AN UPDATED N-SHAPED PRESSURE-TEMPERATURE-TIME PATH FROM THE GROUSE CREEK MOUNTAINS, UTAH: AN EXAMPLE OF AN OROGENIC CYCLE

Past investigations of garnet schist in the Basin Creek area of the Grouse Creek Mountains have established an N-shaped pressure-temperature (PT) path. The burial segments (legs of the N) were determined by numerical thermodynamic simulation of garnet growth zoning from two horizons (upper horizon, UH, and lower horizon, LH) of the schist of Stevens Spring. Garnet growth occurred by different reactions at different conditions in the two horizons due to differences in bulk composition. Prograde decompression between the burial segments was inferred from several lines of evidence, most importantly that the burial segments could not be joined without invoking a significant pressure drop and temperature increase between them. This inference is supported by partial reaction of the older (UH) garnet to form muscovite-free reaction rims, and by secondary prograde reequilibration along the corroded garnet rims. Here we present new calculations of the PT path derived by combining the Gibbs method (program GIBBS by F.S. Spear and others) with the G-minimization approach of de Capitani and Petrakakis (programs THERIAK and DOMINO). Plots generated by DOMINO from the bulk composition allow the garnet core composition to be located in PT space, providing the initial P and T needed for the simulation of garnet growth zoning by GIBBS. The other initial values needed for GIBBS (mineral assemblage, modes and compositions) are calculated by THERIAK using the initial P and T. GIBBS calculates changes in P and T by specifying changes in any two variables (e.g. DXgr, DXalm). This version of the path is more tightly constrained than previous versions; it tracks garnet and muscovite consumption in the UH rocks associated with the decompression segment, and it provides a simple explanation for the production of retrograde staurolite associated with garnet consumption during the development of extensional shear fabrics in the LH rocks. A previously determined Lu-Hf age on the UH garnets indicates growth at 85.5 ± 1.9 Ma and published Th-Pb ages on monazite inclusions that co-crystallized with LH garnets indicate growth began at about 60 Ma, thus bracketing the exhumation at 85-60 Ma. We propose the path represents an example of an orogenic cycle, in which an episode of synconvergent extension associated with mantle delamination occurs between episodes of crustal thickening.