Structural, Isotopic, and Kinematic Analysis of Eclogite-Facies Shear Zones and Associated Structures, Lofoten, North Norway

Abstract

Three variably retrograded, fluid-mediated eclogite occurrences on Flakstadøy, in the Lofoten archipelago of north Norway, display two structurally-controlled styles that are rheological inverses: competent lenses and crystal-plastic shear zones. Lenses of eclogite at Myrland and Storvatnet formed from mafic enclaves and were stretched and rotated in a semi-rigid state within amphibolite-facies shear zones generated during uplift. Post-eclogite shearing almost completely obliterated any primary eclogite-facies deformational fabrics, leaving these retro-eclogites disconnected from their lower-crustal paleostress fields. Amphibolite-facies shear zones encapsulating the eclogite lenses do, however, preserve the paleostress record of their early uplift. Shear zones at Nusfjord are the rheological inverse of the lens types because eclogite was the weak unit, serving as the locus of crystal-plastic deformation. These are true eclogite-facies shear zones where minerals in the gabbronorite host rocks are progressively transformed into eclogite-facies minerals only within the shear zone boundaries, thus, preserving their lower-crustal paleostress fields. The proximity of these three occurrences and their vastly different deformational styles suggests that additional factors beyond lithology control shear zone formation in the lower crust. Structural, petrological, and isotopic information allow insights into deformation and exhumation of the deep-continental crust now exposed in the northern Caledonides. U/Pb analyses of zircon and xenotime from a syenogranite dike that cuts retro-eclogite are reported. The dike intruded its mafic host at 1800 ± 5 Ma and was eclogitized together with it at 478 ± 41 Ma, which places eclogitization early in the Caledonian Orogeny. The east-west orientation of s1 determined from the Nusfjord eclogite shear zones parallels that observed for Caledonian thrust translation upon mainland Norway. Omphacite breakdown textures imply rapid isothermal decompression during initial uplift, followed by slow uplift. Near-vertical s2 at all three occurrences implies continued compressive stress during initial uplift. Emplacement of the overlying Leknes Group at 469-461 Ma implies Lofoten basement resided in the lower crust for at least 17 million years. Cooling dates from hornblende from a retro-eclogite at Nusfjord and muscovites from throughout Lofoten imply the residence of retro-eclogites in the middle crust for 101-190 m.y., which resulted in a high degree of retrogression relative to eclogites from the Bergen Arcs and Western Gneiss Region in southern Norway.