PETROLOGY OF A MULTIPLE META-IGNEOUS INTRUSIVE OUTCROP, TUPPER LAKE, NEW YORK

An outcrop 10.5 miles South of Tupper Lake, New York, along the east side of Route 30, contains numerous metamorphic rock types (Precambrian in age) from pink garnetiferous gneisses to grey garnet-poor gneisses to black hornblende-plagioclase amphibolites. The pink gneisses are comprised of potassium feldspar and quartz, with lesser amounts of ortho- and clinopyroxene, plagioclase, biotite, and magnetite/ilmenite. Scattered garnets, commonly 2-3 mm in diameter, are found throughout the massive pink gneisses, with pockets containing higher densities of garnets. The low concentration of mafic mineral phases, particularly in the garnet rich zones, could be due to metamorphic reaction creating garnet and consuming the mafic phases. Potassium feldspar with perthitic texture suggests a granitic protolith, and the presence of orthopyroxene would make these charnockitic granites. Two hornblende-plagioclase-biotite amphibolites cut through the pink gneisses, suggesting that these were basaltic dikes that cut through the granite.

The grey gneisses become more common on the northern end of the outcrop and contain abundant plagioclase feldspar, hornblende and orthopyroxene, which have mostly been altered to chlorite. Garnet is largely absent, while mafic phases are more abundant. Presence of opx and low quartz content suggests these were mangeritic granites. Locally, hornblende crystals are several cms in diameter.

Pegmatite intrusions commonly form as pockets, up to a meter in diameter. One such pocket contains large potassium feldspar and hornblende grains along with pale green clinopyroxene and pale pink pleochroic orthopyroxene. Because the mineralogy of the pegmatites is similar to the mineralogy of the host rock, they most likely are the result of hot, watery fluids that recrystallized the gneiss in late stages of metamorphism.