Mineral paragenesis of the granulite facies in the Lake George area, New York

The Lake George Area belongs to the southeastern part of the Adirondack Highlands. The rocks of this area show variations in mineral assemblages which are caused by differences in bulk chemical composition. This makes it unsuccessful to use a subdivision of the granulite facies which is suggested by de Waard (1965). The CFM diagram (Abbott, 1982) is used to show the relationship between composition and paragenesis of the Lake George Area. In this study, a different result has been found in the order of partitioning of Fe between coexisting minerals, namely, garnet\u3ehornblende\u3ebiotite\u3eorthopyroxene\u3eclinopyroxene. The hornblende granitic gneiss belongs to a different subfacies from the mafic granulite and charnockitic gneiss according to Abbott\u27s subdivision of the facies. But, following the order of XFe of the minerals from this study, the typical mineral assemblage of this rock, hornblende-garnet-biotite, may belong to the same subfacies as the other rock types of this area. The study of biotites of metapelite in the Lake George Area indicates different substitution mechanisms in different regions of the granulite facies. From the garnet-orthopyroxene (clinopyroxene) thermometer, 710°C was obtained for the highest estimate of the metamorphism. This temperature condition appears to be consistent with the observed mineral assemblages of a lower-grade part of the granulite facies and confirms the isotherm pattern of Bohlen et al. (1985). Cooling ages of 971±17 Ma, 882±7 Ma and 656±17 Ma were obtained for hornblende, biotite and microcline, respectively, with 40Ar/39Ar dating method. With cooling ages of minerals and reasonable closure temperatures for each mineral, average cooling rates are calculated: 2.7°C/Ma for the time interval of peak metamorphism-hornblende, 2.2°C/Ma for hornblende-biotite, and 0.7°C/Ma for biotite-microcline