Geochemistry and tectonic significance of alkalic mafic ...spiercey/Piercey_Research_Site/... · magmatisme mafique et des roches felsiques volcaniques, dérivées de la croûte,

Geochemistry and tectonic significance of alkalicmafic magmatism in the Yukon–Tanana terrane,Finlayson Lake region, Yukon1

Stephen J. Piercey, James K. Mortensen, Donald C. Murphy, Suzanne Paradis,and Robert A. Creaser

Abstract: This paper provides an integrated field and geochemical study of weakly alkalic, �360 Ma mafic rocks fromthe Yukon–Tanana terrane in the Finlayson Lake region, southeastern Yukon. These mafic rocks occur as dykes andsills that crosscut older felsic metavolcanic rocks and metasedimentary rocks (Kudz Ze Kayah unit) or as flowsinterlayered with carbonaceous metasedimentary rocks. The mafic rocks have signatures similar to those of ocean-island basalts, moderate TiO2 and P2O5 contents, elevated high field strength element and light rare earth elementcontents, and εNd350 = +1.1. A subset of the dykes (group 4b) has similar geochemical characteristics but with higherTh/Nb and lower Nb/U ratios, higher Zr and light rare earth element contents, and εNd350 = –2.8. The geochemicaland isotopic attributes of these rocks are consistent with formation from either lithospheric or asthenospheric sourcesduring decompression melting of the mantle, with some rocks exhibiting evidence for crustal contamination (group 4b).The alkalic basalts are interpreted to represent �360 Ma ensialic back-arc rifting and basin generation. It is envisionedthat east-dipping subduction, represented by slightly older magmatic suites (Fire Lake unit), was disrupted bysubduction hinge roll-back, westward migration of arc magmatism, and the onset of back-arc extension. Decompressionmelting of the mantle associated with back-arc generation resulted in mantle melting and the formation of the alkalicbasalts. The spatial association of this mafic magmatism with crustally derived felsic volcanic rocks and containedvolcanogenic massive sulphide mineralization suggests that the associated deposits (Kudz Ze Kayah, GP4F) formedwithin an ensialic back-arc environment.

1744Résumé : Cet article présente une étude intégrée de données de terrain et de géochimie de roches mafiques, ~360 Ma,faiblement alcalines, du terrane Yukon–Tanana dans la région du lac Finlayson, dans le sud-est du Yukon. Ces rochesmafiques se présentent sous forme de dykes et de filons-couches, qui recoupent des roches métavolcaniques et des rochesmétasédimentaires (unité Kudz Ze Kayah) plus âgées, ou sous forme de coulées interstratifiées avec des rochesmétasédimentaires carbonées. Les roches mafiques ont des signatures semblables à celles des basaltes d’îlesocéaniques, des teneurs modérées en TiO2 et P2O5, de hautes teneurs en éléments à champ électrostatique élevé et enéléments de terres rares légers, ainsi qu’un εNd350 = +1,1. Un sous-ensemble des dykes, (groupe 4b), a des caractéristiquesgéochimiques similaires mais avec un rapport Th/Nb plus élevé, un rapport Nb/U plus faible et une plus haute teneuren Zr et en éléments de terres rares légers, ainsi qu’une valeur de εNd350 = –2,8. Les caractéristiques géochimiques etisotopiques de ces roches concordent avec une formation de source lithosphérique ou asthénosphérique durant la fusionde décompression du manteau, alors que quelques roches montrent des évidences d’une contamination crustale (groupe4b). Les basaltes alcalins représenteraient l’extension d’arrière-arc ensialique et la génération de bassins, ~360 Ma. Oncroit que la subduction à pendage vers l’est, représentée par des suites magmatiques légèrement plus âgées (unité FireLake), a été perturbée par le repositionnement de la charnière de subduction, la migration vers l’ouest du magmatismed’arc et le début d’une extension d’arrière-arc. La fusion de décompression du manteau, associée à la générationd’arrière-arc, a conduit à la fusion du manteau et à la formation des basaltes alcalins. L’association spatiale de ce

Can. J. Earth Sci. 39: 1729–1744 (2002) DOI: 10.1139/E02-090 © 2002 NRC Canada

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Received 17 October 2001. Accepted 11 September 2002. Published on the NRC Research Press Web site at http://cjes.nrc.ca on13 December 2002.

Paper handled by Associate Editor L. Corriveau.

S.J. Piercey2,3 and J.K. Mortensen. Mineral Deposit Research Unit, Department of Earth and Ocean Sciences, University ofBritish Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada.D.C. Murphy. Yukon Geology Program, P.O. Box 2703 (F-3), Whitehorse, YT Y1A 2C6, Canada.S. Paradis. Mineral Resources Division, Geological Survey of Canada, 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada.R.A. Creaser. Department of Earth and Atmospheric Sciences, University of Alberta, 126 Earth Sciences Building, Edmonton, ABT6G 1E3, Canada.

1Mineral Deposit Research Unit (MDRU) Contribution P-153.2Present address: Mineral Exploration Research Centre (MERC), Department of Earth Sciences, Laurentian University, Ramsey LakeRoad, Sudbury, ON P3E 2C6, Canada.

3Corresponding author (e-mail: [email protected]).

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magmatisme mafique et des roches felsiques volcaniques, dérivées de la croûte, et la minéralisation de sulfures massifsvolcanogènes qu’on y retrouve portent à croire que les dépôts associés (Kudz Ze Kayah, GP4F) se sont formés dans unenvironnement d’arrière-arc ensialique.

[Traduit par la Rédaction] Piercey et al.

Introduction

The origins and tectonic setting of mafic magmatism withinthe Yukon–Tanana terrane (YTT) of the northern CanadianCordillera have been problematic. Central to these problemshas been the stratigraphic grouping of mafic rocks withoutregard to stratigraphic context, geochemistry, or tectonic setting(e.g., Anvil Allochthon; Tempelman-Kluit 1979). There hasalso been contradictory nomenclature of YTT-associatedmafic rocks. For example, some workers have correlatedmafic rocks of the YTT with the Anvil Allochthon or AnvilAssemblage, considering them allochthonous with respect tothe YTT (Tempelman-Kluit 1979). Other workers haveconsidered some YTT mafic rocks to be stratigraphicallypart of the YTT, whereas other YTT mafic rocks wereconsidered allochthonous and correlated with the SlideMountain terrane (e.g., Mortensen and Jilson 1985; Mortensen1992).

In recent years new stratigraphic mapping, trace elementgeochemistry, and Nd-isotope geochemistry of mafic rocksin the YTT have led to significant new advances in under-standing the tectonic setting and origin of YTT mafic rocks(e.g., Creaser et al. 1997; Grant 1997; Murphy and Piercey1999, 2000; Murphy 2001; Piercey et al. 1999, 2001a,2001b). For example, Creaser et al. (1997) and Grant (1997)illustrated that mafic rocks of the Anvil Assemblage had arc-liketrace element geochemical and Nd isotopic signatures thatwere incompatible with them being correlative to basalts ofthe Slide Mountain terrane, which have mid-ocean-ridge basalts(MORB) like signatures (Nelson and Bradford 1993; Nelson1993; Plint and Gordon 1997). Recent stratigraphic mappingin the YTT as part of the Ancient Pacific Margin NationalGeoscience Mapping Program (NATMAP) Project (Murphy1998, 2001; Murphy and Piercey 1999, 2000; Nelson et al.2000; Colpron and Yukon–Tanana Working Group 2001) hasillustrated the diversity of mafic rocks in the YTT. Thesestudies have shown that mafic rocks are essential constituentsof the YTT but are not necessarily stratigraphically or tem-porally equivalent, nor have they formed in similar tectonic set-tings (Murphy 1998, 2001; Murphy and Piercey 1999, 2000;Nelson et al. 2000; Colpron and Yukon–Tanana WorkingGroup 2001). Furthermore, geochemical data, onstratigraphically and temporally constrained mafic and felsicrocks in the YTT, illustrate diverse geochemical characteristicsfor these YTT rocks (Colpron 2001; Piercey 2001; Pierceyet al. 2001a, 2002) that reflect processes occurring duringmid to late Paleozoic arc magmatism and back-arc basingeneration (Nelson et al. 2000; Colpron 2001; Colpron andYukon–Tanana Working Group 2001; Piercey et al. 2001a,2001b).

In this paper we present and interpret a regional geologicaland geochemical dataset for �360 Ma (late Early Mississippian)within-plate mafic rocks from the Finlayson Lake region.These mafic rocks form part of unit 4 of the Grass Lakessuccession, the oldest succession in the Finlayson Lake

region, and are both underlain by �360 Ma felsic rocks (KudzZe Kayah unit; Piercey 2001) and crosscut by �360 Ma graniticrocks (Grass Lakes suite intrusions; Mortensen 1992). Theunderlying, but broadly coeval, felsic rocks of the Kudz ZeKayah (KZK) unit host the Kudz Ze Kayah and GP4F volcanic-hosted massive sulphide (VHMS) deposits. The geochemicaldata provided in this paper imply that within-plate maficrocks record derivation from enriched mantle sources withinan ensialic back-arc-basin setting. Differences between thesamples are attributed to different levels of crustal contami-nation. The combined geological and geochemical datasetpresented herein aims to (i) document the geological andgeochemical characteristics of these within-plate magmaticrocks, (ii) provide insights into the origins and petrogenesisof the YTT-associated mid-Paleozoic within-plate maficmagmatic rocks, and (iii) ascertain the significance of thisstyle of mafic magmatism in the tectonic evolution of theYTT in the northern Cordillera.

Geologic and stratigraphic setting

The YTT in the Finlayson Lake region (Fig. 1) is composedof deformed and metamorphosed (greenschist to loweramphibolite grade) metasedimentary, metavolcanic, andmetaplutonic rocks (e.g., Tempelman-Kluit 1979; Mortensenand Jilson 1985) (Fig. 2). Despite the structural andmetamorphic overprint on these rocks, a stratigraphicallyintact sequence consisting of three mid to late Paleozoicunconformity-bound successions has been defined: the GrassLakes, Wolverine Lake, and Campbell Range successions(Fig. 2) (Murphy 1998, 2001; Murphy and Piercey 1999,2000). The alkalic rocks of this study occur in the uppermostpart of the Grass Lakes succession.

The Grass Lakes succession consists of unit 1, the FireLake unit (unit 2), the Kudz Ze Kayah (KZK; unit 3) unit,and unit 4 (Figs. 2, 3). The lowermost part of the GrassLakes succession consists of pre-365 Ma quartz-rich, non-carbonaceous metaclastic rocks of unit 1, which are overlainby the �365–360 Ma mafic dominated arc- and back-arcrocks of the Fire Lake unit (Mortensen 1992; Grant 1997;Murphy and Piercey 1999, 2000; Piercey et al. 1999, 2001a).The KZK unit stratigraphically overlies the Fire Lake unitand consists of felsic volcanic and variably carbonaceoussedimentary rocks (Murphy 1998) (Figs. 2, 3). The KZKunit is stratigraphically overlain by interlayered alkalic basaltsand carbonaceous sedimentary rocks of unit 4 (Murphy 1998)(Figs. 2, 3). Unit 4 mafic rocks occur in a variety of formsincluding, most commonly, metre-scale layers of chloriticschist interlayered with carbonaceous phyllitic rocks; locally,volcanic flow textures (massive flows and hyaloclastite) arepreserved. Mafic dykes and sills are commonly found nearthe KZK and GP4F VHMS deposits (Fig. 2). Near thesedeposits, medium-grained diabase sills are interlayered withcarbonaceous phyllite and locally preserve fine intergrowthsof lath-like pseudomorphed plagioclase and pyroxene and

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cumulate textures. Mafic dykes near the KZK deposits arecommonly biotite-rich, weakly altered, and cut the footwalland hanging wall rhyolitic rocks and are therefore interpretedto post-date the hydrothermal system that formed the KZKdeposit. Similarly, near the GP4F VHMS deposit (Fig. 2),numerous mafic dykes cut across the felsic volcaniclasticstratigraphy; however, some of the mafic rocks may beinterlayered flows or volcaniclastic sedimentary material.Biotite-rich (biotitite) dykes are also sporadically presentwithin the Finlayson Lake region; however, the greatestconcentration of the alkalic mafic rocks in the FinlaysonLake region appears to be concentrated near the KZK andGP4F deposits (Fig. 2).

Coeval with and crosscutting the KZK unit and unit 4 arethe �360 Ma (Mortensen 1992) K-feldspar porphyritic tomegacrystic granitoids of the Grass Lakes suite (Figs. 2, 3).These intrusions are interpreted to represent the subvolcanicintrusive complex to the felsic volcanic rocks associatedwith the KZK unit VHMS mineralization (Murphy andPiercey 1999). The stratigraphic relationship of unit 4 to theessentially coeval KZK unit and Grass Lakes suite granitoids(Figs. 2, 3) suggests that the alkalic rocks of the studyare �360 Ma.

The rocks of the Grass Lakes succession, and the entireFinlayson Lake region, have been subject to various degreesof deformation, but most of the deformation post-dates theformation of the rocks in this paper. In the Late Pennsylvanian,

rocks of the Fire Lake unit were displaced approximately30 km from the west-southwest to their present positioneast-northeast along the Money Creek thrust (Figs. 1, 2;Murphy and Piercey 2000; Murphy 2001; Murphy et al.2002). The dominant fabric in the Finlayson Lake region isrelated to Cretaceous ductile deformation and low-displacement,southwest-vergent folding and thrusting (Murphy 1998). AMississippian event of uncertain kinematics has affected theGrass Lakes succession; however, this event also post-datedthe formation of the Grass Lakes succession and is interpretedto represent an intra-arc deformation episode (e.g., Colpronand Yukon–Tanana Working Group 2001).

Lithogeochemistry and neodymium-isotopegeochemistry

Sampling and analytical methodsSamples of mafic volcanic and subvolcanic rocks in this

study were collected during 1 : 50 000 scale regional mapping,and all samples were analyzed at the laboratories of theGeological Survey of Canada, Ottawa (Table 1). Based onfield interpretations, most of the schistose and massiveaphyric (greenstone) rocks samples are interpreted to be ofvolcanic origin. Samples that had relict phenocrysts ofplagioclase have been interpreted to be either plagioclase-phyric flows or high-level intrusions. In both of these cases

Fig. 1. Location of the Finlayson Lake region and VHMS deposits with respect to the Yukon–Tanana terrane, Yukon Territory, Canada(modified from Hunt 1998).

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it is interpreted that these rocks represent, or were very closeto, liquid compositions. All samples had weathered edgesremoved by a diamond saw and were pulverized in a steeljaw crusher. Most of the pulverized material was crushed ina tungsten carbide mill; three samples were pulped in a Cr-steelmill and are denoted as such in Table 1. Samples that havebeen crushed by Cr-steel likely have excess Cr values, whereas

those crushed by tungsten carbide likely have excess Ta.Throughout the paper Ta is not used during geochemicaldiscussions, and the three Cr-contaminated samples are notincluded in discussions when Cr is used. Powders from theaforementioned steps were used in all analytical techniques.

The majority of major elements were determined on fusedbeads by X-ray fluorescence (XRF), with the exception of


Fig. 2. Geological map of the Finlayson Lake region modified after Murphy and Piercey (1999), with locations of different zones ofthe Wolverine VHMS deposit and other VHMS deposits of the Finlayson Lake district.

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H2O and CO2, which were analyzed by infrared spectroscopy,and FeO, which was analyzed by modified Wilson titration.Samples analyzed for trace elements were totally dissolvedusing a combination of nitric, perchloric, and hydrofluoricacids, with a lithium metaborate flux if any residual materialexisted after the first acid attack. These solutions were thenanalyzed for trace elements using a combination of inductivelycoupled plasma – emission spectrometry (ICP–ES: Cr, Ni,Co, Cu, Zn, Ba, La, Pb, Sc, Sr, V, Y, Yb, and Zr if >100 ppm)and inductively coupled plasma mass spectrometry (ICP–MS:remaining rare earth elements (REE), Cs, Rb, Th, U, Ga, Hf,Ta, and Zr if <100 ppm). Further details on the methodologycan be obtained from the Geological Survey of Canada at< http://132.156.95.172/ chemistry >. Analytical precisioncalculated from repeat analyses of internal basaltic referencematerials is given as percent relative standard deviation(%RSD = 100 × (standard deviation)/mean) and yielded valuesof 0.43–6.52% for the major elements; 0.72–8.80% for thetransition elements (V, Ni, Cr, Co); 2.21–5.92% for the highfield strength elements (HFSE; Nb, Zr, Hf, Y, Sc, Ga);2.35–6.96% for the low field strength elements (LFSE) Cs,Rb, Th, and U, but slightly higher for Ba and Sr (1.49–15.75%);

2.15–6.47% for the REE (La–Lu); and 1.12–98.12% for Cu, Pb,and Zn (Piercey 2001). Concentrations of the latter elementswere close to detection limits in the internal reference materials,however, leading to decreased precision and high %RSDvalues (Piercey 2001). Two samples were analyzed for theirNd isotopic composition at the University of Alberta radiogenicisotope facility. Samples were analyzed by thermal ionizationmass spectrometry (TIMS) using the preparation and errortreatment methodology of Creaser et al. (1997). Values forthe Geological Survey of Japan Shin Etsu Nd standardyielded an average value of 143Nd/144Nd = 0.512106 with ananalytical uncertainty of ±0.000012 (1σ), which is interpretedto be the minimum uncertainty estimate of the 143Nd/144Ndfor any particular sample. The Nd isotopic data presentedhere are normalized to 143Nd/144Nd = 0.512107 for the Geo-logical Survey of Japan Shin Etsu standard (R.A. Creaser,unpublished data), which is equivalent to 0.511850 for theLa Jolla standard (Tanaka et al. 2000). Although the age ofthe mafic rocks is �360 Ma, the initial εNd values werecalculated at 350 Ma to facilitate comparison with otherYTT Nd isotopic data (Grant 1997; Creaser et al. 1997).This is further justified as the variations between εNd350 andεNd360 are insignificant within the timescale of the evolutionof the Sm–Nd system (<0.1 ε unit; e.g., Hamilton et al. 1983;Goldstein et al. 1984). Results of the Nd isotopic analyses arepresented in Table 2.

Alteration–metamorphism and element mobilityAlthough attempts were made to collect least altered samples,

field and petrographic data on the mafic rocks of the studyarea indicate that they have been affected by low-gradehydrothermal alteration (seawater alteration) and (or)greenschist-facies regional metamorphism. Even thoughprimary volcanic–intrusive features are readily observable atboth outcrop scale (e.g., cumulate textures) and thin-sectionscale (e.g., relict plagioclase), the primary mineralogy of therocks has largely been replaced. In particular, petrographicexamination indicates that the matrixes of the rocks havebeen replaced by an assemblage of chlorite, tremolite–actinolite,quartz, muscovite, titanite, epidote, and carbonate. Precursorplagioclase and mafic phenocrysts have been replaced bycarbonate, muscovite and epidote, and chlorite and tremolite–actinolite, respectively. The greenschist-facies metamorphicand seafloor conditions that these samples have experiencedimply a high probability that most major elements (e.g.,SiO2, Na2O, K2O, CaO) and LFSE (Cs, Rb, Ba, Sr) havebeen mobilized to various degrees (MacLean 1990). Thelikelihood of element mobility is supported by the metamorphicmineral assemblages present in the rocks, and the high volatilecontents (H2O + CO2 = 2.4–14.6%; Table 1). In contrast, themajor elements Al2O3, TiO2, and possibly P2O5, the transitionelements, HFSE, REE, and Th were likely immobile underthe aforementioned conditions (e.g., Pearce and Cann 1973;Wood 1980; MacLean 1990; Rollinson 1993; Jenner 1996).To test if these elements have remained immobile duringalteration and metamorphism, we have plotted theAl2O3/Na2O alteration index (Spitz and Darling 1978)against key compatible elements, compatible element ratios,and incompatible element ratios used throughout this paper(Fig. 4). In all of these diagrams it is clear that there is verylittle correlation, if any, between these elements and the

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Fig. 3. Schematic stratigraphy of the Finlayson Lake district withthe stratigraphic location of VHMS deposits. Alkalic mafic rocksfrom this study occur near the top of the Grass Lakes successionunit and crosscut the Kudz Ze Kayah and GP4F deposits. Mod-ified after Murphy and Piercey (1999, 2000).

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ample name: P98-8 P98-9ba P98-28A P98-35 P98-36a P98-45 P98-48 P98-47 98DM-141 P98-2 P98-3 P98-5aa P98-7 P98-43

Rock typeb: CS CS BCS MB MB MB CS CS MCS MB CS CS MG BMB

Group: 4a 4a 4a 4a 4a 4a 4a 4a 4a 4b 4b 4b 4b 4b

UTM northing: 6815190 6815954 6803506 6817028 6817394 6822258 6811313 6811497 6817068 6816617 6817360 6818770 6814941 6814176

UTM easting: 414061 413795 419847 425862 425450 414457 411645 413765 431291 414979 414591 414328 414655 414449

SiO2 (wt.%) 42.40 46.80 52.80 40.70 44.20 48.30 44.50 45.90 42.00 51.40 48.90 37.90 50.40 51.40TiO2 2.82 3.57 2.11 1.35 1.63 3.19 1.83 1.85 2.06 2.81 2.78 4.45 1.49 2.77Al2O3 13.90 17.70 15.40 12.50 12.90 14.50 10.30 12.90 14.20 13.40 12.40 18.70 14.30 13.80Fe2O3 1.60 1.40 1.40 1.60 5.80 1.90 1.70 1.40 1.20 2.60 1.50 2.20 3.70 2.90FeO 10.00 10.90 6.60 7.60 3.70 10.50 10.40 9.60 9.10 10.00 10.70 11.50 5.60 9.50MnO 0.21 0.13 0.14 0.09 0.11 0.23 0.22 0.23 0.13 0.19 0.18 0.20 0.16 0.15MgO 5.65 5.08 3.93 6.29 7.36 3.17 13.10 6.15 7.84 4.23 3.09 5.92 7.20 5.56CaO 9.20 2.63 5.93 14.23 19.61 7.09 8.88 9.87 7.26 4.90 7.88 6.38 12.12 4.50Na2O 2.00 2.70 3.40 2.10 0.60 3.90 0.10 1.90 0.20 3.50 4.10 2.40 3.10 0.10K2O 1.11 1.81 3.66 1.44 0.15 1.46 1.50 0.25 2.13 0.10 0.06 1.99 0.09 6.53P2O5 0.40 0.67 0.48 0.16 0.32 0.59 0.25 0.23 0.21 0.52 0.48 0.74 0.16 0.38H2O 4.40 4.60 1.50 3.20 1.90 3.00 4.80 4.30 4.10 3.90 3.00 5.30 2.30 2.40CO2 6.70 1.50 2.00 9.00 2.30 2.30 2.80 5.90 10.50 3.10 5.30 2.40 0.10 0.40Total 101.49 100.69 100.15 101.16 100.98 101.33 101.48 101.58 101.93 101.75 101.57 101.38 101.32 101.39

Cr (ppm) 115 28 98 736 690 <10 558 233 142 62 32 74 402 80Ni 72 21 40 303 350 <10 345 54 58 19 <10 37 59 17Co 48 55 35 44 22 33 57 41 23 36 37 50 36 45Sc 30 13 26 35 21 19 30 37 25 32 33 50 44 34V 288 120 292 177 160 182 209 278 235 317 309 410 255 330Cu 92 <10 12 <10 <10 14 <10 <10 19 16 11 40 56 <10Pb 5 3 4 10 5 20 2 14 3 2 6 2 7 5Zn 78 100 58 106 55 121 128 196 80 147 204 210 66 125Rb 34.0 55.0 82.0 66.0 6.2 32.0 57.0 7.5 47.0 3.8 1.7 69.0 1.4 240.0Cs 0.86 1.60 6.80 7.10 0.41 18.00 4.20 0.66 1.30 0.67 0.60 1.80 0.09 7.70Ba 1036 2300 3038 1283 100 1225 1655 281 2300 86 30 1100 22 570Sr 231 130 323 334 790 464 44 157 250 122 213 190 304 130Ga 20 20 18 12 16 24 15 18 18 22 22 33 17 26Ta 2.50 4.60 4.10 1.70 2.60 4.10 1.80 1.30 1.40 2.20 2.00 2.70 0.94 1.80Nb 39 74 64 26 55 68 28 15 21 30 29 50 12 29Hf 4.8 6.0 3.3 2.2 3.0 6.2 2.9 3.2 2.9 6.2 5.9 9.8 2.1 4.2Zr 212 270 143 88 140 247 111 117 120 259 244 410 75 240Y 46 25 24 16 20 50 21 42 20 50 48 87 29 47Th 3.1 7.0 5.2 1.9 3.8 5.5 2.2 1.5 1.8 6.4 6.4 8.2 1.7 4.7U 0.78 1.40 1.00 0.38 1.30 1.30 0.54 0.44 0.45 1.80 1.40 3.10 0.34 1.70La 31 52 35 14 28 46 20 14 14 34 34 53 12 25Ce 63 110 66 25 60 93 42 31 30 70 72 120 25 54Pr 8.4 12.0 7.3 3.2 7.2 12.0 5.5 4.5 3.6 9.5 9.5 15.0 3.6 6.4

Table 1. Geochemical data for weakly alkaline mafic rocks from unit 4 of the Grass Lakes succession.

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1735Nd 34 52 29 13 29 48 22 20 17 39 38 67 16 29Sm 7.8 9.6 6.1 3.1 5.0 10.0 4.8 5.4 4.3 8.8 8.4 14.0 4.0 7.9Eu 2.7 2.8 1.8 1.0 1.8 3.3 1.5 1.7 1.8 2.5 2.4 4.3 1.4 2.3Gd 8.4 8.1 5.1 3.2 4.7 11.0 4.8 6.4 4.3 9.1 9.0 16.0 4.8 8.5Tb 1.50 1.10 0.79 0.51 0.64 1.60 0.72 1.10 0.65 1.50 1.40 2.50 0.81 1.30Dy 8.6 5.2 4.3 3.0 3.4 8.9 3.9 6.8 3.7 8.5 8.0 14.0 4.8 7.5Ho 1.70 0.87 0.82 0.57 0.65 1.70 0.75 1.40 0.73 1.70 1.60 3.00 1.00 1.60Er 4.0 2.0 2.0 1.4 1.6 4.2 1.8 3.7 1.8 4.3 4.1 7.4 2.6 4.2Tm 0.53 0.27 0.29 0.20 0.23 0.57 0.24 0.54 0.27 0.62 0.58 1.20 0.37 0.63Yb 3.1 1.7 1.8 1.4 1.4 3.8 1.6 3.7 1.7 4.3 4.0 7.0 2.5 4.1Lu 0.41 0.28 0.27 0.20 0.18 0.52 0.23 0.53 0.26 0.59 0.56 1.00 0.34 0.58

Al2O3/TiO2 4.93 4.96 7.30 9.26 7.91 4.55 5.63 6.97 6.89 4.77 4.46 4.20 9.60 4.98Zr/Y 4.61 10.80 5.96 5.50 7.00 4.94 5.29 2.79 6.00 5.18 5.08 4.71 2.59 5.11Zr/TiO2 75.18 75.63 67.77 65.19 85.89 77.43 60.66 63.24 58.25 92.17 87.77 92.13 50.34 86.64Zr/Hf 44.17 45.00 43.33 40.00 46.67 39.84 38.28 36.56 41.38 41.77 41.36 41.84 35.71 57.14Ti/V 58.70 178.35 43.32 45.72 61.07 105.08 52.49 39.89 52.55 53.14 53.94 65.07 35.03 50.32Th/Nb 0.08 0.09 0.08 0.07 0.07 0.08 0.08 0.10 0.09 0.21 0.22 0.16 0.14 0.16La/Smn

c 2.57 3.50 3.70 2.92 3.62 2.97 2.69 1.67 2.10 2.49 2.61 2.44 1.94 2.04Gd/Ybn

c 2.24 3.94 2.34 1.89 2.78 2.39 2.48 1.43 2.09 1.75 1.86 1.89 1.59 1.72Nb/Y 0.85 2.96 2.67 1.63 2.75 1.36 1.33 0.36 1.05 0.60 0.60 0.57 0.41 0.62Nb/U 50.0 52.9 64.0 68.4 42.3 52.3 51.9 34.1 46.7 16.7 20.7 16.1 35.3 17.1Nb/Nb*d 1.34 1.31 1.60 1.71 1.81 1.45 1.43 1.11 1.42 0.67 0.65 0.81 0.90 0.89

Note: All samples are in universal transverse Mercator (UTM) zone 9V (NAD27 datum).aCrushed in a Cr-steel mill.bBCS, biotite-bearing chlorite schist; BMB, biotite-bearing metabasalt; CS, chlorite schist; MB, metabasalt; MCS, muscovite-bearing chlorite schist; MG, metagabbro; wt.%, weight percent.cChondrite normalized to values of Sun and McDonough (1989).dNb/Nb* = (2 × Nb/Nbpm)/(Th/Thpm + La/Lapm), where pm indicates normalized to primitive mantle values of Sun and McDonough (1989).

Sample Suite

143 NdNd144

a 147SmNd144

εNd350b fSm/Nd

b εNd0b

Sm (ppm) Nd (ppm)

P98-9b Group 4a 0.512505 (07) 0.1151 +1.1 –0.41 –2.6 9.50 49.90P98-2 Group 4b 0.512364 (07) 0.1395 –2.8 –0.29 –5.3 8.88 38.48

aUncertainties in the last two decimal places are given in parentheses and are at the 2σm level.bCalculated using 143Nd/144Nd of chondritic uniform reservoir (CHUR) = 0.512638 and 147Sm/144Nd = 0.1966

(Hamilton et al. 1983).

Table 2. Neodymium-isotope geochemical data for mafic rocks from unit 4.

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degree of alteration and metamorphism (Fig. 4). Given theseconstraints, it is inferred that the elements shown in Fig. 4have been controlled by processes other than alteration andmetamorphism and have remained immobile during the latterprocesses (e.g., Pearce and Cann 1973; Wood 1980; MacLean1990; Rollinson 1993; Jenner 1996). Given the likelihood ofmobility of some elements (e.g., LFSEs), discussions in thispaper rely primarily on immobile element systematics.Throughout this paper Zr is chosen as a fractionation index,as this element behaves incompatibly during the fractionationof basaltic liquids and is immobile during low to moderategrades of alteration and metamorphism (e.g., MacLean 1990).

ResultsThe unit 4 mafic rocks are characterized by basaltic affinities

(Fig. 5c) (Zr/TiO2 = 50.3–92.2; Table 1) and weakly tomoderately alkalic Nb/Y ratios (Nb/Y = 0.36–2.96; Table 1).The weak to moderate alkalinity of the rocks is also reflectedby their elevated TiO2 (1.53–4.82%) and P2O5 contents(0.16–0.80%; Table 1), which show a positive correlationwith increasing Zr, suggesting incompatibility of all of theseelements during fractionation (Figs. 6a, 6b); in contrast, Zrexhibits a negative trend with the compatible elements Ni,Cr, and Sc (Figs. 6c, 6d, 6e). The Al2O3/TiO2 ratios of themafic rocks (4.20–9.26; Table 1) are lower than primitivemantle (�21) and normal mid-ocean-ridge basalts (N-MORB≈ 11) but lie between those of ocean-island basalts (OIB ≈ 5)and enriched MORB (E-MORB ≈ 9.5; Sun and McDonough1989) (Table 1).

The immobile trace element features of the unit 4 maficrocks are plotted in a series of discrimination diagrams andprimitive mantle normalized and chondrite-normalized plotsin Figs. 7–9. Based on the Th – Nb – light rare earth element(LREE) relationships in primitive mantle normalized plots,unit 4 rocks can be subdivided into two groups. The group4a samples are characterized by positive Nb anomalies(Nb/Nb* > 1) relative to Th and La (Fig. 7), whereas thegroup 4b samples have weakly negative Nb anomalies(Nb/Nb* < 1) relative to Th and La (Fig. 7). It should benoted that these geochemical groups can only be distinguishedgeochemically, as there are no distinguishable differencesbetween them based on field observations or thin sections.The group 4a samples are characterized by smooth, downward-sloping profiles (Fig. 7a) with signatures similar to those ofOIB (Fig. 7a). The group 4b samples have similar downward-sloping profiles, but the group 4b samples have negative Nbanomalies and higher Th/Nb values (Fig. 7b; Table 1). Thedownward-sloping profiles of the two groups in Fig. 7 areechoed by the REE profiles (Fig. 8), with both groups exhibitingLREE enrichment (La/Smn = 1.94–3.70) and fractionation ofthe middle REE from the heavy rare earth elements (HREE)(Gd/Ybn = 1.40–3.86).

In various discrimination diagrams most of the sampleshave within-plate affinities. The elevated TiO2 contents ofthe unit 4 mafic rocks are reflected by the samples lying onthe OIB–MORB boundary in the Ti–V plot (Fig. 9a). In theTh–Nb–Zr plot, groups 4a and 4b separate systematically,with the group 4a samples lying largely in the field for OIBwith their positive Nb anomalies (Nb/Nb* = 1.11–1.81) andlow Th/Nb (0.07–0.10), whereas the group 4b samples aredisplaced towards the Th apex, reflecting their negative Nb

anomalies (Nb/Nb* = 0.67–0.90) and higher Th/Nb ratios(0.14–0.22) (Fig. 9b; Table 1). Furthermore, the group 4bsamples have Th/Nb ratios that correlate with Zr and La/Smand have higher Th/Yb at a given Nb/Yb content than thegroup 4a suite of rocks, typical of rocks that have beencontaminated by continental crust (Figs. 10a–10c). AlthoughU can be mobile under oxidizing fluid conditions (Brenan etal. 1995), the strong correlation of U with immobile Nb suggeststhat U has been immobile and that the Nb–U systematics ofthe rocks reflect primary values. There is a strong separationin Nb/U ratios between the group 4a samples (Nb/U = 34–68)and the group 4b samples (Nb/U = 17–35), which are displacedtowards values for continental crustal reservoirs (Nb/U ≈ 12;Fig. 10d). Both group 4a and group 4b samples have similarand largely chondritic (�36) to slightly superchondritic (>36)Zr/Hf ratios (e.g., Eggins et al. 1997), ranging from 36.6 to46.7 (average = 41.7) and from 35.7 to 57.1 (average = 43.6),respectively (Table 1).

A sample from the group 4a mafic volcanics yieldedεNd350 = +1.1 and fSm/Nd = –0.41, and a group 4b sampleyielded εNd350 = –2.8 and fSm/Nd = –0.29 (Table 2). TheseεNd350 values are much lower than those for the depletedmantle (DM) reservoir at 350 Ma which has a value ofεNd350 = +9.5 and fSm/Nd = +0.09 (Goldstein et al. 1984).

Discussion

PetrogenesisDeciphering the origin of mafic magmatism within the

YTT requires distinguishing between mantle source featuresversus superimposed features related to fractional crystallizationand (or) other open-system processes (e.g., crustal contamina-tion). For the most part, compatible element contents, such asNi, Cr, and Sc, provide us with significant information aboutthe fractionation history of mafic rocks. Compared to Zr, theincompatible element fractionation monitor, the compatibleelements Ni, Cr, and Sc exhibit an inverse relationship (Fig. 6).This is consistent with fractional crystallization of olivine,spinel, and clinopyroxene. These compatible element features,however, provide little information on the source or subsequentcontamination history of the unit 4 magmatic rocks. In contrast,the immobile and high to moderate incompatible-elementcharacteristics of these rocks are largely unaffected by fractionalcrystallization and thus can provide evidence bearing on themantle source region(s), tectonic setting, and the between-suitecharacteristics of the mafic rocks (e.g., Pearce 1983; Pearceand Peate 1995). For example, the low Al2O3/TiO2 ratiosand high TiO2 and P2O5 values suggest that these rocks havebeen derived from an enriched mantle source region, similarto OIB, at low degrees of partial melting (e.g., LaFlèche etal. 1998). Likewise, an OIB-like source is further supportedby the HFSE and REE characteristics and primitive mantlenormalized patterns of the group 4a rocks (Fig. 7). Theslight variation from OIB in the group 4b signatures likelyreflects crustal contamination of an OIB-type magma, asindicated by the Nd isotopic data and crustally sensitiveincompatible element systematics (see the following text).

Rocks with alkalic affinities and OIB signatures are foundin a variety of continental margin settings including continentalrifts (Goodfellow et al. 1995) and continental arc rift settings(van Staal et al. 1991; Shinjo et al. 1999), and there is

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Fig. 4. Key compatible elements (a, b), compatible element ratio (c), and incompatible element ratios (d–h) used throughout this paperagainst the Al2O3/Na2O alteration index (Spitz and Darling 1978). Further details provided in the text.

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considerable debate as to whether alkalic rocks in theseenvironments come from asthenospheric or subcontinentallithospheric mantle sources. For example, numerous workershave shown that magmas derived from enriched asthenosphericmantle can yield rocks with enriched OIB-like trace elementsignatures (e.g., Sun and McDonough 1989; van Staal et al.1991; Shinjo et al. 1999). Other workers have shown, however,that the subcontinental lithospheric mantle (SCLM) is stronglyenriched in incompatible trace elements (Pearce 1983;McDonough 1990; Hawkesworth et al. 1990; Menzies 1990).Some workers have also suggested that small volumes ofalkaline magmatism represent melts of dominantlylithospheric origin (e.g., LaFlèche et al. 1998), whereaslarger volumes represent asthenospheric-derived melts (e.g.,McKenzie and Bickle 1988).

The geochemical data from unit 4 are compatible with ei-ther an asthenospheric or SCLM source region. For example,the geochemical attributes of the unit 4 rocks are similar tothose of interpreted asthenospheric mantle derived maficrocks in the modern Okinawa Trough back-arc basin (Shinjoet al. 1999) and back-arc basin basalts in the OrdovicianBathurst Mining camp (van Staal et al. 1991). Equally viable,however, is a lithospheric source. For example, the lowεNd350 value of the group 4a sample, as compared with theDM reservoir at 350 Ma (+9.5), is a feature present in someSCLM-derived magmas (Hawkesworth et al. 1990; Menzies1990). The relatively minor volume of magma associatedwith the unit 4 alkalic magmatism in the Finlayson Lakeregion, as inferred from the field distributions of the rocks(Fig. 2), is also partly supportive of an SCLM source (e.g.,LaFlèche et al. 1998). The latter, however, may be a functionof the large volumes of felsic volcanic rocks associated withthese mafic rocks which may have effectively prevented thesurface expression of large volumes of mafic magmatism.For example, these felsic rocks are interpreted to haveformed from basaltic underplating (Piercey et al. 2001b) ofcontinental crust, and as such the unit 4 magmas may havesolidified in subvolcanic magma chambers below these felsicvolcanic rocks and did not erupt on surface. Both suites,therefore, are consistent with either asthenospheric or SCLMorigins.

The group 4a and 4b suites have strong geochemical

similarity; however, the group 4b rocks have weak negativeNb anomalies (Fig. 7), which point to the role of an additionalcomponent in their genesis. Negative Nb anomalies are commonfeatures of rocks associated with subduction zones andincipient back-arc basins, due to the influx of LFSE into themantle source regions from the subducted slab (Hawkins1995; Pearce and Peate 1995; Shinjo et al. 1999). NegativeNb anomalies, however, are also common to the bulk andupper continental crust (e.g., Taylor and McLennan 1985;Wedepohl 1995), and rocks contaminated by continentalcrust can also exhibit such anomalies. In the case of thegroup 4b suite, numerous lines of field and geochemicalevidence argue against an arc origin for their negative Nbanomalies. Their very high HFSE and REE contents andOIB-like signatures (Fig. 7) are dissimilar from those ofmost arc rocks (e.g., Pearce and Peate 1995). Also, unit 4alkalic rocks are associated with abundant HFSE-enriched(A-type) felsic rocks, with extensional synvolcanic faults(Murphy and Piercey 2000) and VHMS mineralization, allof which are interpreted to have formed in an ensialic back-arcrift or back-arc basin (Piercey et al. 2001b). Given theseconstraints, the negative Nb anomaly is unlikely to reflectformation of these rocks within an arc environment proper.Nevertheless, a subducted slab component may have playeda role in the genesis of these rocks. High LFSE/Nb valueshave been recognized in back-arc lavas of modern back-arcenvironments where they have been attributed to fluids ormelts from the subducted slab metasomatizing the back-arcmantle (Hawkins 1995; Shinjo et al. 1999). If the negativeNb anomalies of the group 4b suite are from a subductedslab component, then this slab component must have containedevolved continental crustal material to account for theεNd350 = –2.80 value in the group 4b sample.

The geochemical and isotopic features of the group 4bsamples can be explained equally by crustal contaminationof group 4a magmas during emplacement. Numerous linesof geochemical evidence support this hypothesis. For example,in Th/Yb–Nb/Yb space (Fig. 10), the group 4a samples havea within-plate enrichment trend with variable amounts ofenriched (OIB-like) components in their source. In contrast,group 4b samples lie on a distinctly different trajectory withhigher Th/Yb values at a given Nb/Yb value (Fig. 10). InFig. 10, samples that have been influenced by Th enrichment,either through subducted slab metasomatism or crustalcontamination, exhibit vertical increases with higher Th at agiven Nb content than rocks that have not been influencedby Th enrichment (e.g., Pearce 1983). Crustal contaminationis further supported by Th/Nb–Zr systematics, with group 4bsamples showing higher Th/Nb at a given Zr content, typicalof magmas that have undergone crustal contamination (Fig. 10).In contrast, the group 4a samples have lower Th/Nb at agiven Zr content, typical of uncontaminated mafic magmas(Fig. 10). A similar distribution exists in Th/Nb–La/Smspace, where group 4a samples show a flat distribution withconstant Th/Nb values with variable La/Sm contents, andgroup 4b samples exhibit an increase in Th/Nb values withincreasing La/Sm (Fig. 10). Similarly, the Nb/U systematicsof group 4a (Nb/Uavg = 51.39) samples lie primarily withinthe array for oceanic basalts (Nb/U = 47 ± 10; Hofmann etal. 1986), whereas the Nb/U values for the group 4b samplesare much lower (Nb/Uavg = 21.17), and similar to crustal values

Fig. 5. Modified Winchester and Floyd (1977) Zr/TiO2–Nb/Y di-agram (Pearce 1996).

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(Nb/U = 12.09, Taylor and McLennan 1985; Nb/Uupper crust =10.40 and Nb/Ulower crust = 12.15, Wedepohl 1995) consistentwith interaction with continental crust (Fig. 10). Lastly,group 4b samples contain greater average contents of

crustally enriched immobile trace elements, including Zr,Hf, and the LREE, relative to the group 4a suite (Table 1).Taken together, both suites of rocks from the KZK unit areconsistent with formation from lithospheric or asthenospheric

Fig. 6. Zr plots against (a) TiO2 and (b) P2O5 illustrate the incompatible nature of these elements during magma evolution. The inverserelationships between Zr and Cr, Ni, and Sc are consistent with spinel, olivine, and clinopyroxene crystallization, respectively. Symbolsas in Fig. 4.

11

sources; however, the group 4b suite of mafic rocks probablyexperienced crustal contamination either en route to the surfaceor within crustal-level subvolcanic magma chambers.

Tectonic settingThe geological and geochemical attributes of alkalic rocks

from the YTT support a continental arc rift to ensialic back-arcrift – back-arc basin setting (Piercey et al. 2001b). Numerousworkers have used geological, geochemical, and isotopictechniques to illustrate that the Devonian– Mississippianevolution of the YTT was a product of west-dipping continentalarc magmatism, possibly built on the distal edge of the NorthAmerican margin (e.g., Mortensen 1992 and referencestherein; Creaser et al. 1997, 1999; Grant 1997; Piercey et al.1999, 2001a, 2001b). In the Finlayson Lake region, arcmagmatism is recorded primarily by the rocks of the �365–360 Ma Fire Lake unit and Money Creek thrust sheet (e.g.,Grant 1997; Piercey et al. 2001a) which lie to the south andwest of the area of alkalic magmatism. At �360 Ma (oldestU–Pb age of A-type Grass Lakes granitoid; Mortensen 1992),this arc was rifted and incipient ensialic back-arc basindevelopment occurred (Piercey et al. 2001b). Piercey et al.(2001b) suggested that this rifting was due to westwardmigration of an east-dipping subduction zone (cf. Mortensen1992). The alkalic basalts from this study are likely the

manifestations of this arc-rifting event, with them forming asa result of decompression partial melting (e.g., McKenzieand Bickle 1988) of either enriched lithospheric orasthenospheric mantle sources. The spatial association ofalkaline basalts with HFSE-enriched felsic rocks furthersuggests the possibility that the alkalic magmas formedduring this arc-rifting event may have provided the heatengine to drive crustal partial melting, which formed theassociated felsic rocks (e.g., Huppert and Sparks 1988).

This period of arc rifting in the Finlayson Lake region andwestward migration of YTT arc magmatism is broadlycoincident with the interpreted opening of the SlideMountain Ocean (Nelson 1993; Nelson and Bradford 1993;Creaser et al. 1999) and rifting, alkalic magmatism, clasticsedimentation, and exhalative Pb–Zn–Cu sulphide depositformation within the North American miogeocline (Mortensenand Godwin 1982; Mortensen 1982; Gordey et al. 1987;Paradis et al. 1998). It is suggested that arc-rifting in theYTT in the Finlayson Lake region may have been in responseto, or induced the, initial opening of the Slide MountainOcean (e.g., Nelson 1993; Nelson and Bradford 1993;Creaser et al. 1999). Coincidentally, the far-field stressesfrom the YTT rifting and Slide Mountain Ocean openingresulted in the rifting, magmatism, and exhalative sulphidedeposit formation in the North American miogeocline (e.g.,

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Fig. 7. Primitive mantle normalized plots of (a) group 4a and(b) group 4b samples. Primitive mantle values from Sun andMcDonough (1989). Symbols as in Fig. 4.

Fig. 8. Chondrite-normalized plots of (a) group 4a and (b) group4b samples. Chondrite values from Sun and McDonough (1989).Symbols as in Fig. 7.

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Paradis et al. 1998; Paradis and Nelson 2000). The interpre-tation that these events are related to the westward shiftingof the focus of YTT arc magmatism is also stronglysupported by recent mapping, geochronology, and fossilages from the YTT as part of the Ancient Pacific MarginNATMAP Project. These data illustrate that following YTTarc rifting in the Finlayson Lake region, the majority ofsubsequent arc magmatism occurred in the western portionsof the YTT (Nelson et al. 2000; Colpron and Yukon–TananaWorking Group 2001; Colpron 2001; Roots and Heaman2001). Furthermore, in the Finlayson Lake region, post-arc-riftvolcanic and intrusive rocks have geological and geochemicalfeatures consistent with formation in a back-arc typeenvironment (Piercey et al. 2001b, 2002). Collectively, thesestudies illustrate a complex history of magmatism and tectonicactivity along the margin of North America during theDevonian–Mississippian.

The YTT exhibits complex interrelationships betweenmid-Paleozoic magmatism, large-scale tectonics, and

metallogenic evolution. The results of this study illustratethat alkalic mafic magmatism in the YTT of the FinlaysonLake region is a manifestation of arc rifting and is broadlycoeval with VHMS deposit formation (KZK and GP4Fdeposits; Murphy and Piercey 2000) and HFSE-enrichedfelsic magmatism (Piercey et al. 2001b). These relationshipssuggest that arc rifting and subsequent extension wereimportant regional-scale mechanisms that promoted hydro-thermal system generation within the Finlayson Lake district.The association of alkalic mafic rocks with HFSE-enrichedfelsic rocks (Piercey et al. 2001b) may be a regional-scaletectonomagmatic relationship that identifies potentialVHMS-bearing stratigraphy within the YTT and similarcontinent-margin bimodal-volcanic siliciclastic VHMSenvironments.

Conclusions

The conclusions of this study are as follows:(1) Devono-Mississippian (�360 Ma) mafic rocks (unit 4)

in the Finlayson Lake region are spatially associatedwith VHMS mineralization and HFSE-enriched felsicvolcanic rocks. These alkalic rocks occur as sills anddykes that crosscut felsic volcanic rocks and maficflows that are interlayered with variably carbonaceoussedimentary rocks.

(2) All of the mafic rocks have alkalic, ocean-island basaltlike geochemical signatures with high TiO2 and P2O5contents and elevated high field strength element andlight rare earth element contents. A subset of the maficrocks (group 4b) has similar geochemical characteristicsbut with higher Th/Nb and Nb/Nb* ratios, lower Nb/Uratios, and higher Zr and LREE contents; a group 4bsample yielded an εNd350 value of –2.8 compared with avalue of +1.1 in a group 4a sample. The geochemicaland isotopic features of the alkalic rocks are consistentwith formation from either lithospheric or asthenosphericsources during decompression melting of the mantle.The group 4b mafic rocks are interpreted to be equivalentto the group 4a suite but have been contaminated bycontinental crust.

(3) The alkalic basalts of this study are interpreted to representmagmatism associated with �360 Ma ensialic back-arcrifting and basin generation within the Yukon–Tananaterrane. We speculate that Devono-Mississippian east-dippingsubduction was disrupted in the Finlayson Lake regionby subduction hinge roll-back, westward migration ofarc magmatism, and the onset of back-arc extension.Decompression melting of the mantle associated withback-arc generation resulted in melting and the formationof the studied alkalic basalts. The spatial association ofthis mafic magmatism with volcanic-hosted massivesulphide mineralization suggests that the associateddeposits (Kudz Ze Kayah, GP4F) formed within anensialic back-arc rift to back-arc basin environment.

Acknowledgments

Peter Bélanger of the Geological Survey of Canada isthanked for prompt and reliable analyses of the samplespresented in this paper. Discussions and interaction with

Fig. 9. Discrimination diagrams for the unit 4 mafic rocks.(a) Ti–V plot of Shervais (1982). ARC, arc-related basalt;BABB, back-arc-basin basalt; BON, boninite; IAT, island-arctholeiite; LOTI, low-Ti tholeiite; MORB, mid-ocean-ridge basalt;OFB, ocean-floor basalt; OIB, ocean-island basalt. (b) Th–Zr–Nbplot of Wood (1980). Symbols as in Fig. 4.

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the following individuals are gratefully acknowledged:Maurice Colpron, Tom Danielson, Peter Holbek,Paul MacRobbie, Harlan Meade, Jan Peter, Terry Tucker,and Bill Wengzynowski. Constructive reviews of this manu-script by Ian Coulson and Marc Laflèche and commentsby Louise Corriveau are greatly appreciated. This project isfunded by the Yukon Geology Program (DCM); the Geolog-ical Survey of Canada and Ancient Pacific MarginNATMAP Project (SP); Atna Resources and Expatriate Re-sources; a Natural Sciences and Engineering ResearchCouncil of Canada (NSERC) operating grant (JKM); and anNSERC post-graduate scholarship, the Hickok-Radford Fundof the Society of Economic Geologists, and a GeologicalSociety of America Student Research Grant (SJP). The Ra-diogenic Isotope Facility at the University of Alberta is sup-ported in part by an NSERC Major Facilities Access Grant.During the completion of this manuscript, SJP was funded by

an NSERC operating grant and a grant from the LaurentianUniversity Research Fund.

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Geochemistry and tectonic significance of alkalic mafic ...spiercey/Piercey_Research_Site/... · magmatisme mafique et des roches felsiques volcaniques, dérivées de la croûte,