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

合作研究：西部片麻岩地区剪切带保存的超高压地形剥露记录（挪威）

• 批准号: 1827220
• 负责人: Christian Teyssier
• 金额: $ 28.63万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-07-01 至 2024-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1827220&HistoricalAwards=false
• 关键词: Collaborative; Research; Record; UHP; Terrain

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.

根据板块构造理论，大陆可能会与其他板块碰撞或潜入另一个板块（俯冲）下方，深度超过 100 公里。地球表面存在的岩石含有只能在地球内部深处形成的矿物，这表明大陆俯冲是一个常见的过程。能够将岩石从相当深的地方带回地表的地质过程（也称为折返）仍然是个谜。该项目重点研究世界上最大、暴露程度最好的地区之一（挪威西部片麻岩地区），这些所谓的超高压岩石 (UHP) 都暴露在那里。这项研究将探讨这种挖掘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，因此美国学院和大学的教学藏品中通常不容易找到它们。该项目的教育活动之一是创建一个以超高压岩石为中心的教学模块，其中包括用于岩石学和相关课程的岩石和薄片集。该馆藏和教学模块将与地球科学教育专家合作分发给 15 所学校，以优化影响力和多样性，并帮助管理教学效果和评估。该项目还包括利用来自西部片麻岩地区的实地照片和岩石样本进行展示和活动，向参观内华达大学里诺分校地球科学和矿物工程博物馆的 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。超高压地体折返的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，而且它也是折返地壳反应程度的函数。已经提出了两种超高压地体折返的端元模式：1）形成连贯且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体，因为它被许多 10-100 m 规模的剪切带横切，这些剪切带提供了基于露头规模横切关系和镁铁质（榴辉岩/角闪岩）体和宿主石英长石片麻岩中的变质叠印的清晰的渐进折返历史。这些很大程度上未经研究的剪切带保留了从地幔折返到地壳深度期间压力-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。研究剪切带内的岩石学和地球化学记录的方法包括：（1）详细的现场测绘，从露头到区域尺度，结合整个西部片麻岩地区剪切带内的微观结构分析，以确定矿物的渐进变形-再结晶，包括可数据矿物； (2) 钛石英温压计，用于追踪上升和减压过程中的状况； (3)基于高温和中温计时器的岩石年代学，其基于 U-Pb 测年，使用锆石和独居石作为 (U)HP 路径部分，使用金红石和钛矿作为 HP 到角闪岩相路径部分。结果为挖掘出的超高压地体地质记录中保存的 P-T-t-D 路径提供了重要的新见解，更广泛地了解了超高压地体挖掘的速率和机制。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。