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.