Eclogitization and Shear Zone Kinematics in the Lofoten Islands, Northern Norway

• Main Author: Valery, Catherine
• Format: Text
• Language: English
• Published: W&M ScholarWorks 2018
• Subjects: Geochemistry; Geology; Tectonics and Structure; Flakstadøy; Lofoten; Norway; Nusfjord; Northern Norway
• Online Access: https://scholarworks.wm.edu/honorstheses/1173; https://scholarworks.wm.edu/context/honorstheses/article/2182/viewcontent/FINAL_CatherineValery_HonorsThesis_2018.pdf

This study focused on the structure and petrology of a retrograded eclogite-facies shear zone in Nusfjord, Flakstadøy in the Lofoten Islands, northern Norway. My 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. 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 478 Ma due to metamorphic and deformational events of the Caledonian Orogeny. The shear zone length exceeds 200 m, was 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 plagioclase, amphibole, and garnet, with lesser amounts of biotite, epidote, Fe-Ti oxides, and scapolite. The rocks thus appear to be retrograded, mylonitic amphibolites that were once eclogites based on the recrystallized texture of elongate plagioclase ribbons and relict clinopyroxene grains. Rocks immediately next to the SW boundary of the shear zone lack amphibole, but contain additional clinopyroxene and orthopyroxene. Normative mineralogy indicates gabbroic-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 plagioclase. The 3D fabric analysis is consistent with moderate-strong oblate strain. Assuming simple shear, the relationship between Rs, Θ angles, ...