U-PB ZIRCON GEOCHRONOLOGY OF META-ANORTHOSITES IN THE BITTERROOT RANGE, WESTERN MONTANA: IMPLICATIONS FOR METAMORPHISM AND ANATEXIS DURING CORE COMPLEX FORMATION

Approximately twenty small, metamorphosed anorthosite bodies crop out in the NE portion of the Bitterroot core complex in western Montana. These anorthosite bodies range in size from small pods (4-6 m²) to a relatively large 2.5 km² body along the northern wall of Bass Creek Canyon. These anorthosite bodies are enclosed by sillimanite gneiss that is interpreted to be the high-grade metamorphosed equivalent of the Prichard Formation, the lowermost unit of the Mesoproterozoic Belt Supergroup. The anorthosite bodies have been variably interpreted as tectonically-emplaced pre-Belt Supergroup basement, possibly correlating with the 1.78 Ga Boehl’s Butte anorthosite in northern Idaho, or as a residuum of crustal anatexis of metapelitic rocks during the Cretaceous-Eocene evolution of the plutonic-metamorphic core complex. The larger bodies of anorthosite contain abundant cm- to m-scale Mg-Al-rich schistose layers with kyanite, sillimanite, cordierite, and phlogopite. The anorthosite bodies have a strong spatial association with deformed biotite-quartz diorite, with complete gradation between the two units. Field relationships between the anorthosite, biotite-quartz diorite, and sillimanite gneiss are well exposed. Local zones of sillimanite-K-feldspar metamorphic assemblages, indicating muscovite dehydration melting, are developed around the anorthosites. Preliminary ²⁰⁶Pb/²³⁸U zircon dates by CA-ID-TIMS from one anorthosite sample yield a spread from 68.35 ± 0.5 Ma to 60.98 ± 0.03 Ma, with one grain recording an Eocene date of 49.91 ± 0.03 Ma. This range of ages corresponds with published monazite ages of 64-56 Ma for a sample of sillimanite gneiss to the north of Bass Creek Canyon and zircon dating of main stage plutonism from S-type granitoids of 66-55 Ma. The onset of extension in this region began c. 53 Ma, so the youngest zircon in the anorthosite likely reflects growth during decompression. Further investigation into the petrogenesis of the Bass Creek anorthosites and associated rocks is planned, with LA-ICP-MS zircon dating and trace element analysis from spatially associated sillimanite gneiss and biotite-quartz diorite to determine whether the ages obtained reflect metamorphism and anatexis, and whether there is any evidence preserved in inherited cores for Paleoproterozoic zircon growth.