Polymetamorphic Evolution of the Trans-Hudson Orogen, Baffin Island, Canada: Integration of Petrological, Structural and Geochronological Data

JOURNAL OF PETROLOGY VOLUME 48 NUMBER 2 PAGES 271^302 2007 doi:10.1093/petrology/egl060 Polymetamorphic Evolution of the Trans-Hudson Orogen, Baffin Island, Canada: Integration of Petrological, Structural and Geochronological Data M. R. ST-ONGE*, N. WODICKA AND O. IJEWLIW GEOLOGICAL SURVEY OF CANADA, 601 BOOTH STREET, OTTAWA, ONTARIO, CANADA, K1A 0E8 RECEIVED MAY 26, 2005; ACCEPTED SEPTEMBER 21, 2006; ADVANCE ACCESS PUBLICATION NOVEMBER 3, 2006 Building on the innovative work of England & Thompson (1984), regional metamorphism in convergent orogens has been shown by coupled thermal^mechanical models to result from the complex interplay of several, generally causative, probably interlinked processes that include subduction-driven convergence and tectonic accretion or collision, crustal thickening and deformation, structural and/or erosional exhumation, underthrusting of materials rich in radiogenic heat-producing elements, and magmatic heat advection (e.g. Jamieson et al., 1996, 1998, 2002; Huerta et al., 1998, 1999; and references therein). Concurrently, field-based petrological^structural^ thermochronological studies have successfully documented the fundamental polymetamorphic character of many higher-grade terrains within collisional orogenic belts ranging in age from Eocene to Neoproterozoic (e.g. Jamieson et al., 1995; Vannay & Hodges, 1996; Culshaw et al., 1997; Harrison et al., 1997; Moecher, 1999; Searle et al., 1999, 2003; Walker et al., 1999; Carr et al., 2000; Indares et al., 2000; Wodicka et al., 2000; Fraser et al., 2001; Johnson et al., 2003). In this paper, we extend the field-based integrated petrological^structural^thermochronological study of convergent orogens to the Paleoproterozoic, and are able to relate key petrological aspects of polymetamorphic mineral assemblages to a well-constrained history of crustal deformation and evolution. The focus of our work is a part of southern Baffin Island (Nunavut, Canada) where an oblique crustal cross-section through a continental collision zone of Paleoproterozoic age is superbly preserved and exposed (St-Onge et al., 2002). The exceptional tectonic context afforded by the geology of southern Baffin Island allows for the internally consistent correlation of deformation fabrics, polymetamorphic assemblages, *Corresponding author. Telephone: (613) 995-4935. Fax: (613) 943-5318. E-mail: ß The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@ oxfordjournals.org Supracrustal units metamorphosed at mid-crustal conditions within the Paleoproterozoic Trans-Hudson Orogen are preserved within an obliquely exposed continental collision zone on Baffin Island (Canada). Early granulite-facies assemblages yield thermobarometric data and phase diagram information that define a steep, compressive P^T path segment.These assemblages are bracketed between ca. 1849 and 1835 Ma, and are interpreted to result from (1) heat advection by an 1865 þ4/^2 to 1848
2 Ma Andean-type granitic batholith, and (2) a ca. 1845 Ma crustal thickening event associated with accretion of an intra-oceanic arc terrane. A subsequent regional metamorphic event is characterized by the growth of retrograde, upper amphibolite-facies assemblages that define a clockwise, decompressive P^T path. Mineral growth is bracketed between 1820
1 and 1813
2 Ma, and is localized within deformation zones associated with the 1820 þ4/^3 to 1795
2 Ma collision of the Rae and Superior cratons.The metamorphic history of Baffin Island supports a progressive change from plate-margin to intraplate processes within an evolving convergent orogen during the Paleoproterozoic that is similar to those documented in younger collisional belts. KEY WORDS: polymetamorphism; geochronology; Paleoproterozoic; Trans-Hudson Orogen I N T RO D U C T I O N JOURNAL OF PETROLOGY VOLUME 48 NUMBER 2 FEBRUARY 2007 mineral chemistry, and age data. The correlation of these multiple datasets for southern Baffin Island, as presented in this paper, allows us to propose unambiguous links between thermal events or processes and consequent regional metamorphic associations. We begin by providing the regional tectonic context for our work and describing the main crustal components of the Quebec^Baffin segment of the Himalayan-scale TransHudson Orogen. The large amount of published work pertaining to this Paleoproterozoic collisional orogen (St-Onge et al., 1992, 1999a, 2001, 2002; Scott et al., 2003; and references therein) provides us with the context within which we can evaluate the significance of the detailed petrological and thermochronological data presented here and which is specific to the generally less well understood orogenic internal zone. Key structural and petrological observations outline the deformation and metamorphic histories of the southern Baffin Island area. Integrated thermobarometric, phase diagram, and thermochronological work documents the pressure^ temperature^time^deformation history of the polymetamorphic internal zone of the Trans-Hudson Orogen on Baffin Island. These results distinguish two regionally significant metamorphic events (M1a, M2) and document the thermal evolution of crustal rocks in the hanging wall of a major suture (tectonic zone separating two crustal blocks of contrasting geological history and parentage or provenance) during accretion- and collision-related deformation. We conclude by considering the potential implications of the south Baffin Island results for the thermal evolution of convergent to collisional orogenic belts. of the Trans-Hudson Orogen in the late Paleoproterozoic (ca. 1820^1795 Ma) was similar to the record of collision and indentation of India into SE Asia in the Eocene (beginning at ca. 50.6 Ma; Rowley et al., 2004). An overview of the tectonostratigraphic framework of the Quebec^ Baffin segment of Trans-Hudson Orogen is given below from south to north (St-Onge et al., 2002; Scott et al., 2003, and references therein), to provide a regional context for the southern Baffin Island study area (Fig. 2). G E O L O G I C A L B AC KG RO U N D Ophiolitic and magmatic arc assemblages The Trans-Hudson Orogen (Hoffman, 1988; Lewry & Collerson, 1990) is a Himalayan-scale collisional orogenic belt that extends from the south^central part of the North American continent to its northeastern edge, where it is truncated by the younger Meso- to Neoproterozoic Grenville Orogen (Fig. 1). The Trans-Hudson Orogen separates a partially subducted or underthrust collisional lower plate (the Superior craton) from a collisional upper-plate (assembled in the hanging wall of a subduction zone) collage of Archean crustal blocks that includes the Wyoming craton, the Hearne domain and Rae craton of the northern Churchill plate, and the North Atlantic craton of Labrador and Greenland (Fig. 1). The orogen is over 4600 km in strike-length and, in certain segments, over (...truncated)