Existence of a marginal basin within the Circum-Superior Belt: geochemical evidence from the Churchill-Superior boundary in Manitoba, Canada

Precambrian Research, 49 ( 1991 ) 167-183 167 Elsevier Science Publishers B.V., Amsterdam

Existence of a marginal basin within the Circum-Superior Belt: geochemical evidence from the Churchill-Superior boundary in

Manitoba, Canada

Norman M. Halden Department of Geological Sciences, University of Manitoba, Winnipeg, Man. R3T 2N2 (Canada)

(Received July 18, 1989; revision accepted May 7, 1990)

ABSTRACT

Halden, N.M., 1991. Existence of a marginal basin within the Circum-Superior Belt: geochemical evidence from the Churchill-Superior boundary in Manitoba, Canada. Precambrian Res., 49:167-183.

Mafic volcanic rocks incorporated within the Churchill-Superior Boundary Zone at Ospwagan Lake, Moak Lake, As- scan Lake and the Fox River Belt constitute part of the Circum-Superior Belt. Major and trace element data have been obtained from mafic and ultramafic volcanic rocks from four localities. The data provide a means of comparing and contrasting the geochemistry of the rocks from the portion of the Circum-Superior Belt surrounding the Archean Superior Province in Manitoba with other units from elsewhere in the belt, and with rocks from the adjacent Proterozoic Churchill Province.

Major element geochemical data are consistent with the rocks having originally formed in a oceanic environment. Trace element and rare earth element data suggest that the environment was a marginal basin. AI203/TiO2 ratios indicate that the magmas were derived from a mantle source by small degrees of partial melting. High field strength element patterns show that the mantle source region giving rise to the rocks was relatively uniform in composition.

Trace element, mid-ocean ridge basalt (MORB) and chondrite-normalized patterns indicate similarities in alkali element enrichment for the rocks at Assean and Ospwagan Lakes. Virtually all the rocks have Ti, Zr, Nb, Y and P levels < MORB, which is consistent with the mantle source region having undergone a small degree of partial melting. Zr /P ratios for the rocks from the Fox River Belt are consistently lower than those from the other localities, and the Assean Lake rocks show slight enrichment in light rare earth elements, which may suggest local compositional variability in the mantle source regions supplying the magmas to the Fox River Belt and the Ospwagan, Assean and Moak Lake areas.

Introduction

The Churchill Province and Churchill-Su- perior Boundary Zone (Bell, 1971 ) are located in northern Manitoba, Canada (Fig. 1 ). They form part of the Trans-Hudson Orogen (Hoffman, 1988) and are a complex amal- gamation of tectonic units comprised of deformed intra-oceanic rocks, continental mar- gins and magmatic arcs (Hoffman, 1988). Analogies have been drawn between this orogenic terrane and those produced by plate tectonic processes (Lewry, 1981; Stauffer, 1984; Green et al., 1985); ophiolites and ophiolitic

assemblages are noticeably absent from the re- constructions, although they are present in other segments of the orogen, e.g. the Cape Smith Belt (Scott et al., 1988). Remnants of such rocks would play a pivotal role in the interpretation and elucidation of the Trans- Hudson Orogen in northern Manitoba.

Mafic and ultramafic volcanic rocks outcrop at a number of localities within the Churchill- Superior Boundary Zone (Fig. 2; see Baragar and Scoates, 1981, 1987; Hoffman, 1988). These volcanic assemblages, including intercalated marine sediments (Stephenson, 1974; Scoates, 1981 ) occur at the contact between the

168 N.M. HALDEN

m SEGMENTS OF THE C~ICUM-SU~ERIOR BELT

[ I ~ 0 k m s I

c~ Belt

U

%.

o

Labrador Trough

Fig. 1. Schematic geological map showing the relationship between the Superior Province, the Churchill Province and the Churchill-Superior Boundary Zone, and the distribution of Circum-Superior Belt rocks (after Baragar and Scoates, 1981 ). The southern and southwestern limit of the boundary zone is the limit of late Hudsonian metamorphism overprinted on the Superior craton. Major terrane boundaries within the Churchill Province are indicated as dotted lines; the Churchill Province consists of granitic, metasedimentary and metavolcanic terrane. The Churchill Province in Manitoba is the southern, juvenile part of the Trans-Hudson orogenic terrane (Hoffman, 1988). The box outlines the area shown in Fig. 2.

Proterozoic Churchill Province and reworked Archean basement rocks of the Superior Prov- ince. The volcanic rocks which provide the fo- cus of this study outcrop at Ospwagan Lake, Moak Lake, Assean Lake and along the Fox River Belt (Fig. 1 ). The relationship between the mafic and ultramafic volcanic suites surrounding the Superior craton (Circum-Supe- rior Belt; see Fig. 1 ) has been discussed by Bar- agar and Scoates ( 1981, 1987 ). They suggested that there was sufficient lithological and stratigraphic continuity to relate rocks from the Labrador trough to the volcanic rocks of the Thompson mobile belt in Manitoba despite the severe deformation imposed by the Hudson- ian Orogeny. Identifying the tectonic setting of these mafic and ultramafic assemblages within the Trans-Hudson Orogen presents tantalising problems, and the extensive deformational history has hampered correlation and discrim-

ination of their environment. Hoffman (1988) made direct reference to the tectonic significance of the magmatism being unclear.

A plate tectonic model for the Trans-Hud- son Orogen was proposed by Green et al. ( 1985 ). This model involved the rifting of an Archean craton, which was estimated to have occurred at ca. 2.3-2.4 Ga, and the creation of Proterozoic oceanic crust. Geochronological data, an 1883-Ma U-Pb age for the Fox River Sill (Krogh et al., 1986) and an 1855 Ma Rb- Sr age for metasediments associated with the Ospwagan Lake volcanic rocks (Brooks and Theyer, 1981 ) would indicate that these rocks were not connected with the original rifting event (Krogh et al., 1986). No age data are available at present for the volcanic rocks at Assean Lake, and for the purpose of this study these rocks are assumed to be similar in age to the Fox River Belt.

E X I S T E N C E O F A M A R G I N A L B A S I N W I T H I N T H E C I R C U M - S U P E R I O R B E L T 169

Recent structural and metamorphic studies in the region have resulted in a better under- standing of the evolution of the Churchill-Su- perior boundary in the form of an oblique collision zone (Bleeker, 1990). Bleeker and Macek (1988) have also shown that the relationship between dykes and metasediments and metavolcanic rocks in the boundary zone is locally very complex: dyke-rocks cut both supracrustal and ultramafic bodies at a number of localities. The present arrangement of lithological and tectonic units represents a post- collisional, modified geometry; any original association between units is likely to have been masked by deformation and dislocation along major fault boundaries.

The stratigraphy, petrology and geochemistry of the Fox River Belt was studied by Scoates (1981); its development was associated with rifting and its present attitude is a function of later deformation. The possibility that at least the Fox River Belt was obducted onto the Ar- chean margin cannot be excluded when it is compared with other mafic volcanic suites of similar age, morphology and orogenic environments from elsewhere in the world (Park et al., 1984; Park, 1988; Scott et al., 1988). A consequence of the oblique collision model

proposed by Bleeker (1990) is that rocks of the Churchill Province were thrust onto the Supe- rior margin; such a process could have in- cluded some of the mafic volcanic rocks of the Circum-Superior Belt in Manitoba. An obduc- tion mechanism was proposed for the emplacement of the Purtuniq ophiolite in the Cape Smith Belt (Scott et al., 1988 ).

The objective of this study is to present some of the major and trace element geochemical characteristics of mafic and ultramafic volcanic rocks within the Churchill-Superior Boundary Zone (with a view to determining their tectonic setting) and to compare and contrast these characteristics with those of other similar rocks from the Circum-Superior Belt and mafic volcanic rocks from the Churchill Province. Trace element data have hitherto been unavailable for the Fox River Belt, although some trace element data are available for the Ospwagan volcanic suite (Paktunc, 1984). Major, trace and rare earth element data were obtained from volcanic rocks from Moak Lake and Assean Lake, Man- itoba (see Fig. 2). Trace, and rare earth element concentrations were determined on samples of Fox River Belt rocks (collected by Scoates, 1981 ). A suite of samples from Osp-

: : : : : : ~:: : ' > : ' : ' : ' g a ° w ' : ' : ' : ' : ' : . . . . . . . " . . . . " . g 6 ' ~ ' : r ' . ' - - ~ - ~ : ~ - g 4 v ' , . . . . . . " " ~ l ~ . z ~ " . . . . ~' "

q .~=eoc m s mls i ~ flc vokanics ) 8OUN(3s~RY _ ~ . ~ ~ m s h z [ s ( ~ f . icvokonics I ZONE M~mo*il* ~ gneiss . I

Fig. 2. Schematic geological map of north-central Manitoba, showing the distribution of Proterozoic supracrustal rocks which include the mafic and ultramafic volcanic rocks of the Circum-Superior Belt.

1 7 0 N.M. HALDEN

wagan lake was analysed for major, trace and rare earth elements to retain analytical continuity for comparison with the other suites.

Geological setting of the mafic and ultramafic volcanic rocks

Ospwagan Lake

The geology of the Ospwagan Lake area has been described by Stephenson (1974), and the area has also been mapped by Macek and Rus- sel (1978). The sampling for this study was based upon the later mapping, as this provided a more detailed lithological subdivision of the mafic and ultramafic rocks. Because of intense deformation associated with the Hudsonian Orogeny in the area, neither study addressed the potentially critical nature of the internal structural relationships between the volcanic units. The metavolcanic rocks are structurally interleaved with metasedimentary rocks and strike NE-SW. This orientation is the dominant structural trend within the Churchill-Su- perior Boundary Zone. The orientation is characterised by zones of cataclasis parallel to near-vertical fold axial planes which strike NE- SW and plunge, at the southern end of lower Ospwagan Lake, to the NE. Bleeker and Macek ( 1988 ), Bleeker (1990) and Hubretsge (1980) provided details of the deformational history of the Thompson Belt and the Pikwitonei granulite domain.

The structural setting of the volcanic rocks is complex. In addition to upright folds and shear zones, there are low-angle shear zones (dipping ~20 ° north)between some of the lithological units. These zones are typically narrow ( ~ 10 cm wide) and include small asymmetrical 'S' folds. The folds' axes dip at ~ 20 ° to the NE at the southern end of upper Ospwagan Lake. The low-angle shear zones appear to be deformed by (and hence predate) the upright folding, and may be thrust planes related to an earlier phase of deformation. The fold asymmetry would suggest overthrusting

from the north. The significance of these features cannot be ignored if any stratigraphic significance is to be attached to internal petrol- ogical and geochemical variations within the volcanic pile. Petrogenetic modelling of the Ospwagan volcanic rocks (Paktunc, 1984) suggests that the mafic and ultramafic rocks are related by the low-pressure fractional crystal- lisation of olivine and clinopyroxene.

The rock types range from pillowed and massive ultramafic metavolcanic rocks (including units described as sill-like porphyritic picrites ) to pillowed and massive metabasaltic rocks. Mineralogically, the only 'primary' volcanic features now reported, beyond the pillowed nature of some of the rocks, are altered olivine phenocrysts (Stephenson, 1974) and relict 'quench' textures (Paktunc, 1984). The metamorphosed ultramafic rocks are predom- inantly serpentine, tremolite and chlorite. The meta-basaltic rocks are comprised of hornblende and plagioclase, with minor chlorite.

Assean Lake volcanic rocks

Volcanic rocks at Assean Lake have not been studied to the same extent as those of the Fox River Belt and Ospwagan Lake. The Assean Lake area was mapped by Haugh ( 1969 ). The principal feature, which dominates the geology of the area, is a major shear zone, striking NE- SW, which bisects the lake. Haugh ( 1969 ) did not report any primary igneous structures within a large area of meta-amphibolite at the northern end of Assean Lake that would have been consistent with a volcanic origin. There are, however, a number of occurrences of flattened pillowed volcanic rocks and volcanic breccia units. The Assean Lake area is of inter- est as it lies between Ospwagan Lake (in the Thompson district ), and the Fox River Belt (at the northern Margin of the Superior Prov- ince). It has been suggested that a tentative correlation can be made between the Ospwa-

EXISTENCE OF A MARGINAL BASIN WITHIN THE CIRCUM-SUPERIOR BELT 171

gan Lake sequence of marine metasediments and volcanic rocks and some of the metasedimentary and volcanic units at Assean Lake (R.F.J. Scoates, personal communication, 1988).

The mafic rocks at Assean Lake occur as tectonically isolated boudins and layers, within, and adjacent to, the main shear zone that sep- arates the Churchill and Superior Provinces at this location. The mafic amphibolites along the NE shore of the lake constitute the main body of mafic rocks. At the southern end of the exposure some of the amphibolite layers retain flattened pillow-like features, and have bleached core regions with darker rims, possibly a relict chill feature. Other layers appear to be fragmental in nature, with representatives of both the bleached and darker material present as irregular, often flattened blocks within a homogeneous matrix. The contacts between individual layers or lithological units through- out the lakeshore outcrop are invariably poorly exposed. They are probably tectonic, and so would obscure any original stratigraphic relationships.

Massive and layered amphibolite occur in- "terdigitated with those layers that have retained relict volcanic features. They are volu- metrically more significant than either the pillowed or fragmental units, but their origin (as either flows or sills ) remains enigmatic because of poorly preserved contact relationships and recrystallization. Both amphibolite types tend to be equigranular, and range between coarse- and fine-grained varieties. Oc- casionally, a mineral alignment of amphibole can be seen within intensely schistose zones, normally of the order of a few centimeters wide. The layered amphibolites are distinguished from the massive variety on the basis of layering which is imparted by alternating (on the scale of 1 mm to ~ 2 cm) feldspathic and amphibole-rich layers. The massive amphibolite lacks the layering and is more homogeneous. Mineralogically, the amphibolites consist of actinolitic hornblende and plagioclase with

minor amounts of chlorite and accessory quartz.

Massive and sporadically altered ultramafic rocks also occur (possibly as boudins) at the southern extremity of the mafic rock outcrop. One unit of particular note is a massive coarse- grained orthopyroxene-bearing rock, pale buff in colour, that is cross-cut by dark green anas- tamosing schistose layers. Mineralogically, this unit contains olivine, garnet serpentine and magnesite. This assemblage may represent the metamorphosed equivalent of the surrounding orthopyroxene assemblage. The contacts between this and the other mafic rocks are not exposed. It is possible that this unit could be a tectonically dismembered portion of a differentiated sill, analogous perhaps to what is observed at the Fox River Sill (Scoates, 1981 ). Haugh ( 1969 ) reported altered tremolite- and carbonate-bearing rocks from the SE end of Assean Lake. The mineralogical assemblage and textural features are similar to those described by Park (1983) in altered ultramafic rocks from the Outokumpu assemblage, Finland.

Fox River Belt

The geology of the Fox River Belt has been described by Scoates ( 1981 ). It forms part of the Circum-Superior Belt adjacent to the northern margin of the Superior craton (Bar- agar and Scoates, 1981, 1987). The Fox River Belt volcanic rocks are intercalated with sediments and differentiated sills. Scoates ( 1981 ) subdivided the volcanic rocks into lower and upper volcanic formations. These were further subdivided into massive, layered and pillowed flows. Mineralogically, the rocks contain olivine + clinopyroxene + orthopyroxene + feldspar and display a variety of textures that include spinifex and cumulate assemblages. The rocks of the Fox River Belt have been metamorphosed to low greenschist facies, which is in contrast to the amphibolite facies meta-

172 N.M. HALDEN

morphic assemblages observed at Assean and Ospwagan Lakes.

Moak Lake

The geology of the Moak Lake area was mapped and described by Patterson (1963), and was remapped by Scoates and Macek (1977). Patterson (1963) referred to the sed- imentary rocks, which host mafic volcanic rocks in the vicinity of the Moak Lake mine- site, as being part of the 'Assean Lake Group'. In the immediate area of the Moak Lake mine- site, exposure is poor and precludes any detailed interpretation of structure and stratigraphy. This sequence of metasedimentary rocks (quartzites, phyllites, skarns, mica- schists and iron-formation) occurs between Ospwagan and Moak Lakes. The amphibolite samples analyzed in this study come from an outcrop located 4.5 km SW of the Moak Lake mine-site. Mineralogically, the rocks consist of mainly amphibole and plagioclase feldspar, and biotite and chlorite occur as secondary alteration products after amphibole. The sporadic presence of garnet, in conjunction with the other minerals, would be consistent with amphibolite facies metamorphism. Scoates and Macek (1977) reported igneous layering within the outcrop, and Patterson ( 1963 ) referred to some units of the mafic amphibolites as possibly being pillowed. Observations made during this study did not confirm unequivo- cally the presence of pillowed structures.

Geochemistry

This study reports new major and trace element data for mafic and ultramafic volcanic rocks from Ospwagan, Moak, and Assean Lakes and new trace element and rare earth element (REE) data for the volcanic rocks from the Fox River Belt. A representative selection of the data is given in Table 1. The pre- fixes 'O', 'A86' and 'MK' refer to samples from Ospwagan, Assean and Moak Lakes respec-

tively. Other sample numbers are those for the Fox River Belt, and these correspond to the sample numbers from Scoates ( 1981, table 2 ). Samples from Assean and Ospwagan Lake were collected from available shoreline outcrops.

All samples were prepared as fused glass discs (excluding those of the Fox River Belt ) in the manner described by Harvey et al. (1972) for major element analysis, and as pressed powder pellets (Leake et al., 1969) for trace element analysis. All analyses were performed on an ARL wavelength-dispersive 8420 X-ray spec- trometer, and trace elements were analyzed using the Compton scatter technique.

Major element geochemistry

Many studies aimed at describing the genesis of basaltic rocks have attempted to apply various geochemical criteria to identify and exclude from consideration those rocks that have been affected by weathering (e.g., Gas- karth and Parslow, 1987 ) or crystal fractionation. Baragar and Scoates ( 1987 ), using MgO/ CaO ratios, identified those rocks in the Cir- cum-Superior Belt that were affected by crystal fractionation and accumulation. Rocks from Scoates' ( 1981 ) study that show the effects of crystal accumulation have been excluded from this study. The rocks from Ospwagan, Assean and Moak Lakes all show the effects of amphibolite facies metamorphism, and in many cases the effects of intense deformation; therefore no primary mineralogical reason for excluding any rocks from this study has been, or can be, observed. In addition there are few macroscopic igneous features, including contact relationships, that are well preserved. The problem, ir- respective of the present conditions of the rocks, as to what the rocks were and in what tectonic environment they were developed, remains.

Figure 3 shows the distribution of the data on the classification diagram of Pearce et al. (1975). The data straddle the boundary between oceanic and continental environments.


TABLE 1

Major (wt.% oxide), trace (p.p.m.) and rare earth element (p.p.m.) data for a representative selection of rocks from Ospwagan Lake ('O' prefix ), Assean Lake ('A86' prefix ), Moak Lake ( 'MK' prefix ) and the Fox River Belt ( 57-4, 77-2, 77-4 )

Sample A865 A8611 A8612 A8614 A8680 A8682 57-4 77-2 77-4 MKI MK2 04 06 08

SiO2 52.03 52.75 49.71 48.68 50.46 50.99 46.85 46.50 51.20 50.57 49.65 49.46 48.46 50.60 TiO2 0.94 0.97 0.87 1.25 0.60 0.57 0.76 0.66 0.73 1.15 1.24 0.63 0.66 0.57 AI203 14.97 13.67 13.81 14.91 14.60 13.32 11.79 1 0 . 1 1 12.06 13.19 13.50 10.87 11.00 8.53 Fe203 10.71 9.50 13.48 13.07 11.13 11.74 1.49 1.50 1.62 14.51 14.76 11.21 11.71 9.32 FeO 0.00 0.00 0.00 0.00 0.00 0.00 10.26 9.44 7.29 0.00 0.00 0.00 0.00 0.00 MnO 0.21 0.21 0.26 0.18 0.19 0.23 0.19 0.19 0.15 0.26 0.24 0.16 0.17 0.21 MgO 6.09 5.54 7.62 8.09 7.82 8.43 10.65 15.90 11.50 7.22 6.42 14.12 13.16 14.92 CaO 10.38 10.63 11.29 10.07 11.47 10.62 12.30 10.04 9.24 9.98 9.95 11.11 12.33 14.53 Na20 3.35 1.92 1.82 2.00 1.03 2.12 0.92 1.13 2.75 2.20 2.58 1.27 1.59 0.95 K20 0.63 2.12 0.54 0.34 1.08 0.85 0.04 0.10 0.18 0.17 0.21 1.21 0.55 0.55 P205 0.09 0.09 0.07 0.10 0.04 10.03 0.12 0.07 0.08 0.06 0.09 0.05 0.04 0.03 H:O + 0.00 0.00 0.00 0.00 0.00 0.00 3.94 4.09 2.91 0.00 0.00 0.00 0.00 0.00

Total 99.40 97.30 99.47 98.69 98.42 98.90 99.30 99.73 99.71 99.31 98.64 100.09 99.67 100.71

Rb 8 62 15 8 35 13 4 6 4 0 0 92 13 63 Sr 83 157 128 129 87 100 15 57 117 94 114 78 94 90 Y 28 29 27 30 16 18 18 16 17 25 30 14 19 13 Nb 3 3 0 0 0 0 3 4 3 0 0 4 4 0 Zr 76 83 69 85 35 35 28 22 33 52 64 39 41 26 Ba 140 340 90 130 90 90 50 50 270 50 100 170 80 100 Th 2 3 1 l 1 1 0 0 0 0 0 0 0 0 Ta 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Sc 57 46 53 50 50 48 54 43 49 60 58 44 45 81 Cr 160 120 110 250 360 360 930 1600 1500 26 20 1500 1400 2400 Hf 3 2 3 3 1 1 1 1 1 1 1 1 1 1 Ni 50 50 70 50 110 70 130 320 220 50 80 250 320 180

La 9.3 11.2 8.0 7.3 3.1 2.4 2.8 2.2 1.7 2.8 3.0 2.7 3.0 1.8 Ce 29.0 30.0 23.0 17.0 9.0 7.0 12.0 7.0 6.0 8.0 8.0 8.0 8.0 10.0 Nd 13.0 16.0 11.0 13.0 4.0 4.0 6.0 5.0 3.0 6.0 6.0 5.0 4.0 3.0 Sm 3.3 3.1 2.9 3.3 1.3 1.4 1.6 1.8 1.3 2.2 2.2 1.6 1.5 1.3 Eu 1.1 1.2 0.8 1.0 0.6 0.5 0.6 0.6 0.5 0.8 0.9 0.6 0.5 0.4 Yb 2.8 2.7 2.6 3.4 1.5 1.6 1.7 2.1 1.3 2.5 2.5 1.6 1.6 1.4 Lu 0.4 0.4 0.4 0.5 0.3 0.2 0.3 0.3 0.2 0.4 0.4 0.2 0.3 0.2

Fe is total Fe as Fe~O3 (X-ray fluorescence analysis) except in the case of the Fox River Belt samples. Rb, Sr, Y, Nb and Zr were determined by X-ray fluorescence; other trace and rare earth elements were determined by instrumental neutron activation analysis.

The spread of data towards the K20 apex could be interpreted as the product of alkali element mobility during either weathering or metamorphism. Arculus ( 1987 ) has highlighted the difficulty of interpreting alkali element enrichment by suggesting that alkali elements and alkali earth elements are mobile in the presence of aqueous fluids at the base of the continental crust and in subduction zones - at the point where the magmas are produced. Rocks from each locality occur well within the oceanic field,

which suggests, at least for some, an oceanic affinity. Criteria used in the classification and comparison of the rocks have to depend upon those elements that are considered relatively immobile under conditions of metamorphism and weathering, and major elements to some degree will always be considered suspect.

Sun and Nesbitt ( 1976 ) argued that A1203/ TiO2 and CaO/TiO2 ratios in mid-ocean ridge basalts vary with the degree of partial melting and the nature of the source region being

174 N.M. HALDEN

T I O 2

K2o P,o,

AS.T~AFI t A K E FOX RIVER BELT O Ioyemd omphlbo l i le • layered f low

, m o ~ ,. r ' l m m ~ -

I p;l low fragments .', p;Ik:,vmd •

OSF~NAGAN LAKE M O A K LAKE

- meto-plc,rit e

\ p ; Ik )v~d bosolt

Fig. 3. K20-TiO2-P2Os (wt.% oxide) ternary plot showing the continental and oceanic fields described by Pearce et al. ( 1975 ). The symbols used in this plot for the various rocks are used subsequently in Figs. 3-7 and ! 2.

melted. Figure 4a and b shows the data from this study on Al203/TiO2 and CaO/TiO2 vs. TiO2 plots. Low-TiO2 ( < 0.6%) rock types occur at each locality but represent a minority of the samples analyzed. Sun and Nesbitt ( 1976 ) pointed out that rocks which are low in TiO2 might be expected to have high AI203/TiO2 and CaO/TiO2 ratios (as high as 60). With such values it has been suggested that the rocks could have been associated with incipient spreading close to a subduction zone (cf. Gas- karth and Parslow, 1987 ). For this study, TiO2 values range as high as 1.54%; these values are associated with low Al203/TiO2 and CaO/ TiO2 ratios. These rocks fall in the mid-ocean ridge basalt (MORB) field described by Sun and Nesbitt (1976). The data in this study also show relatively restricted A1203/TiO2 ratios (between 8 and 28 ) and CaO/TiO2 ratios (between 5 and 28 ). The ranges encompass chon-

5O

4O

2o;

2O

I 0

0 0 . 0

A

o \

_ . .. " . . • ..~. ~ ' : . . . ~ . . M O I ~

o - . . . ' .2Z ' .~ . . ' :

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50

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O "K . . . . " " . . . . . . ..... 0 . . . . . ]:

0 . 5 1 .0 1 . 5 2 . 0

T|O a

Fig. 4. CaO/TiO2 vs. TiO2 and AI203/TiO2 vs. TiO2 plots (wt.% oxide). The dotted line indicates the limits of the MORB field described by Sun and Nesbitt ( 1976 ).

dritic values, 20 and 17 respectively, for these ratios. The mantle from which these rocks were derived was probably comparatively primitive and would not have undergone any great degree of partial melting. High TiO2 values allied with restricted ranges for the Al203/TiO2 and CaO/TiO2 ratios were interpreted by Sun and Nesbitt ( 1976 ) to be consistent with an origin in a mid-ocean ridge, an inter-arc basin or an island arc.

The Al203/TiO2 ratios show a much more restricted range than those published by Gas- karth and Parslow ( 1987 ) for the East Amisk volcanic rocks of the Flin Flon belt, which range between 5 and 95. If these very high ratios were indicative of a high degree of partial melting of the mantle in the source region, this would preclude any simple association be-

EXISTENCE O F A M A R G I N A L BASIN W I T H I N T H E CIRCUM-SUPER1OR BELT 175

tween the source region (s) for the East Amisk volcanic rocks and those fringing the Superior craton margin.

The fractionation history of the volcanic rocks at Ospwagan Lake has been discussed by Paktunc (1984). At this locality the dominant influence on major element chemistry appears to have been the fractionation of olivine and clinopyroxene. Baragar and Scoates (1987) came to a similar conclusion; however, this lat- ter work was also aimed at determining the fractionation history of other volcanic rocks in the Circum-Superior Belt. Variations in the CaO content of the rocks is considered to be dependent upon the relative amounts of clinopyroxene and olivine that have fractionated. Baragar and Scoates (1987) used A1203 and TiO2 as measures of evolution of the rocks studied towards more felsic compositions. Fig- ure 5a and b shows plots of MgO/CaO vs. A1203 and TiO2 for comparison with the fields of Baragar and Scoates (1987). The work of Baragar and Scoates (1987) defined two distinct fields for the Ospwagan and Fox River Belt rocks: those influenced by olivine and those influenced by clinopyroxene fractionation. The data from this study show a more or less continuous distribution for the Fox River Belt and Ospwagan volcanic rocks; the only significant difference is the clustering of the data points for the Assean volcanic rocks. Based on these diagrams, the most significant influence on the Fox River Belt and Ospwagan volcanic rocks was the fractionation of olivine. Clinopyroxene probably had a more impor- tant influence on the crystallization history of the volcanic rocks at the Assean Lake locality.

An attempt to discriminate the likely tectonic setting of the volcanic rocks, beyond an oceanic or continental setting, at the four localities on the basis of major element trends does not result in an unambiguous assignment of tectonic environment. The data in Fig. 4a and b overlap the MORB field. An additional limitation to using the CaO/Ti02 vs. TiO2 plot may be the mobility of Ca during weathering

o" 4

2 . 0

1 .5

1 . 0

0 . 5

0 . 0

A !

° ° ~ - o ~, ~ Llac

, .

I I I I 2 3

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10 -- +

5

0 I 1

i i

_ ~ . . . . ~ c p x

• A

A A

I I 2 3

Fig. 5. TiO2 vs. MgO/CaO and A1203 vs. MgO/CaO plots (wt.% oxide). Arrows indicate fractional crystallization vectors for olivine (ol) and clinopyroxene (cpx) used by Baragar and Scoates (1987).

and alteration (see Gaskarth and Parslow, 1987). On the basis of major element data, it is only possible to conclude that the rocks have an oceanic affinity, and may have been derived from a relatively primitive mantle source region. The possibility that their environment of emplacement could have been transitional to either an island-arc or marginal-basin type of setting cannot be excluded.

Trace element geochemistry

Trace element variations provide a means for both a comparison of the rocks, and a po- tential means for tectonic discrimination of the rocks. T h e high field strength elements (HFSE) (Nb, Ti, Zr, Y), are considered to be immobile in aqueous solutions associated with

176 N.M. HALDEN

weathering and metamorphism (Pearce and Norry, 1979).

Discrimination of tectonic setting on the basis of TiO2 vs. Zr (Fig. 6) shows a spread of data within the MORB and arc-lava fields. As the rocks do not reflect a true MORB chemistry, this suggests that they may have been derived in some sort of anomalous or transitional tectonic environment. The data show a similar distribution to those of Pearce et al. ( 1981 ) for the Troodos complex, which was interpreted as being a Cretaceous arc-basin complex, and the Outokumpu assemblage, which appears to have evolved in a Protero- zoic arc-basin type of complex (Park, 1988 ). The data are also shown in Fig. 7, which includes the tectonic discrimination fields of Pearce and Cann ( 1973 ). These diagrams lead to a similar conclusion, i.e. a strong indication of an oceanic affinity for the rocks (to the ex- clusion of any data falling in the within-plate basalt field). However, the data straddle the boundary between the island-arc tholeiite and calc-alkaline basalt fields. Figure 8 is a log Cr vs. log Y discrimination diagram. The data straddle the boundary between the MORB field and the volcanic-arc basalt field. The MORB field encompasses all varieties of MORB, and most of the data plot at the left side of the

Ti/lO0

i

o ~ . J

I ,I I 0 I O0 1000

Log Zr

Fig. 6. Log TiO2 vs. log Zr plot including field boundaries from Pearce ( 1981 ). The bulk of the data plots above the basic vs. evolved boundary and within the arc lava or MORB field.

T I / I O 0

Z r S t / 2

10 Zr Y*3

Fig. 7. Z r - (T i /100 ) - (S r /2 ) and ( T i / 1 0 0 ) - Z r - ( Y × 3 ) ternary plots with the discriminant field boundaries of Pearce and Cann (1973). OFB refers to ocean-floor basalt, IAB to island-arc basalt and CAB to calc-alkaline basalt.

MORB field (i.e. Y values are low); this suggests that the rocks were not an enriched variety of MORB. The two data points for the Moak Lake amphibolites plot with comparatively low Cr values; this could be considered unusual as these units are associated with serpentinites at Moak Lake, which have high Cr abundances.

EXISTENCE OF A MARGINAL BASIN WITHIN THE CIRCUM-SUPERIOR BELT 1 7 7

1 0 0 0 0

I 000

o~ 100

_1

1 0

VOLCANIC A R C

B A S A L T

I i

1 0

L o g Y

i ' - - " x

+~

~. ~ ....

~,~o "~

~ •

i ' ,~ i ',~ '

1 O0

Fig. 8. Log Cr vs. log Y plot (p.p.m.). Field boundaries after Pearce (1982). The data points fall within the area described by the overlap of the volcanic arc basalt and MORB fields.

Figure 9 shows chondrite-normalised REE plots of rocks from the four localities (normalizing values are from Thompson et al., 1983 ).

The patterns for the Ospwagan and Moak Lake rocks and the Fox River Belt rocks are flat, and about 10× chondrites. The data for the As- sean Lake rocks show a slight light REE (LREE) enrichment, similar to transitional type MORB, and a flat heavy REE (HREE) pattern, again in the region of l 0 × chondrites. The patterns are broadly similar to those for other mafic volcanic rocks in the Circum-Su- perior Belt (see Baragar and Scoates, 1987, fig. 8a).

Recent work by Arculus ( 1987 ) has focused on the problems associated with data that straddle the boundaries of discriminant diagrams. The problem is made more complex because alkali earth (AE) elements, REE and HFSE might be fractionated in the presence of hydrous fluids in the region of magma genesis, or through interaction with silicic melts, which would tend to confuse the geochemical signa-

I O0

A . . . . . 1 O layered flow 1 * massive ,, J A pillowed ,, 1

I t t t I J f ; [ z I I I I I I

Le Co Nd Sm [u Gd Oy Yb L~

100

10

I I I I I r I I r I , [ ~ , i

g ASSEAN LAKE

I I I l I I I l J I I I I I t

Lo Ca Nd SiR Eu Gd Dy Yb ' -~

10(3 i ~ i T i i i i i i , , , i i

C OSPWAGAN LAKE O basalt

picrite

t i = a | = = l i 1 A 1 J i i ,

t . e Ce Nd Sm Eu Gd Ov Yb L~

, , , , , l , , , , , , , , ,

D MOAK LAKE

. _ _ . ~ a n - 4

1 J J i e , I I I I I I I I I I

L e Ce Nd Se Eu Gd Oy Vb Lu

Fig. 9. Chondrite-normalized REE plots of selected rocks from the four principal localities. The patterns for the Assean Lake rocks show a distinct LREE enrichment. Normalizing values are from Thompson ( 1983 ).

178 N.M. HALDEN

i , i i i i i i i i i i ] i ; i I ~ I

A ASSEAN LAKE

i i i i i ~ i , , i ~ J i , i i i , t

O i , R b I ~ K N b T l l C e S r f: ' S e Z r H F T I Y Y 'o

,o~ [ 'C . . . . . . . . . . . . . . . . . FOX RIVER BELT

1 0 0

1

1 I I , I I , I ~ L P / I I I T I I I I I I h R b T h K N b T m ~ S r S o Z r H F Y Y b

Oil i i i , q l i E i i i ~ l , , , i i l

100

10

, i i , i i i , , i , i T , , , , , i

I i I I I F I I I I I I I L ~ I I I L

B 4 R b T h K ' N b T m r - e S r P ~ Z r H ~ T i Y Y b

Fig. 10. Chondrite-normalized plots of the data from the rocks at the four principal localities. There is no distinct fractionation of the HFSE and REE. Nb shows slightly elevated values in contrast to distinct Nb depletion which would be typical of volcanic arc basalts (Arculus, 1987 ). Normalizing values are from Thompson et al. (1983).

ture obtained from rocks. Figure 10 shows chondrite-normalised plots of the data. There does not appear to be significant fractionation of the HFSE and REE, which would suggest that neither hydrous fluids or silicic melts played a significant role in the generation of the magmas. There is a slight enrichment of the AE, which might reflect the choice of normalizing factors (significant AE enrichment is a feature of the MORB-normalized plots for the Ospwagan and Assean volcanic suites ).

MORB-normalized plots of the data are shown in Fig. 11. These show relatively flat and depleted HFSE patterns, indicative of fairly constant interelement HFSE ratios. These patterns would be consistent with mineralogically similar source regions giving rise to the magmas under similar conditions of pressure and temperature. Subsequent fractionation histo- ries for the Assean Lake volcanic rocks appear

to be distinct from those from Ospwagan Lake and the Fox River Belt. All the rocks have HFSE patterns < 1 × MORB; this characteristic probably reflects the nature of the source region. Although the source region was interpreted as being primitive in terms of its major element chemistry it may have undergone an earlier melting event under conditions where phases liable to retain these elements (e.g. homblende, garnet, magnetite and rutile) were unstable.

The data for the Ospwagan and Assean Lake suites show a consistent enrichment in K, Rb and Ba (and Th in the case of the Assean Lake samples). The Moak Lake samples show a similar pattern for the incompatible minor and trace elements but lack the relatively high values for K and Rb. The consistency of the patterns for Moak, Ospwagan, and Assean Lakes from Sr to Y mitigates against any radical al-

E X I S T E N C E O F A M A R G I N A L B A S I N W I T H I N T H E C I R C U M - S U P E R I O R B E L T 179

1 0 0 . . . . . . . . . . . . . . . . . 1 0 0 ~ . . . . . . . . . . . . . . . . .

1: I I I I I I I I I I l I ' i i i i

Sr K Rb Be Th T,I Nb Ca P Z r HF ~ l TI Y Yb S¢ CP 5 r K Rb Be Th Tlb t*lb Ce P ZP H f SI¢ Ti Y Yb S¢

loo loo1 ................. OSPWAGAN LAKE

Sr K Rb Ba Th T I Nb Ce P Z r HF Su TI Y Yb Sc Cr S r K Rb B8 Th T8 Nb Ce Z r HF Sm TI Y Yb Sc Cr

Fig. 11. MORB-normalized plots of major and trace element data of selected rocks from the principal localities. HFSE and REE values are consistently less than MORB. Normalizing values are from Pearce ( 1982 ).

teration of these element values on an irregular basis as might be expected with weathering or alteration. The patterns more probably reflect original source region characteristics for the magmas.

Comparison of the data in this study with a wide range of volcanic-arc basalts and mid- ocean basalts (e.g., Pearce, 1982 ) suggests that the basalts were not true MORB, but could have been derived in a volcanic-arc or marginal-basin type of setting, which is consistent with the major element data. In the case of a relationship to arc-volcanism, this would require proximity to a subduction zone. The structural complexity of the Churchill-Supe- rior Boundary Zone precludes any simple association with a subduction zone either along- strike from the present locations of the volcanic assemblages or within the Churchill Province.

A more direct comparison of the data with

1oo I I 1 I i i I I i i i i i i i i i

o Assean * Fox

\'~ ,~ ~,Moak 1o ' ~ / a Ospwagan

I,, R b B a T h K T a NIo C e S r P H F Z r T i Y C~ N i

Fig. ! 2. N-type MORB-normalized plot of a representative selection of rocks showing. Normalizing values are from Tarney et al. ( 1981 ).

N-type MORB inherently includes a comparison with T- and E-type MORB (cf. Saunders, 1984). In this case, the data (Fig. 12) reveal

180 N.M. HALDEN

broad similarities with the patterns for rocks produced in back-arc spreading regimes. The pronounced negative slope to the patterns (high large ion lithophile element (LILE) ratios) is a pattern consistent with back-arc spreading centres (Tarney et al., 1981 ). Most of the data for Nb, Sr, P, Zr, Ti and Y are < 1 X N-type MORB. This clearly differen- tiates the rocks from N- and E-type MORB: E- type MORB might be expected to have element concentrations in excess of 1 x N-type MORB for these elements (up to ~ 15 × N-type MORB for Nb in the case of E-type MORB). This suggests that the majority of the rocks examined in this study were probably derived from a mantle source depleted in these elements, relative to a mantle that could have produced an N-type MORB, one that may have undergone an earlier melting event.

The data for the Fox River Belt (Fig. 11 c) show slightly lower Zr values than the other rock suites. This could be associated with spatial difference in the mantle source region for the Fox River Belt and the other rocks. Taylor and Nesbitt (1988) argued that Z r / P ratios could be used to distinguish the geographic setting of basaltic source regions for some of the Troodos ophiolite basalts. In this case, the source region's spatial variability was a func-

0 . 5

t 0 . 4

0 . 3

0 . 2

0 . 1

0 . 0 0

' ' ' ' I ' '

\ o

6 o

o * * o

4- -" ' l ' "ik . . . . . . . . . . . . . • e " " . . F O X ROVER aELT

:.,A A • A A . . . . .A it -::

I I i I i i I | (30 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 O00

P

Fig. 13. Zr /P vs. P plot. The data from the Fox River Belt are confined to the lower part of the diagram. Zr /P values for the rocks from the other localities might suggest some form of source region enrichment.

tion of zones of increased melting or source enrichment. Assuming that volcanism at the As- scan, Ospwagan and Moak Lake sites and the Fox River Belt was essentially contempora- neous (to preclude temporal variations in source region chemistry) the metabasalts at these sites may have retained this characteristic of their source region (s). The data for this study fall in two distinct fields. Figure 13 illus- trates the distribution of the data for the Fox River Belt, which is restricted to very low values of Zr /P. The horizontal variation may be due to different degrees of source region melting. The higher values for the other rock suites might be related to varying degrees of source region enrichment, potentially associated with source region metasomatism (cf. Taylor and Nesbitt, 1988 ). LREE element enrichment is a characteristic feature of the volcanic rocks from Assean Lake (Fig. 11 a ).

The history of the source region for the volcanic rocks of ' the Circum-Superior Belt in Manitoba is likely to have been complex, in- volving successive, and spatially disparate, melting and enrichment events. Samples of this mantle source region are not available, and the basalts derived from it will retain the only re- cord of its composition. The nature of the en- r ichment that could have yielded the high Zr / P ratios remains to be tested. If the high Z r / P ratios are a feature associated with Zr alone, and the magmas were emplaced during rifting of Archean crust (cf. Scoates, 1981 ), assimi- lation of continental crust cannot be dis- counted. If this was the case, it would appear to be a feature of the Ospwagan, Moak and As- scan Lake rocks, not the Fox River Belt. Fur- ther discrimination of the nature of the source region(s) would benefit from detailed 87Sr/ 86Sr and N d / S m studies.

Discussion

The trace element and REE data indicate that the volcanic rocks of the Churchill-Supe- rior Boundary Zone in Manitoba were not true


MORB. The MORB-normalized data sets show high LIL to HFSE ratios, consistent with marginal basin settings (Tarney et al., 1981). Comparison of the data in this study with data from the Churchill Province (Flin Flon area, Gaskarth and Parslow (1987), and La Ronge domain, Watters and Pearce (1987)) shows some similarities. The data for the La Ronge domain do show some LREE-HFSE fractionation, consistent with what might be expected in a subduction-zone type setting (see Arculus, 1987). The data for this study contrast with those from the basalts at Flin Flon, East Amisk and Annabel Lakes, many of which show a within-plate affinity (Gaskarth and Parslow, 1987). In addition, the data obtained from those areas show a trend towards much higher A1203/TiO2 ratios than those observed in this study. If there had been a genetic connection between the Circum-Superior Belt volcanic rocks in Manitoba and the volcanic rocks in the Flin Flon area, the differences in geochemical characteristics would require structural re- moval of just those rocks that were derived by limited partial melting of a comparatively primitive mantle (i.e. those rocks occurring now at Ospwagan, Assean and Moak Lakes and the Fox River Belt).

The geochemical characteristics of the Cir- cum-Superior Belt rocks examined in this study would be consistent with their formation in a marginal basin. If the rocks at Ospwagan, As- scan and Moak Lakes represent remnants of sill complexes, similar to the Fox River Belt, then similarities in the geochemical characteristics of the rocks would support the idea that they had all evolved in a similar tectonic environment. The possibility that this environment was connected with an island arc is difficult to assess. Such an environment would require the proximity of a subduction zone, which cannot be conclusively demonstrated. The absence of significant HFSE and REE fractionation (as might be related to hydrous fluids or interaction with silicic fluids) mitigates against the immediate proximity of a subduction zone.

The comparatively flat chondrite-normalized element patterns between HFSE and REE suggest that the mantle giving rise to the magmas had not undergone any serious bulk modifica- tion. In addition, the similarities of the patterns from the rocks at each locality indicate that the mantle source regions were broadly similar.

A number of problems remain to be considered. The absence of a true ophiolite could be attributed to an irregular rifting process, or one that did not mature to the point of producing an ocean basin. A rifting environment was proposed by Scoates ( 1981 ) for the origin of the Fox River Belt. Even though there has been considerable deformation at the Superior margin, the continuity of the Circum-Superior Belt is a significant geological feature (Baragar and Scoates, 1987). Assuming that a rifting process was responsible for producing a series of basins marginal to the Archean craton, the question of what was rifted remains. The volcanic rocks could have been emplaced in a region of continental crustal thinning above a rising mantle diapir. Alternatively, the rifting could have been associated with wrench tectonics at the Superior margin, and the volcanic rocks could have been created in a transten- sional type of tectonic environment. Cran- stone and Turek ( 1976 ) placed a minimum age of faulting along the Assean Lake Fault Zone within the Churchill-Superior Boundary Zone at 1806 Ma (Rb-Sr, whole-rock).

The size to which the marginal basin(s) opened is unknown. If the basin became large, then its closure need not require the immediate proximity ofa subduction zone, or the subduction of oceanic crust beneath the Superior craton (for which there is no magmatic evidence). A similar model was proposed for the origin of the Outokumpu assemblage in Fin- land by Park et al. (1984). The subduction zone in that case was separated from any interaction with the continental craton by the marginal basin itself.

182 N.M. HALDEN

Acknowledgements

This project was funded by the M a n i t o b a

D e p a r t m e n t o f Energy and Mines and in par t

by the Na t iona l Science and Engineer ing

Counci l o f C a n a d a A0612. W. Blonski and T.

F o n i o k assisted with the geochemica l analysis.

I t hank G.S. Clark, W. Weber and R.F.J.

Scoates for reviewing the manusc r ip t and of-

feting helpful suggestions. I also thank two very

cons t ruc t ive referees. This pape r is publ i shed

with the permiss ion o f the Minister , M a n i t o b a

D e p a r t m e n t o f Energy and Mines.

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Documents

Existence of a marginal basin within the Circum-Superior Belt: geochemical evidence from the Churchill-Superior boundary in Manitoba, Canada