MINERAL OXYGEN-ISOTOPE RATIOS FOR THE BOEHLS BUTTE-GOAT MOUNTAIN METAMORPHIC COMPLEX, IDAHO - EVIDENCE FOR FAST COOLING

Oxygen isotope fractionations among minerals from kyanite-grade peliti c schists and amphibolites from the Boehls Butte-Goat Mountain metamor phic complex, northern Idaho, may have been produced by a two-stage cl ockwise P-T path upon retrograde cooling, Mineral fractionations betwe en high closure temperature (T-c) minerals and in bimineralic rocks wh ere one mineral has a high T-c yield equilibrium temperatures that con strain peak conditions to the range 700 degrees to 775 degrees C, Peak metamorphism most likely accompanied the development of the Idaho bat holith in Cretaceous time, Stable isotope temperature and cooling rate estimates together with previously published pressure limits and anal ogies to other metamorphic complexes in the region suggest a two-stage cooling trajectory, The earlier stage cooled from Late Cretaceous pea k conditions (8-11 kb, 700 degrees-775 degrees C) at a slow rate (5 de grees C/10(6) yrs) The rate accelerated dramatically in Eocene time, p erhaps to within the range 20 degrees to 30 degrees C/10(6) yrs, in re sponse to rapid uplift and erosional/tectonic thinning that allowed wa ter 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 s et during the later rapid cooling in Eocene time, The first, slower co oling stage P-T trajectory is bracketed by the hydrous pelite solidus and the anhydrous muscovite dehydration melting reaction, and O exchan ge proceeded under anhydrous conditions. Measured mineral fractionatio ns compared to model cooling exchange paths suggest that O exchange in the second, fast stage occurred in the presence of water. The whole r ock delta(18)O values of the schist and amphibolite show whole rock va ries by 3 and 1.7 permil, respectively and have not been homogenized i sotopically, Anorthosite within meters of the schists and amphibolites have delta(18)O values below 0 permil, yet the schists and amphibolit es show no evidence of O-18 depletion.