MICROTEXTURAL EVIDENCE OF MELT AND HIGH TEMPERATURE DEFORMATION MECHANISMS IN A MESOPROTEROZOIC WRENCH ZONE, WET MOUNTAINS, COLORADO

High temperature shear zones developed at 1.44-1.39 Ga in the Wet Mountains, Colorado, during dynamic metamorphism that coincided with A-type to peraluminous granite magmatism. Temperatures exceeded 750°C, determined over a wide area in the Wet Mountains from Grt-Px parageneses in mafic granulites and Kfs-Sil-Bt-Grt assemblages in metapelites. Our research explores the role of melt upon deformation within a sinistral oblique wrench zone that hosts the Oak Creek pluton in the northern Wet Mountains, CO.

In thin section, granitic gneisses form fine-grained aggregates of plagioclase and K-feldspar with interpenetrating lobate grain boundaries ornamented with blebs and rims of optically continuous quartz that represent melt remnants. The microtextures indicate deformation at temperatures near the constituent minerals’ melting temperatures, via diffusive mass transfer/diffusion creep mechanisms, and under high strain rate. The alternation of grain boundary sliding and granular flow mechanisms across the shear zone may reflect contrasts in proportion of melt, with consequences for strain intensity / strain rate variations. Connectivity of optically uniform quartz (remnant melt) from margins to grain interstices, particularly near biotite, suggests that melt formed by biotite dehydration melting leading to feldspar weakening and granular flow.

To aid our understanding of the role of melt in high temperature wrench formation, we compare the microstructures of the Wet Mountains with those from the better known Aston-Hospitalet gneiss dome (Variscan) of the eastern Axial Zone of the Pyrenees, Spain. Both localities expose large variations in crustal depth, allowing us to compare deformed rocks across the temperature spectrum conducive to partial melting. We are examining whether the grain boundary sliding vs melt-assisted granular flow mechanism is prevalent within constrictional segments of the high temperature shear zones.