STRAIN LOCALIZATION DUE TO GRAIN SIZE SENSITIVE CREEP IN THE SCITUATE GRANITE, RHODE ISLAND

The Scituate Granite (Rhode Island, USA) is a foliated granite which has small-scale (1-2 cm wide) shear zones that extend for over several meters, cross-cutting road-cut sized outcrops near the center of the granite body. The shear zones are subparallel to the foliation, indicating both formed during the same deformation event. In order to determine the mechanisms that initiated strain localization in this granite, I have analyzed the microstructures of all phases and quartz water contents both inside and outside of the shear zones. The host rock surrounding the shear zones is composed of coarse (>3500 microns) porphyoclasts of quartz, microcline, albite and biotite. Quartz grains in the host rock are significantly recrystallized and contain chess-board type subgrains. Microcline and albite grains in the host rock contain exsolution lamellae, myrmekites, undulatory extinction and are recrystallized at the edges. These microstructures are generally consistent with dislocation creep. Within the shear zones the recrystallized grains are much finer in size (~80 microns) than outside of the shear zone and, but the phases are commonly mixed. Four-grain junctions are common in both mono-phase zones and mixed phase zones, which is consistent with deformation by grain boundary sliding-accommodated diffusion creep. Water contents in quartz grains were measured using the FTIR (~300 H/10⁶ Si) and were found to be an order of magnitude lower than observed in quartzites deformed at greenschist grade conditions. Modeling of the strengths of the different phases using flow laws indicate that initially, recrystallization of quartz leads to grain size sensitive creep and strain should localize within the new, fine quartz grains, which is consistent with our microstructural observations. As feldspars recrystallize and are incorporated within the shear zones, they also deform by grain size sensitive mechanisms and accommodate significant strain. The low water content in quartz (~300 H/10⁶ Si) would imply that quartz is strong, however it is considerably more deformed than other phases in the granite, indicating that the high water contents necessary for deformation of quartz at greenschist grade conditions are no longer necessary at amphibolite grade conditions.