Collaborative Research: Geochemical Imaging of Post-Pangean Lithospheric Structure in the Southern Appalachians

阿利根期-海西期-瓦里西亚期联合造山运动（约3亿年前）标志着地球上超级大陆（盘古大陆）形成和扩散的最新例子的顶峰。南部阿巴拉契亚山脉记录了盘古大陆形成的关键区域，因为它们标志着三个现代大陆（北美、南美和非洲）的前身的碰撞。盘古大陆随后的破裂产生了这三个大陆，并随着大西洋的扩张而继续。由佛罗里达大学和密西西比州立大学的教职员工和学生组成的研究小组正在对一组侵入结晶岩的花岗岩火成岩体的空间分布、年龄和地球化学成分进行详细的地球化学调查，这些岩浆岩早在十亿年前就在山脉根部形成，现在位于佐治亚州、佛罗里达州、阿拉巴马州和南卡罗来纳州。这些花岗岩体是唯一已知的两个古代海洋（土卫八和瑞克）毁灭的岩浆表现。了解花岗岩体的年龄和起源对于了解大陆的整体生长，特别是了解盘古大陆超大陆旋回期间大陆地壳如何交换至关重要。特别是，他们专注于确定边界以及运作的构造和岩浆过程，这些过程是最终形成盘古大陆的大陆聚合的特征。根据地球物理数据，初步确定了会聚边界，该边界出现在一个称为苏旺尼缝合带的区域；拟议的缝合线与佛罗里达州和佐治亚州边界平行。盘古大陆破裂后建立了新的边界，产生了现代大西洋以及非洲、南美和北美大陆。这种破裂的位置尚不清楚，但受到该项目的进一步限制。参与该项目的教师和学生（研究生和本科生）正在对下大陆地壳熔化产生的花岗岩进行采样，作为对阿勒根期构造作用和岩浆作用之间的时空关系进行综合地质年代学、热年代学和地球化学研究的一部分。这些数据将为泛古大陆碰撞带的岩石圈结构和演化提供独特的约束。所选区域包含大量阿勒根期岩浆岩，这些岩浆侵入了一系列通常被认为是古代北美一部分的前阿勒根期地体，以及一些被认为基本上是冈瓦纳或近冈瓦纳地区（例如非洲或南美洲）的岩浆岩。铀铅锆石地质年代学和花岗岩体的同位素组成正被用于对拟议的苏万尼缝合线两侧进行地球化学成像，以更好地了解不同大陆碎片在 4 维中的并置。这种岩石圈结构的“地球化学成像”是：1）为该地区正在进行的地球物理研究提供了重要的补充，这些研究是北美 EarthScope 计划的使命不可或缺的一部分，2）提供了对主要非岩浆海洋闭合的全球构造之谜的重要见解，3）有助于我们了解墨西哥湾东部石油省的基底结构。该项目支持佛罗里达大学和密西西比州立大学研究人员之间的重要区域科学合作。它致力于培养 STEM 学科的研究生和本科生。由于两所大学的学生人口统计数据，它有可能有助于扩大代表性不足的群体在地球科学领域的参与。这项研究的持续成果正在指南、专业会议研讨会和参考科学出版物中传达。该项目得到了 NSF 构造学和 EarthScope 计划的支持

The combined Alleghanian-Hercynian-Variscan orogeny (approximately 300 million years ago) marks the culmination of Earth?s most recent example of the formation and dispersal of a supercontinent (Pangea). The southern Appalachian Mountains record a critical zone in the formation of Pangea because they mark the collision of the precursors of three modern continents (North America, South America, and Africa). Pangea's subsequent rupture produced these three continents and continues with the expansion of the Atlantic Ocean. The research team of faculty and students from the University of Florida and Mississippi State University are conducting detailed geochemical investigations into the spatial distribution, ages, and geochemical compositions of a suite of granitic igneous bodies that intruded crystalline rocks that formed up to one billion years earlier in the roots of the mountain belt and are now in Georgia, Florida, Alabama, and South Carolina. These granitic bodies are the only known magmatic manifestation of the destruction of two ancient oceans (Iapetus and Rheic). Understanding the age and origin of the granitic bodies is critical to understanding the growth of continents in general and, particularly, how continental crust was exchanged during the Pangean supercontinent cycle. In particular, they are focusing on identifying the boundary and the operative tectonic and magmatic processes that characterized convergence of the continents that ultimately formed Pangea. The convergent boundary has tentatively been identified on the basis of geophysical data to occur in a zone known as the Suwannee suture; the proposed suture lies sub-parallel to the Florida-Georgia border. A new boundary was established after the rupture of Pangea, which produced the modern Atlantic Ocean and the African, South American, and North American continents. The location of this rupture is not known, but is being further constrained by this project.Faculty and students (graduate and undergraduate) involved in this project are sampling the granitic rocks derived from melting of the lower continental crust as part of an integrated geochronologic, thermochronologic, and geochemical study of the spatial and temporal relationships between Alleghanian tectonism and magmatism. These data will provide unique constraints on lithospheric structure and evolution in the Pangean collision zone. The region chosen contains numerous Alleghanian magmatic rocks that intrude a range of pre-Alleghanian terranes typically considered part of ancient North America, as well as some considered to be fundamentally Gondwanan or peri-Gondwanan (e.g., African or South American). The uranium-lead zircon geochronology and isotopic composition of the granitic bodies are being used to geochemically image both sides of the proposed Suwannee suture to better understand the juxtaposition of the different continental fragments in 4-D. This "geochemical imaging" of lithospheric structure is: 1) providing a critical complement to on-going geophysical studies in this region that are integral to the mission of the EarthScope Program in North America, 2) providing important insight into the global tectonic enigma of largely amagmatic ocean closures, and 3) contributing to our understanding of the basement architecture of the eastern Gulf of Mexico petroleum province. The project supports an important regional scientific collaboration between researchers at the University of Florida and Mississippi State University. It is contributing to the training of graduate and undergraduate students in a STEM discipline. Because of the demographics of the student populations at both universities, it has the potential to contribute to the broadening of participation of underrepresented groups in the geosciences. The ongoing results of this research are being communicated in guidebooks, professional meeting symposia, and refereed scientific publications.This project is being supported by the NSF Tectonics and EarthScope Programs