Origin of Mesozoic and Tertiary granite in the western United States and implications for Pre‐Mesozoic crustal structure: 1. Nd and Sr isotopic studies in the geocline of the Northern Great Basin

Abstract

Mesozoic and Tertiary granitic rocks in and adjacent to the northern Great Basin (NGB) in Nevada and Utah display a wide range of initial 143Nd/144Nd (εND) and 87Sr/86Sr (εSr) values which vary regularly with geographic position. From the Klamath Mountains inland 500 km to central Nevada, granite εNd values decrease regularly from +8 to −6 and correlate with εSr values that increase from −20 to +60. In east–central Nevada, near the trace of the Roberts Mountains Thrust (RMT), the εND values decrease from −6 to an average of −18, while εSr becomes highly variable with values generally greater than +100. These isotopic discontinuities correspond to the west‐to‐east facies transition from pelagic clastic sedimentary rocks to shelf carbonates and to the shift in the dominant granite bulk composition from metaluminous to peraluminous. In the easternmost NGB a second discontinuity in esr occurs with values dropping to ∼+60; average εND remains at −18. Combined with known aspects of NGB geology the isotopic data suggest that west of the RMT, granites formed via interaction of magma derived from a LREE‐depleted mantle reservoir with an inland increasing proportion of assimilated continentally derived pelagic sedimentary rock. Variations in 87Rb/86Sr with Sr, and 147Sm/144Nd with εND, indicate that crystal fractionation accompanied assimilation, but that plagioclase was not an important fractionating phase. East of the RMT, granites appear to be primarily derived from Precambrian continental basement with little mantle input. The isotopic discontinuities near the RMT mark the western edge of Precambrian basement and occur 100–200 km east of the 87Sr/86Sr (=0.7060) line of Kistler and Peterman [1973]. The εSr discontinuity in the eastern NGB marks a boundary between Rb‐depleted (granulite?) lower continental crust to the east, and basement that has no ‘depleted’ lower crust to the west. The difference may be a result of lateral variations in metamorphic grade in the orogenic episode at ∼1.7 b.y. ago or a result of ductile attenuation of the lower crust during the late Precambrian continental rifting. The basement age appears to be intermediate between that of Wyoming (2.6 b.y.) and Colorado (1.8 b.y.) but cannot be precisely determined from the granite Nd model ages due to changes in Sm/Nd during granite magma generation. In contrast to the eastern NGB granites, Mesozoic batholith granites of the western United States have a lowest εND of only −8 even where thought to have been emplaced in crystalline basement, a difference which could be related to an inland decreasing flux of mantle‐derived magma into the continental crust in the Mesozoic. A model for subduction‐induced flow in the upper mantle beneath the continent, resulting in perturbations in the thermal structure of the continental lithosphere, is presented to account, in part, for the spatial difference in mantle magma flux and the distribution of silicic magmatism in the western United States. Overall, the NGB granite data support the concept that mantle magmas are added to continents mainly at the margin, with little mantle magma generation, or continental growth, in the interior, and demonstrate the potential of Nd and Sr isotopic data from granites in discerning fundamental features of deep continental crust.