FLUID FLOW AND DEFORMATION WITHIN LOWER CRUSTAL DUCTILE SHEAR ZONES

What role do aqueous fluids play in localizing deformation within the lower crust? In order to gain insight into the mechanisms of fluid flow, shear localization, and the dynamics of syn-metamorphic deformation within deep levels of continental lithosphere, chemical and structural analyses were conducted on several sheared garnet amphibolite samples from the northern Swedish Caledonides. Thermobarometric estimates combined with structural analyses of deformation fabrics indicate that these rocks were deformed and metamorphosed at 10 - 12 kbar pressure (i.e., > 30 km depth), and temperatures of 600 - 700 °C. The presence of significant amounts of Cl (>1000 ppm) in these rocks, combined with textural evidence of hydration reactions, indicates that Cl-rich aqueous fluids played an important role during metamorphism. Hornblende is the major Cl-bearing mineral. Modal analyses across the shear zones show that it is slightly more concentrated inside the shear zone (42-45%) than outside (35-43%). In addition, microprobe analyses show that Cl concentration in hornblende reaches 3 wt% within the shear zone, compared with < 0.5 wt% a few mm outside the zone. The increased Cl in hornblende is associated with increased Fe, Na, K, and Mn and lower Mg and Ti. Changes in bulk chemistry in these rocks in general are minor, but analysis across a centimeter -scale ductile shear zone indicates a high whole-rock Cl concentration (~4650 ppm) at the center of the shear zone, decreasing to ~1200 ppm several cm away from the zone. These data indicate that Cl-rich fluids were concentrated within the shear zone. The Cl-bearing hornblende has reduced grain size and a weak shape fabric sub-parallel to the shear zone boundaries, indicating syndeformational growth and recrystallization. This implies that the fluid-rock reaction event coincided with shearing in these rocks and possibly caused strain localization.