Monazite as a Sensitive Indicator of the Timing and Type of Fluid Activity During Metamorphism

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Ayers and MillerEAR-0126020The susceptibility of monazite to fluid-induced recrystallization has important implications for interpretation of ages measured in-situ by electron or ion microprobe. To refine interpretations, complementary laboratory and field experiments havebeen designed to identify and characterize monazite recrystallization and alteration in granitoids and associated rocks. The solubility of monazite will be measured as a function of pressure, temperature and fluid composition using the double-capsule method to identify the conditions and fluid compositions that lead to enhanced solubility and recrystallization. Experiments run in cold-seal pressure vessels at 400-600 degrees C and 0.1-0.2 GPa will characterize the effects of varying concentrations of potential complexing ligands F, Cl and OH. Large natural crystals will then be run in fluid atidentified conditions of high solubility to characterize the rate of recrystallization and the effect of alteration on intracrystalline zoning and elemental and oxygen isotopic composition. Duplication of zoning styles and oxygen isotope systematics observed in natural monazites will create a firm link between monazite alteration and fluids, establish the conditions under which alteration occurs, and greatly increase confidence in interpretations of monazite ages.Field studies are designed to test the hypothesis that the ages of altered zones in monazites correspond to the timing of fluid influx, and to test important hypotheses specific to those field areas. The effects of changing temperature, fluid composition, and fluid/rock ratio on preexisting monazite will be examined in contact metamorphic aureoles in which magmatic fluids infiltrate monazite-bearing country rocks: the Ireteba pluton in southern Nevada, the Birch Creek pluton in the White Mts. of east-central California, and the Red Hill W-Sn deposit in northern Guangdong Province, Southeast China. Fruitful geochronological studies of monazite and zircon in the Dabie Shan central UHP (ultra-high pressure) zone will be extended to the enigmatic northern Dabie complex to determine the areal extent of UHP metamorphism and the assignment of the northern Dabie to the Yangtze or Sino-Korean blocks.