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

摘要造山，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深部和上地幔的过程驱动的，而不是直接观测到的。认识到这一点，使人们对受到高压和超高压变质作用的地体产生了极大的兴趣，这是因为地壳俯冲到深层的地球动力学问题，随后需要快速挖掘以保存高压/超高压矿物组合。在这一建议中，PI概述了一种旨在确定HP/UHP地体演化历史的新方法，即在含石榴石的榴辉岩相岩石上结合Lu-Hf和Sm-Nd地质年代学，由于高度优先的元素分配，通常会产生相对于Sm-Nd年龄更早的石榴石生长年龄的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限制相结合，这些岩石学限制可以使用伪剖面方法、石榴石尺寸和分布的3D层析成像以及P-T条件的经典岩石学限制。初步研究表明，榴辉岩相岩石的不同体积组成可能具有不同的石榴石生长历史，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- 20nm的石榴石生长的岩石中，Lu-Hf和Sm-Nd年龄应该是高度不同的；相反，一些大块成分预测石榴石生长主要发生在高压/超高压的峰值条件下，在这种情况下，Lu-Hf和Sm-Nd年龄预计是相同的。建议研究的重点是在瑞士和意大利极好地暴露的西阿尔卑斯山泽马特-萨斯蛇绿岩杂岩和大陆蒙特罗萨推覆体。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发掘中的作用。以前对这些和其他西阿尔卑斯山的高压/超高压单元的地质、岩石学和地质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研究之外，Rb-Sr和U-Pb地质年代学将应用于包含关键成分或结构特征的特定单元，我们相信这将使我们能够开始开发西阿尔卑斯山大陆和海洋单元的埋葬和挖掘机制模型。就拟议研究的更广泛影响而言，该项目将使两名博士生与几位欧洲科学家合作，接触从实地到实验室的广泛研究方法。PI认为这是通过跨学科（结构-岩石-地球化学）方法使美国学生接触国际科学界的绝佳机会。他们还希望通过计划参加麦克奈尔成就项目，为来自代表性不足群体的学生提供暑期本科研究经历的机会。

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.