Northeastern Section - 53rd Annual Meeting - 2018

Paper No. 41-4
Presentation Time: 2:50 PM


GONZALEZ, Joseph P., Department of Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244, BALDWIN, Suzanne L., Department of Earth Sciences, Syracuse University, Syracuse, NY 13244, KIM, Jonathan J., Vermont Department of Environmental Conservation, Vermont Geological Survey, 1 National Life Dr, Main 2, Montpelier, VT 05620-3902 and WEBB, Laura E., Geology, University of Vermont, 180 Colchester Ave., Burlington, VT 05405

Metamorphic rocks in the Tillotson Peak Complex, northern Vermont, provide evidence for subduction processes during the Taconic Orogeny (470-450 Ma). Previously documented metamorphic fabrics and 40Ar/39Ar data were interpreted to record deformation from multiple orogenies. A glaucophane total fusion 40Ar/39Ar age constrains the timing of Taconic high pressure (HP) metamorphism as 468 ± 6.4 Ma (Laird et al., 1984). The Burgess Branch Fault Zone (BBFZ), a polydeformed steep eastward dipping normal fault (present day), separates Tillotson Peak blueschists and metapelites from Ottauquechee Formation metapelites. P-T-t histories were derived from samples collected along a transect across the Tillotson Peak Complex and into the BBFZ. Total fusion 40Ar/39Ar geochronology on phengite and K white mica-chlorite multi-equilibrium thermobarometry were used to constrain P-T-t histories. Chemically homogeneous phengites in garnet mica schist from the Hazen’s Notch Formation within the Tillotson Peak Complex yielded pressures of 16-20 kbar assuming 450oC and a 40Ar/39Ar age of 480 ± 1.3 Ma (all ages 1σ). Another garnet mica schist yielded an age of 468 ± 1.8 Ma. A Hazen’s Notch Formation mica schist farther east yielded pressures of 9-12 kbar at 450oC and an age of 478 ± 1.3 Ma. Within the BBFZ, petrographic observations reveal chemical zoning and intergrowths of compositionally variable phengites provide evidence for recrystallization. A BBFZ Hazen’s Notch Formation phengite yielded P-T conditions of 6-8.5 kbar at 370oC, and an age of 407 ± 1.3 Ma. BBFZ Ottauquechee Formation phengites yielded concordant ages of 417 ± 1.2 Ma and 415 ± 1.8 Ma. A Stowe Formation phengite yielded an age of 436.7 ± 1.2 Ma. P-T data indicate that Hazen’s Notch schists may have formed at greater depths than previously determined. The 40Ar/39Ar total fusion ages support previously reported results, and we interpret the 468-480 Ma 40Ar/39Ar ages for Hazen’s Notch schist as possibly related to exhumation of HP metamorphic rocks. In contrast, 407-417 Ma recrystallized BBFZ phengites appear to be unrelated to exhumation of HP lithologies. Future research includes 40Ar/39Ar step heating experiments on Tillotson Peak phengites for P-T-t histories, and in-situ 40Ar/39Ar analyses on recrystallized BBFZ phengites for dating chemical domains.