Northeastern Section - 56th Annual Meeting - 2021

Paper No. 3-1
Presentation Time: 8:05 AM


VALLEY, Peter M., U.S. Geological Survey, Florence Bascom Geoscience Center, Reston, VA 20192 and WALSH, Gregory, United States Geological Survey, Montpelier, VT 05602

The Adirondack Highlands are comprised of voluminous anorthosite-mangerite-charnockite-granite (AMCG) suite metaigneous rocks that intruded during the mid to late Shawinigan Orogeny and were subsequently metamorphosed during the Ottawan Orogeny. Despite subsequent granulite facies metamorphism, many of the contacts within the AMCG suite preserve their original igneous relationships. Recent 1:24,000 scale mapping in the Port Henry quadrangle shows that charnockite series magmas intruded gabbroic and anorthositic gneiss of the Whiteface facies, but there is evidence of magma mingling in certain outcrops. The contact is a broad transition from charnockite to mangerite or syenite to gabbro. Texturally the rock is remains gneissic, but the decrease in quartz content is critical to recognize. The percentage of mafic minerals increasse as the contact is approached, creating a “mafic charnockite” or syenite. This is followed by a zone of mixed felsic and mafic rocks: charnockite or syenite/monzonite interlayered with gabbro or gradational from syenite to gabbro. Small layers of granite, granite pegmatite, and hornblende pegmatite are locally present. Magnetite-rich layers are common near the contact. At one locality the contact is marked by an olivine-magnetite-amphibolite layer 1-2m thick. Rocks in the transition zone vary from intensely sheared, to moderately deformed, or undeformed and retain magmatic flow textures. Leucosomes are present near the contact in both gabbro and charnockite. It is uncertain if these leucosomes are from in-situ melting during regional metamorphism or intrusion of the charnockite magma. Large blocks of anorthosite are present in the charnockite and the foliation is deformed around them. As the contact is crossed the gabbro is locally biotite and hornblende rich.

Petrologically the mixed zone represents a mingling of AMCG magmas and the transition zone clearly shows pyroxene breaking down to amphibole ± biotite. It is unclear if the destabilization of pyroxene is the result of the intrusive nature of the contact or the manifestations of decompression and Ottawan metamorphism. The presence of amphibolite pegmatite in the transition zone would suggest amphibole is related to intrusion. However, hornblende is a common metamorphic mineral throughout the map area.