Collaborative Research: Growth, preservation, and modification of Neoarchean to Paleoproterozoic crust, an example from the Wyoming province, Montana

地球的大陆地壳被认为是在离散的地质时间间隔内形成的，在那里，特殊的地质过程导致了地壳的形成和生长。前寒武纪的这些时间间隔也反映了向现代板块构造过程的过渡，导致较小的地壳碎片碰撞和组装成大陆和超大陆。在蒙大拿州西南部怀俄明州西北边缘的岩石提供了北美大陆形成过程中的这些过程的记录。本项目将采用综合方法，运用岩石学、地球化学和地质年代学工具，确定地壳生长与随后构造作用的相对贡献。这项工作的主要成果将是更好地理解大陆组装期间地壳保存和生长之间的平衡。本计划将透过支持研究生及本科生的研究，促进教学及训练，并将研究结果纳入大学部的实地考察及实验课程。研究和教育的基础设施将通过发展和改进新的地质年代学技术，并通过向学生提供最先进的分析设备的培训来加强。该项目的新年龄数据将整合到蒙大拿州目前的地质测绘工作中，以更好地解释岩石单元的整体年龄和结构关系。拓展工作将包括为在蒙大拿州西南部举办暑期实地课程的全国学校的学生和教师提供非正式的科学讲座。新太古代和古元古代这两个时期代表了北美洲乃至世界大陆生长和组合的主要时期。这些时期被推断为与大陆地壳增强生产的时期相对应。这个项目将研究在这些时间间隔内产生新的大陆地壳的过程，以及重新加工和组装这个地壳的过程。蒙大拿州西南部怀俄明省的西北边缘是这项研究的理想自然实验室，因为它由各种各样的含石榴石的变质沉积岩组成，这些沉积岩适合于压力-温度-时间的研究，同时包含层间变质火成岩，它们的年代很差。本研究将采用新太古代到古元古代岩石的变质相平衡和地质年代学的综合方法，在蒙大拿元沉积地体的Laramide基底核隆起中建立相对的地壳生长和改造。收集的数据将通过变质相平衡、锆石、单氮石和钛矿的铀-铅（U-Pb）地代学和热年代学、锆石和单氮石的铪（Hf）和钕（Nd）同位素分析以及石榴石的镥-铪/钐-钕（Lu-Hf/Sm-Nd）定年来确定岩浆作用和变质作用的条件、时间、持续时间和空间范围。这些工具的集成将提供岩浆和变质历史的时间分辨率，并直接约束怀俄明州西北部的生长和改造历史。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。

Earth's continental crust is thought to have formed over discrete intervals of geologic time, where particular geologic processes resulted in enhanced crust formation and growth. These time intervals in the Precambrian also reflect a transition to modern plate tectonic processes, resulting in collision and assembly of smaller crustal fragments into continents and supercontinents. Rocks at the northwest margin of the Wyoming province in southwestern Montana provide a record of these processes during formation of the North American continent. This project will utilize an integrated approach by applying petrologic, geochemical, and geochronologic tools to establish relative contributions of crustal growth versus subsequent modification by tectonic processes. The major outcomes of this work will be a better understanding of the balance between crustal preservation and growth during continental assembly. This project will promote teaching and training by supporting graduate and undergraduate research, and results of this study will be incorporated into undergraduate class field trips and laboratory curricula. Infrastructure for research and education will be enhanced through the development and refinement of new geochronological techniques, and by providing training for students in state-of-the-art analytical equipment. New age data from this project will be integrated into current Montana state geologic mapping efforts to better interpret the overall ages of rock units and structural relationships. Outreach efforts will include presenting informal science talks for students and faculty from schools nationwide that conduct summer field courses in southwest Montana.Two intervals in the Neoarchean and Paleoproterozoic represent major periods in the growth and assembly of North America as well as continents worldwide. These periods have been inferred to correspond to times of enhanced production of continental crust. This project will examine the processes that have operated during these time intervals to produce new continental crust as well as the processes operating to rework and assemble this crust. The northwest margin of the Wyoming province in southwestern Montana is an ideal natural laboratory for this study because it is comprised of a diverse suite of metasedimentary garnet-bearing lithologies that are amenable to Pressure-temperature-time studies, as well as containing interlayered meta-igneous rocks that are poorly dated. The proposed work will apply an integrated approach of metamorphic phase equilibria and geochronology of Neoarchean to Paleoproterozoic rocks in the Laramide basement-cored uplifts of the Montana Metasedimentary Terrane to establish relative crustal growth versus modification during these time intervals. Data collected will determine the conditions, timing, duration, and spatial extent of magmatism and metamorphism through metamorphic phase equilibria, Uranium-Lead (U-Pb) geo- and thermochronology of zircon, monazite, and titanite, Hafnium (Hf) and Neodymium (Nd) isotopic analysis of zircon and monazite, and Lutetium-Hafnium/Samarium-Neodymium (Lu-Hf/Sm-Nd) dating of garnet. This integration of tools will provide temporal resolution magmatic and metamorphic histories and directly constrain the growth and modification history of the northwestern Wyoming province.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.