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The 15.7 Ma Aztec Wash pluton is located in the central Eldorado Mountains of the Colorado River extensional corridor in southern Nevada, immediately south of the well-known imbricated volcanic sequence that has been widely cited in studies of extensional tectonism (e.g., Anderson, 1971). It is a shallow level (less than or equal to 5 km), essentially bimodal complex, primarily made up of granite (similar to 72 wt % SiO2) and diabase and diorite (similar to 54 wt % SiO2), with minor amounts of more mafic, intermediate, and highly evolved rocks. The mafic and felsic magmas mingled extensively but mixed only to a limited extent. Late synplutonic mafic and felsic dikes represent continuing injection of the same bimodal magmas. The mafic rocks have high incompatible element concentrations (e.g., K2O similar to 3 wt %, Ba similar to 1600 ppm, light rare earth elements 350 x chondrite) and enriched isotopic compositions (epsilon(Nd)-7.5, Sr-87/Sr-86 0.708); generation in ancient, enriched mantle lithosphere with limited subsequent crustal contamination is inferred, The granite is more potassic (similar to 5 wt %) than the mafic rocks, but it has comparable or lower concentrations of most incompatible elements; its isotopic composition (epsilon(Nd)-10, Sr-87/Sr-86 0.710) is intermediate between those of the mafic rocks and local ancient crust. The granites thus indicate hybridization of the crust by mafic magma, but it is unclear whether this hybridization occurred at deeper levels in this magmatic system, or during an earlier mid-Tertiary or Mesozoic magmatic event. Emplacement of the Aztec Wash pluton preceded peak east west extension in the northern Eldorado Mountains (similar to 15.2 Ma (Gans et al., 1994)), but it coincided with at least modest extension as indicated by the uniform NS orientation of the late dikes and the mafic injections into the magma chamber. Total extension and tilting of the pluton after crystallization was minor, in contrast to the east tilted area to the north and west tilted area to the south. Timing and style of extension are thus consistent with the pluton’s location within, and perhaps mechanical influence on, a major accommodation zone, as suggested by Faulds et al. (1990) and Faulds (1994).