Geothermometry and geobarometry in pelitic rocks of north-central Wopmay Orogen (early Proterozoic), Northwest Territories, Canada

Pressure and temperature estimates for the pelitic schists and gneisses along the east flank of the Hepburn Batholith (early Proterozoic) have been determined by means of an electron-microprobe study of coexisting garnet, biotite, plagioclase, and cordierite.

Garnet-biotite geothermometry using recent calibrations yields results between 560 and 765 °C, consistent in spatial distribution with andalusite, sillimanite, and sillimanite–K-feldspar metamorphic zones. Calibrations of the garnet-cordierite equilibrium give temperature estimates on average 23 °C lower. Both sets of temperature estimates for the garnet-cordierite–bearing assemblages are consistent with the theoretical stability field of these rocks.

Geobarometry based on the 3 CaAl₂Si₂O₈ ⇋ Ca₃Al₂Si₃O₁₂ + 2 Al₂SiO₅ + SiO₂ equilibrium outlines a broad pressure culmination with variation from 2.6 to 5.2 kbar. The P-culmination coincides with a structural plunge culmination, as outlined by the change in plunge azimuth of folds of bedding, by a re-entrant in the trace (“half-window”) of a major premetamorphic thrust fault, and by a narrowing of the synmetamorphic funnel-shaped Hepburn Batholith.

Garnet-cordierite geobarometric calibrations generally give pressure determinations in the same range as the grossular-anorthite results. The spread in the garnet-cordierite pressure estimates from various calibrations, however, points to the need for considering the role of water when using the garnet-cordierite geobarometer.