Collaborative Research: Precambrian Crustal Evolution in Western Laurentia: Implications from the Sawtooth Metamorphic Complex

来自路易斯安那州立大学和佛罗里达大学的地球科学家正在合作调查一段独特的前寒武纪（5.5亿年前，约10年）大陆地壳带到地球上。在爱达荷州东部，约50亿年的锯齿状岩基覆盖了美国的地表。这个地壳是独一无二的，因为它位于北美最古老的地壳（暴露在蒙大拿州西南部）和爱达荷州西部北美大陆最近增加的一些地壳（蓝山）之间。这项研究将测试关于这个地壳形成和演化的一系列假设：它是一个独立的地体，在最近的过去被吸收到北美，或者它是否显示出与蒙大拿州西南部暴露的古老（3000年前）地壳的地球化学联系？有没有记录一个人失踪的证据？类似于1000万年前形成北美东部大部分地区的格伦维尔造山运动时期的造山活动？如果它是在前寒武纪被吸积到北美的，它是否能让我们了解到500-700万年前北美洲的分裂？具体来说，该团队将调查变质沉积岩的起源，这些沉积岩携带着沉积时暴露的大陆地壳性质的信息。这个地壳的温度和压力历史，将限制它附属于北美大陆的时间以及它附属于北美大陆的机制。变质火成岩中的元素丰度将为它们形成的环境提供重要线索，而同位素组成将限制新地壳与保存的再循环地壳的数量。位于蒙大拿州西南部的太古宙（2500 m.y.o）地壳和爱达荷州西部的200 m.y.o地壳之间的地壳的很大一部分被埋在更年轻的岩石之下，这使得锯齿地体成为研究北美西部地壳演化的一个非常重要的窗口。这项研究将由一个团队进行，该团队包括来自代表性不足的群体和女性的成员，他们将接受适用于一系列STEM学科的重要定量技术的培训。该团队还将参与LSU地质与地球物理和马拉松石油公司（地球科学多样性丰富计划）之间的公私合作伙伴关系，该合作伙伴关系的重点是增加STEM的参与。公众也将受益于我们与Sawtooth国家休闲区（美国林业局）的合作，从而在Sawtooth国家休闲区分发更好的教育材料。具体而言，该团队（教师和学生，本科生和研究生）将雇用并提供以下培训：1)我们将对锆石进行U/Pb放射性测年，以确定火成岩的原始结晶年龄和沉积在原岩到变质沉积岩中的沉积物的来源，使用激光烧蚀对佛罗里达大学ICP-MS实验室的单个晶体进行取样；2)佛罗里达大学x射线荧光和ICP相结合的元素分析；3) LSU的电子探针实验室将用于测量单个矿物的元素丰度，然后可用于确定锯齿状地壳的埋藏条件和压力-温度路径（热气压计）；4)利用佛罗里达大学和马萨诸塞大学的设施，对结构和岩石学约束的独一石和钛石进行U-Th-Pb（放射性）定年，将为锯齿状地壳中记录的变质事件提供确定的年龄，以发展压力-温度-时间路径，并确定变质作用的构造背景；5) K-Ar系统（40Ar/39Ar）的测量将有助于评估地形最近的后变质期热历史。两所大学的学生将在这些技术上进行交叉训练，作为他们的论文和论文研究的一部分，然后将做好充分的准备，以迎接该国成员提供的技术挑战机会。it’s STEM劳动力。

Earth scientists from Louisiana State University and the University of Florida are collaborating on an investigation of a unique segment of Precambrian (550 million years old, m.y.o.) continental crust brought to the earth?s surface by the ~50 m.y.o. Sawtooth batholith in eastern Idaho. This crust is unique because it lies between some of the oldest crust in North America (exposed in SW Montana) and some of the most recent additions to the North American continent in western Idaho (Blue Mountains). This research will test a wide range of hypotheses for the formation and evolution of this crust: Is it an independent terrane accreted to North America in the recent past, or does it show geochemical ties to the old (3000 m.y.o.) crust exposed in SW Montana? Does it record evidence of a ?vanished? episode of mountain building similar in age to the Grenville orogeny that formed much of eastern North America 1000 m.y. ago? If accreted to North America in the Precambrian, does it provide insight the fragmentation of North America that occurred 500-700 m.y.o ago? Specifically, the team will investigate the origin of metamorphosed sedimentary rocks that carry information about the nature of continental crust exposed at the time they were deposited. The temperature and pressure history of this crust, which will constrain the time at which it was appended to North America and the mechanism by which it was appended. Elemental abundances in the metamorphosed igneous rocks will give important clues to the environment in which they formed, while isotopic compositions will constrain the amount of new crust vs. recycled crust that is preserved. A large fraction of the crust that lies between the Archean (2500 m.y.o.) crust of SW Montana and the 200 m.y.o. crust of western Idaho is buried beneath younger rocks, making the Sawtooth terrane a very important window into crustal evolution in western North America. This research will be carried out by a team that includes members from underrepresented groups and women who will be trained in important quantitative techniques applicable across a range of STEM disciplines. The team will also participate in the public-private partnership between LSU Geology & Geophysics and Marathon Petroleum Corporation (Geoscience Diversity Enrichment Program) that is focused on increasing STEM participation. The public will also benefit by our partnership with the Sawtooth National Recreational Area (U.S. Forest Service), leading to better educational materials distributed in the Sawtooth National Recreational Area. In detail, this team (faculty and students, undergraduate and graduate) will employ and provide training in the following: 1) We will conduct U/Pb radiometric dating of zircons to determine original crystallization ages of igneous rocks and the origin of sediment deposited in protoliths to the metasedimentary rocks using laser ablation to sample individual crystals in the ICP-MS laboratory at the University of Florida; 2) Elemental analyses by a combination of x-ray fluorescence and ICP methods at the University of Florida; 3) The electron microprobe laboratory at LSU will be used to measure elemental abundances in individual minerals, which can then be used to determine the burial conditions and pressure-temperature path of the Sawtooth crust (thermobarometry); 4) U-Th-Pb (radiometric) dating of texturally and petrologically constrained monazite and titanite will provide a definitive age for the metamorphic event(s) recorded in the Sawtooth crust needed to develop Pressure-Temperature-time path(s) and determine the tectonic setting of metamorphism using facilities at the University of Florida and the University of Massachusetts; and 5) Measurement of K-Ar systematics (40Ar/39Ar) will help assess the most recent, post-metamorphic, thermal history of the terrain. Students from both universities will cross-train in these techniques as part of their thesis and dissertation research, and will then be well prepared to meet the technologically challenging opportunities afforded members of the country?s STEM workforce.