What Causes UHT Metamorphism: Lengthscales and Timescales

在超过900°C的温度下经历区域变质作用的地壳岩石发人深省;解释地壳中如何达到如此高的温度是具有挑战性的。核心发展一个令人信服的解释是量化的空间和时间尺度，这种高温占主导地位：短的空间和时间尺度意味着本地化的深成作用或流体流动，而长的尺度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点。然而，在我们对这个优秀的研究领域的理解中，缺少两个关键信息：量化峰值温度如何随深度和特定深度的位置而变化，以及量化温度如何随时间演变。将使用较新的技术检验关于超高温变质作用原因的三种端元假说-弧下俯冲、碰撞增厚和深成岩作用以及极端碰撞增厚：锆中钛、石英中钛和金红石中锆的温度测量和假截面建模，结合激光烧蚀分流铀/钍-独居石和锆石的铅年代和微量元素。由于马达加斯加南部是异常热地壳的典型例子，在那里开发的想法可以在全球应用。这一提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，今天活跃着什么过程？地球的流变性、地形、密度和组成如何？地壳在碰撞过程中会演变成什么样？马达加斯加在东非-南极洲造山带的演变中扮演了什么角色？这些课题是构造学、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。PI将密切互动，并担任导师，以对方的学生。这提供了跨文化的经验，确保该项目的合作影响将是持久的。该项目将为NSF研究生研究员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小学，使未来的潜在科学家了解地球科学。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泛访问者，激光烧蚀系统的发展，以及电感耦合等离子体质谱仪的发展。

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.