Obtaining Age Information on Metamphic Process Using Mn and HREE Diffusion in Garnets

Garnet is the main mineral containing Mn and HREE in metamorphic rocks. Usually Mn and HREE exhibits some zoning in garnets. Both Mn and HREE contents increase with temperature drop. Garnets from the polymetamorphic Belomorian complex (Tupaya Guba region) were studied using ion microprobe. These garnets from the gneisses are related to high pressure granulite stage I2 followed without interruption by high pressure amphibolite stage I3 (Drugova, 1999). The garnets are untypical zoned in HREE as low-grade garnets. HREE drop rimward significantly. The minimum temperatures of metamorphism were determined as 730-760° for garnet core and 680-700° for garnet rim by garnet-biotite thermometer. The pressure was constant - about 6.5-8 kbar. HREE exhibit a Rayleigh fractionation profile, Mn concentration has been homogenised to flat profile (1,18 wt. per cent at the core and increase in the outer rim upto 1,73 wt. per cent). It can be explained only by quick time (approximately 10-15 Ma) estimated for the metamorphic event. HREE diffusion rates more than two orders of magnitude slower than Mn diffusion rate (Van Orman et al., 2002), and more time of metamorphic event is required to homogenise the HREE profiles. The alike results of modelling of trace element and Mn data were presented for garnets from the Zanskar area of the Indian Himalaya - 3-10 Ma estimate for the metamorphic event (Ayres & Vance, 1994). References Drugova, G.M., Principal stages in metamorphic evolution of Chupinskaya series in Belomorsky folded belt. Proc. Rus. Min. Soc., No 3, 49-57 (1999). Van Orman, J. A., Grove, T. L., Shimizu, N. et al., Rare earth element diffusion in a natural pyrope single crystal at 2.8 Gpa. Contribs Mineral. Petrol., Vol. 142, 416-424. (2002). Ayres, M. and Vance, D., Constraints on the thermal evolution of the Indian Himalaya from manganese and erbium distributions in metapelitic garnets. Mineral. Mag., Vol. 58A, 34-35. (1994).