PETROLOGIC AND STRUCTURAL STUDIES OF ECLOGITE-FACIES SHEAR ZONES CUTTING CONTINENTAL BASEMENT ON FLAKSTADØYA, LOFOTEN ISLANDS, NORTH NORWAY

We report petrologic and structural findings on Precambrian (1.8 Ga) granulite-facies crystalline basement rocks cut by eclogite-facies shear zones on the island of Flakstadøya, North Norway. Shear zones may occur individually, ranging in thickness from a millimeter to <10 meters, or as anastomosing networks up to 100 meters in aggregate thickness, cutting massive to gneissic mangerite, gabbro, troctolite, anorthosite, norite and leuconorite. Well-foliated eclogite (gt + omph) in shears cut pristine granulite (plag + opx) shoulder rock. Eclogite facies minerals are variably retrograded to amphibolite-facies assemblages. Petrographic evidence for eclogitization (i.e., pseudomorphs, coronal structures, symplectites, and grain boundary coatings) progressively disappears only centimeters outside of the shear zone boundaries, clearly indicating fluid-mediated neomineralization. Ours are the first structural studies made on the Flakstadøy eclogite shears. Many are classic Ramsay and Wood (1973) style ductile shear zones where massive, undeformed, granulite shoulder rock becomes foliated and swept asymptotically into the ultramylonitic shear. Asymmetric meso- and microfolds, porphyroclasts of garnet and clinopyroxene, and S-C composite fabrics provide unequivocal sense of shear for many individual shear zones. Resilient, massive granulite ‘knockers' are completely encapsulated by the anastomosing shears. Granulite shoulder rock and larger ‘knockers' typically have networks of thin (<1 cm) eclogite shears with complex orientations and senses of shear. Principal metamorphic fabrics are as follows: S0, compositional banding in the granulite facies gneisses that continues into the eclogite- and amphibolite-facies lithologies; S1, eclogite-facies shear foliation and L1 elongation lineation (gt and cpx); and S2 amphibolite-facies shear foliation and L2 elongation lineation (gt, plag, and amph). Post-amphibolite-facies folds, shears, faults and fractures overprint all of the above. We describe how the rocks and structures trace deformation associated with exhumation of the deep-continental crust through progressively higher and higher crustal levels.