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

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Abstract Mountain 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.