LATEST CRETACEOUS TO EARLY EOCENE RENEWED SHORTENING IN THE INTERIOR OF THE SEVIER-LARAMIDE OROGEN: INSITU TH-PB ION MICROPROBE DATING OF MONAZITE

New Th-Pb ages from inclusions in garnet date a period of renewed shortening, following marked Late Cretaceous extensional exhumation, in the hinterland of the Sevier orogenic wedge. Garnets in two horizons of the pelitic schist of Stevens Spring from Basin Creek in the Grouse Creek Mountains, northwest Utah, grew by different reactions at different times along the same P-T path, and provide a record of two periods of thrusting separated by marked decompression and heating: structural studies are consistent with this kinematic history. The younger garnets yield garnet-biotite rim geothermometry temperatures of 635 degrees C, display growth zoning, and Gibbs method numerical simulations of garnet growth indicate a pressure increase from core to rim of 0.9 kbar, indicative of about 3.5 km of structural burial. Th-Pb dating of 8 to 20 micron sized monazite inclusions in three garnets, on the CAMECA ims 1270 ion microprobe at UCLA, shows an overall pattern of ages becoming younger from core to rim. Forty-four analyses from three garnets vary in age from 70 to 46 Ma. Consideration of the diffusivities of Pb in monazite and in garnet, and the metamorphic temperature, leads to the interpretation that the Th-Pb monazite dates are ages of occlusion of monazite in garnet, and thus record the age of garnet growth and the derived P-T path. Interpretation of the monazite ages as occlusion ages have the following implications for western Cordilleran tectonics. (1) Renewed shortening of latest Cretaceous to Early Eocene age, previously unrecognized in the hinterland, was synchronous with shortening on the Absaroka (Late), Medicine Butte, and Hogsback thrusts of the fold-thrust belt and shortening in the Laramide foreland province. Continued thickening within the interior of the Sevier-Laramide orogen occurred during, and may have been dynamically linked to, lengthening of the orogenic wedge by propagation of thrusts towards the foreland. (2) Earlier exhumation interpreted from decompression P-T paths and geologic structures has an upper age limit bracketed by latest Cretaceous growth of the garnet core, and contrary to other published interpretations, predates, but was not coeval with, the bulk of Laramide shortening.