One billion years later: Reactivation of an Archean shear zone or exhumation of a craton?

Cratons form the stable nuclei of continents, built through the accretion of terranes along crustal-scale shear zones by the end of the Archean. These structures can channel fluids and facilitate fluid-rock interaction, often playing a critical role in forming major mineralization processes across Archean cratons. Despite their significance, the mechanisms and timing of potential Paleoproterozoic reactivation events remain poorly understood. This study examines the Neoarchean Storø crustal-scale shear zone in SW Greenland using in-situ Lu–Hf garnet geochronology, Rb–Sr biotite and U–Pb monazite dating of garnet- and sillimanite-bearing schists, garnet-bearing amphibolite and calc-silicate rocks. Garnet porphyroblasts in the schists record an initial metamorphism at c. 2.7 Ga, whereas Lu–Hf garnet ages of c. 2.63 Ga in metabasalt and calc-silicate rocks, along with robust U–Pb garnetite ages of c. 2.64 Ga, indicate a second Neoarchean metamorphic event. These results support a two-stage metamorphic evolution linked to the accretion of the Eoarchean Færingehavn and Mesoarchean Akia Terranes along the Storø shear zone involving lithospheric thickening and stabilization during the late Archean. In contrast, the biotite-defining foliation yields an age of c. 1.7 Ga, which may represent either the reactivation of the shear zone during the assembly of the supercontinent Columbia or the exhumation of the craton. Monazite grains included in garnet porphyroblasts (c. 2.7 Ga) and aligned parallel to the biotite-defining matrix (c. 2.5 Ga) support the interpretation that the biotite ages reflect craton exhumation rather than Paleoproterozoic reactivation of the shear zone. This study underscores that this region of North Atlantic Craton played the role of rigid block during the Paleoproterozoic assembly of the supercontinent Columbia and major Neoarchean tectonic boundaries were not reactivated. Instead, they represent well-preserved Archean shear zones, ideal for studying Archean tectonic processes.