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Project 8525

Neodymium, Strontium and Lead Isotope Geochemistry Project

by Kenneth D. Collerson1 and John F. Lewry1

Collerson, K.D. and Lewry, J.F. (1985): Neodymium. strontium and lead isotope geochemistry project; in Summary of Investigations 1985. Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscel laneous Report 85-5.

Sample collection was completed in June 1985 for a multi-system (Nd, Sr, Pb) whole-rock and mineral isotopic study of selected suites of rocks from critical areas in the Saskatchewan part of the Thelon segment of the Trans­Hudson Orogen (Lewry, 1984; Lewry et al.. 1985). Initial analytical work on selected samples will be carried out during November, 1985 at the Department of Terrestrial Magnetism. Carnegie Institution, Washington. D.C.

This study is intended to complement the U-Pb zircon dating program currently being undertaken at the University of Kansas (Bickford and Van Schmus, this volume). It is expected to provide information concerning the timing of crust-mantle separation events within the Trans-Hudson Orogen, the isotopic character of the contemporary mantle, the distribution of Archean and reworked Archean crustal components, and an evaluation of the scale of diffusion during isotopic homogenization in response to tectonothermal activity. It will eventually include a study of Sm-Nd isotope systematics on a mineralogical and whole-rock scale at different grades of metamorphism, and is expected to yield Sm-Nd radiometric ages of some rocks (primarily basic-ultramafic types) which are not amenable to the U-Pb zircon dating method.

Recent radiogenic isotope studies of Precambrian basement terrains in North American and other cratonic blocks (DePaolo, 1981; McCulloch and Wasserberg, 1978; Patchett and Bridgwater, 1984) allow conclusions to be drawn regarding the age of initial separation of crustal material from the mantle. In the case of Lower Proterozoic orogenic terrains. they permit distinction between regions underlain by Archean continental basement which have been reworked during Early Proterozoic orogeny, and terrains which represent accretion of new crustal material as. for example, oceanic magmatic arcs. Reconnaissance isotopic data for the eastern part of the Thelon Segment in Saskatchewan (Chauvel et al., in press) indicate that terrains east of the Wathaman Batholith represent accretion of primitive Early Proterozoic crustal components, with little involvement of older Archean crust. This contrasts sharply with terrains to the west of the Wathaman Batholith, which on geological evidence are known to be mostly underlain by variably reworked Archean continental basement. Such data impose important constraints on models of Early Proterozoic crustal evolution and orogenic mechanisms.

•Department of Geology, University of Regina. Project contracted to the University of Regina with funds provided under the Saskatchewan component of the Canada-Saskatchewan Subsidiary Agreement on Mineral Development 1984-89.

Phases of this research were also supported by NSERC grants A0594 to K.D.C. and A7426 to J.F.L.

Sample Descriptions

Samples collected this summer are summarized below. Approximate specimen locations are given in brackets. Localities also sampled for U-Pb zi rcon dating by Bickford (Bickford and Van Schmus, this volume) are asterisked.

Western Craton

HUD85-1' (narrows between Wasekamio and Turner Lakes): Junction Granite; a moderately foliated. coarse megacrystic monzogranite.

HUD85-2* (southwest end of Hill Island, Wasekamio Lake) : Blue quartz gneisses; partly retrograded granulite facies dioritic-granodioritic gneisses cut by amphibolite dykes. Both lithologies were sampled.

HUD85-3' (on winter road 1000 m west of west shore of Wasekamio Lake): Banded aplite/pegmatite. intrusive into older retrograded granulites.

HUD85-4 (on C learwater River, south of Fournier Lake) : Layered anorthosite-leucogabbro/norite. Six samples were collected from different parts/layers of the body.

HUD85-7 (northwest end of Fournier Lake): Layered anorthosite-leuconorite-pyroxenite (this locality was included in the 'granulite gneisses' of Scott, 1979). A suite of five samples was collected. ranging from pyroxenite to gabbro/norite to anorthosite.

HUD85-8' (peninsula on east shore of north arm of Fournier Lake): Xenolithic rafts of granulite facies migmatitic tonalitic gneiss within leuconorite-anorthosite host rocks. Both the xenolithic gneiss and the norite were sampled.

HUD85-9' (island in cent ral part of Careen Lake) : Augen granitic gneiss with intense to mylonitic (Hudsonian?) planar/ linear fabric.

HUD85-10* (small island at southwest end of Careen Lake): Less tectonically overprinted heterogeneous gneisses apparently cut by boudinaged pyroxenitic dykes. Both tonalitic gneiss and pyroxenite were sampled.

Mudjatik Domain HU085-5* (istand in north part of Dipper Lake): Uniform charnockitic granite.

HUD85-6* (southeast shore of Dipper Lake): Grey tonalitic orthogneiss.

HUD85-12· (southeast shore of Jim Lake): Orthopyroxene-bearing felsic gneiss with composite fabric.

HUD85-14 (southwest shore of Porter Lake): Laminated amphibolite and ultramafic granulite with orthopyroxene­bearing neosome 'sweats'; probable metavolcanic assemblage.

HUD85-15. (north end of Segment Lake): Finely laminated felsic granulites within mafic (metavolcanic?) gneiss unit; probable felsic-intermediate volcanics.

Wollaston Domain HUD85-16' (narrows north of Horton Island, Nekweaga Bay, southwest Wollaston Lake): Horton Island Granite; post-tectonic biotite leucogranite.

HUD85-17' (south Wollaston Lake shoreline, south of Brandser Island): Slightly megacrystic pink granite; possible anatectic derivative of upper Wollaston Group meta-arkoses.

HU085-18. (small island in northern part of Compulsion Bay): Within the largely Archean Johnson River basement inlier. This outcrop shows older grey tonalitic and pink augen gneisses with some layered supracrustal amphibolite inclusions. apparently cut by later pink granitic gneiss. In one locality, an early mafic dyke is apparently intrusive into the older gneiss assemblage. Samples of the augen gneiss. mafic dyke and later pink gneiss were collected.

HU085-19 (island south of locality 18): Johnson River Inlier; mafic dykes cutting grey augen gneiss with later pink granitic gneiss. The older augen gneiss was sampled.

HUD85-2Q· (northeast end of Linn Island, south Wollaston Lake} : Augened pink granitic gneiss with amphibolite gneiss supracrustal inclusions; part of the Linn Island basement inl ier. Both augen gneiss and amphibolite were sampled.

HUD85-21 (northeast end of largest of Sandy Islands, central Wollaston Lake}: Sandy Islands Gabbro; little deformed, rhythmically layered gabbro-diorite, with local pyroxenitic and syenitic phases and both felsic and mafic xenoliths. Five phases of the pluton and one of the xenoliths were sampled.

Peter Lake Domain

HUD85-22 (small island east of Crane Island, Maclean Bay, north Reindeer Lake}: Hornblende gabbro and gabbroic pegmatite cut by porphyri tic diabase dykes.

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Suites were collected from fine-grained melagabbro, coarse pegmatitic gabbro phases and the diabase dykes.

HUD85-23 (island east of locality 22): Rhythmically layered gabbro with clinopyroxenite nodules (derived from early cumulus phases?) cut by feldspathic diabase dykes. A sample of the clinopyroxenite was collected.

HUD85-24' (shoreline southwest of Crane Island, north Reindeer Lake): Well-layered hornblende gabbro with patchy pegmatitic phases. Both ultramafic hornblendite and anorthositic cumulus layers are present locally within a layered gabbro/melagabbro/porphyritic gabbro body with later irregular hornblende pegmatite phases. Six sample suites of various phases were collected.

HUD85-25' (shoreline southeast of locality 24): Complex outcrop of discontinuously banded augen granitic gneisses, which include mafic gneiss paleosome and are cut by leucocratic granitic gneiss. This complex is cut by mafic dykes. later pink aplite sheets and finally by xenolithic granodiorite. Samples of all phases but the mafic dykes and aplites were collected.

HUD85-26: (shoreline west of Patterson Island, northwest Reindeer Lake}: Coarse-grained leucogabbro, displaying well-developed 10 cm scale rhythmic layering.

Geological Comments The primary objective of the summer's work was sample collection. However. some revisions of previous geological interpretation and a number of additional observations were made in the course of outcrop examination . The most important of these are summarized below:

1) Rocks previously designated 'granulite gneisses' by Scott in the Fournier Lake area of the Western Craton (sample localities 7 and 8) were identified as only moderately deformed orthopyroxene-bearing anorthosites and leuconorites displaying ubiquitous well-developed primary igneous layering and local mafic to ultramafic cumulate layers. Pyroxenitic erratics observed in the same localities may derive from comagmatic phases exposed in situ elsewhere. Semicontinuous, freshly burnt outcrop extending eastward from Fournier Lake to the mapped western boundary of the anorthosite body (locality 3) appears to comprise similar layered plutonic rocks, suggesting that a continuous. major, layered, anorthosite-norite body extends at least 5 km farther west than this boundary. Other rocks similarly designated 'granulite gneisses' in previous mapping elsewhere in this part of the Western Craton, extending northwestward to Lloyd Lake. may require similar reinterpretation, at least in part. The existence of such a major layered plutonic complex in this area has evident economic implications for platinum-group element exploration.

2) Re-examination of predominantly mafic (amphibolitic and pyribolitic) gneiss zones in the Mudjatik Domain supports previous interpretation of these rocks as being derived from metavolcanic rather than intrusive protoliths. Their absence in the main Wollaston Group succession to the east suggests that they may be of Archean rather than Early Proterozoic age (cf. Lewry and Sibbald, 1980).

3) Detailed examination of outcrops within the Johnson River inlier (localities 18 and 19) revealed more complex lithological relationships than hitherto described in the reconnaissance mapping. An early heterolithic gneiss assemblage (possible Archean granitoid basement?) is cut by mafic dykes, which are in turn cut by late granite phases. The dykes may possibly represent late Archean - early Proterozoic inter-orogenic intrusions, predating Early Proterozoic sedimentation and Hudsonian thermotectonism. A similar relationship was documented in the 'older gneiss' terrain in the northeastern part of the Peter Lake Domain, suggesting possible continuity of older basement material between this and the Wollaston inliers.

4) In previous work (eg. Lewry et al., 1980), hornblende in the Peter lake gabbroic bodies was generally interpreted as metamorphic in origin. However, relationships seen in very clean large outcrops around north Reindeer Lake (e.g., localities 22, 23, 24. 26) suggest that these rocks are essentially unmetamorphosed primary hornblende-bearing gabbros derived by crystallization of 'wet' magmas. Primary textural relations are well preserved. In some localities, irregularly distributed, totally undeformed 'gabbroic pegmatite' segregations contain large bladed hornblendes that are unlikely to be the result of metamorphic recrystallization. Elsewhere, large individual hornblende crystals form cumulate layers at the base of graded units. There is generally little field indication of significant metamorphic modification, either of primary textures or mineralogy.

References

Chauvel, C., Arndt, N.T., Todt, W., Kielinzcuk, S. and

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Thom, A., (in press): Formation of 1.9 Ga: Continental Crust in Saskatchewan, Canada: a Nd and Pb isotopic study; Can. J. Earth Sci.

DePaolo, D.J., (1981): Neodymium isotopes in the Colorado Front Range and crust-mantle evolution in the Proterozoic; Nature, v291 , p193-196.

Lewry, J.F. (1984): Continental platforms and suspect terranes in the Lower Proterozoic 'Trans-Hudson Orogen'; in GAG/MAC Joint Annual Meeting, London, Ontario, Programme with Abstracts. p84.

Lewry, J.F. and Sibbald, T.1.1., (1980): Thermotectonic evolution of the Churchill Province in northern Saskatchewan; T ectonophysics, v68, p45-82.

Lewry, J.F., Sibbald, T.1.1 . and Schledewitz, D.C.P., (1985) : Variation in character of Archean rocks in the western Churchill Province and its significance; in Evolution of Archean Supracrustal Sequences. ed. L.D. Ayres, P.C. Thurston, K.D. Card and W. Weber; Geol. Assoc. Can. Spec. Pap. 28. p239-261 .

Lewry, J.F., Roberts, K. and Rees, C.J .. (1980): Geology of the area around Spalding Lake; Sask. Dep. Miner. Resour., Rep. 199.

McCulloch, M.T. and Wasserberg, C.J., (1978): Sm-Nd and Rb-Sr chronology of continental crust formation; Science, v200, p1003-1011.

Patchett, P.J. and Bridgwater, D .. (1984): Origin of continental crust of 1.9-1. 7 Ga defined by Nd isotopes in the Ketilidian terrain of South Greenland; Contrib. Mineral. Petrol., v87, p311-318.

Scott, B.P. (1979): Reconnaissance geology: Upper Clearwater River (part of NTS area 74F); in Summary of Investigations 1979, Sask. Geol. Surv., Misc. Rep. 79-10, p61-67.

Van Schmus, W.R. and Bickford, M.E. (1984) : U-Pb (zircon) geochronology in Saskatchewan; in Summary of Investigations 1984, Sask. Geol. Surv., Misc. Rep. 84-4. p81-87.