Mineral oxygen isotope ratios for the Boehls Butte-Goat Mountain metamorphic complex, Idaho: Evidence for fast cooling

Abstract

Oxygen isotope fractionations among minerals from kyanite-grade pelitic schists and amphibolites from the Boehls Butte-Goat Mountain metamorphic complex, northern Idaho, may have been produced by a two-stage clockwise P-T path upon retrograde cooling. Mineral fractionations between high closure temperature (TC) minerals and in bimineralic rocks where one mineral has a high TC yield equilibrium temperatures that constrain peak conditions to the range 700° to 775°C. Peak metamorphism most likely accompanied the development of the Idaho batholith in Cretaceous time. Stable isotope temperature and cooling rate estimates together with previously published pressure limits and analogies to other metamorphic complexes in the region suggest a two-stage cooling trajectory. The earlier stage cooled from Late Cretaceous peak conditions (8-11 kb, 700°-775°C) at a slow rate (5°C/106 yrs). The rate accelerated dramatically in Eocene time, perhaps to within the range 20° to 30°C/106 yrs, in response to rapid uplift and erosional/tectonic thinning that allowed water infiltration into the schists. Plagioclase-biotite fractionations are small (2.0 ± 0.3 permil) relative to values commonly observed in metamorphic rocks and yield apparent fractionation temperatures well below peak temperatures. The plagioclase-biotite fractionations were set during the later rapid cooling in Eocene time. The first, slower cooling stage P-T trajectory is bracketed by the hydrous pelite solidus and the anhydrous muscovite dehydration melting reaction, and O exchange proceeded under anhydrous conditions. Measured mineral fractionations compared to model cooling exchange paths suggest that O exchange in the second, fast stage occurred in the presence of water. The whole rock δ18O values of the schist and amphibolite show whole rock varies by 3 and 1.7 permil, respectively and have not been homogenized isotopically. Anorthosite within meters of the schists and amphibolites have δ18O values below 0 permil, yet the schists and amphibolites show no evidence of 18O depletion.