How Long Does It Take to Make and Preserve a HP/UHP Terrane? Petrologic and Geochronologic Studies of the Zermatt-Saas and Monte Rosa Units of the Western Alps

造山，特别是与大陆-大陆碰撞有关的造山，基本上是由发生在地壳和上地幔深处的过程驱动的，而不是直接观察到的。认识到这一点，人们对经历了高压和超高压变质作用的地体产生了极大的兴趣，因为与地壳俯冲到深部相关的地球动力学问题，以及随后的快速折返，需要保存HP/UHP矿物组合。在这项建议中，PI概述了一种新的方法，旨在确定高压/超高压地体的进化史，即含石榴石榴辉岩相岩石的Lu-Hf和Sm-ND联合年代学，由于高度优先的元素分配，通常会产生与Sm-Nd年龄相比相对偏向于较早石榴石生长的Lu-Hf年龄。他们打算将详细的地质年代学数据与石榴石生长的岩石学约束结合起来，这些约束可以使用伪剖面法、石榴石大小和分布的3D层析成像以及P-T条件下的经典岩石学约束来提供。根据前期工作，不同榴辉岩相岩石的块体成分具有不同的进积石榴石生长历史，这应该提供明显不同的Lu-Hf和Sm-Nd年龄。例如，在可能经历了10-20英里的岩石中。前进型石榴石生长的Lu-Hf和Sm-ND年龄应相差很大；相反，一些块体成分预测石榴石的生长主要发生在HP/UHP的高峰期，在这种情况下，Lu-Hf和Sm-ND的年龄应该是相同的。其中一个关键问题是这两个高压/超高压单元的相互关系，以及它们在深俯冲和折返中的作用。以前对西阿尔卑斯山这些和其他高压/超高压单元的地质学、岩石学和年代学研究为新提出的研究提供了广泛的解释背景。除了上面提到的Lu-HF和Sm-ND研究，Rb-Sr和U-Pb年代学将应用于包含关键成分或构造特征的特定单元，我们相信这将使我们能够开始开发埋葬和挖掘西阿尔卑斯山大陆和海洋单元的机制模型。就拟议研究的更广泛影响而言，该项目将与几名欧洲科学家合作，使两名博士生接触到从野外到实验室工作的广泛研究方法。国际学生联合会认为这是一个极好的机会，让美国学生通过跨学科(结构-岩石学-地球化学)的方法接触国际科学界。他们还希望通过有计划地参加麦克奈尔成就计划，为来自代表性不足群体的学生提供暑期本科生研究体验的机会。

AbstractMountain building, particularly that associated with continent-continent collisions is fundamentally driven by processes that occur in the deep crust and upper mantle, far from direct observation. Recognition of this has led to a great interest in terranes that have been subjected to high- and ultra-high-pressure metamorphism because of the geodynamic problems associated with subduction of crust to deep levels, followed by the rapid exhumation that is required to preserve HP/UHP mineral assemblages. In this proposal, the PI's outline a new approach aimed at determining the prograde histories of HP/UHP terranes, namely combined Lu-Hf and Sm-Nd geochronology on garnet-bearing, eclogite-facies rocks, which, because of highly preferential elemental partitioning, will generally produce Lu-Hf ages that are relatively biased toward earlier garnet growth as compared to Sm-Nd ages. They intend to combine detailed geochronological data with petrologic constraints on garnet growth that may be provided using the pseudo section approach, 3D tomographic imaging of garnet size and distribution, and classic petrologic constraints on P-T conditions. Based on preliminary work, it seems clear that different bulk compositions of eclogite-facies rocks will have had different prograde garnet growth histories, which should provide distinctly different Lu-Hf and Sm-Nd ages. For example, in rocks that may have experienced 10-20 m.y. of prograde garnet growth, Lu-Hf and Sm-Nd ages should be highly disparate; in contrast, some bulk compositions predict that garnet growth occurs largely during peak HP/UHP conditions, and in that case, Lu-Hf and Sm-Nd ages would be expected to be the same.The focus of the proposed research will be the Zermatt-Saas ophiolite complex and continental Monte Rosa nappe of the Western Alps, which are superbly exposed in Switzerland and Italy. One of the key questions is the interrelation of these two HP/UHP units, and their roles in deep subduction and exhumation. Previous geologic, petrologic, and geochronological studies of these and other HP/UHP units of the Western Alps provide a broad interpretive context for the new proposed studies. In addition to the Lu-Hf and Sm-Nd studies noted above, Rb-Sr and U-Pb geochronology will be applied on specific units that contain key compositional or structural features that we believe will allow us to begin developing mechanistic models for burial and exhumation of continental and oceanic units of the Western Alps.In terms of the broader impacts of the proposed research, the project will expose two Ph.D. students to a broad range of research approaches, from field-based to lab-based work, in collaboration with several European scientists. The PI's view this as an excellent opportunity to expose U.S. students to the international scientific community through an interdisciplinary (structure-petrology-geochemistry) approach. They also hope to have opportunities for summer undergraduate research experiences for students from underrepresented groups through planned participation in the McNair Achievement Program.