SYNDEFORMATIONAL PARTIAL MELTING REACTIONS AND DEFORMATION LOCALIZATION WITHIN ANATECTIC MIGMATITES OF THE WET MOUNTAINS, CENTRAL COLORADO

The Wet Mountains of central Colorado preserve evidence for in situ partial melting preferentially associated with intense deformation at a variety of crustal levels. From north to south, the degree of partial melting increases, with rare muscovite-dehydration reactions in the north and pervasive partial melting via both biotite dehydration and granitic wet melting reactions in the south. Deformation is more uniform and intense in rocks in the southern Wet Mountains than those in the north. Outcrops in the north that do provide evidence for intense deformation show more evidence for partial melting than those that do not, and deformation is localized in the leucosomes.

In rocks that contain garnets formed via biotite dehydration reactions, the timing of melting can be constrained to be syn-late tectonic. These garnets overgrow a pre-existing foliation, defined by sillimanite and/or biotite, however, the leucosome surrounding the garnets is invariably deformed, with quartz showing chessboard extinction and plagioclase displaying continuous undulatory extinction. Some rocks preserve relict melt channels, found along grain boundaries and subgrain boundaries in quartz, and these locally show minor undulatory extinction. In the south, there is evidence for back-reaction with melt, indicated by coronas of plagioclase with biotite or cordierite surrounding garnet and an increase in Mn content towards the rims of garnet. This suggests garnet has been resorbed and has reacted with melt to produce the originally stable assemblage of biotite, plagioclase and quartz +/- sillimanite. Back-reaction with melt occurred while deformation persisted as evidenced by abundant subgrain formation in plagioclase rimming garnet. These observations indicate deformation both preceded and post-dated biotite dehydration melting.

In granitic gneisses, abundant evidence for partial melting includes cuspate grain boundaries, pseudomorphs of melt on grain and subgrain boundaries and at triple junctions. These rocks are highly deformed with chessboard extinction in quartz, high temperature grain boundary migration and some have been mylonitized. Throughout the Wet Mountains deformation is concentrated in areas where melt producing reactions occurred, and melt appears to be localized along deformation-related features.