MELT MICROSTRUCTURES PROVIDE EVIDENCE FOR PERVASIVE PARTIAL MELTING WITHIN SHEAR ZONES AND SURROUNDING COUNTRY ROCK

Late shear zones within the Lost Creek Gneiss (LCG) of the Grenville-aged Llano Uplift, central Texas, contain relict melt that formed contemporaneously with shearing. Prior to shear zone formation, the LCG underwent polyphase, penetrative deformation at uppermost amphibolite facies. Although surrounding country rock appears melt free, microstructures show the same melt textures as melt-present and melt-absent shear zones do, indicating the entire rock experienced partial melting in situ late in the deformation history.

Textures indicative of partial melting within all LCG samples include: serrate/cuspate grain boundaries, string of beads texture, former melt present along grain boundaries and along tilt walls in quartz and rare feldspars. Additionally, abundant myrmekite and intergranular albite rimming plagioclases adjacent to K-feldspar are most likely melt-related textures. The similarity in melt textures within the surrounding rock to those in late shear zones, indicates all melting was late. Further evidence is the preservation of these delicate melt textures in penetratively deformed gneisses and the presence of very fine blebs of melt along grain boundaries and some tilt walls in quartz that show a chessboard subgrain pattern indicative of earlier high T deformation. Intergranular albite rims on plagioclase also provide evidence for early melt-solid interactions, as evidenced by tilt walls cutting across these rims and the presence of albite rims on plagioclase within original K-feldspar augen.

Melt present shear zones (as determined in the field) only display subtle differences in thin section from those that are melt-absent or the surrounding country rock; they have less mica, less well-aligned felsic phases, are less well-foliated, more equigranular and are coarser grained. Thus, field relationships may suggest that melt is found only in areas of segregation and localization, but microstructures may show that partial melting is far more pervasive.