Megacrystic potassium feldspar gneiss

Megacrystic potassium feldspar gneiss

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This lithology is characterized by 1-4 inch long megacrysts of microperthite in a groundmass of quartz, oligoclase, hornblende, orthopyroxene, garnet, and oxide. Although the contacts of this unit are everywhere conformable, the rock is believed to represent a metamorphosed igneous plutonic rock. Similar lithologies are present at other stratigraphic horizons in the Adirondacks. In the Northwest Adirondack Lowlands, Buddington referred to a similar lithology as the Hermon Granite Gneiss. The present samples would be more appropriately referred to as metamorphosed quartz-monzonites. Sporadic occurrences of orthopyroxene indicate that, locally at least, the lithologies are charnockitic.
Thin section shows plagioclase, quartz, hornblende and orthopyroxene.

1980

Western Minerals Inc.

Adirondack Highlands-collected from a road cut 2.25 miles north of Hamilton-Fulton counties line at sharp curve on NY Route 10, approximately 5.5 miles north of Junction wjth Route 29A.

Precambrian

Purchased from Western Minerals Inc.

Rooster Hill Formation

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