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俯冲到另一个板块下（俯冲），深度超过100公里。地球表面的岩石中含有只有在地球内部深处才能形成的矿物质，这表明大陆俯冲是一个普遍的过程。能够将岩石从相当深的地方带回地面的地质过程，也被称为挖掘，仍然是一个谜。该项目的重点是研究世界上最大、暴露程度最高的地区之一（挪威西部片麻岩地区），这些所谓的超高压岩石（UHP）就暴露在这里。这项研究将研究这种挖掘过程在过去是如何发生的。由于在北美尚未发现大块的超高压岩石，它们通常不容易在美国学院和大学的教学收藏中找到。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，其中包括岩石和薄片集，用于岩石学和相关课程。收集和教学模块将与地球科学教育专家合作分发到15所学校，以优化影响和多样性，并帮助管理教学效率和评估。该项目还包括一个展示和活动，利用野外照片和西部片麻岩地区的岩石样本，向参观内华达大学里诺分校地球科学与矿物工程博物馆的K-12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推广活动方面的培训。超高压地层的挖掘速率和机制仍然知之甚少，因为超高压矿物组合的保存是零星的，这也是挖掘出的地壳反应程度的一个功能。提出了超高压地体挖掘的两种端元模式：1)教育一个相干的、反应性差的岩石圈板块；2)高反应性超高压岩石体沿俯冲通道回流。挪威西部片麻岩地区是研究这些过程的理想地，因为它被许多10-100米尺度的剪切带所横切，根据露头尺度的横切关系和基性（榴辉岩/角闪岩）体和主石英场片麻岩的变质覆印，提供了一个清晰的渐进发掘历史。这些很大程度上未被研究的剪切带保存了从地幔到地壳深度挖掘过程中压力-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挖掘模型的主要候选者。研究剪切带岩石学和地球化学记录的方法包括：(1)从露头到区域尺度的详细野外测绘，结合整个西片麻岩地区剪切带内的微结构分析，确定矿物的渐进变形-再结晶，包括可测定矿物；(2)石英钛测温法追踪上升和减压过程；(3)以U- pb测年为基础的高温和中温岩石年代学，以锆石和独石为路径的U- HP部分，金红石和钛矿为路径的HP -角闪岩相部分。研究结果为挖掘出的超高压地层的地质记录中保存的P-T-t-D路径提供了重要的新见解，并更广泛地了解了超高压地层挖掘的速率和机制。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识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.