Lutetium-Hafnium Ages and the P-T History of High-Pressure Rocks in the Appalachians

Essene/MukasaEAR-0087448我们的研究旨在开发技术，以解决在洋壳俯冲或随后的大陆碰撞期间曾经被深埋的岩石组的地质历史。 在北美东部阿巴拉契亚山脉的一次古老的造山活动——塔科尼克造山运动期间，所考虑的岩石都被埋藏在 40-60 公里深处。 该地区经历了复杂的构造历史，发生了三起造山事件，这些造山事件与显生宙海洋物质或奇异地体的增生有关：大约在 1997 年的塔科尼山。 450 Ma，阿卡迪亚大约。 400 Ma，阿勒格尼阶开始于 320 Ma。 一些构造切片包含的物质也是古代北美的一部分，并在大约 1.1 Ga 前的格伦维尔时间经历了更古老的高级变质作用。 在任何此类反复变形、埋藏和断层的地体中，通常很难确定地识别任何给定事件的时间，并且需要强大的地质年代学和温压技术来识别矿物的生长，而不是在随后的造山运动期间识别矿物的冷却或重置。 我们的工作将采用 Lu-Hf 同位素测年技术对具有高封闭温度的矿物（石榴石、单斜辉石、金红石和锆石）进行分析，这些元素可以从高压岩石（榴辉岩和石榴石麻粒岩）中分离出来，在某些情况下，这些岩石经历了后续的低品位事件（角闪岩相变质作用）。 这项工作将与新的温压技术相结合，旨在最大限度地减少大多数榴辉岩中发现的高方差组合中遇到的问题。 这些方法将与我们的合作者正在进行的测绘、结构分析和 U-Pb 地质年代学相结合，以更好地确定阿巴拉契亚山脉逆冲断面的早期构造历史的时间。

Essene/MukasaEAR-0087448Our research is directed toward developing techniques that will address the geological history of suites of rocks that were once deeply buried during subduction of ocean crust or during subsequent continental collision. The rocks under consideration all were buried to depths of 40-60 km during an ancient mountain-building event, the Taconic Orogeny, in the Appalachian Mountains of eastern North America. This area has undergone a complex tectonic history, with three mountain-building events linked to accretion of oceanic materials or exotic terranes in the Phanerozoic: the Taconic at ca. 450 Ma, the Acadian at ca. 400 Ma, and the Alleghenian beginning at 320 Ma. Some tectonic slices contain materials that were also part of ancient North America and underwent a much older high-grade metamorphism in Grenville time about 1.1 Ga ago. In any such repeatedly deformed, buried and faulted terrane, it is often difficult to identify with certainty the timing of any given event, and robust geochronogical and thermobarometric techniques are needed to identify the growth of minerals rather than their cooling or resetting during some subsequent orogeny. Our work will apply the Lu-Hf isotopic dating technique on minerals with high blocking temperatures for these elements (garnet, clinopyroxene, rutile and zircon) that can be separated from high-pressure rocks (eclogites and garnet granulites) which, in some cases, have undergone subsequent lower grade events (amphibolite facies metamorphism). This effort will be combined with new thermobarometric techniques designed to minimize the problems encountered in high-variance assemblages found in most eclogites. These approaches will be combined with mapping, structural analysis and U-Pb geochronology being done by our collaborators to better establish the timing of the early tectonic history of thrust slices in the Appalachians.