STRUCTURE, PETROLOGY, AND GEOCHRONOLOGY OF ORTHOGNEISSES IN THE SOUTHERN SHELVING ROCK QUADRANGLE, EASTERN ADIRONDACKS, NEW YORK

The Adirondack Mountains have a complex geologic history that includes multiple phases of metamorphism, deformation, and intrusion. This study focuses on the southern Shelving Rock Quadrangle in the eastern Adirondacks. Field mapping shows that this area is dominated by orthogneisses with a minor quartzite unit and a discontinuous impure marble unit. Two lithologically distinct orthogneisses were studied in detail. The first is a well foliated, fine to medium grained, mafic to intermediate gneiss that commonly contains white plagioclase, dark red garnet, clinopyroxene, and hornblende. Orthopyroxene and biotite may occur as subordinate minerals. Megacrystic garnet crystals are locally present. The appearance of the garnet pyroxene hornblende gneiss varies between massive and compositionally homogeneous to compositionally layered. Isochemical models of this unit created using the Theriak Domino software suggest the pressure temperature conditions of metamorphism are consistent with upper amphibolite to lower granulite facies. The second orthogneiss is a well foliated, medium to coarse grained, garnet-bearing, quartzofeldspathic gneiss containing white to pink K-feldspar with dark grey quartz ribbons and small, dark red garnet. This unit contains K-feldspar crystals with recrystallized sigma tails that show top towards the northwest, indicating compressional shearing. Two distinct populations of lineations were seen in the garnet-bearing quartzofeldspathic gneiss. The first trends toward the east with moderate to shallow plunge. The second trends southeast/northwest with nearly horizontal plunge; this lineation population is associated with the thrust-sense sigma porphyroclasts. Zircons from two samples of the garnet-bearing quartzofeldspathic gneiss were analyzed using LA-ICP-MS in order to constrain the timing of intrusion and metamorphism. Results show igneous crystallization ages mainly from 1180 to 1130 Ma and metamorphic ages predominantly from 1070 to 1020 Ma. These dates suggest intrusion during the end of the Shawinigan Orogeny as part of the Anorthosite-Mangerite-Charnockite-Granite suite of intrusions and metamorphism during the Ottawan Orogeny.