1449
Mississippian volcanic assemblage conformably overlying Cordilleran miogeoclinal strata, Turnagain River area, northern British Columbia, is not part of an accreted terrane1 Philippe Erdmer, Mitchell G. Mihalynuk, Hubert Gabrielse, Larry M. Heaman, and Robert A. Creaser
Abstract: A Paleozoic volcanic assemblage exposed in northern British Columbia, near the Turnagain River, previously considered to be part of an accreted terrane, was reported to be in depositional contact with a part of the Cordilleran miogeocline. This paper presents an integrated field, U–Pb geochronology, Sm–Nd isotopic, and geochemical study across the basal contact of the volcanic assemblage. Strongly evolved εNd(T) values, between –13 and –21, from samples of lower Paleozoic sedimentary rocks exposed below the volcanic rocks, and correlated with Atan – Kechika – Road River – Earn strata of the miogeocline farther east, support a North American miogeoclinal affinity, consistent with previously established regional stratigraphic and structural relationships. Nd isotopic data from the volcanic assemblage contrast significantly with data from the sedimentary rocks and record a mantle source (εNd(T) values between +4.0 and +7.0), consistent with a magmatic arc or back arc; negative Nb anomalies are similarly compatible with either arc- or back-arc-related magmatism. A concordant 339.7 ± 0.6 Ma U–Pb zircon date was obtained from the volcanic assemblage. The mixed gradational contact between the miogeoclinal and volcanic rocks is marked by interlayering of finely laminated grey and green phyllites on the scale of centimetres, with no evidence of a tectonic contact. Bedding at the contact is folded into tight outcrop-scale folds that are intruded by an Early Jurassic (187.5 ± 2.9 Ma) granodiorite. On the basis of all available evidence, the contact is interpreted as a facies transition. The Mississippian volcanic assemblage may link the miogeocline with the early development of an Angayucham – Slide Mountain basin. Résumé : Un assemblage volcanique (Paléozoïque) qui affleure dans le nord de la Colombie-Britannique, à proximité de la rivière Turnagain, était antérieurement considéré comme faisant partie d’un terrane accrété; on rapportait qu’il était en contact sédimentaire avec une partie du miogéocline de la Cordillère. Le présent article présente une étude de champ intégré, de géochronologie U–Pb, des isotopes Sm–Nd et de géochimie à travers le contact à la base de l’assemblage volcanique. Des valeurs εNd(T) fortement évoluées, entre –13 et –21, provenant d’échantillons de roches sédimentaires (Paléozoïque inférieur) affleurant sous les roches volcaniques, et corrélées avec les strates Atan – Kechika – Road River – Earn du miogéocline plus à l’est, supportent une affinité miogéoclinale nord-américaine qui concorde avec les relations stratigraphiques et structurales établies antérieurement. Les données de l’isotope Nd provenant de l’assemblage volcanique contrastent de manière significative avec les roches sédimentaires et enregistrent une source du manteau (valeurs εNd(T) entre +4,0 et +7,0), ce qui concorde avec un arc magmatique ou une région arrière-arc. Des anomalies Nb négatives sont aussi compatibles avec un magnétisme d’arc ou d’arrière-arc. Une datation U–Pb concordante de 339,7 ± 0,6 Ma sur un zircon a été obtenue de l’assemblage volcanique. Le contact graduel mixte entre le miogéocline et les roches volcaniques est marqué par une interstratification de phyllites grises et vertes finement laminées d’échelle centimétrique, sans évidence d’un contact tectonique. Au contact, le litage est plissé en des plis serrés à l’échelle de l’affleurement, lesquels sont recoupés par une granodiorite datant du Jurassique précoce (187,5 ± 2,9 Ma). Selon toutes les évidences disponibles, le contact serait un faciès de transition. L’assemblage volcanique datant du Mississippien pourrait relier le miogéocline et le développement précoce d’un bassin Angayucham – Slide Mountain. [Traduit par la Rédaction]
Erdmer et al.
1465
Received 14 June 2004. Accepted 19 April 2005. Published on the NRC Research Press Website at http://www.cjes.nrc.ca on 24 October 2005. Paper handled by Associate Editor S. Hanmer. P. Erdmer,2 L.M. Heaman, and R.A. Creaser. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada. M.G. Mihalynuk. British Columbia Ministry of Energy and Mines, P.O. Box 9320, Station Provincial Government, Victoria, BC V8W 9N3, Canada. H. Gabrielse. Geological Survey of Canada, Pacific Division, 101-605 Robson Street, Vancouver, BC V6B 5J3, Canada. 1 2
Geological Survey of Canada Contribution 2005329. Corresponding author (e-mail:
[email protected]).
Can. J. Earth Sci. 42: 1449–1465 (2005)
doi: 10.1139/E05-045
© 2005 NRC Canada
1450
Introduction The outboard part of the Cordilleran miogeocline in northern British Columbia consists of Neoproterozoic to middle Paleozoic platformal and off-shelf sedimentary strata. Ophiolitic and volcanic-arc-dominated rock assemblages occur to the west and are locally in thrust contact with subjacent miogeoclinal strata. In the absence of stratigraphic links, the evidence of thrusting has supported a general interpretation of oceanic and arc assemblages in the Cordillera as terranes representing offshore crust accreted to the ancestral North American plate. Geological relationships east of Dease Lake (Fig. 1) have been highlighted as examples of interaction between accreted crust and the ancestral margin (Gabrielse 1991). Proposed accreted rocks were assigned to the Quesnel and Stikine arc terranes (or Quesnellia and Stikinia, respectively) and the Cache Creek and Slide Mountain oceanic terranes. Rocks of the Sylvester allochthon, a klippe overlying Paleozoic strata of the ancestral North American margin, are in part included in the Slide Mountain terrane (see Wheeler et al. 1991; Gabrielse et al. 1991). In the same region, however, low-grade Paleozoic to Triassic metavolcanic rocks appeared to be in conformable contact with miogeoclinal strata, for a strike length of more than 15 km (Gabrielse 1998a). That observation is significant because, if the contact is locally stratigraphic, it precludes an accreted origin for a volcanic assemblage. To test whether the volcanic assemblage could be a facies linked to the ancient North American continent, we examined geological relationships near the contact. This paper presents lithologic and structural field data, U–Pb, Sm–Nd, and Rb–Sr isotopic data, and whole-rock geochemical data collected along a roughly 10 km transect across the contact southeast of the Turnagain River, -70 km east of Dease Lake. The results support the existence of a stratigraphic contact, and therefore of a facies transition between volcanic and miogeoclinal strata within the outer margin. They are compatible with the development of a continental-margin arc or back arc in Mississippian time.
Can. J. Earth Sci. Vol. 42, 2005 Fig. 1. Location and tectonic setting of the Cry Lake National Topographic System (NTS) 1 : 250 000 scale map area and the Turnagain River – Faulkner Creek study area.
Previous work Geological mapping at a regional scale was first conducted by the Geological Survey of Canada (see Gabrielse 1962). A review of later work and descriptions of the geology of the Cry Lake and Dease Lake 1 : 250 000 scale National Topographic System (NTS) map areas formed the basis of a terrane interpretation of regional geological evolution (Gabrielse 1998a, 1998b). Rocks in the southwestern part of the study area (Fig. 2) were described regionally as “…black, phyllitic, pyritic slate, chloritic, foliated greenstone, creamy weathering, in part flowbanded, quartz-eye, schistose volcanics, local ribbon chert, and minor pods and lenses of carbonate” (Gabrielse 1998a, p. 48), that resembled parts of the Cache Creek, Slide Mountain, and Quesnel terranes; although included in Quesnellia, they were described as “…of uncertain terrane assignment” or part of “a terrane of undefined affinity….”. The nature of their eastern boundary with sedimentary strata correlated with the miogeocline was inconsistent with a terrane assignment, as no fault could be recognized.
The volcanic assemblage overlies sedimentary strata on the east, which include micaceous quartzite, slate, phyllite, and limestone (mapped regionally as the Lower Cambrian Rosella and Boya formations, Gabrielse 1998a); argillaceous limestone and shale (Upper Cambrian to Lower Ordovician © 2005 NRC Canada
Erdmer et al.
1451
Fig. 2. Geological map of the Turnagain River – Faulkner Creek study area, showing sample locations for U–Pb dating and Sm–Nd analysis. Location of the cross section in Fig. 4 is shown as A–B.
© 2005 NRC Canada
1452
Kechika Formation); and black calcareous slate, shale, and siltstone (originally mapped as Road River Formation by Gabrielse 1998b; shown as undivided Lower Ordovician to possibly Devonian–Mississippian Road River and Earn groups in Fig. 2).
Geological setting and field data We collected outcrop data and samples from a folded stratigraphic succession, in a cross-strike transect along two mountain ridges providing good bedrock exposure, near the confluence of Faulkner Creek and the Turnagain River (see Fig. 2). At its northern end, the transect included strata correlated with part of the Cambrian Boya Formation. There, the rocks are overprinted by the metamorphic aureole of the Cassiar batholith, a composite pluton of late Early Cretaceous age (see Driver et al. 2000) and more than 250 km long, that separates the Boya Formation correlatives from Neoproterozoic rocks of the Ingenika Group to the east. The Boya Formation correlatives include micaceous siltstone and distinct mica-rich and quartzite layers up to 10 cm thick that form up to 20% of the rock. They are discontinuously exposed up to a change in the slope that matches the projection of the lower contact of the Kechika Formation, exposed several kilometres both to the north and south. The Kechika Formation is not exposed in the study area but is inferred to underlie the covered interval. Where the slope steepens south of it, graphitic phyllite, correlated with part of the undivided Road River and Earn groups, is well exposed. It is locally calcareous, with white calcite veins and grey metre-scale, lime-rich lenses. Cubic pyrite crystals up to 2 cm across are common and locally form up to 5% of the rock. Graphitic phyllite is exposed almost to the crest of the ridge, which is underlain by light green phyllite in an isolated fold keel. The green phyllite includes small dark to pale green domains that appear to be fine-grained clasts with diffuse outlines. They locally give the rock a mottled appearance, and its protolith is interpreted as a tuffaceous sediment. The green phyllite is part of a volcanic greenstone assemblage that constitutes a marked change with the underlying strata typical of the miogeocline. A calcareous tuff up to 200 m thick extends northwestward from a local summit (spot elevation 1898 m (peak 1898) in Fig. 2). It is thermally metamorphosed for several tens of metres near its contact with a granodiorite to tonalite pluton. This intrusive body, previously assumed to be Late Triassic (see Gabrielse 1998b), is Early Jurassic on the basis of zircon dating (see “U–Pb geochronology”). Graphitic phyllite, correlated with the undivided Road River and Earn groups, is well exposed to the south for more than 1 km. About 225 m south of a saddle in the ridge (saddle at locality “Camp Col” in Fig. 2), several layers of light green tuffaceous phyllite occur within the upper 50 m of the graphitic phyllite and mark the same change upward into a volcanic greenstone-bearing assemblage as seen in the fold keel on the ridge to the north. Individual green phyllite layers are up to 1.5 m thick and are interbedded with minor slate, pyritic green wacke, and light green pencil phyllite to slate with a locally granular surface and relict feldspar crystals reaching a few millimetres across. Bedrock exposure is nearly continuous in this interval. The last (uppermost) layer of
Can. J. Earth Sci. Vol. 42, 2005
graphitic phyllite in the slope grades over < 2 m into a unit of interlayered green tuff and tan carbonate (Fig. 3) about 80 m thick. The tuff and carbonate unit is overlain by a massive to weakly banded fine-grained intermediate green mafic unit several metres thick. The rock is weakly foliated at the thinsection scale and consists of chlorite, actinolite, epidote, albite, and minor quartz; local clots of chlorite up to a few millimetres across appear to be pseudomorphs of relict primary phenocrysts, possibly augite. The unit is interpreted as a metavolcanic greenstone. The greenstone is in turn overlain by a 600 m interval of green phyllitic tuff. Quartz and plagioclase phenocrysts or aggregates 5–10 mm across are common at the top of the tuff, which is in turn overlain by micaceous quartzite (-80%), tuffaceous phyllite (-10%), pitted limey layers (-5%), and graphitic argillaceous phyllite layers (-5%). These rock units persist along strike for several kilometres to the west of the area shown in Fig. 2. No evidence of a tectonic discontinuity exists between any of the rock units. The characteristics of the contact between the graphitic strata and the volcanic assemblage are those of a stratigraphic transition resulting from continuous deposition with minor lateral interfingering. The overall map pattern results from repetition in kilometrescale folds (Fig. 4), with the youngest units exposed to the south. In the Boya Formation correlative strata to the north, bedding is deformed together with an early schistosity (S1), and both are commonly crenulated and overprinted by a second cleavage (S2). The S2 cleavage is associated with northeastwardverging folds with tens of metres of amplitude that may record drag on the southwest limb of a large antiform. Mineral lineation, intersection lineations, and minor fold and crenulation hinges generally plunge gently southeast. The graphitic strata correlated with the undivided Road River and Earn groups also display an early cleavage overprinted by a crenulation cleavage; additional fabrics are locally developed. Near the top of the unit, folds with wavelengths of several tens of metres are outlined by calcareous tuff. The tuff and limestone interpreted to outline the keel of a syncline on peak 1898 (see Fig. 3) are correlated with the interbedded tuff and limestone unit south of locality Camp Col. An exception to the general southeast plunge is seen on the northern side of the tuff belt, where it is intruded by granodiorite. In that area, asymmetric small shear zones, quartz fibres adjacent to rotated pyrite porphyroblasts, and asymmetric feldspar–quartz porphyroclasts all record top down to the northwest motion, i.e., bulk flow parallel with the fold axis. This may be the reason why the belt of tuffaceous rocks apparently confined to the synclinal keel is wider to the northwest. The minimum age of folding is constrained by the Early Jurassic age of granodiorite, which cuts the syncline keel. A maximum age of the youngest regional deformation is provided by a deformed Middle Jurassic granite dyke (see “U–Pb geochronology”).
Sample selection for U–Pb dating Three samples were analyzed for U–Pb zircon dating (PE00-70, PE00-75, and PE00-78, see locations in Fig. 2). The first (PE00-70) was selected to establish the age of the granodiorite pluton previously inferred to be of Late Triassic age (unit Ltrgd of Gabrielse 1998b). The fact that granodiorite © 2005 NRC Canada
Erdmer et al.
1453
Fig. 3. Interbedded green tuff and pale weathering limestone near the base of the metavolcanic succession. Hammer for scale.
Fig. 4. (A) Cross section along line A–B (see location in Fig. 2), and (B) projected profile normal to estimated mean fold axial direction at the same scale. Abbreviations of geological units as in Fig. 2.
bodies of Triassic age are common in Quesnellia, and generally not characteristic of the ancient North American margin, made the dating of this body of particular interest. The rock is chloritized, medium-grained hornblende granodiorite to tonalite–diorite that is weakly to strongly foliated, locally with abundant secondary epidote. The second sample (PE00-75) was selected to establish the age of the volcanic greenstone assemblage. The sample is of a pale green to white, coarse-grained, epidote–chlorite– actinolite-bearing felsic schist. It contains quartz and plagioclase crystals whose abundance and concentration suggest that the
protolith was a plagioclase and quartz-eye bearing porphyry. Its chemical composition is consistent with an andesite protolith (see later in the text). Gradation between fine- and coarsegrained layers is typical. A penetrative stretching lineation is defined by rods and extended aggregates of grey-blue quartz and chalky white plagioclase. The third sample (PE00-78) was taken from a 5–10 m wide granitic dyke that cuts the metavolcanic assemblage and was selected to test its possible correlation with the nearby Cassiar batholith; the dyke cuts lithologic layering and foliation in the volcanic rocks. The coarse-grained © 2005 NRC Canada
1454
subporphyritic rock is mainly quartz, orthoclase, and muscovite, with minor plagioclase. It displays calcite and sericite alteration, and a weak to locally strong, grain-shape-defined foliation. Thus, it constrains the maximum age of latest penetrative deformation.
U–Pb geochronology The samples selected for U–Pb zircon analysis were pulverized using standard crushing techniques (jaw crusher and disk mill). The methods for isolating zircon, extracting uranium and lead, and measuring isotopic compositions on a VG354 thermal ionization mass spectrometer followed those described by Heaman et al. (2002). All dates were calculated using linear regression or weighted average algorithms available in the Isoplot software (Ludwig 1990). The uranium decay constants used were those recommended by Jaffey et al. (1971). The U–Pb zircon isotopic results are presented in Table 1 and are plotted in a concordia diagram in Fig. 5. All errors quoted in Table 1 are reported at one sigma (1σ). PE00-70 This sample of granodiorite yielded abundant apatite and pyrite but only a small amount of tiny zircon grains (20–50 µm in the longest dimension), fewer than 200 crystals in the least magnetic fractions. The zircon grains are of a single population, consisting of light tan to colourless euhedral prisms or parts of prisms with length to width ratios between 3:1 and 5:1. The U–Pb results for five small multigrain zircon fractions (6–30 grains per fraction) are presented in Table 1 and displayed in a concordia diagram in Fig. 5. Zircon from this sample contained moderate to high uranium (676– 1032 ppm) with consistently low Th/U (0.07–0.12). The 206 Pb/238U dates are similar and fall in the range 183.9– 192.3 Ma. A regression line constructed to pass through analyses 2–4 yields a lower intercept of 187.5 ±2.9 Ma, which is considered the best estimate of the time of granodiorite crystallization, i.e., Early Jurassic. The upper intercept is 1105 ±440 Ma and could indicate the presence of a component of Precambrian inherited Pb in the zircon population. PE00-75 This sample contained a moderate amount of zircon, in two distinct colour populations: (i) colourless, the dominant type; and (ii) pink–tan to brown. The majority of zircon crystals are colourless irregular fragments. A small proportion are equant, multifaceted, and euhedral (this includes both colour types). The U–Pb results for three single zircon crystals and a small multigrain fraction (consisting of seven colourless fragments) are presented in Table 1 and shown in a concordia diagram in Fig. 5. The fractions were selected to encompass the range of grain morphologies and colours present. Despite their differences in colour and morphology, all four zircon fractions contain low to moderate uranium amounts (88– 197 ppm) and have consistent Th/U ratios (0.30–0.42). The four fractions generally plot within error of the concordia curve (Fig. 5) and display remarkably consistent 206Pb/238U dates between 338.8 and 340.9 Ma. The weighted average 206 Pb/238U date of 339.7 ± 0.6 Ma using all four fractions is considered to be the best estimate of the formation age of this volcanic unit, i.e., Early Mississippian.
Can. J. Earth Sci. Vol. 42, 2005
PE00-78 A small amount of zircon was recovered from this sample ( 0.706 have been interpreted to record assimilation of, or genesis from, older crust; values in the range 0.703–0.704 are characteristic of mantle-derived magmas, such as those erupted in volcanic arcs (Kistler and Peterman 1978). Additional 143Nd/144Nd values for samples PE00-74 and PE00-75 were obtained from the MUN laboratory. They are identical within error to the results in Table 2 (listed here in parentheses for comparison): PE00-74 yielded 143Nd/144Nd = 0.512937 (0.512890), and PE00-75 yielded 143Nd/144Nd = 0.512743 (0.512706). Values of 143Nd/144Nd were also obtained for two additional samples from the same units along strike that were the subject of whole-rock geochemical analysis (see the next section); sample MMI00-13-13 (along strike from sample PE00-74) yielded 143Nd/144Nd = 0.513083, and sample MMI00-13-11 (along strike from sample PE00-75) yielded 143Nd/144Nd = 0.513031.
Whole-rock geochemical data We analyzed four samples of Mississippian volcanic rocks for major, trace, and rare-earth elements (REE) in an attempt to characterize the tectonic affinity of the parent magma. The samples are representative of the few homogeneous dark green volcanic strata in the study area without sedimentary interbeds. Two samples were collected from the most massive unit on the western ridge (from the same outcrops as PE00-74 and PE00-75; sample PE00-75 yielded a U–Pb zircon date of -340 Ma, see “U–Pb geochronology”), and two samples from the eastern ridge (MMI00-13-11 and MMI00-13-13). The data are given in Table 3 and plotted in Fig. 6. Figure 6A is a plot of alkalis versus silica. All of the samples are subalkaline. One sample from each ridge falls in the basaltic andesite field, with the second west ridge sample plotting as an andesite and that from the east ridge plotting as a basalt. Both alkalis and silica are likely to be
1457
mobile in metamorphosed rocks such as these, but on the basis of elements generally considered immobile (Fig. 6B), the samples plot in corresponding fields. The REE pattern shows light-REE (LREE) enrichment with respect to normal midocean-ridge basalt (N-MORB; 2–5 times La), and heavy REEs (HREEs) compatible with N-MORB values (Fig. 6C). A normalized plot (Fig. 6D) shows large ion lithophile element (LILE) enrichment relative to N-MORB and variable Nb and Ti values. The western samples show enrichment in LILEs and depletion in Nb and Ti, typical of either arc magmatism or mantle-derived magma with crustal contamination. Elevated LILEs are consistent with either tectonic setting, and elevated values of both Th and Hf may indicate crustal contamination. The eastern samples display a similar REE pattern: relatively enriched in LREE, with HREE typical of N-MORB. In the normalized plot (Fig. 6D), they are enriched in Nb and Ti compared with the western samples, and one of the samples displays elevated LILEs; this may indicate a separate origin, or an uncontaminated equivalent. Discrimination between modern petrogenetic environments is shown by elemental abundances in basalt. Compositions of ancient basalts can be compared with modern ones to interpret the tectonic environment in which they formed. The Th–Hf/3–Nb/16 discrimination plot of Wood (1980) separates basalts generated at a destructive plate margin (arc) from those formed in other environments (Fig. 6E). The west ridge samples fall into the arc field, and the east ridge samples fall in the field of within-plate basalt and enriched MORB (E-MORB). Plotting Th–Hf/3–Ta yielded nearly identical results (not shown in Fig. 6). In a plot of Zr/4–Y–Nb/2 after the method of Meschede (1986), the east ridge samples fall in the field of within-plate tholeiite and volcanic-arc basalt (Fig. 6F). One of the west ridge samples also plots in this field; the other plots in the field of volcanic-arc basalt and N-MORB. For comparison, we have plotted data from an intraoceanicarc setting in which ensialic crustal contamination is not a factor, the Marianas axial ridge (Hawkins et al. 1990). Rocks from that environment show similar arc and MORB geochemical variability. We conclude that, on the basis of four samples, we are not able to distinguish confidently between arc and back-arc environments, particularly with ensialic crustal contamination a likely factor. The lack of alkaline compositions, however, as indicated by both major and immobile trace elements, may be significant. It would not support an ensialic rift-related or back-arc origin. The arc composition displayed by the western ridge samples and the abundance of modal quartz in the structurally highest volcanic units are both compatible with an arc origin.
Discussion The correlation of the sedimentary strata beneath the volcanic assemblage with the miogeocline to the east is subject to a number of uncertainties: the rocks cannot be traced along strike or across the Cassiar batholith, there is no fossil age control, and ε Nd values overlap those of some pericratonic rocks included in the Yukon–Tanana terrane to the west (e.g., Creaser et al. 1997). Although none of these uncertainties can be entirely resolved, we favour the simplest possible regional interpretation, i.e., that the strata represent a western facies © 2005 NRC Canada
1458
Can. J. Earth Sci. Vol. 42, 2005 Table 3. Major, trace, and rare-earth element analyses of four samples of volcanic rocks. Sample No.: Same as:
MMI00-13-4 PE00-74
MMI00-13-6 PE00-75
MMI00-13-11 No PE sample PE00-75
MMI00-13-13 a
No PE sampleb PE00-74
NTS map area:
104 I/7
104 I/7
104 I/7
104 I/7
UTM easting:
513104
513087
514704
514795
UTM northing:
6477645
6477080
6475647
6476020
Oxides (wt.%) SiO2 TiO2 Al2O3 Fe2O3(t) FeO(t) MnO MgO CaO Na2O K2O P2O5 LOI Total
52.31 0.61 17.04 9.32 0.00 0.15 5.03 7.50 4.23 0.43 0.10 2.75 99.47
58.15 0.94 14.59 8.19 0.00 0.12 3.03 4.15 4.96 0.67 0.25 4.59 99.64
48.13 1.67 15.17 9.81 0.00 0.17 5.80 10.27 4.30 0.05 0.20 4.17 99.74
49.16 1.57 18.02 9.21 0.00 0.10 7.65 3.42 4.50 0.75 0.23 4.98 99.59
Normalized data (100% anhydrous) SiO2 54.66 TiO2 0.64 Al2O3 17.81 FeO(t) 8.67 MnO 0.16 MgO 5.26 CaO 7.84 Na2O 4.42 K2O 0.45 P2O5 0.10 Total 100.00
61.76 1.00 15.49 7.75 0.13 3.22 4.41 5.27 0.71 0.27 100.00
50.94 1.77 16.06 9.24 0.18 6.14 10.87 4.55 0.05 0.21 100.00
52.52 1.68 19.25 8.76 0.11 8.17 3.65 4.81 0.80 0.25 100.00
Trace elements, XRF (ppm) Rb 9.35 Sr 264.24 Ba 411.12 Y 27.24 Zr 169.89 Nb 5.32 V 230.28 Cr 17.37 Ni 0.00 Cu 66.95 Zn 58.54
5.93 320.34 175.67 15.23 54.91 2.47 262.04 70.96 0.00 50.70 26.12
0.50 198.02 0.00 27.30 161.05 11.62 250.51 124.83 17.11 53.91 36.38
17.55 144.34 568.54 28.64 178.58 12.33 219.32 213.83 65.44 0.00 39.15
Trace elements, ICP–MS (ppm) Cl 0.00 Ba 331.48 Y 21.56 Zr 118.24 Hf 4.05 Ta 0.27 Th 1.67
47.92 126.42 12.61 46.02 1.72 0.13 1.22
0.00 22.33 21.89 122.80 3.59 0.68 0.50
0.00 380.29 23.43 133.63 3.46 0.72 0.58
© 2005 NRC Canada
Erdmer et al.
1459 Table 3 (concluded). Sample No.: Same as:
MMI00-13-4 PE00-74
MMI00-13-6 PE00-75
MMI00-13-11 No PE sample PE00-75
MMI00-13-13 a
No PE sampleb PE00-74
NTS map area:
104 I/7
104 I/7
104 I/7
104 I/7
UTM easting:
513104
513087
514704
514795
6477080
6475647
6476020
4.91 11.26 1.70 7.94
6.87 18.03 2.86 13.98
7.67 18.97 2.82 13.06
Rare-earth elements, ICP–MS (ppm) Sm 5.45 Eu 1.58 Gd 5.42 Tb 0.81 Dy 4.94 Ho 0.97 Er 3.01 Tm 0.42 Yb 2.78 Lu 0.42
2.33 0.75 2.69 0.43 2.85 0.58 1.88 0.26 1.80 0.27
4.19 1.53 4.96 0.80 5.19 1.03 3.18 0.43 2.82 0.42
3.89 1.43 4.75 0.78 5.35 1.08 3.45 0.47 3.16 0.47
Isotopic ratios 87 Sr/86Sr 2σ 143 Nd/144Nd 2σ
0.704740 0.000014 0.512743 0.000013
0.704932 0.000011 0.513031 0.000010
0.704656 0.000013 0.513083 0.000027
UTM northing: 6477645 Rare-earth elements, ICP–MS (ppm) La 10.63 Ce 26.61 Pr 4.27 Nd 20.32
0.705209 0.000028 0.512937 0.000007
Note: Sample pulps prepared at British Columbia Geological Survey Branch using a chrome steel ring and puck mill. Analyses performed at Memorial University of Newfoundland ICP–MS laboratory (rare earth, isotopes, and trace elements) and at Cominco Laboratory, Vancouver, British Columbia (XRF major oxides). a same unit along strike as PE00-75. b same unit along strike as PE00-74.
of the miogeocline to the east, by placing most weight on the following observations: (i) the northeasternmost exposures of sedimentary rocks in the study area are separated from western exposures of identical Cambrian strata, typical of the miogeocline, by an intervening granitic unit of the Cassiar batholith, there only about 8 km wide (Gabrielse 1998b); (ii) the sedimentary stratigraphy beneath the volcanic assemblage can be readily matched with all of the Cambrian to Devonian–Mississippian succession of the closest part of the miogeocline to the east; (iii) the evolved ε Nd values of the sedimentary strata (–21 to –5) are most consistent with those of miogeoclinal strata to the east (see Garzione et al. 1997), and they do not fully encompass the range of ε Nd values in nearby displaced terranes (e.g., Creaser et al. 1997); and (iv) the strata bear little resemblance in lithology, metamorphic grade, plutonic rock content, and structure to rocks of the Dorsey complex, the closest pericratonic rocks in the Yukon– Tanana terrane on the west side of the Kutcho fault, about 75 km to the northwest (e.g., Nelson and Friedman 2004). The Mississippian volcanic rocks, in turn, do not lithologically resemble those in nearby terranes such as Quesnellia and Stikinia (Gabrielse 1998a). A correlation with rocks of
the Cache Creek terrane is also tenuous, because no Mississippian magmatic rocks of intermediate composition like those in the study area are known in the Cache Creek terrane, nor are Mississippian siliciclastic strata. For several hundred kilometres along strike, the contact between Quesnellia and miogeoclinal strata is marked by faults. In the Dease Lake and Cry Lake map areas, these are the Klinkit, Hottah, and Kutcho structures (see Gabrielse 1998a). The Klinkit fault dips moderately to the south and appears to have had dominantly thrust displacement. The Hottah fault is not seen in outcrop; it has been interpreted as a thrust (Gabrielse 1985). The Kutcho fault appears to accommodate dextral strike-slip offset of Early Cretaceous and(?) earlier age (Gabrielse 1985). Rocks on either side of the Kutcho fault fit the definition of terranes, i.e., successions whose mutual stratigraphic and spatial relationships at their time of formation are uncertain or completely unknown. In the adjacent McDame, Jennings River, and Cry Lake map areas, cryptic faults of large displacement have been interpreted to place shale and chert in the basal Slide Mountain allochthon on shale in the miogeocline (Gabrielse 1963, 1969, 1998a, 1998b). A cryptic fault (or fault zone) would © 2005 NRC Canada
1460
Can. J. Earth Sci. Vol. 42, 2005
Fig. 6. Chemical compositional plots for volcanic samples (solid dots are for samples from the west ridge, half-filled circles are for samples from the east ridge, and open circles are Marianas trough spreading ridge from Hawkins et al. 1990). (A) Alkalis (Na2O + K2O) versus SiO2 with the alkaline–subalkaline fields of Irvine and Baragar (1971) overlain by rock classification fields of Cox et al. (1979). P– N, phonolite–nephelinite; P–T, phonolite–tephrite; B+T, bajaite–tephrite. (B) Rock classification based on immobile elements (after Winchester and Floyd 1977). Alk–Bas, alkaline basalt; Bsn/Nph, basanites and nephelinites; Com/Pant, comendite–pantellerite. (C) Primitive-mantlenormalized rare-earth element variation diagram (average MORB from Pearce 1975; normalizing factors from Sun and McDonough 1989). N-MORB, normal mid-ocean ridge basalt; OIB, oceanic-island basalt. (D) Primitive mantle (PM) normalized multielement variation diagram (normalizing factors from Sun and McDonough 1989). (E) Th–Hf/3–Nb/16 ternary plot. A, N-type MORB; B, E-type MORB and tholeiitic within-plate basalt (WPB) and differentiates; C, alkaline WPB and WPB and differentiates; D, destructive platemargin basalts and differentiates. Fields from Wood (1980). (F) Nb × 2 – Zr/4 – Y discrimination diagram after Meschede (1986). AI– AII, within-plate alkaline basalt; AII–C, within-plate tholeiite; B, P-MORB; D, N-MORB; C–D, volcanic-arc basalt.
constitute the basal emplacement structure of an accreted terrane, most probably Quesnellia, which is exposed to the west and separated from rocks in the study area by the Kutcho fault. The age of tectonic emplacement of the Quesnel arc elsewhere in the Cordillera has been estimated at between 186 and -180 Ma (e.g., Nixon et al. 1993; Ferri 1997). In the study area, a 187.5 ± 2.9 Ma granodiorite cuts across the folded contact between the graphitic phyllite and the base of the volcanic assemblage (Fig. 2) and may indicate that the contact is older than the emplacement of Quesnellia. Nonetheless, because the timing of deformation remains elusive, the uncertainty on the 187 Ma date is permissive of the volcanic assemblage being part of an allochthonous slice of Quesnellia. On the other hand, no evidence of tectonic emplacement is visible in the study area. The conclusion that the contact is depositional is compatible with all available evidence, and we favour the simplest interpretation, in which the volcanic assemblage records a magmatic-arc or back-arc setting at the western margin of the miogeocline in Mississippian time. The volcanic assemblage forms part of a belt of Mississippian to Permian magmatic-arc rocks above older North American continental-margin rocks which extends southward to central British Columbia (Nina Creek Group and Lay Range Assemblage, see Ferri 1997). The Nina Creek Group (thought to be part of the Slide Mountain terrane) and the Lay Range Assemblage (Quesnellia) are linked by similar geochemical and temporal constraints (Ferri 1997) and show a back-arcbasin to magmatic arc relationship. A tectonic model for the margin in which a back-arc basin formed inboard of a Devonian–Mississippian arc built on rifted crust is consistent with the geochemical change inferred in the Turnagain River area. Geochemical characteristics of Slide Mountain mafic volcanic rocks, however, although biased towards voluminous Permian strata, show them to be mainly N-MORB (e.g., Ferri 1997; Tardy et al. 2001), and therefore indicative of a mature ocean basin. Although the data are also permissive of a marginal basin setting, there is no evidence of back-arcbasin development to the east of the Turnagain River area anywhere in the miogeocline. To the north, near the British Columbia – Yukon boundary, a large thickness of deformed Mississippian volcanic rocks of continental-arc affinity appears to depositionally overlie quartz-rich miogeoclinal strata and may constitute the oldest relict of the Quesnel arc (Mihalynuk and Logan 2000). The volcanic assemblage near the Turnagain River may record the local end of arc activity within the miogeocline in the late Early Mississippian. The evidence of an arc on the ancestral North American plate margin permits a number of inferences. One is that
middle Paleozoic east-dipping subduction took place beneath the North American margin at this locality; magmatism was relatively younger (-340 Ma) than where it is inferred in the miogeocline farther south (e.g., rhyolitic to dacitic Exshaw tuff, -363 Ma; Richards et al. 2002). Another inference is that the magmatic arc was built at the edge of continental crust, as interpreted from 87Sr/86Sr data (Table 3). Sr isotopic data from Mesozoic plutons (Armstrong 1988) show similar values and support the interpretation that a convergent plate margin existed here in both Mississippian and Early Jurassic times. In the Cassiar Mountains near the Kechika River, -100 km east of the Turnagain River area, latest Devonian alkalic volcanic rocks overlie Upper Devonian to Mississippian miogeoclinal strata (see, e.g., Gabrielse 2003). They include tuffaceous green alkalic rocks lithologically similar to those near the Turnagain River. They have yielded a U–Pb zircon date of 362 Ma and show that comparable tectonic conditions prevailed at the time. In the Pelly Mountains, -400 km to the north, Mississippian alkaline volcanic rocks lie on miogeoclinal strata (e.g., Gordey 1981; Mortensen 1982). They have been interpreted to record extension in the western part of the miogeocline that may be related to arc magmatism farther outboard, along strike with the rocks near the Turnagain River. The largest known Cenozoic back-arc basin at a continental margin is the Sea of Japan. It is of a size comparable to the proposed Slide Mountain – Angayucham basin (e.g., Mihalynuk 1999) and may have undergone a similar evolution. In the Sea of Japan, graben formation and widespread alkalic volcanism more than 400 km inboard of the plate edge (Liu et al. 2001) preceded back-arc-basin initiation by -60 million years. In the Cordillera, Ordovician to Middle Devonian volcanic rocks (Fig. 7) occur at least 300 km inboard from the plate edge in the miogeocline. We do not attribute the Ordovician magmatism to an arc, mainly because no arc strata that are old enough are known anywhere east of the Alexander terrane in the Cordillera; it is most likely a late manifestation of opening of the Panthalassa Ocean. Early Devonian quartz-rich magmatic rocks are known in Stikinia, however, and those may represent an arc behind which the Devonian volcanic strata in the miogeocline were erupted far inboard of the plate margin, as in the Sea of Japan. Neoproterozoic volcanic rocks in the Cordillera (780– 690 Ma; e.g., Ferri et al. 1999) have been interpreted to record initial (failed?) rifting of the plate margin; a rift to drift transition took place in the Eocambrian (570–555 Ma; e.g., Colpron et al. 2002; Bond and Kominz 1984). Subduction © 2005 NRC Canada
Erdmer et al.
1461
© 2005 NRC Canada
1462
Can. J. Earth Sci. Vol. 42, 2005 Fig. 7. Representative distribution of latest Proterozoic and Paleozoic magmatic rocks along the ancient margin in the Canadian Cordillera, listed in order of increasing age. Pennsylvanian–Permian: 1, Campbell Range basalt Pen-Perm (Hunt 2002; Harms in Plint and Gordon 1997), E-MORB (±OIB) and N-MORB; 2, alkalic and calc-alkalic dikes and sills (two suites) = Slide Mountain feeders, 281 Ma (Lower Permian), Ferri and Friedman (2002), MORB – E-MORB, back arc to arc (?). Early–Middle Mississippian: 3, Turnagain River, this study, 339.7 ± 0.6 Ma. Late Devonian – Early Mississippian: 4, Quesnel Lake gneiss, 357.2 ± 1.0 Ma, Ferri et al. (1999), W suite Sri = 0.72–0.75, peraluminous, two-mica, S-type, E suite Sri = 0.703–709, I-type (arc) and coeval; 5, Gilliland tuff, 357 Ma (F. Ferri, personal communication, 2003); 6, Kudz Ze Kayah felsic metavolcanic rocks, -360 Ma, ensialic back arc composition (Piercey et al. 2002); 7, Wolverine Lake succession, 345 Ma (Piercey et al. 2002); 8, Fyre Lake succession, 365–360 Ma, Piercey et al. (2002), subalkaline boninitic basalt–andesite, tholeiitic-alkaline OIB, and basaltic andesite; MORB >> MORB to back-arc or fore-arc setting (Sebert and Hunt 1999); 9 and 11, part of Pelly Mountains Mississippian volcanic belt; 10, Late Devonian – Early Mississippian (Mortensen in Hunt 2002); high-K metaluminous calc-alkalic trachyte and rhyolite (Hunt 2002), interbedded with and overlying Earn Group strata; 12, volcanic tuff in Earn Group at Marg deposit; sparse geochemical data for rhyodacite–dacite show possibly rift-related and within-plate granite affinity (Hunt 2002); 13, Eagle Bay volcanic unit, 360 ± 6 Ma (Bailey et al. 2001); 14, Downie Creek orthogneiss, 354 Ma (Logan and Friedman 1997); 15, Exshaw rhyolitic to dacitic tuff, -363 Ma (Richards et al. 2002); 16, foliated tonalite in Dorsey assemblage (= Yukon–Tanana terrane), 349.9 ± 4.2 Ma, Nelson et al. (1998). Lower Ordovician to Middle Devonian: 17, Middle Ordovician to Middle Devonian Marmot Formation (Misty Creek Embayment) alkalic basalt (WPB), Cecile et al. (1982); 18, Early to Middle Ordovician mafic volcanic rocks along the northern margin of the Selwyn Basin (see Souther 1991). Cambrian to Devonian: 19, mafic alkalic and potassic volcanic rocks (see Goodfellow et al. 1995). Late Proterozoic – Eocambrian: 20, Gataga volcanic rocks, 689 Ma (Ferri et al. 1999); 21, Hamill volcanic rocks, 570 Ma, within-plate alkali basalt (Colpron et al. 2002); 22, Spa Creek igneous clasts, granite to granodiorite, 555.6 ± 2.5 Ma (Erdmer et al. 2001).
beneath the North American margin began in the Middle to Late Devonian, as indicated by magmatic activity in the miogeocline, and was followed by the separation of correlative rocks and the opening of a back-arc basin. The Mississippian volcanic rocks near the Turnagain River, the youngest reported within the miogeocline, were apparently stranded east of the main oceanic basin. Closure of that basin is recorded by widespread Middle Permian magmatism in the separated part of the continental margin (e.g., Mortensen 1992), which forms the basement of the Quesnel arc farther south. In central and northern British Columbia, the Quesnel arc edifice was thrust over the continental margin between 186 and 180 Ma (Nixon et al. 1993). Such a tectonic overlap is compatible with the observed shift in ε Nd values in Jurassic granodiorite and granite in the study area (from +6.5 to –11.5 between -188 and 171 Ma). Both evolved crustal and primitive mantle sources
are indicated by these plutons, which may record either a continental-margin setting with multiple magma sources or contamination.
Conclusions An Early Mississippian volcanic assemblage (U–Pb zircon date of 339.7 ± 0.6 Ma) near the Turnagain River is in stratigraphic contact with underlying miogeoclinal strata correlated with the ancient North American margin. The contact is marked by interlayering of finely laminated phyllites on the scale of centimetres. The volcanic assemblage has isotopic characteristics more primitive than those of the underlying miogeoclinal strata, and arc to back-arc chemistry. The contact was folded at the outcrop scale into tight folds before intrusion of an Early Jurassic granodiorite (187.5 ± 2.9 Ma). On the basis of its lithology, isotopic characteristics, and age, the © 2005 NRC Canada
Erdmer et al.
volcanic assemblage may be the youngest known representative of a mid-Paleozoic arc built on the continental margin. Volcanic-arc rocks younger than Mississippian lie farther outboard and are restricted to Quesnellia and outer terranes. Devonian–Mississippian volcanism in the miogeocline is mainly extension related (e.g., Earn Group); in deformed pericratonic rocks, it is arc related and compatible with subduction at the plate margin that was widespread by the Late Devonian and peaked in the Early Mississippian. The Mississippian volcanic rocks near the Turnagain River lie on outboard miogeoclinal strata and are probable relicts of the arc associated with the opening of a Slide Mountain – Angayucham basin.
Acknowledgments Research was supported by the Natural Sciences and Engineering Research Council of Canada (Grant No. 750-00) and Lithoprobe. We acknowledge assistance in the field from F. Devine and comments by Journal reviewers F. Ferri and S. Piercey and Associate Editor S. Hanmer, whose suggestions helped sharpen the interpretation of results.
References Armstrong, R.L. 1988. Mesozoic and early Cenozoic evolution of the Canadian Cordillera. In Processes in continental lithospheric deformation. Edited by S.P. Clark, Jr., C. Burchfiel, and J. Suppe. Geological Society of America, Special Paper 218, pp. 55–91. Bailey, S.L., Paradis, S., and Johnston, S.T. 2001. New insights into metavolcanic successions and geochemistry of the Eagle Bay assemblage, south-central British Columbia. In Current research 2001-A8. Geological Survey of Canada, Paper 2001-A8, 16 p. Bond, G.C., and Kominz, M. 1984. Construction of tectonic subsidence curves for the early Paleozoic miogeocline, southern Canadian Rocky Mountains: implications for subsidence mechanisms, age of breakup, and crustal thinning. Geological Society of America Bulletin, 95: 155–173. Cecile, M., Fritz, W.H., Norford, B.S., and Tipnis, R.S. 1982. The lower Paleozoic Misty Creek Embayment, Selwyn Basin, Yukon and Northwest Territories. Geological Survey of Canada, Bulletin 335. Colpron, M., Logan, J.M., and Mortensen, J.K. 2002. U–Pb zircon age constraint for late Neoproterozoic rifting and initiation of the lower Paleozoic passive margin of western Laurentia. Canadian Journal of Earth Sciences, 39: 133–143. Cox, K.G., Bell, J.D., and Pankhurst, R.J. 1979. The interpretation of igneous rocks. George Allen and Unwin, London, UK. Creaser, R.A., Erdmer, P., Stevens, R.A., and Grant, S.L. 1997. Tectonic affinity of Nisutlin and Anvil assemblage strata from the Teslin tectonic zone, northern Canadian Cordillera: constraints from neodymium isotope and geochemical evidence. Tectonics, 16: 107–121. Driver, L.A., Creaser, R.A., Chacko, T., and Erdmer, P. 2000. Petrogenesis of the Cretaceous Cassiar batholith, Yukon–BC, Canada: implications for magmatism in the North American Cordilleran interior. Geological Society of America Bulletin, 112: 1119–1133. Erdmer, P., Heaman, L., Creaser, R.A., Thompson, R.I., and Daughtry, K.L. 2001. Eocambrian granite clasts in southern British Columbia shed light on Cordilleran hinterland crust. Canadian Journal of Earth Sciences, 38: 1007–1016. Ferri, F. 1997. Nina Creek Group and Lay Range Assemblage,
1463 north-central British Columbia: remnants of late Paleozoic oceanic and arc terranes. Canadian Journal of Earth Sciences, 34: 854–874. Ferri, F., and Friedman, R. 2002. New U–Pb and geochemical data from the Barkerville Subterrane. In Slave – Northern Cordillera Lithospheric Evolution (SNORCLE) Transect and Cordilleran Tectonics Workshop Meeting, Sidney, B.C., 21–24 February 2002. Compiled by F. Cook and P. Erdmer. Lithoprobe Report No. 82, p. 75. Ferri, F., Rees, C., Nelson, J.A., and Legun, A. 1999. Geology and mineral deposits of the northern Kechika Trough between Gataga River and the 60th parallel. British Columbia Ministry of Energy and Mines, Geological Survey Branch, Bulletin 107. Gabrielse, H. 1962. Geology, Cry Lake, British Columbia. Geological Survey of Canada, Map 42-1962, scale 1 : 253 440. Gabrielse, H. 1963. McDame map-area, Cassiar District, British Columbia. Geological Survey of Canada, Memoir 319. Gabrielse, H. 1969. Jennings River map-area, British Columbia. Geological Survey of Canada, Paper 68-55. Gabrielse, H. 1985. Major dextral transcurrent displacements along the Northern Rocky Mountain Trench and related lineaments in north-central British Columbia. Geological Society of America Bulletin, 96: 1–14. Gabrielse, H. 1991. Late Paleozoic and Mesozoic terrane interactions in north-central British Columbia. Canadian Journal of Earth Sciences, 28: 947–957. Gabrielse, H. 1998a. Geology of Cry Lake and Dease Lake map areas, north-central British Columbia. Geological Survey of Canada, Bulletin 504. Gabrielse, H. 1998b. Geology, Cry lake, British Columbia. Geological Survey of Canada, Map 1907A, scale 1 : 250 000. Gabrielse, H. 2003. Geology, Kechika River, British Columbia. Geological Survey of Canada, Open File 1633. Gabrielse, H., Monger, J.W.H., Wheeler, J.O., and Yorath, C.J. 1991. Part A. Morphogeological belts, tectonic assemblages and terranes. In Geology of the Cordilleran Orogen in Canada. Edited by H. Gabrielse and C.J. Yorath. Geological Survey of Canada, Geology of Canada, no. 4, Chap. 2, pp. 15–28. (Also Geological Society of America, The Geology of North America, Vol. G-2.) Garzione, C.N., Patchett, P.J., Ross, G.M., and Nelson, J. 1997. Provenance of Paleozoic sedimentary rocks in the Canadian Cordilleran miogeocline: a Nd isotopic study. Canadian Journal of Earth Sciences, 34: 1603–1618. Goldstein, S.L., O’Nions, R.K., and Hamilton, P.J. 1984. A Sm/Nd isotopic study of atmospheric dusts and particulates from major river systems. Earth and Planetary Science Letters, 70: 221–236. Goodfellow, W.D., Cecile, M.P., and Leybourne, M.I. 1995. Geochemistry, petrogenesis, and tectonic setting of lower Paleozoic alkalic and potassic volcanic rocks, Northern Canadian Cordilleran Miogeocline. Canadian Journal of Earth Sciences, 32: 1236–1254. Gordey, S.P. 1981. Stratigraphy, structure and tectonic evolution of the southern Pelly Mountains in the Indigo Lake area, Yukon Territory. Geological Survey of Canada, Bulletin 318. Hawkins, J.W., Lonsdale, P.F., Macdougall, J.D., and Volpe, A.M. 1990. Petrology of the axial ridge of the Mariana Trough back arc spreading center. Earth and Planetary Science Letters, 100: 226–250. Heaman, L.M., Erdmer, P., and Owen, J.V. 2002. U–Pb geochronologic constraints on the crustal evolution of the Long Range Inlier, Newfoundland. Canadian Journal of Earth Sciences, 39: 845–865. Hunt, J.A. 2002. Volcanic-associated massive sulphide (VMS) mineralization in the Yukon–Tanana terrane and coeval strata of the North American miogeocline in the Yukon and adjacent areas. © 2005 NRC Canada
1464 Indian and Northern Affairs Canada, Exploration and Geological Services Division, Yukon Region, Bulletin 12. Irvine, T.N., and Baragar, W.R.A. 1971. A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Earth Sciences, 8: 523–548. Jaffey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C., and Essling, A.M. 1971. Precision measurements of half-lives and specific activities of 235U and 238U. Physics Reviews, 4: 1889–1906. Kistler, R.W., and Peterman, Z.E. 1978. Reconstruction of crustal blocks of California on the basis of initial strontium isotopic compositions of Mesozoic granitic rocks. US. Geological Survey, Professional Paper 1071. Liu, J., Han, J., and Fyfe, W.S. 2001. Cenozoic episodic volcanism and continental rifting in northeast China and possible link to Japan Sea development as revealed from K–Ar geochronology. Tectonophysics, 339: 385–401. Logan, J.M., and Friedman, R.M. 1997. U–Pb ages from the Selkirk Allochthon, Seymour Arm map area, southeast British Columbia (82M/8 and 9). In Geological fieldwork 1996. British Columbia Ministry of Energy and Mines, Paper 1997-1, pp. 17–23. Longerich, H.P., Jenner, G.A., Fryer, B.J., and Jackson, S.E. 1990. Inductively coupled plasma – mass spectrometric analysis of geological samples: a critical evaluation based on case studies. In Microanalytical methods in mineralogy and geochemistry. Edited by P.J. Potts, C. Dupuy, and J.F.W. Bowles. Chemical Geology, 83: 105–118. Ludwig, K.R. 1990. Isoplot: A plotting and regression program for radiogenic isotopic data. US. Gelogical Survey, Open File Report 90-91, 39 p. Meschede, M. 1986. A method of discriminating between different types of mid-ocean ridge basalts and continental tholeiites with the Nb–Zr–Y diagram. Chemical Geology, 56: 207–218. Mihalynuk, M.G. 1999. Geology and mineral resources of the Tagish Lake area (NTS 104M/8, 9, 10E, 15 and 104N/12W), northwest British Columbia. British Columbia Ministry of Energy and Mines, Geological Survey Branch, Bulletin 105. Mihalynuk, M.G., and Logan, J.M. 2000. Big Salmon complex and Stikine terrane stratigraphic and deformational age comparisons, northwestern British Columbia. Geological Society of America, Cordilleran Section, 1990 Annual Meeting, Abstracts with Programs, 32(6): A31. Mortensen, J.K. 1982. Geological setting and tectonic significance of Mississippian felsic metavolcanic rocks in the Pelly Mountains, southeastern Yukon Territory. Canadian Journal of Earth Sciences, 19: 8–22. Mortensen, J.K. 1992. Pre-mid-Mesozoic tectonic evolution of the Yukon–Tanana terrane, Yukon and Alaska. Tectonics, 11: 836–853. Nelson, J., and Friedman, R. 2004. Superimposed Quesnel (late Paleozoic – Jurassic) and Yukon–Tanana (Devonian–Mississippian) arc assemblages, Cassiar Mountains, northern British Columbia: field, U–Pb, and igneous petrochemical evidence. Canadian Journal of Earth Sciences, 41: 1201–1235. Nelson, J.A., Harms, T.A., and Mortensen, J.K. 1998. Extensions and affiliates of the Yukon–Tanana terrane in northern British Columbia. In Geological fieldwork 1997, a summary of field activities and current research. British Columbia Ministry of Employment and Investment, Geological Survey Branch, Paper 1998-1, pp. 7.1–7.12. Nixon, G.T., Archibald, D.A., and Heaman, L.M. 1993. 40 Ar/39Ar and U–Pb geochronometry of the Polaris Alaskan-type complex, British Columbia: precise timing of Quesnellia – North America interaction. Geological Association of Canada – Mineralogical Association of Canada, Program with Abstracts, 18: A76.
Can. J. Earth Sci. Vol. 42, 2005 Pearce, J.A. 1975. Basalt geochemistry used to investigate past tectonic environments on Cyprus. Tectonophysics, 25: 41–67. Piercey, S.J., Mortensen, J.K., Murphy, D.C., Paradis, S., and Creaser, R.A. 2002. Geochemistry and tectonic significance of alkalic mafic magmatism in the Yukon–Tanana terrane, Finlayson Lake region, Yukon. Canadian Journal of Earth Sciences, 39: 1729–1744. Piercey, S.J., Murphy, D.C., Mortensen, J.K., and Creaser, R.A. 2004. The mid-Paleozoic initiation of the Northern Cordilleran marginal back-arc basin: geological, geochemical and neodymium isotopic evidence from the oldest mafic magmatic rocks in Yukon– Tanana Terrane, Finlayson Lake district, southeast Yukon, Canada. Geological Society of America Bulletin, 116: 1087–1106. Plint, H.E., and Gordon, T.M. 1997. The Slide Mountain Terrane and the structural evolution of the Finlayson Lake Fault Zone, southeastern Yukon. Canadian Journal of Earth Sciences, 34: 105–126. Richards, B.M., Ross, G.M., and Utting, J. 2002. U–Pb geochronology, lithostratigraphy and biostratigraphy of tuff in the Upper Famennian to Tournaisian Exshaw Formation: evidence for a mid-Paleozoic magmatic arc on the northwestern margin of North America. In Carboniferous and Permian of the world. Edited by L.V. Hills, C.M. Henderson, and E.W. Bamber. Canadian Society of Petroleum Geologists, Memoir 19, pp. 158–207. Sebert, C., and Hunt, J.A. 1999. A note on preliminary lithogeochemistry of the Fire Lake area. In Yukon exploration and geology 1998. Edited by C.F. Roots and D.S. Emond. Indian and Northern Affairs Canada, Exploration and Geological Services Division, Yukon Region, pp. 139–142. Souther, J.G. 1991. Volcanic regimes. In Geology of the Cordilleran Orogen in Canada. Edited by H. Gabrielse and C.J. Yorath. Geological Survey of Canada, Geology of Canada, no. 4, Chap. 14 (Also Geological Society of America, The Geology of North America, Vol. G-2.), pp. 457–490. Stacey, J.S., and Kramers, J.D. 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters, 26: 207–221. Steiger, R.H., and Jäger, E. 1977. Subcommission on geochronology: convention in the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36: 359–362. Sun, S.S., and McDonough, W.F. 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the ocean basins. Edited by A.D. Saunders and M.J. Norry. Geological Society,(London), Special Publication 42, pp. 313–345. Tardy, M., Lapierre, H., Struik, L.C., Bosch, D., and Brunet, P. 2001. The influence of mantle plume in the genesis of the Cache Creek oceanic igneous rocks: implications for the geodynamic evolution of the inner accreted terranes of the Canadian Cordillera. Canadian Journal of Earth Sciences, 38: 515–534. Unterschutz, J., Creaser, R.A., Erdmer, P., Thompson, R.I., and Daughtry, K.L. 2002. North American margin origin of Quesnel terrane strata in the southern Canadian Cordillera: inferences from geochemical and Nd isotopic characteristics of Triassic metasedimentary rocks. Geological Society of America Bulletin, 114: 462–475. Wheeler, J.O., Brookfield, A.J., Gabrielse, H., Monger, J.W.H., Tipper, H.W., and Woodsworth, G.J. 1991. Terrane map of the Canadian Cordillera. Geological Survey of Canada, Map 1713A, scale 1 : 2 000 000. Winchester, J.A., and Floyd, P.A. 1977. Geochemical discrimination of different magma series and their differentiation products using immobile elements. Chemical Geology, 20: 325–343. Wood, D.A. 1980. The application of a Th–Hf–Ta diagram to © 2005 NRC Canada
Erdmer et al. problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary volcanic province. Earth and Planetary Science Letters, 50: 11–30.
Appendix A: Whole-rock analytical procedures Major element analysis of rocks by X-ray fluorescence spectrometry (XRF) Sample preparation procedure A predetermined amount of 150–200 mesh sample pulp is roasted to determine the loss on ignition (LOI). Two grams of the roasted sample is fused in a platinum–gold crucible with a commercial lithium tetraborate flux. The molten material is cast in a graphite mold. XRF analysis: whole rock The fusion disks are analyzed using a Siemens SRS-200 sequential X-ray spectrometer with a chromium tube for 11 elements that include CaO, K2O, P2O5, SiO2, Al2O3, MgO, Na2O, Fe2O3, TiO2, MnO, and Ba. All elements analyzed are corrected for absorption effect, enhancement effect, line overlap, background correction, and LOI. The calibration curves for whole rock analysis are set up using synthetic and commercial standards with certified analytical data. Quality control and statistics Every 20 fusion disks prepared include one in-house or commercial standard and one repeat sample. Every 10 samples analyzed include two commercial standards. The lower limit of detection (or two times standard deviation) for whole rock analysis is 0.01%, except 0.1% for SiO2. Trace elements analysis by XRF Sample preparation procedure Finely milled rock pulp is milled with boric acid for 3 min. The milled samples are then pressed at high pressure to produce 40 mm pressed pellets. XRF trace analysis Different excitation X-ray tubes are employed to analyze different trace elements. The use of different X-ray tubes allows for maximum intensity and higher resolution analytic peak. One or two backgrounds are run with the sample. All trace element analysis calibration curves are set up by using certified commercial standards. Calculations are used to compensate for absorption and enhancement effects. Multiple curves are used for each trace element to cover different ranges of analysis. Quality control and statistics For each 20 samples prepared, one is a repeat. Every 10 samples analyzed includes one commercial standard. For trace
1465
element analysis, curves are recalibrated for each new batch of unknown samples to minimize equipment drift. Inductively coupled plasma – mass spectrometry (ICP–MS) analysis The results are given in parts per million. In addition to the quantitative determinations of the REE, Y, and Th, data are given for Zr, Nb, Ba, Hf, and Ta. The latter elements are generally collected quantitatively (particularly Nb and Ta) by the digestion procedure but on occasion may not be (particularly Zr, Hf, and Ba). Both Zr and Ba can be confirmed by the XRF data (Table 3), and Ta XRF values are consistent with Nb values. Grinding was accomplished in a steel mill, negating potential Ta–Nb contamination. Details of the procedure are given in Longerich et al. (1990). Analytical technique for Nd and Sr at MUN The analyses were performed using the following method (M. Poujol, written communication, 2005). Approximately 0.1 g of rock power is dissolved in Savilex Teflon beakers using a mixture of concentrated HF–HNO3 acids. A mixed 150 Nd/149Sm spike is added to each sample prior to acid digestion. Both sample and spike are weighed on a highprecision balance. After 5 days of digestion, the solution is evaporated to dryness and then taken up in 6N HCl acid for 2 days. The solution is then dried and taken up in 2.5N HCl and loaded on cationic exchange chromatography using AG50W-X8 resin to collect the REE fractions on one hand and Sr and Rb on the other hand (this chemistry is done twice to purify the Sr). The REE fractions are then purified and Sm and Nd are isolated using a secondary column loaded with Eichrom Ln resin. Sr is separated with the resin Spec Sr. All reagents are purified to ensure a low contamination level. The measured total chemical blanks range between 40 and 90 pg and are considered negligible. Sm and Nd concentrations and Sr and Nd isotopic compositions are analyzed using a multicollector Finnigan Mat 262 mass spectrometer in static mode. Nd isotopic ratios are normalized to 146Nd/144Nd = 0.7219. The reported values are adjusted to the La Jolla Nd standard. The in-run precisions on Nd isotopic ratio are given at the 95% confidence level. Errors are < 0.002% in the Nd isotopic compositions and are estimated to be lower than 0.1% errors in the 147Sm/144Nd ratio. Sr isotopic ratios are normalized to 88/86 = 8.375209. Reported Sr values are adjusted to the NBS 987 standard. Errors in the Sr isotopic compositions are < 0.002%. The ε Nd values are calculated using 147Sm/144Nd = 0.1967 and 143 Nd/144Nd = 0.512638 values for the present-day chondrite uniform reservoir (CHUR). The 147Sm decay constant is 6.54 × 10–12 year–1 (Steiger and Jäger 1977), TDM is calculated both with respect to a depleted mantle with an ε Nd(0) value of +10 isolated from the CHUR since 4.55 Ga following a linear evolution and with respect to the De Paolo mantle model.
© 2005 NRC Canada
