TIMING OF DEFORMATION AND METAMORPHISM OF GRENVILLIAN METAPELITE, ROCK AND BEAR PONDS AREA, EASTERN ADIRONDACK MOUNTAINS, TICONDEROGA, NY, USA

Integrating field observation with petrochronology is critical for understanding the tectonometamorphic evolution of the North American Grenville Province. Despite methodological advances in petrologic modeling and geochronology, incorporating these data into larger tectonic models of the Adirondack Mtns. remains particularly challenging due to the presence of multiple tectonic events, including the 1190 – 1140 Ma Shawinigan and 1090 – 1030 Ma Ottawan orogenies. Targeted field work in a well-exposed region south of the Marcy anorthosite massif was performed to assign dates to deformation components. The Rock and Bear Ponds area is a dome of tight-to-isoclinally folded graphitic-sulfidic metapelite in structural contact with amphibolite/metagabbro. Fold generations are orthogonal and partitioned such that the northern area is dominated by earlier D₂ deformation and an E-W S₂ and the southern by D₃ deformation and a N-S S₃. Observed assemblages are qz + pl + bi + sil + kfs + grt + gr ± py in metapelite and hbl + qz + pl + cpx + grt ± ilm in metagabbro. Metapelitic garnet is anhedral, lacks compositional zoning, and overgrows a strong sub-horizontal (S₁) fabric. Mineralogical textures and petrologic modeling suggest garnet, alkali-feldspar, and melt were produced by breakdown of quartz, plagioclase, aluminosilicate, and biotite at peak conditions of ~700 °C and 6-8 kbar (Bohlen et al., 1987; Storm and Spear, 2005). High-Y monazite in association with the garnet-hosted S₁ fabric yield ca. 1150 Ma dates. Monazite observed in S₂-aligned biotite and feldspar have high-Y (+ U and HREE) cores that yield dates of 1073 ± 9 Ma and are surrounded by low Y, U, HREE domains (1049 ± 10 Ma) with irregular high-Y rims (1016 ± 10 Ma). These data suggest early garnet growth over a strong (ca. 1150 Ma – Shawinigan) D₁ fabric. D₂ and D₃ folding (doming) occurred during the deformational phase of the Ottawan Orogeny, 1080 – 1060 Ma. High-Y and U rims are interpreted to reflect garnet breakdown and anatexis during 1050 – 990 Ma extension and exhumation. The Mesoproterozoic metapelite of the Rock and Bear Ponds area record an intense polydeformational history and therefore provide a valuable window into episodic mid-crustal deformation and metamorphism during the assembly of the Adirondack Mtns. and supercontinent Rodinia.