Collaborative Research: How do ultrahigh pressure metamorphic sheets form and exhume? A case study from the Tso Morari complex, India

超高压变质岩为研究大陆碰撞开始时发生在约100公里深处的地球过程提供了难得的机会。印度北部的左莫拉里杂岩是这些超高压地体中最大、最年轻的地体之一。这个杂岩的大小和年轻，再加上过去十年的分析进展，使我们能够收集广泛地理区域内独特精确的变质压力、温度、年龄和结构测量结果。这些空间分布的数据集将使我们能够区分相互竞争的模型，以确定这些超高压岩石是如何形成的，以及在大陆碰撞期间是如何被挖掘出来的。这项工作将通过支持2名博士生和2-6名本科生的定向研究和教育努力，促进科学和美国的利益。博伊西州立大学每年将有多达25名地质专业学生参加矿物学和岩石学课程，华盛顿州立大学每年将有25名地质专业学生参加构造地质学课程，华盛顿州立大学每年将有1500名本科生参加普通地质课程。博伊西州立大学和华盛顿州立大学都为不同寻常的人群提供服务，其中有大量的拉丁裔和退伍军人学生。Co-Pi Long在当地社区大学招收学生，这将使研究成果和教育机会传播给更广泛的地区受众。这两个PI还将在印度瓦迪亚研究所展示结果，接触到广泛的国际人口和国际先进的科学。超高压变质岩形成在柯石英稳定区之内或之上，并在许多造山带中观察到。超高压岩石形成和折返的机制是一个长期存在的构造学问题。根据印度左莫拉里杂岩的资料，我们将检验由超高压变质作用的热-力学模型得出的两个重要假说：1)超高压旋回(埋藏到峰值P-T条件和折返到~1 Gpa)发生在C.5Myr，2)超高压岩石被埋藏、折返和作为相干片层侵位。具体地说，我们拟回答以下问题：1)左莫拉日杂岩在什么时间、什么压温条件下变质，超高压变质作用持续多久？2)角闪岩相叠加在什么时间、什么压温条件下发生，左莫拉日杂岩保持热多久？3)冷却速度是多少？4)在什么温度范围内发生了与折返有关的剪切作用？5)压力-温度-变形-时间路径的空间分布是什么？收集的数据将包括使用包裹体和边缘温压计的压力-温度条件，前超高压石榴石核心和超高压石榴石边缘的Lu-Hf年龄，任何高U金红石包裹体的U-Pb年龄，角闪石和白云母的40Ar/39Ar年龄，石英c轴张开角度测温，以及石榴石中化学梯度的扩散模拟。压力-温度-形变-时间路径将从俯冲和折返运移方向两端的至少8个地点收集。这些路径的系统沿途输运差异将区分超高压单元是作为一个相干的片层(如目前假设的那样)变质和剥离，还是作为多个或复杂的结构。结果将与超高压变质作用的不同热-机械模型相结合，使用单一或近端露头的多个计时器，以最大限度地减少地理外推，确定沿走向和跨走向的年龄，与已发表的热-机械模型进行直接比较，并确定左莫拉里杂岩何时发生中地壳侵位。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Ultrahigh pressure metamorphic rocks provide a rare opportunity to investigate Earth processes that occur at depths of c. 100 km at the onset of continental collisions. The Tso Morari complex in northern India represents one of the largest and youngest of these ultrahigh pressure terranes. The size and youth of this complex, coupled with analytical advances over the last decade, permit us to collect uniquely precise metamorphic pressures, temperatures, ages, and structural measurements across a wide geographic region. These spatially distributed datasets will allow us to discriminate among competing models for how such ultrahigh pressure rocks form and are exhumed during continental collisions. This work will advance science and US interests by supporting the directed research and education efforts of 2 PhD students and 2-6 undergraduate students. Additional educational benefits will accrue to as many as 25 geology majors per year at Boise State University in mineralogy and petrology classes, 25 geology majors per year at Washington State University in a structural geology class, and 1500 undergraduates per year at Washington State University in general geology classes. Both Boise State and Washington State Universities serve an unusual demographic with high Latino and military veteran student populations. Co-PI Long recruits students at local community colleges, which will allow research results and educational opportunities to be disseminated to a broader regional audience. Both PIs will also present results at the Wadia Institute, India, reaching a broad international demographic and advancing science internationally.Ultrahigh pressure metamorphic rocks form within or above the coesite stability field and are observed in many orogens. The mechanisms by which ultrahigh pressure rocks form and exhume is a long-standing problem in tectonics. Based on data collected from the Tso Morari complex in India, we will test two important hypotheses that derive from thermal-mechanical models of ultrahigh pressure metamorphism: 1) Ultrahigh pressure cycles (burial to peak P-T conditions and exhumation to ~1 GPa) occur in c. 5 Myr, and 2) Ultrahigh pressure rocks are buried, exhumed, and emplaced as coherent sheets. Specifically, we propose to answer the following questions: 1) At what time and at what pressure-temperature conditions were Tso Morari complex rocks metamorphosed, and how long did ultrahigh pressure metamorphism last? 2) At what time and at what pressure-temperature conditions did amphibolite-facies overprinting occur, and how long did the Tso Morari complex rocks remain hot? 3) What was the rate of cooling? 4) Over what temperature range did exhumation-related shearing occur? 5) What is the spatial distribution of pressure-temperature-deformation-time paths? Data collected will include pressure-temperature conditions using inclusion and rim thermobarometry, Lu-Hf ages on pre-ultrahigh pressure garnet cores and ultrahigh pressure garnet rims, U-Pb ages on any high-U rutile inclusions, 40Ar/39Ar ages of hornblende and muscovite, quartz c-axis opening angle thermometry, and diffusion modeling of chemical gradients in garnet. Pressure-temperature-deformation-time paths will be collected from at least 8 localities located at extremes along the subduction and exhumation transport directions. Systematic along-transport differences in these paths will discriminate whether the ultrahigh pressure unit was metamorphosed and exhumed as a coherent sheet (as currently assumed) or as multiple or complex structures. Results will be integrated with diverse thermal-mechanical models of UHP metamorphism using multiple chronometers in single or proximal outcrops to minimize geographic extrapolation, determine ages along- and across-strike, make direct comparisons to published thermal-mechanical models, and identify when mid-crustal emplacement of the Tso Morari complex occurred.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.