REFINED BAROMETRY IN KINZIGITIC GNEISSES, SOUTHEASTERNMOST ADIRONDACKS

Kinzigitic gneisses exposed along Route 4 south of Whitehall NY contain the essential mineral assemblage quartz + plagioclase + perthite + garnet + biotite + sillimanite. Accessory minerals include ilmenite or rutile, zircon and/or monazite and rare hercynitic spinel. This assemblage is compatible with upper amphibolite-granulite facies transition conditions. Previous work has demonstrated that these rocks underwent metamorphism and deformation synchronously with partial melting. Quantitative thermometry of kinzigites, and thermometry and barometry on other assemblages from the area, resulted in estimated peak temperatures of about 710 C and pressures of about 6-7 kbar, commensurate with the work of Bohlen and others (1985). In the current study, detailed microprobe analyses of the kinzigite were done for the purpose of determining pressures using the assemblage garnet-sillimanite-plagioclase-quartz. Garnet is abundant and locally quite coarse grained, up to a few cm in diameter. Sillimanite is present in some but not all layers. Where present it is coarse-grained and in some cases has a crystallographic preferred orientation with C-axes parallel to other mineral lineations and meso- and megascopic fold axes. Plagioclase and quartz are present throughout the unit. Garnet is almandine- and pyrope-rich, with low grossular component and almost no spessartine. Garnets are faintly zoned, with rims slightly higher in Fe and Ca and lower in pyrope than cores. Plagioclase compositions vary only slightly and systematically around X(anorthite)=0.17. Using these data and the calibrations of Ghent and of Koziol and Newton gives equilibration pressure of about 6 kbar using garnet cores and 7 kbar using garnet rims. Data must be considered preliminary but if the upswing in Ca to the rim of the garnets is consistent throughout the samples, then we consider these data evidence in favor of Whitney's anticlockwise metamorphic P-T-t path for the region.