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

One of the largest exposures of Precambrian crystalline rock in the Basin a nd Range province of the southwestern USA is the Gold Butte block of the so uth 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 t hick that was exhumed by a deeply penetrating normal fault during Miocene e xtension. To test this interpretation as well as the use of the newly devel oped titanite (U-Th)/He thermochronometer, we examined the low temperature thermal history of the Gold Butte block with the aparite and titanite (U-Th )/He and muscovite Ar-40/Ar-39 thermochronometers. Apatite He ages average 15.2 +/- 1.0 (2 sigma) Ma throughout the block, indicating that the entire section was warmer than 70 degrees C prior to Miocene exhumation. Titanite He ages increase from 18.6 +/- 1.5 Ma near the paleobottom (west) end of th e 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 observe d at about 9.3 km paleodepth, and is interpreted as a fossil He partial ret ention zone for titanite. Assuming a pre-exhumation geotherm of 20 degrees C/km (consistent with earlier apatite fission track work), this depth would have corresponded to 196 degrees C prior to exhumation, indicating that la boratory-derived He diffusion characteristics for titanite that yield a clo sure temperature of about 200 degrees C are applicable and correct. Muscovi te Ar-40/Ar-39 ages are 1.0-1.4 Ga near the paleotop of the block, and 90 M a near the paleobottom. Together with (207)pb/(206)pb ages on apatite and t itanite, and an earlier apatite fission track transect across the Gold Butt e block, our data indicate that the continental crust at the western edge o f 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 ev ent clearly affected the mid-crust (deepest portions of Gold Butte), which may reflect accelerated cooling or a brief heating and cooling cycle at thi s time, after which gradients returned to about 20 degrees C/km prior to ra pid 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 southwes tern USA. (C) 2000 Elsevier Science B.V. All rights reserved.