Tectonothermal Evolution of the Middle Crust in the Trans-Hudson Orogen, Baffin Island, Canada: Evidence from Petrology and Monazite Geochronology of Sillimanite-bearing Migmatites

Hall Peninsula, on SE Baffin Island, Canada, exposes a lithologically diverse mid-crustal section of the Paleoproterozoic Trans-Hudson Orogen, which features Paleoproterozoic supracrustal strata overlying Archean crystalline basement. Recent mapping, combined with petrological studies and U–Pb monazite dating, provides new insights into the tectonothermal history of the internal zone of the orogen, as well as mid-crustal orogenic processes in general. Peak metamorphic grade is constrained to progressively increase westward from mid-amphibolite- to granulite-facies conditions and three regional deformation events are recognized: east-directed thin-skinned crustal shortening (D1); east-vergent, thick-skinned thrusting and folding (D2); late-orogenic, north–south-directed, thick-skinned folding (D3). Equilibrium phase diagram modeling constrained by garnet compositional transects in pelite indicates peak mid-amphibolite-facies conditions on the eastern peninsula of ∼720–740°C and 6·25–7·25 kbar, followed by cooling and late muscovite growth. In accordance with field evidence for extensive biotite dehydration melting, peak metamorphic conditions of granulite-facies pelite on the western peninsula are estimated at ∼810–890°C and ∼6·1–7·35 kbar. In situU–Pb monazite geochronology (laser ablation inductively coupled plasma mass spectrometry) constrains the timing of peak metamorphism and concurrent D1deformation throughout the peninsula to between c.1850 and 1825 Ma, coincident with the accretion of crustal blocks and arc terranes during the amalgamation of the orogenic upper plate. A younger population of c.1800 Ma monazite implies fluid-assisted dissolution–reprecipitation of older monazite, possibly related to the terminal collision of the amalgamated upper plate Churchill domain with the lower plate Superior craton. Combined with published observations, the integrated geochronological and petrological data suggest that the crustal section exposed on Hall Peninsula remained at elevated temperatures (>550°C) for ∼100 Myr after the thermal peak. Despite significant crustal thickening and sustained high temperatures, which are comparable with those of large, hot orogens, the crustal section records no evidence of mid-crustal ductile (channel) flow or rapid exhumation during orogenic collapse.