Petrogenesis of Mid-Tertiary Synextensional Volcanic Rocks in the Basin and Range Province, East-central Nevada and Adjacent Utah

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This project will undertake a detailed petrologic and isotopic study of volcanic rocks associated with mid-Tertiary, large- magnitude extension in the eastern Basin and Range Province. The goals are to: 1) completely characterize this moderate- to high-K calcalkalic suite; 2) petrogenetically model compositional changes in the suite with continued extension; and 3) evaluate the relative chemical and isotopic contributions of the continental crust and underlying mantle. At the onset of extension (-39 Ma), it appears that volcanism was crudely bimodal (silicic andesite to dacite, and high-silica rhyolite) and was characterized by small eruptions from scattered vents. At -35 Ma, volcanism shifted to dacite to low-silica rhyolite compositions erupted as voluminous, areallly extensive tuffs and lavas. Existing models for magmatism in the region suggest a contribution ranging from zero to as much as 50% of mantle- derived magmas. The compositional and isotopic diversity of the suite suggest a complex petrogenetic history involving several magma sources during early stages of magmatism, followed by large-scale mixing of these magmas in the middle (?) crust with continued extension and heat input. The proposed work is multifaceted, including field, petrologic and isotopic studies which will compliment ongoing research efforts in the area dealing with the structure, metamorphism, and isotopic composition of the basement rocks.