Isotopic and Petrologic Investigations Into the Development of a Young (~100 Ma) Sialic Crustal Section-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 sialic crustal sections in the world. Compositional and structural patterns observed in this section in conjunction with geophysical data pose important problems on sialic crustal genesis and the nature of lower continental crust. NSF funds are sought for the PI and two graduate students for isotopic, petrologic and field studies into such problems. The funds sought constitute the main basis of support and scientific utilization of a recently acquired (NSF-Caltech cost-share) multicollector thermal ionization mass spectrometer. Some specific aims of the proposed research include (1) isotopic, geochemical 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 prebatholithic framework based on regional map relations; (3) application of appropriate thermobarometers to deep-level exposures as a test for a gross crustal structure model and for constraints on the physical conditions of peak metamorphism and crustal melting; and (4) construction of mixing models between the young mantle component and pre-existing crustal materials and the application of these models to large-scale problems of batholith petrogenesis and emplacement. The mixing phenomena will be studied mainly by Nd-Sr isotopic techniques in well- understood deep-level domains which exhibit melting and mixing relations with the aid of selected oxygen isotope and trace element data. 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. This research will also shed new light on auriferous ore generation that was widespread along the western wall of the Sierra Nevada batholith.