PRELIMINARY MAPPING RESULTS FROM THE PORT HENRY QUADRANGLE, ADIRONDACK MOUNTAINS, NEW YORK

The Port Henry quadrangle lies on the eastern edge of the Adirondack Highlands. The Adirondack Highlands are comprised of voluminous AMCG metaigneous rocks that intruded the Highlands during the mid to late Shawinigan Orogeny and were subsequently metamorphosed during the Ottawan Orogeny. Preliminary 1:24,000-scale bedrock mapping in the Port Henry quadrangle shows that many of the contacts between orthogneiss units preserve their original intrusive igneous relationships, despite subsequent granulite facies metamorphism. Recent and historical debate has centered around Ottawan deformation and metamorphism in the Adirondack Highlands. At Port Henry, charnockite to biotite granite gneiss intruded gabbroic and anorthositic gneiss. The contact between the mafic and felsic units is irregular, interfingered and contains a wide zone of possible contact metamorphic effects. At the contact, the gabbroic rock is altered to a clinopyroxene-magnetite rich rock that often lacks plagioclase. Gabbro and anorthosite show mutually crosscutting relationships, block structure, and magma mingling textures. Coronitic garnet-bearing leucogabbro/gabbroic anorthosite contains megacrysts of plagioclase; no end-member anorthosite was observed. Magnetite-bearing granite intruded the gabbro and gabbroic anorthosite. The relationship between the granite and charnockite is more complicated, in part due to similarities in appearance between the two rocks. At some localities, the magnetite bearing granite contains xenoliths of charnockite, and in other places the contact looks gradational. The magnetite granite contains minor garnet near the contact, while the charnockite near the contact is magnetite rich.

Post-emplacement Ottawan deformation appears limited. Post-intrusive deformation and modification consists primarily of doming and migmatization, and upright folding. Upper amphibolite to granulite facies Ottawan metamorphism, characterized by the growth of amphibole, biotite, garnet, corona textures, migmatite production and the disappearance of orthopyroxene, is attribute to decompression melting from high temperature and pressures during orogenic collapse of the Grenville orogen.