Abstract
The Lofoten Islands of northern Norway are one of two areas on Earth where eclogite-facies pseudotachylite has been found. Pseudotachylite-bearing eclogite shear zones indicate brittle faulting below the brittle/ductile transition existing at approximately 40 km depth in mafic lithosphere. Localized eclogite-facis shear zones, containing omphacite and garnet, have been found in various types of host rock on the islands of Flakstadøy, Vestvagøy, and Austvagøy. These ecogite shear zones are thought to be the result of deep crustal processes during Caledonian subduction and continent-continent collision. Pseudotachylite venis within these eclogite shear zones were observed in two locations on Flakstadøy near Flakstad and Nusfjord. At both locations, pseudotachylite- and non-pseudotachylite-bearing eclogite shear zones were sampled with a one-inch core drill. Using Fe-Mg exchange between garnet and omphacite, eclogitization took place at approximately 1.5 GPa and 680 degrees Celsius or approximately 45 km depth. Miineral chemistry and microstructural analyses of the host rock to shear zone transitions will provide information on the relationship between deformation and high-grade metamorphism. The microstructural relationship between deformation and high-grade metamorphism. The microstructural relationships between the eclogite shear zones and pseudotachylite formation will be analyzed using electron backscatter diffraction (EBSD) at San Francisco State University. We expect mineral chemistry an dEBSD analyses will distinguish between deformation-controlled or fluid-induced eclogite-facies P-T conditions in the lower crust. Previous attempts to date the eclogite shear zones have been hamptered by a retrograde amphibolite-facies overprint durign post-Caledonian extension and exhumation. We will use 40Ar/39Ar to date crystal-free portions (i.e. unretrogressed) of pseudotachylite veins and Rb-Sr or Lu-Hf to date garnet an omphacite from the eclogite shear zones to better constrain relative timing of metamorphism and pseudotachylite formation. Understanding the relationship between metamorphism and deformation will provide insight to the deep crustal mechanisms related to tectonic collisions and provide information to better model lower crustal processes.