February Dinner Meeting and Speaker: Joe Gonzales (Syracuse Univ) "Pressure-temperature-time histories across the Burgess Branch Fault Zone, northern Vermont" and Mariana Rhoades (St. John Fisher College), "Historic Quarries and the Stone Industry..."

Joe Gonzales (2017 CNYAPG Student Scholarship recipient), "Pressure-temperature-time histories across the Burgess Branch Fault Zone, northern Vermont"

Abstract

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 (footwall) blueschists and metapelites from Ottauquechee Formation metapelites (hanging wall). 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 thermobarometry were used to constrain P-T-t. 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 an 40Ar/39Ar age 480 ± 1.3 Ma (1). 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 intergrowth of different composition phengites suggesting partial to complete recrystallization. A BBFZ Hazen’s Notch Formation phengite yielded recrystallization P-T as 6-8.5 kbar at 370oC and an age of 407 ± 1.3 Ma. BBFZ Ottauquechee Formation phengites yielded 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 known. The 40Ar/39Ar total fusion ages are concordant with those of previous studies, and 468-480 Ma ages in Hazen’s Notch schist are 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, instead they are possibly related to the late-reactivation deformation.

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Mariana Rhoades will introduce her new book "HISTORIC QUARRIES & THE STONE INDUSTRY ERIE COUNTY, NY, 1820-1930"