Fluid controlled metamorphism of eclogitic pseudotachylite-bearing shear zones, Flakstadøy, northern Norway

The Lofoten Islands in northern Norway represent the lower crust in the Baltica- Laurentia collision during the Caledonian Orogeny -460 Ma. The island of Flakstadoy consists o f -1.8 Ga granulite-facies gneisses and AMCG units cut by discrete Caledonian shear zones, some containing pseudotachylite veins. A combination of petrography, mineral chemistry, and electron backscatter diffraction analyses show a spatial relationship between pseudotachylite formation, shear zone development, fluid infiltration, and metamorphism. Microfractures and backscatter imaging indicate that pseudotachylite-forming processes seismically shattered the rocks, reducing grain sizes and opening fluid pathways. Evidence of eclogite-facies metamorphism in omphacite and/or albite-diopside symplectite is found exclusively in the shear zones. A strong amphibolite-facies overprint in the shear zones replaced eclogite with amphibolite; a strong alignment of amphibole 0 0 1 parallel to the shear zone fabric indicates fluid availability during shearing. A lack of amphibole growth in the host rock indicates they remained effectively dry. I propose that co-seismic pseudotachylite formation opened fluid pathways in an otherwise effectively dry, impermeable granulitic crust. The advection of fluids led to localized strain and eclogite-facies metamorphism in otherwise metastable granulites. Permeability was maintained through localized strain during exhumation leading to a nearly complete amphibolite-facies overprint in the shear zones.