Age trends in garnet-hosted monazite inclusions from upper amphibolite facies schist in the northern Grouse Creek Mountains, Utah

We performed in situ Th–Pb dating of monazite in upper amphibolite facies pelitic schist from the Grouse Creek Mountains in northwest Utah. Sixty-six ages from inclusions in four garnet grains range from 37 to 72 Ma and decrease with radial distance from garnet cores. The age range of 30 matrix monazite grains overlaps and extends to younger ages than inclusions (25–58 Ma). The monazite grains are not intersected by cracks in the garnets, through which dissolution, reprecipitation or Pb loss might occur, and are generally too small (<20 μm) to allow for more than one age determination on any one grain. Processes that might explain inclusion ages that decrease with radial distance from garnet cores include: (1) Pb diffusion in monazite, (2) dissolution and reprecipitation of monazite, and (3) co-crystallization of monazite and garnet. After consideration of these possibilities, it is concluded that the co-crystallization of monazite and garnet is the most plausible, with monazite neoblasts deriving REEs from the breakdown of muscovite. Garnet ages derived by regression of the inclusion ages and assuming a constant rate of volume increase during garnet growth yield model ages with a maximum difference between core and rim of 22 m.y.