Helium and argon thermochronometry of the Gold Butte block, south Virgin Mountains, Nevada

Abstract

One of the largest exposures of Precambrian crystalline rock in the Basin and Range province of the southwestern USA is the Gold Butte block of the south Virgin Mountains, about 15 km west of the Colorado Plateau. It has been interpreted as a largely continuous crustal cross-section about 15–20 km thick that was exhumed by a deeply penetrating normal fault during Miocene extension. To test this interpretation as well as the use of the newly developed titanite (U–Th)/He thermochronometer, we examined the low temperature thermal history of the Gold Butte block with the apatite and titanite (U–Th)/He and muscovite ^(40)Ar/^(39)Ar thermochronometers. Apatite He ages average 15.2±1.0 (2σ) Ma throughout the block, indicating that the entire section was warmer than 70°C prior to Miocene exhumation. Titanite He ages increase from 18.6±1.5 Ma near the paleobottom (west) end of the block, to 195±15 Ma near the paleotop (east) end. A rapid change from mid-Tertiary to increasingly older titanite He ages to the east is observed at about 9.3 km paleodepth, and is interpreted as a fossil He partial retention zone for titanite. Assuming a pre-exhumation geotherm of 20°C/km (consistent with earlier apatite fission track work), this depth would have corresponded to 196°C prior to exhumation, indicating that laboratory-derived He diffusion characteristics for titanite that yield a closure temperature of about 200°C are applicable and correct. Muscovite ^(40)Ar/^(39)Ar ages are 1.0–1.4 Ga near the paleotop of the block, and 90 Ma near the paleobottom. Together with ^(207)Pb/^(206)Pb ages on apatite and titanite, and an earlier apatite fission track transect across the Gold Butte block, our data indicate that the continental crust at the western edge of the Colorado Plateau resided at moderate geothermal gradients (and slowly declined in temperature) from 1.4 Ga to about 100–200 Ma. A 90 Ma cooling event clearly affected the mid-crust (deepest portions of Gold Butte), which may reflect accelerated cooling or a brief heating and cooling cycle at this time, after which gradients returned to about 20°C/km prior to rapid exhumation in the Miocene. This work thus supports previous structural and thermochronologic studies that suggest that the Gold Butte block is the thickest largely continuous cross-section of crust exposed in the southwestern USA.

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