Tectonic setting of the Taltson magmatic zone at1.9–2.0 Ga: a granitoid-based perspective1

Thomas Chacko, Suman K. De, Robert A. Creaser, and Karlis Muehlenbachs

Abstract: The Paleoproterozoic Taltson magmatic zone is one of the key tectonic features of western Laurentia. Theexisting tectonic model for the belt envisions its formation by subduction of oceanic crust beneath a continental mar-gin, followed by direct collision between formerly separate crustal blocks. We tested this model by comparing the largegeochemical and isotopic database available for Taltson magmatic zone granitoids with similar databases forPhanerozoic granitoid suites from different tectonic environments. The comparison reveals that the early granitoid suiteof the Taltson magmatic zone, which had been ascribed to the subduction phase of orogenesis, lacks the mantle signa-ture apparent in granitoids of Phanerozoic continental-margin arc settings. Instead, both early and late suites appear tohave an intracrustal origin, similar to Mesozoic and Cenozoic granitoids of the Cordilleran interior of western NorthAmerica, which formed in the distant hinterland of a convergent plate margin. In light of these findings, we propose analternative tectonic model, which envisions formation of the Taltson magmatic zone in a plate-interior rather than aplate-margin setting. Modern-day examples of this setting are found in the mountain belts of central Asia, such as theTian Shan, which are located many hundreds of kilometres away from the plate margin. The critical feature of thesebelts that make them an appealing analogue for the Taltson magmatic zone is that there is no subduction zone closelyassociated with their formation. Rather, magmatism occurs in response to thickening of crust in the continental interior.

Résumé : La zone magmatique Taltson, datant du Paléoprotérozoïque, est l’un des paramètres tectoniques caractéristi-ques de la Laurentie occidentale. Selon le présent modèle tectonique, la ceinture a été formée par la subduction de lacroûte océanique sous la marge continentale, suivie d’une collision directe entre des blocs crustaux auparavant séparés.Nous avons mis ce modèle à l’épreuve en comparant la grande base de données géochimiques et isotopiques disponiblepour les granitoïdes de la zone magmatique Taltson et des bases de données similaires pour les suites granitoïdes duPhanérozoïque de divers environnements tectoniques. Cette comparaison révèle que la suite granitoïde précoce de lazone magmatique Taltson, qui a été attribuée à la phase de subduction de l’orogenèse, n’a pas la signature du manteauque l’on voit dans les granitoïdes d’environnements d’arcs à la marge du continent. Au lieu, les suites précoce et tar-dive semblent toutes deux avoir une origine intra-crustale, semblable aux granitoïdes du Mésozoïque et du Cénozoïquede la cordillère interne de l’Amérique du Nord occidental qui se sont formés loin dans l’arrière-pays d’une bordure deplaque convergente. Compte tenu de ces découvertes, nous proposons un autre modèle tectonique, soit la formation dela zone magmatique Taltson à l’intérieur d’une plaque plutôt qu’en bordure d’une plaque. Des exemples visibles de nosjours de ce milieu se retrouvent dans les ceintures de montagnes de l’Asie centrale, telles que le Tian Shan, qui sontlocalisées à des centaines de kilomètres des bordures de plaque. La caractéristique clef de ces ceintures, et qui en faitun analogue attrayant pour la zone magmatique Taltson, est qu’il n’y a pas de zone de subduction étroitement associéeà leur formation. Plutôt, le magmatisme a lieu en réponse à un épaississement de la croûte à l’intérieur du continent.

[Traduit par la Rédaction] Chacko et al. 1609

Introduction

Tectonic models for Paleoproterozoic orogenic belts arecommonly derived from two classical Phanerozoic ana-logues: the ocean–continent collisional setting of the Northand South American Cordillera and the continent–continentcollisional setting of the Himalayas. The lithological, struc-

tural, and geochemical characteristics of rocks in these twomodern-day, convergent, plate-margin settings provide atemplate for interpreting broadly similar features in Paleo-proterozoic terranes. Importantly, both continental-arc andcollisional settings are ideal for the production of graniticmagmas, and each setting produces a geochemically and iso-topically distinctive suite of granitoids (e.g., Clarke 1992;Pitcher 1993). Thus, in principle, the nature of graniticmagmatism in Paleoproterozoic orogenic belts can be usedas a guide for recognizing the tectonic environment in whichthese belts formed. This approach, however, requires thatdetailed comparisons be made between Paleoproterozoicgranitoids and their putative Phanerozoic counterparts. Un-fortunately, such comparisons are not always made. Instead,Proterozoic granitoid belts with a broadly “calc-alkaline”chemical signature are attributed to formation in a continen-tal-margin arc, and peraluminous granitoids to formation ina continent–continent or arc–continent collision. These cur-

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Received September 20, 1999. Accepted March 10, 2000.Published on the NRC Research Press Web site onOctober 24, 2000.Paper handled by Associate Editor M. Stauffer.

T. Chacko,2 S.K. De, R.A. Creaser, and K. Muehlenbachs.Department of Earth and Atmospheric Sciences, University ofAlberta, Edmonton, AB T6G 2E3, Canada.

1Lithoprobe Publication 1145.2Corresponding author (e-mail: Tom.Chacko@ualberta.ca).

sory comparisons can be deceptive as granitoids with thesame general characteristics are not restricted to the twoplate-margin settings outlined above, but can form in severaltectonic environments (e.g., Clarke 1992; Pitcher 1993). Amore reliable indication of tectonic setting comes from mak-ing careful geochemical and isotopic comparisons betweenPaleoproterozoic granitoid suites and well-documentedPhanerozoic suites from a number of different tectonic set-tings.

In this paper, we take such an approach in evaluating thetectonic environment responsible for the formation of the1.9–2.0 Ga Taltson magmatic zone (TMZ) of northwesternCanada (Fig. 1). The existing tectonic model for the TMZ is

an extension of a model first proposed for the Thelon tec-tonic zone, the northward continuation of the TMZ (Gibband Thomas 1977; Hoffman 1987, 1988). The models forthe Thelon tectonic and the Taltson magmatic zones arebased on a Himalayan analogue and rely in part on the gen-eral character of granitic magmatism. More specifically, theearly, calc-alkaline (I-type) granitoid suite present in thesebelts is proposed to have formed in a continental-margin arcsetting during closure of an ocean basin that formerly sepa-rated the crustal blocks flanking the belts. The later,peraluminous (S-type) granitoid suite is believed to haveformed after basin closure in a continent–continentcollisional setting. We tested this model by comparing the

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Fig. 1. Generalized tectonic map of western Laurentia (modified after Hoffman 1989; Ross et al. 1991; McDonough et al. 1995). GSLand STZ refer to the Great Slave Lake shear zone and Snowbird tectonic zone, respectively. Areas to the west and south of the dashedline are buried beneath Phanerozoic sedimentary cover. The geological details of the outlined area are given in Fig. 2.

large geochemical and isotopic database available for TMZgranitoids, with similar databases available for Phanerozoicgranitoid suites from different tectonic environments. Thecomparison reveals that the early granitoid suite of the TMZlacks the mantle component that is apparent in the granitoidsof Phanerozoic continental-margin arc settings. Instead, bothearly and late suites of TMZ granitoids appear to have anintracrustal origin, similar to granitoids found in modern-daycollisional settings. In light of these findings, we propose analternative tectonic model for the TMZ and discuss its impli-cations for the tectonic evolution of western Laurentia atca. 2.0 Ga.

Regional geology

The Taltson magmatic zone makes up the southern part ofthe 2500 km long Taltson–Thelon (2.0–1.9 Ga) orogenic belt(Bostock et al. 1987; Hoffman 1988, 1989). The TMZ isbounded to the west by the 2.0–2.4 Ga Buffalo Head terraneand to the east by the Archean Churchill Province (Fig. 1)(Ross et al. 1991). The 300 km exposed section of the TMZin Alberta and the Northwest Territories comprisesgranitoids, metasedimentary gneisses, granitic basementgneisses, and amphibolites (Fig. 2).

Three decades of geological mapping and supporting geo-chronological studies in the Precambrian shield of northeast-ern Alberta, which includes the TMZ (Godfrey 1986;Baadsgaard and Godfrey 1967, 1972), have served as thefoundation for later structural, petrological, geochemical,and isotopic studies in the area (Godfrey and Langenberg1978; Nielsen et al. 1981; Langenberg and Nielsen 1982;Goff et al. 1986; McDonough et al. 1993, 1995; De et al.2000). Work has also focussed on the northward extensionof the TMZ into the Northwest Territories (Bostock et al.1987, 1991; Bostock and Loveridge 1988; Thériault 1992).Collectively, these studies have shown that the TMZ is dom-inated by 1.99–1.93 Ga granitoid rocks (Fig. 2). These in-clude several suites of biotite and hornblende-biotite granitesand granodiorites; the 1986 ± 2 Ma Deskenatlata suite(Bostock et al. 1987), the 1971 ± 4 Ma Colin Lake suite, andthe 1963 ± 6 Ma Wylie Lake suite (McDonough et al. 1995).In this paper, we group these three suites together, becausethey show similar ranges in mineralogical, major element,Nd, Pb, and O isotope compositions (data from Thériault1992; Forest 1999; De et al. 2000), and refer to them as theearly TMZ granitoids. This early phase of metaluminous tomoderately peraluminous magmatism was followed by alarge volume of moderately to strongly peraluminous mag-matism, which we refer to as the late TMZ granitoids. Thelatter include the 1955–1933 Ma biotite ± garnet ±cordierite ± spinel granitoids of the Slave and Konth suites(Bostock et al. 1987; Bostock and Loveridge 1988; Bostocket al. 1991) and the 1928 ± 2 Ma biotite ± garnet granitoidsof the Arch Lake suite (McNicoll et al. 1993).

The 2.0–1.9 Ga TMZ granitoids intrude basement rockswith a complex pre-2.0 Ga history. This lithologically di-verse group of rocks consists primarily of 2.27–2.44 Gahornblende–biotite orthogneisses, amphibolites, and para-gneisses (Bostock et al. 1991; van Breemen et al. 1992;McNicoll et al. 1994). Also present are numerous bodies ofhigh-grade, pelitic, metasedimentary rocks, some of which

are found as enclaves in the Slave and Konth suitegranitoids (Godfrey 1986; Bostock et al. 1987; Thériault1992; Chacko and Creaser 1995; Grover et al. 1997). On thebasis of a U–Pb zircon and monazite study, Bostock and van

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Fig. 2. Generalized geological map of the exposed section of theTaltson magmatic zone (TMZ) compiled from the maps ofGodfrey (1986), Bostock et al. (1987), McDonough et al. (1995),and Berman and Bostock (1997). The two major suites of TMZgranitoids discussed in the text are the 1.99–1.96 Ga early suite(Deskenatlata, Colin Lake, and Wylie Lake granitoids) and the1.95–1.93 Ga late suite (Slave, Konth, and Arch Lakegranitoids). GSL, CL, and LL denote the Great Slave Lake,Charles Lake, and Leland Lake shear zones, respectively.

Breemen (1994) concluded that these metasedimentary rocksare the dismembered remnants of a once continuous sedi-mentary basin, the Rutledge River basin, which was depos-ited and metamorphosed between 2.17 and 2.04 Ga. Theyproposed that the basin formed in response to rifting alongthe western margin of the Churchill Province prior to thecompressional event that led to the intrusion of the mainTMZ granitoids.

Comparisons

The Phanerozoic granitoid suites chosen for comparisonwith the TMZ include those formed by subduction of anoceanic slab beneath a continental margin and those formedby continent–continent collision with crustal thickening. Werefer to these as subduction-related and collision-relatedgranitoids, respectively. These particular granitoid suiteswere selected because there is general agreement regardingthe tectonic environment in which the granitoids formed andalso because of the availability of a large body of geochemi-cal and isotopic data. The subduction-related suites are theCoastal batholith of Peru; the magmatic rocks of the CentralVolcanic Zone of southern Peru, Bolivia, and northern Chile;the Southern California and Sierra Nevada batholiths andCoast Plutonic Complex of the North American Cordilleranmargin; and the precollisional Trans-Himalayan batholith ofPakistan, India, and Tibet. In all of these areas, magmatismis believed to be directly related to subduction of oceaniccrust beneath a continental margin (e.g., DePaolo 1981;James 1982; Crawford and Searle 1992), as has been pro-posed for the early phase of orogenesis in the TMZ. The col-lision-related suite chosen for comparison is the lateMesozoic to early Cenozoic Cordilleran Interior granitoidsof western North America, which include the Cassiarbatholith of British Columbia and Yukon, the mid-Cretaceous batholiths of southeastern British Columbia, andthe Idaho batholith. Although there remains some contro-versy, the growing consensus is that Cordilleran Interiorgranitoids are the product of crustal thickening and associ-ated intracrustal melting in the distant hinterland of a con-vergent plate margin (Patiño Douce et al. 1990; Pitcher1993; Brandon and Lambert 1994; Foster and Fanning 1997;Driver et al. 2000). In effect, these granitoids formed in aplate-interior collisional setting rather than in a plate-margincollisional setting, like the present-day Himalayas.

In the following figures, data from the Phanerozoicgranitoid suites indicated above are compared to data forTMZ granitoids, as reported in Goff et al. (1986), Thériault(1990, 1992), De et al. (2000), and Forest (1999). Shown forcomparison are data for Paleoproterozoic granitoids from theUngava segment of the Trans-Hudson Orogen of north-central Canada, which is also interpreted to have formed in acontinental-margin arc setting (Dunphy and Ludden 1998).

Major elementsTrace-element discrimination diagrams are commonly

used to infer the tectonic setting of ancient granitoid suites(e.g., Pearce et al. 1984). Such diagrams, however, can leadto erroneous classification because of the tendency of thetrace-element characteristics of granitoids to mimic those oftheir source rocks (Morris and Hooper 1997). Thus, discrim-

ination based on trace-element compositions alone may re-flect the tectonic setting in which the source rocks ratherthan the granitoids themselves formed. In our view, a morereliable first-order indication of tectonic setting comes fromconsidering the major-element compositions of a large suiteof granitoid samples from a particular terrane. For example,Kerr (1989) demonstrated the value of a simple normativeclassification approach (Streckeisen and LeMaitre 1979) indistinguishing between granitoids formed in subduction-zone and nonsubduction-zone settings. This approach, whichis the normative equivalent of the conventional InternationalUnion of Geological Sciences classification scheme forgranitoids, contrasts the lithological diversity of granitoidsin the two tectonic settings.

In their classification, Streckeisen and LeMaitre (1979)used two sets of normative parameters, the degree of silicaoversaturation and anorthite:orthoclase ratios (Fig. 3). Earlyand late TMZ granitoids differ somewhat in their distribu-tion of samples; the late granitoid suites comprise primarilygranites and alkali feldspar granites, whereas the early suitesalso contain a significant proportion of monzogranites andgranodiorites. Both sets of TMZ granitoids are, however,clearly different than Phanerozoic subduction-relatedbatholiths. More specifically, the latter have more than 30%of samples plotting in the quartz monzodiorite, monzo-diorite, tonalite, quartz diorite, diorite, quartz gabbro, andgabbro fields (fields 11–17). In contrast, the early TMZgranitoids, which supposedly formed in a subduction-zoneenvironment, have less than 10% of samples plotting inthese fields. It should be noted that no attempt was made toexclude tonalitic, dioritic, or gabbroic rocks in the samplingprocedure. In fact, most of the samples from the Colin andWylie Lake suites were obtained in a systematic grid sam-pling program, and thus should be representative of therange of compositions present in that granitoid suite (Goff etal. 1986). We interpret the paucity of these samples to reflecta minimal contribution of mantle-derived magmas (low Kand Si) to the overall igneous activity in the TMZ. This situ-ation differs from modern-day continental arc environments,where magmatic activity is triggered by the influx of mantle-derived magmas into the crust. Interestingly, the Paleo-proterozoic granitoids of the Ungava segment of the Trans-Hudson Orogen, which have also been ascribed to formationin a continental-margin arc (Dunphy and Ludden 1998), doresemble granitoids from Phanerozoic subduction suites(Fig. 3). This observation suggests no fundamental temporalchange in the nature of subduction-zone magmatism fromPaleoproterozoic to Phanerozoic times. Rather, the observeddifference between early TMZ granitoids and Phanerozoicsubduction-related granitoids more likely results from a dif-ference in tectonic setting.

Collectively, the distribution of early and late TMZgranitoid samples (Fig. 3) closely resembles the distributionin the Cordilleran Interior granitoids of western NorthAmerica, where the overwhelming abundance of granitesand granodiorites is related to a predominantly intracrustalsource region for the magmas (Brandon and Lambert 1994;Driver et al. 2000). Similarly, De et al. (2000) proposed thatboth early and late granitoid suites of the southern TMZwere derived exclusively from crustal source rocks. Theysuggested that the differences between the two suites re-

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Fig. 3. Comparison of early and late suites of TMZ granitoids with Phanerozoic examples of subduction-related and nonsubduction-related granitoids using the CIPW normative equivalent to the IUGS classification scheme for granitoids (Streckeisen and LeMaitre1979). Also shown are data for granitoids from the Paleoproterozoic Ungava segment of the Trans-Hudson Orogen. Sources of data:TMZ (Goff et al. 1986; Thériault 1990, 1992; De et al. 2000; Forest 1999); Ungava segment of the Trans-Hudson Orogen (Dunphyand Ludden 1998). Phanerozoic subduction-related batholiths: Peninsular Ranges batholith of southern California (Baird and Meisch1984); Trans-Himalaya batholith (Honneger et al. 1982; Debon et al. 1986, 1987; Harris et al. 1988a; Crawford and Searle 1992;Ahmad et al. 1998); Coastal batholith of Peru (Pitcher et al. 1985). Phanerozoic collision-related batholiths: Cordilleran Interior gran-ites of North American (Hyndman 1983, 1984; Hyndman and Foster 1988; Miller and Barton 1990; Brandon and Lambert 1993, 1994;Driver et al. 2000). Parameters: ANOR = normative (An/(Or+An)) × 100, and Q = normative (Q/(Q+Ab+Or+An)) × 100. Fields: 1, al-kali feldspar granite; 2, alkali feldspar quartz syenite; 3, alkali feldspar syenite; 4, granite; 5, quartz syenite; 6, syenite; 7,monzogranite; 8, quartz monzonite; 9, monzonite; 10, granodiorite; 11, quartz monzodiorite; 12, monzodiorite; 13, tonalite; 14, quartzdiorite; 15, diorite; 16, quartz gabbro; and 17, gabbro.

sulted from differences in the nature of their crustal sourcerocks (metaigneous versus metasedimentary) rather than todifferences in tectonic setting.

Neodymium isotopesComparisons of Nd isotope data lead to similar conclu-

sions (Fig. 4). The Cordilleran Interior granitoids show ex-clusively negative εNd(T) values, with the majority ofsamples having values between –5 and –15 (Driver et al.2000). Importantly, the values for these granitoids closelymatch those of the local crust, consistent with the interpreta-tion that the granitoids were derived solely from thesecrustal source rocks with little or no mantle contribution(Brandon and Lambert 1994; Driver et al. 2000). In contrast,the Phanerozoic subduction-related granitoid suites haveboth positive and negative epsilon values and typically plotbetween values for depleted mantle and local crust (e.g.,Boily et al. 1989; Crawford and Searle 1992). This fact re-flects a significant mantle contribution to the granitoidsalong with variable amounts of crustal contamination. Aswas the case with the major-element comparison, both earlyand late TMZ granitoid suites compare much more favour-ably with Cordilleran Interior granitoids than with subduction-related granitoids in that theirεNd(T) values are exclusivelynegative and overlap values reported for the local crust (Deet al. 2000; Forest 1999). In contrast to the TMZ, granitoidsfrom the Ungava segment of the Trans-Hudson Orogen aresimilar to Phanerozoic subduction-related suites (Dunphyand Ludden 1998).

Oxygen isotopesOxygen-isotope data for TMZ granitoids reveal the same

pattern with regard to the relative contributions of crust ver-sus mantle to the granitoids. Theδ18O values of crustal rocksare generally higher than those of mantle-derived rocks be-cause many crustal rocks have protoliths with a prehistory ofwater–rock interaction at low temperatures, a process thattypically leads to an increase in18O content. Thus,granitoids derived exclusively from crustal source rocks tendto have higherδ18O values than granitoids derived from acombination of crust and mantle sources (e.g., Taylor andSheppard 1986). This is apparent in Fig. 5, wherePhanerozoic subduction suites show a wide range ofδ18Ovalues, from mantle-like values of 5.5‰ to higher values re-flecting the oxygen-isotope composition of the local crust.The Cordilleran Interior granitoids, on the other hand, aredominated by samples with high18O values, with the over-whelming majority havingδ18O values greater than 8.5‰.Like the Cordilleran Interior granitoids, both early and lateTMZ granitoid suites are characterized by highδ18O values,with less than 10% of the samples havingδ18O values below8.5‰. These results again are consistent with an intracrustalorigin for 1.9–2.0 Ga magmatism in the TMZ.

Proposed tectonic model for the TMZ

The geochemical and isotopic comparisons outlined aboveindicate that the currently accepted tectonic model ofsubduction, followed by Himalayan-style collision, does notadequately explain the geochemical or isotopic character ofgranitic magmatism in the TMZ. In particular, there is little

evidence to suggest that the early phase of TMZ magmatismwas directly related to subduction of oceanic crust beneath acontinental margin, such as occurred to form the Trans-Himalayan batholith and other subduction-related batholiths.Rather, both early and late-stage TMZ granitoids were de-rived from crustal sources. We suggest, therefore, that a re-evaluation of the existing tectonic model is required.

Hoffman’s (1987, 1988) tectonic model for the Thelonorogen and later extensions of this model to the TMZ (Rosset al. 1991; Thériault and Ross 1991; Thériault 1992) pro-posed a plate-margin setting, analogous to the convergentplate margin that gave rise to the Himalayas. We propose in-stead that the TMZ, and perhaps also the Thelon orogen,formed in a plate-interior setting, such as is currently activein the mountain belts of central Asia. The central Asianbelts, which include the Altyn Tagh, Nan Shan, and TianShan, are a product of the India–Asia collision, but are lo-cated 500–2000 km away from the plate margin (Molnar andTapponier 1975; Windley 1995) (Fig. 6). Although they oc-cur at the plate interior, a number of features in these beltsresemble those at convergent-plate margins. These featuresinclude significant crustal thickening, laterally extensivethrust and strike-slip faults, and both large and small fore-land basins spatially associated with the rising mountainbelts (Windley et al. 1990; Yin et al. 1998). Windley (1995)suggested that one of these belts, the Tian Shan, is currentlyundergoing high-grade metamorphism and partial melting ofits lower crust as a result of thickening-induced elevation ofthe geotherm. The shallow erosional level that characterizesthis very young orogenic belt precludes direct assessment ofthis proposal. Nevertheless, the Tian Shan may serve as amodern-day analogue for older, more deeply erodedintracontinental orogenic belts, such as the Cordilleran inte-rior belt of western North America, where the metamorphicand plutonic core is exposed.

We propose that the Taltson–Thelon orogen is broadlyanalogous to the Tian Shan (Fig. 6). In part, our proposal re-lates to a suggestion by Thompson and Henderson (1983)that the Thelon tectonic zone represents a zone ofintracratonic thrusting. However, unlike that earlier study,we place our observations in a plate tectonic context. In ourmodel, the TMZ and Thelon orogen correspond to the east-ern and western Tian Shan, respectively. The two segmentsof the Tian Shan are separated by a system of right-lateralstrike-slip faults (Yin et al. 1998), analogous to the GreatSlave Lake shear zone. In contrast to Hoffman’s (1987)model, the Slave Province in our model is not the primaryindenter corresponding to India. Rather, it is analogous tothe Tarim block, a rigid crustal block that acts as a second-ary indenter, transferring stresses from the India–Asia platemargin into the Asian hinterland (Molnar and Tapponier1975; Neil and Houseman 1997). As a result of northwardmovement of the Tarim block, the crust of the Tian Shan hasbeen thickened by thrusting to 50–55 km (Roecker et al.1993; Cotton and Avouvac 1994). A similar crustal thicknesscan be inferred for the TMZ at ca. 2.0–1.9 Ga by combiningpaleopressure estimates of 5–8 kbar (1 kbar = 100 MPa)(Chacko and Creaser 1995; Grover et al. 1997; Berman andBostock 1997) with an assumed present-day crustal thick-ness of 30–40 km. In our model, the Buffalo Head terrane(Fig. 1) occupies a Tibetan plateau position, an older block

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Fig. 4. Comparative plots ofεNd versus time for selected (A) Phanerozoic and (B) Paleoproterozoic granitoid suites. Note that neitherthe early nor late TMZ granitoids compare favourably with Phanerozoic subduction-related batholiths. Sources of data: TMZ (Thériault1992; De et al. 2000; Forest 1999); Ungava segment of the Trans-Hudson Orogen (Dunphy and Ludden 1998); Cordilleran marginbatholiths of North America (Southern California, Sierra Nevada batholiths and Coast Plutonic Complex) (DePaolo 1981; Pickett andSaleeby 1994; Cui and Russell 1995); Coastal batholith, Peru (Boily et al. 1989); Trans-Himalayan batholith (Harris et al. 1988b;Crawford and Searle 1992); Cordilleran Interior granitoids (Fleck 1990; Brandon and Lambert 1993, 1994; Patchett et al. 1998; Driveret al. 2000). CHUR, condritic uniform reservoir.

of crust that has been variably reworked over a broad area atca. 2.0 Ga. Finally, the rise of the Tian Shan over the last 24million years (Yin et al. 1998) has resulted in the formationof several foreland basins (e.g., Turpan and Tarim basins).We suggest that these are analogous to the Kilohigok,Athapuscow, and Nonacho basins associated with theTaltson–Thelon orogen (Fig. 6).

From a granitoid-based perspective, the most compellingfeature of a plate-interior tectonic setting is that it obviatesthe need for a subduction zone directly associated with for-mation of the mountain belt. For example, there was no LateMesozoic to Cenozoic subduction zone associated with for-mation of the central Asian mountain belts; deformation andcrustal thickening occurred entirely in the continental inte-rior. This feature is consistent with the observation that bothearly and late TMZ granitoids lack the significant mantlecomponent that characterizes subduction-zone granitoids. In-stead, the TMZ granitoids had an intracrustal origin, similar

to the Cordilleran interior granitoids of western North Amer-ica, which also formed in the continental hinterland.

Our proposed model has significant implications for thetectonic evolution of western Laurentia in Paleoproterozoictime. If the Taltson–Thelon orogenic belt does not mark thelocation of a plate boundary at ca. 2.0 Ga, then the crustalblocks flanking the belt (Buffalo Head terrane, Slave andChurchill cratons) either formed and assembled in theArchean (i.e., they represent part of a single, variably re-worked Archean supercontinent), or the blocks formed sepa-rately, but achieved their present configuration between 2.5and 2.0 Ga. The conclusions of a recent study by Aspler andChiarenzelli (1998) favour the first possibility. On the basisof similarities in 2.5–2.1 Ga sedimentary successions andwidespread magmatic activity at 2.5–2.1 Ga in the Superior,Churchill, Slave, and Wyoming cratons, Aspler andChiarenzelli postulated that these cratons represent the west-ern part of an Archean supercontinent (“Kenorland” of Wil-

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Fig. 5. Histograms comparing the oxygen isotope composition of magmatic rocks from (A) Central Volcanic Zone (CVZ), Andes, andTrans-Himalaya batholith, (B) Cordilleran Margin batholiths of North America, (C) North American Cordilleran Interior, and (D) TMZ.Note the paucity of granitoid samples withδ18O values <8.5‰ in the TMZ and Cordilleran Interior compared to that found insubduction-related batholiths of the Andes, North American Cordilleran margin, and Trans-Himalaya. Dashed line atδ18O = 8.5 ‰shown for reference. Sources of data: TMZ (Forest 1999; De et al. 2000); Cordilleran Margin (Taylor and Silver 1978; Silver et al.1979; Magaritz and Taylor 1976; Masi et al. 1981); Central Volcanic Zone (James 1982; Longstaffe et al. 1983; Harmon et al. 1984);Trans-Himalaya (Debon et al. 1986); Cordilleran Interior (Fleck and Criss 1985; Criss and Fleck 1987; Fleck 1990; Brandon and Lam-bert 1994; Driver et al. 2000).

liams et al. 1991). Furthermore, they suggested that thissupercontinent experienced protracted continental breakupbetween 2.5 and 2.1 Ga, culminating in dispersal of conti-nental fragments between 2.1 and 2.0 Ga. Following onHoffman’s (1988, 1989) model, they envisioned reassemblyof the continent by subduction and collisional processes be-tween 2.0 and 1.8 Ga. The last part of this geological historyis not consistent with our conclusions regarding the tectonicsetting of 2.0–1.9 Ga TMZ magmatism. Namely, none of theTMZ granitoids bears a subduction imprint, implying thatproposed pre-2.0 Ga rifting in the TMZ area (i.e., formationof the Rutledge River basin) did not result in the opening ofan ocean basin, but remained ensialic in character. However,the basic idea that the Slave and Churchill cratons and Buf-falo Head terrane were contiguous entities by Neoarcheantimes is entirely consistent with the hypothesis that theTaltson–Thelon orogen formed in an intraplate tectonic set-ting.

The alternative possibility, that the assembly of crustalfragments took place between 2.5 and 2.0 Ga, is analogous

to the situation in central Asia. In the case of central Asia,the Tarim block is believed to have accreted onto the south-ern margin of Asia in the late Paleozoic, the Tian Shanmarking the location of the Carboniferous suture (Windleyet al. 1990). Thus, present-day deformation in the Tian Shanis reactivating an ancient suture. Similarly, it is possible that1.9–2.0 Ga deformation, metamorphism, and magmatism inthe TMZ occurred at the site of an older plate boundary. Inthis regard, we note that 2.27–2.44 Ga granitoids andorthogneisses have been recognized along the entire lengthof the TMZ and may also be present in the Thelon orogen(Bostock and Loveridge 1988; van Breemen et al. 1987,1992; McNicoll et al. 1994). Available geochemical datafrom the southern TMZ (Goff et al. 1986) suggest more arcaffinities for the older granitoids than for any of thegranitoids discussed in this paper. Thus, these oldergranitoids may represent the vestiges of an older continen-tal-margin arc (Bostock and van Breemen 1994), which wasreactivated at 1.9–2.0 Ga to form the dominant granitoids ofthe TMZ. We propose that reactivation occurred while the

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Fig. 6. (A) Simplified tectonic map of the Indo–Asian collisional system and, in particular, the plate-interior mountain belts of centralAsia (modified after Yin et al. 1998). The abbreviations H and TB in the inset map correspond to the Himalayas and Tarim Basin, re-spectively. (B) Drawn to the same scale is our proposed configuration of western Laurentia at ca. 1.95–2.0 Ga. In our model, the Taltsonmagmatic zone (TMZ) and the Thelon tectonic zone (TTZ) correspond to the eastern and western Tian Shan, respectively. The Slavecraton corresponds to the Tarim block and the Buffalo Head terrane (BH) to the Tibetan plateau. The Kilohigok (K), Athapuscow (A),and Nanacho (NO) basins correspond to the various foreland basins associated with the Tian Shan (e.g., Turpan basin).

ancient suture was in a plate-interior setting, such as theTian Shan rather than in a Himalayan-type plate-margin set-ting.

Our tectonic model implies that the plate boundary ofwestern Laurentia at ca. 2.0 Ga was to the west of the Slavecraton and Buffalo Head terrane, but its precise location isdifficult to constrain on the basis of available data. The ma-jor orogenic belts of westernmost Laurentia, the Great Bearand Fort Simpson belts, are characterized by magmatic agesless than 1.9 Ga (Hildebrand et al. 1987; Hoffman 1988 andreferences therein), and thus are too young to be the platemargin associated with the formation of the Taltson–Thelonbelt. The plate boundary may be marked by the poorly ex-posed Hottah terrane and subsurface Ksituan magmatic belt,both of which contain magmatic rocks of 1.9–2.0 Ga age(Ross et al. 1991; Bowring and Grotzinger 1992; Villeneuveet al. 1993) (Fig. 1). However, the most recent tectonic mod-els proposed for these two areas favour their formation in awest-dipping subduction zone outboard of the Slave cratonand Buffalo Head terrane (Thériault and Ross 1991;Bowring and Grotzinger 1992). This hypothesis is not gener-ally compatible with the model proposed in the presentstudy, which views these two blocks as part of the overridingrather than the subducting plate associated with the platemargin. The issue of the actual plate boundary of westernLaurentia at ca. 2.0 Ga remains unresolved.

Proposed tests of the model

The principal difference between proposed and existingtectonic models for the Taltson–Thelon orogen concerns theposition of the belt within the overall collisional zone; theearlier model views the belt as forming at a plate margin,whereas our model views it as a zone of intraplate deforma-tion. We propose the following tests to evaluate the twomodels. Our model is based on the geochemical and isotopiccharacteristics of granitic rocks in the TMZ. To our knowl-edge, no comparable database exists for Thelon granitoids. Ifour model is correct, however, the magmatic rocks of theThelon orogen should resemble those of the TMZ. That is,the early phase granitoids of the Thelon, like those of theTMZ, should not carry a subduction-zone signature, butshould be broadly similar to the Cordilleran Interiorgranitoids.

Secondly, in the earlier model, the poorly known region tothe east of the Thelon orogen, the so-called Queen Maud up-lift (Fig. 6), represents an eroded Tibetan-type plateau(Hoffman 1987). In the modern-day Tibetan plateau, over-thickening of crust and possible lithospheric delamination isbelieved to have induced high-grade metamorphism and par-tial melting in the middle and lower crust (Windley 1995and references therein; Hirn et al. 1997). Accordingly, if aneroded Tibetan plateau model is applicable to the QueenMaud uplift, the rocks of this zone should be extensively re-worked during formation of the Thelon orogen at ca. 2.0 Ga.In contrast, our model predicts that this region should not begreatly affected by 2.0 Ga metamorphism as it occupies aposition corresponding to present-day Siberia and Mongolia,which is north of most of the deformation associated withthe India–Asia collision. These alternative predictions can be

tested through geochronological studies in the Queen Mauduplift.

Conclusions

Our goal in this paper was to compare the geochemicaland isotopic compositions of Taltson magmatic zonegranitoids with those of Phanerozoic granitoids fromsubduction-related and collision-related settings. None of theTMZ granitoids resembles those found in subduction-zoneenvironments. More specifically, the felsic and isotopicallyevolved nature of both early and late stage TMZ granitoidsindicates derivation from exclusively crustal sources, in con-trast to the combined crust–mantle sources that characterizesubduction-zone granitoids. TMZ granitoids share manysimilarities to the Cordilleran Interior granitoids of westernNorth America, which formed largely by intracrustal meltingin the distant hinterland of a convergent plate margin. Wepropose, therefore, that a plate-interior setting, such as theCordilleran Interior, is a better analogue for the TMZ thanthe plate margin setting envisioned in the currently acceptedtectonic model. Modern-day examples of this setting occurin the mountain belts of central Asia, which are a product ofthe India–Asia collision, but are located many hundreds ofkilometres away from the plate margin. To date, tectonicmodels for the Taltson magmatic zone and other Paleo-proterozoic orogenic belts in western Laurentia have fo-cussed exclusively on plate-margin settings. In light of thefindings of the present study, we suggest that plate-interiorsettings also be considered.

Acknowledgments

We are grateful to Dave Forest for permission to use un-published data from his B.Sc. thesis in our compilation ofdata for TMZ granitoids. Hewitt Bostock provided helpfuldiscussions and samples. We thank Michael Fisher for hishelp in drafting Figs. 1 and 2. A Natural Sciences and Engi-neering Council of Canada (NSERC) Lithoprobe (AlbertaBasement Transect) grant to KM and a NSERC Researchgrant to TC supported this work. We are grateful to LarryHeaman for an informal review of the paper and JonathanPatchett, Barrie Clarke, and Gerry Ross for formal journalreviews. We are solely responsible for remaining errors.

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