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

JOURNAL OF Journal of Petrology, 2016, Vol. 57, No. 8, 1437–1462 PETROLOGY doi: 10.1093/petrology/egw046 Original Article Tectonothermal Evolution of the Middle Crust in the Trans-Hudson Orogen, Baffin Island, Canada: Evidence from Petrology and Monazite Geochronology of Sillimanite-bearing Migmatites D. R. Skipton1*, M. R. St-Onge2, D. A. Schneider1 and C. R. M. McFarlane3 1 Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, ON, K1N 6N5, Canada; Geological Survey of Canada, Natural Resources Canada, Ottawa, ON, K1A 0E8, Canada and 3Department of Earth Sciences, University of New Brunswick, Fredericton, NB, E3B 5A3, Canada 2 *Corresponding author. E-mail: Received June 25, 2015; Accepted July 18, 2016 ABSTRACT 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, thickskinned 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 625–725 kbar, followed by cooling and late muscovite growth. In accordance with field evidence for extensive biotite dehydration melting, peak metamorphic conditions of granulitefacies pelite on the western peninsula are estimated at 810–890 C and 61–735 kbar. In situ U–Pb monazite geochronology (laser ablation inductively coupled plasma mass spectrometry) constrains the timing of peak metamorphism and concurrent D1 deformation 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 midcrustal ductile (channel) flow or rapid exhumation during orogenic collapse. Key words: Trans-Hudson Orogen; P–T–t evolution; equilibrium phase diagram; in situ monazite geochronology; LA-ICP-MS C The Author 2016. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: V 1437 1438 Journal of Petrology, 2016, Vol. 57, No. 8 INTRODUCTION TECTONIC SETTING The THO separates the lower plate Superior craton from an upper plate collage of Archean crustal blocks (Churchill plate; Hoffman, 1988; Lewry & Collerson, 1990). The Churchill plate in the Quebec–Baffin segment of the THO (Fig. 2) consists of the Rae craton and several microcontinents that accreted to the Rae margin between c. 1880 and 1840 Ma, prior to the collision of the Churchill plate with the Superior craton at 1820– 1795 Ma (St-Onge et al., 2009). The Meta Incognita microcontinent (MIM; Fig. 2), which includes much of Hall Peninsula N ? North Atlantic n dso -Hu Wyoming ns Tra Superior n ea ok n Pe Yavapai G re nv ill e ea rn e -R ae Slave MIM H Cordillera The Trans-Hudson Orogen (THO) is a collisional belt that extends along-strike for 3000 km from the western USA to Greenland and has a long-lived accretionary and collisional history between c. 1920 and 1800 Ma (Fig. 1; Hoffman, 1988; Lewry & Collerson, 1990). Characterized by significant crustal thickening and highgrade metamorphism, the THO conforms in many ways to the definition of a large, hot orogen (Beaumont et al., 2006). It has been likened to the Himalayan Orogen (StOnge et al., 2006), and is considered a prototype for modern accretionary processes (Corrigan et al., 2009; St-Onge et al., 2009). The tectonic history of the Quebec–Baffin segment of the THO has been well documented by previous studies (e.g. Lucas, 1989; St-Onge et al., 2007; Corrigan et al., 2009), but tectonometamorphic investigations of Hall Peninsula (Figs 1 and 2), SE Baffin Island, have been limited by the lack of comprehensive field and geochronological studies. The peninsula provides a well-exposed oblique section of thrust-imbricated supracrustal cover sequences and crystalline basement, and an opportunity to compare mid-crustal tectono-metamorphic processes in the internal zone of the THO with those of similarly large and/ or hot orogens. In this contribution, we integrate the results of recent mapping, petrographic studies and mineral analyses, equilibrium phase diagram modeling and in situ U–Pb monazite geochronology (laser ablation inductively coupled plasma mass spectrometry; LA-ICPMS) to decipher the Paleoproterozoic tectonometamorphic history of the internal zone of the THO. Our results indicate that this segment of the orogen reached peak metamorphism at mid-amphiboliteto granulite-facies conditions under moderate pressures at c. 1850–1825 Ma, with concurrent east-vergent crustal shortening. The data suggest a regional, lateorogenic tectonothermal event at c. 1800 Ma, and together with previous geochronology (Scott, 1999) imply elevated temperatures (>550 C) until c. 1730 Ma. The apparent absence of channel flow and orogenic collapse suggests large-scale rheological or tectonic differences between the THO and similar large and/or hot orogens, which may have implications for understanding the development of these processes. Atlantic Ocean 400 km 1.92-1.80 Ga orogens Churchill domain (Trans-Hudson Orogen upper-plate) Post-1.80 Ga orogens and cover Archean Superior craton (Trans-Hudson Orogen lower-plate) Archean North Atlantic craton Fig. 1. Simplified tectonic map of North America showing the extent of the Trans-Hudson Orogen, the Superior craton and the Churchill domain, including the Hearne, Rae, Wyoming and Slave cratons. Hall Peninsula on Baffin Island is indicated on the Meta Incognita microcontinent (MIM), outlined in white. Modified after Hoffman (1988) and St-Onge et al. (2007). southern Baffin Island, accreted to the Rae margin between c (...truncated)