Precambrian rocks near Lake Hopatcong, New Jersey, form a part of the intensely deformed and metamorphosed Reading Prong. The Lake Hopatcong area is divisible into several northeast-trending fault blocks, each of which contains a mappable stratigraphic sequence of paragneisses and granitic or syenitic rocks.

The paragneisses generally are well foliated and well layered. They consist chiefly of biotite-feldspar-quartz gneisses and quartz-oligoclase leucogneisses that are interpreted as metamorphosed potassium-rich sandstones and quartz keratophyres, respectively. A thin well-foliated unit of biotite-plagioclase gneiss is thought to be a metamorphosed sill of gabbroic anorthosite.

The granitic and syenitic rocks generally form thick, regionally concordant sheets. They are typically foliated and are composed chiefly of microcline microperthite and plagioclase (or mesoperthite), quartz, and iron-rich hornblende and clinopyroxene. These foliated granitic and syenitic rocks are viewed as syntectonic magmatic intrusives. One regionally discordant, unfoliated sheet of clinopyroxene quartz syenite is probably a late tectonic magmatic intrusive.

Mineral assemblages in Lake Hopatcong paragneisses may be assigned to the hornblende granulite subfacies of metamorphism. The presence of Ca-bearing mesoperthite in biotite-feldspar-quartz gneiss indicates that metamorphic temperatures exceeded 700° C, and the assemblage garnet-sillimanite-quartz without cordierite indicates that load pressure was greater than 2.5 kb. The rocks have thus probably been buried to depths in excess of 10 km.

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