Carolyn Relf, DIAND NWT Geology Division, Yellowknife. NWT
The Northern Slave and Western Churchill Provinces are currently the two best-underatood geologic domains in Nunavut; this is a result of both extensive mineral exploration, and geological mapping efforts over the past two decades. Both areas are underlain by Archean granite-greenstone terranes which host a wealth of mineral occuffences, but their tectonic histories are remarkably different. This paper provides a brief geolottic synthesis of the two areas, highlighting the similarities and differences between them.
Northern Slave Province:
The Northern Slave Province underlies a significant portiots of Nunavut’s Kitikmeot Region. Much of the area has seen recent bedrock geological mapping complemented by strategic thematic studies. A special volume ofCJES (Bleeker and Davis, 1999) was recently published summarizing results of much of this recent work. The summary presented below draws largely from contributions to this volume.
The Slave Province contains c. 4.0 to 1.8 Ga gneisses. unconformably overlain by a ca. 2.8 Ga cover sequence of quartzite, conglomerate, BIF and minor volcanic rocks that are interpreted to represent initial rifling and subsidence of the underlying crust. Together the gneisses and associated cover sequence comprise what is commonly referred to as "basement" in the Slave Province. The basement is structurally overlain by 2.72-2.70 Ga tholeiitic basalts which record variable amounts of crustal contamination, and are therefore interpreted as a break-up sequence (Dostal and Corcoran 1998, Bleeker et al. 1999). The base of the tholeiitic package is locally characterized by mylonites (e.g. Kusky 1991; Northrup et al. 1999), suggesting an allocthonous relationship to basement. Ca. 2.69-2.66 Ga calc-alkaline "arc-like" volcanic rocks, associated plutons and turbidites overlie the tholeiites. Deposition of the turbidites overlapped with and followed eruption of the volcanic rocks. Regional tectonothermal events across the province are recorded by development of multiple folds and fabrics, and growth of ca~ 2.62-260 Ga high T/low P metamorphic assemblages in supracrustal rocks. Between 2.62 and 2.58 Ga, the Slave Province saw widespread emplacement of late-to post-tectonic mesaluminous to peraluminous granitoids (van Breemen et al. 1992), and localized deposition of conglomerates containing ca. 2.6 Ga detrital zircons. Uplift of two granulite fades domains at ca. 2.6 Ga, and the discovery of two late Archean (ca. 2.95 Gal) carbonatites suggest crustal extension occurred at least locally following the regional thermal peak, and provides a mechanism for erosion of ca 2.6 Ga material (ReIf and Stubley 1996)1
U-Pb, Pb-Pb and Nd isotopic data from across the Slave Province reveal that basement underlies the western part of the province, but is absent in the east (Bowring et al. 1989; Thorpe et al. 1992; Davis and Hegner 1992; Bleeker and Davis 1999). This age dichotomy led workers to propose tectonic models involving westward accretion of greenstone belts against a basement terrane (eg. Fyson and Helmstaedt 1989; Hoffman 1989, Kusky 1989, 1990; Bleeker and Ketchum 1990). Much of the recent work in the Slave Province has focussed on refining this model. Recent studies have better delineated the extent of basement and identified tectonothermal events within it (Ketchum and Bleeker 1998, Emon et al. 1999; ReIf et al. 1999). Bleeker and Ketchum. (1998) suggested the basement cover sequence is autochthonous to parautochthonous, and are continuing to gather data to support this interpretation Other studies include examining the timing and kinematics of sub-greenstone belt mylonite zones, studying stratigraphic relationships within the turbidites, sorting out relationships between deposition and tectonism, identifying the cause of late, pan-Slave plutonism (lithosphere delamination? crustal thickening?), and determining the nature of the underlying lithospheric mantle and its relationship (if any) to crustal features.
Western Churchill Province:
The Western Churchill Province, in the Kivaliq Region of Nunavut, is currently the focus of a 5-year. multi-disciplinary NATMAP initiative. The initiative involves bedrock mapping supported by geochemical, geochronological, structural, stratigraphic and P-T studies. At the beginning of the project, one of our stated goals was to "lift the veil of Paleoproterozoic tectonothermal overprinting’, in order to resolve the Archean history of the area (greenstone belt deposition, plutonism, mineralization). While we have had some success, we have also discovered that the "veil’ is much more complex than previously realized, and a comprehensive, testable tectonic model for the Western Churchill is not yet within our grasp. Nevertheless, a clearer picture of the area is emerging, and the boundary conditions of various tectonothermal events are becoming clearer. With two years remaining in the NATMAP, our emerging tectonic picture can be refined by strategic studies, and priorities for future work can be identified. Much of the supporting data for the synthesis below is presently unpublished, but will be presented at the GeoCanada 2000 conference this spring.
While Hoffman’s (1989) subdivision of the Western Churchill Province into the continental Rae and juvenile, oceanic Hearne domains still holds, subsequent work suggest a tripartite subdivision, in which the Hearne is further subdivided into northern and southern sub-domains. Definition of these three (sub)domains is based on Archean lithological associations, geochemical characteristics, distribution of plutonic rocks, and the superposition of Paleoproterozoic tectonothermal events. The earliest recorded events in the three (sub)domains are the eruption of greenstone belts and emplacement of associated synvolcanic plutons between ca. 2.72 and 2.68 Ga. Volcanic assemblages in the Rae domain are distinguished from those in the Hearne by a quartzite-komatiite association and the presence of Mesoarchean detritus, suggesting a rifted continental margin setting for the Rae. Zaleski et al. (unpublished data) have speculated on the possibility of a plume as the driving mechanism for rifting. In contrast, trace element and isotopic analyses of rocks from the Hearne domain led Sandeman et al. (unpublished data) to propose an ensirnatic setting for these rocks. Volcanic rocks and cogenetic plutons from the northern Hearne sub-domain have chemical signatures similar to modern back-arc settings, whereas those from the southern Hearne preserve arc-like characteristics. It is therefore tempting to speculate on the two Hearne sub-domains as an ocean arc/back arc pair formed outboard of the rifted Rae continental margin at ca. 2.69-2.68 Ga.
Archean greenstones are not the only rocks that support this tripartite division. Late Archean (ca.2.6 Ga) granites intrude the Rae and northern Hearne sub-domains, but are absent in the southern Hearne, indicating that by ca. 2.6 Ga, the northern Hearne "back arc" was aecreted to the Rae continent, while the southern Heame sub-domain was apparently not. Following this event, ca. 2.55-2.50 Ga metamorphism and tectonic imbrication affected much of the northern Hearne sub-domain.
Evidence for this event is restricted to the northern Hearne sub-domain. Another event that was apparently restricted to the northern Hearne sub-domain was high pressure (10-13 kbar) metamorphism at ca. 1.9 Ga. This event is enigmatic, as is lacks any regional penetrative fabrics, despite requiring crustal thickening on the order of >20 km. Berman et al. (unpublished data) suggested that thickening may have occurred via thrust stacking of discrete crustal blocks, possibly in response to shortening along the Talton-Thelon orogen. The problem with this model is that post- 1.9 Ga sediments (upper Hurwitz Group) overlie pre-1.9 Ga sediments (lower Hurwitz Group) with apparent conformity (Aspler et al., unpublished data) in the adjacent southern Hearse sub-domain: how could the lower sedimentary sequence survive crustal thickening of this magnitude-without creating an angular discordance between the upper and lower Hurwitz packages? Alternatively, Berman’s data do allow the possibility that high P metamorphism was part of the ca. 2.5 Ga event. In this model, thickening and burial at ca. 2.5 Ga was followed by thrust reactivation at ca. 183 Ga, exposing the high P rocks at the base of a tilted crustal section. Further thermobarometric data will help to discriminate between these two models.
The Rae, northern Hearne and southern Hearne (sub) domains appear to have acted as a single, coherent tectonic entity from ca. 1.83 Ga, suggesting they were assembled in their present configuration by this time. All three (sub)domains are stitched by ca. 1:83 Ga granitoid plutons, and saw intrusion and eruption of mantle-derived ca. 1.83 Ga ultrapotassic magma (Christopher Island Formation) during the initiation of the Dubawnt Supergroup basins. Between ca. 1.82 and 1.81 Ga, a number of shear zones were formed and/or reactivated across the Churchill Province, including the Snowbird Tectonic Zone, the Amer and Wager Bay shear zones. Ca. 1.83 Ga appears to mark a major change in the behaviour of the Churchill craton.
Tectonothermal events preceding ca. 1.83 Ga affected only one or two sub-domains of the Western Churchill Province, and were therefore likely controlled at intra-crustal scales. In contrast, post-I .83 Ga events transected sub-domain boundaries, and included eruption of mantle-derived magmas, suggesting controls at a lithospheric scale. Both scales of reworking are absent in the Slave Province, which remained essentially intact after ca. 2.6 Ga cratonization.
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