INTERGRANULAR DIFFUSION KINETICS OF FE AND MG DURING RETROGRADE METAMORPHISM OF A PELITIC GNEISS FROM THE ADIRONDACK MOUNTAINS

Pelitic granulite gneiss from the Irving Pond Formation in the souther n Adirondack Highlands, New York, contains chemical zoning in Fe and M g in both individual mineral grains and over a distance of millimeters in the polycrystalline matrix. Garnet porphyroblasts are zoned with i ncreasing X(Alm) and decreasing X(Prp) towards crystal rims. Compositi ons of individual biotite grains in the matrix vary systematically wit h distance from the garnet porphyroblasts up to about 1 mm, with the h ighest Mg concentrations in grains adjacent to the garnet and progress ive Fe-enrichment in grains further away. We have constructed numerica l models of the distribution of Fe and Mg components obtained during t he retrograde reaction and diffusion history of this rock based on the mineral assemblage, modes, textural information, and the preserved zo ning trends. These simulations indicate that the effective rate of int ergranular transport of Fe and Mg components through the polycrystalli ne matrix was only one order of magnitude faster than the rate of volu me diffusion in garnet. The relatively slow intergranular kinetics ind icate that grain boundaries did not provide rapid diffusion pathways. Absence of an interconnected fluid phase along grain edges and grain b oundary annealing likely inhibited intergranular diffusivities during cooling. Our models also suggest that the mineral assemblage experienc ed metamorphic temperatures in excess of 700 degrees C, followed by ra pid cooling. This T-t evolution is quite different than determined for the ca. 1060-1000 Ma regional granulite-facies metamorphism in the Ad irondack Highlands. Fe-Mg distribution in the gneiss likely reflects a n earlier metamorphic episode. h