ECLOGITE-FACIES PSEUDOTACHYLITE IN DEEP CRUSTAL ROCKS EXPOSED IN NORTHERN NORWAY

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–facies shear zones (ESZ), containing omphacite and garnet, have been found in various types of host rock on the islands of Flakstadøy, Vestvagøya, and Austvagøya. These ESZ are thought to be the result of deep crustal processes during Caledonian subduction and continent-continent collision. Pseudotachylite veins within these ESZ were observed in two locations on Flakstadøy, near Flakstad and Nusfjord. At both locations, pseudotachylite- and non-pseudotachylite-bearing ESZ 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° C or approximately 45 km depth. Mineral 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 the ESZ and pseudotachylite formation will be analyzed using electron backscatter diffraction (EBSD) at San Francisco State University. We expect mineral chemistry and EBSD analyses will distinguish between deformation-controlled and fluid-induced eclogite-facies metamorphism in otherwise metastable host rocks. Based on field observations, the ESZ and pseudotachylite formed coseismically, indicating brittle faulting occurred at eclogite-facies P-T conditions in the lower crust. Previous attempts to date the ESZ have been hampered by a retrograde amphibolite–facies overprint during 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 and omphacite from ESZ 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.