Hai Thanh Tran Et Al. 2008

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Precambrian Research 167 (2008) 171185

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Precambrian Research

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Provenance and tectonic setting of Paleoproterozoic metasedimentary rocksalong the eastern margin of Hearne craton: Constraints from SHRIMPgeochronology, Wollaston Group, Saskatchewan, Canada

Hai Thanh Tran a , Kevin M. Ansdell b , , Kathryn M. Bethune c , Ken Ashton d , Mike A. Hamilton e , 1a Faculty of Geology, Hanoi University of Mining and Geology, Dong Ngac, Tu Liem, Hanoi, Viet Namb Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canadac Department of Geology, University of Regina, Regina, Saskatchewan S4S 0A2, Canadad Saskatchewan Geological Survey, Regina, Saskatchewan S4S 0A2, Canadae Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada

a r t i c l e i n f o

Article history:Received 15 October 2007Received in revised form 25 July 2008Accepted 2 August 2008

Keywords:SaskatchewanTrans-Hudson OrogenWollaston GroupSHRIMPDetrital zirconsProvenance

a b s t r a c t

Single detrital zircon grains from various partsof the Wollaston Group, a Paleoproterozoic metasedimen-tary succession depositedalongthe southeastern margin of the Hearne Province,northern Saskatchewan,Canada, wereanalyzedby SHRIMP UPb geochronological techniques. Zircon analysesare mostly concor-dant and yield ages ranging from ca. 2800 to 1780Ma, although distinct age populations were detectedin all samples. The stratigraphically oldest sample (Geoch 4) is dominated by a bimodal distribution of zircon ages (ca. 1.90 and 2.42.6 Ga), which is similar to that preserved in the sample (Geoch 2) fromthe middle portion of the Wollaston Group. The stratigraphically youngest sample (Geoch 9) contains ca.2.1 Ga zircons, as well as zircons with thesameagesas observed in Geoch 4 and Geoch2. Zirconagesolderthan 2450 Ma appearto be consistent with theage of theHearne Province basement, suggesting that partof thesedimentarydetrituswas locally derived.Zircons with ages in the24302350Ma range, found in allsamples,may havebeen derived from a more distant source, such as RaeProvince rocks that were affectedby the recently identied Arrowsmith orogeny. Signicant amounts of 19201880 Ma zircon grains arefound in all samples; these are interpreted to represent sedimentary detritus derived from juvenile vol-canic terranes. Zircons younger than 1860Ma are interpreted to be the product of post-Wollaston Groupthermal overprinting. Our data, together with eld relationshipsand geochemical data, suggest thatmostof the preserved Wollaston Group was deposited in a back-arc to foreland basin environment. It receiveddetritus from both Archean continental crust to the west and a juvenile continental magmatic arc, likelylocated to the east, as the youngest zircon ages are not consistent with the age of Taltson Orogen rocks tothe west.

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1. Introduction

Studies of stratigraphic sequences deposited in basinal set-tings adjacent to active continental margins have revealed twomajor sources for the detrital sediments: one is the stable con-tinental interior whereas the other is a younger magmatic beltadjacent to the basin (e.g., Condie, 1997 ). These diverse sourcescan allow detailed reconstructions of the location of basins andthe tectonic processes operative in the mountain belts that wereformerlythe sitesof thesedimentarybasins.The evolutionof 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 Geol-ogy, University of Toronto, Toronto, ON M5S 3B1, Canada.

graphic records formed in relatively young sedimentary basins isquite advanced, because the combination of lithologic composi-tion, sedimentary structure, and relative dating using the fossilrecord has led to accurate reconstruction of the nature of sed-imentary successions. In contrast, historical reconstructions of Precambrian metasedimentary records are hampered by a lack of precise chronological constraints. In addition, most Precambrianbelts have undergone medium- to high-grade metamorphism aswell as polyphase deformation, which has destroyed primary sed-imentary structures, and are also deeply eroded.

The Wollaston Group is a multiplydeformed,upper amphiboliteto granulite facies Paleoproterozoic metasedimentary successionexposed along the western margin of the Trans-Hudson Orogen innorthern Saskatchewan ( Fig. 1). It has been exhumed from signif-icant depths, and the succession is likely not complete. Althoughrecent studies ( Delaney,1994; Delaney et al.,1995, 1996,1997; Tran

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doi: 10.1016/j.precamres.2008.08.003
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172 H.T. Tran et al. / Precambrian Research 167 (2008) 171185

Fig. 1. (A) Location of the Trans-Hudson Orogen (THO) and other tectonic elements of the Canadian Shield in North America (modied from Hoffman, 1988 ); (B) majortectonic divisions of the exposed Precambrian crust in Saskatchewan and Manitoba, Canada; symbols: (1) Virgin River Shear Zone (Snowbird Tectonic Zone), (2) HansonLake 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-centralexposedportion of the Cree LakeZone in Saskatchewan; rectangular boxes areareas of detailed study fromwhich sampleswere obtainedfor thisstudy.The reader is referredto Tran (2001) for descriptions of these areas.

and Yeo, 1997; Tran and Smith, 1999; Tran et al., 1998, 1999; Yeoand Savage, 1999 ) have led to a better understanding of the rel-ative 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 adistal continental highland and depositedin a passive marginal set-

ting (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 arcand/or foreland basin on an active continental margin (e.g., Lewryand Collerson, 1990; Tran and Smith, 1999; Tran et al., 2000 )? YeoandSavage(1999) suggest thatno detrituswould have beenderivedfrom the juvenile arc rocks of the Trans-Hudson Orogen, whereasTran et al. (2003) suggest that these rocks may have provided sig-

nicant detritus based on whole rock Nd isotopic data.


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H.T. Tran et al. / Precambrian Research 167 (2008) 171185 173

Understanding of the geological framework for the westerncratonic margin of the Trans-Hudson Orogen (e.g., Cree LakeZone; Lewryand Sibbald, 1977 ) and bounding tectonostratigraphicdomains (e.g., Peter Lake, Rottenstone and La Ronge domains)has improved considerably over the last decade as a consequenceof 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 etal., 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 ). Inparticular, studies by Chauvel et al. (1987) , Hegneret al. (1989) , Tranet al. (2003) , and Clarke et al. (2005) indicate that Paleoprotero-zoic mac and granitoid intrusive rocks record variable degrees of contamination by Archean and older Paleoproterozic crust, whichunderlines the complex evolutionary history of the crust along thewestern margin of the Trans-Hudson Orogen.

Collectively, these sets of geochemical, isotopic,and geochrono-logical data have characterized potential source areas for variousPaleoproterozoic sedimentary successions in this part of the Trans-Hudson Orogen, including the Wollaston Group. UPb dating of detrital minerals offers another important tool for deciphering theageand source of sedimentary strata. This paper reports the resultsof detrital mineral geochronology on stratigraphically controlledsamples from Wollaston Group. These new UPb geochronologi-cal data, collected using the SHRIMP II at the Geological Survey of Canada, offer important insights intothe provenance of the Wollas-ton Group, which in turn bears on thetectonic setting of deposition.Specically, the precise SHRIMP UPb zircon data enable resolu-tion of two different metasedimentary sources for the WollastonGroup, which in turn can be linked to two major stages of basinevolution. Thesetwo stages offercontrols on tectonicand paleogeo-graphical reconstruction of this Precambrian mountain belt in theperiod leading up to, and following terminal (continentcontinent)collision.

2. Geological setting

The Wollaston Group occurs within the southern part of the exposed Hearne Province (Cree Lake Zone) in northernSaskatchewan ( Fig. 1) where it overlies and is interfolded withhighly remobilized Archean (or dominantly Archean) basementrocks ( Lewry and Sibbald, 1980 ). Subordinate syn-rift metased-imentary, bimodal volcanic, and layered intrusive rocks of theintervening ca. 2.1 Ga Needle Falls Group ( Ray, 1979 ) are locally sit-uated above the basement and below the Wollaston Group. TheWollaston Group comprises a wide variety of complexly deformed,mainly upper amphibolite- to granulite-facies rocks, whose strati-

graphic subdivisionand lithotectonic relationsare complicated andregionally variable (e.g., Annesley et al., 2005 ). It is exposed mainlyin theWollastondomain ( Lewryand Sibbald,1980 ; Fig. 1) butisalsopreserved as discontinuous bands, commonly tectonically interca-lated with the Archean basement felsic gneisses, in the westernpart of the Cree Lake Zone (Mudjatik and Virgin River domains,Fig. 1 ). The basement assemblage is thought to be of Neoarchean toearliest Paleoproterozoic age (e.g., Wanless and Loveridge, 1978;Ray and Wanless, 1980; Bickford et al., 1990, 1994; Annesley etal., 1992, 1997, 1999; Hamilton et al., 2000 ), whereas the overlyingmetasedimentary cover is Paleoproterozoic ( Ansdell et al., 2000 ).

In the study area ( Fig. 1C), the Wollaston Group can be dividedinto Lowerand Uppersubgroups, which areseparated by a regionalunconformity ( Fig. 2; Tran, 2001 ). These two subgroups may be

subdivided into three smaller units, termed sequences, that form

continuous vertical successions bounded at the top and bottom byeither major discontinuities or unconformities ( Fig. 2). The Lowersubgroup, comprising Sequences I and II, consists of various rocktypeswith a distinctive spatialdistribution. Graphite-rich rocks andgarnet-orthopyroxene-amphibole gneisses, which are interpretedas silicate facies iron-formationsformingthe basal partof SequenceI, occur in narrow zones in the western and easternmost Wollastondomain, whereas quartzite units in Sequence I occur only along theeastern margin ( Tran, 2001 ). Stratigraphically above them, rocksof Sequence II include thick, ne-grained siliciclastic rocks ( Fig. 2),interpreted as turbiditic deposits. These are dominant in the east-ern Wollaston domain and contrast with the thick, shallow water,siliciclastic units and associated calcareous rocks that are preva-lent in the western part of the domain. Only some of the units inthe uppermostpart of Sequence II are laterally extensive, and theseare mainly tyrogenous deposits ( Tran, 2001 ).

The Upper subgroup (Sequence III) includes mostly molasse-type sedimentary rocks, ranging from talus (i.e., fanglomerateand conglomerate, Fig. 2) and arkose to carbonate and evaporitedeposits. These are interpreted to have been deposited in uvial-alluvial, restricted marine to lacustrine environments ( Tran, 2001 ).The non-marine talus deposits form a wedge-shaped, upward-coarsening succession reaching more than 1000 m in thickness inthe east and rapidly wedging out westward ( Delaney et al., 1995;Tran and Yeo, 1997; Tran et al., 1998 ). The Upper subgroup com-prises signicant detritus that appear similar to, and are possiblyderived from, the Lower subgroup ( Delaney et al., 1995; Tran andYeo, 1997; Tran, 2001 ).

In earlier studies the Wollaston Group was interpreted as ashallow-water, miogeosynclinal succession deposited on the con-tinental shelf of a subsiding cratonic margin (e.g., Money, 1968;Money et al., 1970; Ray and Wanless, 1980; Stauffer, 1984; Lewryet al., 1985; Coombe, 1994 ). Although some workers (e.g., Ray andWanless,1980; Lewry, 1981; Lewryet al., 1985; Stauffer, 1984 ) sug-gested that the Cree Lake Zone evolved from a passive to an activecontinental margin with the formation of a magmatic belt along itseastern margin during the Paleoproterozoic, their models impliedthat the Wollaston Group was a wholly passive margin successionthatreceivedall of its detritusfrom theolderArcheancratonic high-land to the west (present-day coordinates). In contrast, Lewry et al.(1985) suggested that the Wollaston Group wasoverlainby shallowwater to continental foreland basin clastic wedge deposits derivedfrom terranes in the Trans-Hudson Orogen to the east. Expand-ing on this idea, Lewry and Collerson (1990) suggested that theupper feldspathic and/or calcareous quartzite and meta-arkose of the Wollaston Group could represent synorogenic foreland, ratherthan passive margin, deposits, although they did not provide anyevidence for this.

3. Analytical procedures

In this study, three samples of medium- to coarse-grainedpsammitic rocks with heavy mineral layering were collected fromdifferent stratigraphic levels in the Wollaston Group. Geoch 4, rep-resenting the lowermost stratigraphic level, is from near the baseof Sequence II of the Lower subgroup ( Fig. 2) while Geoch 2, rep-resenting intermediate stratigraphic levels, is from the uppermostpart of the Sequence II of the Lowersubgroup. The stratigraphicallyhighest sample, Geoch 9, was collected from the package of uncon-formably overlying molasse-type sedimentary rock that compriseSequence III (Upper subgroup) ( Fig. 2 ). In each case, between 5 and30 kg of fresh rock was collected. Initial preparation of the sam-ples took place in the Department of Geological Sciences at the

University of Saskatchewan.The samples werejaw-crushed,swing-


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Fig. 2. Idealized stratigraphic sections fromthe easternWollaston Domain, and location of geochronological samples. Thesectionswere 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 dened by Tran (2001) . (*) Stratigraphic section made for easternmost partof area 1 only. Not to scale. See Tran (2001) for detailed rock description.

milled, sieved, and were then passed over a Wiley table to obtainheavy mineral concentrates. Magnetic and paramagnetic mineralswere then removed from the heavy mineral concentrates using ahandmagnet and standardFrantzmagnetic-separation techniques.Methylene iodide (MEI, density = 3.33 g/cm 3 ) was then used tosep-arate zircon from less dense minerals, prior to a nal stage of magnetic separation. This nal stage was performed to separatemetamict zircons, which are typically more magnetic, from non-magnetic zircon, as the latter are more likely to yield concordantanalyses ( Krogh, 1982 ). However, paramagnetic as well as non-

magnetic zircon crystals were also analyzed so as to reduce the

potential for sample biasing ( Sircombe and Stern, 2002 ). Zirconcrystals had minimum and maximum dimensions ranging from


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coated with high purity Au, and then imaged with a CambridgeInstruments scanning electron microscope equippedwith cathodo-luminescence (CL) and backscatter detectors in order to identifycompositional zoning and fracturing. CL imaging carried out priorto ion probe analysis was indispensable in identifying complexgrowth zoning features in the zircons, and permitted targetingof discrete domains of either primary magmatic or secondarymetamorphiccrystallization.Generally,the coresof grains were tar-geted, although rims on some grains were also analyzed to conrmthat they represent metamorphic overgrowths.

UPb analyses were conducted using the sensitive high-resolution ion microprobe (SHRIMP) technique at the J.C. RoddickSHRIMP II Ion Microprobe Lab, GSC ( Stern, 1996 ). Analytical anddata reduction procedures are described in detail by Stern (1997) .Fifty-two spots were analyzed on 49 grains for Geoch 2, 40 spots on34grainsforGeoch4,and71spotson54grainsforGeoch9( Table1 ).An O- primary beam, whose strength was varied from 14.5 to 3.8nA, was focused to yield an elliptical spot size using Kohlerionfocusing( Stern, 1999 ). In some cases,several analyseswere per-formedon a singlegrainin orderto check forage variations.In mostcases, the data are either concordant or slightly discordant within a1 error; quotedagesare based onthe 207 Pb/ 206 Pbratios.Analyticaldata 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 morphologi-cal populations, namely stubby subhedral prisms with reasonablypreserved faces, or equant shapes with only minor preservationof facets ( Fig. 3). Euhedral zircons are rare, and most common inthe conglomerate unit of the Upper subgroup (sample Geoch 9).Most of the zircons display surface abrasion, although a few areneedle-shaped, and some still retain prismatic faces and pyrami-dal terminations ( Fig. 3B). Most are small (100 m) being preserved predominantly in sampleGeoch 9. These zircons are interpreted to represent igneous grainsthat have been variably abraded during transport. Rare, very small,colourless, transparent, equant, and rounded and multifacetedgrains ( Fig. 3A) are also present in all samples; the fomer are inter-preted as metamorphic, whereas the latter maybe either metamor-phic or igneous in origin. Overall, euhedral zircons were probablyderived from more local sources, whereas rounded zircon grainswere derived from a greater distance or were repeatedly reworked.

CL imaging ( Fig. 4) revealed that most of the zircon grains con-tain numerous cracks, inclusions, and complex zoning patterns.Although some retain near-perfect concentric magmatic zoning

and inclusions ( Fig. 4A), most are dominated by distinctive coreswith generally euhedral, concentric zoning diagnostic of igneoushabit. They are terminated by one or more generations of irregularzones or rims (e.g., Fig. 4B, D and E). It is likely that the detrital zir-cons in the Wollaston Group, especially those of small grain-size,mayhave at leastpartlybeenrecrystallizedand altered during peakmetamorphism. This may have reached more than 800 C in thestudy area ( Tran, 2001 ), which is close to the closure temperatureof zircon ( Lee et al., 1997; Cherniak and Watson, 2000 ). The zon-ing patterns in some zircon may therefore be ascribed to partial orcomplete recrystallization of igneous zircon or new growth of zir-con during metamorphism (e.g., Heaman and Parrish, 1991; Harleyet al., 2007 ). However, redistribution of Pb within the zircon mayhave occurred at lower temperatures as well (e.g., Pidgeon et al.,

1998 ).

4.2. Analytical results

Most of the analytical results representing individual spot anal-yses are concordant or generally not greater than 4% discordant(at the 1 error level) ( Fig. 5 ; Table 1 ). However, even though a fewgrains have relatively homogeneous ages, those in which there aredistinct cores and rims typically yield very different ages ( Fig. 4).Many of the zircon rims have a high U contents and low Th/Uratios, 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 agepopulations ( Table 1 ; Fig. 5) and this is highlighted by the proba-bility plots in Fig. 6. Sample Geoch 4 was taken from a psammiteunit towards the base of the Lower subgroup of the WollastonGroup. Thirty-six analyses were obtained, 28 of which were in therange from 2400 to 2600 Ma, with the most common measuredage from this sample being ca. 2540 Ma. The remainder clusteredat around 1900 Ma with a range of 18401925Ma ( Fig. 6a). TheNeoarchean/earliest Paleoproterozoic ages were typicallyobtainedfrom cores of complex grains ( Fig. 4E). Some of the younger (ca.1900Ma) grains show core-rim relationships attributed to meta-morphic overprinting. For example, Fig. 4D shows the age of newzircon (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 topof the Lowersubgroup, and the fty-two analyses obtained yieldeda broadlybimodaldistribution( Fig. 6B). Nozircons were found withan age of between 1920and 2360 Ma.The youngest zircons includethree grains with an agerange of 17901820Ma, two grains with anage of ca. 1865 Ma, and 15 grains in the range of 18801920 Ma. Thelatter include zircons showing excellent magmatic growth zoning.Theolderzircon populations in this sampleinclude four grainswithages of between 2360 and 2400 Ma, although most grains range inage between 2450 and 2570 Ma.

SampleGeoch9 was taken from a conglomerate near thebase of theUppersubgroupof theWollaston Domain, andyielded themostdiverse suite of ages. The youngest zircon ages range from 1785to 1835 Ma, and are commonly from overgrowths on older grains(Fig. 4E). The 18801920 Ma age range is also common. There is asignicantly older group of zircons that yield ages between 2350and2585Ma. Geoch 9 is distinct in that it also contains three grainsin the 20502080 Ma agerange, and six grains with ages older than2600Ma, including one that has an age of 2835 Ma, which is theoldest zircon found in this study.

In general, each sample contains two distinct age groups (ca.1.9 and 2.5Ga) separated by a considerable age gap. The overallage distribution among the samples denes several distinctive agegroups. The oldest population of zircon (>2600 Ma) is restrictedto sample Geoch 9 ( Fig. 6C). All samples are dominated by zircon

in the 24502600 Ma age-range. The percentage of zircons in thisage range, relative to zircons from other age ranges in a particularsample, 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 percentageof grains in this age range then increases slightly, as represented insample Geoch 9 (conglomerate; Fig. 6C).

The23502400Ma zirconpopulation occursin very small quan-tities in samples Geoch 2 and Geoch 9 ( Figs. 6 B and C), whereas theca. 2050 Ma zircons are found only in Geoch 9. 19201870Ma zir-cons are present in all samples but the percentage of zircon withthis age appears highest in the middle of the stratigraphic section(Fig.6 ). Geoch 9 appears tohave more zircons that are younger than1900 Ma, whereas thoselower in the stratigraphicsection appear to

have more zircons in the range 19001920 Ma,although this obser-


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Table 1SHRIMP UThPb analytical data of detrital zircon of Wollaston Group

No. Spot [U] ppm [Th] ppm [Pb] ppm Th/U Age (Ma) Conc. (%)208 Pb/ 232 Th 1 206 Pb/ 238 U 1 207 Pb/ 235 U 1 207 Pb/ 206 Pb 1

Sample Geoch 41 4147.1 257 96 86 0.385 1853.3 55.3 1762.0 19.7 1798.5 15.3 1841.0 20.4 962 439.2 17 9 43 0.527 1903.6 105.0 1880.5 39.7 1867.4 45.1 1852.9 79.2 1023 445.2 353 245 140 0.718 2005.6 35.4 1923.4 21.5 1897.9 13.8 1870.2 14.5 1034 445.1 348 189 124 0.559 1919.4 38.7 1798.0 18.0 1833.1 14.0 1873.3 18.8 965 439.3 90 56 33 0.638 1847.5 44.0 1843.6 20.0 1865.3 24.5 1889.6 43.9 986 459.1 175 95 67 0.559 1932.5 62.4 1931.6 25.0 1916.4 17.0 1899.9 19.8 1027 468.1 240 142 88 0.614 1838.1 33.8 1845.8 22.6 1872.2 13.1 1901.6 7.9 978 462.1 809 468 295 0.598 1931.2 33.3 1818.6 17.6 1859.2 10.5 1904.9 7.4 969 490.1 573 678 253 1.221 1968.5 21.1 1917.0 16.9 1914.5 10.2 1911.8 8.6 100

10 459.2 241 150 94 0.645 1919.5 31.8 1930.7 18.8 1923.1 10.6 1914.8 6.4 10111 490.2 169 122 67 0.742 1941.8 56.5 1909.1 24.6 1914.9 16.6 1921.2 18.7 9912 439.1 111 82 43 0.762 1857.6 83.3 1852.4 61.8 1885.4 34.7 1922.0 13.4 9613 488.1 76 46 40 0.628 2438.0 150.5 2428.6 84.7 2413.9 65.7 2401.5 87.9 10114 444.1 791 47 338 0.061 2519.0 380.0 2268.2 202.7 2365.6 182.4 2450.6 252.2 9315 4132.1 1322 210 636 0.164 2543.0 34.9 2451.9 20.8 2458.6 16.8 2464.2 23.2 10016 414.1 267 121 143 0.467 2627.9 42.8 2521.4 29.8 2503.0 15.7 2488.0 12.2 10117 422.1 140 48 74 0.351 2458.0 81.5 2565.1 26.9 2524.8 21.9 2492.5 30.6 10318 465.1 165 64 84 0.396 2551.3 58.8 2453.4 30.9 2481.7 20.0 2504.9 22.7 9819 484.1 348 471 211 1.397 2411.4 29.1 2403.2 20.8 2460.3 11.4 2507.8 9.0 9620 489.1 314 71 159 0.232 2627.5 50.3 2522.2 24.5 2518.1 30.6 2514.7 49.3 10021 4116.1 96 58 53 0.617 2603.0 55.0 2510.1 28.0 2518.4 151.2 2525.0 274.2 9922 436.1 65 5 31 0.082 2578.9 776.5 2480.5 57.6 2505.8 59.5 2526.4 90.5 9823 495.1 347 243 191 0.724 2482.7 48.5 2469.8 24.1 2502.6 13.6 2529.4 12.4 9824 425.1 241 103 122 0.441 2470.5 54.4 2426.8 21.1 2483.1 10.2 2529.5 4.0 9625 416.1 160 131 94 0.847 2593.3 47.8 2546.9 37.9 2538.8 17.6 2532.4 6.1 10126 469.1 431 340 247 0.815 2500.3 36.6 2522.0 22.3 2527.9 11.2 2532.6 7.2 10027 446.1 86 41 45 0.491 2499.4 65.4 2472.0 25.8 2506.1 13.8 2533.8 10.7 9828 49.1 154 54 83 0.362 2624.1 85.5 2577.7 51.2 2554.7 33.9 2536.5 40.2 10229 4149.1 420 137 215 0.338 2479.0 52.4 2495.3 32.8 2518.4 20.0 2537.0 21.1 9830 4150.1 726 16 354 0.023 2420.6 303.3 2536.0 66.2 2539.6 148.8 2542.5 258.6 10031 463.1 655 286 338 0.452 2522.2 42.6 2449.4 22.8 2500.9 10.9 2543.0 3.6 9632 418.1 228 69 123 0.315 2586.7 39.4 2613.8 26.5 2575.3 13.6 2545.2 10.4 10333 45.1 180 149 105 0.857 2534.6 33.6 2533.4 23.2 2540.7 11.8 2546.6 8.1 10034 430.1 177 57 91 0.335 2427.2 76.2 2505.3 36.6 2529.2 25.1 2548.4 30.4 9835 48.1 313 168 171 0.556 2536.4 133.5 2524.2 57.7 2541.3 55.9 2555.0 82.9 9936 438.1 121 41 66 0.348 2590.3 161.7 2595.0 66.8 2577.0 53.6 2562.9 73.1 10137 457.1 105 77 62 0.752 2608.8 55.9 2577.0 24.0 2569.9 15.9 2564.3 18.8 101

38 4126.1 161 57 83 0.367 2674.6 101.8 2479.1 25.5 2531.1 16.8 2573.2 19.3 9639 4121.1 183 69 97 0.388 2508.5 48.3 2534.1 22.2 2560.3 11.0 2581.0 6.4 9840 496.1 138 63 75 0.475 2584.7 60.3 2544.4 32.5 2580.1 16.9 2608.2 12.4 98

Sample Geoch 21 296.1 83 86 34 1.058 1832.2 50.2 1845.1 41.9 1820.2 25.4 1791.8 20.7 1032 2118.1 967 481 349 0.513 1910.3 29.8 1847.0 19.0 1831.6 12.5 1814.1 13.3 1023 22.1 166 82 59 0.512 1813.9 29.8 1830.6 18.5 1823.8 10.8 1816.1 7.0 1014 2129.1 360 590 173 1.693 1968.4 35.3 1909.4 18.9 1886.6 10.6 1861.7 6.3 1035 2109.2 277 148 102 0.554 1975.5 50.4 1860.5 18.4 1864.8 17.1 1869.5 27.2 1006 2122.1 455 265 170 0.600 1982.3 31.2 1854.7 16.9 1866.6 12.2 1879.9 15.1 997 269.2 716 528 284 0.762 1966.4 36.6 1902.3 22.3 1893.0 17.4 1882.8 24.2 1018 2116.1 515 365 195 0.733 1886.7 43.3 1837.1 18.8 1863.3 11.6 1892.6 9.7 979 2112.3 468 214 157 0.472 1792.2 39.8 1742.3 20.4 1812.6 14.4 1894.4 16.1 92

10 2120.1 525 297 195 0.584 1891.4 34.7 1863.7 17.5 1879.3 11.3 1896.6 11.3 9811 213.2 121 80 47 0.685 1935.6 57.0 1893.9 20.3 1898.8 15.2 1904.1 19.9 10012 249.2 188 95 69 0.519 1841.9 37.1 1868.3 26.3 1886.2 17.2 1906.1 17.5 9813 2147.1 922 1588 448 1.779 1937.7 23.3 1905.1 17.2 1905.9 10.1 1906.8 7.5 10014 2107.2 389 244 152 0.647 1969.2 48.6 1925.4 18.2 1917.0 12.5 1907.9 14.8 10115 2134.1 908 1077 399 1.226 1952.2 21.7 1910.8 18.1 1911.2 10.0 1911.7 4.8 10016 222.2 12 6 4 0.504 1814.9 337.7 1875.0 34.1 1893.1 91.1 1912.9 184.8 9817 2147.2 597 883 284 1.528 1992.0 24.2 1949.3 18.0 1931.7 9.8 1912.9 4.7 10218 264.3 508 129 177 0.262 1840.1 42.2 1887.0 21.0 1899.6 13.1 1913.3 11.7 9919 264.2 324 69 116 0.219 1975.5 69.2 1954.5 20.5 1936.9 14.2 1918.0 17.2 10220 2134.3 587 623 243 1.096 1894.2 33.1 1857.6 17.8 1884.1 10.3 1913.4 6.3 9721 264.1 399 228 201 0.590 2383.3 30.9 2364.0 21.9 2377.8 14.4 2389.6 16.7 9922 277.1 936 133 427 0.146 2691.1 39.1 2361.4 20.7 2362.1 10.4 2362.7 5.1 10023 263.1 997 150 452 0.156 2400.4 39.0 2352.0 20.3 2359.1 11.1 2365.3 8.9 9924 26.1 70 52 38 0.769 2489.8 39.2 2405.5 27.4 2400.0 17.4 2395.4 19.4 10025 286.1 84 36 41 0.446 2527.3 59.5 2360.4 23.7 2397.7 15.2 2429.6 16.9 9726 264.4 242 167 132 0.711 2494.2 49.8 2465.8 22.0 2456.9 10.6 2449.5 4.8 10127 217.1 1266 59 579 0.048 2386.6 70.7 2407.2 22.7 2440.5 14.8 2468.4 16.8 9828 218.1 77 80 44 1.078 2448.1 111.4 2409.7 46.7 2450.2 25.7 2484.0 20.7 9729 261.1 717 81 344 0.117 2407.8 41.3 2472.8 21.5 2480.4 11.3 2486.6 8.4 9930 2137.1 102 35 49 0.357 2375.9 52.8 2378.9 23.0 2437.7 14.6 2487.2 15.6 96


7/15


Table 1 ( Continued )

No. Spot [U] ppm [Th] ppm [Pb] ppm Th/U Age (Ma) Conc. (%)

208 Pb/ 232 Th 1 206 Pb/ 238 U 1 207 Pb/ 235 U 1 207 Pb/ 206 Pb 1

31 2102.1 358 315 204 0.910 2574.6 34.3 2451.9 23.3 2475.1 12.4 2494.1 9.3 9832 25.1 130 67 68 0.531 2422.7 62.4 2474.7 30.4 2488.7 17.1 2500.2 15.4 9933 2131.1 162 67 84 0.431 2734.3 113.8 2460.6 23.5 2484.7 15.9 2504.4 19.0 9834 254.1 229 92 120 0.412 2591.0 58.1 2501.4 26.5 2506.3 13.9 2510.3 10.4 10035 27.1 152 55 76 0.375 2437.8 57.9 2437.7 33.6 2480.3 19.6 2515.4 18.7 9736 236.1 490 320 268 0.675 2489.6 35.4 2477.6 22.5 2498.4 11.0 2515.4 5.6 9937 298.1 1286 841 710 0.676 2503.2 30.2 2498.9 21.5 2510.1 10.5 2519.1 5.5 9938 260.1 173 109 91 0.655 2362.4 39.1 2425.2 28.7 2477.8 15.7 2521.2 12.6 9639 2141.1 79 37 41 0.478 2520.7 104.2 2448.2 40.0 2489.6 20.3 2523.6 12.0 9740 2130.1 132 74 71 0.577 2540.7 55.6 2501.9 23.7 2514.9 12.7 2525.5 10.4 9941 259.1 187 55 95 0.303 2516.4 72.4 2487.5 25.9 2510.4 12.9 2529.1 7.7 9842 2146.1 149 69 81 0.474 2623.8 49.9 2535.4 31.3 2532.4 17.2 2529.9 15.3 10043 272.1 341 195 186 0.590 2584.3 38.0 2492.0 23.5 2513.1 12.1 2530.2 8.2 9944 2151.1 352 149 183 0.438 2502.2 42.2 2473.2 24.7 2504.7 14.5 2530.3 14.1 9845 267.1 268 131 148 0.503 2679.2 50.8 2566.0 25.6 2549.4 12.1 2536.2 5.4 10146 250.1 133 87 73 0.679 2526.8 50.3 2477.2 26.0 2515.3 14.7 2546.1 13.3 9747 23.1 63 30 32 0.484 2284.1 50.5 2414.4 28.2 2489.2 18.7 2550.8 21.4 9548 253.1 101 58 55 0.593 2438.6 49.9 2482.5 23.9 2524.8 12.1 2559.0 7.4 9749 226.1 160 130 94 0.839 2645.1 72.6 2534.5 31.2 2549.1 15.7 2560.7 10.3 9950 248.1 181 124 104 0.710 2624.7 70.3 2558.5 24.0 2559.8 13.2 2560.8 11.8 10051 238.1 78 60 44 0.798 2470.9 73.0 2486.0 41.4 2528.4 22.9 2562.6 19.4 9752 2103.1 153 59 80 0.399 2521.1 79.6 2491.9 33.3 2534.6 19.7 2568.9 19.5 97

Sample Geoch 91 911.1 965 140 310 0.149 1569.1 763.9 1821.1 21.9 1804.1 64.3 1784.5 135.0 1022 933.2 684 125 222 0.189 1788.7 34.5 1812.3 17.1 1806.5 9.8 1799.8 5.2 1013 922.2 626 165 213 0.273 1863.2 34.7 1849.0 21.6 1832.1 14.8 1813.0 17.2 1024 9128.1 647 148 216 0.237 1850.8 43.4 1833.0 17.1 1823.9 10.4 1813.5 8.8 1015 916.1 719 223 243 0.320 1863.9 31.7 1819.0 16.7 1817.6 9.2 1815.9 2.9 1006 961.1 855 200 282 0.242 1913.5 25.0 1808.2 16.4 1812.1 9.6 1816.6 6.1 1007 991.1 739 247 247 0.345 1805.6 21.1 1794.0 16.3 1805.0 9.5 1817.6 5.8 998 9110.1 1190 111 394 0.097 1856.7 37.8 1879.5 17.7 1851.2 22.9 1819.6 42.2 1039 9146.1 834 40 264 0.050 2041.7 36.8 1818.8 17.4 1819.9 10.2 1821.2 6.7 100

10 954.1 830 239 279 0.297 1779.1 33.5 1822.2 18.2 1823.7 10.5 1825.5 6.0 10011 973.1 1077 180 336 0.173 1746.6 33.0 1752.8 15.9 1786.4 10.8 1825.9 11.5 9612 969.2 921 267 322 0.300 1935.2 31.8 1880.1 18.1 1856.1 13.6 1829.2 18.4 10313 938.2 341 6 106 0.018 1663.3 196.8 1812.5 19.3 1820.4 12.8 1829.4 13.4 9914 954.2 731 103 246 0.145 1934.9 44.5 1877.8 17.5 1856.9 16.3 1833.7 26.2 10215 9103.1 921 109 302 0.123 1899.8 57.9 1849.7 17.4 1842.6 10.1 1834.6 6.9 10116 918.2 1125 127 374 0.116 1978.6 90.2 1870.7 17.6 1853.8 9.6 1834.9 3.6 10217 931.2 479 7 153 0.015 1770.1 191.6 1851.9 20.0 1845.8 13.1 1839.0 13.9 10118 996.2 871 163 295 0.193 1903.7 39.0 1873.8 20.2 1857.8 13.1 1840.0 13.5 10219 9136.1 484 317 187 0.676 1902.8 29.6 1903.3 16.9 1886.5 10.5 1868.2 9.8 10220 9107.2 359 145 123 0.416 1809.6 36.1 1793.6 18.6 1832.6 14.9 1877.3 20.5 9621 9144.2 537 247 208 0.474 2035.9 28.0 1974.4 18.2 1928.2 10.2 1878.9 6.8 10522 9110.2 372 162 140 0.450 2017.9 32.8 1940.8 19.7 1912.1 11.6 1881.2 9.4 10323 9136.2 944 968 375 1.059 1824.5 36.4 1807.5 24.2 1846.9 20.8 1891.5 30.8 9624 9144.1 558 215 198 0.397 1997.3 28.5 1852.7 20.0 1871.1 11.4 1891.5 5.9 9825 972.1 478 250 183 0.539 1912.4 28.9 1933.0 18.0 1923.4 10.7 1913.1 8.7 10126 936.1 541 294 210 0.561 2026.8 31.3 1937.1 18.4 1927.4 11.1 1917.1 9.5 10127 998.2 36 19 16 0.539 2264.0 194.3 2157.7 32.0 2104.5 49.2 2052.9 89.4 10528 998.3 23 14 9 0.610 2039.9 76.2 1941.9 28.1 1998.7 21.2 2057.8 27.2 9429 998.1 22 15 9 0.693 2040.8 124.5 2026.9 37.0 2053.3 32.8 2080.0 48.9 9730 918.1 25 14 12 0.573 2453.7 180.1 2254.2 45.9 2305.6 39.3 2351.4 55.7 9631 990.2 672 12 301 0.018 2388.4 217.2 2391.0 64.9 2396.9 44.5 2402.0 53.2 10032 9134.1 688 475 368 0.713 2410.2 37.8 2439.6 24.5 2423.3 12.2 2409.7 6.8 10133 912.1 267 94 131 0.366 2594.7 64.3 2390.8 28.1 2420.3 15.6 2445.2 13.0 9834 968.1 453 92 216 0.210 2415.9 62.0 2418.6 22.7 2448.0 11.7 2472.5 7.6 9835 922.1 143 58 75 0.419 2591.3 74.1 2501.4 39.9 2492.4 25.4 2485.1 28.6 10136 970.1 149 65 72 0.453 2405.3 40.5 2326.4 20.6 2414.6 58.5 2489.8 103.8 9337 996.1 839 236 433 0.291 2589.6 52.0 2533.7 25.0 2511.4 14.7 2493.4 14.9 10238 931.1 155 77 81 0.515 2506.9 64.4 2458.9 24.0 2478.8 14.0 2495.1 13.5 9939 975.1 293 114 149 0.403 2525.9 39.1 2449.8 22.7 2477.2 21.9 2499.8 32.6 9840 948.1 152 54 73 0.366 2249.0 81.9 2355.2 52.3 2435.9 35.2 2504.1 40.1 9441 94.1 828 330 439 0.412 2565.5 34.6 2532.3 28.0 2517.6 23.7 2505.7 33.6 10142 926.1 57 61 34 1.106 2562.6 61.1 2484.2 23.4 2499.1 13.5 2511.1 13.0 9943 991.2 258 102 135 0.407 2530.7 66.4 2515.3 31.9 2514.1 18.9 2513.2 19.1 10044 9128.2 159 65 84 0.423 2529.2 59.6 2511.9 24.3 2515.0 13.9 2517.5 13.4 10045 9139.1 167 61 87 0.374 2497.4 47.1 2511.1 22.4 2515.4 15.9 2518.9 20.2 10046 944.1 129 55 65 0.443 2483.5 77.2 2408.2 23.6 2469.0 12.5 2519.4 8.6 9647 941.1 296 123 152 0.429 2388.8 43.3 2460.2 25.1 2494.7 14.9 2522.9 14.9 9848 964.1 174 77 88 0.458 2428.3 55.3 2420.4 24.1 2477.4 13.2 2524.5 10.6 9649 9111.1 109 55 58 0.519 2450.3 48.5 2475.8 26.4 2507.8 42.3 2533.7 70.7 9850 986.1 351 215 193 0.634 2514.2 51.8 2505.5 30.0 2523.0 18.4 2537.1 19.5 99


8/15


Table 1 ( Continued )

No. Spot [U] ppm [Th] ppm [Pb] ppm Th/U Age (Ma) Conc. (%)

208 Pb/ 232 Th 1 206 Pb/ 238 U 1 207 Pb/ 235 U 1 207 Pb/ 206 Pb 1

51 9125.1 260 138 145 0.550 2631.8 33.8 2575.9 23.7 2556.2 11.8 2540.6 7.8 10152 9146.2 43 28 23 0.667 2428.3 126.3 2456.2 35.9 2503.3 19.7 2541.8 16.1 9753 976.2 563 273 308 0.501 2577.2 32.4 2551.7 22.2 2546.7 10.7 2542.7 5.7 10054 976.1 1026 1151 670 1.159 2711.9 28.3 2639.4 22.7 2586.3 10.4 2545.0 4.4 10455 9116.1 419 169 211 0.416 2418.5 31.8 2416.3 21.0 2489.4 10.5 2549.6 5.3 9556 988.1 91 94 58 1.068 2689.0 169.5 2622.2 45.2 2584.6 21.3 2555.2 10.3 10357 974.1 100 75 54 0.773 2407.4 70.3 2423.6 25.1 2498.2 14.6 2559.4 13.5 9558 925.1 153 55 77 0.373 2431.2 39.0 2440.1 24.6 2507.7 12.8 2563.0 8.4 9559 9103.2 65 50 37 0.789 2591.9 105.2 2519.3 58.6 2545.7 38.6 2566.8 44.4 9860 9100.1 123 55 69 0.461 2751.1 85.2 2606.7 25.0 2586.4 40.8 2570.4 68.0 10161 933.1 229 79 120 0.357 2536.2 64.5 2515.6 26.3 2547.2 15.3 2572.5 14.8 9862 928.1 226 81 120 0.369 2632.0 59.5 2532.9 24.1 2557.7 16.4 2577.4 19.8 9863 930.1 947 520 538 0.568 2611.2 30.0 2598.7 22.8 2587.7 10.8 2579.1 5.4 10164 9122.1 287 112 154 0.403 2580.5 42.8 2549.7 23.1 2570.4 11.7 2586.8 8.0 9965 932.1 272 105 148 0.398 2606.0 59.4 2574.4 25.4 2581.4 13.5 2586.8 11.0 10066 951.1 27 21 17 0.809 2719.3 123.6 2658.0 42.3 2628.3 24.8 2605.4 25.7 10267 959.1 292 116 167 0.408 2712.8 67.7 2680.3 25.4 2639.7 14.7 2608.8 15.2 10368 952.1 84 63 50 0.780 2707.4 46.4 2572.5 28.2 2597.1 16.5 2616.3 16.4 9869 961.2 152 45 83 0.304 2673.9 71.0 2625.4 28.7 2640.5 15.1 2652.1 12.1 9970 938.1 129 87 75 0.697 2532.8 47.8 2602.1 31.6 2631.2 18.5 2653.6 18.6 9871 982.1 41 34 29 0.857 2915.1 105.5 2885.6 38.2 2856.3 26.5 2835.6 33.0 102

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 propagationof 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.18401790 Ma, is present in all samples and is commonly from therims 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 constraintson the source of sedimentary detritus in the Wollaston Group

and thereby determine whether this sedimentary package wasdeposited in a passive or active continental margin setting. Asdescribed above, a wide range of detrital zircon ages was detectedin the three samples ( Fig. 6), and it is assumed that the ages arerepresentative of the age of the terrane from which the zirconswere derived. There might be a concern that, because of the smallnumber of analyses, thestudymissed zircons that are less common(cf. Vermeesch, 2004 ). However, regardless of the total number of analyses, it is the occurrence of a 19201880 Ma population that is

particularly relevant to the development of a model for the originof the Wollaston Group.

The Wollaston Group was deposited on the margin of theArchean core of the Hearne Province and is now adjacent to ter-ranes (e.g., La Ronge belt) that areinterpretedto have formed in thePaleoproterozoic Manikewan Ocean ( Ansdell, 2005 ). The Wollas-ton Group was deformed and metamorphosed during the 1.8 GaHudsonian orogeny, and, following crustal thickening initiated atabout 1850Ma, is deemed to have attained peak metamorphicconditions at about 1815 Ma ( Fig. 7; Tran, 2001; Annesley et al.,1997, 2005 ). The 18401790 Ma zircons measured in this study areassumed to represent new zircon growth during peak metamor-phism. Consistent with this, most of these ages were obtained fromthe rims of zircon grains, whose cores yielded signicantly olderages.

The sedimentarydetritus of the WollastonGroup can be inferredto include at least four sources. Zircons older than 2450Ma areundoubtedly derived from basement rocks ( Fig. 7). The age of thebasement complex throughout the Cree Lake Zone of the Hearne

Fig. 3. Photomicrograph of representativezircon grains fromWollaston Group. (A) Representative zircons of differingcolour, shape, and size in Geoch 4. Large, dark,roundedgrains (1) areprobably of detrital origin whereas small, homogeneous,and transparentgrains (2) maybe metamorphic. Zircons with distinct cores (3) and concentriczoningare common in all samples and probably represent igneous or modied 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.


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Fig. 4. (A) Photomicrograph of subhedral, prismatic grain with concentric zoning pattern interpreted as igneous zoning. (B) Cathodoluminescence (CL) image of zirconshowing 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 eachsample 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. 2450Ma ( Ray and Wanless, 1980;Collerson et al.,1988;Bickfordet al., 1988, 1990, 1994, 2001; Annesley et al., 1992, 1997; Orrell etal., 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 low-ermost part of the Wollaston Group. A reduction of the >2400 Mazircon population in Sample Geoch 2 ( Fig. 6B), which is located inthe middle of the stratigraphic section ( Fig. 2), suggests an increasein the input of younger detritus making up the sediment composi-tion. The increase in the proportion of these Neoarchean zircons inSample Geoch 9 (from conglomerate unit above an unconformity)appears to be the result of the stripping and recycling of under-lying units and the basement complex as a consequence of theiruplift and unroong 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. 2350to less than 2450 Ma are relatively uncommon, but do occur in allsamples. Hamilton et al.(2000) also recognizeda ca. 24752400 Mazircon population in a quartzitic unit equivalent to Trans (2001)Unit 11 ( Fig. 2 ) in the vicinity of Duddridge Lake to the northeast of thestudy area.Detrital zircons ranging inage from 2.3to2.5 Gahavealso been found in a conglomerate in the Southern Indian Domain,to the east in Manitoba ( Rayner and Corrigan, 2004 ). This domainalso contains a ca. 1886 Ma quartz diorite containing 2.42.5 Gainherited zircons, and Rayner and Corrigan (2004) suggest thattheseagessuggest derivation fromSask craton crustalthoughthereare signicantly 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. 24502300 Ma zircon ages have


10/15


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 RaeProvince west of Uranium City effectively forms the substrate tothe Taltson magmatic arc. Previous workers reported ages rang-ing between 2500 and 2150Ma for rocks forming the basementto the Taltson magmatic arc (see Villeneuve et al., 1993; Aspler andChiarenzelli,1997 f or summary ofage).Recent mapping andrelatedgeochronology (e.g., Ashton et al., 2000; Hartlaub et al., 2006 ),however, indicates ages of ca. 27002600 and 23502300Ma formetaplutonic rocks in this region, with superimposed metamor-phic events at ca. 2350 Ma and ca. 19301920Ga. In addition, insitu UPb analysis of monazites in the Committee Bay Belt in theRae Province of Nunavut has identied a period of deformationand metamorphism at ca. 2350Ma, 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 ). TheLate Archean and earliest Paleoproterozoic zircons are thus mostlikely derived from cratonic terranes to the west and north of thepresent-day Wollaston Domain.

Theca. 2050 Mazirconpopulationappearsonlyin SampleGeoch9, and is approximately the same age as parts of the syn-rift Nee-dleFalls Groupin eastern Wollaston Domain ( Fig. 7 ; MacNeil, 1998;Ansdell et al., 2000 ). Fieldrelationshipsdemonstrated thatthe con-glomerate unit lying above an unconformity (see Tran, 2001 ) wasmost likely formed by cannibalization of the lower parts of theWollaston Group, the Needle Falls Group, and the basement com-plex. Therefore the simplest explanation is that the 2050 Ma zirconpopulation in the conglomerate unit was probably derived fromunroongof thesyn-riftassemblages thatcomprise theNeedle FallsGroup.

The 19201880Ma zircon population is similar in age to theca. 19101880 Ma rhyolitic volcanics ( Baldwin et al., 1987; VanSchmus et al., 1987; Bickford et al., 1990 ), and ca. 19201890 Matonalitic intrusiverocks ( Lewry etal., 1987; VanSchmus et al., 1987;Coolican, 2001 ) that are abundant in the Rottenstone Domain andthe 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 geochemi-cal data ( Tran, 2001; Tran et al., 2003 ) suggest a possible afnitybetween the sedimentary rocks in the eastern Wollaston Domainand the Rottenstone/La Ronge Domain in the east. The presenceof continental molasse deposits above an unconformity separatingthe lower and upper part of Wollaston Group suggests a period of uplift and tectonic instability during the late stages of its deposi-tion. The increasing abundance of volcanogenic rocks toward thetop ofthis group ( Sibbald, 1979, 1983; Tran, 2001 ) further suggeststhat volcanic rocks derived from nearby active magmatic arc(s) hadbecomea sourcefor parts of the Wollaston Group. Furthermore,Ndisotopicstudies( Tranetal.,2003 ) also show strongevidence ofmix-ing of detritus from more primitive, mantle-derived sources withmore evolved crustal sources. Nd values of the Wollaston Groupat the inferred time of deposition ranges from 6.8 to 3.4, muchhigherthanthe valueof 15.3 forthe basementcomplex( Tranet al.,2003 ). Similarly, trace element compositions indicate a wide rangeof detritus derived from basaltic toandesitic magmaticarcs to felsicupper continental crust, and were used to suggest that most of theWollaston Group wasdeposited in an active continental margin set-ting from basement-derived and juvenile mantle-derived sources(Tran et al., 2003 ).

Thus, the simplest explanation is that the 19201880Ma zir-cons were derived from the volcanic rocks of the western part of the Reindeer Zone, and most likely the Rottenstone or La Rongedomains ( Figs. 7 and 8 ). The greater abundance of zircon of this agein the youngest stratigraphic units may indicate an increase in sup-ply of juvenile detritus to the basin during the late stages of basin

evolution.An alternative explanation thatwas proposed by Yeo and Savage

(1999) was that thesediments of theWollaston Groupwerederivedsolely by erosion of the Taltson Magmatic Zone, and that the detri-tus was carried across the Rae and Hearne basement rocks bylarge river systems before emptying onto the margin of the Hearnecraton. The TaltsonMagmatic Zone consists of 25002140 Ma base-ment rocks and 19901920Ma arc and syn-collisional granitoidrocks ( Fig. 7; Theriault, 1990; Bostock and van Breemen, 1994; vanBreemen and Aspler, 1994; van Breemen et al., 1992; McDonoughand McNicoll, 1997; Villeneuve et al., 1993; De et al., 1997; Chackoet al., 2000 ). Intrusive and volcanic rocks younger than 1920 Mahave yet to be identied in this zone, whereas this study has shownthat 19901920Ma zircon is absent in the sedimentary rocks of

the Wollaston Group ( Fig. 6). This, in combination with the lower


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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 and2550 6. The sample locations are shown on the simplied stratigraphic column, which has the same legend as in Fig. 2.

Nd values of magmatic rocks from the Taltson Magmatic Zone(Theriault, 1990, 1992 ) when compared to that of the WollastonGroup ( Tran et al., 2003 ), suggest that the Wollaston Group was notderived by mixing of detritus from an Archean provenance withdetritus solely from the Taltson Magmatic Zone.

Yeo and Savage (1999) also proposed that ca. 1900Ma zirconcould have been derived from 1900 to 1920Ma granulite grade

rocks along the Snowbird Tectonic Zone (STZ). The STZ has beeninterpreted as an intracontinental granulite-grade shear zone thataffected rocks varying in age from 3.2 to 2.6 Ga and which haddeveloped within the interior of the Churchill Province by theNeoarchean ( Hanmer et al., 1994, 1995 ). In contrast, Hoffman(1988) had suggested that the STZ represented a Paleoproterozoicsuture between the Rae and Hearne cratons based on truncation of regional geophysical trends, and Ross et al. (2000) suggested thatthe STZwas active at the same time as 1.851.82Ga arc magmatismin the Alberta basement. Further work using UPb geochronologyof monazites has shown that the STZ was affected by granulite andeclogite grade metamorphism at ca. 1.9 Ga ( Baldwin et al., 2003 ),and forms the southern end of a regionally extensive zone of highmetamorphic grade rocks that extend northeastwards along theboundary between the Rae and Hearne provinces (e.g., Sanborn-

Barrie et al., 2001 ). These rocks could be the potential source of ca.1900Ma zircons. However, this requires evidence that these rockswere exposed at surface so that they could represent a potentialsource of detritus. The exhumation of the STZ along shear zoneshas been constrained by structural and geochronological data tohave been initiated at about 1.83Ga ( Mahan et al., 2003; Mahanand Williams, 2005 ), and thus it is unlikely that the rocks now

presently exposed at surface in the STZ were a potential sourceof detritus for the Wollaston Group. The lack of ca. 3.0 Ga zirconsalso lends support to this conclusion. Thus we suggest that the1920 Ma and younger detrital zircon population in the WollastonGroup was most likely derived from a local, juvenile source relatedto the magmatic arcs outboard of the Hearne margin, namely theRottenstoneLa Ronge arc system ( Figs. 7 and 8 ).

The interpretation that the younger detrital zircon populationwas derived from magmatic arcs to the east of the Hearne mar-gin opens up the possibility that some of the older zircons couldhave also been sourced from that direction. The Sask craton, whichis exposed in structural inliers within the Glennie Domain andthe Hanson Lake block ( Fig. 1), comprises rocks that show evi-dence of signicant zircon growth between 2425 and 2525Ma,as well as older (2.8, 2.9 and 3.1Ga) 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 f or thedistribution 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 Saskcraton was the source of some of the oldest Paleoproterozoic andNeoarchean zircons, then it must have been in an appropriate geo-graphical location for detritus to have been transported to the siteof deposition of the Wollaston Group. However, in order to explain

the deformational history within the Reindeer Zone, the Sask cra-ton is interpreted to have been transported on a separate tectonicplate anddid notinteract withthe Paleoproterozoicrocks preservedalong the Hearne margin until after the deposition of the Wollas-ton 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) Modelshowing the basin conguration during the deposition of most of the Wollaston Group. The proposed model suggests that the Wollaston Group contains detritus derivedfrom 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 directlyfrom the Sask craton.

Even though detrital zircons in this study are not interpreted tohave been derived from the STZ, the range of ages obtained fromdetrital zircons fromthe Wollaston Group does provide constraintson the role of the STZ in the development of Laurentia. For exam-ple, in the discussion above it is suggested that the 23502450 Mapopulation of zircons is derived from rocks present within theRae Province, and that potentially they grew during magmaticand metamorphic activity associated with the Arrowsmith Orogen(Bermanet al., 2005 ). This is signicant as this wouldimply that theRaecratonwas incontactwith theHearne cratonat thetimeof Wol-laston Group deposition. Thus, the STZ must represent an Archeansuture and not one that formed during the Paleoproterozoic, assuggested by Hanmer et al. (1995) .

5.2. Timing of deposition

The Wollaston Group unconformably overlies the Needle FallsGroup and thus the maximum depositional age of the former isconstrained by the upper age limit of the Needle Falls Group.The age of 2072 2 Ma obtained for the Needle Falls Group vol-canic rocks ( Ansdell et al., 2000 ) is therefore considered theoldest possible age for deposition of the Lower subgroup of theWollaston Group. The Wollaston Group is deformed, metamor-phosed, andcut bypre-peakmetamorphic intrusivebodies; termedthe Grey Granite Suite by Annesley et al. (2005) . Annesley andMadore (1991) and Annesley et al. (2005) indicate that these intru-sive rocks are ca. 1840 Ma, which thus represents the minimumage for deposition of the Wollaston Group. However, these mag-matic bodies represent the last gasps of arc magmatism alongthe Hearne craton margin, which is considered to have culmi-nated in the formation of the Wathaman batholith, the mainphases of which crystallized between 1865 and 1855Ma ( VanSchmus et al., 1987; Meyer et al., 1992 ). The uplift associatedwith the intrusion of the Wathaman batholith may have led towidespread uplift, including the adjacent foreland basins in thewest. The extensive Wollaston Group marine sedimentation musthave ended by that time. Therefore, the upper depositional limitof the Wollaston Group is pinned by the age of the Wathamanbatholith.

The Wollaston Groupitself canbe subdivided into several strati-graphic units ( Fig. 2). The ca. 1920 Mazircon is common toall of thesamples indicating that rock units equivalent to or younger thanthat of sampleGeoch 4 (Unit 14 of Tran, 2001 ; Fig. 2) weredepositedafter 1920Ma. This in turn suggests that only the quartzite andassociated silicate-facies iron formation units of Sequence 1 of theLower subgroup of the Wollaston Group ( Fig. 2; Tran, 2001 ), whichappearto lack zirconyounger than 2400 Ma ( Hamilton et al., 2000 ),could predate 1920Ma. The Sequence 1 rocks are interpreted to

represent a passive margin sequence, deposited from ca. 2100 to1920 Ma along the margin of the Hearne craton.

SequenceII includes samplesGeoch 2 and4 thatcontainvariableamounts of ca. 19201880Ma zircon. The increasing abundanceof ca. 19201880Ma zircon from the lower to upper part of thissequence suggests an increase of arc-related detritus. An activetectonic regime was already in place along the eastern margin of the Hearne craton by as early as 1890Ma ( Baldwin et al., 1987;Coolican, 2001 ) or at least pre-1870Ma ( Corrigan et al., 1999 ). Wetherefore envisage an active, possible back-arc basin environmentfor Sequence II ( Fig. 8).

Sequence III, comprising talus-type molasse deposits, is sepa-rated 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. 1880Ma

zircons in the two samples from the underlying Lower subgroup,but is older than that of the Wathaman batholith (ca. 1860Ma).We attribute uplift of the lower part of the Wollaston Group andthe basement, and formation of the overlying progressive uncon-formity, to tectonic activity associated with the transformation of aback-arc to a foreland basin. The accumulation of molasse depositsin the Cree Lake Zone for a period of ca. 20 m.y. between 1880 and1860Ma, is similar to the length of time that most modern forelandbasins collect detritus (e.g., Windley, 1995 ).

6. Conclusion

This study suggests that the Wollaston Group was built fromdetritus derived from diverse source rocks, ranging from old conti-nental crust to younger, possibly juvenile volcanic arc rocks. Thereis strong evidence that the magmatic arc-related juvenile detritusis not restricted to the upper part but also occurs in the lowerparts of the Wollaston Group stratigraphic section. Nd isotopicdata of Tran et al. (2003) demonstrated that a large portion of the Wollaston Group detritus was likely derived from continentalmagmatic sources, and this is supported by the presence of sig-

nicant amounts of zircons ranging in age from 1920 to 1880Maobtained in this study. Thus, most of the Wollaston Group is con-sidered to have been deposited in a basin adjacent to a magmaticarc (e.g., Dewey and Bird, 1970 ), within which there was a mixedsupply of both craton-derived and arc-derived detritus. Since therewere no suitable volcanic arcs to the west during the deposition of the Wollaston Group (e.g., Theriault, 1990; Hoffman, 1990; Lewryand Collerson, 1990 ), the juvenile detritus was likely derived fromthe Rottenstone-La Ronge Magmatic Arc complex (see Lewry andStauffer, 1990; Bickford et al., 1990 ). Field relationships ( Tran et al.,1998, 2001; Delaney et al., 1995 ) suggest that at least the upperpart of the Wollaston Group was deposited in an active tectonicsetting, 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 andIII) was depositedin a back-arc toforeland basin setting,contemporaneous withthe build-up of nearby volcanic arcs ( Fig. 8; see also Tran et al., 2000, 2001, 2003 ).Overall, we conclude that most of the Wollaston Groupin the studyarea was deposited in an active continental margin basin setting(i.e., back-arc to foreland basin), which contrasts with interpreta-tions that proposed that the Wollaston Group was whollya passivemargin depositional sequence that received detritus from Archeanbasement.

This study shows that in addition to eld and geochemical datasets, systematic dating of detrital minerals in multiply deformedandhigh-grademetamorphicmetasedimentary successionsprovesa powerful tool in unravelling the tectonic setting of orogenic ter-ranes that evolved from formerly depositional basins.

Acknowledgements

This paper is dedicatedto thememory of John F. Lewry. Financialsupport for this study was provided by Natural Science and Engi-neering Research Council of Canada (NSERC), Cameco Corporation,Cogema Resources Inc. (now Areva Canada), and PNC Exploration(Canada)Co. Ltd.through an NSERC-IndustryCollaborativeresearchgrant. Saskatchewan Energy and Mines provided invaluable logis-tical support for eld study. The assistance of Richard Stern andNatalie Morrisette in the SHRIMP laboratory at the GeologicalSurvey of Canada is appreciated. This work is part of the seniorauthors Ph.D. thesis at the University of Regina. Reviews by PatBickford,RichardStern,LarryHeaman, andPeter Cawoodareappre-

ciated.


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