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，大陆可以与其他大陆碰撞或潜入另一个板块（俯冲）下超过100公里的深度。地球表面的岩石中含有只有在地球内部很深的地方才能形成的矿物，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，也被称为剥露作用，仍然是个谜。该项目的重点是研究世界上最大和暴露最好的地区之一（挪威西部片麻岩地区），这些所谓的超高压岩石（UHP）暴露在那里。这项研究将审查过去挖掘过程是如何发生的。由于在北美尚未发现大块的超高压岩石，因此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所学校分发收集和教学模块，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，使用来自西部片麻岩地区的实地照片和岩石样本，向参观内华达州-里诺大学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，将对研究生和本科生进行研究和推广活动方面的培训。由于超高压矿物组合的保存是零星的，而且它也是一个函数，如何反应的折返地壳的速率和机制仍然知之甚少。已经提出了两种端元模式的折返的超高压岩石：1）教育的连贯性，因此反应性差的岩石圈板片;和2）返回流的身体高活性超高压岩石沿着俯冲通道。挪威西部片麻岩区是研究这些过程的理想场所，因为它被许多10-100 m规模的剪切带横切，根据出露规模的横切关系和镁铁质（榴辉岩/斜长角闪岩）体和寄主石英长石质片麻岩中的变质叠加，提供了清晰的渐进折返历史。这些大部分未经研究的剪切带保留了从地幔到地壳深处的折返过程中压力-温度-时间-变形（P-T-t-D）条件的最佳记录，因此是评估拟议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）详细的野外测绘，从露头到区域尺度，结合西部片麻岩区剪切带内的显微结构分析，以确定矿物（包括可定年矿物）的渐进变形-重结晶;（2）石英中的钛温度压力计，以跟踪上升和减压期间的条件;（3）基于高温和中温计时器的岩石年代学研究，以锆石和独居石为（U）高压部分，以金红石和钛铁矿为高压-角闪岩相部分的U-Pb定年，为研究保存在超高压岩石地质记录中的P-T-t-D通道提供了新的重要信息，该奖项反映了NSF的法定使命，并被认为值得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来支持。