AGE OF METAMORPHISM AND PRESSURE-TEMPERATURE PATH FROM METAMORPHOSED DUNDERBERG SHALE IN THE WOOD HILLS, EASTERN NEVADA

The Wood Hills, located in eastern Nevada in the hinterland of the Sevier orogenic belt, includes sedimentary rocks of Cambrian to Devonian age that underwent burial, folding, and metamorphism associated with Sevier orogenesis. We studied the metamorphosed Cambrian Dunderberg Shale, a pelitic schist with the mineral assemblage garnet-staurolite-kyanite-plagioclase-muscovite-biotite-quartz, to determine the age of garnet growth, the conditions of metamorphism, and pressure-temperature path. Garnets are generally <2 mm and comprise about 3% of the rock’s volume. Determinations of pressure-temperature (P-T) conditions were evaluated using isochemical plots (pseudosections) calculated using DOMINO and the thermodynamic dataset of Holland and Powell, and GASP/garnet-biotite thermobarometry (calibrations of Holdaway). Thermobarometry indicated conditions of 610-630 °C and pressures of 6.8-7.1 kb. The pressures determined are about 1 kb higher than estimated by plotting the isopleths for the garnet core compositions (Xpy, Xgr, Xal, and Xsp) on an isochemical plot. Conditions of 625 °C and 6.6 kb, consistent with thermobarometry within uncertainties, fall inside the rock’s mineral assemblage field on an isochemical plot, and were used as starting conditions for garnet growth simulations to determine P-T paths. Using the program GIBBS, which performs calculations based on Duhem’s theorem, and selecting as monitors the changes in mole fraction of grossular (ΔXgr) and moles garnet (ΔMgar), it was determined that garnets grew core-to-rim during 10 °C of temperature increase while pressure decreased 500 bars then increased 400 bars. The path suggests that garnets grew during a transition from exhumation to burial. The timing of this path was determined using the Lu-Hf method of garnet dating. Four garnet fractions and two whole rocks yielded an isochron age of 82.8 ± 1.1 Ma (MSWD=15). Approximately 130 km to the northeast, in the Basin Creek area of the Grouse Creek Mountains, previously studied garnets in Neoproterozoic metasedimentary rocks (schist of Steven’s Spring) preserve a P-T path indicative of prograde burial and yielded a similar age of 85.5 ± 1.9 Ma. The similar age supports the hypothesis from previous studies that both areas experienced burial by the Windermere thrust at 82-86 Ma.