HIGH-CL AQUEOUS FLUID INFILTRATION AND STRAIN LOCALIZATION IN A GARNET AMPHIBOLITE FROM THE SEVE NAPPE IN THE NORTHERN SWEDISH CALEDONIDES

Many mechanisms have been described as causing strain localization and ductile shear zone formation. They include strain localization along pre-existing structures, competency contrasts of different phases, various recrystallization processes, and the introduction of fluid.

Discrete cm-scale garnet amphibolite shear zones, developed and metamorphosed under upper amphibolite facies conditions (650-750 °C, 10-13 kbar), contain high-Cl hornblende relative to hornblende in the adjacent wall rock. This difference in Cl content reflects different fluid/rock interactions; namely, infiltration by a high-Cl aqueous fluid that facilitated strain localization in a changing stress regime. This Cl-fluid was further channeled along the developing shear zones by positive feedback between fluid flow and deformation. This scenario is supported by low-Cl core (0.40 wt %) and high-Cl rim (0.76 wt %) zoned hornblende only found within the shear zones, as well as by a higher abundance of hornblende within the shear zone (31.2 ± 4.3% vs. 18.0 ± 4.5%). Shear zone hornblende displays a shape preferred orientation indicating that infiltration of the Cl-bearing fluid was syn-kinematic with respect to shear zone formation and metamorphism. Hornblende is the only major phase that records this Cl-rich fluid event. Scapolite is common in these rocks, and some grains contain Cl, but scapolite has partially replaced plagioclase and textural evidence strongly suggest that scapolite obtained Cl from nearby hornblende.

Field relationships, the pressure and temperatures of garnet amphibolite equilibration, augmented by published geochronology, indicate that these shear zones are associated with Seve Nappe emplacement following the Finnmarkian phase of the Caledonide orogeny.