Date Thesis Awarded

5-2018

Document Type

Honors Thesis

Degree Name

Bachelors of Science (BS)

Department

Geology

Advisor

Dr. Christopher Bailey

Committee Members

Dr. Brent Owens

Dr. Jordan Walk

Abstract

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, and shear strain (γ) yield a displacement estimate of ~57 m across the zone. However, intrafolial folds and back rotated structures require zone normal shortening and general shear. The average vorticity number for the zone corroborated this, with Wm = 0.9, denoting general shear, and thus adjusts the displacement estimate to be ~30 m. Thin sections showed clear retrogression, preserved flow of recrystallized plagioclase into drawn-out ribbons, garnet rimming between the amphibole and plagioclase ribbons, albite twinning, and heavily fractured garnets.

These data are interpreted to indicate that fluid-mediated strain weakening under eclogite-facies metamorphic conditions caused dynamic recrystallization of feldspars and shear zone formation within a gabbroic-gabbronoritic host rock.

Available for download on Saturday, May 02, 2020

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