A WET AFFAIR: THE MIDDLE OROGENIC CRUST OF THE TRANS-HUDSON OROGEN, BAFFIN ISLAND, NUNAVUT

The Hall Peninsula on southeastern Baffin Island, Nunavut, is part of the Paleoproterozoic Trans-Hudson orogen, which separates the lower plate Superior craton from an upper plate collage of Archean crustal blocks (Churchill plate). Rocks now exposed across much of the peninsula represent the orogenic middle crust, where conditions during peak M2 metamorphism were 630-650°C and 5.9-6.2 kbar. M2 overprints early M1 granulite facies assemblages in Paleoproterozoic metasedimentary rocks that dominate the western peninsula, with M1 characterized by widespread anatexis, producing Grt ± Crd-bearing leucogranite melt at ca. 1.87 Ga. Archean orthogneisses and inlying Paleoproterozoic rocks of the eastern peninsula preserve a record of initial higher pressure conditions, as witnessed by sillimanite pseudomorphs after kyanite, and high-Ca garnet cores containing Na-rich plagioclase inclusions. Pervasive fluids were likely present during tectonism, driving melting reactions and the recrystallization of minerals to accommodate strain. Notably, despite good rock exposure, no regionally-significant high strain ductile shear zones have been observed. Rather, the dominant style of deformation in the middle orogenic crust of the Trans-Hudson orogeny comprises east-vergent fold trains and subordinate imbricate thrusting. In situ LA-ICPMS U-Pb analyses of monazite in garnet porphyroblasts and in fabric-defining matrix minerals from across Hall Peninsula suggest synchronous amphibolite facies metamorphism and deformation at ca. 1.84-1.83 Ga. Late- to post-orogenic N-S compression is evidenced as large open folds that plunge shallowly to the west. Water remained present in the system, allowing growth of retrograde biotite under amphibolite facies conditions, and our regional thermochronology reveals a protracted period of cooling and unroofing. The Hall Peninsula can be used as a window into mid- to lower-crustal orogenic processes that remain largely hidden in the Himalayan-Tibetan system and comparable modern mountain belts.