COMPOSITION AND AGE OF MONAZITE IN PARAGNEISSES FROM THE LARSEMANN HILLS, PRYDZ BAY, EAST ANTARCTICA

The rare-earth phosphate monazite, (REE, Th, Ca)(P, Si, S)O4, can be used to estimate the temperature of metamorphism, oxygen fugacity and age of crystallization. It is a widespread accessory in granulite-facies paragneisses of the Larsemann Hills, Prydz Bay, East Antarctica. The current study is based on monazite from a cordierite-hercynite-biotite-monazite±xenotime segregation in biotite gneiss and from quartzite containing biotite, magnetite, apatite (several modal percent), and, in places, plagioclase, cordierite and xenotime. Monazite grains are commonly zoned in Y, Th and U. Solvus thermometry on monazite (average of 6 grains) associated with coarse-grained xenotime in the segregation gave 689±50 °C (Pyle et al., 2001, J. Petrol., 42, 2083) and 757±45 °C (Heinrich et al., 1997, J. metamorph. Geol., 15, 3); maximum temperatures for monazite associated with fine-grained xenotime in a quartzite are 607 °C and 682 °C, respectively. Gadolinium distribution temperatures (Gratz & Heinrich, 1998, Eur. J. Mineral., 10, 579) are 728±28 °C (segregation) and 636 °C (quartzite). The temperatures are somewhat lower than standard estimates of 800-860 °C determined by other methods and reported in the literature. Compositional data on the sulfur fit very closely to the coupled substitution Ca + S = REE + P, with up to about 5.5 mole % CaSO4 component. SO3 content reaches 1.89 wt%, almost as much as reported from the highly oxidized emery of the Cordlandt complex, New York (Tracy, 2004, annual GSA mtg. abstract). This relationship shows that oxygen had a high chemical potential in the metamorphic system. Only the segregation monazite contains sufficient Th for dating with the electron microprobe. Age maps show older cores, but zoning is complex. Very preliminary quantitative analyses gave a bimodal distribution with one cluster of ages lying mostly between 850 and 970 Ma; the second, between 520 and 600 Ma. Overall, results support previous findings indicating two distinct metamorphic events in the Larsemann Hills area.