STRUCTURAL STUDIES OF RARE ECLOGITE-FACIES SHEAR ZONES CUTTING BALTIC CONTINENTAL BASEMENT, LOFOTEN ISLANDS, NORWAY

Field and petrographic observations are reported for rare, shear-zone eclogites exposed on Flakstadøy in the Lofoten archipelago, north Norway. Previous workers reported Lofoten eclogites to occur in lenses of sheared anorthosite and gabbro (mafic xenoliths?) forming part of the Baltic Precambrian continental basement. We report additional occurrences where eclogite formed within ductile shear zones that cut 1.8 Ga, granulite-facies, mangeritic basement gneiss, an unusual style of occurrence shared only with Caledonian shear zone eclogites of the allochthonous Bergen Arcs of western Norway. We examined field exposures, polished slabs, and thin sections to begin characterizing the geometry, kinematics, timing, and crustal conditions under which the shear zones formed. The eclogites occur in isolated occurrences, the largest body extending slightly more than one kilometer along strike, scattered roughly 3-4 km from each other. Each locality has a main shear zone, the widest being ~30 m across strike. Subsidiary shears formed both synchronous with the main shear zone and also cut it. Amphibolite-facies retrogression is variable but strong and it does not appear to affect the pristine, granulite-facies shoulder rocks outside of the shears. The peak eclogite assemblage is garnet + omphacite + magnetite + rutile. In sheared samples, omphacite is deformed and recrystallized in thin, ultramylonitic shear bands. Porphyroblasts of omphacite are rimmed by tiny garnets. Regional relations and existing isotopic data suggest that, like the Bergen Arcs eclogites, the Lofoten eclogites formed at some yet undetermined time during the early Caledonian orogeny (Cambrian - Early Silurian?). One locality contains muscovite-bearing granitoids that cut the eclogite shears but were emplaced prior to hydration and retrogression under amphibolite-facies conditions, affording an opportunity to further constrain timing relations. In another locality, eclogite shears are cut by numerous, post-Caledonian brittle shears, faults, and fractures related to formation of the Modern Norwegian shelf.