Geochemistry, petrogenesis, and tectonic setting of lower Paleozoic alkalic and potassic volcanic rocks, Northern Canadian Cordilleran ~iogeoclinel
I
Wayne D. Goodfellow, Mike P. Cecile, and Matthew I. Leybourne
Abstract: The Northern Canadian Cordilleran Miogeocline developed intermittently during the early Paleozoic and hosts alkalic and ultrapotassic volcanic rocks that are spatially restricted in thin beds and lenses and isolated volcanic piles. On the basis of geochemistry and geographic location, these volcanic rocks are subdivided into five main groups. Group I rocks (Porter Puddle and Macmillan rocks) are potassic basanites characterized by high Nb, Ce, and NbIY and low ZrINb. They are chemically similar to the Mountain Diatreme, indicating a genetic link. Group 11 rocks (Porter Puddle, Niddery, and Macmillan rocks) are also potassic but have lower abundances of Nb and Ce, higher ZrINb, and lower NbIY. Group I11 rocks (Vulcan and Itsi Lakes) are also potassic but are chemically variable, have lower contents of high field strength elements (HFSE) than the groups I and I1 rocks, and contain elevated Ba contents. Groups 1-111 are characterized by mica (biotite and phlogopite) phenocrysts, sanidine, augite, and Ba-feldspar, a mineral assemblage typical of ultrapotassic lavas. Group IV (Whale Mountain) alkali basalts are the least enriched in the large ion lithophile elements and have relatively low contents of HFSE compared with Groups I and I1 basalts. Groups I-111 are consistent with partial melting of a previously metasomatized lithospheric mantle that was variably enriched in Ba, Nb, and Ce, whereas the group IV rocks are more typical of asthenospherically derived oceanic island basalt partial melts. The geochemistry of the volcanic rocks is consistent with paleotectonic models of the Selwyn Basin. The Selwyn Basin is a passive continental rift that underwent episodic extension and associated subsidence throughout the lower Paleozoic. Alkalic volcanism, and spatially and temporally associated Ba and base metal mineralization, is concentrated along rift-parallel normal faults, particularly where these faults are offset by transform faults. Resume : Le MiogCoclinal de la Cordillkre du nord du Canada a connu un dkeloppement Bpisodique durant le PalBozolque prCcoce; il inclut des roches volcaniques alcalines et ultrapotassiques, lesquelles sont confinBes spatialement des lits minces et des lentilles et des piles volcaniques isolCes. Ces roches volcaniques sont subdivisCes en cinq groupes principaux Btablis sur la base de leur gtochimie et de leur localisation gkographique. Les roches du groupe I (roches de Porter Puddle et Macmillan) sont des basanites potassiques caractBris6es par des valeurs BlevCes de Nb, Ce et NbIY et des rapports faibles de ZrlNb. Elles sont chimiquement similaires a celles de Mountain Diatreme, ce qui indique un lien gCnCtique. Les roches du groupe 11 (roches de Porter Puddle, Niddery et Macmillan) sont potassiques Bgalement, mais elles fournissent des concentrations plus faibles de Nb et Ce, des rapports ZrlNb plus Clevis et Nb/Y plus faibles. Les roches du groupe I11 (les lacs Vulcan et Itsi) sont potassiques Bgalement, cependant elles sont chimiquement variables, leurs 6lCments a forte BlectronBgativitB (HFSE) sont moins abondants que dans les roches des groupes I et 11, et les teneurs en Ba sont BlevCes. Les groupes 1-111 sont caractBrisCs par des phBnocristaux de mica (biotite et phlogopite), de la sanidine, de l'augite et du feldspath-Ba, un assemblage minCralogique typique des laves ultrapotassiques. Les basaltes alcalins du groupe IV (Whale Mountain) sont les moins enrichis en BlBments a grand rayon ionique (LILE), et leurs teneurs en BlBments h forte BlectronBgativitB (HFSE) sont relativement plus faibles par
Received August 22, 1994. Accepted March 15, 1995.
W.D. Goodfellow2 and M.I. L e y b ~ u r n e .Geological ~ Survey of Canada, 601 Booth Street, Ottawa, ON KIA OE8, Canada. M.P. Cecile. Geological Survey of Canada, 3303-33rd Street NW, Calgary, AB T2L 2A7, Canada.
'
Geological Survey of Canada Contribution 62794. Corresponding author (e-mail:
[email protected]). Present address: Ottawa-Carleton Geoscience Centre, Department of Geology, University of Ottawa, Ottawa, ON KIN 6N5, Canada.
Can. J. Earth Sci. 32: 1236-1254 (1995). Printed in Canada / Imprim6 au Canada
Goodfellow et al.
1237 comparaison avec les basaltes des groupes I et 11. Les groupes 1-111 sont compatibles avec l'hypothkse d'une fusion partielle d'un manteau lithosphkrique, antkrieurement mktasomatisk et variablement enrichi en Ba, Nb et Ce, tandis que les roches du groupe IV sont plus typiques des fusions partielles de basaltes alcalins (OIB) dkrivks de l'asthknosph'ere. La gkochimie des roches volcaniques est cohkrente avec les modkles palkotectoniques dkveloppks pour le bassin de Selwyn. Le bassin de Selwyn est un rift continental passif, qui a subi des kvknements kpisodiques d'extension et de subsidence associke, au travers le PalCozoYque infkrieur. Le volcanisme alcalin, et la minkralisation de mktaux de base et Ba qui lui est spatialement et temporairaement associke, est concentrk le long des failles norrnales parallkles au rift, particulikrement oh ces failles sont dkcalkes par les failles transformantes. [Traduit par la rMaction]
Introduction The western edge of ancestral North America was a passivetype continental margin during the latest Proterozoic to Late Devonian. The Southern Canadian Cordilleran Miogeocline was initiated in Late Proterozoic to Cambrian time (see Devlin and Bond 1988; Bond and Kominz 1988). Evidence of Late Proterozoic extensional tectonics is provided by the approximately 780 Ma Redstone River and Coppercap formations, which are interpreted, based on rapid facies and thickness changes, to have formed in a series of grabens (Eisbacher 1985; Aitken 1993). Minor basaltic volcanic rocks and voluminous sills dated at approximately 778 Ma (Jefferson and Parrish 1989) are unconformably overlain by the Redstone River Formation (Helmstaedt et al. 1979; Eisbacher 1981). The Mount Harper volcanic complex in the Ogilvie Mountains is also of Late Proterozoic age (75 1 f 25 Ma; Roots and Parrish 1988). Evidence of postCoppercap Formation latest Proterozoic extensional tectonism consists of a thick sequence of coarse clastic sediment of the Windemere Supergroup exposed in the Mackenzie Mountains (Eisbacher 1981, 1985; Aitken 1993). These rocks are composed of a mixture of coarse-grained quartz sandstone and grit, shale, siltstone, and carbonate, which are interpreted to have complex lateral facies relationships (Eisbacher 1981). The Selwyn Basin, Nasina Basin, Misty Creek Embayment, and North Yukon Basin in the Yukon and Northwest Territories, the Kechika Trough in British Columbia, and the western edge of their adjoining carbonate platforms make up the Northern Canadian Cordilleran Miogeocline (NCCM). These sedimentary basins host Cambrian to Devonian alkalic volcanic rocks erupted onto the western margin of the ancestral North America Miogeocline (Fig. 1) (see Cecile and Norford 1991, 1993, for details of paleogeographic elements). The occurrence of lower to middle Paleozoic alkalic volcanic rocks over most of the miogeocline is evidence for widespread extensional tectonics from Cambrian to Devonian time in the Northern Cordillera. Many of these rocks contain high Ba contents and host barium silicates (hyalophane and celsian), suggesting a genetic link between extensional tectonism, magmatism, hydrothermal activity, and the formation of seafloor Zn -Pb -barite deposits found in many locations in the Northern Cordillera (Goodfellow and Jonasson 1986; Goodfellow 1987). In addition, microdiamonds have been recovered from the Mountain Diatreme (Godwin and Price 1986). In this study, we discuss the petrochemistry of volcanic rocks from seven Cambrian to Devonian volcanic
and related intrusive occurrences in the NCCM, evaluate the petrogenesis as a function of tectonic style and magma sources, and assess the temporal and spatial relationship between crustal extension, magmatism, and hydrothermal activity.
Volcanic occurrences The NCCM contains more than 20 separate occurrences of alkali-rich volcanic and intrusive rocks of Cambrian to Middle Devonian age (Figs. 1 and 2). Three major volcanic episodes can be defined: Cambrian, Early to Middle Ordovician, and Middle to Late Devonian. The Cambrian volcanic rocks occur from the platform edge to near the western margin of lower Paleozoic basinal strata. The second episode consists of a broad belt of Ordovician volcanic rocks that extends from the edge of the eastern platform westward into the adjacent inner basin (see Fig. 2 for names). The third volcanic episode is composed of Devonian volcanic rocks that occur in the inner basin area. On a palinspastic restoration of the NCCM (Fig. I), the Cambrian volcanic rocks are most common in the Middle and Outer Basin Belt, whereas most of the Middle Ordovician and Devonian occurrences are in the Platform Margin and Inner Basin Belt (Fig. 1). The eastern part of the Selwyn Basin is considered to be underlain by attenuated continental crust because late tectonic Mesozoic plutons intruded into Paleozoic strata in this area have high initial 87Sr/86Srratios (Armstrong 1979). Volcanic rocks form thin, continuous and discontinuous, lenticular belts or volcanic piles that are laterally restricted. Feeder sills and dyke intruding nearby sedimentary rocks are associated with some volcanic occurrences. The volcanic rocks consist of submarine flows and volcaniclastic rocks, but where the volcanic sequence is thick or distinct volcanic centres have been identified, the volcanic rocks typically shoal upward into strata containing conglomerate, local carbonate beds, and unconforrnities (e.g., Porter Puddle Complex, Misty Creek Embayment; Fig. 3). Volcanic breccia is common in the upper part of this complex (Fig. 4). The Vulcan volcanic rocks are a succession of pyritic amygdaloidal basalt flows and tuffs intercalated with Early -Middle Ordovician transitional to basin dolostones and limestones (Sapper Formation, see Fig. 17 of Gordey and Anderson 1993). The Itsi Lake occurrence is relatively minor and consists of mafic tuffs and related intrusives that are hosted by Silurian graptolitic shales of the Selwyn Basin near the Itsi Range, Selwyn Mountains.
-
............... ....-.... L
4 1 p-epusse ~ sesueunmo an!sruau! pqelaJ JO a!uealo~ qozooled ~ a m op l eaJe JO al!S
sapy u!seq ueunl!g pue ue!qnopJO snouoqipove 40 uo!gnqup!p lweua3
'3661 'ZE ' P A '!as Yll'33
'r 'ue3
861 1
Goodfellow et at.
1239
Fig. 1. Distribution of lower and middle Paleozoic mafic volcanic and intrusive occurrences in the Northern Canadian Cordilleran Miogeocline (paleogeography adapted from Cecile and Norford 1991). Palinspastic restoration of the Northern Canadian Cordilleran Miogeocline was carried out as follows. Restore 500 km of right-lateral offset on Tintina Fault. The northwestern basin area north of 68" restores northward 40-50%. The remaining area north of 65" restores westward by about 20% starting from the position of the Mackenzie River. South of 65" and north of 60°, the area between the Mackenzie River and the Yukon border restores westward by about 20%, and west of there by 40-50%. South of 60" the platform facies restores from its edge westward by 20% and the basin facies by 30-50%.
The Macrnillan volcanic rocks are discontinuous occurrences of Devonian tuffs, volcaniclastics, and minor flows interbedded with Devonian shale and limestone of the Selwyn Basin (see Cecile and Abbott 1992). Volcanic diamictite occurs interbedded with Pb -Zn sulphides at the Boundary Creek occurrence west of the Jason deposit. The Niddery volcanic rocks consist of several hundred metres of interstratified basalt, tuff, volcaniclastic sandstones, siltstones, and mudstone with minor tuff, shale, and limestone. Gabbro sills containing biotite-phlogopite crystals intrude locally. These strata directly overlie a distinct unit of maroon and green argillite dated as Lower(?) Cambrian (Hofmann and Cecile 1981) and are interstratified with a Cambrian succession of argillite with minor quartzite, limestone, shale, and chert (Cecile and Abbott 1992). The Porter Puddle Complex is a volcanic centre in the southeastern Misty Creek Embayment. The Misty Creek Embayment is a marine rift feature (Cecile 1982) with two stages of rifting (White and McKenzie 1988). The first stage is latest Early Cambrian to Early Ordovician in age but has no known volcanism associated with it. The second is late Early Ordovician to Early Silurian in age, and the bulk of volcanism in the Mistv Creek Embavment is associated with the late-Early and ~ i d d l ~e r d o v i c i kinitial fill of this phase (Marmot and Duo Lakes formations, Fig. 1). The Porter Puddle Complex is represented by two volcanic episodes of Middle Ordovician (Marmot Formation) and Late Silurian to Early Devonian (unnamed) age. A gap in biostratigraphy suggests a possible unconformity between the two (Fig. 3). The Marmot Formation consists of mafic lapilli tuff to finegrained breccia, sandstone, siltstone, and argillite, massive amygdaloidal flows, conglomerate, sills, and minor coarse breccia and pillow breccia. The Porter Puddle Complex includes several sills and dykes that cut both the Rabbitkettle and Duo Lake formations, a discontinuous 60 m thick unit of monolithic unsorted angular volcanic breccia and abundant volcanic conglomerate. These units are unique to this complex and indicate proximity to a volcanic centre. The Mountain Diatreme is 50 km to the east of the Misty Creek volcanic centre. It is a small mafic intrusive body that outcrops on the floor and walls of a southward-opening cirque, is 600 m in diameter, and consists of an outer rusty weathering zone and an inner green breccia core. The Mountain Diatreme intrudes Upper Cambrian to Lower Ordovician transitional to basin carbonate strata and Upper Ordovician to Lower Silurian platformal carbonates (Cecile 1982). McArthur et al. (1980) established a minimum age for the emplacement of the diatreme as early Middle Ordovician on the basis of conodonts extracted from a carbonate xenolith in the centre of the intrusive complex. However, the intrusion of diatreme breccia 60 m into the basal Mount Kindle Formation on the east side of the complex indicates that the diatreme is Late Ordovician to Early Silurian in age. In addi-
tion, a sill extending south from this intrusion has baked Late Ordovician corals. More recent studies suggest that the diatreme is Early Silurian in age based on K- Ar and Rb- Sr dating of phlogopite yielding emplacement ages of 445 f 15 and 427 Ma, respectively (Godwin and Price 1986). The Dempster volcanic rocks consist of alkalic basalt and minor rhyolite flows and breccias that directly overlie Lower(?) Cambrian maroon argillite which, like the argillite beneath the Niddery occurrences, are Oldhamia bearing (Thompson and Roots 1982; Roots 1988). The Dempster volcanic rocks underlie Middle Ordovician black chert, and a pod of limestone in the upper part of the succession yielded conodonts identified by R. S. Tipnis (personal communication, 1980) as Early Ordovician. The lower part of the Dempster volcanic rocks are considered subaqueous, whereas the upper part includes subaerial breccias (Roots 1988). The Whale Mountain volcanic rocks outcrop in an east west-trending belt in the British Mountains of the northern Yukon and northeastern Alaska. They consist of a mixture of tuffs, basalt flows, and volcaniclastic rocks. Dutro et al. (1971, 1972) recovered Late Cambrian trilobites from a limestone in the basal part of this succession.
Analytical methods Silicate minerals in 20 polished thin sections were analyzed by a Cameca-Camebax Microbeam electron microprobe utilizing on-line PAP reduction at the Geological Survey of Canada. Operating conditions were 15 kV electron acceleration potential, 40 s counting time, and 10 nA sample current for feldspar. Mineral standards used were sodium chloride (Na) , potassium bromide (K) , quartz (Si) , magnetite (Fe) , corundum (Al) , wollastonite (Ca), and sanidine @a). For the other silicates, counting times were 20 s at 10 nA sample current for Na, K, Mg, and Fe, and 10 s at 30 nA for Al, Si, Ca, Ti, Mn, and Cr. The mineral standards used were labradorite (Na, Al, Si), orthoclase (K), diopside (Mg, Ca), fayalite (Fe), rutile (Ti), rhodochrosite (Mn, Zn), and chromite (Cr). Chemical compositions were determined at the Geological Survey of Canada as follows: X-ray fluorescence on fused disks prepared by mixing sample powders with lithium tetraborate, lithium fluoride, and ammonium nitrate for SO2, Ti02, Al2O3, K20, Na20, CaO, MgO, P2O5, MnO, Fe203T(total iron), Ba, Sr, Rb, Zr, Ba, Nb, Ce, La, and Y; atomic absorption spectrometry for Ba, Cr, Sr, V, Zn, Cu, Pb, Ni, Co, Ag, Mo, Cd, As, Sb, Se, and Mo; ion selective electrode for F; fluorometer on a disk made by fusion of sample aliquot with NaF, Na2C0, and K2CO3 for U; spectrophotometer for C1; combustion and wet chemical methods for FeO, Fe203, H20, C02, total organic carbon (TOC), and S. Total sulphur was determined by evolving sulphur as an oxide by combustion in an induction furnace
rig. 2. Age of autochthonous lower Paleozoic mafic volcanic and intrusive rocks, NCCM, and adjacent Alaska. Occurrences are listed from south to north.
Goodfellow et al.
1241
Fig. 3. Stratigraphic sections through the Porter Puddle Complex, a volcanic complex in the southeastern part of the Misty Creek Embayment. Age identifications by B.S. Norford, A.W. Norris, T.T. Uyeno, and R.V. Tipnis. SECTION MIDDLE. WRT PORTER PUDDLE COMPLEX U T.M.Zn9
Base at 437100 m E. 7115100 m N Top at 436050 m E. 7115050 m N (Sedion 178, new data)
Mked tUT sedimentary rock
E a
,"hay-
smm or
with
Limestone with
a
Vobanlolastk
shale and breccia Shale
sandstone
rn
Volcsnic breccia CJA~~-LI%I~.Geochemical EX1114 sample number
[r
Volcanicwic eong)omer* With sanjSbrr
and concentrations measured iodometrically in an automated titrator. Total carbon was determined by combustion in an induction furnace, and the resultant C 0 2 was absorbed in a nonaqueous solution and titrated with a leco titrator with sodium methylate. Additional trace elements on selected samples were measured by inductively coupled plasma emission spectroscopy (ICP -ES) and by inductively coupled plasma - mass spectrometry (ICP-MS) (Nb, Ce) at the Geological Survey of Canada.
Petrography and mineral chemistry Volcanic rocks used in this study have been subdivided into four main groups, based on bulk chemical compositions and geographic location. Groups 1-111 are potassic -ultrapotassic, whereas group IV rocks are alkalic. Although many classification schemes are based on petrographic criteria, we
asa an now. some amygdaloidal
r n Sectbn n~
"I"
Stratlgraphlc columns are combinations of of symbols shown above.
t"
lnmmplefe
~
i
w
have chosen to classify these rocks on bulk chemical criteria because (i) the rocks are variably altered and primary mineralogy is commonly not obvious; and (ii) alkalic and potassic rocks typically contain uncommon mineral assemblages, making petrographic classification schemes difficult at best (Foley et al. 1987).
Group I (Porter Puddle, Macmillan, and Mountain Diatreme) The Macmillian rocks are typically highly altered dykes, flows, and tuffs. The alteration assemblage consists of extensive carbonate, sericite, chlorite, and leucoxene as a result of late-stage hydrothermal activity. Trace quantities of apatite, sphene, pyrite, and chalcopyrite, and more rarely, quartz, are also observed. The Porter Puddle flows, tuffs, and dykes are commonly less severely altered. In some samples,
Can. J. Earth Sci. Vol. 32, 1995 Fig. 4. Volcanic breccia from the upper part of the Porter Puddle Complex (Institute of Sedimentary and Petroleum Geology (ISPG) photograph 1300-17).
Fig. 5. Hand specimen of amygdaloidal basalt with biotite-phlogopite phenocrysts from the Marmot Formation, Porter Puddle Complex. (ISPG photograph 1616-2).
primary mineral assemblages are preserved and include plagioclase, pyroxene, hornblende, and opaques, with chlorite pseudomorphs after olivine. Primary biotite -phlogopite phenocrysts are observed in some samples (Fig. 5). Analyzed feldspars include albite and K-feldspar; plagioclase was only rarely observed. The lack of plagioclase is probably a reflection of the intensity of hydrothermal alteration, although the occurrence of K-feldspar is consistent with the potassic nature of these rocks. Primary clinopyroxene phenocrysts from group I rocks are calcium and magnesium rich and classify as augites
(Fig. 6). Augite compositions and the absence of low-Ca pyroxene are characteristic of alkalic and ultrapotassic volcanic rocks (O'Brien et al. 1991) and support an alkalic affinity for the volcanic rocks of the Selwyn Basin. Most of the clinopyroxene phenocrysts are titanium rich (TiOz = 0.72-4.31 wt. %), also consistent with an alkalic affinity. The Mountain Diatreme has a matrix typically composed of a fine crystalline mass of carbonate with 5- 10% pyrite and other sulfides. Autoliths are composed of an opaque groundmass hosting flow-textured feldspar laths and 5 - 10% anhedral to subhedral phenocrysts of diopside and horn-
Goodfellow et al.
Fig. 6. Ternary classification of pyroxenes in groups I and IV volcanic rocks. NCCM. Ca2Si20,
blende. Phlogopite, biotite, and pyroxene crystals are also present. Phlogopite and biotite phenocrysts are Ti rich, with Ti02 contents ranging from 3.24 to 9.47 wt. % in mica from volcanic rocks and from 1.02 to 5.13 wt. % in mica from the Mountain Diatreme breccia (Table 1). The phlogopite and biotite Mg# ranges from 68.3 to 85.1 and from 40.4 to 64.5, respectively. Celsian and hyalophane are present in several samples from different volcanic centres in the Selwyn Basin. The Ba component of the feldspars ranges from C~0.43to C~51.7 (Table 1). The anorthite and albite components are typically low and range up to Ab50.5 and An11.7, respectively.
Group I1 (Porter Puddle, Niddery, and Macmillan) Group I1 rocks are mineralogically similar to the group I rocks. They typically occur as variably altered dykes, flows, and tuffs. The alteration assemblage consists of extensive carbonate, sericite, chlorite, and leucoxene. Less common alteration minerals include apatite, sphene, pyrite, and chalcopyrite. As with the group I rocks, the Porter Puddle flows, tuffs, and dykes are commonly less altered than the Macmillan and Niddery rocks. Primary mineral assemblages include plagioclase, pyroxene, hornblende, opaques, rare biotitephlogopite, and olivine pseudomorphed by chlorite. K-feldspar and albite are the most common feldspars preserved in these rocks, which is probably a function of alteration. Microprobe analyses show that the micas are Ti rich, with Ti02 contents ranging from 7.25 to 9.12 wt. % . The phlogopite and biotite Mg# ranges from 67.1 to 79.8 and from 50.5 to 64.9, respectively. Hyalophane is also present in several
samples. Only one sample was analyzed by electron microprobe, which yielded a Ba component of to C S ~(Table ~ . ~ 1). The albite and anorthite components are typically low and range up to Ab50.5 and An11,7, respectively.
Group I11 (Vulcan and Itsi lakes) Group 111 rocks include flows, dykes, tuffs, and gabbros. Typically, they are moderately to highly altered, with the alteration assemblage consisting of carbonate, chlorite, and sericite. Primary mineralogy, where observed, is similar to the previous groups. Significance of mineralogy in potassic groups I -111 The mineralogy of the groups 1-111 rocks from the Selwyn Basin is consistent with the geochemical classification as potassic-ultrapotassic (discussed below). The flows and dykes contain Ti-rich phlogopite and biotite phenocrysts and groundmass crystals. Potassic and ultrapotassic rocks are the only volcanic series to commonly host magmatic micas (biotite and phlogopite) and they are typically Ti rich (Wilson 1989; O'Brien et al. 1991; MacDonald et al. 1992). Primary hyalophane has previously been reported as phenocrysts in mafic phonolites, differentiated leucitebearing lavas, and potassic ultramafic - shonkinitic rocks (Larsen 1981). Ba-rich sanidine (up to 11.1 wt. % BaO), which coexists with augite and phlogopite, has also been reported from ultrapotassic minettes from Montana (O'Brien et al. 1991; MacDonald et al. 1992). In the case of volcanic rocks from the Selwyn Basin, the association of Ba-feldspars with ultrapotassic rocks is consistent with a primary origin,
44.72
101.04
96.55
81.54
50.05 1.42 1.51 0.07 1.81 15.90 0.56 7.89 21.02 0.75 0.06
BP0118 I Pyroxene
45.11 1.88 6.17 0.38 4.88 1.13 0.02 13.66 23.21 0.04 0.07
Pyroxene
BP0115 I
83.49
100.12
0.04
51.66 0.72 2.89 0.29 1.39 4.67 0.13 16.74 21.59
789006 IV Proxene
56.04
101.19
50.46 1.16 1.77 0.06 1.21 13.87 0.40 10.57 21.52 0.11 0.06
+
789015 IV Pyroxene
85.14
97.36
9.45
6.70 0.00 21.54 0.01
37.94 5.13 16.59
50.46
97.04
1.31 19.43 0.18 11.10 0.13 0.56 7.45
34.66 7.25 14.97
71.82
94.17
2.05 10.18 0.11 14.56 0.81 0.93 8.15
34.00 8.26 15.12
+
40.44
92.88
6.15
0.01 24.97 0.07 9.51
33.82 4.49 13.86
CJA80-248-8 CJA79-48-54 CJA79-48-51 SL0152 I 11 11 111 Phlogopite Biotite Phlogopite Biotite
1.04 7.61 16.00 99.54
0.86 8.31 15.03 99.05
0.23
21.99
22.34 0.57
52.63
51.98
CJA77-40-4 CJA77-40-4 I I Ba-feldspar Ba-feldspar
0.69 5.34 6.89 2.88 100.71
0.11
21.34
63.46
CJA79-48-71 I Ba-feldspar
n
0.67 12.42 7.17 100.62
1.02
20.25
59.99
Ba-feldspar
CJA79-48-54
+
Note: Pyroxene analyses normalized to 4 cations and 6 oxygens by converting Fez+ to Fe3+ where necessary. Fe" distributed to achieve a cation sum of 2 in the tetrahedral site. Mg# calculatad using total Mg and Fe cations in the octahedral sites (100 MgIMg Fez+ Fe3+).Mica analyses normaliztd to 22 101 equivalents with enough Fez+ converted to Fe3+ [iv] to sum the tetrahedral site to 8 cations. Mg# subsequently calculated using h e Mg:Fe'- ratio of the cations in the octahedral site (100 MglMg Fez+).Ba-feldspar analyses normalized to 16 [O].
Mg#
si ( P P ~ ) Al[iv] Fe3+[iv] Ti Al[vi] Cr Fe3+[vi] Fez+ Mn Mg Ca Na K Ba Total
FeO MnO MgO CaO Na,O K2O BaO Total
Crz03
A1z03
SiO, (wt. 570) TiO,
Mineral:
G ~ O U ~ :
Sample No.:
Table 1. Representative mineral analyses of volcanic rocks from NCCM.
Goodfellow et al. although this is difficult to prove because of the overprinting effects of hydrothermal alteration and metamorphism. If the hyalophane is magmatic, it would support the classification of these rocks as potassic. It is possible that some or all of the hyalophane is hydrothermal in origin, related to BaPb -Zn mineralization in the Selwyn Basin. Celsian and (or) hyalophane are associated with the Howards Pass (Goodfellow and Jonasson 1986), Tom (Goodfellow and Rhodes 1990), and Jason Pb - Zn (Turner 1990) and Gary North barite deposits in the Selwyn Basin. The association of celsian and Ba base metal mineralization has also been reported for the Aberfeldy deposit in Scotland (Fortey and BeddoeStephens 1982).
The NCCM volcanic and igneous rocks commonly contain high C02 and H20 contents due to carbonate and chlorite alteration, respectively (Table 2). Element contents discussed in the text and presented in diagrams have been normalized to a volatile-free basis. Clearly, the highest volatile contents are in large part the result of alteration, but many samples have F abundances well above 0.1 % , suggesting that some of the volatile content is primary, consistent with the alkalic to ultrapotassic nature of the rocks. The presence of phlogopite, biotite, and possibly primary calcite in some samples suggests that high K20, F, C1, C02, and H 2 0 contents partly reflect primary minerals in many of the rocks. The following discussion of chemical variations in the NCCM volcanic rocks concentrates on the more immobile elements. Furthermore, ratios of immobile elements and ternary plots are commonly used to minimize the effects of volume or mass changes related to hydrothermal alteration. Ratio plots also eliminate errors of classification that may have been introduced by the normalization of elements to a volatile-free basis when a component of the volatiles represents primary compositions. Despite the degree of alteration, primary geochemical trends are evident.
The Macmillan group I rocks are highly altered, with C02 ranging from 11.40 to 25.4 wt. %. Si02 contents are very low (28.97-42.51 wt. %). The rocks are mafic with high Cr (267- 1327 ppm) and Ni (126-483 ppm) contents. They are also characterized by highly enriched LILE and elevated immobile elements contents, e.g., Nb (106-208 ppm) and Ce (155 -269 pprn), moderate enrichment of less incompatible high field strength elements (HFSE), e.g., Zr (155 269 ppm) and Ti02 (2.52 - 3.18 wt. %), but low contents of Y (22 -33 ppm) and the heavy rare earth elements (HREE), e.g., Yb (0.92 -2.07 pprn). This trend is evident from the steep slopes for elements plotted on mid-oceanic ridge basalt (MORB) normalized plots (Fig. 9A). The low mobility of most HFSEs, despite obvious alteration, is indicated by the lack of scatter on normalized plots, whereas the large variability of Ba, K20, and Sr demonstrates the susceptibility of large ion lithophile elements (LILE) to alteration (Fig. 9A). On the MORB-normalized diagram, it is evident that there are two groups of samples; one group is more primitive with elevated Cr and Ni contents (Fig. 9A). Compared with average Mountain Diatreme rocks, the Macmillan group I rocks have similar immobile incompatible element contents, and NbIY and ZrINb ratios, but are generally more primitive with higher Cr and Ni abundances. The Porter Puddle Complex group I rocks are commonly less altered than the Macmillan group I rocks, based on C 0 2 (0.2- 17.70 wt. %) and H20 contents. SiOz is highly variable and ranges from 40.21 to 56.69 wt. %. The rocks are typically mafic, though less primitive on average than the Macmillan group I rocks, with Cr ranging from 38 to 694 ppm, and Ni from 52 to 291 ppm. The Nb to Yb MORBnormalized pattern for Porter Puddle Complex group I rocks is similar to other group I rocks, with elevated Nb (124307 ppm) and Ce (91-389 ppm), moderate Zr (142 - 349 ppm) and Ti02 (0.86-5.30 wt. %), and low Y (9 -30 pprn). Less scatter of mobile LILE in the Porter Puddle Complex group I rocks is consistent with lower volatile contents (Fig. 9A). Four samples of the Mountain Diatreme green breccia were analyzed. These samples are altered, with C 0 2 contents ranging from 13.28 to 25.47 wt. % and Si02 ranging from 33.3 1 to 34.00 wt. % . The samples have fairly coherent patterns on a MORB-normalized diagram, and the shape of the pattern is similar to both the Porter Puddle Complex group I and Macmillan group I rocks (Fig. 9A). The Mountain Diatreme samples analyzed in this study are very similar to previously published analyses (Godwin and Price 1986), although the present study samples have lower P2O5 and higher loss on ignition (LOI).
Group I (Porter Puddle, Macmillan, and Mountain Diatreme) These rocks are highly alkalic basanites -nephelinites (Fig. 7) characterized by high Nb contents (typically > 100 ppm) and low ZrINb ratios ( 1 (Wilson 1989). Other chemical characteristics of group I1 ultrapotassic rocks (Foley et al. 1987) include elevated Ba, Ti02, CaO, and Nb, negative K20 spikes, low P205/Ti02ratios, and low Si02. These chemical attributes are also characteristic of the Northern Cordilleran groups I and I1 volcanic rocks (e.g., note the high Nb in Fig. 8, and high Ba and low K20 in Fig. 9). In contrast, groups I and I11 ultrapotassic rocks of Foley et al. (1987), which are influenced by subduction processes, are typically depleted in Nb, Ta, Ti, Ba, and P and enriched in Rb, Th, and K relative to chondrite. The classification of the majority of the volcanic rocks in the NCCM as group II ultrapotassic is therefore consistent with the tectonic setting deduced from other lines of evidence.
Group IV volcanic rocks of the NCCM have chondritenormalized patterns very similar to the Cameroon line of alkalic rocks (Fitton and Dunlop 1985). Group IV rocks are distinguished from other groups of the NCCM volcanic rocks by NaO > K20 in most cases, and lower overall abundances of incompatible HFSE. On a Ce/Y versus Zr/Nb plot (Fig. lo), groups I and I1 volcanic rocks of the NCCM plot well above the OIB field, indicating a source more enriched in the LILE and LREE compared with OIB. These chemical characteristics are consistent with partial melting of a previously metasomatized lithospheric mantle. Group IV rocks plot within the OIB field, again indicating that they are more asthenospheric in character than the other groups. The group 111 volcanic rocks show scatter, plotting within and above the OIB field, suggesting that they represent variable mixing between lithospheric and asthenospheric partial melts. The OIB character of the group IV volcanic rocks of the NCCM may indicate greater lithospheric attenuation at that location, allowing passage of the 0IB-type magmas to the surface. For example, Gibson et al. (1993) found that on the rift flanks and shoulders of the Rio Grande Rift, K-rich volcanism dominated, whereas at the rift zone axis tholeiitic and alkalic basalts with compositions similar to those of OIB were erupted. Volcanism within the Selwvn Basin can therefore be modelled within the framework6f an episodically subsiding passive margin type continental rift. Upwelling asthenosphere and (or) decompression due to deep fractures associated with rift-parallel-normal and transform faults casued partial melting of old, metasomatized (Ba, Nb, light rare earth element enriched) subcontinentallithosphere. This partial melt produced the groups I and I1 Northern Cordilleran volcanics rocks. Where crustal attenuation was sufficiently advanced, the partial melting of an upwelling OIB-type asthenosphere produced more LILE- and HFSE-depleted magmas, such as the group IV Northern Cordilleran volcanic~rocks. Group I11 rocks are more depleted in the incompatible elements than groups I and 11, and are transitional t o group IV in trace element ratios.
Conclusions The lower Paleozoic volcanic rocks of the Yukon territory are alkalic to ultrapotassic. On the basis of geochemistry and geographic location, the rocks studied were subdivided into four main groups. Group I rocks (Porter Puddle Complex, Macmillan volcanic rocks, and Mountain Diatreme) are characterized by high Nb, Ce, and Nb/Y and low Zr/Nb. Group I1 rocks (Porter Puddle Complex, Niddery, and Macmillan volcanic rocks) are also potassic, but have lower contents of Nb and Ce, higher Zr/Nb, and lower Nb/Y. Group 111 rocks (Vulcan and Itsi Lakes) are chemically variable, contain highly elevated Ba contents, and are also potassic ultrapotassic, but have lower HFSE than the groups I and I1 rocks. Groups 1-111 are also characterized by mica (biotite and phlogopite) phenocrysts, sanidine, salite, and Ba-feldspar. Group IV (Whale Mountain) alkali basalts (Na20 > K20) are the least enriched in LILE and have relatively low HFSE compared with groups I and I1 basalts. The composition of groups I-111 is consistent with the partial melting of a previously metasomatized lithospheric
Can. J. Earth Sci. Vol. 32, 1995 mantle source that was variably enriched in Ba, Nb, and Ce. Group IV rocks, however, are more typical of asthenospherically derived OIB partial melts. The geochemistry and petrogenesis of the Northern Cordilleran volcanics are consistent with paleotectonic models of the Selywn Basin. The Selwyn Basin is a passive continental rift that was episodically reactivated and underwent several stages of subsidence from Cambrian to Late Devonian time. Alkalic volcanism and spatially and temporally associated SEDEX Ba-Pb-Zn deposits are localized along riftparallel and transform faults that have been reactivated. This model for the tectonic evolution of the Paleozoic Cordilleran margin is consistent with earlier models proposed on the basis of sedimentary facies and thicknesses, and the history of thermal subsidence of the margin.
Acknowledgments The authors thank V. Ansell for assisting with the petrography and mineral chemistry. C . Gregoire and G.E.M. Hall are thanked for supervising the chemical analyses. W e are also grateful to E. Anderson, L.A. Dunworth, J. Pell, S. Goff, and D. Francis for reviewing the paper and offering many suggestions, many of which were incorporated into the final version of this paper.
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