Quantifying Microstructure Development in Small-scale Shear Zones in Gabbro
Mentor 1
Dyanna M. Czeck
Location
Union Wisconsin Room
Start Date
5-4-2019 1:30 PM
End Date
5-4-2019 3:30 PM
Description
The Grassy Portage Sill (GPS), located in the Rainy Lake region of northwestern Ontario, is an approximately 2.7 billion year old gabbroic intrusion that was caught within a major deformation zone related to granite-greenstone terrane formation. Previous research indicates regional deformation consisted of both right-lateral and shortening components. This deformation was accommodated in a variety of ways by different rock units. Strong rock units like the GPS generally are void of penetrative foliation, but have localized cm- scale ductile shear zones. We are analyzing one shear zone within a GPS metagabbro that is approximately 1 cm in width. It contains strong hornblende, biotite, and chlorite alignment inside the shear zone compared to isotropic mineral fabric outside of the shear zone. Understanding the microscopic structure and mineralogy of small shear zones like this one will provide a better understanding of shear zone dynamics. For analysis, our sample was prepared into petrographic thin sections that span the deformation from outside the shear zone gradually into the shear zone. Changing mineralogy and microstructure were quantified across the strain gradient by point counting. The analysis shows that our samples consist of plagioclase and hornblende, with minor amounts of biotite, chlorite, pyroxene and quartz. Plagioclase concentration decreases and quartz and hornblende concentrations increase towards the shear zone. Quartz appears in veins that indicate fracturing and fluid intrusion. Plagioclase microstructures including undulose extinction, bulged grain boundaries, fracturing, recrystallized edges and subgrain boundaries all increase in occurrence towards the shear zone indicating deformation by dislocation creep and brittle fracturing. Within the shear zone the plagioclase is completely recrystallized. Quartz grains also show some instances of undulose extinction, which increase towards the shear zone. Together, the microstructures indicate that deformation was accommodated by ongoing ductile dislocation creep processes with intermittent embrittlement likely due to fluid infiltration.
Quantifying Microstructure Development in Small-scale Shear Zones in Gabbro
Union Wisconsin Room
The Grassy Portage Sill (GPS), located in the Rainy Lake region of northwestern Ontario, is an approximately 2.7 billion year old gabbroic intrusion that was caught within a major deformation zone related to granite-greenstone terrane formation. Previous research indicates regional deformation consisted of both right-lateral and shortening components. This deformation was accommodated in a variety of ways by different rock units. Strong rock units like the GPS generally are void of penetrative foliation, but have localized cm- scale ductile shear zones. We are analyzing one shear zone within a GPS metagabbro that is approximately 1 cm in width. It contains strong hornblende, biotite, and chlorite alignment inside the shear zone compared to isotropic mineral fabric outside of the shear zone. Understanding the microscopic structure and mineralogy of small shear zones like this one will provide a better understanding of shear zone dynamics. For analysis, our sample was prepared into petrographic thin sections that span the deformation from outside the shear zone gradually into the shear zone. Changing mineralogy and microstructure were quantified across the strain gradient by point counting. The analysis shows that our samples consist of plagioclase and hornblende, with minor amounts of biotite, chlorite, pyroxene and quartz. Plagioclase concentration decreases and quartz and hornblende concentrations increase towards the shear zone. Quartz appears in veins that indicate fracturing and fluid intrusion. Plagioclase microstructures including undulose extinction, bulged grain boundaries, fracturing, recrystallized edges and subgrain boundaries all increase in occurrence towards the shear zone indicating deformation by dislocation creep and brittle fracturing. Within the shear zone the plagioclase is completely recrystallized. Quartz grains also show some instances of undulose extinction, which increase towards the shear zone. Together, the microstructures indicate that deformation was accommodated by ongoing ductile dislocation creep processes with intermittent embrittlement likely due to fluid infiltration.
