INVESTIGATION INTO THE P/T CONDITIONS OF MULTIPLE FRICTIONAL MELT VEIN GENERATIONS HOSTED BY A DEEP-CRUSTAL SHEAR ZONE
The Cora Lake shear zone is a 4-6 km wide and >90 km long granulite to upper amphibolite-facies (1.06 – 0.8 GPa, 800-700 °C) sinistral strike-slip structure that was active in an intracontinental setting during the earliest stages of the Trans-Hudson Orogeny (~1.88 Ga). The shear zone hosts an extensive network of variably deformed and recrystallized PST. One sample contains four distinct generations of PST; the oldest three (1-3) are plastically deformed and contain chemically distinct garnet populations, whereas the youngest generation (4) shows no deformation and very little re/neo- crystallization. Generations 1-3 contain sinistral shear sense indicators, and the presence of neoblastic micron-sized orthopyroxene, garnet and plagioclase in the earliest generation argues strongly for the synkinematic formation of PST networks within the shear zone while in the deep crust. Preliminary thermobarometry of the earliest generation using the Al-in-Opx thermometer and the Grt-Opx-Pl-Qz barometer indicate minimum PST formation conditions of 0.74 GPa and 760 °C. Thermobarometric calculations and equilibrium phase diagrams for each of the three earliest PST veins can provide a rare window into the timing and conditions of alternating brittle and ductile processes in the lower crustal levels of an intracontinental strike-slip structure.