和钛矿的岩石年代学。这一结果为保存在出土的超高压地体的地质记录中的P-T-t-D路径提供了重要的新见解，并更广泛地揭示了超高压地体出土的速率和机制。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

According to the theory of plate tectonics, continents can collide against others or dive under another plate (subduction) to depths of over 100 km. The existence of rocks at the Earth's surface that have minerals that can only be formed at great depths in the Earth's interior suggests that continental subduction is a common process. The geological processes that are able to bring rocks from considerable depth back to the surface, also known as exhumation, remain enigmatic. This project focuses on a study of one of the largest and best-exposed area in the world (the Western Gneiss Region of Norway) where these so-called ultrahigh pressure rocks (UHP) are exposed. This study will examine how this process of exhumation may have occurred in the past. Because large blocks of UHP rocks have yet to be discovered in North America, they are typically not readily available in teaching collections at U.S. colleges and universities. One of the education activities in this project will be to create a teaching module centered on UHP rocks that includes rock and thin section sets to be used in petrology and related courses. The collections and teaching module will be disseminated to 15 schools in collaboration with geoscience education specialists in order to optimize impact and diversity, and help manage teaching effectiveness and assessment. The project also includes a display and activities using field photos and rock samples from the Western Gneiss Region to explain plate tectonics and subduction zones to K-12 students visiting the Earth Science and Mineral Engineering Museum housed at University of Nevada - Reno. In addition, graduate and undergraduate students are to be trained in both research and outreach activities. The rates and mechanisms of exhumation of UHP terranes are still poorly known because the preservation of UHP mineral assemblages is sporadic, and it is also a function of how reactive the exhumed crust was. Two end-member modes of exhumation of UHP terranes have been proposed: 1) education of a coherent and thus poorly reactive lithospheric slab; and 2) return flow of a body of highly reactive UHP rocks along the subduction channel. The Western Gneiss Region of Norway is an ideal terrane to study these processes because it is transected by numerous 10-100 m scale shear zones that offer a clear history of progressive exhumation based on outcrop-scale crosscutting relationships and metamorphic overprinting in both the mafic (eclogite/amphibolite) bodies and the host quartzofeldspathic gneiss. These largely unstudied shear zones preserve the best record of pressure-temperature-time-deformation (P-T-t-D) conditions during exhumation from mantle to crustal depths and are therefore prime candidates for evaluating the proposed exhumation models. Methods that investigate the petrologic and geochemical record within the shear zones include: (1) detailed field mapping, from outcrop to regional scale, incorporating microstructural analyses within shear zones from across the Western Gneiss Region to determine progressive deformation-recrystallization of minerals, including datable minerals; (2) Titanium-in-quartz thermobarometry to trace conditions during ascent and decompression; and (3) petrochronology based on high- and medium-temperature chronometers based on U-Pb dating using zircon and monazite for the (U)HP part of the path and rutile and titanite for the HP to amphibolite-facies portion of the path. Results provide significant new insight into the P-T-t-D paths that are preserved in the geologic record of exhumed UHP terranes, and more generally into the rates and mechanisms of UHP terrane exhumation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.