Abstract

The Willow Spring Diorite, in the Black Mountains of the central Death Valley extended terrain, yields a U-Pb zirocn age of 11.6 ±0.2 Ma. 40Ar-39Ar analyses of hornblende and U-Pb analyses of sphene from this sample give ages of about 10 Ma, indicating that the batholith remained above about 500 °C for about 1.5m.y. after crystallization. Geologic relations indicate that the diorite was exposed to erosion by about 5 Ma, bracketing the evolution of the diorite within the time between onset of extension and uplift of the Black Mountain crustal block. Initial 87Sr/86Sr ratios range from 0.7060 (mafic diorite) to 0.7083 (felsic diorite) in samples collected from an area 200 x 100 m. These data, combined with structural and petrologic evidence, suggest that the batholith represents a rare view of a mid-crustal zone of mixing between mantle-derived magma and crustal material, often suggested to exist on the basis of observations of intermediate volcanic rocks. The Black Mountains may therefore expose a cross section through a continental rift magmatic system, from partially contaminated mafic to intermediate intrusive rocks in the deep crust up to their volcanic equivalents. The relatively low initial 87Sr/86Sr and high ϵNd(-1.4) of the diorite, which is within Proterozoic basement with ϵNd ∼-18, is consistent with significant amounts mantle input in continental rifts inferred from geophysical data and measurement of He isotopic ratios. Such additions to the crust in continental rifts may represent a significant process crustal growth. Futhermore, the emplacement of igneous bodies with a large mantle component may help reconcile the large crustal pull apart in the Basin and Range (in excess of 140 km) with the fact that the crust still has no normal thickness of about 30-35 km.

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