Abstract

The northern part of the Elizabethtown and Port Henry quadrangles, which includes the largest surficial exposure of olivine metagabbro in the Adirondack Precambrian rocks, was mapped at a scale of 1:15,840. The area is dominated by meta-igneous rocks of four types, which probably intruded in the following sequence (from earliest to latest): granitic gneiss, anorthosite, garnet-pyroxene gneiss, and metagabbro. The metagabbroic complex is a multiple intrusion forming a basinlike structure, with the central part still covered by a roof complex of granitic gneisses. The limited scale of the in situ differentiation could not produce the observed range of compositions; therefore, differentiation prior to intrusion is postulated. The observed differentiation trend toward lower silica content can be explained by pyroxene fractionation. Intrusive relationships are evident from locally preserved chilled margins in gabbros and from the development of hybrid zones in the surrounding granitic gneisses; these zones are especially common in the roof complex. Granulite facies metamorphism produced garnet-bearing mineral assemblages, but igneous assemblages and textures are still well preserved.

The garnet-pyroxene gneiss is characterized by the metamorphic assemblage plagioclase + garnet + clinopyroxene and by the presence of blue plagioclase mega-crysts. The gneiss forms intrusive sill-like bodies, usually in contact with anorthosite. In some places, the contact is transitional, suggesting a possible comagmatic origin.

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