U-Pb geochronology, geochemistry, and isotopic composition of granitoids across the Nolan-Zemlak domain boundary in the SW Rae craton, Laurentia: Evidence for a late Neoarchean suture reworked during Arrowsmith orogeny

Highlights

• Petrogenesis of Archean granitoids.

• New U-Pb/Lu-Hf/geochemical data from granitoids in Tazin Lake region, SW Rae Craton.

• The ca. 2.6 Ga Nolan suite was metamorphosed at ca. 2.52 Ga.

• Metamorphism related to emplacement of adjacent ca. 2.52 Ga Zemlak suite.

Abstract

The Nolan-Zemlak domain boundary in the southwestern Rae Province separates ca. 2.6 Ga granodioritic to granitic rocks (Nolan suite) of the Nolan lithotectonic domain to the north from an assemblage of ca. 2.52 Ga ultramafic-intermediate plutonic rocks (Zemlak suite) and ca. 2.3 Ga granitoids, and derived gneisses, of the Zemlak domain to the south, the latter representing part of the Taltson basement complex (TBC). The restriction of the Zemlak suite to south of the domain boundary suggests that its emplacement was structurally controlled, an inference supported by recognition of pre-2.52 Ga deformation in Nolan domain. This paper reports the results of U-Pb and Lu-Hf isotopic analysis of zircon, coupled with multi-element whole-rock geochemical analysis, for the Nolan and Zemlak suites, plus a younger suite of leucogranites that stitch the domain boundary. Both the Nolan and Zemlak suites have geochemical-isotopic signatures consistent with subduction-related magmatism, but the ca. 2.52 Ga Zemlak suite is distinguished by anomalously low Th concentrations, attributed to minimal sediment flux into the mantle wedge. Zircon rims from Nolan granites record ca. 2.52 Ga metamorphism ascribed to intrusion of the Zemlak suite. Regionally, these observations are best explained by the subduction-related accretion of the Zemlak terrane, part of a larger Paleo-Mesoarchean crustal block now embedded within the TBC, to a ‘proto-Rae’ cratonic core. A model is presented wherein circa 2.64–2.58 Ga (Nolan age) continental arc magmatism was generated via north-dipping subduction below the proto-Rae craton leading to eventual collision/accretion of the outboard Paleo-Mesoarchean block. Collisional suturing occurred by ca. 2.57 Ga, as recorded by both local and regional metamorphic ages. Following collision, subduction is inferred to have stepped outward, directed beneath the now amalgamated Nolan and Paleo-Mesoarchean (Zemlak/TBC) terrane with further arc magmatism represented by the ca. 2.52 Ga Zemlak suite. Thereafter, arc magmatism continued along the west-southwest margin of Rae craton from ca. 2.47 Ga to 2.4 Ga, likely the expression of Arrowsmith orogeny. Terminal collision ending the Arrowsmith orogenic cycle resulted in reactivation of the boundary zone and formation of the South Tazin Lake shear zone coeval with 2.37 Ga metamorphism and leucogranite emplacement.

Introduction

The Rae Province, forming the western half of the Western Churchill Province (Fig. 1a) and core of Laurentia, has long been recognized for its complex tectonic history. In recent years, perspectives on the tectonic history of this Province have changed dramatically with recognition that it has a unique history, markedly different from neighbouring Archean blocks within Laurentia, and may have been derived from an independent landmass, Nunavutia (Pehrsson et al., 2013). The notion of an independent Rae craton developed in part from recent recognition that, in addition to Thelon-Taltson orogenesis, the craton’s western flank was affected by a unique, older period of early Paleoproterozoic tectonic activity, the 2.5–2.3 Ga Arrowsmith orogeny (Berman et al., 2018, Berman et al., 2015, Berman et al., 2013). The Arrowsmith orogeny was first proposed by Berman et al. (2005) based on recognition of relatively older (ca. 2.36 Ga) regional metamorphism and deformation in the Committee Bay belt of Nunavut, well into the Rae interior (Fig. 1a). The effects of Arrowsmith orogeny are now known to stretch from northwestern Saskatchewan (Hartlaub et al., 2007, Ashton et al., 2007, Ashton et al., 2009a, Bethune et al., 2013) northward to the Queen Maud and Boothia Peninsula regions of Nunavut (Schultz et al., 2007, Berman et al., 2013), for a potential strike length of ~ 1500 km. Arrowsmith orogenesis is proposed to have involved an Andean-type accretionary margin with two distinct pulses of tectono-metamorphic activity at 2.54–2.45 Ga and 2.39–2.28 Ga (Berman et al., 2013, Pehrsson et al., 2013). The older tectonism is thought to correlate with the collision of a ‘Rae-family’ of cratons to form the Paleoproterozoic supercraton Nunavutia, whereas the younger tectonism marks subsequent accretionary processes along the margins of this supercraton (Pehrsson et al., 2013).

The recognition of more than one major phase of convergent margin tectonism along the western Rae margin strongly suggests that older, Arrowsmith-aged suture zones should exist inboard of (i.e., east of, present-day coordinates) the 2.0–1.92 Ga Thelon-Taltson orogen (Bethune, 2014, Bethune et al., 2016). Furthermore, these inferred sutures are likely difficult to discern, owing to the overprinting effects of both Arrowsmith orogenesis and subsequent Thelon-Taltson orogeny. This paper examines one possible candidate for an Arrowsmith-age suture, the boundary between the Nolan and Zemlak lithotectonic domains of northwestern Saskatchewan (Fig. 1b). This boundary separates dioritic to granodioritic orthogneisses of the Zemlak domain in the south, from relatively undeformed granitoid rocks of the Nolan domain in the north. The boundary is marked by a regional (km-scale) high-strain zone, herein referred to as the South Tazin Lake shear zone (STLsz). The Nolan domain granitoids show a progressive southward increase in strain toward this boundary-focused shear zone, which is marked by mylonitization and onset of partial melting in the form of lit-par-lit sheets and map-scale bodies of leucogranite (Ashton et al., 2005). The gneissic rocks of the Zemlak domain south of the STLsz are broadly similar in character to those within the southwest Beaverlodge domain east of the Black Bay fault (Fig. 2). Collectively, these gneisses are inferred to represent an eastward extension of the Mesoarchean to Paleoproterozoic gneisses of the Taltson basement complex hosting the 1.99–1.93 Ga Taltson magmatic suite to the west and south (McNicoll et al., 2000, McDonough et al., 2000, Ashton et al., 2005, Bethune et al., 2010, Pană, 2010, Ashton et al., 2014, Card et al., 2014).

Previous geochronological and isotopic studies of granitoids in this region reinforce that rocks on either side of the domain boundary-focused STLsz have distinctive histories. While granitoid rocks of the Nolan domain, north of the boundary, fall exclusively within the 2.64–2.58 Ga age-range (Van Schmus et al., 1986, Ashton et al., 2007), rocks of this age are rare south of the domain boundary. Instead, granitoids in the Zemlak and Beaverlodge domains have ages of ca. 3.0 Ga, ca. 2.7 Ga, ca. 2.52 Ga and ca. 2.33 Ga with only one known ca. 2.6 Ga occurrence (Hartlaub et al., 2005, Hartlaub et al., 2007, Ashton et al., 2007, Ashton et al., 2009a, Ashton et al., 2014, Card et al., 2016). Directly along the domain boundary, mylonitized ca. 2.6 Ga Nolan granite is in structural contact with ca. 2.52 Ga granodioritic orthogneiss of the Zemlak domain (Ashton et al., 2007, Ashton et al., 2014). These age distinctions in granitoid rocks are also matched by differences in Sm-Nd isotopic characteristics, reported more fully below.

On the basis of these marked differences, Ashton et al. (2014) proposed that the Nolan-Zemlak domain boundary potentially marks a cryptic suture, separating two crustal blocks: a block to the north dominated by 2.7–2.6 Ga crust, referred to as the ‘proto-Rae craton’, and a block to the south, comprising Mesoarchean to Paleoproterozoic components of the Taltson basement complex. Although the exact timing of the putative collision between these blocks is uncertain, previous work has detected 2.37 to 2.34 Ga metamorphism and/or thermal effects, coeval with the second of the two phases of Arrowsmith tectonism, on both sides of the domain boundary (Koster and Baadsgaard, 1970, Ashton et al., 2009a, Bethune et al., 2013); and more recently documented within the coincident STLsz (Deane et al., 2020, Deane, 2020). Ca. 2.3 Ga granitoid plutons have also been identified in the Zemlak domain (Ashton et al., 2007) and are correlative with the well-studied syn- to post-collisional granites of the western Beaverlodge domain (Hartlaub et al., 2007). These 2.33–2.29 Ga granites postdate the 2.37–2.34 Ga peak of the Arrowsmith orogeny by up to 40 Myr. In contrast, the nature of the pre-2.37 Ga magmatic activity, when early accretionary events associated with Arrowsmith orogeny, including the aforementioned collision, may have occurred, is poorly constrained. In this paper we therefore investigate the geochemistry, age, and isotopic character of granitoid suites along the Nolan-Zemlak domain boundary in the Tazin Lake area (Fig. 2) to gain insight into their genesis. Through this in-depth study, we aim more specifically to determine how the southwest Rae entity evolved through the critical window of time from emplacement of late Neoarchean granitoids (ca. 2.64–2.58 Ga) of the Nolan domain into the latest Neoarchean and early Paleoproterozoic, when 2.52 and 2.33–2.29 Ga granitoids ascribed to Arrowsmith orogeny were generated in domains to the south, and what role the Nolan-Zemlak domain boundary, and the spatially coincident STLsz, played in this process.

Given the significant knowledge gap associated with the geological record of the region preceding and approaching Arrowsmith orogeny within the southern Rae, we focus on two key suites within this window, the 2.64–2.58 Ga granites of the Nolan domain and the ca. 2.52 Ga dioritic to granodioritic rocks of the Zemlak domain. The ca. 2.3 Ga syn- to post-collisional granites were also considered in our regional survey; however, as they are part of the late Arrowsmith history, new age and geochemical data on these rocks is reserved for a companion paper (Cloutier et al., in prep.). While not the focus of new analytical work, we also consider the role played by the oldest, ca. 3.0 Ga granitoid suite (Hartlaub et al., 2004, Hartlaub et al., 2005), prevalent within the Beaverlodge domain and inferred to underlie the Zemlak domain (Ashton et al., 2016, Card et al., 2016). Lastly, we investigate part of a widespread suite of leucogranites that crosscuts rocks on both sides of the STLsz. To date, this ‘stitching’ suite has yielded poorly constrained Taltson ages, generally with significant inheritance (Hartlaub et al., 2005).

This study involved laser ablation split stream inductively coupled plasma mass spectrometry (LASS-ICPMS) on zircons from four previously dated and four newly collected samples of these granitoid suites for coupled U-Pb and Lu-Hf isotopic analysis. U-Pb and Lu-Hf isotopes were used to determine crystallization ages and to identify any metamorphic or Pb-loss events, as well as to evaluate the degree of crustal contamination. Whole-rock major, trace, and rare earth element geochemistry of the three suites further constrains the petrogenesis and tectonic setting of emplacement of the granitoid rocks and complements the isotopic data. This data builds upon previously published Sm-Nd isotopic data (Hartlaub et al., 2005, Ashton et al., 2014, Ashton et al., 2016, Ashton et al., 2017a, Ashton et al., 2017b, Card et al., 2016) of respective granitoid suites.