In Situ Lu–Hf Garnet Dating of Archean Deep Crust Granulites from the Polymetamorphic Grenville Front Tectonic Zone

Abstract

Recent advances in geochronological techniques now allow the ability to efficiently decipher the timing and duration of geological processes in complex high-grade polymetamorphosed orogenic terranes. This is the case of the Grenville Front Tectonic Zone, which truncates the Superior Craton to the southeast. The zone exposes parautochtonous Archean rocks that underwent mid- to high-pressure granulite facies metamorphism of uncertain age. The metamorphic assemblages have been either interpreted as Archean and associated with the final stages of the Superior craton assembly, or as the result of Mesoproterozoic Grenvillian metamorphism, based on cross-cutting relationships and traditional geochronology methods such as U-Pb zircon and 40Ar-39Ar mica dating. Herein, we revisit the extent of the Grenvillian metamorphic overprint in the parautochtonous domain and provide new age constraints for granulite-facies metamorphic assemblages through in-situ garnet dating within migmatitic paragneiss, migmatitic orthogneiss, and mafic granulites, combined with in-situ trace element mapping. Six samples, which show bell-shaped and occasionally sharp and oscillatory lutetium growth zoning in garnet, yield garnet Lu-Hf isochrons with identical Archean dates of c. 2.6 Ga. Sparce analyses of material trend toward Grenvillian ages (c. 1 Ga) in one sample from which garnet shows lutetium zoning consistent with post-growth fluid-assisted disturbance. Overall, our results indicate that the widespread granulite-facies metamorphism within the Grenville Front Tectonic Zone is dominantly late Neoarchean in age, unveiling a rare exposure of Archean lower crust in the southeastern Superior Craton. Our results also point towards a limited Grenvillian metamorphic overprint though the spatial extent and precise thermal conditions of this metamorphism are still unknown. The results presented herein demonstrate the potential of in-situ isotopic geochronology on rock-forming minerals like garnet in polymetamorphic terranes.