TRANSITIONAL GRANULITE-FACIES MAFIC XENOLITHS IN THE 2.8 GA TRONDHJEMITE-TONALITE-GRANODIORITE (TTG) GNEISSES OF THE EASTERN BEARTOOTH MOUNTAINS, MT/WY: EVIDENCE FOR VARIABLE FLUID INTERACTIONS

Most of the mafic xenoliths found in the dominant 2.8 Ga TTG gneisses of the eastern Beartooth Mountains (MT/WY) are amphibolites that may contain orthopyroxene inclusions in calcic amphiboles. However, some relatively uncommon mafic xenoliths contain granulite-facies mineral assemblages inter-layered with upper-amphibolite-facies mineral assemblages. The granulite-facies assemblages are typically quartz + plagioclase (An_50-80) + orthopyroxene + magnetite + pargasitic amphibole +/- clinopyroxene +/- garnet +/- biotite +/- ilmenite whereas the amphibolite-facies layers contain quartz + plagioclase + pargasitic amphibole + magnetite +/- orthopyroxene. Thermobarometry of the transitional granulites indicate conditions of 720-800°C and 5-8 kbar. Where felsic veins from the 2.8 Ga TTG igneous rocks cut the mafic xenoliths, the mafic assemblages can be rehydrated in some instances (i.e. orthopyroxene converts to amphibole) and in other instances exhibit dehydration (i.e. amphibole converts to orthopyroxene). When rehydration occurs, there are many cases in which vermicular epidote forms on the margins of plagioclase and titanite forms after ilmenite. Thermometry suggests that this feature is generated at lower temperature ~650-700°C. Evidence from backscattered electron images, electron microprobe data, and optical cathodoluminescence imaging suggests that fluid with variable activity of H₂O is one of the primary controlling factors for the mineral assemblages and textures observed. The variable activity of H₂O in the layered granulite/amphibolite rocks are likely controlled by high-grade, channelized fluids. A late generation of K-feldspar or biotite nucleates on the margins of some plagioclase and orthopyroxenes, evidence for a hydrating fluid that is likely derived from the cross-cutting felsic veins; this signals minor-to-major metasomatic changes to the mafic xenoliths.