Seeing through the Ottawan Overprint, Adirondack Mtns., NY: Integrating Microstructural Analysis, Geothermobarometry, and in-situ Monazite Petrochronology
Creators: name: Mistikawy, Justin 
Publisher: name: University of Massachusetts Amherst
Year: 2020

description: Integrating field observation with petrochronology is critical for understanding the tectonometamorphic evolution of the North American Grenville Province. Despite methodological advances in geothermobarometry and geochronology, incorporating these data into larger models of the Adirondack Mountains remains particularly challenging due to the presence of multiple generations of deformation, primarily related to the ca. 1190 – 1140 Ma Shawinigan and ca. 1090 – 1020 Ma Ottawan Orogenies (McLelland et al.,2013). The Rock and Bear Ponds area is a dome of tight-to-isoclinally folded metapelites in structural contact with orthogneiss. Fold generations are orthogonal and partitioned such that the northern area is dominated by an earlier episode of D2 deformation and an E-W S2 fabric and the southern by D3 deformation and a N-S S3 fabric. Observed assemblages include Qtz + Pl + Kfs + Bt + Sil + Grt + Gr ± Py ± Mnz ± Zr in metapelite and Hbl + Pl + Grt + Qtz + CPx ± Ilm in metagabbro. Metapelitic garnet is anhedral and overgrows a strongly transposed S1 fabric. A population of small high-Y monazite cores are associated with S1 and yield a mean weighted date of 1174 ± 5 Ma. Monazite observed in S2-defining phases and matrix have very high-Y & HREE cores that yield dates of 1068 ± 7 Ma and are surrounded by low-Y & HREE mantles (1048 ± 4 Ma) with irregular high-Y rims (1023 ± 6 Ma). These data suggest garnet growth followed the transposition of a strong Shawinigan S1 fabric during D2 and D3 folding events, ca. 1090 – 1070 Ma. The timing of this shortening is interpreted to coincide with the early Ottawan Orogeny, ca. 1090 – 1050 Ma. Geothermobarometric calculations of S2-associated phases constrains peak metamorphic conditions to 700 – 750 ± 50 °C and 6.5-7.5 ± 1 kbar; these data are well in agreement with those reported in multiple studies, thereby suggesting that regional PT calculations reflect Ottawan tectonometamorphic conditions (Bohlen et al., 1985; Spear & Markussen, 1997; Storm & Spear, 2005). High-Y & HREE rims are also observed and interpreted to reflect garnet breakdown ca. 1050 – 990 Ma during decompression and orogenic collapse, which has become increasingly reported in the eastern Adirondack Mountains over the last decade (Wong et al., 2012; Chiarenzelli et al., 2017; Regan et al., 2019; Williams et al., 2019). The Mesoproterozoic metapelite of the Rock and Bear Ponds area record an intense polydeformational history and therefore provide a valuable window into episodic middle-to-lower crustal deformation and metamorphism. The integration of focused microstructural observation with geothermobarometric and timing constraints has provided much insight into the structural evolution of the Adirondack Mountains.
Integrating field observation with petrochronology is critical for understanding the tectonometamorphic evolution of the North American Grenville Province. Despite methodological advances in geothermobarometry and geochronology, incorporating these data into larger models of the Adirondack Mountains remains particularly challenging due to the presence of multiple generations of deformation, primarily related to the ca. 1190 – 1140 Ma Shawinigan and ca. 1090 – 1020 Ma Ottawan Orogenies (McLelland et al.,2013). The Rock and Bear Ponds area is a dome of tight-to-isoclinally folded metapelites in structural contact with orthogneiss. Fold generations are orthogonal and partitioned such that the northern area is dominated by an earlier episode of D<sub>2</sub> deformation and an E-W S<sub>2</sub> fabric and the southern by D<sub>3</sub> deformation and a N-S S<sub>3</sub> fabric. Observed assemblages include Qtz + Pl + Kfs + Bt + Sil + Grt + Gr ± Py ± Mnz ± Zr in metapelite and Hbl + Pl + Grt + Qtz + CPx ± Ilm in metagabbro. Metapelitic garnet is anhedral and overgrows a strongly transposed S<sub>1</sub> fabric. A population of small high-Y monazite cores are associated with S<sub>1</sub> and yield a mean weighted date of 1174 ± 5 Ma. Monazite observed in S<sub>2</sub>-defining phases and matrix have very high-Y & HREE cores that yield dates of 1068 ± 7 Ma and are surrounded by low-Y & HREE mantles (1048 ± 4 Ma) with irregular high-Y rims (1023 ± 6 Ma). These data suggest garnet growth followed the transposition of a strong Shawinigan S<sub>1</sub> fabric during D<sub>2</sub> and D<sub>3</sub> folding events, ca. 1090 – 1070 Ma. The timing of this shortening is interpreted to coincide with the early Ottawan Orogeny, ca. 1090 – 1050 Ma. Geothermobarometric calculations of S<sub>2</sub>-associated phases constrains peak metamorphic conditions to 700 – 750 ± 50 °C and 6.5-7.5 ± 1 kbar; these data are well in agreement with those reported in multiple studies, thereby suggesting that regional PT calculations reflect Ottawan tectonometamorphic conditions (Bohlen et al., 1985; Spear & Markussen, 1997; Storm & Spear, 2005). High-Y & HREE rims are also observed and interpreted to reflect garnet breakdown ca. 1050 – 990 Ma during decompression and orogenic collapse, which has become increasingly reported in the eastern Adirondack Mountains over the last decade (Wong et al., 2012; Chiarenzelli et al., 2017; Regan et al., 2019; Williams et al., 2019). The Mesoproterozoic metapelite of the Rock and Bear Ponds area record an intense polydeformational history and therefore provide a valuable window into episodic middle-to-lower crustal deformation and metamorphism. The integration of focused microstructural observation with geothermobarometric and timing constraints has provided much insight into the structural evolution of the Adirondack Mountains.