IDENTIFYING INHERITED "RESTITE" AND MAGMATIC COMPONENTS IN GRANITE: APPLICATION OF THE MONAZITE DATING TECHNIQUE

Three distinct microprobe dates have been obtained from monazite grains in the Devonian biotite-muscovite +/- garnet Wallamatogus granite. Dates were determined from multiple spot analyses of individual grains within thin sections, thus preserving the spatial context of each grain. All dates are reported as the mean +/- two times the standard error of the mean. Preliminary results define: 1) a population of grains with dates ranging from 436 +/- 17 Ma to 408 +/- 7 Ma, 2) a second population with dates of 385 +/- 17 Ma (consistent with the "accepted" crystallization age), and 3) a population with dates of 308 +/- 22 Ma. Garnet-biotite pairs from micaceous enclaves and from cores of garnet crystals in the granite record liquidus temperatures of ~800^oC, whereas garnet rims yield near solidus temperatures of ~600^oC. Deformed garnet-biotite pairs record temperatures of 400-450^oC. Monazite grains with dates older than the "accepted" crystallization age are exclusively found within biotite crystals. This raises the possibility that both the monazite and the biotite crystals were inherited "restite" from the source or were incorporated from the crust during transport and/or emplacement. Monazite grains with dates that closely correspond with the "accepted" crystallization age occur in quartz and primary muscovite and are considered to be magmatic. The youngest population is interpreted to record the timing of brittle deformation that affected the pluton. "Inherited/xenocrystic" and "magmatic" monazite grains occur both in the granite and in micaceous enclaves found within the granite. This suggests that micaceous enclaves in the Wallamatogus granite are accumulations comprised of a combination of both "restite" or xenocrysts and primary magmatic crystals. The ubiquitous occurrence of "inherited/xenocrystic" monazite grains indicates that these grains are an intrinsic component of the magma, and opens the possibility that a significant amount of minerals and components in the Wallamatogus granite may also have been incorporated into the magma from source or country rocks during the cycle of granite genesis.