PETROLOGIC CHARACTERIZATION OF HIGH PRESSURE METAMORPHIC ROCKS ALONG THE BURGESS BRANCH FAULT ZONE AT THE TILLOTSON PEAK COMPLEX, VERMONT

The Burgess Branch Fault Zone (BBFZ), is a north-south trending high angle normal fault that bounds the Tillotson Peak Complex on the eastern limb of the Green Mountain Anticlinorium of north central Vermont (Kim et al., 1999). Blueschists and eclogites in the Tillotson Peak Complex are one of the few exhumed high pressure (HP) metamorphic rock suites preserved from the Taconic orogeny and occur in the footwall of the BBFZ. It has been proposed that the Prospect Rock Fault forms the structurally higher thrust bounding the Taconic tectonic wedge from the cover rocks. Hinterland directed extension along the BBFZ exhumed the HP rocks in the core of the wedge with the cover rocks structurally above the Prospect Rock Fault (Lamon 2001). This study aims to delineate the pressure and temperature (P-T) conditions, timing, and exhumation rate of rocks in the hanging wall, foot wall, and within the BBFZ. Oriented samples were collected parallel and perpendicular to the BBFZ to determine the P-T-time(t)- history of exhumed rocks in the Tillotson Peak Complex. Petrographic observations identified white micas that define three generations of deformational fabrics. Characterization of a suite of samples from the BBFZ, hanging wall, and foot wall was done by making WDS X-ray compositional maps and taking spot analyses. Data was processed using XMapTools (Lanari et al., 2013). Three X-ray maps and 120 spot analyses were used to determine pressure estimates based on the Massonne and Schreyer, (1987) phengite thermobarometer. P-T conditions in the foot wall and within the BBFZ are ~8.0-8.9 kbar, assuming a previously determined temperature of 450 ± 100 °C (Laird et al, 1981). The hanging wall preserves pressure conditions of ~6 kbar corresponding to temperatures of ~300 °C, utilizing chlorite thermometry. These findings suggest a pressure difference across the BBFZ implying that hanging wall rocks were at shallower depths than footwall rocks, in agreement with tectonic models that call for exhumation of HP metamorphic rocks during the Taconic orogeny via down to the east normal faulting. Compositionally characterized white micas will be analyzed using the ⁴⁰Ar/³⁹Ar technique and compared with apatite fission track data to determine exhumation rates for Taconic HP rocks in the hanging wall, foot wall, and within the BBFZ.