KINEMATIC, METAMORPHIC, AND GEOCHRONOLOGIC CONSTRAINTS ON INTRUSION, DEFORMATION, AND METAMORPHISM SOUTHEAST OF DACY CLEARING, SHELVING ROCK QUADRANGLE, EASTERN ADIRONDACK MOUNTAINS, NEW YORK

Protomylonitic to mylonitic garnet-bearing, K-feldspar porphyroclastic orthogneisses and migmatitic paragneisses occur along a north-trending ridge to the east and south of Dacy Clearing in the Shelving Rock quadrangle in the eastern Adirondacks. These rocks are part of a well-foliated sequence of generally north-striking, gently to moderately east-dipping paragneisses and orthogneisses. Additional paragneiss lithologies include marble, quartzite, and garnet-biotite quartzofeldpathic gneiss. The orthogneisses range in composition from gabbroic to granitic. Most units in this sequence contain a moderate to strong, gently plunging, E-SE-trending lineation. Kinematic indicators within these gneisses suggest top to the west, thrust-sense motion. K-feldspar sigma porphyroclasts have polycrystalline tails but retain megacrystic cores. Metamorphic mineral assemblages in various lithologies are consistent with upper amphibolite to lower granulite facies metamorphism. We analyzed zircons from three samples by LA-ICP-MS in order to constrain the timing of intrusion, deformation, and metamorphism of these rocks. Preliminary interpretation of ²³⁸U/²⁰⁶Pb zircon data from samples 16TG244, a garnet-biotite gneiss with salmon-colored K-feldspar porphyroclasts, and 16TG241, a garnet-biotite gneiss with white K-feldspar porphyroclasts yields crystallization ages of 1153 ± 6 Ma (n=21, U/Th=2.7) and 1160 ± 9 Ma (n=14, U/Th=3.3). A third garnet-biotite quartzofeldspathic gneiss of uncertain affinity yielded an age of 1143 ± 10 Ma (n=16, U/Th=5.5). These ages suggest these rocks are part of the AMCG igneous suite. There are two possible interpretations for the timing of the west-directed, thrust-sense deformation suggested by the asymmetric sigma porphyroclasts in the orthogneisses. The current challenge is to distinguish whether this phase of deformation occurred either during the waning stages of the Shawinigan Orogeny (ca. 1200-1140 Ma) or during the Ottawan Orogeny (1090-1050 Ma). Additional research involving in-situ U/Pb zircon and/or U-Th-Pb monazite data is needed to constrain the timing of this phase of deformation.