STRUCTURAL AND ISOTOPIC STUDIES OF ECLOGITE-FACIES SHEAR ZONES AND ASSOCIATED PSEUDOTACHYLITES IN LOWER-CRUSTAL CONTINENTAL BASEMENT, LOFOTEN ISLANDS, NORWAY: DEEP CRUSTAL SEISMIC FAULTS(?)

Field, petrographic and initial isotopic data are reported for shear zone eclogites and related pseudotachylites and granulites in Lofoten-Vesterålen, Norway. Traversing across strike traces the amphibolite-granulite isograd in Vesterålen to eclogite-facies rocks in the western Lofoten islands. Eclogites occur as lenses in sheared anorthosite, mangerite, metagabbro and orthogneiss forming part of the Baltic Precambrian basement. Eclogite shear assemblages contain omphacite-garnet but are variably retrograded; hypersthene, clinozoisite, rutile and biotite are found in samples that escaped amphibolite-grade retrogression. Our 40Ar/39Ar dating of hornblende from a retrograded Lofoten eclogite indicates eclogitization prior to 433 Ma. We also report analyses of 10 single zircons from felsic veins associated with one eclogite shear zone on Flakstadøy. These veins contain quartz, muscovite and plagioclase and have been sheared with the eclogite indicating pre/syn-kinematic emplacement. All zircons appear to be Paleoproterozoic (~1800 Ma) but with variable minor lead loss during the Caledonian orogeny. Most plot close to the upper intercept of the best fit discordia. Based on the existing data 490±85 Ma (lower intercept) is the best age we could attain. This predates eclogitization of the Western Gneiss Region of west Norway (425-400 Ma) but may be of “early” Caledonian age like those of the Bergen Arcs (500-450 Ma). Pseudotachylites occur in association with some eclogites and also in the large (>150 km2) ‘Heier’s zone’ along the amphibolite-granulite transition. Lofoten pseudotachylites formed both before and after ductile deformation as evidenced by isoclinally folded or rotated pseudotachylite veins in/near shear zones as well as veins cross-cutting the mylonitic foliation. The Heier’s zone pseudotachylites contain hypersthene microlites. The observation that pseudotachylite only occurs with the eclogite shears, combined with initial microprobe studies, suggests that these are deep-crustal seismic faults similar to ones found in the Bergen Arcs. New 40Ar/39Ar dates on muscovite from amphibolite-granulite transitional rocks in Heier’s zone are Cambro-Ordovician (440-570 Ma), reflecting bulk closure and/or partial resetting, compatible with an Ordovician paleomagnetic pole position for the pseudotachylites (Olesen et al., 1997).