Metamorphism in the Adirondacks. I. Petrology, Pressure and Temperature |
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Authors: | BOHLEN, STEVEN R. VALLEY, JOHN W. ESSENE, ERIC J. |
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Affiliation: | 1Department of Earth and Space Sciences, State University of New York Stony Brook, New York 11794 2Department of Geology and Geophysics, University of Wisconsin Madison, Wisconsin 53706 3Department of Geological Sciences, University of Michigan Ann Arbor, Michigan 48109 |
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Abstract: | Grenville Supergroup sediments and suites of pre- and syn-tectonicigneous rocks have been metamorphosed to the upper amphiboliteand granulite facies in the Adirondacks of northern New Yorkduring the Grenville orogeny about one billion years ago. Magnetite-ilmenite, alkali feldspar-plagioclase, calcite-dolomiteand garnet-clinopyroxene thermometry indicate that metamorphictemperatures (T) increase from about 650 ?C in the area westand northwest of Gouverneur to 700750 ?C near Coltonand along the Lowlands-Highlands boundary to 750800 ?Cin areas within and around the Marcy anorthosite massif. Thepresence of grossular-rich garnet + quartz without wollastonite+ plagioclase in calc-silicate rocks and the apparent absenceof metamorphic ferropigeonite in charnockites restrict maximummetamorphic T to less than 800850 ?C. Metamorphic pressures (P), determined from coexisting pyrite-pyrrhotite-sphalerite,garnet-rutile-sillimanite-ilmenite-quartz, fayalite-quartz-ferrosilite,fayalite-anorthite-garnet, ferrosilite-anorthite-garnet-quartz,kyanite-sillimanite, anorthite-garnet-sillimanite-quartz andthe stability of akermanite, are 6?57?0 kb near Gouverneurand increase to 7?58?0 kb in the central Adirondack Highlands. The above P-T data deduced from diverse mineralogical/chemicalsystems are interpreted as peak or near-peak conditions forAdirondack metamorphism. The compositions of thin retrograderims on garnets indicate a post-peak-metamorphic P-T path forthe Adirondacks with appreciable cooling (200300?) beforedecompression. Peak and retrograde P-T conditions inferred forthe Adirondacks are similar to numerous other granulite terranessuggesting that similar tectonothermal events are necessaryfor the formation of many granulite belts. |
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