A POSSIBLE EXAMPLE OF PULVERIZED GRANITE FROM THE BRITTLE-DUCTILE TRANSITION

We describe a 100–200-m-wide brittle damage zone in foliated granite flanking the brittle-ductile, strike-slip Kellyland fault zone (KFZ) in eastern Maine that exhibits many characteristics common to pulverized crystalline rocks in the upper crust. The damage zone is marked by pervasive fracturing. But, pre-fracture grain shapes are preserved, and there is little or no rotation or shear offset across fractures. Fractured feldspar grains exhibit curviplanar bands or branching networks of very fine-grained recrystallized feldspar in healed fractures; many dilitational fracture networks filled by fine-grained biotite or, locally, quartz; intergranular fractures extending into adjacent feldspar and quartz grains; and fracture-controlled weathering that obscures or obliterates primary features. Initial color cathodoluminescence (CL) images and X-ray composition maps of feldspar reveal many 1–30-mm-wide, bands of low-luminescence material with the same chemical composition as host grains. These bands are pervasive down to the 100-mm scale and are likely healed fractures. Quartz in the damage zone exhibits many healed intra- and intergranular fractures marked by 5–50-mm-wide trails of neoblasts that cut across deformation bands, subgrains, and bands of dynamically recrystallized quartz. Quartz also hosts local dilitational fractures filled with fine-grained biotite. Initial CL images of quartz also reveal many low-luminosity bands that may be healed fractures. Networks of recrystallized biotite filling dilitational fractures in feldspar and quartz can be traced to highly contorted, partially recrystallized relict biotite grains. Damage intensity increases approaching the KFZ, and the damage zone grades into a ~5-m-wide cataclasite zone that is itself overprinted by a 100–200-m-wide ultramylonite zone. Fractured granite and cataclasite also are cut by many small mylonitic shear zones derived from cataclasite and, locally, probable pseudotachylyte. Overprinting of damage-zone and brittle fault rocks by mylonitic fabrics indicates fracturing occurred at the granite brittle-ductile transition. We have not observed similar damage features in the poorly exposed metasedimentary rocks juxtaposed with granite across the KFZ in this area. This indicates the damage zone may be asymmetrical.