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

Alleghanian-Hercynian-Variscan造山带（大约3亿年前）标志着地球的顶峰？这是超大陆（盘古大陆）形成和扩散的最新例子。阿巴拉契亚山脉南部记录了盘古大陆形成的关键地带，因为它们标志着三个现代大陆（北美，南美和非洲）的前身碰撞。盘古大陆随后的破裂产生了这三个大陆，并继续扩大大西洋。来自佛罗里达大学和密西西比州立大学的教师和学生组成的研究小组正在对一套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，这些火成岩体侵入了10亿年前在山脉根部形成的结晶岩，现在位于格鲁吉亚州、佛罗里达州、亚拉巴马州和南卡罗来纳州。这些花岗岩体是两个古老海洋（Iapetus和Rheic）毁灭的唯一已知岩浆表现。了解花岗岩体的年龄和起源对于了解大陆的生长至关重要，特别是大陆地壳在盘古超大陆循环期间如何交换。特别是，他们正在集中精力确定边界和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会聚的特征。根据地球物理数据，会聚边界暂时被确定为发生在一个被称为Suwannee缝合线的区域;拟议的缝合线与佛罗里达州-乔治亚州边界平行。新的边界是在盘古大陆破裂后建立的，它产生了现代大西洋和非洲，南美和北美大陆。该断裂的位置尚不清楚，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）正在对来自下陆壳熔融的花岗岩进行取样，作为对Alleghanian构造作用和岩浆作用之间的空间和时间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。这些数据将提供独特的约束岩石圈结构和演化的盘古碰撞带。所选地区包含大量的阿勒格尼岩浆岩，这些岩浆岩侵入了一系列通常被认为是古北美一部分的前阿勒格尼岩浆岩，以及一些被认为是冈瓦纳或冈瓦纳周边的岩浆岩（例如，非洲或南美洲）。花岗岩体的铀-铅锆石地质年代学和同位素组成正在被用来对拟议的Suwannee缝合线两侧进行地球化学成像，以更好地了解四维中不同大陆碎片的并置。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重要补充，这些研究是北美地球镜计划（EarthScope Program）使命的组成部分，2）为大部分无岩浆海洋闭合的全球构造之谜提供重要见解，3）有助于我们理解墨西哥湾石油省东部的基底结构。该项目支持佛罗里达大学和密西西比州立大学的研究人员之间的重要区域科学合作。它有助于培养STEM学科的研究生和本科生。由于这两所大学的学生人口统计数据，它有可能有助于扩大地球科学中代表性不足的群体的参与。这项研究正在进行的结果正在指南，专业会议研讨会和参考科学出版物中进行交流。

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