Durations and Rates of High Temperature Metamorphism During Archean Orogenesis: Implications for Early Earth's Tectonics

太古代代表了地球历史上的一个重要时期，在此期间产生了大量的大陆地壳，现代板块构造过程被认为已经开始。太古代的岩石圈包含了稳定的大陆岩石圈块体，它们构成了地球的基础。地球上的大陆已经生长和演化，岩石学证据表明，它们最早的历史通常至少涉及一个极端热流时期（地壳温度超过900 °C）。在这种高温至超高温（HT/UHT）条件下的变质作用有助于产生致密、坚固、相对无水的岩石，这些岩石构成了地球的大部分地壳，因此，导致极端加热的机制可能是克拉通稳定所必需的。不同的地壳加热机制发生在诊断深度，在不同的时间尺度，并与可区分的地质特征。由极端地壳加热形成的变质岩石提供了这些特征的可解释的记录，本项目试图利用这些信息来阐明导致地球早期稳定大陆形成的过程。我们的目标是量化两个具有对比地质特征的太古代岩石的绝对深度，温度和变质作用的持续时间。来自怀俄明州和上级火山的岩石，分别暴露在美国MT的Beartooth山脉和加拿大MB的Pikwitonei Granulite Domain，都记录了约27亿年前的HT/UHT条件，但变质作用发生在不同的长度尺度上，可能在不同的深度和不同的持续时间。本研究将联合收割机的多种年代学方法（石榴石中的Lu-Hf和Sm-Nd，独居石和锆石中的U-Pb）与微量元素特征和扩散速度测量相结合，以限定地壳加热的时间尺度。热力学建模将确定达到最高温度的深度，并揭示冷却和/或挖掘后的路径片段。地质年代学和热力学建模的整合将对这些压力（深度）-温度路径的持续时间施加限制。这些结果将提供洞察机制负责HT/UHT变质作用在每个岩石圈，和风格的板块构造过程中运作的太古代。该项目将支持一名研究生和一名本科生，并将启动PI之间的新的研究伙伴关系，他们都是早期的职业科学家，并总理NSF资助的实验室。

The Archean represents a significant period in Earth's history during which large volumes of continental crust were generated and modern plate tectonic processes are thought to have initiated. Archean cratons contain stable blocks of continental lithosphere that form the foundation from which Earth?s continents have grown and evolved, and petrologic evidence suggests that their earliest histories generally involved at least one period of extreme heat flow (with crustal temperatures in excess of 900 °C). Metamorphism at such high to ultrahigh temperature (HT/UHT) conditions helped to produce the dense, strong, relatively anhydrous rocks comprising most of the Earth's cratonic crust, and the mechanisms that lead to extreme heating may thus be required for craton stabilization. Different mechanisms for crustal heating occur at diagnostic depths, over various timescales, and are associated with distinguishable geologic features. Metamorphic rocks formed by extreme crustal heating provide an interpretable record of these characteristics and this project seeks to use this information to elucidate the processes that resulted in the formation of Earth's early, stable continents.We aim to quantify the absolute depths, temperatures, and durations of metamorphism in two Archean terranes with contrasting geological characteristics. Rocks from the Wyoming and Superior cratons, exposed respectively in the Beartooth Range, MT, USA, and the Pikwitonei Granulite Domain, MB, Canada, both record HT/UHT conditions at ~2.7 billion years ago, but metamorphism occurred over different length-scales, and possibly at different depths and over different durations. This study will combine several geochronological techniques (Lu-Hf and Sm-Nd in garnet and U-Pb in monazite and zircon) with trace element characterization and diffusion speedometry in order to constrain the timescales of crustal heating. Thermodynamic modeling will determine the depths at which maximum temperatures were attained and reveal segments of the paths that were followed upon cooling and/or exhumation. Integration of the geochronology and thermodynamic modeling will place constraints on the durations of these pressure (depth)-temperature paths. These results will provide insights into the mechanisms responsible for HT/UHT metamorphism in each terrane, and the style of plate tectonic processes that operated in the Archean. This project will support one graduate student and one undergraduate, and will initiate new research partnerships between the PIs, both of whom are early career scientists, and premier NSF-funded laboratories.