DEFORMATION AND METAMORPHISM IN THE CROSSETT POND AREA, EASTERN ADIRONDACK MOUNTAINS, NY

Recent high resolution geochronologic data has led to significant revision of tectonic models for the geologic evolution of the Adirondack Mountains. Current models include the Elsevirian Orogeny (1245-1225 Ma), the Shawinigan Orogeny (1200-1160 Ma), intrusion of the AMCG suite (1160-1140 Ma), the Ottawan Orogeny (1090-1050 Ma), and post-Ottawan extensional collapse (<1050 Ma). Granulite facies metamorphism and associated deformation accompanied both Shawinigan and Ottawan orgenies. In order to better understand the nature of Shawinigan versus Ottawan deformation and metamorphism, a field-based study, including mapping, structural analysis, and petrologic analysis was conducted in the Crossett Pond area, located in the southern half of the Shelving Rock quadrangle. This area was chosen because earlier mapping suggests that this area is located in the nose of a W-NW trending, kilometer scale fold involving AMCG related rocks. We mapped a variety of rock types including pyroxene-bearing felsic gneisses with variable amounts of hornblende and garnet, weakly to strongly deformed metagabbroic to leucogabbroic rocks, white, garnet-bearing granitic rocks, marbles, and biotite-bearing paragneisses. The rocks in the southern portion of the area are well foliated with a consistent orientation of 285/30 with a strong lineation that variably plunges gently to the east or west. Rocks in the northern part of the area are not as strongly foliated and are variably oriented. Mineral assemblages in the mafic-leucogabbroic rocks include Grt-Cpx-Hbl-Pl+/-Opx suggesting metamorphism under upper amphibolite to granulite facies metamorphic conditions. These rocks document a period of high-grade metamorphism and deformation after the emplacement of the AMCG magmas (ca. 1155 Ma). In the context of the tectonic model above, this would either be at the very end of the Shawinigan Orogeny or during the Ottawan Orogeny.