Development of late Paleozoic volcanic arcs in the Canadian Cordillera: an example from the Klinkit ...

Development of late Paleozoic volcanic arcs in theCanadian Cordillera: an example from the KlinkitGroup, northern British Columbia and southernYukon

Renée-Luce Simard, Jaroslav Dostal, and Charlie F. Roots

Abstract: The late Paleozoic volcanic rocks of the northern Canadian Cordillera lying between Ancestral North Americato the east and the accreted terranes of the Omineca belt to the west record early arc and rift magmatism along thepaleo-Pacific margin of the North American craton. The Mississippian to Permian volcano-sedimentary Klinkit Groupextends discontinuously over 250 km in northern British Columbia and southern Yukon. The two stratotype areas are asfollows: (1) in the Englishman Range, southern Yukon, the English Creek Limestone is conformably overlain by thevolcano-sedimentary Mount McCleary Formation (Lower Clastic Member, Alkali-Basalt Member and VolcaniclasticMember), and (2) in the Stikine Ranges, northern British Columbia, the Screw Creek Limestone is conformably overlain bythe volcano-sedimentary Butsih Formation (Volcaniclastic Member and Upper Clastic Member). The calc-alkali nature ofthe basaltic volcaniclastic members of the Klinkit Group indicates a volcanic-arc setting ((La/Yb)N = 2.77–4.73), withlittle involvement of the crust in their genesis (εNd = +6.7 to +7.4). Alkali basalts in the Mount McCleary Formation((La/Yb)N = 12.5–17.8) suggest periodic intra-arc rifting events. Broadly coeval and compositionally similar vol-cano-sedimentary assemblages occur in the basement of the Mesozoic Quesnel arc, north-central British Columbia, and inthe pericratonic Yukon–Tanana composite terrane, central Yukon, suggesting that they all represent pieces of a singlelong-lived, late Paleozoic arc system that was dismembered prior to its accretion onto Ancestral North America. Therefore,Yukon–Tanana terrane is possibly the equivalent to the basement of Quesnel terrane, and the northern Quesnel terranehas a pericratonic affinity.

Résumé : Les roches volcaniques du Paléozoïque tardif de la Cordillère canadienne septentrionale, qui se trouve entrel’Amérique du Nord ancestrale à l’est et les terranes accrétés de la ceinture Omineca à l’ouest, enregistrent du magmatismed’arc et de distension précoce le long de la bordure paléo-Pacifique de l’Amérique du Nord. Les roches volcano-sédimentairesdu Groupe de Klinkit (Mississippien à Permien) s’étendent sur plus de 250 km dans le nord de la Colombie-Britanniqueet le sud du Yukon. Les deux régions stratotypes sont : (1) dans la chaîne Englishman, du sud du Yukon, les rochesvolcano-sédimentaires de la Formation de Mount McCleary (membre inférieur clastique, membre basalte alcalin etmembre volcano-clastique) reposent en concordance sur le calcaire English Creek; (2) dans la chaîne Stikine du nordde la Colombie-Britannique, les roches volcano-sédimentaires de la Formation de Butsih (membre volcano-clastique etmembre supérieur clastique) reposent en concordance sur le calcaire de Screw Creek. La nature calco-alcaline desmembres volcano-clastiques basaltiques du Groupe de Klinkit indique un environnement d’arc volcanique ((La/Yb)N =2,77–4,73); la croûte aurait été peu impliquée dans la genèse de ces roches volcano-clastiques (εNd= +6,7 à +7,4). Lesbasaltes alcalins dans la Formation de Mount McCleary ((La/Yb)N = 12,5–17,8) suggèrent des événements périodiquesde dérive intra-arc. On retrouve des assemblages volcano-sédimentaires, généralement contemporains et de compositionsemblable, dans le socle de l’arc Quesnel (Mésozoïque), dans le centre-nord de la Colombie-Britannique et dans le terranecomposite péricratonique de Yukon–Tanana, au centre du Yukon. Cela suggère qu’ils représentent tous des pièces d’unsystème d’arc unique de longue durée, au Paléozoïque tardif, qui a été démembré avant son accrétion à l’Amérique duNord ancestrale. Le terrane Yukon–Tanana est donc possiblement l’équivalent du socle du terrane de Quesnel et leterrane de Quesnel nordique aurait une affinité péricratonique.

[Traduit par la Rédaction] Simard et al. 924

907

Can. J. Earth Sci. 40: 907–924 (2003) doi: 10.1139/E03-025 © 2003 NRC Canada

Received 8 October 2002. Accepted 6 March 2003. Published on the NRC Research Press Web site at http://cjes.nrc.ca on20 June 2003.

Paper handled by Associate Editor J.D. Greenough.

R.-L. Simard.1 Department of Earth Sciences, Dalhousie University, Halifax, NS B3H 3J5, Canada.J. Dostal. Department of Geology, Saint Mary’s University, Halifax, NS B3H 3C3, Canada.C.F. Roots. Geological Survey of Canada, Yukon Geology Program, Box 2703 K-10, Whitehorse, YT Y1A 2C6, Canada.

1Corresponding author (e-mail: [email protected]).

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Introduction

The Canadian Cordillera is generally considered to be acollage of allochthonous oceanic and pericratonic terranesthat were accreted to the western margin of the North Americancraton during the Mesozoic (Monger and Irving 1980;Coney et al. 1980; Monger et al. 1982; Gabrielse and Yorath1991). In the northern Canadian Cordillera, late Paleozoicvolcano-sedimentary sequences are an important part of theseterranes. The magmatic sequences, as well as their surroundingstratigraphy, are well documented in intra-oceanic arc terranessuch as Quesnel and Stikine (e.g., Monger et al. 1991), whereasthose of pericratonic affinities, like in the Yukon–Tananacomposite terrane, are ill-defined (Mortensen 1992). Knowledgeof the geological and geochemical evolution of late Paleozoicmagmatic sequences in pericratonic terranes is needed tomake reasonable paleogeographic reconstruction of the northernCanadian Cordillera and to determine their tectonomagmatichistory. Furthermore, age and compositional data may suggesta spatial, temporal, or genetic relationship between pericratonicand intra-oceanic terranes in late Paleozoic time.

The comparison and correlation of various time-correlativemagmatic suites in northern British Columbia and southernYukon is a fundamental step toward resolving the evolutionof the northern Canadian Cordillera. The volcanic part of theLate Mississippian to Permian Klinkit Group represents oneof these magmatic sequences. Previously linked to the oceanicSlide Mountain terrane and the pericratonic Dorsey andYukon–Tanana terranes (Wheeler et al. 1991; Monger et al.1991; see historical background in Appendix A for details),this well preserved volcano-sedimentary sequence provides arare opportunity to document the pre-accretion history ofthis part of the Canadian Cordillera.

The purpose of this paper is to define a new stratigraphicunit, the Klinkit Group, and to characterize its geologicalsetting, stratigraphy, petrography, and geochemistry, particularlywith respect to its volcanic units. The distinct geochemicalsignature of these volcanic units can be used to indirectlydiscriminate between the allochtonous terranes in the areaand to study their relationships. The data presented here areused to evaluate the petrogenesis and tectonic setting of thesevolcanic rocks and to compare the Klinkit Group with age-correlative volcano-sedimentary sequences of both pericratonicand intra-oceanic terranes in the northern Canadian Cordillera.The derived paleogeographic and tectonic reconstructionsuggests that a major Paleozoic arc system developed atop arelatively thin continental crust and was structurally dis-membered prior to, or at some stage during, its emplacementonto the western margin of North America.

Geological setting of the Klinkit Group

The Klinkit Group (proposed new Group name; seeAppendix A) lies along the western side of the Omineca belt

in the northern Canadian Cordillera (Fig. 1). Along its length,the Omineca belt is composed of complexly deformed andmetamorphosed sedimentary and volcanic rocks intruded byJurassic and Cretaceous plutons (Gabrielse et al. 1991; Monger1999). This belt is dominated by fragments of the Proterozoicto Paleozoic North American continental margin, togetherwith terranes2 of mostly pericratonic affinities (Gabrielse etal. 1991). However, in northern British Columbia and southernYukon, the main terranes of this belt are the pericratonic3

Yukon–Tanana composite terrane, the oceanic4 Slide Mountainterrane and smaller exposures of the intra-oceanic5 Quesnelterrane (Fig. 1).

The Klinkit Group is a part of the pericratonic Yukon–Tananacomposite terrane, which structurally overlies rocks of theNorth American continental margin. In Yukon, the Yukon–Tanana terrane is composed of variably metamorphosedsedimentary and volcanic successions with abundant dioriticto granitic intrusions of dominantly Mississippian age(Mortensen 1992).

The rocks of the Klinkit Group extend discontinuously fromnorthern British Columbia into southern Yukon, a strike lengthof over 250 km (Fig. 1). This volcano-sedimentary sequenceis characterized by the predominance of volcaniclastic rocksbut includes carbonates of Visean to Bashkirian age(Mississippian to Early Pennsylvanian) in the lowermostpart. Roots et al. (2002) reported an Early Permian age(U–Pb zircon; 281 ± 2 Ma) for the volcaniclastic rocks andinferred that Klinkit volcanism began in the Carboniferousand continued into Permian times. The group is unconformablyoverlain by the Triassic Teh Clastic succession (Fig. 2), asedimentary unit of continental affinity (Creaser and Harms1998; Mihalynuk et al. 2000; Colpron and Yukon–TananaWorking Group 2001).

The Klinkit Group lies within a composite allochthon thathas been thrust eastward onto the continental rocks of theNorth America (Nelson 2000; Nelson and Friedman, inpreparation6). To the west and northwest, the Klinkit Groupis in inferred fault contact with the Late Devonian volcano-sedimentary Big Salmon Complex of the Yukon–Tananacomposite terrane (Fig. 1; Mihalynuk et al. 1998, 2000). Tothe southwest, the Klinkit Group is faulted against Triassicarc strata of the Quesnel terrane.

The Klinkit Group shows important lithological changesalong strike, including a variation of the relative abundanceand stratigraphic position of the associated siliciclasticsediments. Two of the most continuous exposures revealdistinct stratigraphic successions within the group.

In the Englishman Range of southern Yukon, the KlinkitGroup is subdivided into two formations (Figs. 2, 3): (1) theEnglish Creek Limestone (proposed new Formation name;see Appendix A), and (2) the overlying volcano-sedimentaryMount McCleary Formation (proposed new Formationname; see Appendix A). The Mississippian English CreekLimestone is a light grey, finely crystalline, locally crinoidal

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2 Terranes: Fault-bounded crustal blocks that have distinct lithologic and stratigraphic successions and that have geologic histories differentfrom neighbouring terranes (Schermer et al. 1984).

3 Pericratonic terrane: Terrane showing stratigraphic affinities with the margin of the craton (Gabrielse et al. 1991), in this case North America.4 Oceanic terrane: Terrane stratigraphy mainly records the evolution of an ocean.5 Intra-oceanic terrane: Terrane stratigraphy mainly records the evolution of an intra-oceanic arc.6 Nelson, J., and Friedman, R. Interrelatedness among displaced pericratonic and island-arc terranes of the Canadian Cordillera: illustrationsfrom the Cassiar Mountains of northern British Columbia. In preparation.



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Simard et al. 909

Fig. 1. Generalized terrane map of the eastern Canadian Cordillera. Modified after Wheeler et al. (1991). The inset shows the spatialrelationship of the main late Paleozoic volcano-sedimentary sequences of the northern Canadian Cordillera and the surrounding terranes.



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limestone with minor dolostone, chert nodules and quartzarenite (Ml unit of Gordey and Stevens 1994). It is conformablyoverlain by the Mount McCleary Formation, composed ofthree members (Figs. 2, 3). From bottom to top, they are asfollows: (i) the Lower Clastic Member, a 60-m-thick,dominantly siliciclastic sequence of interbedded sandstone,thin siltstone intervals, local conglomeratic lenses and carbonatebeds, (ii) the Alkali-Basalt Member, 5–10-m-thick lenses ofdark-green mafic volcanic rocks, and (iii) the VolcaniclasticMember, a > 150-m-thick sequence of volcaniclastics andminor siliciclastic beds. In the Mount McCleary area, the

Klinkit Group structurally overlies highly deformed siliciclasticsediments of probable continental affinity (Gordey 1992; Gordeyand Stevens 1994).

In the second area, the Stikine Ranges of northern BritishColumbia (Figs. 2, 4), the Klinkit Group includes the ScrewCreek Limestone (Poole 1956; Mihalynuk et al. 2000; Rootset al. 2002) conformably overlain by the volcano-sedimentaryButsih Formation (proposed new Formation name; seeAppendix A; Roots et al. 2002). The Screw Creek Lime-stone is a prominent light-coloured reef and debris-flowcarbonate of Visean to Bashkirian age (Mississippian to Early

Fig. 2. Schematic stratigraphic sections of the Klinkit Group. Left-hand section, Stikine Ranges, northern British Columbia; Right-handsection, Englishman Range, southern Yukon.



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Pennsylvanian; Poole 1956). The Butsih Formation is composedof two units: (i) the Volcaniclastic Member, a > 250-m-thickdominantly volcaniclastic sequence interbedded with sandstone,argillite, chert, and minor limestone, and (ii) the thin UpperClastic Member, < 100 m thick, composed of siliciclasticand epiclastic rocks. In the Stikine Ranges, the KlinkitGroup conformably overlies siliciclastic sedimentary rocksof continental affinity called the Swift River succession(Nelson 2001). In at least one locality, the Triassic TehClastic succession (Roots et al. 2002) lies in angular uncon-formity on top of the Klinkit Group (Figs. 2, 4; Simard et al.2002).

Stratigraphy and petrography of thevolcano-sedimentary formations of theKlinkit Group

Butsih FormationThe Volcaniclastic Member of the Butsih Formation is the

thickest volcanic unit of the southern part of the KlinkitGroup. It consists of several 100-m-thick, fining-upwardsequences of volcaniclastic beds ranging from lithic-tuff orbreccia to crystal-tuff to ash deposits (Fig. 5A), togetherwith minor interbedded siltstone beds. Its overall thicknessis > 300 m (Simard et al. 2001a, 2001b).

Breccias at the base of some fining-upward sequences(Fig. 5A) are composed of plagioclase-phyric clasts (up to

4 cm in diameter) enclosed in a plagioclase crystal-richquartzo-feldspathic matrix. This breccia facies is typically < 8 mthick and grades upward into coarse tuff.

The main lithology of the Volcaniclastic Member is medium-to coarse-grained crystal- and lithic-tuffs. They are composedof variable proportions of crystals and lithic fragments enclosedin a fine devitrified quartzo-feldspathic matrix. The crystalfraction of those tuffs includes plagioclase, hornblende, andin some cases clinopyroxene. The lithic fraction containsmainly plagioclase-phyric or plagioclase- and hornblende-phyricbasaltic fragments, a lesser amount of relic plagioclase-phyricdevitrified glass shards, and rare fine-grained siliciclastic andrecrystallized quartz (chert?) clasts. The fine-grained volcan-iclastic material (ash) includes small broken crystals (quartzand plagioclase) in a quartzo-feldspathic, carbonate or clay-rich matrix. The clasts are usually subrounded and presentlow sphericity. Towards the top of each sequence, volcaniclasticbeds become progressively more diluted by nonvolcanicclastics, such as chert clasts and subrounded quartz grains,and the matrix becomes more clay-rich.

The Upper Clastic Member, which upwards contains in-creasing amounts of sandstone, carbonate, and dark argilliteover the volcaniclastic beds, reflects a gradual decrease involcanic input. Metres-thick sequences of massive and cross-bedded layers of epiclastic and siliciclastic sediments havebeen observed in places (Fig. 5B). In contrast, the overlyingTriassic sedimentary rocks of the Teh Clastic succession arecomposed of interbedded black argillite, dark siltstone,sandstone, and chert, with minor pebble conglomerate andlimestone beds.

Mount McCleary FormationThe Mount McCleary Formation is composed of three

members (Fig. 2).(i) The Lower Clastic Member is composed of quartz-rich

sandstone interbedded with minor siltstone horizons and localconglomerate lenses. It displays an increase in volcaniclasticbeds and limestone lenses towards the top.

(ii) The Alkali-Basalt Member is a laterally discontinuousunit of finely bedded volcaniclastic beds and massive porphyriticlava flows. The volcaniclastic beds are composed of intercalatedplagioclase-rich crystal-tuff and ash-tuff layers. Althoughprimary mineralogy has been obliterated by low-grade meta-morphism, original magmatic textures are preserved in thelava flows. Phenocrysts, probably of clinopyroxene and (or)hornblende originally, are replaced by actinolite and set in achlorite–actinolite groundmass.

(iii) The Volcaniclastic Member of the Mount McClearyFormation is petrographically indistinguishable from equivalentrocks of the Butsih Formation. It is a thick pile of massive,coarse crystal- and lithic-tuff beds, characterized by abundantplagioclase and hornblende crystals, and basaltic clasts togetherwith minor rounded quartz grains and other exotic clasts. Avery fine recrystallized quartzo-feldspathic matrix is observedthroughout this member.

Geochemistry

Analytical techniques, alteration, and samplingForty-eight representative rock samples were selected for

geochemical analyses from a suite of over 200 specimens

Simard et al. 911

Fig. 3. Geological map of the Mount McCleary area, EnglishmanRange, southern Yukon. The Mount McCleary Formationconformably overlies the English Creek Limestone to the north.This coherent stratigraphy is truncated by mid-Cretaceous granitein the northeast.



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collected during the mapping of the Klinkit Group. Majorand trace-element compositions of 13 representative7 samplesfrom the Klinkit Group are given in Table 1.

Major and some trace (Rb, Sr, Ba, Zr, Y, Cr, Ni, V, Cu,Zn) elements were determined by X-ray fluorescence at theGeochemical Centre, Saint Mary’s University, Halifax, NovaScotia. The major elements along with V, Cr, Ba, Ni, Zn, Sr,and Zr were measured on fused glass beads. Other traceelements were determined on pressed pellets. Accuracy forthis method for silica is within 0.5%. The error is < 1% forthe other major elements. For trace elements, the accuracy iswithin 5%.

Additional trace elements (the rare-earth elements (REE),Ta, Hf, Nb, Th) were analyzed by inductively coupledplasma – mass spectrometry (ICP–MS) at the ActivationLaboratories Ltd., Ancaster, Ontario, involving fusion usinglithium borate-lithium tetraborate to insure total dissolutionof the sample. Precision is better than 6% for trace elements,and accuracy is better than 5% (Young 2002).

Four of these samples from the Butsih Formation weresubsequently selected for Nd isotope analysis. Sm and Ndabundances were determined by isotope dilution massspectrometry at the Department of Earth Sciences, CarletonUniversity, Ontario. Analytical technique, as well as accuracyand precision for the technique, is described by Cousens(1996, 2000). In general, the 147Sm/144Nd ratios are repro-ducible to 1% (Cousens 2000). Measured 143Nd/144Nd valueswere normalized to a natural 146Nd/144Nd ratio of 0.72190.Epsilon values (εNd) (Table 2) were calculated assuming anage of 281 Ma for the Butsih Formation (Roots et al. 2002).

After careful field screening and petrographic analyses,only the least altered and metamorphosed samples from thevolcanic rocks of the Klinkit Group were selected for geo-chemical analyses. Most major elements, high field-strengthelements (HFSE), REE, Th, and transition elements arethought to be immobile under low-grade metamorphism

(Winchester and Floyd 1977). The consistency of variouscompositional trends using both mobile and immobile elementsin the volcanic rocks of the Klinkit Group, and their similaritiesto those of modern igneous rocks suggest that most majorand trace elements were not significantly affected by alterationprocesses, and that the distribution of these elements reflecttheir primary magmatic distribution.

Petrographic study of the sample selected for geochemicalanalyses was done to minimize the effect of their clastic nature;only the crystal- and lihic-tuff samples showing < 3% ofnonvolcanic material were considered. To evaluate the effectof the presence of siliciclastic material on the geochemicalanalyses of volcaniclastic rocks, few samples from the UpperClastic Member of the Butsih Formation showing over 10%of non-volcanic material (e.g., chert clasts, rounded quartzgrains) were processed along with the volcaniclastic samplesof the Volcaniclastic Member (Table 1). The relative increaseof siliciclastic over volcaniclastic material in the sample ischaracterized by higher SiO2, Zr, and LOI (loss on ignition)values, along with lower MgO, CaO, Cr, and Ni values(Table 1; Figs. 6–8).

Butsih FormationThe volcanic rocks of the Butsih Formation plot entirely

within the subalkaline field of Winchester and Floyd (1977;Fig. 6), with SiO2 contents between 49.9% and 51.9%(LOI-free). Their Mg# (MgO/(MgO + FeOtot) in mol%) aretypically around 0.60. This moderately low Mg# comparedto primitive melts suggests that these rocks underwent moderatefractional crystallization. Although they exhibit a calc-alkalineaffinity (Figs. 7A, 7B, 10), the rocks also display a tholeiiticfractionation trend (Fig. 7C), as shown by a slight increaseof TiO2 with differentiation, suggesting that the rocks aretransitional between tholeiitic and calc-alkaline. Zirconium,the incompatible trace element that is considered to be immobileduring alteration processes (Winchester and Floyd 1977),

Fig. 4. Geological map of the Butsih Creek area, Stikine Ranges, northern British Columbia. The rocks of the Klinkit Group conform-ably overlie those of the Swift River succession and are disconformably overlain by the Triassic Teh Clastic succession.

7 Contact the first author for a complete data set of the Klinkit Group.



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Simard et al. 913

was used as an index of differentiation (Figs. 7C, 8). The Ticontent, as well as the Ti/Zr and Ti/V ratios (Figs. 7C, 7D),of the volcanic rocks resemble those of modern island-arctholeiitic basalts (e.g., Gamble et al. 1995).

Some major and trace elements show systematic fractionationtrends when plotted against Zr (eg., Figs. 7C, 8). A decreaseof CaO, Mg, Cr, Ni, and CaO/Al2O3 values, while Zr increasessuggests fractionation of clinopyroxene and plagioclase (Fig. 8).An increase of TiO2 and V with increasing Zr argues againstfractionation of Fe–Ti oxides (Fig. 7D).

The REE patterns of the Butsih Formation (Fig. 9) show amoderate light REE (LREE) enrichment ((La/Yb)N = 2.77–4.73)with a flat heavy REE (HREE) pattern, suggesting that thesource did not contain residual garnet. The rocks were probablyproduced from partial melting in the spinel peridotite field,at < 60 km depth (White et al. 1992). The mantle-normalizedtrace-element patterns (Fig. 10) display pronounced negativeNb and Ti anomalies, which are characteristic of subduction-related magmas (e.g., Hawkesworth et al. 1979; Pearce1983). The subduction-related nature of the rocks is alsosupported by their Th–Hf–Nb distribution (Fig. 7B), whichis indicative of volcanic arc rocks.

The εNd values of these rocks (Table 2) are high and positive(from +6.7 to +7.4), suggesting a primitive magma sourcewith at most only little involvement of continental crust inthe magma genesis (Fig. 11). The εNd value for the samplefrom the Upper Clastic Member (+ 6.7; Table 2) is similar tothose of the Volcaniclastic Member, suggesting derivationfrom the same source.

Mount McCleary Formation

Volcaniclastic MemberThe geochemical signature of the Volcaniclastic Member

of the Mount McCleary Formation is very similar to theequivalent unit in the Butsih Formation, suggesting that theywere both subduction-related, and probably derived from thesame sources. The Mount McCleary Volcaniclastic Memberrocks plot in the andesite field of Winchester and Floyd(1977; Fig. 6), with SiO2 contents around 55.8% (LOI-free).Both volcaniclastic members of the Klinkit Group representcalc-alkaline magmatism, as shown in the Zr–Ti–Y diagramof Pearce and Cann (1973; Fig. 7A). They also share thesame moderate LREE enrichment ((La/Yb)N = 4.1–4.2) witha flat HREE pattern on the REE diagram (Fig. 9), as well asthe same mantle-normalized trace-element patterns (Fig. 10)with the strongly negative Nb and Ti anomalies.

Alkali-Basalt MemberThe two samples analysed from the volcanic rocks of the

Alkali-Basalt Member plot in the alkaline basalt andbasanite fields of Winchester and Floyd (1977; Fig. 6) with

Fig. 5. Detailed stratigraphic sections of the Butsih Formation.(A) Measured section of the Butsih Formation Volcaniclastic Memberin the Stikine Ranges. (B) Section showing the unconformity betweenthe Upper Clastic Member of the Butsih Formation and the TehClastic succession. (C) Detail of the Upper Clastic Member justbelow the unconformity. Grain-size scale indicates v (very), f(fine), m (medium), c (coarse), b (breccia).



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Butsih Formation Mount McCleary Formation

Volcaniclastic Member

Upper Clastic

Member

Volcaniclastic

Member

Alkali-Basalt

Member

Sample: 33-2 36-1a2 36-A S-A2 S-36-G 34-a21 35-5 28-2b 28-2c S-15-3 S-16-4 S-16-11 S-15-4

SiO2 (wt.%) 50.74 51.72 51.17 49.49 50.00 50.79 50.20 55.02 56.64 55.84 55.87 48.02 45.08TiO2 0.96 1.02 0.77 0.98 0.86 0.95 0.82 0.88 0.84 0.969 0.947 2.160 3.722Al2O3 16.92 15.94 14.81 16.48 18.16 17.46 17.24 16.43 16.81 17.19 17.25 19.67 13.58Fe2O3 9.22 9.35 9.20 9.67 9.09 9.23 8.34 7.91 7.72 9.04 8.61 12.75 12.82MnO 0.15 0.17 0.18 0.18 0.16 0.14 0.14 0.14 0.13 0.137 0.137 0.278 0.299MgO 6.24 6.40 7.31 7.41 6.35 6.26 6.85 3.60 3.35 4.42 3.84 2.58 7.72CaO 10.23 8.49 11.97 10.52 10.53 10.63 11.44 6.38 6.52 6.66 8.08 7.42 14.55Na2O 4.46 4.96 3.44 2.48 2.51 3.33 2.90 3.42 3.78 3.71 3.89 4.90 0.94K2O 0.16 0.17 0.56 0.90 1.18 0.15 0.78 1.71 1.18 1.57 0.94 0.96 0.18P2O5 0.22 0.20 0.15 0.22 0.18 0.19 0.19 0.19 0.19 0.21 0.21 0.90 0.48LOI 0.64 0.40 0.71 0.75 0.68 0.37 0.78 3.49 2.26 0.47 0.26 0.08 0.46Mg #a 0.57 0.58 0.61 0.60 0.58 0.57 0.62 0.47 0.46 0.49 0.47 0.29 0.54Total 99.94 98.82 100.26 99.08 99.70 99.49 99.68 99.17 99.42 100.22 100.03 99.72 99.83

Cr (ppm) 175 161 208 208 139 175 166 61 49 53 57 60 405Ni 64 57 61 89 55 72 62 17 17 56 40 89 132Co 37 37 35 38 34 34 34 30 30 23 21 25 33V 187 192 175 200 192 194 176 158 159 221 205 155 375Cu 134 79 91 79 97 78 6 81 90 78 46 72 26Zn 87 82 79 81 77 74 56 81 77 94 87 168 186Rb 1 2 8 12 20 2 9 38 28 37 47 47 12Ba — 20 174 263 261 22 282 842 447 838 822 1050 121Sr 205 122 409 484 352 439 544 482 308 223 307 1980 671Ga 17 17 16 18 20 19 18 20 18 19 20 26 21Ta — 0.47 — 0.39 — 0.35 — — — 0.21 0.31 8.32 4.40Nb 4.27 8.20 2.56 6.88 4.28 6.45 4.24 6.00 4.92 2.22 5.01 112.99 58.17Hf 2.80 2.74 2.15 2.74 2.40 2.80 2.36 — 3.35 3.10 3.29 6.65 5.57Zr 101 106 75 101 87 103 86 144 116 121 125 343 232Y 20.57 20.84 18.20 20.20 18.57 20.25 18.32 22.00 20.72 23.18 22.89 40.87 26.18Th 1.60 1.75 1.25 1.51 1.41 1.57 1.48 2.00 3.01 1.96 2.17 10.53 5.06La 11.33 13.37 7.36 12.04 10.36 11.96 10.07 13.00 15.44 13.36 13.47 83.73 36.91Ce 26.94 31.06 17.58 28.58 24.77 28.84 23.96 — 33.01 25.76 26.03 121.76 64.05Pr 3.90 4.25 2.54 4.04 3.53 4.12 3.42 — 4.30 3.78 3.81 14.61 8.46Nd 17.86 18.82 11.79 18.48 16.14 18.44 15.64 17.00 18.00 16.45 16.51 52.70 35.10Sm 4.39 4.50 3.20 4.56 4.06 4.44 4.06 — 3.93 3.97 3.99 9.17 7.68Eu 1.25 1.32 0.98 1.37 1.30 1.38 1.31 — 1.18 1.29 1.36 3.49 2.75Gd 4.31 4.32 3.40 4.45 4.08 4.35 3.87 — 3.91 4.40 4.19 7.17 6.93Tb 0.64 0.66 0.53 0.65 0.60 0.63 0.58 — 0.62 0.67 0.65 1.02 0.95Dy 3.86 3.93 3.26 3.87 3.47 3.82 3.37 — 3.79 4.04 4.00 6.56 5.37Ho 0.77 0.79 0.67 0.74 0.69 0.74 0.69 — 0.77 0.85 0.84 1.29 0.98Er 2.24 2.18 2.00 2.10 1.95 2.09 1.95 — 2.20 2.45 2.46 3.73 2.60Tm 0.31 0.32 0.29 0.30 0.29 0.30 0.30 — 0.33 0.37 0.36 0.51 0.35Yb 2.00 2.03 1.91 2.02 1.86 1.94 1.87 — 2.09 2.31 2.31 3.38 2.12Lu 0.33 0.31 0.29 0.30 0.28 0.30 0.28 — 0.33 0.35 0.35 0.52 0.28

Note: 33-2, 36-1a2, 36-A, S-A2, S-36-G, 34-a21, 35-5, S-15-3, and S-16-4, coarse-grained crystal- and lithic-tuffs; 28-2b and 28-2c, epiclastic materialwith > 20% nonvolcanic material, such as quartz grains and metamorphosed clays; S-15-4, porphyritic alkali-basalt; S-16-11, layered crystals- and ash-tuff.

aMg# = MgO/(MgO + FeOtot) in mol%.

Table 1. Representative analyses of the Klinkit Group rocks.

Sample Nd (ppm) Sm (ppm)

147

144

SmNd

143 NdNd144

aεNd (281 Ma)

S-A2 18.48 4.56 0.1486 0.51293 + 7.4S-28-2C 18.00 3.93 0.1352 0.512868 + 6.7S-34-A2 18.44 4.44 0.1497 0.512907 + 6.9S-35-5 15.64 4.06 0.1505 0.512928 + 7.3S-36A 11.79 3.20 0.1585 0.512929 + 7.1

Note: S-A2, S-34-A2, S-35–3, and S-36A, Volcaniclastic Member of the Butsih Formation; S-28-2c,Upper Clastic Member of the Butsih Formation.

a 143Nd/144Nd at present.

Table 2. Nd isotopic composition of the Klinkit Group rocks.



SiO2 ranging between 45.4% and 48.2% (LOI-free). Thesamples also plot in the alkaline within-plate basalts field ofTh-Hf-Nb diagram (Fig. 7B). Their alkaline nature is supported

by the high Ti/V ratio (Fig. 7D), as well as by the high contentof incompatible elements, such as Th, Nb, and Zr.

The REE patterns (Fig. 9) have a strong LREE enrichment((La/Yb)N = 12.5–17.8), typical of ocean-island basalts. Themantle-normalized trace-element patterns (Fig. 10) display astrong enrichment in the highly incompatible elements.Mantle-normalized incompatible trace-element patterns(Fig. 10) show smooth profiles that increase with increasingelement incompatibility and peak at Nb. These patterns aresimilar to those of ocean-island basalts (e.g., alkaline basaltsof Hawaii; Figs. 9, 10; Wilson 1989). Geochemically, theserocks closely resemble the alkaline basalts of the LittleSalmon succession of the Yukon–Tanana terrane in centralYukon (Figs. 1, 6, 7, 9; Colpron 2001).

Petrogenesis of the Klinkit Group

The volcanic rocks of the Klinkit Group are mostlyarc-derived. The similarities in relative element abundancesof the volcaniclastic members of the Butsih and MountMcCleary formations, as well as their similar temporal andstratigraphic associations with Carboniferous limestones,suggest that they are genetically related, despite being200 km apart at present.

Both volcaniclastic members show similar calc-alkalinemagmatism with relatively low abundance of highly incom-patible elements. The relatively high Mg#, as well as themoderate values of Cr (between 100 and 200 ppm) and Ni(< 100 ppm) indicate that these basaltic rocks are moder-ately differentiated.

The low concentration of incompatible elements, such asTh and Zr, in these volcanic rocks, as well as their relativelylow SiO2 content and their highly positive εNd values suggestno significant crust contamination. The Th/La ratio is a sen-sitive indicator of crust contamination for which the averageprimitive mantle and the total continental crust values are0.12 and 0.22, respectively (Taylor and McLennan 1985).Both volcaniclastic members of the Butsih and the MountMcCleary formations present Th/La ratios ranging from 0.13to 0.17, which suggest crust contamination. Higher Th/Laratio values of the Upper Clastic Member (0.15–0.2) couldhave been caused by the addition of continentally derivedsiliciclastic sediments to the volcaniclastic rocks. However,the highly positive εNd values of the Upper Clastic Member,as well as its position just outside the mixing line betweenthe depleted mantle (Slide Mountain ocean; Fig. 11) and thecrustal material (Alberta basement; Fig. 11) formed by thecontinentally derived sediments of the Yukon–Tanana terrane(Nitsulin assemblage; Creaser et al. 1997; Fig. 11), argueagainst significant crust interaction with old crustal material.

Based on geochemical and geological evidence, thevolcaniclastic members of the Klinkit Group can be interpretedas part of a primitive arc erupted either through relativelyyoung crust that consists of slightly older arc basement, orthat was rapidly emplaced through coated conduits and (or)relatively thin continental crust without significant crustcontamination. Similar interpretations have been proposedfor mafic volcanic rocks of the Anvil Assemblage (Creaseret al. 1997), Little Salmon succession (Colpron 2001) of theYukon–Tanana terrane, and the Upper Mafic Tuff of the Lay

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Simard et al. 915

Fig. 6. Zr/TiO2 vs. SiO2 (wt.%) diagram (Winchester and Floyd1977). Note the similarity of (A) the Klinkit samples to (B) boththe subalkaline rocks of the Lay Range Assemblage (from Ferri1997) and the alkaline basalts of the Little Salmon succession(from Colpron 2001). The Klinkit Group includes bothsubalkaline and alkaline mafic volcanic rocks. Sub-AB,subalkaline basalt; AB, alkaline basalt; TA, trachyandesite; BTN,basanite, trachyte, nephenite; T, trachyte; Ph, phonolite.



© 2003 NRC Canada

Range Assemblage of the Harper Ranch sub-terrane, basementto Quesnel terrane (Ferri 1997).

Concentrations of several trace elements in the basalts ofthe Alkali-Basalt Member of the Mount McCleary Formationare such that they cannot be derived from the same source orparental magma as those of the Volcaniclastic Member. Theirgeochemical characteristics, including the lack of negativeNb and Ti anomalies, are consistent with their derivationfrom asthenospheric sources, possibly from episodic intra-arc rifting events.

Reconstruction of the paleovolcanicenvironment of the Klinkit Group

In the Butsih Creek area, the Klinkit volcanics weredeposited atop continentally derived siliciclastic sedimentsof the Swift River succession which represents sedimentationinto a marginal basin older than the Late Carboniferous –Pennsylvanian Screw Creek Limestone.

The fining-upward sequences within the VolcaniclasticMember (Fig. 5A) resemble megaturbidite deposits typical


Fig. 7. Geochemical characteristics of the volcanic rocks of the Klinkit Group. (A) Zr–(Ti/100)–(Y·3) diagram of Pearce and Cann (1973).(B) Th–(Hf/3)–(Nb/16) diagram of Wood et al. (1979). (C) Ti–Zr diagram of Winchester and Floyd (1977). LKT, low-potassium tholeiites;CAB, calc-alkaline basalts; OFB, ocean-floor basalts. This diagram also gives information on possible fractionation trends with differentiationof rock sequence. (D) V–(Ti/1000) diagram of Shervais (1982). V/Ti < 20 typical of arc, V/Ti > 50 typical of alkaline magmatism.Representative analyses of the volcanic rocks of the Lay Range assemblage (Ferri 1997) and the Little Salmon succession (Colpron2001) are shown for comparison. Symbols as in Fig. 6.



of subaqueous, below-wave-base depositional setting ofvoluminous volcaniclastic turbidity currents. Volcaniclasticmegaturbidite sedimentation units can be in the order of100-m-thick and include abundant coarse, dense components(McPhie et al. 1993), which are a typical feature of thevolcaniclastic members of the Klinkit Group. These currentsare responsible for resedimentation of a wide variety ofunconsolidated, primary volcaniclastic and volcanogenicsedimentary deposits, or can be directly fed by syn-eruptivepyroclastic flows, volcanic debris avalanches, volcaniclasticdebris flows and lahars (McPhie et al. 1993). In some cases,such syn-eruptive deposits are overlain by thinner beddedvolcaniclastic turbidite sequences (Bull and Cas 1991). Thesuccession of massive and cross-bedded layers ofvolcaniclastic and siliciclastic sediments of the Upper ClasticMember rocks of the Butsih Formation can be interpreted asthese. Despite uncertain origin, the latter clearly representsthe gradual post-eruptive readjustment of the sedimentarytransport and depositional processes by means of mass-flowresedimentation (McPhie et al. 1993). The significant thicknessof volcaniclastic sediments covering the reef–debris-flowlimestone suggests a deep or constantly subsiding basin near

an active volcano. The absence of intrusions, lava flows, andvery coarse volcanic breccias in this arc-related volcanicsequence further supports the distal nature of thevolcaniclastic rocks of the Klinkit Group.

Arc volcanism was active in the Early Permian but ceasedby Triassic time as the volcaniclastic rocks were covered bythe nonvolcanic, continentally derived deep-water sedimentsof the Triassic Teh Clastic succession (Roots et al. 2002).

The northern part of the Klinkit Group, and possibly otherparts of the Klinkit arc that were not preserved, experiencedalkaline-basalt magmatism. The scarcity of those lavassuggests that they accompanied episodic intra-arc riftingevents.

Klinkit age-correlative successions

The Klinkit Group is one of several late Paleozoic volcanicsuccessions that form a linear belt within Yukon–Tananaterrane (Figs. 1, 12). Representative analyses from some ofthese are shown in the discriminant diagrams (Figs. 6–10).

The late Paleozoic volcano-sedimentary sequence mostsimilar to the Klinkit Group in the eastern part of the Cordillera

© 2003 NRC Canada

Simard et al. 917

Fig. 8. Variation of (A) Cr (ppm), (B) MgO (wt.%), (C) CaO (wt.%), and (D) CaO/Al2O3 relative to Zr (ppm) in the volcaniclasticmembers of the Klinkit Group. Symbols as in Fig. 6.



© 2003 NRC Canada


is the Lay Range Assemblage (Ferri 1997), 400 km to thesouth in northern central British Columbia (Figs. 1, 12;Nelson 1997; Harms and Stevens 1996). It is a tectonicallydismembered volcanic complex that could be a displacedfragment from the Klinkit arc, as their general stratigraphyand petrochemical nature are very similar (Figs. 6–10, 12).

The Lower Sedimentary Division of the Lay Range As-semblage resembles the siliciclastic Swift River successionwhich conformably underlies the Klinkit Group. Both arecharacterized by heterogeneous siliciclastic lithologies andthe probable continental derivation of at least some of therocks (Creaser and Harms 1998; Creaser et al. 1997; Ferri1997).

Mid-Mississippian to Early Pennsylvanian limestone formsan important marker just above both of these sedimentarysequences (Fig. 12). It marks the change from deep basinsedimentation with sporadic siliciclastic influx to an activevolcanic island-arc environment (Nelson 2001).

Like the volcaniclastic members of the Klinkit Group, theUpper Mafic Tuff Division of the Lay Range Assemblage iscomposed mainly of mafic crystal-lithic and lapilli tuffs withminor siliciclastic input. However, the Upper Mafic Tuff

Division also contains several volcanic flows suggestingmore proximal facies than what is preserved within theKlinkit volcaniclastic members. The volcanism of the UpperMafic Tuff Division is interpreted to be of Permian age(Ferri 1997), consistent with the Early Permian age obtainedfrom the Volcaniclastic Member of the Butsih Formation.The geochemical signature of the Upper Mafic Tuff Divisionis almost identical to the signature of the volcaniclastic membersof the Klinkit Group (Figs. 6–10). This geochemical resem-blance, as well as their age correlation and stratigraphicsimilarities (Fig. 12), implies that they were both formed inthe same subduction-related environment, probably derivedfrom the same sources.

Unlike the Klinkit Group, which is unconformably overlainby the Triassic Teh Clastic succession of continental affinity,the volcanics of the Lay Range Assemblage are conformablyoverlain by a thick limestone straddling the Permian–Triassicboundary and Mesozoic volcanic rocks of the Quesnel arc(Fig. 12; Gabrielse and Yorath 1991, Ferri 1997). Thisconformable sequence demonstrates that the Lay RangeAssemblage is the basement of the Mesozoic Quesnel arc(Ferri 1997).

Fig. 9. Chondrite-normalized REE patterns for the volcanic rocksof the Klinkit Group. For comparison, shaded fields of representativeanalyses of the volcanic rocks of (A) the Lay Range Assemblage(Ferri 1997) and (B) the Little Salmon succession (Colpron 2001)are shown. Normalizing values from Sun (1982). Symbols as inFig. 6.

Fig. 10. Mantle-normalized incompatible trace-element patternsfor the volcanic rocks of the Klinkit Group. For comparison,shaded fields of representative analyses of the volcanic rocks of(A) the Lay Range Assemblage (Ferri 1997) and (B) the LittleSalmon succession (Colpron 2001) are shown. Normalizing val-ues from Sun and McDonough (1989). Symbols as in Fig. 6.



© 2003 NRC Canada

Simard et al. 919

In addition, detrital quartz and zircons of Proterozoic agewere recovered from the Upper Mafic Tuff division of theLay Range Assemblage (Ferri 1997), suggesting that continentalcrust was supplying detritus. Early Proterozoic and Paleozoicinheritence on zircon fractions recovered from Early Permianfelsic volcanics of the Upper Mafic Tuff Division indicatesthat Lay Range magma erupted through older continentalcrust (F. Ferri, written communication to the first author,2002). In contrast, the primitive nature of the Klinkit magmawith no or very small involvement of continental crust couldhave resulted from its rapid emplacement through thincontinental crust.

Like in the Klinkit Group, Lay Range volcanic rocksinclude some alkaline flows at the base of the Upper MaficTuff division (F. Ferri, written communication to the firstauthor, 2002). Alkaline magmatism was also present in at

least one other correlative late Paleozoic volcanic sequenceof the Yukon–Tanana composite terrane, the Little Salmonsuccession (Figs. 6–10, 12; Colpron 2001).

Tectonic implications

In Late Devonian time, the western margin of North Americahad a thin and stretched continental edge (Monger 1999).Outboard of the continent was a strip of continental sediments,and possibly an underlying fragment of continental crust,atop which volcanic arcs formed (Fig. 13). These are definedin the north as the Yukon–Tanana composite terrane (Mortensen1992; Colpron and Yukon–Tanana Working Group 2001),and in the south as the basement of the Mesozoic Quesnelarc (Gabrielse and Yorath 1991; Ferri 1997). These LateDevonian to Early Mississippian arc successions include theBig Salmon complex (Mihalynuk et al. 1998, 2000; Colpronand Yukon–Tanana Working Group 2001; Figs. 12, 13) ofnorthern British Columbia, the Little Kalzas succession ofcentral Yukon (Colpron 2001; Colpron and Yukon–TananaWorking Group 2001) and the Fire Lake Unit of the GrassLake succession of the Finlayson Lake (Piercey et al. 2002)in the Yukon–Tanana composite terrane.

Between these late Paleozoic arc-related rocks and theancient North American cratonic rocks are discontinuousexposures of Slide Mountain rocks – remnant of a spreadingocean that probably developed as a marginal back-arc typebasin throughout Early- and mid-Mississippian time(Tempelman-Kluit 1979; Gabrielse 1991; Nelson 1993). Inthe Stikine Ranges of northern British Columbia, the ButsihFormation rests on continentally derived sediments of theSwift River succession, representing a marginal basin olderthan Late Carboniferous – Pennsylvanian to the east of theKlinkit arc (Colpron and Yukon–Tanana Working Group 2001;Nelson 2000). The Swift River basin may be the northernprecursor of the Slide Mountain ocean (Fig. 13). The north-ernmost exposures of the pericratonic belt is found in theFinlayson Lake region, southeastern Yukon, where the KudzZe Kayah Unit of the Grass Lake succession and the Wolverinesuccession also recorded back-arc volcanic activity at thattime (e.g., Piercey et al. 2003, 2002, 2001). Ocean-floorspreading in Pennsylvanian time is found in the CampbellRange succession, Finlayson Lake region, southeastern Yukon,unconformably lying on the back-arc complex (Colpron andYukon–Tanana Working Group 2001).

The Klinkit arc is part of another volcanic pulse developedatop this arc system from Pennsylvanian to Permian time(Fig. 13). Episodes of mafic arc volcanism at that time alsooccurred in the Little Salmon Lake area (Colpron 2001), inthe Boswell and Semenof hills in central Yukon, and in theUpper Mafic Tuff Division of the Lay Range Assemblage incentral northern British Columbia (Ferri 1997; Fig. 12). Thisbelt of penecontemporaneous volcanism, which in its northernpart is part of the Yukon–Tanana composite terrane and tothe south forms the basement to the Quesnel terrane, suggeststhat the Yukon–Tanana composite terrane may be the northernextension of the basement of the Quesnel terrane. Therefore,the northern part of the Quesnel terrane may be consideredpericratonic as it formed atop continentally derived rocks,instead of the former intra-oceanic interpretation.

Fig. 11. εNd vs. (A) SiO2 (wt.%) and (B) 147Sm/144Nd for the rocksof the Butsih Formation, Klinkit Group. Values of εNd were calculatedat an age of 281 Ma. Shown for comparison are the compositionsof Slide Mountain oceanic basalts (Smith and Lambert 1995), maficcontinental volcanic-arc rocks of the Anvil assemblage, Nitsulinsedimentary rocks of the Yukon–Tanana composite terrane in Teslinarea, Yukon (Creaser et al. 1997), and the average Alberta basement(Thériault and Ross 1991).



© 2003 NRC Canada

920 Can. J. Earth Sci. Vol. 40, 2003F

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© 2003 NRC Canada

Simard et al. 921

Alkaline magmatic rocks are present in most of latePaleozoic successions of the northern Canadian Cordillera(e.g., Colpron 2001; Ferri 1997), suggesting that intra-arcrifting may have occurred throughout this major arc systemduring Pennsylvanian time. However, only the southernmostsequence, the Lay Range Assemblage, records the overlyingMesozoic arc volcanism of the Quesnel arc. In the Klinkitarea, the continentally derived Triassic Teh Clastic successionis preserved, and contemporaneous volcanic rocks either werenot present or are not preserved. The continental signature ofthe Teh sediments suggests that this part of the arc system,and possibly the Yukon–Tanana composite terrane as a whole,was close to the North American craton in Triassic time.

Conclusion

The rocks of the Klinkit Group in northern British Columbiagive new insights on the late Paleozoic tectonic history ofthe northern Canadian Cordillera. The volcanic rocks werepart of an arc system erupted through possibly thin continentalcrust between mid-Mississippian and Permian time thatexperienced some episodes of intra-arc rifting. This arcvolcanism shed significant volcaniclastic debris into thesurrounding basin, which constitute the Klinkit Group. ByTriassic time the Klinkit part of this arc system was proximalto North America as indicated by the continental nature ofthe sediments overlying the Klinkit rocks.

Volcanic rocks of the Klinkit Group strongly resemblethose of the upper Lay Range Assemblage, the basement ofthe Quesnel terrane. The late Paleozoic arc-related rocks ofpericratonic Yukon–Tanana composite terrane may then be anorthern equivalent to the basement of the southern latePaleozoic to Mesozoic Quesnel arc. The northern Quesnelterrane therefore shows pericratonic affinity.

Acknowledgments

This research was supported by the Natural Sciences andEngineering Council of Canada operating grant to J. Dostal,Lithoprobe, and by the Geological Survey of Canada underthe Ancient Pacific Margin National Geoscience MappingProgram (NATMAP) Project. We thank JoAnne Nelson forinvaluable discussions and critical reading of an earlier versionof the manuscript and Fil Ferri, Kirstie Simpson, CynthiaDusel-Bacon, Stephen J. Piercey, and John Greenough forcareful and constructive review of the manuscript.

Appendix A

Historical backgroundRegional mapping in northern British Columbia and

southern Yukon in the last part of the century (Poole 1956;Poole et al. 1961; Gabrielse 1969; Abbott 1981) set the basisof the Terrane Map of the Canadian Cordillera (Wheeler etal. 1991) in this area. Rocks of the new proposed KlinkitGroup were there associated either to the Dorsey terrane tothe east in the case of the sedimentary rocks, or to the SlideMountain terrane to the west, on the basis of their maficvolcanic contents (Wheeler et al. 1991; Monger et al. 1991).

Subsequent mapping by Harms and Stevens of this area(Harms and Stevens 1995, 1996; Stevens and Harms 1995;Stevens 1996) extinguished the possibility of a fault separating

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© 2003 NRC Canada


the Slide Mountain terrane from the Dorsey terrane in thearea (Harms and Stevens 1995). They also proposed newnomenclature to replace the former Dorsey terrane based onboth lithological and metamorphic–structural character, andrelated those new assemblages either to the pericratonicYukon–Tanana composite terrane or to the intra-oceanicQuesnel terrane (Harms and Stevens 1995; Harms andCreaser 1997; Stevens and Harms 2000).

The present paper proposes a revised nomenclature forthis part of the northern Canadian Cordillera. The assem-blages of Harms and Stevens (1995) are now subdivided intothe following: (1) the Ram Creek assemblage (Harms andStevens 1995, 1996; Stevens and Harms 1995, 1996; Stevens1996); (2) the Dorsey assemblage (Harms and Stevens 1995,1996; Stevens and Harms 1995, 1996; Stevens 1996); (3) theKlinkit Group8 (new proposed unit; see text section “Stratotypeinformation” for previous work on these rocks), whichincludes two distinct stratigraphies (Fig. 4) (i) the ButsihFormation (new proposed unit) and Screw Creek Limestone(Poole 1956), from the Stikine Ranges, northern BritishColumbia and (ii) the Mount McCleary Formation (newproposed unit) and the English Creek Limestone (newproposed unit) from the Englishman Range, southern Yukon;(4) the Swift River succession (Stevens and Harms 1996;Nelson 2001); and (5) the Triassic Teh succession (Colpronand Yukon–Tanana Working Group 2001).

Stratotype informationType areas for the different members of the Butsih Formation

(Fig. 2) are located in the Stikine Ranges, northern BritishColumbia. The Volcaniclastic Member is best exposed in theheadwaters of Butsih Creek and Teh Creek, about 15 kmsouth-southwest of the Simpson Peak and 25 km northwestof Klinkit Lake (W 71°31′58′ ′ , N 59°35′01′ ′). It consists ofsteep sections 100–200 m high on both sides of a prominentpeak (Fig. 5A). This unit was previously included in “Klinkitassemblage” of Harms and Stevens (1995), as “Tuffite”(Mihalynuk et al. 2000), and as the “Butsih unit” by Rootset al. (2002). The Upper Clastic Member is best exposedabout 5 km north on the same ridge (W71°30′48′ ′ ,N59°36′12′ ′). It was described as the “Transitional Unit” byMihalynuk et al. (2000) and the “Bigfoot unit” by Roots etal. (2002).

The type area for the Mount McCleary Formation and theEnglish Creek Limestone is well exposed on the northeast-facingcliff section beside a tarn 3 km south of the headwaters ofEnglish Creek and 5 km north of Mount McCleary in theEnglishman Range, southern Yukon (W66°04′26′ ′ , N60°21′4′ ′).The cliff exposes the entire stratigraphy with the EnglishCreek Limestone at the base overlain by the Mount McClearyFormation and its four members. Rocks near the base arethermally metamorphosed by the mid-Cretaceous granite im-mediately to the east. These units have not been previouslynamed, but are shown as part of the Mv and Ml units,respectively, on the map by Gordey and Stevens (1994).

References

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