What Causes UHT Metamorphism: Lengthscales and Timescales

在温度超过900°C时经历区域变质作用的地壳岩石令人深思；解释地壳如何达到如此高的温度是一项挑战。要提出一个令人信服的解释，关键是对如此高温盛行的时空尺度进行量化：较短的时空尺度意味着局部的岩浆作用或流体流动，而较长的时空尺度则表明更大规模的过程。马达加斯加南部是研究超高温变质作用的关键地区。然而，在我们对这个优秀的研究领域的理解中，有两个关键信息缺失：峰值温度如何随深度和特定深度的位置变化的量化，以及温度如何随时间演变的量化。超高温变质成因的三个端元假说——弧下俯冲、碰撞增厚和岩浆作用，以及极端碰撞增厚——将使用相对较新的技术进行测试：锆英钛、石英钛、金红石锆测温和伪剖面建模，并结合激光烧蚀分裂流铀/钍-铅测年以及独居石和锆石的微量元素。由于马达加斯加南部是地壳异常热的典型例子，在那里形成的理论可以应用于全球。这一建议旨在了解异常热的地壳是如何发展的，但它也将影响以下问题：在像西藏这样的热碰撞造山带下面，今天有什么活动？地球的流变学、地形、密度和组成是如何变化的？地壳在碰撞中演化？马达加斯加在东非-南极洲造山带的演化中扮演了什么角色？这些主题涉及构造学、岩石学、地质年代学、地球动力学、大地测量学和地球物理学的交叉学科。这个项目汇集了来自美国、马达加斯加、澳大利亚和法国的科学家。这些pi将密切互动，并作为彼此学生的导师。这提供了跨文化的经验，确保这个项目的合作影响将是持久的。该项目将为一名NSF研究生和几名本科生提供培训和研究机会，并使我们能够继续访问当地的小学，让潜在的未来科学家接触地球科学。我们的激光烧蚀分流技术的持续发展将有利于我们实验室的广大访客，激光烧蚀系统的发展，以及电感耦合等离子体质谱的发展。

Crustal rocks that have undergone regional metamorphism at temperatures greater than 900°C are thought provoking; explaining how such high temperatures are attained in the crust is challenging. Central to developing a cogent explanation is quantifying the spatial and temporal scales over which such high temperature prevailed: short spatial and temporal scales imply localized plutonism or fluid flow, whereas long scales indicate larger scale processes. Southern Madagascar represents a key place for studying ultrahigh-temperature metamorphism. There are, however, two key pieces of information missing from our understanding of this otherwise excellent study area: a quantification of how peak temperature varied with depth and with position at a specific depth, and a quantification of how temperature evolved with time. Three endmember hypotheses for the cause of ultrahigh-temperature metamorphism--subduction beneath an arc, collisional thickening and plutonism, and extreme collisional thickening--will be tested using relatively new techniques: titanium-in-zircon, titanium-in-quartz, and zirconium-in-rutile thermometry and pseudosection modeling, in conjunction with laser-ablation split-stream uranium/thorium-lead dates and trace elements of monazite and zircon.Because Southern Madagascar is an archetypal example of exceptionally hot crust, ideas developed there can be applied globally. This proposal is targeted toward understanding how exceptionally hot crust develops, but it will also impact the following questions: What processes are active today beneath hot collision orogens such as Tibet? How do the rheology, topographic, density, and composition of Earth?s crust evolve during collisions? What role did Madagascar play in the evolution of the East African-Antarctica Orogen? These topics are of cross-disciplinary interest in tectonics, petrology, geochronology, geodynamics, geodesy and geophysics. This project brings together scientists from the U.S., Madagascar, Australia, and France. The PIs will interact closely and serve as mentors to each other's students. This provides cross-cultural experiences that ensure that the collaborative impact of this project will be long lasting. This project will provide training and research opportunities for an NSF Graduate Fellow, several undergraduates, and enable us to will continue visiting local elementary schools to expose potential future scientists to Earth science. Continued development of our laser-ablation split-stream technique will benefit the broad range of visitors to our laboratory, the development of laser-ablation systems, and the development of inductively coupled plasma mass spectrometers.