PETROGRAPHIC ANALYSIS OF AN ADIRONDACK GNEISS

A petrographic study was undertaken of an outcrop of Precambrian rocks located in the Adirondack Mountains about 10 miles south of Tupper Lake, New York, along Route 30. The outcrop consists of marble that is sandwiched between two gneissic units, a garnetiferous quartz-feldspar-biotite gneiss at the south end and a quartz-feldspar-biotite gneiss at the north end. Zones containing approximately 80% quartz in both gneissic units, in conjunction with the interlayered marble, suggest a sedimentary protolith for this outcrop. The rocks therefore provide a good look at the metasedimentary host rocks in a terrane dominated by metamorphosed intrusive rocks.

The marble contains varying concentrations of diopside, calcite, quartz and feldspar, indicative of a protolith that varied from a limey mudstone to a siliceous limestone. Both gneisses contain a few relict orthopyroxene grains showing alteration to biotite. These Opx-bearing gneisses formed at peak metamorphic conditions and are indicative of granulite facies metamorphism at known Adirondack Highland P-T conditions. Symplectic intergrowths of biotite with quartz, and feldspar with quartz, suggest simultaneous co-nucleation during a late stage or post-Grenville retrograde event.