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被深埋的岩石套的地质史。研究中的岩石都是在北美东部阿巴拉契亚山脉的塔科尼克造山运动期间被埋在40-60公里深的地方。该地区经历了复杂的构造历史，有三次造山事件与显生界海洋物质或外来地体的积累有关：约450 Ma的塔克期，约400 Ma的阿卡迪期，以及320 Ma开始的阿勒格期。一些构造切片包含的物质也是古代北美的一部分，大约在1.1年前在格伦维尔经历了一次更古老的高级变质作用。在任何这种反复变形、埋藏和断裂的地体中，往往很难确定任何特定事件的时间，需要强大的地质年代学和温度气压测量技术来确定矿物的生长，而不是在随后的一些造山作用期间对其进行冷却或重新启动。我们的工作将对这些元素(石榴石、单斜辉石、金红石和锆石)具有较高封闭温度的矿物应用Lu-Hf同位素测年技术，这些矿物可以从高压岩石(榴辉岩和石榴石麻粒岩)中分离出来，在某些情况下，这些高压岩石经历了较低级别的事件(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在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、构造分析和U-Pb年代学相结合，以更好地确定阿巴拉契亚地区逆冲岩片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.