Chemical Signatures of Igneous and Metasedimentary Rocks from the Pelican Slide Area: Implications for Their Sources and Tectonic

Environments

M 1 M S 1 _2 G 3 . 1 1 . Sun , .R. tauffer, J.F. Lewry , . Edwards , R. Kernch and T.K. Kyser

Sun, M., Stauffer, M.R., Lewry, J.F., Edwards, G., Kerrich, R. and Kyser, T.K. (1991 ): Chemical ;,ignatures of igneous and metasedimentary rocks from the Pelican Slide area: Implications for their sources and tectonic environments; in Summary of In­vestigations 1991, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 91-4.

The Trans-Hudson Orogen (Hottman, 1981) is the largest coherently preserved and exposed Early Proterozoic orogenic belt in the world. In northern Sas­katchewan and Manitoba, it mainly comprises two con­trasting subdivisions, the ensialic Cree Lake Zone and the juvenile Reindeer Zone (Stauffer, 1984). The latter in­corporates disparate lithostructural domains dominated by Early Proterozoic arc volcanics, volcanogenic greywacke-pelite assemblages and plutons of various ages. Recent work (Lewry et al., 1990) suggests that most of the exposed Reindeer Zone southeast of the Wathaman batholith is essentially allochthonous, com­prising imbricated thrust packages and nappe sheets soled by initially gently dipping ductile high-strain zones. This allochthonous pile may be tectonically emplaced across more-or-less continuous lower plate Archean con­tinental basement along a fundamental basal movement zone.

An exposed part of such a basal sole to the Early Proterozoic allochthons may be represented by the 'Pelican Slide' (Lewry et al., 1989), located in the Han­son Lake block (Figure 1). This feature comprises an early-tectonic, heterogeneous ductile high-strain to mylonitic assemblage of high metamorphic grade or­thogneisses and paragneisses which structurally overlie, and envelop the mainly monzocharnockitic Sahli and MacMillan Point Archean 'window' forming the core of the Hanson Lake block (Macdonald, 1974). This largely mylonitic package separates Archean basement from the overlying Early Proterozoic allochthons which form higher structural levels of the Hanson Lake block and the adjoining Kisseynew and Flin Flon domains. Al­though complexly refolded, the Pelican Slide is thought to have been an initially gently dipping zone, along which regional lateral displacement on the order of tens or possibly hundreds of kilometres may have occurred (Lewry et al., 1989). Thus, the Pelican Slide may be one of the most important structures in this part of the Trans­Hudson Orogen.

The Pelican Slide includes both migmatitic metasedimen­tary genisses and a broad range of mafic to granodioritic-granitic orthogneisses displaying extremely variable strain intensity and of widely differing age rela-

(1) Department of Geological Sciences, University of Silskatchcwan. (2) Departmenl of Geology, University of Re9ina. (3) University of Alhat:Jasca, Alberta.

162

tive to the ductile shear zone fabric. They include pre­kinematic, early- to late synkinematic and post-kinematic major plutons, minor intrusive sheets and anatectic melt fractions (Lewry, 1990).

In June, 1991, 70 samples were collected from the Pelican Slide zone (Figure 1 ), including metasediments, pre-, syn- and post-tectonic intrusions and anatectic phases. Our objective is to study the geochemistry, pres­sure-temperature-time and fluid evolution (PTtf) of this zone as part of the Trans-Hudson LITHPROBE project. The first step toward evaluating the PTtf history is detailed study of the petrographic and geochemical character of the rocks involved, initial results of which are reported here.

1 . Samples in this Study

a) Archean Inlier

Two samples from the Sahli Granite and one from the McMillan Point Granite were selected. The samples are -2500 Ma old charnockitic granites (Lewry et al, 1987).

b) Proterozoic Intrusions

Proterozoic intrusions include granite, granodiorite, and tonalite, to quartz diorite, and have U-Pb zircon ages be­tween 1870 and 1835 Ma (Lewry et al, 1987). The granodiorite and tonalite have strain fabrics increasing in intensity toward the Archean inlier and some are mylonitic; accordingly they have been considered to be early-tectonic plutons. The high-strain pinkish granites in­truding the granodiorite are interpreted as syntectonic in­trusions, whereas the relatively undeformed rocks of the Jan Lake Granite suite are considered to be post-tec­tonic. Three granodiorites, one tonalite, and one quartzdiorite representing early-tectonic plutons, two pinkish syntectonic granites, and two Jan Lake granite samples were selected for this study.

Summary of Investigations 1991

Figure 1 • Simplified geological map of the Jan Lake area, after Lewry et al. {1989), illustrating sample locations. Lithological units as follows: 1-Archean granitoid inliers; 2-Pelitic gneiss assemblage; 3-Quartofeldspathic gneisses; 4-Mixed homblendic and porphyrocfastic gneisses; 5-Kisseynew Gneisses (high­grade pelitic-psmmopelitic); 6-Lower grade metasediments along the Tabbernor Fault zone; 7-Hanson Lake volcanics (low to medium grade) and plutons; PN­Pelican Narrows settlement; JL-Jan Lake resort; SG-Sahli Granite; MPG-Mc­Millian Point Granite.

Saskatchewan Geological Survey

c) Pegmatites

Pegmatites with different intensities of tectonic fabric are widely distributed in the area. According to their fabric and intrusive relationships, they have been recognized as syn-, late-, and post-tec­tonic pegmatitic dykes, which are inten­sely deformed, mildly deformed and un­deformed respectively. Four syntec­tonic, one late-tectonic, and two post­tectonic pegmatites were chosen for chemical study.

d) Mafic Rocks

Mafic rocks in the region include arc volcanics between 1.89 and 1.87 Ga (Rb-Sr dates, Watters and Armstrong, 1985), and mafic intrusions of various dimensions that have ages of 1.88 to 1. 79 Ga or younger (Sm-Nd isochron dates, Hegner et al, 1989). Two vol­canic rocks and five samples from mafic dykes intruded by the Proterozoic granodiorite have been considered as representative of pre- and early-tectonic mafic rocks respectively.

Five samples from mafic dykes intrud­ing the Proterozoic granodiorite are in­terpreted as syn-tectonic dykes. These dykes are either intruded by late peg­matite or disrupted by tectonic move­ments.

A (200 to 300 m diameter) pyroxenite body with a round outcrop shape is regarded as post-tectonic; two samples have been analyzed. This body is not deformed and has a chilled margin. The country rocks are cracked around the dyke edges.

e) Metasedimentary Rocks of the Pelican Slide

Eleven samples of metasediments were taken from shoreline exposures along the south part of Pelican Lake (Figure 1). At outcrop scale, the metasediments are generally contorted and injected by variable amounts of leucosome, provid­ing gross evidence for metamorphic conditions sufficient to produce melting (diatexite). Based on mineralogical and textural variations, most of the original sediments were probably mixtures of pelites and psammites. The texture of all samples is granob!astic. The typical metape!itic assemblage is plagioclase +quartz+ biotite +garnet+ K­feldspar±cordierite±sillimanite± musco­vite + accessory minerals.

163

Sillimanite, invariably coexisting with cordierite, occurs as fine needles in plagioclase, larger isolated euhedral crystals, polycrystalline knots, and fibrolite replacing biotite. In sample PS-21, one anhedral sillimanite grain is overgrown by cordierite. Cordierite is anhedral, and in some samples is partly to completely pinitized. Cor­dierites analyzed by electron microprobe have molar ratios for Fe/Fe+ Mg from 0.34 to 0.40.

The mineral assemblage noted above records peak metamorphism within the upper amphibolite facies. The paucity of prograde muscovite, however, suggests that metamorphism may have reached the amphibolite­granulite transition. The Fe/Fe+ Mg ratio of cordierite (0.34 to 0.40) provides a rough upper limit for pressure of 4.5 to 5 kb where cordierite coexists with garnet and biotite. More detailed analysis of the P-T conditions of metamorphism, using garnet-biotite geotherrnometry and garnet-anorthite and garnet-cordierite geobarometry, is currently in progress.

2. Geochemistry of Igneous Rocks from the Pelican Slide

Major elements were determined by conventional X-ray fluorescence spectrometry, and trace elements by induc­tively-coupled plasma mass spectrometry (ICP-MS) at the University of Saskatchewan. Reproducibility is generally better than 5 percent for major elements, and 5 to 10 percent for trace elements depending on abun­dance.

a) Archean Inlier

The Sahli and the McMillan Point Granites have mildly fractionated LREE enriched patterns with Lan -100, Ybn = 5 to 20, and Lan/Ybn = 6 to 17 (Figure 2a). This pat­tern is similar to that of Archean high-Al203 tonalite­trondhjemite-granodiorite (TTG), e.g. the English River granite (Condie, 1981). The granites are relatively en­riched in K, Rb, Ba, Th, and Ce (Figure 3a), and are compositionally similar to granites formed in modern arc environments. They also plot in the Volcanic Arc Granite (VAG) field in Rb vs. (Y+Nb) (Figure 4), Ta vs. Yb, and Rb vs. {Yb+ Ta) discrimination diagrams.

b) Early-tectonic Granitoids

Granodiorite

Three granodiorite samples with different strain inten­sities have REE patterns and Lan/Ybn ratios (9 to 12) similar to the Archean inlier, but with about half the total REE contents (Figure 2a). The granodiorites are also similar to the Archean granites in trace element distribu­tions (Figure 3a), but contain lower incompatible ele­ment contents than the former. They also plot in the VAG field in discriminant diagrams (Figure 4).

Quartz diorite

One quartz diorite is comparable to the Archean granite inlier in terms of REE and trace element distributions

164

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A). PRE- & EARLY-TECTONIC GRA:-JITOIDS

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Figure 2 - Chondrite normalized REE distribution of A) Archean and early-, B) syn·, and C) late- and post-tectonic granitoids.

(Figures 2a and 3a), and plots in the VAG field in dis­criminant diagrams (Figure 4).

Tonalite

One tonalitic sample has a highly fractionated REE pat­tern with Lan/Ybn = 14.B, and a pronounced positive Eu anomaly (Figure 2a). The tonalite is enriched in LILE (large ion lithophile elements), but depleted in HFS (high field strength) elements (Figure 3a), and plots in the VAG field (Figure 4).

Summary of Investigations 1991

1000

A). PRE- & EARLY-TECTONIC GRANITOIDS

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Sr K Rb Ba Tb Ta Nb Ce P Zr Hr Sm Ti Y Yb

Figure 3 - Ocean ridge granite normalized diagrams for A) Ar­chean and early-, BJ syn-, and CJ late- and post-tectonic granitoids. Normalizing values from Pearce et al. (1984) and Harris et al., (1986).

c) Syn-tectonic Granitoids

Pink Granite

Two pink granites have REE patterns similar to the Ar­chean inlier, but with a slight negative Eu anomaly; one sample is more LREE enriched, where Lan/Lun = 34 (Figure 2b). These rocks are characterized by high Ce and low Nb contents (Figure 3b), and plot in the VAG field (Figure 4) .

Saskatchewan Geological SuNey

1000 ~ ~;==~~ -n/7-• l r r.~*x : WPG .. :

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Figure 4 - Granite d iscrimination d iagram (after Pearce et al., 1984) showing pre-, early-, syn-, and late- and post-tectonic granitoids.

Syn-tectonic Pegmatites

Four syn-tectonic pegmatites share highly fractionated REE patterns (Lan/Ybn = 10 to 180), two have pronounced negative Eu anomalies, one has a positive Eu anomaly, and one no Eu anomaly (Figure 2b). These rocks are generally characterized by high Ce and low Nb, Y, and Yb (Figure 3b). They plot in the VAG field and near the boundary of VAG and syn-collisional granite fields (Figure 4) .

d) Late-tectonic Pegmatites

One late-tectonic pegmatite is very depleted in all REE, with a negative Eu anomaly and unfractionated REE (Lan/Lun ::::: 1) (Figure 2c). The late-tectonic pegmatite is characterized by high K, Rb, and Th, low Ba and HFS elements (Figure 3c), and plots in the VAG field (Figure 4).

e) Post-tectonic Granitoids

Post-t ect onic Pegmatite

Two post-tectonic pegmatites have fractionated REE dis­tributions where Lan/ Ybn = 72 to 88, and pronounced negative Eu anomalies (Figure 2c). These pegmatites are characterized by high Th, Ce, and Sm, and low Ba, Nb, Zr, Hf, Y and Yb contents (Figure 3c). They plot in the fields of VAG and syn-collisional granites (Figure 4).

Jan Lake Granite

One sample from the Jan Lake Granite is depleted in all the REE, with a pronounced negative Eu anomaly, and

165

Lan/Ybn = 3 (Figure 2c). It is enriched in K, Rb, and Th, and depleted in Ba and HFS elements (Figure 3c). It plots in the syn-collisional field (Figure 4).

f) Mafic Rocks

Mafic rocks from the Pelican Slide area are mostly sub­alkaline and tholeiitic. They plot dominantly in the VAB (volcanic arc basalt) field in many discriminant diagrams (e.g. Th vs. Ta). These mafic rocks have flat mildly to LEE enriched REE patterns (Figure 5a, b, and c). There is no significant Eu anomaly. The REE character can be explained either by fractional crystallization of olivine and pyroxene, or by crustal contamination. Some of the

Q) ...., -~ ... 100

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..c::: u "-..::.: JO

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A). Pre-tectonic mafics

VOLCA:s/ICS • PS-LA c PS-lB

DIKE

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C). Post-tectonic mafics

La Ce Pr Nd Sm. Eu Gd Tb Dy

• PS-3A o PS-3B

no Er Till Yb Lu

Figure 5 - Chondrite-normalized AEE pattern of A) pre- and early-, B) syn-, and C) late- and post-tectonic mafic rocks.

166

pre- and early-tectonic mafic rocks have Ta, Nb, but not Ti, depletions on normalized diagrams; this feature has been interpreted to signify an arc environment.

The pre- and early-tectonic mafic rocks have Nb, Zr, and Y depletions, and plot in the field of subduction-re­lated basalts on the Ce/Pb-Nb/Th diagram (Figure 6, Condie, 1990). The La-La/Nb diagram (Jahn, 1990) shows a significant involvement of Archean continental material in the formation of the pre- and early-tectonic mafic rocks (Figure 7). We infer that they were formed in a volcanic arc-like environment proximal to an Archean continent.

The syn-tectonic mafic rocks are not depleted in Nb, but are enriched in Th, and depleted in Zr and Ti. They mainly plot off defined fields in discriminant diagrams (e.g. Figure 6). This could be due to element mobility

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Figure 6 - Ce/Pb vs. Nb/Th diagram (after Condie, 1990) show­ing pr1;r and early-, syn-, and late- and post-tectonic mafic rocks.

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La (ppm)

Figure 7- La/Nb vs. La diagram (after Jahn, 1990) showing pre­and early-, syn, and late- and post-tectonic mafic rocks.

Summary of Investigations 1991

during metamorphism. It appears that they were less contaminated by Archean continental material during their formation (Figure 7).

Samples from the post-tectonic mafic intrusions have flat depleted REE patterns (Figure 5c) and a trace ele­ment distribution similar to oceanic island arcs. They also plot in the field for modern IAB, away from the Ar­chean continent field (Figure 7).

g) Summary

Early and syn-tectonic granitic rocks from the Pelican Slide area have a VAG (volcanic arc granite) character, whereas late-stage pegmatites evolved towards the syn­collisional granite field. The Archean granites are LREE and K, Rb, Ba, Th enriched, as is the early-tectonic quartz diorite. Granodiorites are compositionally similar, but feature lower incompatible element contents. An early-tectonic tonalite, however, has a positive Eu anomaly and depletion in HFS elements.

The syn-tectonic pink granites and pegmatites are more REE fractionated, and have high Th, Ce, and low Nb contents. A late-tectonic pegmatite is depleted in all REE and HFS elements, has a negative Eu anomaly, and Th enrichment. Post-tectonic pegmatites are charac­terized by high Rb, Th, Ce, Sm and low Nb. The post­tectonic Jan Lake Granite is depleted in Ba, REE and HSF elements, and enriched in K and Rb relative to all other granites.

Mafic rocks from the Pelican Slide area are all subduc­tion-related.

3. Discussion

This preliminary geochemical work has shown sig­nificant differences in pre-, early-, syn-, late- and post-tec­tonic granitoids, providing a solid basis for further study of the pressure, temperature, time, and fluid evolution. It is not clear whether the Sahli and McMillan Point Granites are autochthonous or para-autochthonous Ar­chean continental basement, or alternatively whether the granites are part of an allochthonous basement thrust slice transported within the high-strain sequence. The chemical signatures of granodiorite and quartz diorite are similar to the Archean granites, and the pre-, and early-tectonic mafic rocks may have a significant involve­ment of Archean material. This may lend support for the allochthonous explanation for the Archean inlier. Final resolution of this question awaits further detailed studies.

4. Future Objectives

Further work will address the pressure, temperature, time, and fluid evolution of the Pelican Slide using ap­propriate geobarometers, geothermometers and geochronology. Nd isotopic data will help to reveal the sources of rocks, and model magma genesis.

Saskatchewan Geological Survey

5. Acknowledgments

This study was funded by a Trans-Hudson LITHOPROBE grant, and NSERC Infrastructure grant to R. Kerrich and T. K. Kyser. We thank J. Jain for assis­tance with ICP-MS.

6. References

Condie, K.C. (1981): Archean Greenstone Belts; Elsevier, 381pp.

____ , (1990): Geochemical characteristics of Precambrian basaltic greenstones; in Hall, R.P. and Hughes, D.J. (eds.), Early Precambrian Basic Magmatism, Blackie and Son, Glasgow, p40-55.

Harris, N.B.W., Pearce, J.A. and Tindle, A.G. (1986); Geochemi­cal characteristics of collision-zone magmatism; in Coward, M.P. and Ries, A.C. (eds.), Collision Tectonics, Geel. Soc., Spec. Publ. no. 19, p67-81.

Hegner, E., Kyser, T.K. and Hulbert, L. (1989): Nd, Sr, and O isotopic constraints on the petrogenesis of mafic in­trusions in the Proterozoic Trans-Hudson Orogen of central Canada; Can. J. Earth Sci., v26, p1027-1035.

Hoffman, P.F. (1981): Autopsy of Athapuscow Aulacogen: a failed arm affected by three collisions; in Cambell, F.H.A. (ed.). Proterozoic Basins of Canada, Geo!. Surv. Can., Pap. 81-10, p97-102.

Jahn, B.M. (1990): Early Precambrian basic rocks of China; in Hall, R.P. and Hughes, D.J. (eds.), Early Precambrian Basic Magmatism, Blackie and Son, Glasgow, p294-316.

Lewry, J.F. (1990): Bedrock geology, Tulabi-Church Lake area: derivation and significance of porphyroclastic gneis­ses in the Pelican Window; in Summary of Investigations 1990, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 90-4, p36-43.

Lewry, J.F., Macdonald, R., Livesey, C., Meyer, M., Van Schmus, R. and Bickford, M.E. (1987): U-Pb geochronol­ogy of accreted terranes in the Trans-Hudson Orogen, northern Saskatchewan, Canada; in Pharaoh, T.C., Beckin­sale, R.D. and Rickard, D. (eds.). Geochemistry and Mineralization of Proterozoic Volcanic Suites, Geel. Soc. Spec. Pub!. no. 33, p147-166.

Lewry, J.F., Macdonald, R. and Stauffer, M.R. (1989): The development of highly strained rocks in the Pelican Win­dow during high-grade metamorphism and pervasive anatexis; in Summary of Investigations 1989, Sas­katchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 89-4, p58-65.

Lewry, J.F., Thomas, D.J., Macdonald, R. and Chiarenzell, J. (1990): Structural relations in accreted terranes of the Trans-Hudson Orogen, Saskatchewan: telescoping in a collisional regime?, in Lewry, J.F. and Stauffer, M.R., (eds.}, The Early Proterozoic Trans-Hudson Orogen of North America: Geel. Assoc. Can., Spec. Pap. 37, p75-94.

Macdonald, R. {1974): Pelican Narrows (west) area: reconnais­sance geological survey of 63M-2(W); in Summary Report of Field Investigations by the Saskatchewan Geological Survey, Sask. Dep. Miner. Resour., p30-37.

Pearce, J.A., Harris, N.B.W. and Tindle, A.G. (1984): Trace ele­ment discrimination diagrams for the tectonic interpreta­tion of granitic rocks; J. Petrol., v25, p956-983.

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Stauffer, M.R. (1984): Manikewan: an Early Proterozoic ocean in central Canada, its igneous history and orogenic closure; Precam. Res., v25, p257-281.

168

Watters, B.A. and Armstrong, R.L. {1985): Rb-Sr study of metavolcanic rocks from the La Range and Flin Flan Domains, northern Saskatchewan; Can. J. Earth Sci. , v22, p452-324.

Summary of Investigations 1991