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

我们的研究方向是开发技术，以解决在海洋地壳俯冲或随后的大陆碰撞期间曾经深埋的岩系的地质历史。研究中的岩石都是在古造山活动——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公里深处的。该地区经历了复杂的构造史，显生宙有三次造山事件与海洋物质或外来地体的增生有关：约450 Ma的塔尼克期、约400 Ma的阿卡迪亚期和始于320 Ma的阿勒格尼期。一些构造薄片包含的物质也是古北美的一部分，并在大约1.1亿年前的格伦维尔时期经历了更古老的高级变质作用。在任何这种反复变形、埋藏和断裂的地块中，通常很难确定任何给定事件的确切时间，需要强大的地质年代学和热气压技术来确定矿物的生长，而不是在随后的一些造山运动中确定它们的冷却或重置。我们的工作将把Lu-Hf同位素定年技术应用于这些元素（石榴石、斜辉石、金红石和锆石）的高阻断温度矿物上，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，这些岩石经历了随后的低品位事件（角闪岩相变质作用）。这项工作将与新的热压测量技术相结合，旨在最大限度地减少在大多数榴辉岩中发现的高变异组合中遇到的问题。这些方法将与我们的合作者所做的制图、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冲断断层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.