Thermochronology and Geochemistry of Lower Crustal Xenoliths, Central Mongolia: Formation and Evolution of the Deep Crust in an Intracontinental Setting

关于地球，我们还不了解的一件事是为什么以及如何在远离板块边界的地方形成高海拔。在这样一个地形异常的地区，有来自塔里亚特的年轻熔岩。蒙古境内携带着所谓“捕虏体”形式的罕见下地壳样本。这些捕虏体可以帮助回答有关下地壳的变化如何反过来控制地表地形的问题。高地貌的形成和破坏可以控制生态系统和生物多样性的演变，并可以改变气候模式。特别是，了解蒙古高地貌形成的时间对于了解北半球古气候演化具有重要意义，因为蒙古中部的高海拔地区目前对于北太平洋风暴的形成非常重要。为了了解蒙古中部下地壳的形成时间、形成和热演化，Tariat下地壳捕虏体的这个多学科项目将使用热压测量、地质年代学、热年代学以及常量元素、微量元素和同位素地球化学。温压法将用于限制捕虏体的平衡温度压力。样品中观察到的大量锆石将通过 U-Pb 激光烧蚀 ICP-MS 和 ID-TIMS 进行测年，以确定下地壳形成和任何变质作用的时间。就杭爱地区由地壳根部支撑而言，限制根部的年龄将有助于限制岩石隆起的时间，并通过推断和建模限制地表隆起的时间。全岩主量元素和微量元素地球化学以及 Sr、Nd、Hf 和 Pb 同位素分析将有助于限制形成下地壳的过程。下地壳造山后的长期热演化将通过对这些样品中可能存在的许多痕量相（包括独居石和磷灰石）进行 U-Pb 分析来评估。与正在进行的蒙古地球动力学研究一起，这项工作是博士学位的一部分。论文将确定蒙古中部高地貌的时间和演化，更广泛地说，将增进我们对可以在大陆内部产生高地貌的地球动力学过程的认识。

One thing we do not yet understand about Earth is why and how high elevations can form far from the boundaries of the tectonic plates. Within one such region of anomalous topography, young lavas from Tariat. Mongolia carry in them rare samples of the lower crust in the form of what are called ?xenoliths?. These xenoliths can help answer questions about how changes in the lower crust might in turn control surface topography. The formation and destruction of high topography can serve as a control on the evolution of ecosystems and biodiversity, and can alter climate patterns. In particular, understanding when high topography formed in Mongolia has significant ramifications for understanding northern-hemisphere paleoclimate evolution because high-elevation regions in central Mongolia are currently important for the nucleation of northern-Pacific storms.To understand the timing, formation and thermal evolution of the lower continental crust in central Mongolia this multidisciplinary project of Tariat, lower-crustal xenoliths will use thermobarometry, geochronology, thermochronology, and major-element, trace-element, and isotope geochemistry. Thermobarometry will be used to constrain the equilibration temperatures pressures of the xenoliths. Abundant zircon observed in the samples will be dated by U-Pb laser ablation ICP-MS and ID-TIMS to determine the timing of lower-crustal formation and any metamorphism. To the extent that the Hangay region is supported by a crustal root, constraining the age of the root will help place constraints on the timing of rock uplift and, by inference and by modeling, surface uplift. Whole-rock major- and trace-element geochemistry as well as Sr, Nd, Hf, and Pb isotope analyses will help constrain what processes formed the lower-crust. The post-orogenic, long term thermal evolution of the lower crust will be assessed using U-Pb analyses of a number of trace phases likely to be present in these samples, including monazite and apatite. Together with ongoing studies of Mongolian geodynamics, this work, which is part of a Ph.D. dissertation, will determine the timing and evolution of the high topography in central Mongolia, and more broadly, advance our knowledge about geodynamic processes that can produce high topography in continental interiors.