A COMPARISON OF PRESSURE-TEMPERATURE-TIME HISTORIES ACROSS THE BURGESS BRANCH FAULT ZONE, NORTHERN VERMONT

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 450^oC 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 450^oC 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 370^oC, 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.