CORRELATION BETWEEN TH-PB ION MICROPROBE AGES OF MONAZITE INCLUSIONS IN GARNET AND GARNET GROWTH ZONING FROM UPPER AMPHIBOLITE FACIES SCHIST: IMPLICATIONS FOR GARNET DATING

We have conducted in situ Th-Pb ion microprobe dating of monazite inclusions in garnet and matrix grains in upper amphibolite facies pelitic schist from the Grouse Creek Mountains of northwestern Utah, in the interior of the Sevier orogenic belt. The garnets grew core-to-rim from ~600°C to ~635°C from a reaction involving the breakdown of staurolite and muscovite and growth of sillimanite and plagioclase. The ages of 67 monazite inclusions distributed among four garnets decrease from core to rim. Ages extrapolated to the garnet cores range from 81.0 ± 9.0 to 61.7 ± 6.1 (2s) Ma and extrapolated to the rims range from 51.5 ± 7.2 to 37.9 ± 6.2 Ma. Thirty-six ages determined from matrix monazites range from 23.9 ± 6.8 to 58.4 ± 12.5 Ma (1s). Explanations for the age correlation include Pb diffusion in monazite, dissolution and reprecipitation of monazite, and co-crystallization of monazite and garnet. If rapid Pb diffusion allowed isotopic equilibrium to be maintained between monazite grains and the matrix, then isotopic closure imposed by armoring at the time of occlusion may explain the age trends; matrix grains would record younger ages. Arguing against this interpretation is the fact that several small matrix grains record older ages than many larger inclusions. If dissolution and reprecipitation of monazite were very active at the time of garnet growth, armoring by garnet would impose isotopic closure by shielding them from further recycling; however, age-correlative sources of magmatic fluids are not present in the area. Metamorphic fluids derived from pelitic schist would not ordinarily be sufficiently acidic or alkaline to dissolve monazite. The co-crystallization of monazite and garnet, with monazite neoblasts deriving REEs from the breakdown of muscovite, is the most plausible explanation. Fluids from the nearby 27Ma Vipont granite may have been responsible for the variable resetting of matrix grains to younger ages. The age trends in garnet are thus interpreted as garnet growth ages and, because garnet growth occurred over a pressure increase of 0.9 kbar, they record a previously unrecognized period of Late Cretaceous to early Eocene thrust burial in the interior of the Sevier orogenic belt.