Upper Triassic Takla Group volcanic rocks,Stikine Terrane, north-central British Columbia:geochemistry, petrogenesis, and tectonicimplications1

J. Dostal, V. Gale, and B.N. Church

Abstract: The Upper Triassic Takla Group volcano-sedimentary assemblage is part of the Stikine Terrane of theIntermontane Belt in the Canadian Cordillera and covers an area of more than 30 000 km2 in a belt up to 50 km wideand more than 800 km long. In the McConnell Creek area of north-central British Columbia, the assemblage consistsof plagioclase–clinopyroxene-phyric, dominantly basaltic to andesitic flows and pyroclastic rocks, interlayered withvolcanogenic sedimentary rocks. Compositionally, the volcanic rocks are intermediate between tholeiitic and calc-alkaline. Their mantle-normalized trace element patterns are characterized by a moderate large-ion lithophile elementenrichment and Nb and Ti depletion, suggesting that magmatism occurred in a volcanic-arc setting. Flat, heavy rareearth element chondrite-normalized patterns with (La/Yb)n ratios from 2 to 4.5 suggest that the parent magma wasproduced by mantle melting in the spinel stability field. The low Sr isotopic ratios (87Sr/86Sri ≈ 0.7033–0.7043) andpositive εNd values (� +7) indicate that an older sialic crust was not involved in their genesis. A coeval andcompositionally similar volcano-sedimentary assemblage, also of the Takla Group, occurs in the adjacent QuesnelTerrane, in fault contact with the Stikinian Takla Group. Chemical resemblances between the Takla Groups of theStikine and Quesnel terranes suggest that the volcanic assemblages may have had similar source compositions and melthistories. These results emphasize larger scale similarities between the Stikine and Quesnel terranes and suggest theUpper Triassic volcanic suites represent different fragments of the same early Mesozoic arc system.

Résumé : L’assemblage volcano-sédimentaire du Groupe de Takla, du Trias supérieur, représente une partie du terranede Stikine dans le Domaine intermontagneux de la Cordillère au Canada, il couvre une aire dépassant 30 000 km2 etforme une ceinture d’une largeur maximum de 50 km et une longueur de plus de 800 km. Dans la région deMcConnell Creek du nord-central de la Colombie-Britannique, l’assemblage est formé de plagioclase–clinopyroxène-phyriques, avec prédominance de coulées de composition basaltique à andésitique et de roches pyroclastiques,interstratifiées avec des roches sédimentaires volcanogéniques. Les roches volcaniques ont une compositionintermédiaire entre tholéiitique et calco-alcaline. Les diagrammes des éléments en traces normalisés au manteau sontcaractérisés par un enrichissement modéré en éléments lithophiles à grand rayon ionique, et par un appauvrissement enNd et Ti, suggérant que l’activité magmatique s’est manifestée dans un contexte d’arc volcanique. Les diagrammes àprofil plat des terres rares lourdes normalisés aux chondrites, avec les rapports (La/Yb)n variant de 2 à 4,5 suggèrentque le magma parental fut produit par la fusion du manteau sous les conditions du champ de stabilité du spinelle. Lesfaibles rapports isotopiques (87Sr/86Sri ≈ 0,7033–0,7043) et les valeurs positives de εNd (� +7) indiquent l’absence detoute participation d’une croûte sialique plus ancienne à la genèse des magmas. Un assemblage volcano-sédimentairecontemporain de composition similaire, attribué aussi au Groupe de Takla, apparaît dans le terrane de Quesnelladjacent, en contact de faille avec le Groupe de Takla stikinien. Les ressemblances géochimiques des groupes de Taklade la Stikinie et des terranes de la Quesnellie suggèrent que les assemblages volcaniques avaient peut-être des sourcesde compositions similaires et des histoires magmatiques identiques. Ces résultats mettent en évidence l’existence desimilitudes à grande échelle dans les terranes de la Stikinie et de la Quesnellie, et suggèrent que les suites volcaniquesdu Trias supérieur représentent différents fragments d’un même système d’arc du Mésozoïque précoce.

[Traduit par la Rédaction] Dostal et al. 1494

Introduction

Current theories indicate that the Canadian segment of theCordillera is a collage of tectono-stratigraphic terranes; how-ever, the history and evolution of individual terranes remain

controversial (e.g., Monger et al. 1991; Gehrels and Kapp1998). Stikinia, the largest of these terranes, extends thelength of the Canadian Cordillera from close to the Wash-ington border (49°N) to Alaska. It is allochthonous and wasaccreted to North America by the Middle Jurassic. Further

Can. J. Earth Sci. 36: 1483–1494 (1999) © 1999 NRC Canada

1483

Received December 18, 1998. Accepted April 26, 1999.

J. Dostal.2 Department of Geology, Saint Mary’s University, Halifax, NS B3H 3C3, Canada.V. Gale. Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada.B.N. Church. Geological Survey Branch, B.C. Ministry of Employment and Investment, Victoria, BC V8W 9N3, Canada.

1British Columbia Geological Survey Branch Contribution 63; Lithoprobe Publication 979.2Corresponding author (e-mail: jdostal@shark.stmarys.ca).

knowledge of the tectonic origin of this terrane will aid inconstraining the accretionary history of the northern Cordil-lera.

The Upper Triassic Takla Group volumetrically composesa large portion of Stikinia, covering an area greater than30 000 km2 in a belt up to 50 km wide and more than800 km long. It is well exposed in the McConnell Creekmap area (Fig. 1), the type area for the group, located360 km northwest of Prince George in north-central BritishColumbia (Monger and Church 1977). The group consists ofbasaltic to andesitic flows and pyroclastic rocks, volcano-genic sandstones, and argillites that were deposited underconditions varying from subaerial to submarine (Church

1975). In the McConnell Creek map area, the assemblage iscut by the northern extension of the Pinchi fault, a majordextral strike-slip fault of the Canadian Cordillera (Fig. 1).This fault also forms the boundary between Stikinia andQuesnellia, another allochthonous terrane of theIntermontane Belt which also hosts rocks included in theTakla Group. The relationship between the Takla Group ofStikinia (STG) and Quesnellia (QTG) remains uncertain.Furthermore, the composition of volcanic rocks of the TaklaGroup, particularly in the Stikine Terrane, is poorly known.The purpose of this paper is to petrographically describe andgeochemically characterize the STG volcanic rocks in theMcConnell Creek area using major and trace element analy-

© 1999 NRC Canada

1484 Can. J. Earth Sci. Vol. 36, 1999

Fig. 1. Generalized geological map of the McConnell Creek area. Modified after Monger and Church (1977). The Pinchi fault marksthe terrane boundary of Stikinia (west) and Quesnellia (east). Paleozoic rocks include rocks of the Cache Creek and Asitka groups andLay Range Assemblage; intrusive rocks include Jurassic and younger bodies; metamorphic rocks include rocks of the Quesnellia TaklaGroup and related intrusions. The Hazelton Group is composed of Lower to Middle Jurassic arc-related volcano-sedimentarysequences.

ses and Sr–Nd isotopes, and to discuss their petrogenesisand tectonic setting. In addition, an evaluation of the rela-tionship between the STG and the QTG will contribute toour understanding of the Mesozoic tectonic history of thispart of the Canadian Cordillera.

Geological settingMonger and Church (1977) divided the Takla Group in the

type area into three major units, the Dewar, Savage Moun-tain, and Moosevale formations. Fossil evidence indicatesthat the deposition of these rocks took place between the lateCarnian to early Norian. The Dewar Formation consistsmainly of dark, thinly bedded tuffaceous siltstones andargillite underlying and, in part, equivalent in age to the Sav-age Mountain Formation, which comprises as much as3000 m of pillowed and massive basalt, breccia, and tuff.The effusive rocks of these formations are dominantly ba-saltic and may occur with coarse, platy plagioclase and (or)clinopyroxene phenocrysts. Pillows are commonly less than1 m in diameter and are tightly packed or set in a tuff–breccia matrix of the same composition. Massive lava flowsare on average 10 m thick and are mostly intercalated withbrecciated basalt. These formations are overlain by theMoosevale Formation, which is composed of up to 1800 mof marine and nonmarine mafic to intermediate volcani-clastic deposits, with clasts similar to the Savage Mountainporphyritic units, and rare lava flows. These units were lo-cally deformed into a broad fold pattern with northwest–southeast-trending fold axes, intruded by numerous Jurassicgranitic bodies and regionally metamorphosed to zeolite andprehnite–pumpelleyite facies (Monger 1977).

The STG, which lies to the west of the Pinchi fault(Fig. 1), unconformably overlies the Lower Permian AsitkaGroup which is composed of chert, limestone, and felsic vol-canic rocks. Elsewhere, the STG is in fault contact with theunderlying Cache Creek Group, a marine volcano-sedimentary sequence that is highly deformed and typically

metamorphosed to the greenschist facies (Monger 1977).The QTG is stratigraphically underlain by the Permo-Pennsylvanian Lay Range Assemblage (Ferri 1997). Thislast assemblage comprises an internally imbricated faultslice, composed mainly of phyllite, chlorite schist, and meta-volcanic rocks, that separates the QTG from Precambrianbasement rocks (Nelson and Bellefontaine 1996). A regionalunconformity separates both the STG and the QTG from theoverlying Lower to Middle Jurassic volcano-sedimentary as-semblages (Monger et al. 1991; Lang et al. 1995).

Analytical technique

Mineral compositions were determined at the Departmentof Earth Sciences of Dalhousie University using a JEOLSuperprobe 733 equipped with four wavelength-dispersivespectrometers and one energy-dispersive spectrometer andoperated with a beam current of 15 kV at 5 nA. Fifty-twoSTG samples collected by B.N. Church during mapping ofthe McConnell Creek area (Monger and Church 1977) wereanalyzed for major and selected trace elements (Ba, Sr, Rb,Zr, Nb, Y, Cr, Ni, V, Zn, and Ga) by X-ray fluorescence atthe Regional Geochemical Centre at Saint Mary’s Univer-sity. Fourteen of these samples were then chosen for theanalysis of rare-earth elements (REE), Th, U, Hf, Zr, Nb,and Y by an inductively coupled plasma – mass spectrome-ter at the Memorial University of Newfoundland. Themethod was described by Longerich et al. (1990). The preci-sion and accuracy of the data have been reported by Dostalet al. (1986, 1994). The analytical error of the determina-tions is 2–10% for the trace elements and less than 5% formajor elements. The representative mineral and whole-rockanalyses are given in Tables 1 and 2, respectively. A com-plete set of analytical data is given in Gale (1996), and sam-ple locations are given in Table A1 of the Appendix.

Five of these samples were subsequently selected for Nd-and Sr-isotope analyses. Sm, Nd, Rb, and Sr abundances and

© 1999 NRC Canada

Dostal et al. 1485

Clinopyroxene Plagioclase

W-26 W-31 W-34 W-36 W-37 NW-29 S-15 S-19 S-26 CS-3 NW-29 W-27 S-25

SiO2 (wt.%) 52.11 53.72 51.03 52.75 49.71 50.13 49.52 53.43 50.49 51.39 52.46 54.04 54.35TiO2 0.58 0.26 0.54 0.39 0.61 0.50 0.99 0.27 0.97 0.39Al2O3 5.14 1.74 2.62 2.30 4.79 4.56 5.61 2.24 3.19 3.00 30.36 28.69 28.15Cr2O3 0.38 0.18 0.30 0.31 0.60 0.18FeO 9.48 5.00 10.72 6.33 7.87 7.40 7.04 5.21 10.59 6.83 1.25 0.94 1.05MgO 12.82 16.74 15.52 16.58 15.07 14.68 14.03 17.02 14.33 15.70CaO 18.65 22.65 19.21 21.49 21.59 21.18 22.13 21.77 20.23 22.80 12.51 12.06 11.49Na2O 0.42 0.20 0.53 0.40 0.35 0.33 0.42 0.38 0.59 0.35 3.99 4.28 4.55K2O 0.23 0.42 0.45Total 99.20 100.69 100.17 100.42 100.30 99.09 99.74 100.92 100.57 100.46 100.80 100.43 100.04

Wo 42.5 45.5 39.1 43.4 44.3 44.7 46.9 44.0 41.7 45.6En 40.6 46.7 43.9 46.6 43.1 43.1 41.4 47.8 41.0 43.7Fs 16.9 7.8 17.0 10.0 12.6 12.2 11.7 8.2 17.3 10.7Ab 36.1 38.2 40.6An 62.5 59.3 56.7Or 1.4 2.5 2.6

Notes: Savage Mountain Formation samples: W-26, W-27, W-31, W-34, W-36, W-37, NW-29, and CS-3; Moosevale Formation samples: S-15, S-19, S-25, and S-26.

Table 1. Representative compositions of clinopyroxene and plagioclase from the Stikine Takla Group volcanic rocks.

©1999

NR

CC

anada

1486C

an.J.

Earth

Sci.

Vol.36,

1999

Savage Mountain Formation Moosevale Formation

W-26 W-28 W-31 W-35 W-36 W-37 W-39 NW-77 NW-92 NW-14 CS-2 CS-9 S-32 S-74 S-153 S-15 S-19

SiO2 (wt.%) 47.06 54.96 49.31 46.31 49.21 48.64 51.66 51.25 54.45 47.91 48.17 47.00 47.18 48.04 55.31 47.60 46.95TiO2 0.98 0.76 0.72 0.61 0.89 0.83 0.77 1.03 1.18 0.90 0.73 0.62 1.01 0.99 0.74 1.37 0.63Al2O3 15.72 12.13 11.47 9.61 13.39 14.28 13.35 16.18 17.71 12.51 19.64 3.98 18.17 15.25 18.36 16.61 9.70FeO* 10.31 7.79 10.39 11.82 11.35 10.45 7.11 7.58 8.35 11.05 7.64 9.90 10.36 9.48 7.11 11.27 11.38MnO 0.21 0.22 0.18 0.20 0.18 0.16 0.10 0.14 0.17 0.19 0.13 0.17 0.21 0.23 0.18 0.17 0.19MgO 7.84 5.44 7.56 13.44 7.24 7.01 4.32 3.38 3.13 9.33 5.77 15.84 4.93 7.28 4.10 5.55 11.25CaO 9.75 9.90 11.73 10.53 9.86 9.80 9.00 7.76 7.40 11.01 11.99 19.42 8.82 6.07 4.03 7.89 12.03Na2O 2.65 4.48 3.00 1.22 3.25 3.22 2.85 4.19 3.25 1.72 2.70 0.10 3.10 3.94 6.78 4.16 2.53K2O 1.12 1.53 1.25 2.01 1.56 1.12 1.81 0.97 1.67 1.32 0.31 1.02 1.95 0.10 1.17 0.03P2O5 0.20 0.19 0.22 0.20 0.29 0.28 0.31 0.39 0.27 0.18 0.04 0.33 0.21 0.23 0.22 0.16LOI 3.80 1.50 3.10 4.10 2.10 2.80 8.50 5.80 1.60 3.10 1.80 2.60 3.80 5.10 3.00 3.40 3.70Total 99.64 98.90 98.93 100.05 99.32 98.59 99.78 98.67 99.17 99.22 98.92 99.63 98.93 98.54 99.94 99.41 98.55Mg# 57.5 55.4 56.4 66.9 53.2 54.5 51.0 44.3 40.0 60.1 57.4 74.0 45.9 57.8 50.7 46.7 63.8

Cr (ppm) 139 244 214 774 124 94 105 53 10 332 103 657 13 127 5 29 624Ni 48 46 45 210 38 37 53 28 7 99 76 103 20 44 9 28 143V 291 234 282 257 369 378 253 295 208 288 248 195 283 314 158 392 271Zn 87 56 92 95 93 100 111 50 50 119 91 100 104Rb 33 22 23 31 26 24 41 28 26 24 4 0 25 27 2 16 5Ba 242 209 369 293 375 263 507 138 911 339 139 3 345 509 64 291 12Sr 403 146 330 176 547 441 451 428 458 291 596 31 600 343 301 286Ga 16 10 12 13 14 17 16 19 8 24 19 22 20Nb 3.5 2.6 2.4 1.6 3.2 2.4 4.5 7.0 6.0 4.0 0.6 0.3 4.3 3.1 4.4 5.0Hf 1.78 1.58 1.46 1.09 1.63 1.47 2.04 0.46 0.59 2.34 1.70 2.56Zr 61 54 43 33 46 44 66 78 114 48 11 11 77 56 83 71 39Y 17 14 13 10 14 14 17 16 24 14 6 7 17 16 20 21 12Th 0.79 0.78 1.05 1.05 1.19 1.72 1.62 0.07 0.03 1.45 0.91 1.25U 0.46 0.49 0.36 0.59 0.50 0.83 2.79 0.04 0.02 0.62 0.57 0.63La 5.61 5.81 6.58 6.14 7.35 10.24 11.06 8.00 17.00 7.00 2.08 0.77 12.33 7.72 9.25Ce 14.02 13.60 14.57 13.52 17.18 23.27 21.24 46.00 35.00 20.00 4.88 2.70 27.20 17.36 21.71Pr 2.01 1.98 2.10 1.90 2.52 3.30 3.00 0.75 0.57 3.60 2.43 2.90Nd 9.48 9.38 9.74 8.66 12.20 15.20 13.04 3.79 3.45 16.38 11.32 13.05Sm 2.70 2.57 2.67 2.35 3.31 3.63 3.18 1.19 1.32 3.91 3.09 3.44Eu 0.98 0.88 0.84 0.76 0.98 1.10 1.05 0.57 0.47 1.45 1.25 1.06Gd 3.27 2.84 2.75 2.44 3.47 3.35 3.22 1.40 1.68 4.21 3.54 3.78

Table 2. Representative analyses of the Stikine Takla Group rocks.

Sr- and Nd-isotope ratios were determined by isotopedilution mass spectrometry in the AURIF laboratory of theMemorial University of Newfoundland. A description of theanalytical technique was given by Kerr et al. (1995). Mea-sured 143Nd/144Nd values were normalized to a natural146Nd/144Nd ratio of 0.7219. The LaJolla standard, analyzedas part of every run, yielded average 143Nd/144Nd =0.511849 ± 9. Epsilon values (εNd) (Table 3) assume mod-ern 143Nd/144NdCHUR = 0.512638 and 147Nd/144NdCHUR =0.1967. The 87Sr/86Sr was corrected using 86Sr/88Sr =0.1194. Replicate runs for the NBS 987 Sr standard gave87Sr/86Sr = 0.710250 ± 11. Initial Sr isotopic ratios and εNdvalues were calculated assuming an age of 220 Ma for theTakla Group (Monger and Church 1977; Haq and VanEysinga 1987).

Petrography and mineral chemistry

STG effusive volcanic rocks range in composition frombasaltic to andesitic and contain varying proportions ofclinopyroxene, plagioclase, and olivine phenocrysts. Porphy-ritic rock matrices are hypocrystalline to holocrystalline andconsist of cryptocrystalline material (altered glass) andmicrocrystalline clinopyroxene, plagioclase, magnetite, andminor ilmenite. Effusive rock matrices are commonlychloritized. Pilotaxitic texture is present in some samplesand is defined by the subparallel alignment of micro-crystalline plagioclase laths. Seriate textures are common inplagioclase and clinopyroxene crystals.

Clinopyroxene (augite) is present in all porphyritic effu-sive samples, composing on average �32 vol.%. Based oncolour differences, two types of clinopyroxene can be distin-guished. The most common is light yellow–green and occursas individual grains as well as rims in zoned clinopyroxenephenocrysts. The less abundant type, which comprises �10%of the total amount of clinopyroxene, is colourless and oc-curs as cores in zoned clinopyroxene phenocrysts.Compositionally, colourless cores are enriched in Ca, Mg,and Cr and are depleted in Fe, Al, and Ti relative to the yel-low–green rims. Values of the clinopyroxene end memberspecies vary from 38.7 to 50.9 for enstatite, 4.9 to 18.0 forferrosilite, and 36.0 to 46.6 for wollastonite. Core to rimcompositional changes appear gradational. The simple andgradual zonation sequence from more primitive to moreevolved composition for the Takla Group pyroxenes is mostlikely a result of the compositional evolution of the melt.The lack of zoning of some of the more primitive colourlessclinopyroxene phenocrysts, as well as the selective occur-rence of zoning, could be indicative of magma chamber het-erogeneity. The low Cr and Ti contents of theclinopyroxenes (Table 1) are typical of those hosted byorogenic basalts (Fig. 2). Al and Ti concentrations in theSTG clinopyroxenes overlap the fields of tholeiitic and calc-alkaline orogenic basalts (Fig. 2).

Plagioclase comprises on average �12 vol.% in basalticrocks and in andesitic rocks it is the dominant phenocrystphase (15–25 vol.%). Its composition usually ranges fromAn51 to An64. Some basaltic samples contain pseudomorphsof olivine typically 2 mm × 1 mm which constitute �5 vol.%of the samples.

© 1999 NRC Canada

Dostal et al. 1487

Sav

age

Mou

ntai

nF

orm

atio

nM

oose

vale

For

mat

ion

W-2

6W

-28

W-3

1W

-35

W-3

6W

-37

W-3

9N

W-7

7N

W-9

2N

W-1

4C

S-2

CS

-9S

-32

S-7

4S

-153

S-1

5S

-19

Tb

0.49

0.44

0.44

0.37

0.48

0.48

0.51

0.21

0.28

0.60

0.55

0.60

Dy

3.12

2.85

2.82

2.15

3.09

3.06

3.24

1.28

1.72

3.57

3.45

3.65

Ho

0.65

0.59

0.54

0.43

0.61

0.56

0.63

0.27

0.31

0.72

0.71

0.78

Er

1.85

1.68

1.54

1.25

1.70

1.57

1.90

0.67

0.84

2.01

1.99

2.20

Tm

0.25

0.25

0.20

0.17

0.23

0.24

0.29

0.09

0.10

0.29

0.28

0.32

Yb

1.67

1.57

1.33

1.09

1.47

1.39

1.89

0.59

0.68

1.87

1.76

2.12

Lu

0.25

0.23

0.20

0.17

0.21

0.19

0.27

0.08

0.09

0.28

0.25

0.31

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es:

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W-3

1,W

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W-3

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amyg

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salt;

NW

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W-7

7,S-

32,

and

S-74

,cl

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yrox

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plag

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ase-

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salt;

NW

-92,

apha

nitic

basa

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site

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S-2,

pillo

wla

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basa

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S-9,

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S-15

3,cl

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plag

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Mg#

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/MgO

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0 tot

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).

Tab

le2

(con

clud

ed).

Pyroclastic samples of the STG include crystal lithic tuffs,lapilli tuffs, and volcanic breccias. They are heterolithic withangular to subangular clasts of exclusively volcanogenic ma-terial. In the pyroclastic rocks of the Savage Mountain For-mation, fragments chiefly consist of plagioclase–augite-phyric basalt while plagioclase-phyric basalt and andesitefragments predominate in the pyroclastic rocks of theMoosevale Formation.

Alteration

Minor and trace element geochemistry indicates that thecomposition of the STG volcanic rocks has been meta-somatically altered (Gale 1996). This process could have oc-curred during zeolite-facies metamorphism. Several samplesyielded elevated loss on ignition (LOI) values and displayedunusually wide ranges in K and Rb concentrations, probablyreflecting the mobility of the alkali metals in aqueous fluids(Table 2). Conversely, the concentrations of most major ele-ments, high-field-strength elements (HFSE), REEs, and tran-sition elements are thought to reflect primary magmaticdistributions. When these elements are plotted against Zr,which is considered to be a good indicator of the fraction-ation and is apparently immobile under most metamorphicconditions (e.g., Winchester and Floyd 1977), they displaystrong correlations (Gale 1996). Remobilization duringmetamorphism is unlikely to produce such consistent results.The consistency of these trends and their similarities tothose of modern volcanic rocks suggest that the distributionof these elements reflects the original composition of theSTG volcanic rocks.

Geochemistry

Major and trace elementsMajor and trace element compositions of representative

volcanic rocks from the STG are given in Table 2. The SiO2content ranges from 47 to 60 wt.% (LOI free), although inthe majority of the rocks it varies from 48 to 52 wt.%. In thecase of the Takla Group samples, major element plots tradi-tionally used to determine alkalinity, such as the (Na2O +K2O) versus SiO2 diagram of Irvine and Baragar (1971), areunreliable due to the mobility of the alkali elements. As a re-sult, the SiO2 versus Zr/TiO2 plot (Fig. 3) of Winchester andFloyd (1977) based upon relatively immobile elements wasused to assess the degree of alkalinity and differentiation involcanic rocks. The Takla Group samples plot entirely

within the subalkaline field. A slight increase of TiO2 andSiO2 with the FeO*/MgO ratio (where FeO* equals totaliron expressed as FeO) suggests that the samples possesstholeiitic characteristics but are transitional betweentholeiitic and calc-alkaline rocks (Fig. 4). The rocks of theSavage Mountain Formation have, on average, lower con-tents of SiO2 and stronger tholeiitic affinities than theMoosevale rocks. Compared to the effusive samples,pyroclastic rocks are generally more enriched in SiO2. Thisprobably reflects the greater tendency for pyroclastic erup-tion in andesites.

Several major and trace elements show systematic frac-tionation trends when plotted against SiO2 (Fig. 5). An in-crease of Al2O3 accompanied by the decrease of CaO andMgO while SiO2 increases suggests fractionation ofclinopyroxene (Fig. 5). The trends are also reflected by thesharp decrease of the CaO/Al2O3 ratio. The trends flatten ata SiO2 value of about 51 wt.%, probably reflecting the onsetof feldspar fractionation. The sharp decline of Ni and Crover a SiO2 range of 47–52 wt.% supports the early fraction-ation of olivine and clinopyroxene (Fig. 5). The Cr/Ni ratiodeclines over a SiO2 range of 48–52 wt.% and then flattens,suggesting that crystallization of clinopyroxene at a fasterrate than olivine was subsequently followed by plagioclase-dominated fractionation. Generally, the P2O5 content in-creases at a higher rate over a SiO2 range of 47–51 wt.%, af-ter which it continues to increase at a lower rate (Gale1996). This reflects the suppression and subsequent onset ofapatite fractionation. The early crystallization sequence ofthe magma was dominated by clinopyroxene and, to a lesserextent, olivine fractionation. Later stages were controlled bythe fractionation of plagioclase, apatite, and Fe–Ti oxideminerals.

Chondrite-normalized REE patterns in the Takla Groupvolcanic rocks (Fig. 6) are slightly enriched in light REE(LREE) with slopes that show a complete range of (La/Yb)nratios from 2 to 4.5. The slight LREE enrichments of thesepatterns are intermediate between those of island-arctholeiitic basalt (e.g., standard rock JB-2, O-Shima, Japan)and calc-alkaline andesite (e.g., U.S. Geological Surveystandard rock AGV-1, Guano Valley, Oregon, in Fig. 6), em-phasizing the intermediate composition of the Takla Grouprocks. The relatively flat heavy REE segment of the patternindicates that the source did not contain residual garnet. Themelt was probably produced in the spinel peridotite stabilityfield of the mantle, at a depth of less than 60 km (White etal. 1992).

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1488 Can. J. Earth Sci. Vol. 36, 1999

Rb(ppm)

Sr(ppm)

87

86

RbSr

87

86

SrSr

87

86

SrSr

i

Sm

(ppm)Nd(ppm)

147

144

SmNd

1

144

43 NdNd εNd

W-26 31.09 399.83 0.21684(20) 0.704403(14) 0.703724 2.70 9.57 0.17395(23) 0.513004(6) 7.78W-31 22.51 328.37 0.18432(8) 0.704923(19) 0.704346 2.58 9.33 0.17102(2) 0.512922(6) 6.26W-36 23.08 550.33 0.11695(6) 0.704690(14) 0.704324 3.34 11.79 0.17497(8) 0.512960(7) 6.90CS-2 4.26 595.70 0.01992(2) 0.703630(14) 0.703567 0.99 3.15 0.19373(17) 0.512967(17) 6.51CS-9 1.01 31.29 0.08996(1) 0.703602(12) 0.703320 1.31 3.55 0.22836(21) 0.513064(18) 7.41

Notes: Measured 87Sr/86Sr ratios normalized to 86Sr/88Sr = 0.1194. The Nd-isotope ratios and εNd values normalized to 144Nd/146Nd = 0.7219. The 2σprecision in the last listed place for the measure of isotopic ratios is given in parentheses. εNd calculated at T = 220 Ma, using the chondritic uniformreservoir (CHUR) ratios of 143Nd/144NdCHUR = 0.51238 and 147Sm/144NdCHUR = 0.1967. Precision of concentrations ±1%. Savage Mountain Formation lava(W-26, W-31, W-36) and pillow lava (CS-2) and dike (CS-9).

Table 3. Sr and Nd isotopic compositions of the Takla Group rocks.

Mantle-normalized trace element patterns of the Taklasamples (Fig. 7) reveal an enrichment of large-ion lithophileelements (LILE) such as Th, Ba, and LREE relative to heavyREE and HFSE and distinct troughs at Nb and Ti, character-istic of subduction-related magmas (e.g., Hawkesworth et al.1979; Pearce 1983). Compared with primitive island-arctholeiites, the Takla samples have higher contents of incom-patible trace elements and their mantle-normalized patternsare more fractionated.

Sr and Nd isotopesMeasured Sr and Nd isotopic data for five volcanic rocks

of the STG are given in Table 3, together with age-corrected(220 Ma) εNd values and initial Sr isotopic ratios. The dataare displayed in a εNd versus 87Sr/86Sri diagram in Fig. 8, inwhich fields for mid-ocean-ridge basalts (MORB) and is-land-arc volcanics (IAV) as well as rocks from the StikineTerrane (Samson et al. 1989) have been plotted for compari-son. The field of IAV (Samson et al. 1989) includes regionsfor volcanics uncontaminated by subducted continental sedi-ments (e.g., the Aleutians, the Marianas) and rocks stronglycontaminated by sediments (e.g., Sunda arc). From the isoto-pic data, Samson et al. (1989) inferred that many crustalrocks of the Stikine Terrane were derived from thelithospheric mantle. This probably also applies to the Taklarocks. The range of Sr and Nd isotopic ratios in the analyzedsamples is limited. All samples plot within the uncontami-nated IAV field characterized by positive εNd values and the

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Dostal et al. 1489

Fig. 3. Zr/TiO2 vs. SiO2 diagram of Winchester and Floyd(1977) for the rocks of the Stikine Takla Group. Sub-AB,subalkaline basalt; AB, alkaline basalt; TrAn, trachyandesite;Bas, basanite; Trach, trachyte; Neph, nephelinite. � , SavageMountain Formation; +, Moosevale Formation.

Fig. 4. (A) FeOt/MgO vs. TiO2 and (B) FeOt/MgO vs. SiO2 forthe volcanic rocks of the Stikine Takla Group. The lineseparating the calc-alkaline and tholeiitic fields in (B) is afterMiyashiro (1974). �, Savage Mountain Formation; +, MoosevaleFormation.

Fig. 2. (A) (Ti + Cr) vs. Ca and (B) Ti vs. Al (in number ofcations per six oxygens) discrimination diagrams of Letterier etal. (1982) comparing the composition of clinopyroxenes from theStikine Takla Group basaltic rocks with compositional fields forclinopyroxenes from nonorogenic tholeiites and orogenic basalts(A) and from calc-alkaline and tholeiitic basalts (B), respectively.�, Savage Mountain Formation; +, Moosevale Formation.

low initial Sr isotopic ratios (Fig. 8) and show no evidenceof continental influence.

Comparison of the geochemistry of STG and QTGGeochemical data (major and several trace elements) on

the neighbouring QTG were reported by Minehan (1989).Effusive rocks of the QTG range in composition from ba-saltic to dacitic, whereas STG effusive rocks are limited tobasaltic to andesitic composition. A detailed comparisonshows that the basaltic rocks of both units are remarkablysimilar (Gale 1996). The Ti–Y–Zr diagram of Pearce andCann (1973) displays close overlap of the two samplegroups (Fig. 9), consistent with an arc-related setting forboth groups and derivation from a similar source composi-tion. A comparison of the mantle-normalized trace elementdistribution of STG and QTG rocks (Fig. 10) shows a signif-icant variation in LILE abundance between the two units, re-flecting enrichment of these elements in STG duringalteration; however, abundances of the more immobile trace

elements of the two units (Nb, Zr, Ti, and Y) are very simi-lar, suggesting derivation from a similar melt source.

Discussion

Petrogenesis and depositional environmentVariations of several trace elements in the basalts, particu-

larly those of REEs, and the variations of the Sr and Nd iso-topic ratios cannot be readily explained by fractionalcrystallization and (or) variable degrees of partial melting,but are consistent with derivation from a heterogeneousmantle source and (or) the effect of crustal contamination.We favour an interpretation that the STG basaltic rocks wereprobably derived from a heterogeneous mantle source char-acterized by a high LILE to HFSE ratio, as such ratios alsocharacterize island-arc magma sources (e.g., Pearce 1983).High εNd and low 87Sr/86Sr ratios bordering the field forMORB (Fig. 8) argue against substantial continental crustcontamination.

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1490 Can. J. Earth Sci. Vol. 36, 1999

Fig. 5. Variations of SiO2 vs. CaO, Al2O3, MgO, CaO/Al2O3, Ni, and Cr in the volcanic rocks of the Stikine Takla Group. �, SavageMountain Formation; +, Moosevale Formation.

Eruption of the STG volcanic rocks in a mature island-arcsetting is also consistent with lithological features, particu-larly the dominance of basaltic over andesitic compositions,and the association of these volcanic rocks with a shallowmarine sedimentary assemblage. Also, the isotope data fromthe STG volcanic rocks support the inference of Samson etal. (1989) that Stikinia was not close to a continent duringmagmatic activities. In contrast, the QTG volcanic rockswere erupted in proximity to a continental land mass (Ferri1997; Ferri et al. 1994).

Tectonic implicationsThe Takla Group, although divided by the Pinchi fault,

was originally mapped as a single assemblage (Lord 1948;Monger 1977). Sequences east and west of the fault com-prise similar volcanic and sedimentary rocks overlain by Ju-rassic volcano-sedimentary assemblages. Studies of the STGand QTG indicate that both Takla Group volcanic rockswere erupted in a volcanic-arc environment and were de-rived from a similar mantle source, although the QTG hostsgreater volumes of more differentiated rocks and waserupted closer to a continental land mass. The presence ofHalobia sp. (upper Carnian – lower Norian) in both se-quences indicates that they are time equivalent (Monger1977; Monger and Church 1977).

The Takla sequences differ in metamorphic grade acrossthe Pinchi fault, ranging from lower greenschist (Quesnellia)

on the east to prehnite–pumpellyite and zeolite facies(Stikinia) on the west. Also, the sequences have differentbasement assemblages; the STG is juxtaposed with the Pa-leozoic Asitka and Cache Creek groups by unconformity andfaulting, respectively, whereas the QTG stratigraphicallyoverlies the Lay Range Assemblage (Ferri 1997). It may bethat these differences in composition and metamorphic gradeof the Takla groups across the Pinchi fault can, in part, beaccounted for by movement along the fault, which duringthe Tertiary has an estimated strike-slip displacement in theorder of 200 km (Gabrielse 1991).

The Takla Group is one of the manifestations of the UpperTriassic volcanism in the two tectono-stratigraphic terranes,Stikinia and Quesnellia. The other manifestations are presentin the Stuhini Group of Stikinia and the Nicola Group inQuesnellia. Compositionally, the Takla Group volcanic rocksare comparable to volcanic rocks from the western part ofthe Nicola Group (Fig. 9) with which they also overlap inage (Mortimer 1987) and exhibit lithological similarities(e.g., the presence of clinopyroxene-phyric basalts). Similar-ities between the STG and the Nicola Group have implica-tions for palinspastic reconstructions, since restoration of thedextral strike-slip on the Pinchi and other faults may actually

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Dostal et al. 1491

Fig. 6. Chondrite-normalized REE patterns for the volcanic rocksof the Stikine Takla Group. (A) Savage Mountain Formation(W-31 and W-36) and Moosevale Formation (S-153).(B) Clinopyroxene-phyric basalt sample W-26 (Savage MountainFormation), which displays a typical REE pattern of the TaklaGroup, is compared to a typical island-arc tholeiite from theO-Shima volcano in Japan (JB-2, standard rock of the JapaneseGeological Survey; Govindaraju 1994) and a typical calc-alkalineandesite from the Guano Valley, Oregon (AGV-1, U.S.Geological Survey standard rock; Govindaraju 1994). The Taklapattern is intermediate between the tholeiitic and calc-alkalinesamples. Normalizing values after Sun (1982).

Fig. 7. Mantle-normalized incompatible trace elementabundances in the volcanic rocks of the Stikine Takla Group.(A) Savage Mountain Formation (W-28 and W-31) andMoosevale Formation (S-153). (B) Clinopyroxene-phyric basaltsample W-26 (Savage Mountain Formation), which displays apattern typical of the Takla Group, is compared to a typicalisland-arc tholeiite from the O-Shima volcano and a high-Al arcbasalt from the Fuji volcano in Japan (JB-2 and JB-3,respectively, standard rocks of the Japanese Geological Survey;Govindaraju 1994). Normalizing values and the element orderafter Sun and McDonough (1989).

place the STG beside the Nicola Group (D. Thorkelson, per-sonal communication, 1999). Likewise, the Takla volcanicrocks are similar to the Stuhini Group volcanics of Stikinia(Fig. 9) which outcrop in northern British Columbia(Souther 1991; Thorkelson 1992). Some similarities in theearly Mesozoic rock suites of Stikinia and Quesnellia havealso been noted in several regional studies of the NorthAmerican Cordillera (Lang et al. 1995; Mihalynuk et al.1994; Wernicke and Klepacki 1988; Mortimer 1986, 1987).

On a regional scale, the Pinchi fault is believed to repre-sent the collisional boundary between the Stikine andQuesnel terranes which has been in existence since the Mid-dle Jurassic. Stikinia and Quesnellia are two allochthonous,predominantly arc-related terranes separated by the mainlyoceanic Cache Creek Terrane, which hosts an exotic Tethyanfauna. In the McConnell Creek area, the Cache CreekTerrane might be absent. Several models exist to explain theamalgamation of these terranes. They include the riftingmodel (e.g., Tempelman-Kluit 1979; Mortensen 1992),strike-slip model (e.g., Gehrels and Kapp 1998), overthrustmodel (e.g., Coney 1989; Smith and Gehrels 1991; Robackand Walker 1995), escape model (e.g., Wernicke andKlepacki 1988), and enclosure model (e.g., Nelson and

Mihalynuk 1993; Mihalynuk et al. 1994). Although theavailable data cannot indicate the most likely model, thesimilarities of the two Takla units emphasize the conceptthat early Mesozoic magmatic processes in the Stikine andQuesnel arcs were remarkably similar. In fact, the similari-ties of the Upper Triassic volcanic rocks of the Quesnelliaand Stikinia in chemical composition, lithology, and agesuggest that they represent different segments of the samelower Mesozoic arc system. However, the presence of conti-nentally derived sediments in the Quesnel basement rocksindicates that the two arcs may not have shared the exactsame history or that they were founded on different types ofcrust, a situation similar to the present-day Aleutian arc,which can be traced from North American continental litho-sphere westward onto oceanic crust.

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1492 Can. J. Earth Sci. Vol. 36, 1999

Fig. 8. εNd vs. 87Sr/86Sri for the volcanic rocks of the StikineTakla Group (×). Initial isotope ratios were calculated at an ageof 220 Ma. Shown for comparison are the compositions of mid-ocean-ridge basalts (MORB) (Zindler and Hart 1986) and island-arc volcanic rocks (IAV) (Samson et al. 1989) and thecomposition of mantle-derived continental crust of the StikineTerrane of the Canadian Cordillera (Samson et al. 1989). �,felsic volcanics; �, mafic volcanics; �, felsic plutonics; �,mafic plutonics; �, sediments.

Fig. 9. Zr – (Ti/100) – (Y × 3) diagram of Pearce and Cann(1973). (A) Comparison of typical basalts from Stikine (�) andQuesnel (+) Takla Group. The data for QTG are from Minehan(1989). (B) Shows the field for basalts of types 2 and 3 from thecentral and western belts of the Nicola Group (Mortimer 1987)and the basalts of the Stuhini Group (�) (Thorkelson 1992).Fields for various basaltic types after Pearce and Cann (1973):A, low-K tholeiite; B, low-K tholeiite, ocean-floor basalt, andcalc-alkaline basalt; C, calc-alkaline basalt; D, within-platebasalt.

Conclusions

The Upper Triassic Takla Group volcano-sedimentary as-semblage of the Stikine Terrane in the McConnell Creekarea (north-central British Columbia) contains basaltic toandesitic effusive and pyroclastic rocks. The volcanic rocksare compositionally intermediate between tholeiitic and calc-alkaline. They define geochemical trends indicating earlycrystallization of olivine and clinopyroxene followed byplagioclase-dominated fractionation. The basaltic rocks wereprobably derived from a heterogenous mantle enriched inLILE relative to HFSE in the spinel stability field. The lowSr isotopic ratios and positive εNd values of the lavas indi-cate that an older sialic crust did not play a role in their gen-esis. The rocks were emplaced in a mature island-arc setting.

A coeval and compositionally similar volcano-sedimentary assemblage occurs in the Quesnel Terrane, infault contact with the Takla Group of the Stikine Terrane.The geochemistry of the volcanic rocks of the two assem-blages confirms the similarity of Late Triassic volcanism inStikinia and Quesnellia in British Columbia. However, an in-crease in the abundance of andesite and dacite in the QTGsuggests relative proximity during eruption to the NorthAmerican craton compared to the STG. The Takla Groupvolcanics are similar in age, composition, and lithology tothe rocks of the Stuhini Group of Stikinia and to a part ofthe Nicola Group of Quesnellia. This study supports the con-cept that major parts of the Stikine and Quesnel terranes aredistal parts of the same arc complex, or adjacent belts in acomposite arc.

Acknowledgments

This research was supported by the Natural Sciences andEngineering Research Council of Canada operating grant(Lithoprobe grant) to J. Dostal and by the British ColumbiaGeological Survey Branch. We thank John Greenough andDerek Thorkelson for careful and constructive reviews and

Filippo Ferri for critical reading of an earlier version of themanuscript.

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Appendix

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Sample No. Lat. N Long. W

W-26 56°34.65′ 126°33.98′W-27 56°34.55′ 126°34.18′W-28 56°34.50′ 126°34.48′W-31 56°34.01′ 125°34.51′W-34 56°33.69′ 126°34.53′W-35 56°33.64′ 126°34.63′W-36 56°33.59′ 126°34.73′W-37 56°33.54′ 126°34.93′W-39 56°33.44′ 126°35.33′NW-14 56°45.03′ 126°35.27′NW-29 56°45.31′ 126°33.82′NW-77 56°45.31′ 126°35.74′NW-92 56°44.91′ 126°34.79′CS-2 56°48.71′ 126°30.61′CS-3 56°49.27′ 126°31.56′CS-9 56°46.00′ 126°29.81′S-15 56°37.57′ 126°39.95′S-19 56°37.84′ 126°39.94′S-25 56°37.37′ 126°40.65′S-26 56°37.42′ 126°40.64′S-32 56°36.49′ 126°39.73′S-74 56°36.06′ 126°40.05′S-153 56°35.68′ 126°40.07′

Table A1. Sample locations (NTS sheet 94D/10).