Petrogenetic and Tectonic Studies in an Exposed Cross- Section of Young (approx. 100 Ma) Continental Crust - Southern Sierra Nevada, California

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The southern Sierra Nevada offers an oblique section through young batholith-generated sialic crust from surface (volcanic) levels to deep (granulitic) levels. Regional mapping and Pb/U zircon geochronology reveal structural continuity through this crustal section for igneous-metamorphic assemblages of ~100 Ma, making it one of the youngest exposed continental crustal sections in the world. Compositional and structural patterns observed in this section in conjunction with geophysical and volcanic xenolith data pose important problems on sialic crustal genesis and the nature of lower continental crust. Some specific aims of the proposed research include (1) isotopic, petrochemical and petrographic characterization of an apparent major mantle-derived igneous suite that was added to the crust at ~100 Ma, and consideration of its role in the formation of a granulitic lower crust; (2) study of crustal melting phenomena imposed by this mantle igneous suite on a variety of pre-existing crustal materials that are known to characterize much of the pre- batholithic framework based on regional map relations; (3) con- struction of mixing models between the young mantle-derived component and pre-existing crustal materials and the application of these models to large-scale problems of batholith petrogenesis; (4) application of appropriate thermobarometers to deep-level exposures as a refinement of the crustal section model and for placing constraints on the physical conditions of peak metamorphism and crustal melting; and (5) structural and geochronological analysis of selected ductile shear zones and fault systems which appear to be intimately related to batholith emplacement and volcanic venting. Large-scale goals of this research are to model the major components and material transport patterns that contributed to the production of the southern Sierra sialic crustal column and to consider the implications of such phenomena for broader issues of continent structure and evolution.