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Description
U-Pb zircon data indicate that the Emigrant Gap composite pluton crystallized about 163 Ma. North of I-80 the pluton can be subdivided into a two-pyroxene diorite unit and hornblende granodiorite unit. The contact between these two units is gradational Additionally, petrologic study indicates that pyroxenes in the two-pyroxene diorite unit are magmatically replaced by hornblende in a broad band adjacent to the contact. Geochemical analyses of the two units reveals smooth variation among major and trace elements. These observations and data indicate that the two-pyroxene diorite unit is genetically related to the hornblende granodiorite unit. The emplacement of the Emigrant Gap composite pluton produced a hornblende hornfels facies metamorphic aureole in the adjacent Shoo Fly Complex. Intense strain associated with the intrusion of the pluton is characterized by a composite foliation (S-1 ), which was produced by mimetic growth, rotational and irrotational strain. S-1 progressively swings from northwesterly trends outside the metamorphic aureole to a northeast trend inside the aureole, becoming concordant with the margin of the Emigrant Gap composite pluton. This composite foliation contains both a Paleozoic and a Mesozoic component The Paleozoic component is older than about 375 Ma based on the observation that it does not occur within the Bowman Lake batholith (375 ± 10 Ma). The Mesozoic component becomes dominant when approaching the pluton, and is the same age as the pluton based on porphyroblast-cleavage relationships. Structures near the pluton formed as a result of oblique dextral-sense shear. A domainally developed crenulation cleavage (S-2) formed as a result of progressive deformation in the aureole. Possible mechanisms for deformation include, ballooning of the pluton during emplacement, response of thermally softened wall rocks to a weak regional stress field, or gravitational sagging. Additionally, porphyroblast-cleavage relationships indicate that ballooning of the pluton may have been an early deformational mechanism. The lack of any structures younger than 163 Ma (Middle Jurassic) implies that either, the Late Jurassic Nevadan orogeny did not produce penetrative structures or is not a regionally recognized deformational event.
