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

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对前寒武纪(密苏里州5.5亿年前)的一段独特的化石进行调查。大陆地壳带到了地球上？S的地表被约50英里。爱达荷州东部的锯齿岩基。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)和北美大陆的一些最近增加的爱达荷州西部(蓝山)之间。这项研究将检验关于该地壳形成和演化的一系列假说：它是最近在北美增加的一个独立的地体，还是表明它与旧的(3000英里/年)有关。蒙大拿州西南部裸露的地壳？它有记录一个人失踪的证据吗？与形成北美东部大部分地区的格伦维尔造山运动在年代上相似的造山期。很久以前？如果是在前寒武纪的北美，它能洞察500-700英里前发生的北美的碎裂吗？具体地说，该团队将调查变质沉积岩的来源，这些沉积岩携带着有关沉积时暴露的大陆地壳性质的信息。地壳的温度和压力历史，这将限制它被附加到北美的时间和附加机制。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再循环地壳的保存数量。位于太古宙(2500英里)之间的一大部分地壳。蒙大拿州西南部的地壳和200英里。爱达荷州西部的地壳埋藏在较年轻的岩石下，使锯齿地体成为了解北美西部地壳演化的一个非常重要的窗口。这项研究将由一个小组进行，该小组包括来自代表性不足群体的成员和妇女，她们将接受适用于一系列STEM学科的重要量化技术的培训。该团队还将参与路易斯安那州立大学地质学和地球物理学与马拉松石油公司(地球科学多样性浓缩计划)之间的公私合作伙伴关系，该伙伴关系的重点是增加STEM的参与。公众还将受益于我们与锯齿国家娱乐区(美国林务局)的合作伙伴关系，从而在锯齿国家娱乐区分发更好的教育材料。具体地说，这个团队(教职员工、本科生和研究生)将在以下方面进行聘用和提供培训：1)我们将在佛罗里达大学的电感耦合等离子体质谱仪实验室使用激光烧蚀技术对单个晶体进行元素分析，以确定火成岩的原始结晶年龄和沉积在原岩中的沉积物的来源；2)佛罗里达大学的X射线荧光和电感耦合等离子体方法相结合的元素分析；3)路易斯安那州立大学的电子探针实验室将用于测量单个矿物中的元素丰度，然后可用于确定锯齿状地壳的埋藏条件和压力-温度路径(热压法)；4)利用佛罗里达大学和马萨诸塞州大学的设施，对结构和岩石约束的独居石和钛铁矿进行U-Th-Pb(放射性)测年，将为锯齿状地壳中记录的变质事件(S)提供确切的年龄，以建立压力-温度-时间路径(S)并确定变质作用的构造背景；K-Ar系统学(40Ar/39Ar)的测量将有助于评估该地区最新的变质后热史。两所大学的学生将在这些技术方面进行交叉培训，作为他们论文和学位论文研究的一部分，然后将做好充分准备，以迎接国家成员--S科技工作者--面临的技术挑战机会。

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.