Southeastern Section - 67th Annual Meeting - 2018

Paper No. 21-4
Presentation Time: 9:00 AM


VALERY, Catherine Anne, BAILEY, Christopher M. and OWENS, Brent E., Department of Geology, College of William & Mary, Williamsburg, VA 23187

This study focused on the structure and petrology of a retrograded eclogite-facies shear zone in Nusfjord, Flakstadøy in the Lofoten Islands, Norway. Our goals were to: 1) evaluate the nature of the protoliths both within and next to the shear zone; 2) quantify the deformation kinematics of the shear zone; 3) determine the physical conditions at which the eclogitization occurred.

The shear zone developed in a gabbronorite host rock hypothesized to be part of the Anorthosite-Mangerite-Charnockite-Granite Suite, which was emplaced ~1.8 Ga. Shear zone formation and eclogitization has been reported to have occurred at 487 Ma due to metamorphic and deformational events of the Caledonian Orogeny. The shear zone length exceeds 200 m, is 3 m in thickness, and has sharp boundaries that strike 295° and dip ~80° NE. Foliation within the zone is oriented 290° 85° NE with a gently plunging elongation lineation. Kinematic indicators are consistent with left-lateral strike-slip. Mylonitic rocks in the zone are dominated by pl, amp, and grt, with lesser amounts of bt, ep, Fe-Ti oxides, and scp. The rocks thus appear to be retrograded amphibolites that were once eclogites based on the recrystallized texture of elongate pl ribbons and relict cpx grains. Rocks immediately next to the shear zone lack amp, but contain additional cpx and opx. Normative mineralogy indicates gabbronoritic protoliths for rocks both within and next to the shear zone, with intermediate Mg-number ~44.

Whole rock composition of sheared rocked and un-sheared adjacent rocks are similar, suggesting little chemical change/volume loss from deformation. Chondrite-normalized REE patterns for both are LREE-enriched (avg. LaN/LuN = 5.1), and all samples display prominent positive Eu-anomilies. The latter feature suggests that the protoliths were cumulates enriched in pl.

The 3D fabric analysis is consistent with general oblate strain. Assuming simple shear, the relationship between Rs, Θ values, and γ yield a displacement estimate of 54 m across the zone. However, intrafolial folds and back rotated structures require zone normal shortening and general shear. Thin sections showed clear retrogression, preserved flow of recrystallized pl into drawn-out ribbons, garnet rimming between the amp and pl ribbons, albite twinning, and heavily fractured garnets.