Repeated metamorphism and deformation localized in a shear zone recording the formation-subduction-exhumation history of the continental crust

Abstract

<p>A prominent natural laboratory to deduce the interplay of seismic and aseismic deformation in the lower continental crust is exposed on the Lofoten archipelago (northern Norway). A key feature to unravel its tectonic history is the ~600 m thick Ramberg-Flakstad shear zone (RFS) that is interpreted as a retrogressed eclogite-facies shear zone. However, the rest of the lower crustal section preserves evidence of cyclicity between seismic rupture (pseudotachylytes) and viscous shear at amphibolite-facies conditions, while the record of high-pressure deformation and metamorphism is less clearly preserved. The RFS is thus a key structure to understand the subduction-exhumation history of the Lofoten crustal section, providing insight into the localization of metamorphism and strain during orogenesis. Here we report field observations combined with mineral chemical, microstructural, and textural observations of this long-lived multistage shear zone. The shear zone is heterogeneous with the main foliation wrapping around weakly to non-foliated blocks. These blocks are dissected by millimeter to centimeter-thick shear zones. The RFS is hosted by Paleoproterozoic gabbroic rocks that were intruded by anorthositic and charnockitic plutons at ~1.8 Ga. Granulite-facies metamorphism, indicated by the crystallization of garnet, recrystallization of orthopyroxene, and a locally preserved migmatitic fabric is likely related to pluton emplacement. Later eclogite-facies metamorphism (age disputed) is evidenced by inclusions of omphacitic clinopyroxene in garnet and clinopyroxene + plagioclase symplectites after omphacite within the main foliation. Inclusion distributions in garnet are patchy and electron backscatter diffraction (EBSD) analysis reveals that individual garnet grains can be divided into multiple domains, indicating various growth phases. The main foliation is dominantly formed by the preferred orientation of amphibole and plagioclase, consistent with amphibolite-facies P-T conditions reported from shear zones and pseudotachylytes elsewhere in Lofoten. The symplectites after omphacite are aligned with this main foliation but internally preserve a vermicular microstructure indicating that retrogression actually occurred statically after alignment. Additionally, plagioclase within the symplectites is more albitic than in the matrix, precluding that significant element redistribution occurred during or after retrogression. Lastly, the main fabric is crosscut by undeformed (to locally weakly folded) pegmatite dykes of Caledonian age which provides a lower age boundary on RFS deformation at ~413 Ma. These observations indicate that the RFS is long-lived (~1.4 Ga), established during Proterozoic granulite-facies metamorphism and repeatedly exploited as a site of metamorphism at varying P-T conditions, hydration/dehydration reactions, and deformation. Key minerals and mineral assemblages reveal these modifications through a history of stable lower continental crust, subduction, and exhumation.</p>