QUANTIFYING EXHUMATION AND COOLING OF THE EOCENE ANACONDA METAMORPHIC CORE COMPLEX, WESTERN MONTANA

The Anaconda metamorphic core complex, western Montana, is bounded on the east by the Anaconda detachment. This gently east dipping (10-30°) detachment system stretches 100 km from the Flint Creek and Anaconda Ranges south into the Big Hole Valley. In the eastern Anaconda Range the detachment is characterized by a 300-500 m thick greenschist facies mylonitic shear zone. The mylonite is comprised of stretched granitoids with shallowly east dipping mineral lineations consistently oriented at 102-108°, indicating a top to the ESE extension direction. Structurally below the detachment upper amphibolite grade, attenuated Mesoproterozoic Belt Supergroup strata are intruded by Cretaceous-Tertiary plutons and sills. A brittle faulted carapace of unmetamorphosed Tertiary volcanic and volcaniclastic rocks overlies the detachment. 40Ar/39Ar mica ages from detachment level plutons and mylonites progressively young to the east, ranging from 51.9±1 to 38.5±1 Ma. These ages suggest extension along the detachment and cooling of the footwall began by ~52 Ma and continued until at least ~38 Ma. A synkinematic pluton within the mylonite reports a U-Pb (SHRIMP II) zircon age of 47.1±.14 Ma and a 40Ar/39Ar biotite cooling age of 38.5±1 Ma indicating a cooling rate of ~35°C/Ma during this period of extension. Furthermore, 40Ar/39Ar mica ages suggest a slip rate of <1 km/Ma for the Anaconda detachment. Immediately west and beneath the Eocene greenschist facies mylonite and exposed detachment the metamorphic grade abruptly increases. Belt rocks exhibit higher-grade, transposed metamorphic fabrics with orientations that contrast those of the mylonite to east. New thermobarometry from metapelitic strata within a km-scale ductile shear zone in the west reports metamorphic conditions of 700°C and 3-4 kbar. A granodiorite pluton overprints this shear zone and reports a 40Ar/39Ar biotite cooling age of 78.7±.8 Ma indicating motion along the shear zone had ceased and cooled below 325°C by ~79 Ma. These data indicate portions of the footwall were not affected by Eocene detachment and mylonite in the east and that not all mylonitic rocks in the Anaconda metamorphic core complex were deformed during Eocene time.