ISOTOPIC SOURCE CONSTRAINTS ON LATE CRETACEOUS CORDILLERAN INTERIOR MAGMATISM: THE WHITE ROCK WASH PLUTON, NEVADA

In eastern Mojavia, the grossly extended Colorado River extensional corridor (GREC) was characterized by voluminous synextensional bimodal magmatism that modified the Paleoproterozoic bedrock of SW Laurentia in the Miocene. In the Newberry Mountains of southernmost Nevada, the Mojavia basement was intruded by the 68.5 Ma, two-mica White Rock Wash pluton (WRWP), prior to the Miocene magmatic event (Miller et al. 1997, Haapala et al. 2005). The WRWP is circular (≥ 5 km in diameter) and comprises leucocratic, peraluminous granite with two magmatic micas and plagioclase dominant over alkali feldspar. The WRWP cuts sharply across the Precambrian wall rocks and also includes marginal pegmatite. Geochemically, the WRWP is moderately peraluminous, high in silica (71.5-73.5 wt.%) and Sr (180-550 ppm), very low in the HREE (Gd through Lu 4-8 ppm), and is rather similar to the other Late Cretaceous-Paleocene peraluminous plutons described from the CREC region. However, the WRWP is leucocratic throughout and not bimodal as, for instance, the Iriteba pluton in the Eldorado Mountains farther north is (Kapp et al. 2002). The WRWP has ε_Nd (at 65 Ma) of -15.6 to -16.4 (granite), Sr_i of 0.71215-0.71255 (granite) and 0.71369-0.71731 (pegmatite), μ₂ (S&K second-stage ²³⁸U/²⁰⁴Pb) of 9.64-9.74, ω₂ (S&K second-stage ²³²Th/²⁰⁴Pb) of 41.8-43.0, and S&K second-stage model age of 469-507 Ma (feldspar from granite and pegmatite). The WRWP feldspars define, together with whole-rock and feldspar fractions from the Precambrian basement rocks of the Newberry Mountains, a ²⁰⁷Pb/²⁰⁶Pb isochron of 1692 ± 170 Ma (MSWD 0.38). This is compatible with a major crustal (Mojavia-type) source component in the WRWP. The exposed Precambrian wall rock is, however, less radiogenic in Nd [ε_Nd (at 65 Ma) -18.7 to -21.7] and more radiogenic in Sr [⁸⁷Sr/⁸⁶Sr (at 65 Ma) 0.7269-0.7349] and thus the Mojavia upper crust cannot have been the sole source of the WRWP. A minor less mature source component has been suggested for the Iriteba pluton, based on Jurassic cores of zircon (Kapp et al. 2002). Variation in lower crust isotope composition (and overall mantle separation age) across the Cordilleran Interior could also have resulted in the observed variability.