Provenance and tectonic setting of Paleoproterozoic metasedimentary rocks along the eastern margin of Hearne craton: Constraints from SHRIMP geochronology, Wollaston Group, Saskatchewan, Canada

Precambrian Research 167 (2008) 171–185

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Provenance and tectonic setting of Paleoproterozoic metasedimentary rocks along the eastern margin of Hearne craton: Constraints from SHRIMP geochronology, Wollaston Group, Saskatchewan, Canada Hai Thanh Tran a , Kevin M. Ansdell b,∗ , Kathryn M. Bethune c , Ken Ashton d , Mike A. Hamilton e,1 a

Faculty of Geology, Hanoi University of Mining and Geology, Dong Ngac, Tu Liem, Hanoi, Viet Nam Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada Department of Geology, University of Regina, Regina, Saskatchewan S4S 0A2, Canada d Saskatchewan Geological Survey, Regina, Saskatchewan S4S 0A2, Canada e Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada b c

a r t i c l e

i n f o

Article history: Received 15 October 2007 Received in revised form 25 July 2008 Accepted 2 August 2008 Keywords: Saskatchewan Trans-Hudson Orogen Wollaston Group SHRIMP Detrital zircons Provenance

a b s t r a c t Single detrital zircon grains from various parts of the Wollaston Group, a Paleoproterozoic metasedimentary succession deposited along the southeastern margin of the Hearne Province, northern Saskatchewan, Canada, were analyzed by SHRIMP U–Pb geochronological techniques. Zircon analyses are mostly concordant and yield ages ranging from ca. 2800 to 1780 Ma, although distinct age populations were detected in all samples. The stratigraphically oldest sample (Geoch 4) is dominated by a bimodal distribution of zircon ages (ca. 1.90 and 2.4–2.6 Ga), which is similar to that preserved in the sample (Geoch 2) from the middle portion of the Wollaston Group. The stratigraphically youngest sample (Geoch 9) contains ca. 2.1 Ga zircons, as well as zircons with the same ages as observed in Geoch 4 and Geoch 2. Zircon ages older than 2450 Ma appear to be consistent with the age of the Hearne Province basement, suggesting that part of the sedimentary detritus was locally derived. Zircons with ages in the 2430–2350 Ma range, found in all samples, may have been derived from a more distant source, such as Rae Province rocks that were affected by the recently identified Arrowsmith orogeny. Significant amounts of 1920–1880 Ma zircon grains are found in all samples; these are interpreted to represent sedimentary detritus derived from juvenile volcanic terranes. Zircons younger than 1860 Ma are interpreted to be the product of post-Wollaston Group thermal overprinting. Our data, together with field relationships and geochemical data, suggest that most of the preserved Wollaston Group was deposited in a back-arc to foreland basin environment. It received detritus from both Archean continental crust to the west and a juvenile continental magmatic arc, likely located to the east, as the youngest zircon ages are not consistent with the age of Taltson Orogen rocks to the west. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Studies of stratigraphic sequences deposited in basinal settings adjacent to active continental margins have revealed two major sources for the detrital sediments: one is the stable continental interior whereas the other is a younger magmatic belt adjacent to the basin (e.g., Condie, 1997). These diverse sources can allow detailed reconstructions of the location of basins and the tectonic processes operative in the mountain belts that were formerly the sites of the sedimentary basins. The evolution of strati-

∗ Corresponding author. Fax: +1 306 966 8593. E-mail address: [email protected] (K.M. Ansdell). 1 Present address: Jack Satterley Geochronology Laboratory, Department of Geology, University of Toronto, Toronto, ON M5S 3B1, Canada. 0301-9268/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2008.08.003

graphic records formed in relatively young sedimentary basins is quite advanced, because the combination of lithologic composition, sedimentary structure, and relative dating using the fossil record has led to accurate reconstruction of the nature of sedimentary successions. In contrast, historical reconstructions of Precambrian metasedimentary records are hampered by a lack of precise chronological constraints. In addition, most Precambrian belts have undergone medium- to high-grade metamorphism as well as polyphase deformation, which has destroyed primary sedimentary structures, and are also deeply eroded. The Wollaston Group is a multiply deformed, upper amphibolite to granulite facies Paleoproterozoic metasedimentary succession exposed along the western margin of the Trans-Hudson Orogen in northern Saskatchewan (Fig. 1). It has been exhumed from significant depths, and the succession is likely not complete. Although recent studies (Delaney, 1994; Delaney et al., 1995, 1996, 1997; Tran

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Fig. 1. (A) Location of the Trans-Hudson Orogen (THO) and other tectonic elements of the Canadian Shield in North America (modified from Hoffman, 1988); (B) major tectonic divisions of the exposed Precambrian crust in Saskatchewan and Manitoba, Canada; symbols: (1) Virgin River Shear Zone (Snowbird Tectonic Zone), (2) Hanson Lake Block, (3) Tabbernor Fault Zone, dashed lines are major fault/shear zones, shaded area are lakes; closed box is the area of (C); (C) tectonic divisions of the south-central exposed portion of the Cree Lake Zone in Saskatchewan; rectangular boxes are areas of detailed study from which samples were obtained for this study. The reader is referred to Tran (2001) for descriptions of these areas.

and Yeo, 1997; Tran and Smith, 1999; Tran et al., 1998, 1999; Yeo and Savage, 1999) have led to a better understanding of the relative relationships and stratigraphic order of lithologic members, its depositional setting, age, and provenance are still problematic. Was the detritus comprising the Wollaston Group derived from a distal continental highland and deposited in a passive marginal setting (e.g., Yeo and Savage, 1999) or was at least part of the detritus

derived from a more proximal source and deposited in a back arc and/or foreland basin on an active continental margin (e.g., Lewry and Collerson, 1990; Tran and Smith, 1999; Tran et al., 2000)? Yeo and Savage (1999) suggest that no detritus would have been derived from the juvenile arc rocks of the Trans-Hudson Orogen, whereas Tran et al. (2003) suggest that these rocks may have provided significant detritus based on whole rock Nd isotopic data.

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Understanding of the geological framework for the western cratonic margin of the Trans-Hudson Orogen (e.g., Cree Lake Zone; Lewry and Sibbald, 1977) and bounding tectonostratigraphic domains (e.g., Peter Lake, Rottenstone and La Ronge domains) has improved considerably over the last decade as a consequence of greater understanding of the age and isotopic composition of various Paleoproterozoic sedimentary and magmatic assemblages, including the Wollaston Group, and underlying basement rocks (e.g., Ray and Wanless, 1980; Lewry et al., 1987; Van Schmus et al., 1987; Baldwin et al., 1987; Collerson et al., 1988; Bickford et al., 1988, 1990, 1994, 2001; Annesley et al., 1992, 1997; Hamilton et al., 2000; Ansdell et al., 2000; Coolican, 2001; Rayner et al., 2005). In particular, studies by Chauvel et al. (1987), Hegner et al. (1989), Tran et al. (2003), and Clarke et al. (2005) indicate that Paleoproterozoic mafic and granitoid intrusive rocks record variable degrees of contamination by Archean and older Paleoproterozic crust, which underlines the complex evolutionary history of the crust along the western margin of the Trans-Hudson Orogen. Collectively, these sets of geochemical, isotopic, and geochronological data have characterized potential source areas for various Paleoproterozoic sedimentary successions in this part of the TransHudson Orogen, including the Wollaston Group. U–Pb dating of detrital minerals offers another important tool for deciphering the age and source of sedimentary strata. This paper reports the results of detrital mineral geochronology on stratigraphically controlled samples from Wollaston Group. These new U–Pb geochronological data, collected using the SHRIMP II at the Geological Survey of Canada, offer important insights into the provenance of the Wollaston Group, which in turn bears on the tectonic setting of deposition. Specifically, the precise SHRIMP U–Pb zircon data enable resolution of two different metasedimentary sources for the Wollaston Group, which in turn can be linked to two major stages of basin evolution. These two stages offer controls on tectonic and paleogeographical reconstruction of this Precambrian mountain belt in the period leading up to, and following terminal (continent–continent) collision.

2. Geological setting The Wollaston Group occurs within the southern part of the exposed Hearne Province (Cree Lake Zone) in northern Saskatchewan (Fig. 1) where it overlies and is interfolded with highly remobilized Archean (or dominantly Archean) basement rocks (Lewry and Sibbald, 1980). Subordinate syn-rift metasedimentary, bimodal volcanic, and layered intrusive rocks of the intervening ca. 2.1 Ga Needle Falls Group (Ray, 1979) are locally situated above the basement and below the Wollaston Group. The Wollaston Group comprises a wide variety of complexly deformed, mainly upper amphibolite- to granulite-facies rocks, whose stratigraphic subdivision and lithotectonic relations are complicated and regionally variable (e.g., Annesley et al., 2005). It is exposed mainly in the Wollaston domain (Lewry and Sibbald, 1980; Fig. 1) but is also preserved as discontinuous bands, commonly tectonically intercalated with the Archean basement felsic gneisses, in the western part of the Cree Lake Zone (Mudjatik and Virgin River domains, Fig. 1). The basement assemblage is thought to be of Neoarchean to earliest Paleoproterozoic age (e.g., Wanless and Loveridge, 1978; Ray and Wanless, 1980; Bickford et al., 1990, 1994; Annesley et al., 1992, 1997, 1999; Hamilton et al., 2000), whereas the overlying metasedimentary cover is Paleoproterozoic (Ansdell et al., 2000). In the study area (Fig. 1C), the Wollaston Group can be divided into Lower and Upper subgroups, which are separated by a regional unconformity (Fig. 2; Tran, 2001). These two subgroups may be subdivided into three smaller units, termed ‘sequences’, that form

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continuous vertical successions bounded at the top and bottom by either major discontinuities or unconformities (Fig. 2). The Lower subgroup, comprising Sequences I and II, consists of various rock types with a distinctive spatial distribution. Graphite-rich rocks and garnet-orthopyroxene-amphibole gneisses, which are interpreted as silicate facies iron-formations forming the basal part of Sequence I, occur in narrow zones in the western and easternmost Wollaston domain, whereas quartzite units in Sequence I occur only along the eastern margin (Tran, 2001). Stratigraphically above them, rocks of Sequence II include thick, fine-grained siliciclastic rocks (Fig. 2), interpreted as turbiditic deposits. These are dominant in the eastern Wollaston domain and contrast with the thick, shallow water, siliciclastic units and associated calcareous rocks that are prevalent in the western part of the domain. Only some of the units in the uppermost part of Sequence II are laterally extensive, and these are mainly tyrogenous deposits (Tran, 2001). The Upper subgroup (Sequence III) includes mostly molassetype sedimentary rocks, ranging from talus (i.e., fanglomerate and conglomerate, Fig. 2) and arkose to carbonate and evaporite deposits. These are interpreted to have been deposited in fluvialalluvial, restricted marine to lacustrine environments (Tran, 2001). The non-marine talus deposits form a wedge-shaped, upwardcoarsening succession reaching more than 1000 m in thickness in the east and rapidly wedging out westward (Delaney et al., 1995; Tran and Yeo, 1997; Tran et al., 1998). The Upper subgroup comprises significant detritus that appear similar to, and are possibly derived from, the Lower subgroup (Delaney et al., 1995; Tran and Yeo, 1997; Tran, 2001). In earlier studies the Wollaston Group was interpreted as a shallow-water, miogeosynclinal succession deposited on the continental shelf of a subsiding cratonic margin (e.g., Money, 1968; Money et al., 1970; Ray and Wanless, 1980; Stauffer, 1984; Lewry et al., 1985; Coombe, 1994). Although some workers (e.g., Ray and Wanless, 1980; Lewry, 1981; Lewry et al., 1985; Stauffer, 1984) suggested that the Cree Lake Zone evolved from a passive to an active continental margin with the formation of a magmatic belt along its eastern margin during the Paleoproterozoic, their models implied that the Wollaston Group was a wholly passive margin succession that received all of its detritus from the older Archean cratonic highland to the west (present-day coordinates). In contrast, Lewry et al. (1985) suggested that the Wollaston Group was overlain by shallow water to continental foreland basin clastic wedge deposits derived from terranes in the Trans-Hudson Orogen to the east. Expanding on this idea, Lewry and Collerson (1990) suggested that the upper feldspathic and/or calcareous quartzite and meta-arkose of the Wollaston Group could represent synorogenic foreland, rather than passive margin, deposits, although they did not provide any evidence for this.

3. Analytical procedures In this study, three samples of medium- to coarse-grained psammitic rocks with heavy mineral layering were collected from different stratigraphic levels in the Wollaston Group. Geoch 4, representing the lowermost stratigraphic level, is from near the base of Sequence II of the Lower subgroup (Fig. 2) while Geoch 2, representing intermediate stratigraphic levels, is from the uppermost part of the Sequence II of the Lower subgroup. The stratigraphically highest sample, Geoch 9, was collected from the package of unconformably overlying molasse-type sedimentary rock that comprise Sequence III (Upper subgroup) (Fig. 2). In each case, between 5 and 30 kg of fresh rock was collected. Initial preparation of the samples took place in the Department of Geological Sciences at the University of Saskatchewan. The samples were jaw-crushed, swing-

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Fig. 2. Idealized stratigraphic sections from the eastern Wollaston Domain, and location of geochronological samples. The sections were developed for areas 1 and 2 in Fig. 1; I, II, III are allostratigraphic sequences after Tran (2001); number in bracket of Legend is rock unit defined by Tran (2001). (*) Stratigraphic section made for easternmost part of area 1 only. Not to scale. See Tran (2001) for detailed rock description.

milled, sieved, and were then passed over a Wilfley table to obtain heavy mineral concentrates. Magnetic and paramagnetic minerals were then removed from the heavy mineral concentrates using a hand magnet and standard Frantz magnetic-separation techniques. Methylene iodide (MEI, density = 3.33 g/cm3 ) was then used to separate zircon from less dense minerals, prior to a final stage of magnetic separation. This final stage was performed to separate metamict zircons, which are typically more magnetic, from nonmagnetic zircon, as the latter are more likely to yield concordant analyses (Krogh, 1982). However, paramagnetic as well as nonmagnetic zircon crystals were also analyzed so as to reduce the

potential for sample biasing (Sircombe and Stern, 2002). Zircon crystals had minimum and maximum dimensions ranging from <50 to 200 ␮m, respectively, and representatives of the full grain size spectrum were analyzed. Selected grains of each sample were mounted with Geological Survey of Canada (GSC) laboratory zircon standards (BR266 with 206 Pb/238 U isotope dilution age = 559 Ma, and Kipawa Syenite zircon with isotope dilution age = 993 Ma; Stern, 1997) in an epoxy grain mount (IP151) at the GSC (Ottawa, Canada). They were then ground and polished to reveal grain centres. The grains were photographed in transmitted and reflected light, cleaned,

H.T. Tran et al. / Precambrian Research 167 (2008) 171–185

coated with high purity Au, and then imaged with a Cambridge Instruments scanning electron microscope equipped with cathodoluminescence (CL) and backscatter detectors in order to identify compositional zoning and fracturing. CL imaging carried out prior to ion probe analysis was indispensable in identifying complex growth zoning features in the zircons, and permitted targeting of discrete domains of either primary magmatic or secondary metamorphic crystallization. Generally, the cores of grains were targeted, although rims on some grains were also analyzed to confirm that they represent metamorphic overgrowths. U–Pb analyses were conducted using the sensitive highresolution ion microprobe (SHRIMP) technique at the J.C. Roddick SHRIMP II Ion Microprobe Lab, GSC (Stern, 1996). Analytical and data reduction procedures are described in detail by Stern (1997). Fifty-two spots were analyzed on 49 grains for Geoch 2, 40 spots on 34 grains for Geoch 4, and 71 spots on 54 grains for Geoch 9 (Table 1). An O- primary beam, whose strength was varied from ∼14.5 to ∼3.8 nA, was focused to yield an elliptical spot size using Kohler ion focusing (Stern, 1999). In some cases, several analyses were performed on a single grain in order to check for age variations. In most cases, the data are either concordant or slightly discordant within a 1 error; quoted ages are based on the 207 Pb/206 Pb ratios. Analytical data are provided in Table 1. The Isoplot program of Ludwig (2003) was used to generate concordia and probability density plots.

4. Results 4.1. Zircon morphology Most zircon grains fall into one of two principal morphological populations, namely stubby subhedral prisms with reasonably preserved faces, or equant shapes with only minor preservation of facets (Fig. 3). Euhedral zircons are rare, and most common in the conglomerate unit of the Upper subgroup (sample Geoch 9). Most of the zircons display surface abrasion, although a few are needle-shaped, and some still retain prismatic faces and pyramidal terminations (Fig. 3B). Most are small (<60 ␮m), with the rare larger grains (>100 ␮m) being preserved predominantly in sample Geoch 9. These zircons are interpreted to represent igneous grains that have been variably abraded during transport. Rare, very small, colourless, transparent, equant, and rounded and multifaceted grains (Fig. 3A) are also present in all samples; the fomer are interpreted as metamorphic, whereas the latter may be either metamorphic or igneous in origin. Overall, euhedral zircons were probably derived from more local sources, whereas rounded zircon grains were derived from a greater distance or were repeatedly reworked. CL imaging (Fig. 4) revealed that most of the zircon grains contain numerous cracks, inclusions, and complex zoning patterns. Although some retain near-perfect concentric magmatic zoning and inclusions (Fig. 4A), most are dominated by distinctive cores with generally euhedral, concentric zoning diagnostic of igneous habit. They are terminated by one or more generations of irregular zones or rims (e.g., Fig. 4B, D and E). It is likely that the detrital zircons in the Wollaston Group, especially those of small grain-size, may have at least partly been recrystallized and altered during peak metamorphism. This may have reached more than 800 ◦ C in the study area (Tran, 2001), which is close to the closure temperature of zircon (Lee et al., 1997; Cherniak and Watson, 2000). The zoning patterns in some zircon may therefore be ascribed to partial or complete recrystallization of igneous zircon or new growth of zircon during metamorphism (e.g., Heaman and Parrish, 1991; Harley et al., 2007). However, redistribution of Pb within the zircon may have occurred at lower temperatures as well (e.g., Pidgeon et al., 1998).

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4.2. Analytical results Most of the analytical results representing individual spot analyses are concordant or generally not greater than ±4% discordant (at the 1 error level) (Fig. 5; Table 1). However, even though a few grains have relatively homogeneous ages, those in which there are distinct cores and rims typically yield very different ages (Fig. 4). Many of the zircon rims have a high U contents and low Th/U ratios, which is sometimes characteristic of metamorphic zircon (Heaman and Parrish, 1991; Rubatto, 2002; Harley et al., 2007; Fig. 4F; Table 1). Each of the analyzed samples contains zircons of different age populations (Table 1; Fig. 5) and this is highlighted by the probability plots in Fig. 6. Sample Geoch 4 was taken from a psammite unit towards the base of the Lower subgroup of the Wollaston Group. Thirty-six analyses were obtained, 28 of which were in the range from 2400 to 2600 Ma, with the most common measured age from this sample being ca. 2540 Ma. The remainder clustered at around 1900 Ma with a range of 1840–1925 Ma (Fig. 6a). The Neoarchean/earliest Paleoproterozoic ages were typically obtained from cores of complex grains (Fig. 4E). Some of the younger (ca. 1900 Ma) grains show core-rim relationships attributed to metamorphic overprinting. For example, Fig. 4D shows the age of new zircon (1852 Ma) relative to the age of the core of the grain (ca. 1900 Ma). Sample Geoch 2 was taken from an arkosic unit towards the top of the Lower subgroup, and the fifty-two analyses obtained yielded a broadly bimodal distribution (Fig. 6B). No zircons were found with an age of between 1920 and 2360 Ma. The youngest zircons include three grains with an age range of 1790–1820 Ma, two grains with an age of ca. 1865 Ma, and 15 grains in the range of 1880–1920 Ma. The latter include zircons showing excellent magmatic growth zoning. The older zircon populations in this sample include four grains with ages of between 2360 and 2400 Ma, although most grains range in age between 2450 and 2570 Ma. Sample Geoch 9 was taken from a conglomerate near the base of the Upper subgroup of the Wollaston Domain, and yielded the most diverse suite of ages. The youngest zircon ages range from 1785 to 1835 Ma, and are commonly from overgrowths on older grains (Fig. 4E). The 1880–1920 Ma age range is also common. There is a significantly older group of zircons that yield ages between 2350 and 2585 Ma. Geoch 9 is distinct in that it also contains three grains in the 2050–2080 Ma age range, and six grains with ages older than 2600 Ma, including one that has an age of 2835 Ma, which is the oldest zircon found in this study. In general, each sample contains two distinct age groups (ca. 1.9 and 2.5 Ga) separated by a considerable age gap. The overall age distribution among the samples defines several distinctive age groups. The oldest population of zircon (>2600 Ma) is restricted to sample Geoch 9 (Fig. 6C). All samples are dominated by zircon in the 2450–2600 Ma age-range. The percentage of zircons in this age range, relative to zircons from other age ranges in a particular sample, is highest at lowest stratigraphic levels (sample Geoch 4), and lowest at middle stratigraphic levels (Sample Geoch 2, Fig. 6B). However, towards the top of the Wollaston Group, the percentage of grains in this age range then increases slightly, as represented in sample Geoch 9 (conglomerate; Fig. 6C). The 2350–2400 Ma zircon population occurs in very small quantities in samples Geoch 2 and Geoch 9 (Figs. 6B and C), whereas the ca. 2050 Ma zircons are found only in Geoch 9. 1920–1870 Ma zircons are present in all samples but the percentage of zircon with this age appears highest in the middle of the stratigraphic section (Fig. 6). Geoch 9 appears to have more zircons that are younger than 1900 Ma, whereas those lower in the stratigraphic section appear to have more zircons in the range 1900–1920 Ma, although this obser-

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Table 1 SHRIMP U–Th–Pb analytical data of detrital zircon of Wollaston Group No.

Spot

[U] ppm

[Th] ppm

[Pb] ppm

Th/U

Age (Ma)

Sample Geoch 4 1 4–147.1 2 4–39.2 3 4–45.2 4 4–45.1 5 4–39.3 6 4–59.1 7 4–68.1 8 4–62.1 9 4–90.1 10 4–59.2 11 4–90.2 12 4–39.1 13 4–88.1 14 4–44.1 15 4–132.1 16 4–14.1 17 4–22.1 18 4–65.1 19 4–84.1 20 4–89.1 21 4–116.1 22 4–36.1 23 4–95.1 24 4–25.1 25 4–16.1 26 4–69.1 27 4–46.1 28 4–9.1 29 4–149.1 30 4–150.1 31 4–63.1 32 4–18.1 33 4–5.1 34 4–30.1 35 4–8.1 36 4–38.1 37 4–57.1 38 4–126.1 39 4–121.1 40 4–96.1

257 17 353 348 90 175 240 809 573 241 169 111 76 791 1322 267 140 165 348 314 96 65 347 241 160 431 86 154 420 726 655 228 180 177 313 121 105 161 183 138

96 9 245 189 56 95 142 468 678 150 122 82 46 47 210 121 48 64 471 71 58 5 243 103 131 340 41 54 137 16 286 69 149 57 168 41 77 57 69 63

86 43 140 124 33 67 88 295 253 94 67 43 40 338 636 143 74 84 211 159 53 31 191 122 94 247 45 83 215 354 338 123 105 91 171 66 62 83 97 75

0.385 0.527 0.718 0.559 0.638 0.559 0.614 0.598 1.221 0.645 0.742 0.762 0.628 0.061 0.164 0.467 0.351 0.396 1.397 0.232 0.617 0.082 0.724 0.441 0.847 0.815 0.491 0.362 0.338 0.023 0.452 0.315 0.857 0.335 0.556 0.348 0.752 0.367 0.388 0.475

Sample Geoch 2 1 2–96.1 2 2–118.1 3 2–2.1 4 2–129.1 5 2–109.2 6 2–122.1 7 2–69.2 8 2–116.1 9 2–112.3 10 2–120.1 11 2–13.2 12 2–49.2 13 2–147.1 14 2–107.2 15 2–134.1 16 2–22.2 17 2–147.2 18 2–64.3 19 2–64.2 20 2–134.3 21 2–64.1 22 2–77.1 23 2–63.1 24 2–6.1 25 2–86.1 26 2–64.4 27 2–17.1 28 2–18.1 29 2–61.1 30 2–137.1

83 967 166 360 277 455 716 515 468 525 121 188 922 389 908 12 597 508 324 587 399 936 997 70 84 242 1266 77 717 102

86 481 82 590 148 265 528 365 214 297 80 95 1588 244 1077 6 883 129 69 623 228 133 150 52 36 167 59 80 81 35

34 349 59 173 102 170 284 195 157 195 47 69 448 152 399 4 284 177 116 243 201 427 452 38 41 132 579 44 344 49

1.058 0.513 0.512 1.693 0.554 0.600 0.762 0.733 0.472 0.584 0.685 0.519 1.779 0.647 1.226 0.504 1.528 0.262 0.219 1.096 0.590 0.146 0.156 0.769 0.446 0.711 0.048 1.078 0.117 0.357

Conc. (%) ±1

206

1853.3 1903.6 2005.6 1919.4 1847.5 1932.5 1838.1 1931.2 1968.5 1919.5 1941.8 1857.6 2438.0 2519.0 2543.0 2627.9 2458.0 2551.3 2411.4 2627.5 2603.0 2578.9 2482.7 2470.5 2593.3 2500.3 2499.4 2624.1 2479.0 2420.6 2522.2 2586.7 2534.6 2427.2 2536.4 2590.3 2608.8 2674.6 2508.5 2584.7

55.3 105.0 35.4 38.7 44.0 62.4 33.8 33.3 21.1 31.8 56.5 83.3 150.5 380.0 34.9 42.8 81.5 58.8 29.1 50.3 55.0 776.5 48.5 54.4 47.8 36.6 65.4 85.5 52.4 303.3 42.6 39.4 33.6 76.2 133.5 161.7 55.9 101.8 48.3 60.3

1832.2 1910.3 1813.9 1968.4 1975.5 1982.3 1966.4 1886.7 1792.2 1891.4 1935.6 1841.9 1937.7 1969.2 1952.2 1814.9 1992.0 1840.1 1975.5 1894.2 2383.3 2691.1 2400.4 2489.8 2527.3 2494.2 2386.6 2448.1 2407.8 2375.9

50.2 29.8 29.8 35.3 50.4 31.2 36.6 43.3 39.8 34.7 57.0 37.1 23.3 48.6 21.7 337.7 24.2 42.2 69.2 33.1 30.9 39.1 39.0 39.2 59.5 49.8 70.7 111.4 41.3 52.8

208

232

Pb/

Th

±1␴

207

1762.0 1880.5 1923.4 1798.0 1843.6 1931.6 1845.8 1818.6 1917.0 1930.7 1909.1 1852.4 2428.6 2268.2 2451.9 2521.4 2565.1 2453.4 2403.2 2522.2 2510.1 2480.5 2469.8 2426.8 2546.9 2522.0 2472.0 2577.7 2495.3 2536.0 2449.4 2613.8 2533.4 2505.3 2524.2 2595.0 2577.0 2479.1 2534.1 2544.4

19.7 39.7 21.5 18.0 20.0 25.0 22.6 17.6 16.9 18.8 24.6 61.8 84.7 202.7 20.8 29.8 26.9 30.9 20.8 24.5 28.0 57.6 24.1 21.1 37.9 22.3 25.8 51.2 32.8 66.2 22.8 26.5 23.2 36.6 57.7 66.8 24.0 25.5 22.2 32.5

1845.1 1847.0 1830.6 1909.4 1860.5 1854.7 1902.3 1837.1 1742.3 1863.7 1893.9 1868.3 1905.1 1925.4 1910.8 1875.0 1949.3 1887.0 1954.5 1857.6 2364.0 2361.4 2352.0 2405.5 2360.4 2465.8 2407.2 2409.7 2472.8 2378.9

41.9 19.0 18.5 18.9 18.4 16.9 22.3 18.8 20.4 17.5 20.3 26.3 17.2 18.2 18.1 34.1 18.0 21.0 20.5 17.8 21.9 20.7 20.3 27.4 23.7 22.0 22.7 46.7 21.5 23.0

238

Pb/

U

±1

207

1798.5 1867.4 1897.9 1833.1 1865.3 1916.4 1872.2 1859.2 1914.5 1923.1 1914.9 1885.4 2413.9 2365.6 2458.6 2503.0 2524.8 2481.7 2460.3 2518.1 2518.4 2505.8 2502.6 2483.1 2538.8 2527.9 2506.1 2554.7 2518.4 2539.6 2500.9 2575.3 2540.7 2529.2 2541.3 2577.0 2569.9 2531.1 2560.3 2580.1

15.3 45.1 13.8 14.0 24.5 17.0 13.1 10.5 10.2 10.6 16.6 34.7 65.7 182.4 16.8 15.7 21.9 20.0 11.4 30.6 151.2 59.5 13.6 10.2 17.6 11.2 13.8 33.9 20.0 148.8 10.9 13.6 11.8 25.1 55.9 53.6 15.9 16.8 11.0 16.9

1841.0 1852.9 1870.2 1873.3 1889.6 1899.9 1901.6 1904.9 1911.8 1914.8 1921.2 1922.0 2401.5 2450.6 2464.2 2488.0 2492.5 2504.9 2507.8 2514.7 2525.0 2526.4 2529.4 2529.5 2532.4 2532.6 2533.8 2536.5 2537.0 2542.5 2543.0 2545.2 2546.6 2548.4 2555.0 2562.9 2564.3 2573.2 2581.0 2608.2

20.4 79.2 14.5 18.8 43.9 19.8 7.9 7.4 8.6 6.4 18.7 13.4 87.9 252.2 23.2 12.2 30.6 22.7 9.0 49.3 274.2 90.5 12.4 4.0 6.1 7.2 10.7 40.2 21.1 258.6 3.6 10.4 8.1 30.4 82.9 73.1 18.8 19.3 6.4 12.4

96 102 103 96 98 102 97 96 100 101 99 96 101 93 100 101 103 98 96 100 99 98 98 96 101 100 98 102 98 100 96 103 100 98 99 101 101 96 98 98

1820.2 1831.6 1823.8 1886.6 1864.8 1866.6 1893.0 1863.3 1812.6 1879.3 1898.8 1886.2 1905.9 1917.0 1911.2 1893.1 1931.7 1899.6 1936.9 1884.1 2377.8 2362.1 2359.1 2400.0 2397.7 2456.9 2440.5 2450.2 2480.4 2437.7

25.4 12.5 10.8 10.6 17.1 12.2 17.4 11.6 14.4 11.3 15.2 17.2 10.1 12.5 10.0 91.1 9.8 13.1 14.2 10.3 14.4 10.4 11.1 17.4 15.2 10.6 14.8 25.7 11.3 14.6

1791.8 1814.1 1816.1 1861.7 1869.5 1879.9 1882.8 1892.6 1894.4 1896.6 1904.1 1906.1 1906.8 1907.9 1911.7 1912.9 1912.9 1913.3 1918.0 1913.4 2389.6 2362.7 2365.3 2395.4 2429.6 2449.5 2468.4 2484.0 2486.6 2487.2

20.7 13.3 7.0 6.3 27.2 15.1 24.2 9.7 16.1 11.3 19.9 17.5 7.5 14.8 4.8 184.8 4.7 11.7 17.2 6.3 16.7 5.1 8.9 19.4 16.9 4.8 16.8 20.7 8.4 15.6

103 102 101 103 100 99 101 97 92 98 100 98 100 101 100 98 102 99 102 97 99 100 99 100 97 101 98 97 99 96

235

Pb/

U

206

Pb/

Pb

±1

H.T. Tran et al. / Precambrian Research 167 (2008) 171–185

177

Table 1 (Continued ) No.

Spot

[U] ppm

[Th] ppm

[Pb] ppm

Th/U

Age (Ma) 208

31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

Pb/232 Th

Conc. (%) ±1

206

Pb/238 U

±1␴

207

Pb/235 U

±1

207

Pb/206 Pb

±1

2–102.1 2–5.1 2–131.1 2–54.1 2–7.1 2–36.1 2–98.1 2–60.1 2–141.1 2–130.1 2–59.1 2–146.1 2–72.1 2–151.1 2–67.1 2–50.1 2–3.1 2–53.1 2–26.1 2–48.1 2–38.1 2–103.1

358 130 162 229 152 490 1286 173 79 132 187 149 341 352 268 133 63 101 160 181 78 153

315 67 67 92 55 320 841 109 37 74 55 69 195 149 131 87 30 58 130 124 60 59

204 68 84 120 76 268 710 91 41 71 95 81 186 183 148 73 32 55 94 104 44 80

0.910 0.531 0.431 0.412 0.375 0.675 0.676 0.655 0.478 0.577 0.303 0.474 0.590 0.438 0.503 0.679 0.484 0.593 0.839 0.710 0.798 0.399

2574.6 2422.7 2734.3 2591.0 2437.8 2489.6 2503.2 2362.4 2520.7 2540.7 2516.4 2623.8 2584.3 2502.2 2679.2 2526.8 2284.1 2438.6 2645.1 2624.7 2470.9 2521.1

34.3 62.4 113.8 58.1 57.9 35.4 30.2 39.1 104.2 55.6 72.4 49.9 38.0 42.2 50.8 50.3 50.5 49.9 72.6 70.3 73.0 79.6

2451.9 2474.7 2460.6 2501.4 2437.7 2477.6 2498.9 2425.2 2448.2 2501.9 2487.5 2535.4 2492.0 2473.2 2566.0 2477.2 2414.4 2482.5 2534.5 2558.5 2486.0 2491.9

23.3 30.4 23.5 26.5 33.6 22.5 21.5 28.7 40.0 23.7 25.9 31.3 23.5 24.7 25.6 26.0 28.2 23.9 31.2 24.0 41.4 33.3

2475.1 2488.7 2484.7 2506.3 2480.3 2498.4 2510.1 2477.8 2489.6 2514.9 2510.4 2532.4 2513.1 2504.7 2549.4 2515.3 2489.2 2524.8 2549.1 2559.8 2528.4 2534.6

12.4 17.1 15.9 13.9 19.6 11.0 10.5 15.7 20.3 12.7 12.9 17.2 12.1 14.5 12.1 14.7 18.7 12.1 15.7 13.2 22.9 19.7

2494.1 2500.2 2504.4 2510.3 2515.4 2515.4 2519.1 2521.2 2523.6 2525.5 2529.1 2529.9 2530.2 2530.3 2536.2 2546.1 2550.8 2559.0 2560.7 2560.8 2562.6 2568.9

9.3 15.4 19.0 10.4 18.7 5.6 5.5 12.6 12.0 10.4 7.7 15.3 8.2 14.1 5.4 13.3 21.4 7.4 10.3 11.8 19.4 19.5

98 99 98 100 97 99 99 96 97 99 98 100 99 98 101 97 95 97 99 100 97 97

Sample Geoch 9 1 9–11.1 2 9–33.2 3 9–22.2 4 9–128.1 5 9–16.1 6 9–61.1 7 9–91.1 8 9–110.1 9 9–146.1 10 9–54.1 11 9–73.1 12 9–69.2 13 9–38.2 14 9–54.2 15 9–103.1 16 9–18.2 17 9–31.2 18 9–96.2 19 9–136.1 20 9–107.2 21 9–144.2 22 9–110.2 23 9–136.2 24 9–144.1 25 9–72.1 26 9–36.1 27 9–98.2 28 9–98.3 29 9–98.1 30 9–18.1 31 9–90.2 32 9–134.1 33 9–12.1 34 9–68.1 35 9–22.1 36 9–70.1 37 9–96.1 38 9–31.1 39 9–75.1 40 9–48.1 41 9–4.1 42 9–26.1 43 9–91.2 44 9–128.2 45 9–139.1 46 9–44.1 47 9–41.1 48 9–64.1 49 9–111.1 50 9–86.1

965 684 626 647 719 855 739 1190 834 830 1077 921 341 731 921 1125 479 871 484 359 537 372 944 558 478 541 36 23 22 25 672 688 267 453 143 149 839 155 293 152 828 57 258 159 167 129 296 174 109 351

140 125 165 148 223 200 247 111 40 239 180 267 6 103 109 127 7 163 317 145 247 162 968 215 250 294 19 14 15 14 12 475 94 92 58 65 236 77 114 54 330 61 102 65 61 55 123 77 55 215

310 222 213 216 243 282 247 394 264 279 336 322 106 246 302 374 153 295 187 123 208 140 375 198 183 210 16 9 9 12 301 368 131 216 75 72 433 81 149 73 439 34 135 84 87 65 152 88 58 193

0.149 0.189 0.273 0.237 0.320 0.242 0.345 0.097 0.050 0.297 0.173 0.300 0.018 0.145 0.123 0.116 0.015 0.193 0.676 0.416 0.474 0.450 1.059 0.397 0.539 0.561 0.539 0.610 0.693 0.573 0.018 0.713 0.366 0.210 0.419 0.453 0.291 0.515 0.403 0.366 0.412 1.106 0.407 0.423 0.374 0.443 0.429 0.458 0.519 0.634

1569.1 1788.7 1863.2 1850.8 1863.9 1913.5 1805.6 1856.7 2041.7 1779.1 1746.6 1935.2 1663.3 1934.9 1899.8 1978.6 1770.1 1903.7 1902.8 1809.6 2035.9 2017.9 1824.5 1997.3 1912.4 2026.8 2264.0 2039.9 2040.8 2453.7 2388.4 2410.2 2594.7 2415.9 2591.3 2405.3 2589.6 2506.9 2525.9 2249.0 2565.5 2562.6 2530.7 2529.2 2497.4 2483.5 2388.8 2428.3 2450.3 2514.2

763.9 34.5 34.7 43.4 31.7 25.0 21.1 37.8 36.8 33.5 33.0 31.8 196.8 44.5 57.9 90.2 191.6 39.0 29.6 36.1 28.0 32.8 36.4 28.5 28.9 31.3 194.3 76.2 124.5 180.1 217.2 37.8 64.3 62.0 74.1 40.5 52.0 64.4 39.1 81.9 34.6 61.1 66.4 59.6 47.1 77.2 43.3 55.3 48.5 51.8

1821.1 1812.3 1849.0 1833.0 1819.0 1808.2 1794.0 1879.5 1818.8 1822.2 1752.8 1880.1 1812.5 1877.8 1849.7 1870.7 1851.9 1873.8 1903.3 1793.6 1974.4 1940.8 1807.5 1852.7 1933.0 1937.1 2157.7 1941.9 2026.9 2254.2 2391.0 2439.6 2390.8 2418.6 2501.4 2326.4 2533.7 2458.9 2449.8 2355.2 2532.3 2484.2 2515.3 2511.9 2511.1 2408.2 2460.2 2420.4 2475.8 2505.5

21.9 17.1 21.6 17.1 16.7 16.4 16.3 17.7 17.4 18.2 15.9 18.1 19.3 17.5 17.4 17.6 20.0 20.2 16.9 18.6 18.2 19.7 24.2 20.0 18.0 18.4 32.0 28.1 37.0 45.9 64.9 24.5 28.1 22.7 39.9 20.6 25.0 24.0 22.7 52.3 28.0 23.4 31.9 24.3 22.4 23.6 25.1 24.1 26.4 30.0

1804.1 1806.5 1832.1 1823.9 1817.6 1812.1 1805.0 1851.2 1819.9 1823.7 1786.4 1856.1 1820.4 1856.9 1842.6 1853.8 1845.8 1857.8 1886.5 1832.6 1928.2 1912.1 1846.9 1871.1 1923.4 1927.4 2104.5 1998.7 2053.3 2305.6 2396.9 2423.3 2420.3 2448.0 2492.4 2414.6 2511.4 2478.8 2477.2 2435.9 2517.6 2499.1 2514.1 2515.0 2515.4 2469.0 2494.7 2477.4 2507.8 2523.0

64.3 9.8 14.8 10.4 9.2 9.6 9.5 22.9 10.2 10.5 10.8 13.6 12.8 16.3 10.1 9.6 13.1 13.1 10.5 14.9 10.2 11.6 20.8 11.4 10.7 11.1 49.2 21.2 32.8 39.3 44.5 12.2 15.6 11.7 25.4 58.5 14.7 14.0 21.9 35.2 23.7 13.5 18.9 13.9 15.9 12.5 14.9 13.2 42.3 18.4

1784.5 1799.8 1813.0 1813.5 1815.9 1816.6 1817.6 1819.6 1821.2 1825.5 1825.9 1829.2 1829.4 1833.7 1834.6 1834.9 1839.0 1840.0 1868.2 1877.3 1878.9 1881.2 1891.5 1891.5 1913.1 1917.1 2052.9 2057.8 2080.0 2351.4 2402.0 2409.7 2445.2 2472.5 2485.1 2489.8 2493.4 2495.1 2499.8 2504.1 2505.7 2511.1 2513.2 2517.5 2518.9 2519.4 2522.9 2524.5 2533.7 2537.1

135.0 5.2 17.2 8.8 2.9 6.1 5.8 42.2 6.7 6.0 11.5 18.4 13.4 26.2 6.9 3.6 13.9 13.5 9.8 20.5 6.8 9.4 30.8 5.9 8.7 9.5 89.4 27.2 48.9 55.7 53.2 6.8 13.0 7.6 28.6 103.8 14.9 13.5 32.6 40.1 33.6 13.0 19.1 13.4 20.2 8.6 14.9 10.6 70.7 19.5

102 101 102 101 100 100 99 103 100 100 96 103 99 102 101 102 101 102 102 96 105 103 96 98 101 101 105 94 97 96 100 101 98 98 101 93 102 99 98 94 101 99 100 100 100 96 98 96 98 99

178

H.T. Tran et al. / Precambrian Research 167 (2008) 171–185

Table 1 (Continued ) No.

Spot

[U] ppm

[Th] ppm

[Pb] ppm

Th/U

Age (Ma) 208

51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

9–125.1 9–146.2 9–76.2 9–76.1 9–116.1 9–88.1 9–74.1 9–25.1 9–103.2 9–100.1 9–33.1 9–28.1 9–30.1 9–122.1 9–32.1 9–51.1 9–59.1 9–52.1 9–61.2 9–38.1 9–82.1

260 43 563 1026 419 91 100 153 65 123 229 226 947 287 272 27 292 84 152 129 41

138 28 273 1151 169 94 75 55 50 55 79 81 520 112 105 21 116 63 45 87 34

145 23 308 670 211 58 54 77 37 69 120 120 538 154 148 17 167 50 83 75 29

0.550 0.667 0.501 1.159 0.416 1.068 0.773 0.373 0.789 0.461 0.357 0.369 0.568 0.403 0.398 0.809 0.408 0.780 0.304 0.697 0.857

Pb/232 Th

2631.8 2428.3 2577.2 2711.9 2418.5 2689.0 2407.4 2431.2 2591.9 2751.1 2536.2 2632.0 2611.2 2580.5 2606.0 2719.3 2712.8 2707.4 2673.9 2532.8 2915.1

Conc. (%) ±1

206

33.8 126.3 32.4 28.3 31.8 169.5 70.3 39.0 105.2 85.2 64.5 59.5 30.0 42.8 59.4 123.6 67.7 46.4 71.0 47.8 105.5

2575.9 2456.2 2551.7 2639.4 2416.3 2622.2 2423.6 2440.1 2519.3 2606.7 2515.6 2532.9 2598.7 2549.7 2574.4 2658.0 2680.3 2572.5 2625.4 2602.1 2885.6

Pb/238 U

±1␴ 23.7 35.9 22.2 22.7 21.0 45.2 25.1 24.6 58.6 25.0 26.3 24.1 22.8 23.1 25.4 42.3 25.4 28.2 28.7 31.6 38.2

207

Pb/235 U

2556.2 2503.3 2546.7 2586.3 2489.4 2584.6 2498.2 2507.7 2545.7 2586.4 2547.2 2557.7 2587.7 2570.4 2581.4 2628.3 2639.7 2597.1 2640.5 2631.2 2856.3

±1 11.8 19.7 10.7 10.4 10.5 21.3 14.6 12.8 38.6 40.8 15.3 16.4 10.8 11.7 13.5 24.8 14.7 16.5 15.1 18.5 26.5

207

Pb/206 Pb

2540.6 2541.8 2542.7 2545.0 2549.6 2555.2 2559.4 2563.0 2566.8 2570.4 2572.5 2577.4 2579.1 2586.8 2586.8 2605.4 2608.8 2616.3 2652.1 2653.6 2835.6

±1 7.8 16.1 5.7 4.4 5.3 10.3 13.5 8.4 44.4 68.0 14.8 19.8 5.4 8.0 11.0 25.7 15.2 16.4 12.1 18.6 33.0

101 97 100 104 95 103 95 95 98 101 98 98 101 99 100 102 103 98 99 98 102

Ages calculated using the 204-method for common Pb correction (Stern, 1997). Uncertainties are reported at the 1 sigma level and are calculated by numerical propagation of all known sources of error (Stern, 1997). “Conc.” (concordance) = 100 × (206 Pb/238 U age)/(207 Pb/206 Pb age).

vation is based on very few analyses. The youngest age-group, ca. 1840–1790 Ma, is present in all samples and is commonly from the rims of zoned grains (e.g., Fig. 4E). 5. Discussion 5.1. Provenance of the Wollaston Group The primary objective of this study was to provide constraints on the source of sedimentary detritus in the Wollaston Group and thereby determine whether this sedimentary package was deposited in a passive or active continental margin setting. As described above, a wide range of detrital zircon ages was detected in the three samples (Fig. 6), and it is assumed that the ages are representative of the age of the terrane from which the zircons were derived. There might be a concern that, because of the small number of analyses, the study missed zircons that are less common (cf. Vermeesch, 2004). However, regardless of the total number of analyses, it is the occurrence of a 1920–1880 Ma population that is

particularly relevant to the development of a model for the origin of the Wollaston Group. The Wollaston Group was deposited on the margin of the Archean core of the Hearne Province and is now adjacent to terranes (e.g., La Ronge belt) that are interpreted to have formed in the Paleoproterozoic Manikewan Ocean (Ansdell, 2005). The Wollaston Group was deformed and metamorphosed during the ∼1.8 Ga Hudsonian orogeny, and, following crustal thickening initiated at about 1850 Ma, is deemed to have attained peak metamorphic conditions at about 1815 Ma (Fig. 7; Tran, 2001; Annesley et al., 1997, 2005). The 1840–1790 Ma zircons measured in this study are assumed to represent new zircon growth during peak metamorphism. Consistent with this, most of these ages were obtained from the rims of zircon grains, whose cores yielded significantly older ages. The sedimentary detritus of the Wollaston Group can be inferred to include at least four sources. Zircons older than 2450 Ma are undoubtedly derived from basement rocks (Fig. 7). The age of the basement complex throughout the Cree Lake Zone of the Hearne

Fig. 3. Photomicrograph of representative zircon grains from Wollaston Group. (A) Representative zircons of differing colour, shape, and size in Geoch 4. Large, dark, rounded grains (1) are probably of detrital origin whereas small, homogeneous, and transparent grains (2) may be metamorphic. Zircons with distinct cores (3) and concentric zoning are common in all samples and probably represent igneous or modified igneous grains. Scale bar is approximately 30 ␮m. (B) The zircon in sample Geoch 9 is predominantly euhedral to subhedral suggesting either less reworking or derivation from more local sources. Scale bar is approximately 300 ␮m. See text for discussion.

H.T. Tran et al. / Precambrian Research 167 (2008) 171–185

179

Fig. 4. (A) Photomicrograph of subhedral, prismatic grain with concentric zoning pattern interpreted as igneous zoning. (B) Cathodoluminescence (CL) image of zircon showing possible multiple zoning in which the core (1) is overgrown by two rims (2) and (3). CL images (C and D) and photomicrograph (E) of a selected zircon from each sample showing core-rim relationships and age of spot analyses. (F) Variation of Th/U ratios with age for zircons (n = 30) for which core and rim ages were obtained.

Province has been widely documented to be equal to or older than ca. 2450 Ma (Ray and Wanless, 1980; Collerson et al., 1988; Bickford et al., 1988, 1990, 1994, 2001; Annesley et al., 1992, 1997; Orrell et al., 1999; Hamilton et al., 2000; Rayner et al., 2005; Hartlaub et al., 2006). The high percentage of zircon of this age in sample Geoch 4 (Fig. 6A) suggests dominantly cratonic-derived detritus in the lowermost part of the Wollaston Group. A reduction of the >2400 Ma zircon population in Sample Geoch 2 (Fig. 6B), which is located in the middle of the stratigraphic section (Fig. 2), suggests an increase in the input of younger detritus making up the sediment composition. The increase in the proportion of these Neoarchean zircons in Sample Geoch 9 (from conglomerate unit above an unconformity) appears to be the result of the stripping and recycling of underlying units and the basement complex as a consequence of their uplift and unroofing at later stages of Hudsonian orogenesis (Tran, 2001). This process also appears to have exposed older basement

rocks (>2600 Ma) that were not available as a source of detritus for sedimentary rocks lower in the succession. Earliest Paleoproterozoic zircons ranging in age from ca. 2350 to less than 2450 Ma are relatively uncommon, but do occur in all samples. Hamilton et al. (2000) also recognized a ca. 2475–2400 Ma zircon population in a quartzitic unit equivalent to Tran’s (2001) Unit 11 (Fig. 2) in the vicinity of Duddridge Lake to the northeast of the study area. Detrital zircons ranging in age from 2.3 to 2.5 Ga have also been found in a conglomerate in the Southern Indian Domain, to the east in Manitoba (Rayner and Corrigan, 2004). This domain also contains a ca. 1886 Ma quartz diorite containing 2.4–2.5 Ga inherited zircons, and Rayner and Corrigan (2004) suggest that these ages suggest derivation from Sask craton crust although there are significantly older rocks within the Sask craton (Ansdell et al., 2005) that do not appear to be represented in the Wollaston Group detrital zircon suite. However, ca. 2450–2300 Ma zircon ages have

180

H.T. Tran et al. / Precambrian Research 167 (2008) 171–185

Fig. 5. Concordia diagrams of zircon analyses from lower (A: eoch 4) to upper (C: Geoch 9) parts of the Wollaston Group.

also been reported for the basement rocks in the Cree Lake Zone (Bickford et al., 1988; Annesley et al., 1997) and farther west, along the eastern margin of the Rae Province near Uranium City (e.g., Ashton et al., 2002; Hartlaub et al., 2006). That part of the Rae Province west of Uranium City effectively forms the substrate to the Taltson magmatic arc. Previous workers reported ages ranging between 2500 and 2150 Ma for rocks forming the basement to the Taltson magmatic arc (see Villeneuve et al., 1993; Aspler and Chiarenzelli, 1997 for summary of age). Recent mapping and related geochronology (e.g., Ashton et al., 2000; Hartlaub et al., 2006), however, indicates ages of ca. 2700–2600 and 2350–2300 Ma for metaplutonic rocks in this region, with superimposed metamorphic events at ca. 2350 Ma and ca. 1930–1920 Ga. In addition, in situ U–Pb analysis of monazites in the Committee Bay Belt in the Rae Province of Nunavut has identified a period of deformation and metamorphism at ca. 2350 Ma, now termed the Arrowsmith orogeny, which developed as a result of collisional activity along

the western margin of the Rae craton (Berman et al., 2005). The Late Archean and earliest Paleoproterozoic zircons are thus most likely derived from cratonic terranes to the west and north of the present-day Wollaston Domain. The ca. 2050 Ma zircon population appears only in Sample Geoch 9, and is approximately the same age as parts of the syn-rift Needle Falls Group in eastern Wollaston Domain (Fig. 7; MacNeil, 1998; Ansdell et al., 2000). Field relationships demonstrated that the conglomerate unit lying above an unconformity (see Tran, 2001) was most likely formed by cannibalization of the lower parts of the Wollaston Group, the Needle Falls Group, and the basement complex. Therefore the simplest explanation is that the 2050 Ma zircon population in the conglomerate unit was probably derived from unroofing of the syn-rift assemblages that comprise the Needle Falls Group. The 1920–1880 Ma zircon population is similar in age to the ca. 1910–1880 Ma rhyolitic volcanics (Baldwin et al., 1987; Van Schmus et al., 1987; Bickford et al., 1990), and ca. 1920–1890 Ma tonalitic intrusive rocks (Lewry et al., 1987; Van Schmus et al., 1987; Coolican, 2001) that are abundant in the Rottenstone Domain and the western La Ronge and Lynn Lake belt (e.g., Lewry et al., 1985; Coombe et al., 1986; Baldwin et al., 1987; Zwanzig et al., 1999). Field relationships (Tran, 2001; Tran et al., 1998) and geochemical data (Tran, 2001; Tran et al., 2003) suggest a possible affinity between the sedimentary rocks in the eastern Wollaston Domain and the Rottenstone/La Ronge Domain in the east. The presence of continental molasse deposits above an unconformity separating the lower and upper part of Wollaston Group suggests a period of uplift and tectonic instability during the late stages of its deposition. The increasing abundance of volcanogenic rocks toward the top of this group (Sibbald, 1979, 1983; Tran, 2001) further suggests that volcanic rocks derived from nearby active magmatic arc(s) had become a source for parts of the Wollaston Group. Furthermore, Nd isotopic studies (Tran et al., 2003) also show strong evidence of mixing of detritus from more primitive, mantle-derived sources with more evolved crustal sources. εNd values of the Wollaston Group at the inferred time of deposition ranges from −6.8 to −3.4, much higher than the value of −15.3 for the basement complex (Tran et al., 2003). Similarly, trace element compositions indicate a wide range of detritus derived from basaltic to andesitic magmatic arcs to felsic upper continental crust, and were used to suggest that most of the Wollaston Group was deposited in an active continental margin setting from basement-derived and juvenile mantle-derived sources (Tran et al., 2003). Thus, the simplest explanation is that the 1920–1880 Ma zircons were derived from the volcanic rocks of the western part of the Reindeer Zone, and most likely the Rottenstone or La Ronge domains (Figs. 7 and 8). The greater abundance of zircon of this age in the youngest stratigraphic units may indicate an increase in supply of juvenile detritus to the basin during the late stages of basin evolution. An alternative explanation that was proposed by Yeo and Savage (1999) was that the sediments of the Wollaston Group were derived solely by erosion of the Taltson Magmatic Zone, and that the detritus was carried across the Rae and Hearne basement rocks by large river systems before emptying onto the margin of the Hearne craton. The Taltson Magmatic Zone consists of 2500–2140 Ma basement rocks and 1990–1920 Ma arc and syn-collisional granitoid rocks (Fig. 7; Theriault, 1990; Bostock and van Breemen, 1994; van Breemen and Aspler, 1994; van Breemen et al., 1992; McDonough and McNicoll, 1997; Villeneuve et al., 1993; De et al., 1997; Chacko et al., 2000). Intrusive and volcanic rocks younger than 1920 Ma have yet to be identified in this zone, whereas this study has shown that 1990–1920 Ma zircon is absent in the sedimentary rocks of the Wollaston Group (Fig. 6). This, in combination with the lower

H.T. Tran et al. / Precambrian Research 167 (2008) 171–185

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Fig. 6. Relative probability plots showing the distribution of age ranges of zircon populations of Geoch 4 (A), Geoch 2 (B), and Geoch 9 (C). All the ages are 207 Pb/206 Pb ages, and the peaks are labeled to provide an indication of the ages of zircon populations. Using the Unmix ages routine in Isoplot v.3 (Ludwig, 2003) yields the following ages (million years ± 2 sigma) for the two most dominant age modes in each sample: Geoch 4: 1905 ± 7 and 2540 ± 4; Geoch 2: 1908 ± 5 and 2514 ± 4; Geoch 9: 1835 ± 3 and 2550 ± 6. The sample locations are shown on the simplified stratigraphic column, which has the same legend as in Fig. 2.

εNd values of magmatic rocks from the Taltson Magmatic Zone (Theriault, 1990, 1992) when compared to that of the Wollaston Group (Tran et al., 2003), suggest that the Wollaston Group was not derived by mixing of detritus from an Archean provenance with detritus solely from the Taltson Magmatic Zone. Yeo and Savage (1999) also proposed that ca. 1900 Ma zircon could have been derived from 1900 to 1920 Ma granulite grade rocks along the Snowbird Tectonic Zone (STZ). The STZ has been interpreted as an intracontinental granulite-grade shear zone that affected rocks varying in age from 3.2 to 2.6 Ga and which had developed within the interior of the Churchill Province by the Neoarchean (Hanmer et al., 1994, 1995). In contrast, Hoffman (1988) had suggested that the STZ represented a Paleoproterozoic suture between the Rae and Hearne cratons based on truncation of regional geophysical trends, and Ross et al. (2000) suggested that the STZ was active at the same time as 1.85–1.82 Ga arc magmatism in the Alberta basement. Further work using U–Pb geochronology of monazites has shown that the STZ was affected by granulite and eclogite grade metamorphism at ca. 1.9 Ga (Baldwin et al., 2003), and forms the southern end of a regionally extensive zone of high metamorphic grade rocks that extend northeastwards along the boundary between the Rae and Hearne provinces (e.g., Sanborn-

Barrie et al., 2001). These rocks could be the potential source of ca. 1900 Ma zircons. However, this requires evidence that these rocks were exposed at surface so that they could represent a potential source of detritus. The exhumation of the STZ along shear zones has been constrained by structural and geochronological data to have been initiated at about 1.83 Ga (Mahan et al., 2003; Mahan and Williams, 2005), and thus it is unlikely that the rocks now presently exposed at surface in the STZ were a potential source of detritus for the Wollaston Group. The lack of ca. 3.0 Ga zircons also lends support to this conclusion. Thus we suggest that the 1920 Ma and younger detrital zircon population in the Wollaston Group was most likely derived from a local, juvenile source related to the magmatic arcs outboard of the Hearne margin, namely the Rottenstone–La Ronge arc system (Figs. 7 and 8). The interpretation that the younger detrital zircon population was derived from magmatic arcs to the east of the Hearne margin opens up the possibility that some of the older zircons could have also been sourced from that direction. The Sask craton, which is exposed in structural inliers within the Glennie Domain and the Hanson Lake block (Fig. 1), comprises rocks that show evidence of significant zircon growth between 2425 and 2525 Ma, as well as older (2.8, 2.9 and 3.1 Ga) gneissic rocks (Chiarenzelli

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Fig. 7. Comparison of age ranges of the dominant detrital zircon populations from the study area (Cree Lake Zone), which are marked by the circles (also see Fig. 6 for the distribution of ages in each sample), with potential source terranes and tectonic environments. Grey arrows represent possible sources for zircons. See text for discussion.

et al., 1998; Ashton et al., 1999; Rayner et al., 2005). If the Sask craton was the source of some of the oldest Paleoproterozoic and Neoarchean zircons, then it must have been in an appropriate geographical location for detritus to have been transported to the site of deposition of the Wollaston Group. However, in order to explain

the deformational history within the Reindeer Zone, the Sask craton is interpreted to have been transported on a separate tectonic plate and did not interact with the Paleoproterozoic rocks preserved along the Hearne margin until after the deposition of the Wollaston Group (Fig. 8; Ansdell, 2005; Corrigan et al., 2005). Thus it is

Fig. 8. (A) Schematic section showing the position of the study area and the Wollaston Group relative to tectonic elements that may have provided detritus. (B) Model showing the basin configuration during the deposition of most of the Wollaston Group. The proposed model suggests that the Wollaston Group contains detritus derived from the Archean rocks of the Hearne and Rae provinces, and the active arc environments developing along the margin of the Hearne craton during the Paleoproterozoic. The Wollaston Group is interpreted to have been deposited in a basin in a back-arc tectonic setting relative to this Paleoproterozoic arc.

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unlikely that zircons in the Wollaston Group were derived directly from the Sask craton. Even though detrital zircons in this study are not interpreted to have been derived from the STZ, the range of ages obtained from detrital zircons from the Wollaston Group does provide constraints on the role of the STZ in the development of Laurentia. For example, in the discussion above it is suggested that the 2350–2450 Ma population of zircons is derived from rocks present within the Rae Province, and that potentially they grew during magmatic and metamorphic activity associated with the Arrowsmith Orogen (Berman et al., 2005). This is significant as this would imply that the Rae craton was in contact with the Hearne craton at the time of Wollaston Group deposition. Thus, the STZ must represent an Archean suture and not one that formed during the Paleoproterozoic, as suggested by Hanmer et al. (1995). 5.2. Timing of deposition The Wollaston Group unconformably overlies the Needle Falls Group and thus the maximum depositional age of the former is constrained by the upper age limit of the Needle Falls Group. The age of 2072 ± 2 Ma obtained for the Needle Falls Group volcanic rocks (Ansdell et al., 2000) is therefore considered the oldest possible age for deposition of the Lower subgroup of the Wollaston Group. The Wollaston Group is deformed, metamorphosed, and cut by pre-peak metamorphic intrusive bodies; termed the Grey Granite Suite by Annesley et al. (2005). Annesley and Madore (1991) and Annesley et al. (2005) indicate that these intrusive rocks are ca. 1840 Ma, which thus represents the minimum age for deposition of the Wollaston Group. However, these magmatic bodies represent the last gasps of arc magmatism along the Hearne craton margin, which is considered to have culminated in the formation of the Wathaman batholith, the main phases of which crystallized between 1865 and 1855 Ma (Van Schmus et al., 1987; Meyer et al., 1992). The uplift associated with the intrusion of the Wathaman batholith may have led to widespread uplift, including the adjacent foreland basins in the west. The extensive Wollaston Group marine sedimentation must have ended by that time. Therefore, the upper depositional limit of the Wollaston Group is pinned by the age of the Wathaman batholith. The Wollaston Group itself can be subdivided into several stratigraphic units (Fig. 2). The ca. 1920 Ma zircon is common to all of the samples indicating that rock units equivalent to or younger than that of sample Geoch 4 (Unit 14 of Tran, 2001; Fig. 2) were deposited after 1920 Ma. This in turn suggests that only the quartzite and associated silicate-facies iron formation units of Sequence 1 of the Lower subgroup of the Wollaston Group (Fig. 2; Tran, 2001), which appear to lack zircon younger than 2400 Ma (Hamilton et al., 2000), could predate 1920 Ma. The Sequence 1 rocks are interpreted to represent a passive margin sequence, deposited from ca. 2100 to 1920 Ma along the margin of the Hearne craton. Sequence II includes samples Geoch 2 and 4 that contain variable amounts of ca. 1920–1880 Ma zircon. The increasing abundance of ca. 1920–1880 Ma zircon from the lower to upper part of this sequence suggests an increase of arc-related detritus. An active tectonic regime was already in place along the eastern margin of the Hearne craton by as early as 1890 Ma (Baldwin et al., 1987; Coolican, 2001) or at least pre-1870 Ma (Corrigan et al., 1999). We therefore envisage an active, possible back-arc basin environment for Sequence II (Fig. 8). Sequence III, comprising talus-type molasse deposits, is separated from the underlying sequences by an unconformity (Tran, 2001). The age of this sequence (represented by sample Geoch 9) is probably younger than 1880 Ma, given the presence of ca. 1880 Ma

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zircons in the two samples from the underlying Lower subgroup, but is older than that of the Wathaman batholith (ca. 1860 Ma). We attribute uplift of the lower part of the Wollaston Group and the basement, and formation of the overlying progressive unconformity, to tectonic activity associated with the transformation of a back-arc to a foreland basin. The accumulation of molasse deposits in the Cree Lake Zone for a period of ca. 20 m.y. between 1880 and 1860 Ma, is similar to the length of time that most modern foreland basins collect detritus (e.g., Windley, 1995). 6. Conclusion This study suggests that the Wollaston Group was built from detritus derived from diverse source rocks, ranging from old continental crust to younger, possibly juvenile volcanic arc rocks. There is strong evidence that the magmatic arc-related juvenile detritus is not restricted to the upper part but also occurs in the lower parts of the Wollaston Group stratigraphic section. Nd isotopic data of Tran et al. (2003) demonstrated that a large portion of the Wollaston Group detritus was likely derived from continental magmatic sources, and this is supported by the presence of significant amounts of zircons ranging in age from 1920 to 1880 Ma obtained in this study. Thus, most of the Wollaston Group is considered to have been deposited in a basin adjacent to a magmatic arc (e.g., Dewey and Bird, 1970), within which there was a mixed supply of both craton-derived and arc-derived detritus. Since there were no suitable volcanic arcs to the west during the deposition of the Wollaston Group (e.g., Theriault, 1990; Hoffman, 1990; Lewry and Collerson, 1990), the juvenile detritus was likely derived from the Rottenstone-La Ronge Magmatic Arc complex (see Lewry and Stauffer, 1990; Bickford et al., 1990). Field relationships (Tran et al., 1998, 2001; Delaney et al., 1995) suggest that at least the upper part of the Wollaston Group was deposited in an active tectonic setting, directly related to closure of the sedimentary basin. All of their lines of evidence lend support to the suggestion that most of Wollaston Group (e.g., Sequences II and III) was deposited in a backarc to foreland basin setting, contemporaneous with the build-up of nearby volcanic arcs (Fig. 8; see also Tran et al., 2000, 2001, 2003). Overall, we conclude that most of the Wollaston Group in the study area was deposited in an active continental margin basin setting (i.e., back-arc to foreland basin), which contrasts with interpretations that proposed that the Wollaston Group was wholly a passive margin depositional sequence that received detritus from Archean basement. This study shows that in addition to field and geochemical data sets, systematic dating of detrital minerals in multiply deformed and high-grade metamorphic metasedimentary successions proves a powerful tool in unravelling the tectonic setting of orogenic terranes that evolved from formerly depositional basins. Acknowledgements This paper is dedicated to the memory of John F. Lewry. Financial support for this study was provided by Natural Science and Engineering Research Council of Canada (NSERC), Cameco Corporation, Cogema Resources Inc. (now Areva Canada), and PNC Exploration (Canada) Co. Ltd. through an NSERC-Industry Collaborative research grant. Saskatchewan Energy and Mines provided invaluable logistical support for field study. The assistance of Richard Stern and Natalie Morrisette in the SHRIMP laboratory at the Geological Survey of Canada is appreciated. This work is part of the senior author’s Ph.D. thesis at the University of Regina. Reviews by Pat Bickford, Richard Stern, Larry Heaman, and Peter Cawood are appreciated.

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