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1117 Low-pressure regional -facies to -facies of the Paleoproterozoic Thompson Nickel Belt, Manitoba

Chris G. Couëslan and David R.M. Pattison

Abstract: The Thompson Nickel Belt is a ca. 35 km × 400 km northeast-trending segment of the northwest margin of the Archean in Manitoba, bounded to the west by the Paleoproterozoic Reindeer Zone. The belt was metamor- phosed and deformed during the Trans-Hudson (ca. 1.9–1.7 Ga). assemblages in metamorphosed , aluminous , igneous , formation, and ferruginous wacke define regional metamorphic domains, separated by mineral , that are subparallel to the strike of the belt and to regional-scale D3 structures. An elongate, ca. 5 km × 73 km, central zone of middle amphibolite-facies rocks is characterized by the following: -bearing mineral assemblages in containing combinations of , , and ; muscovite-free, staurolite + + -bearing mineral assemblages in ; -bearing mineral assemblages in mafic metaigne- ous rocks; and grunerite-bearing mineral assemblages in iron formation. Pressure–temperature (P–T) conditions of the mid- dle amphibolite-facies zone are ca. 550–620 °C and 3.0–5.0 kbar (1 kbar = 100 MPa), with pressure increasing to the northeast. The middle amphibolite-facies zone is bordered to the east and west by an upper amphibolite-facies zone, ca. 5 km wide on the east and ca. 3–5 km on the west. The upper amphibolite-facies zone is characterized by variably migma- titic K- + sillimanite-bearing mineral assemblages in pelites; migmatitic, garnet + cordierite + sillimanite-bearing min- eral assemblages in greywackes; orthopyroxene-free, hornblende-bearing mineral assemblages in mafic rocks; and orthopyroxene-bearing mineral assemblages in iron formations. Pressure–temperature conditions of the upper amphibolite-facies zone are ca. 640–710 °C and 3.0–5.5 kbar in the southeast, and 675–755 °C and 4.5–6.0 kbar in the northwest. The outermost is of the granulite facies, characterized by migmatitic garnet + cordierite + K-feldspar-bearing assemblages in pelites and greywackes, orthopyroxene + clinopyroxene ± garnet-bearing mineral assemblages in mafic rocks, and orthopyroxene + K-feldspar-bearing mineral assemblages in iron formation in which is unstable. Pressure–temperature conditions of the granulite-facies zone are ca. 775–830 °C and 5.0–7.0 kbar. The P–T paths in the Thompson Nickel Belt appear to be broadly clockwise, except for some domains where they are close to isobaric. The peak P–T conditions, combined with local but widespread development of andalusite, imply relatively steep geothermal For personal use only. gradients of ca. 33–51 °C/km during metamorphism. Regional bathozones (domains of uniform peak-metamorphic pressure) correspond in general but not in detail with the metamorphic-facies zones. They reveal an increase in pressure towards the northeast, suggesting greater degrees of postmetamorphic exhumation in that region. Microstructural analysis sug- gests that peak metamorphism coincided with, and possibly outlasted, the D2 deformation event. Metamorphic isograds were deformed by D3–D4 structures. These features are consistent with a tectonic model in which the Superior craton moved in a northwest or west-northwest direction relative to the Reindeer Zone, with greatest convergence and tectonic burial occurring at the Thompson promontory. Résumé : La ceinture nickélifère de Thompson est un segment de la bordure nord-ouest du craton Supérieur (Archéen), au Manitoba; elle est de tendance nord-est et mesure environ 35 km sur 400 km; elle est limitée à l’ouest par la zone Reindeer (Paléoprotérozoïque). La ceinture a été métamorphisée et déformée durant l’orogenèse Trans-Hudson (∼1,9 à 1,7 Ga). Les assemblages minéralogiques dans des pélites métamorphisées, des grauwackes alumineux, des roches ignées mafiques, des formations de fer et des wackes ferrugineuses définissent des domaines métamorphiques régionaux, séparés par des isogra- des minéralogiques qui sont subparallèles à la direction de la ceinture et aux structures D3 d’échelle régionale. Une zone

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 centrale allongée, d’environ 5 km sur 73 km, formée de roches de faciès amphibolite, est caractérisée par : des assemblages minéralogiques à muscovite dans des pélites contenant diverses combinaisons de staurotide, d’andalousite et de sillimanite; des assemblages dans des grauwackes, sans muscovite, mais comportant de la staurotide + de la cordiérite et des grenats; des assemblages minéralogiques comportant de la hornblende dans des roches mafiques méta-ignées et des assemblages minéralogiques à grunérite dans une formation de fer. Les conditions de température et de pression dans la zone de faciès

Received 13 September 2011. Accepted 16 May 2012. Published at www.nrcresearchpress.com/cjes on 11 September 2012. Paper handled by Associate Editor Kathryn M. Bethune. C.G. Couëslan* and D.R.M. Pattison. Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada. Corresponding author: C.G. Couëslan (e-mail: [email protected]). *Current address: Manitoba Geological Survey, Manitoba Innovation, Energy, and Mines, 360-1395 Ellice Avenue, Winnipeg, MB R3G 3P2, Canada.

Can. J. Earth Sci. 49: 1117–1153 (2012) doi:10.1139/E2012-029 Published by NRC Research Press 1118 Can. J. Earth Sci., Vol. 49, 2012

amphibolite sont d’environ 550 à 620 °C et 3,0 à 5,0 kbar (1 kbar = 100 MPa); la pression augmentant vers le nord-est. La zone de faciès amphibolite moyen est limitée à l’est et à l’ouest par une zone de faciès amphibolite supérieur d’une largeur d’environ 5 km du côté est et de 3 à 5 km du côté ouest. La zone de faciès amphibolite supérieur est caractérisée par des assemblages minéralogiques migmatiques variables de feldspath K + sillimanite dans des pélites; des assemblages minéralo- giques migmatiques de grenat + cordiérite + sillimanite dans des grauwackes; des assemblages minéralogiques à horn- blende, sans orthopyroxène, dans des roches mafiques et des assemblages minéralogiques à orthopyroxène dans des formations de fer. Les conditions de température et de pression dans la zone de faciès amphibolite supérieur sont d’environ 640 à 710 °C et 3,0 à 5,5 kbar dans le sud-est et de 675 à 755 °C et 4,5 à 6,0 kbar au nord-ouest. La région métamorphique la plus externe est de faciès granulite; elle est caractérisée par des assemblages de grenat migmatique + cordiérite + felds- path K dans des pélites et des grauwackes, des assemblages minéralogiques d’orthopyroxène + clinopyroxène ± grenat dans des roches mafiques et des assemblages minéralogiques à orthopyroxène + feldspath K dans une formation de fer dans la- quelle la biotite est instable. Les conditions de température et de pression de la zone de faciès granulite sont d’environ 775 à 830 °C et 5,0 à 7,0 kbar. Les courbes T–P dans la ceinture nickélifère de Thompson semblent en général dans le sens des aiguilles d’une montre, sauf pour quelques domaines où les courbes tendent vers des isobares. Les conditions de pointe de T–P, combinées au développement localisé mais largement distribué d’andalousite, impliquent des gradients géothermiques plutôt abrupts d’environ 33 à 51 °C/km durant le métamorphisme. Les bathozones régionales (domaines de pointe uniforme de pression métamorphique) correspondent en général, mais pas dans les détails, aux zones de faciès métamorphique. Ces bathozones révèlent une augmentation de pression vers le nord-est, suggérant plus d’exhumation post-métamorphique dans cette région. Une analyse microstructurale suggère que le sommet du métamorphisme ait coïncidé avec l’événement de dé- formation D2, se prolongeant possiblement au-delà de l’événement. Les isogrades métamorphiques ont été déformés par des structures D3–D4. Ces caractéristiques concordent avec un modèle tectonique selon lequel le craton du Supérieur s’est dé- placé vers le nord-ouest ou vers l’ouest-nord-ouest par rapport à la zone de Reindeer; le point de plus grande convergence et d’enfouissement tectonique se produisant au promontoire Thompson. [Traduit par la Rédaction]

Introduction Regional The Thompson Nickel Belt (TNB) forms a segment of the The TNB is largely underlain by reworked Archean Superior Boundary Zone of Manitoba (Bell 1971; Bleeker of the Superior craton (Fig. 2). The gneiss is interpreted to be 1990). It occurs on the northwestern margin of the Archean derived from the Pikwitonei Granulite Domain (Hubregtse Superior craton, which was tectonically reworked during the 1980). Although the Archean are traditionally

For personal use only. Trans-Hudson orogeny by collision with the Paleoproterozoic thought to be orthogneiss, a possibly Archean supracrustal arc-derived of the Reindeer Zone (Fig. 1; Bleeker sequence has been recognized on Paint and Wintering lakes 1990; Ansdell 2005). The TNB trends 030° and has a width (Böhm 2005; Couëslan 2009). The entire package of Archean of 30–40 km and a strike length of almost 400 km; however, gneiss was subjected to amphibolite- to granulite-facies con- the southern one-third to half of the belt is covered by Paleo- ditions from ca. 2.72 to 2.64 Ga (Mezger et al. 1990; Hea- zoic limestone (Burnham et al. 2009). man et al. 2011). Temperature and pressure estimates based The Trans-Hudson Orogen represents one of the most ex- on geothermobarometry for Pikwitonei granulite near the tensively studied Paleoproterozoic orogens (Lewry and TNB margin vary from 700 to 900 °C and 8 to 12 kbar Stauffer 1990; Hajnal et al. 2005). The TNB portion has (1 kbar = 100 MPa) (Paktunc and Baer 1986; Mezger et al. been subject to a number of regional mapping, stratigraphic, 1990; Heaman et al. 2011). structural, isotopic, and ore-deposit studies (Fueten and The Pikwitonei were exhumed prior to the dep- Robin 1989; Bleeker 1990; Gapais et al. 2005; Macek et al. osition of the Paleoproterozoic supracrustal rocks of the Osp- 2006; Böhm et al. 2007; Zwanzig et al. 2007; Burnham et al. wagan Group (Scoates et al. 1977; Bleeker 1990; Zwanzig et 2009; Machado et al. 2011a, 2011b). However, a belt-wide al. 2007). The Ospwagan Group metasedimentary sequence

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 study of regional metamorphism has been lacking, and the consists of a fining upward siliciclastic sequence (Manasan latter is necessary to assess geothermal gradients and depth Formation), grading into calcareous metasedimentary rocks of tectonic burial in relation to the deformation events that of the Thompson Formation. The Thompson Formation is affected the . overlain by deeper basin siliciclastic and chemical metasedi- This paper consists of three main sections. The first mentary rocks of the Pipe Formation, which grade into the section describes the mineral assemblages observed in rocks Setting Formation, a coarsening upward siliciclastic package. of various bulk compositions from which three main meta- The Setting Formation is finally capped by a thick sequence morphic-facies domains, separated by isograds, are defined. of mafic to ultramafic metavolcanic rocks of the Bah Lake The second section estimates peak metamorphic conditions Assemblage (Bleeker 1990; Zwanzig et al. 2007). Paleopro- within these domains using equilibrium assemblage dia- terozoic detrital grains have been extracted from the grams. The third section discusses the distribution of the Manasan and Setting formations, yielding maximum ages for metamorphic domains and isograds in relation to metamor- deposition of 2235 ± 45 and 1974 ± 50 Ma, respectively phic depth zones (bathozones). The paper finishes with a dis- (Bleeker and Hamilton 2001; Burnham et al. 2009; Machado cussion of the tectonic implications of these findings. et al. 2011a). A minimum age for the Ospwagan Group is

Published by NRC Research Press Couëslan and Pattison 1119

provided by amphibolitized dykes interpreted to be part of Fig. 1. Simplified tectonic-elements map of Manitoba (adapted from the Molson dyke swarm, and the possibly cogenetic nickel Corkery et al. 1994, Burnham et al. 2009). Abbreviations: KD, Kis- ore-bearing ultramafic sills, which intruded the Ospwagan seynew Domain; SBZ, Superior Boundary Zone; TNB, Thompson Group supracrustals at all stratigraphic levels at ca. 1880 Ma Nickel Belt; TP, Thompson Promontory. (Bleeker 1990; Burnham et al. 2009; Heaman et al. 2009; Scoates et al. 2010). During the Trans-Hudson orogeny, the Ospwagan Group was affected by four main deformational events. The D1 event predates the intrusion of the ca. 1880 Ma mafic mag- matism; however, the resulting deformation is poorly defined. It has been interpreted as an east-verging, nappe-forming event (Bleeker 1990), or a more upright folding event (Burn- ham et al. 2009). The F1 folds can only be observed where suitable markers are present. The D2 event postdates the in- trusion of the Molson dykes and caused refolding and tight- ening of the F1 folds and resulted in either east-verging (Bleeker 1990; White et al. 2002), or southwest-verging (Burnham et al. 2009), isoclinal to recumbent F2 folds. The D2 event is considered coincident with peak metamorphic conditions of middle amphibolite to granulite facies in the TNB, as demonstrated by the following: the wholesale reor- ientation of almost all preexisting fabrics into regionally pen- etrative S2 and L2 fabrics; the isoclinal to recumbent nature of the regional ductile folding (Bleeker 1990; Burnham et al. 2009); and microstructures (more later in the text). The main regional (S2) is subparallel to S0 layering of Ospwa- gan Group rocks and the highly attenuated Archean gneissos- ity (SA) (Paktunc and Baer 1986; Bleeker 1990; Zwanzig et al. 2007; Burnham et al. 2009). The D2 event was likely related to the collision of the Superior craton with the al- lochthonous Kisseynew Domain of the Reindeer Zone at ca. 1840–1800 Ma (Ansdell 2005; Burnham et al. 2009). The D3 event resulted in tight, vertical to steeply southeast-dipping

For personal use only. isoclinal F3 folds (Fig. 3). Infolded Archean gneiss and Osp- wagan Group rocks typically form elongate chains of north- east-trending, doubly plunging F3 synforms and antiforms (Bleeker 1990; Burnham et al. 2009). zones with subvertical stretching lineations parallel many of the F3 sure estimates ranging between 4 and 7 kbar (Russell 1981; limbs (Bleeker 1990). The D3 event is generally considered Paktunc and Baer 1986; Bleeker 1990; Burnham et al. to be accompanied by retrograde, -facies metamor- 2009). Fueten et al. (1986) estimated metamorphic conditions phism (Bleeker 1990); however, recent investigations suggest of 575–625 °C and 2.5–5.75 kbar at Pipe II mine based on peak metamorphic conditions locally prevailed into D3 (ca. the metapelitic mineral assemblage (Qtz) + sillimanite 1770–1760 Ma, Zwanzig 1998). The D3 deformation event is (Sil) + biotite (Bt) + muscovite (Ms) + (Pl) ± attributed to northwest–southeast shortening and regional, garnet (Grt) and the presence of earlier relict andalusite. southeast-side-up, sinistral transpression (Bleeker 1990; Granulite underlying the Paint Lake area has been interpreted Burnham et al. 2009). The isoclinal nature of F2 and F3 re- as either preserved Archean, Pikwitonei-type granulite (Rus- sulted in a coplanar relationship between S2 and S3 along F3 sell 1981; Paktunc and Baer 1986) or as a Hudsonian granu- Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 fold limbs. Tightening of F2–F3 structures continued during lite-facies overprint (Fueten and Robin 1989; Bleeker 1990). D4, which was accompanied by localized retrograde greens- Pressure and temperature estimates for these rocks range chist metamorphism along mylonitic and brittle cataclastic from 700 to 900 °C and from 9 to 10 kbar (Russell 1981; shear zones commonly indicating southeast-side-up, sinistral Paktunc and Baer 1986; Bleeker 1990). movement (Bleeker 1990; Burnham et al. 2009). An alterna- The adjacent Kisseynew Domain is part of the Reindeer tive tectonic model has been presented that promotes long- Zone. It represents either a back-arc or fore-arc basin under- lived transpressional from ca. 1850 to 1750 Ma, lain by predominantly turbidite derived, metagreywacke of possibly as late as 1720 Ma (Gapais et al. 2005; Burnham et the Burntwood Group (Ansdell et al. 1995; Zwanzig et al. al. 2009; Machado et al. 2011a). The new metamorphic re- 1997, 2007). The Grass River Group is a shallow-water clas- sults of this study have implications for these models. tic sequence of metaconglomerate to arkosic metasandstones Thirty years of metamorphic studies of various parts of the that is present along the eastern margin of the Kisseynew TNB have provided a range of temperature estimates for Domain. It is believed to be contemporaneous with the depo- Hudsonian metamorphism from 530 to 575 °C in the Ospwa- sition of the Burntwood Group (ca. 1.86 to <1.83 Ga; Per- gan Lake area to 600–750 °C elsewhere in the belt, and pres- cival et al. 2005; Zwanzig et al. 2007). The boundary

Published by NRC Research Press 1120 Can. J. Earth Sci., Vol. 49, 2012

Fig. 2. map of the Thompson Nickel Belt and adjacent portions of the Kisseynew and Pikwitonei Granulite domains (adapted from Corkery et al. 1994, Macek et al. 2006). For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

between the Kisseynew Domain and the TNB is no longer gneisses and an Ospwagan Group-like cover sequence with well defined. Structural domes up to 60 km west of the TNB, zircon populations typical of Superior craton rocks (Percival in the eastern Kisseynew Domain, are cored by Archean et al. 2006; Zwanzig et al. 2006). Interleaving between Osp-

Published by NRC Research Press Couëslan and Pattison 1121

Fig. 3. Simplified structural map of the Thompson Nickel Belt. The structural data are sourced from Bleeker (1990), Zwanzig (1998), Zwan- zig and Böhm (2002), and Burnham et al. (2009). Abbreviations: BMZ, Burntwood Mylonite Zone; GRL, Grass River Lineament; SBF, Superior Boundary Fault; SLFZ, Setting Lake Fault Zone; TS, Thompson Structure. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

wagan Group and Burntwood Group rocks in the northern Peak metamorphic conditions in the adjacent Kisseynew part of the TNB suggests early thrusting from the Kisseynew Domain have been estimated at 700–800 °C and 4.5– Domain onto the TNB along a low-angle D2 fault (Zwanzig 6.8 kbar, with peak metamorphic assemblages in the Burnt- et al. 2006, 2007; Böhm et al. 2007). The TNB and adja- wood Group containing Qtz + Pl + K-feldspar (Kfs) + cent parts of the Kisseynew Domain therefore likely have a Bt + Grt + cordierite (Crd) + Sil + (Spl) (Gordon shared tectonic history starting with the D2 event (Zwanzig 1984, 1989; Growdon et al. 2006). More recent studies and Böhm 2002; Percival et al. 2006; Zwanzig et al. 2006). have suggested higher temperatures and pressures of ca.

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Table 1. Modal percentage of rock, forming in samples discussed in the text.

Zone: MA MA MA UA UA UA UA UA UA UA G G G G Sample: OP-06-27C PP-08-08Ba MY-07-03A PP-06-28Ca SP-02-01 TM-06-14A2 CC-786 SL-07-29C MN-07-17A CC-771 HY-06-22A 108-09-415 CC-602 108-08-226 Strat. unit: Op Op Bw Os Om Op Op Gr Bw Bw Bw Md Op Pt Composition: pel pel grwk spel spel pel pel ark grwk grwk grwk diab IF sif Fe-wk And — 2.5 — — — — —— — —— — —— Ap tr. — tr. ————0.2 0.3 tr. tr. tr. 0.4 0.7 Bt 19.5 32.6 31.8 34.1 24.6 34.5 27.8 9.0 20.9 43.6 12.9 0.3 19.1 17.8 Chl 0.9b — tr.b — 1.2b — 0.2b —— ——— —— Crd ——3.0 —————6.6 tr. 12.4 ——— Gr tr. tr. tr. ——tr. 1.1 — 0.6 — 0.8 ——— Grt 1.1 0.9 9.3 0.8 — 0.8 6.6 — 3.1 7.2 11.2 tr. 8.0 0.5 Ilm 0.6 1.3 0.3 tr. 0.6 tr. tr. 1.4 tr. 0.4 tr. 0.6 tr. Kfs ——— 11.4 16.8 2.0 2.7 6.8 ——17.3 — 2.8 1.7 Ms 26.6 13.0 — 1.2b 33.0b 9.1b — 1.4b —————— Pl 12.6 7.6 27.4 9.8 5.2 14.7 27.5 11.8 29.4 18.4 22.1 32.2 tr. 34.9 Po tr. 0.9 0.4 0.6 1.6 0.3 tr. 0.3 1.2 Qtz 36.2 35.6 27.5 35.4 16.8 29.0 24.2 63.4 38.1 24.0 22.5 7.0 25.1 30.2 Sil 2.4 0.7 6.9 1.2 8.3 9.6 6.0 0.9 6.4 0.8 ——— St 2.4 3.2 tr. —————tr. tr. ———— Cpx ——— —————— ——7.3 —— Hbl ——— —————— ——37.1 —— Opx ——— —————— ——14.9 43.4 14.1 Ttn ——— —————— ——0.3 ——

Total 99.9 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 99.9 Note: Mineral abbreviations are from Kretz (1983). Row headings: Zone, metamorphic-facies zone; MA, middle amphibolite-facies zone; UA, upper amphibolite-facies zone; G, granulite-facies zone; Strat. unit, stratigraphic unit; Op, Ospwagan Group, Pipe Formation; Bw, Burntwood Group; Os, Ospwagan Group, Setting Formation; Om, Ospwagan Group, Manasan Formation; Gr, Grass River Group; Md, Molson Dyke; Pt, Paint Lake sequence; Composition; pel, pelite; grwk, greywacke; spel, semipelite; ark, ; diab, diabase; IF sif, -facies iron formation; Fe-wk, ferruginous wacke. tr., trace amounts present; —, not present. a.J at c. o.4,2012 49, Vol. Sci., Earth J. Can. aFrom Couëslan et al. (2011). ulse yNCRsac Press Research NRC by Published bInterpreted to be a retrograde phase. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 For personal use only. oëlnadPattison and Couëslan Table 2. Summary of mineral analyses from samples discussed in the text.

Zone: MA MA MA UA UA UA UA G G G G G Sample: OP-06-27C PP-08-08Ba MY-07-03A PP-06-28Ca TM-06-14A2 CC-786 CC-771 PT-07-18A 108-09-415 CC-601 CC-602 HY-06-22A Strat. unit: Op Op Bw Os Op Op Bw Pt Md Op Op Bw Composi- pel pel grwk spel pel pel grwk pel diab pel IF sif grwk tion: Bt Mg# 0.45 0.40 0.47 0.39 0.44 0.51 0.51 0.67 0.52 0.48 0.35 0.54 Ti 0.168 0.188 0.207 0.293 0.278 0.356 0.342 0.147 0.622 0.587 0.366 0.596 Grt (r) Mg# 0.11 0.10 0.17 0.10 0.13 0.18 0.22 0.41 0.23 0.22 0.13 0.19

XGrs 0.04 0.04 0.03 0.03 0.04 0.03 0.05 0.02 0.19 0.04 0.08 0.03

XSps 0.08 0.07 0.02 0.15 0.15 0.13 0.02 0.01 0.04 0.02 0.04 0.01 Grt (c) Mg# 0.08 0.10 0.17 0.13 0.18 0.23 0.22 0.40 n.z. 0.30 0.13 0.28

XGrs 0.20 0.04 0.04 0.03 0.04 0.03 0.08 0.06 n.z. 0.04 0.09 0.03

XSps 0.15 0.08 0.04 0.15 0.12 0.10 0.03 0.01 n.z. 0.02 0.04 0.01 Pl

XAn 0.25 0.21 0.28 0.28 0.25 0.26 0.27 0.16 0.76 0.38 0.33 0.26 Kfs

XOr n.p. n.p. n.p. 0.90 0.64 0.85 n.p. 0.74 n.p. 0.85 0.87 0.84 St Mg# 0.14 0.12 0.06 n.p. n.p. n.p. 0.16 0.38 n.p. n.p. n.p. n.p. Zn# 0.019 0.016 0.039 n.p. n.p. n.p. 0.063 0.026 n.p. n.p. n.p. n.p. Crd Mg# n.p. n.p. 0.62 n.p. n.p. n.p. n.a. 0.79 n.p. 0.67 n.p. 0.68 Spl Mg# n.p. n.p. n.p. n.p. n.p. n.p. n.p. 0.39 n.p. n.p. n.p. n.p. Zn# n.p. n.p. n.p. n.p. n.p. n.p. n.p. 0.042 n.p. n.p. n.p. n.p. Opx Mg# n.p. n.p. n.p. n.p. n.p. n.p. n.p. n.p. 0.51 n.p. 0.31 n.p. Al n.p. n.p. n.p. n.p. n.p. n.p. n.p. n.p. 0.042 n.p. 0.045 n.p. Cpx Mg# n.p. n.p. n.p. n.p. n.p. n.p. n.p. n.p. 0.67 n.p. n.p. n.p. Ca n.p. n.p. n.p. n.p. n.p. n.p. n.p. n.p. 0.877 n.p. n.p. n.p. Al n.p. n.p. n.p. n.p. n.p. n.p. n.p. n.p. 0.081 n.p. n.p. n.p.

ulse yNCRsac Press Research NRC by Published Hbl Mg# n.p. n.p. n.p. n.p. n.p. n.p. n.p. n.p. 0.57 n.p. n.p. n.p. Si n.p. n.p. n.p. n.p. n.p. n.p. n.p. n.p. 6.693 n.p. n.p. n.p. Na + K n.p. n.p. n.p. n.p. n.p. n.p. n.p. n.p. 0.442 n.p. n.p. n.p. Ti n.p. n.p. n.p. n.p. n.p. n.p. n.p. n.p. 0.133 n.p. n.p. n.p. Note: Mineral abbreviations are from Kretz (1983). Row headings: Zone, metamorphic-facies zone; MA, middle amphibolite-facies zone; UA, upper amphibolite-facies zone; G, granulite-facies zone; Strat. unit, stratigraphic unit; Op, Ospwagan Group, Pipe Formation; Os, Ospwagan Group, Setting Formation; Bw, Burntwood Group; Md, Molson Dyke; Pt, Paint Lake sequence; Composition; pel, pelite; grwk, greywacke; spel, semipelite; diab, diabase; IF sif, silicate-facies iron formation; Grt (r), garnet rim analyses; Grt (c), core analyses; Al, cations per formula unit assuming six ; Ca, cations per formula unit assuming six oxygen; Mg#, Mg/(Mg + Fe); Na + K, cations per formula unit assuming 23 oxygen; Si, cations per formula unit assuming 23 oxygen; Ti, cations per formula unit assuming 22 oxygen for 2+

biotite and 23 oxygen for hornblende; Zn#, Zn/(Zn + Fe + Mn + Mg). n.p., not present; n.z., no zonation; n.a., not analyzed. 1123 aFrom Couëslan et al. (2011). Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 For personal use only. 1124

Table 3. Whole-rock geochemical analyses of samples discussed in the text.

Zone: MA MA MA UA UA UA UA UA UA UA G G G G a a Sample: OP-06-27C PP-08-08B MY-07-03A PP-06-28C SP-02-01 SL-07-29C TM-06-14A2 CC-786 MN-07-17A CC-771 108-09-415 108-08-226 HY-06-22A CC-602 Strat. unit: Op Op Bw Os Om Gr Op Op Bw Bw Md Pt Bw Op avg. Op avg. Bw Composition: pel pel grwk spel spel ark pel pel grwk grwk diab Fe-wk grwk IF sif pel grwk SiO2 64.61 65.11 62.16 67.56 63.54 81.78 60.36 51.52 58.53 54.05 49.24 60.89 61.60 60.74 59.65 58.86 Al O 18.74 16.93 16.30 16.39 16.47 8.01 19.90 21.68 19.09 18.28 13.68 13.96 17.41 8.55 20.04 17.86 2 3T Fe2O3 5.73 6.49 7.78 6.42 6.60 3.73 8.94 11.85 8.64 14.83 12.56 11.65 8.65 23.17 7.45 10.08 MnO 0.086 0.042 0.052 0.078 0.038 0.027 0.164 0.496 0.059 0.191 0.186 0.100 0.071 0.379 0.122 0.087 MgO 1.87 1.91 2.97 1.85 1.78 0.81 2.65 3.69 3.19 4.31 8.29 4.53 3.04 4.77 2.20 3.56 CaO 0.97 0.97 2.34 0.78 0.27 1.44 1.07 0.85 1.78 1.48 11.33 3.40 2.01 1.27 0.94 1.96 Na2O 1.99 1.88 2.86 1.60 0.70 1.27 1.78 1.39 2.79 1.40 1.01 2.86 2.93 0.08 1.62 2.62 K2O 4.05 3.63 2.35 4.47 8.18 1.48 3.80 3.61 2.89 2.89 0.79 1.97 2.93 1.68 4.32 2.65 TiO2 0.627 0.482 0.765 0.752 0.849 0.420 0.894 0.999 0.840 1.334 1.069 0.693 0.837 0.364 0.688 0.983 P2O5 0.05 0.08 0.16 0.05 0.09 0.08 0.05 0.03 0.14 0.06 0.09 0.32 0.14 0.11 0.07 0.13 LOI 2.12 3.06 1.36 0.89 1.49 0.60 1.41 4.01 1.76 1.63 1.02 0.14 0.64 bdl. —— Total 100.80 100.58 99.09 100.84 100.01 99.05 101.02 100.27 99.70 100.63 99.25 100.50 100.39 100.40 97.10 98.78 S 0.019 0.215 0.016 0.055 —— 0.331 0.717 0.010 0.092 — 0.017 bdl. 0.171 0.434 0.022 C 0.03 0.27 0.02 0.03 —— 0.20 0.38 0.03 0.09 ——0.49 0.08 0.25 0.16 Mg# 0.39 0.37 0.43 0.36 0.35 0.30 0.37 0.38 0.42 0.37 0.57 0.44 0.41 0.29 0.38 0.41

T Note: All constituents are reported in wt%. Total Fe is reported as Fe2O3 . Row headings: Zone, metamorphic-facies zone; MA, middle amphibolite-facies zone; UA, upper amphibolite-facies zone; G, granulite-facies zone; Strat. unit, stratigraphic unit; Op, Ospwagan Group, Pipe Formation; Bw, Burntwood Group; Os, Ospwagan Group, Setting Formation; Om, Ospwagan Group, Manasan Formation; Gr, Grass River Group; Md, Molson Dyke; Pt, Paint Lake sequence; avg. Op, average Pipe Formation pelite (sample size n = 16, Table S61); avg. Bw, average Burntwood Group aluminous greywacke (n = 11, Table S71); Composition; pel, pelite; grwk, greywacke; spel, semipelite; ark, arkose; diab, diabase; Fe-wk, ferruginous wacke; IF sif, silicate-facies iron formation; Mg#, Mg/(Mg + Fe). —, not analyzed; bdl., below detection limit. aFrom Couëslan et al. (2011). a.J at c. o.4,2012 49, Vol. Sci., Earth J. Can. ulse yNCRsac Press Research NRC by Published Couëslan and Pattison 1125

Fig. 4. Metamorphic-facies map of the Thompson Nickel Belt and adjacent Kisseynew Domain. Solid black lines define indicated facies-zone boundaries, dashed lines define inferred boundaries, and dotted lines indicate limits of data. Line X–X′ indicates the position of the schematic cross section in Fig. 22d. 1 kbar = 100 MPa. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

850 °C and 8 kbar for orthopyroxene-bearing Burntwood graphic microscopy studies (Table 1). From these, 12 Group rocks (Growdon 2010). samples were selected for mineral chemical analysis. Mineral analyses were obtained on the JEOL JXA-8200 Superprobe Analytical methods at the University of Calgary Laboratory for Electron Mi- crobeam Analysis (UCLEMA), Calgary, Alberta. A summary Metamorphic mineral assemblages were identified in out- of key mineral chemical parameters is presented Table 2. crop and diamond drillcore, and by more detailed petro- Complete analyses of representative minerals, along with

Published by NRC Research Press 1126 Can. J. Earth Sci., Vol. 49, 2012

Fig. 5. (a) Fibrous sillimanite (Sil) pseudomorph after andalusite in pelite of the Pipe Formation at the city of Thompson. The pseudomorph is transected by an S2 shear band, and much of the sillimanite is replaced by fine-grained, white (WM). (b) Andalusite (And)-bearing quartz (Qtz) vein hosted in pelite of the Pipe Formation at Ospwagan Lake. The quartz vein is wrapped by the S2 foliation. (c) Photomicro- graph of potassium feldspar in pelite of the Pipe Formation at Thompson Mine. The K-feldspar (Kfs) is partially replaced by skeletal inter- growths of quartz + muscovite (Ms), and fine-grained, massive white mica. (d) Diamond drillcore from the Thompson structure. A band of tape marks a section of semipelite of the Thompson Formation containing fibrous sillimanite knots that are pseudomorphs after poikiloblastic andalusite. The tape is ∼2.5 cm wide. (e) Photomicrograph of garnet (Grt) enclosed by cordierite (Crd) in pelite from the Pipe Formation; the sample is from the granulite-facies zone west of Mystery Lake. (f) Photomicrograph of K-feldspar in pelite from Paint Lake. The K-feldspar is partially replaced by skeletal intergrowths of quartz and biotite (Bt). (g) Sillimanite pseudomorphs after idioblastic andalusite in pelite from Paint Lake. (h) Composite photomicrograph perpendicular to a pseudomorphed andalusite from the previous photograph. The andalusite is replaced by polycrystalline sillimanite, which in turn, is partially replaced by cordierite.

1 analytical details, are found in supplementary Tables S1–S5. The S2 foliation is locally folded by F3 folds and crosscut by Fourteen samples were sent to Activation Laboratories Ltd., aS3 foliation defined by reoriented muscovite and local bio- Ancaster, Ontario, for whole-rock geochemical analysis tite. Retrograde chlorite is locally observed. It typically occurs (Table 3). Major elements were analyzed by lithium as a replacement of biotite and, along with muscovite, as a metaborate–tetraborate fusion – inductively coupled plasma replacement of staurolite. The chlorite is commonly randomly (ICP). Total S and C were analyzed by infrared spectroscopy oriented; however, it can define a weak S3 foliation. using an induction furnace. At the city of Thompson, pelite of the Pipe Formation con- tains of staurolite and garnet, and knots of fi- Mineral assemblages and metamorphic brous sillimanite. The staurolite is commonly xenoblastic domains with rims of randomly oriented medium- to coarse-grained muscovite. The muscovite rims commonly retain the poikilo- Three distinct are recognized in the blastic texture of the original staurolite. The sillimanite knots TNB: middle amphibolite, upper amphibolite, and granulite. locally retain the outlines of prisms and are likely pseudo- The metamorphic facies are oriented subparallel to the re- morphs after andalusite (Fig. 5a). The sillimanite may be ra- gional strike of the TNB (Fig. 4). The mineral assemblages dial or randomly oriented at the cores of the knots, but is that define these facies are presented in the following text, typically oriented parallel to S at the rim. grouped according to protolith. Protoliths include pelites, alu- 2 At Ospwagan Lake, pelite of both the Pipe and Setting for- minous greywackes, mafic igneous rocks, iron formations, mations contain porphyroblasts of staurolite and garnet. and ferruginous wackes (although all the rocks are metamor- phosed, the prefix “meta” has been omitted from rock names Staurolite porphyroblasts are wrapped by the S2 foliation. to improve readability). All mineral abbreviations in the text The garnet locally forms inclusions in the staurolite, and rare For personal use only. are from Kretz (1983). idioblastic garnet faces overgrow the S2 foliation, sug- gesting protracted growth of garnet. Quartz veins are locally Pelite and semipelite bulk compositions present in pelite of the Pipe Formation. Pinch-and-swell The majority of pelite and semipelite samples are from the structures are a common characteristic of the quartz veins Manasan, Pipe, and Setting formations of the Paleoprotero- which are oriented parallel to, and are wrapped by, the S2 fo- zoic Ospwagan Group. Metamorphic assemblages must liation. Coarse-grained, idioblastic andalusite is commonly therefore be the result of Paleoproterozoic metamorphism. present within the quartz veins (Fig. 5b), but has not been The exceptions to this are the samples from the Paint Lake observed in the groundmass of the . area, which are part of a metasedimentary sequence of uncer- At the Pipe II mine, an eastward transition occurs from tain age. Because the Paint Lake sequence does not correlate pelite containing the mineral assemblage Qtz + Ms + Bt + to any of the known Paleoproterozoic supracrustal groups in Pl + Sil, with or without St, And, and Grt, to pelite contain- the region, Couëslan (2009) suggested a tentative Archean ing Qtz + Bt + Sil + Kfs + Pl, with or without Grt (Couë- age for the metasedimentary package. All rocks of pelite and slan et al. 2011). This transition indicates a change from semipelite bulk composition contain in their assemblage middle amphibolite-facies to upper amphibolite-facies peak Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 Qtz + Pl + Bt + Ms (at lower grade) or Kfs (at higher metamorphic conditions, with the metamorphic grade in- grade). Accessory phases commonly include pyrrhotite (Po), creasing towards the east as outlined in Couëslan et al. 2011 graphite (Gr), (Ilm), apatite (Ap), (Tur), (Fig. 4). Porphyroblasts of andalusite are common in the and monazite (Mnz). middle amphibolite-facies rocks, and are wrapped by the S2 foliation that typically contains fine-grained fibrous silliman- Middle amphibolite facies ite. Staurolite is locally observed as inclusions in andalusite. Pelitic and semipelitic rocks containing the assemblage Qtz + Ms + Bt + Pl, with or without Grt, Sil, andalusite Upper amphibolite facies (And), and staurolite (St), are amongst the lowest metamor- Ospwagan Group pelitic and semipelitic rocks containing phic grade rocks in the TNB and indicate middle amphibo- the assemblage Bt + Qtz + Pl + Sil + Kfs, with or without lite-facies conditions. Porphyroblasts are typically wrapped Grt, are indicative of upper amphibolite-facies conditions. by, and contain inclusion trails parallel to, the S2 foliation. The rocks are generally migmatitic; however, nonmigmatitic,

1Supplementary data are available with the article through the journal Web site at http://nrcresearchpress.com/doi/suppl/10.1139/e2012-029.

Published by NRC Research Press Couëslan and Pattison 1127 For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

Kfs + Sil-bearing pelite is present in the Pipe mine area and the absence of leucocratic segregations in these rocks, the K- north of Setting Lake. In nonmigmatitic rocks, the K-feldspar feldspar is interpreted to have grown from muscovite breakdown forms equant, xenomorphic porphyroblasts. Combined with under subsolidus conditions. The highest grade semipelite from

Published by NRC Research Press 1128 Can. J. Earth Sci., Vol. 49, 2012

Fig. 6. Prograde mineral-assemblage map of the north half of the Thompson Nickel Belt for pelite and semipelite bulk compositions. Solid black lines define indicated assemblage-zone boundaries, dashed lines define inferred boundaries, and dotted lines indicate limits of data. The numbered sample locations are referred to in the text. Abbreviations as in Fig. 5. Pl, plagioclase; Spl, spinel; St, staurolite. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

the Pipe mine contains Qtz + Bt + Pl + Sil + Kfs; leuco- Garnet porphyroblasts are generally subidiomorphic to xeno- some is prominent in both outcrop and , sugges- morphic and wrapped by S2. K-feldspar occurs in the ground- tive of (Couëslan et al. 2011). mass and leucosome. It can be partially overprinted by In migmatitic rocks, migmatitic layering and irregular myrmekite, fine-grained white mica, randomly oriented mus- patches of leucosome are typically foliated and wrapped by covite, and skeletal quartz–muscovite or rarely quartz–biotite S2 and are interpreted as syndeformational; however, exam- intergrowths (Fig. 5c). Fibrous to fine-grained prismatic silli- ples of massive leucosome transecting the S2 fabric do occur. manite occurs intergrown with biotite and locally forms dis-

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Fig. 7. Prograde mineral-assemblage map of the south half of the Thompson Nickel Belt for pelite and semipelite bulk compositions. Solid black lines define indicated assemblage-zone boundaries, dashed lines define inferred boundaries, and dotted lines indicate limits of data. The numbered sample location is referred to in the text. The position of the Kfs + Grt + Crd assemblage zone west of Setting Lake is from Bailes and McRitchie (1978). Abbreviations as in Figs. 5 and 6. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

continuous laminae; it also occurs as discrete flattened knots Granulite facies and as elongate masses with a shape locally reminiscent of Migmatitic pelites containing the mineral assemblage andalusite (Fig. 5d). Sillimanite may be partially replaced by Qtz + Bt + Kfs + Crd + Pl + Grt + Sil are characteristic of retrograde muscovite and fine-grained white mica. Retro- metamorphism at granulite-facies conditions. The subassem- grade muscovite can also occur as grains that are ran- blage Kfs + Grt + Crd is commonly used to define the gran- domly oriented or oriented along S3.S3 muscovite is locally ulite facies in metapelites (e.g., Pattison et al. 2003). associated with chlorite. Staurolite and spinel, along with rare , occur as inclusions

Published by NRC Research Press 1130 Can. J. Earth Sci., Vol. 49, 2012

Fig. 8. (a) Photomicrograph of a staurolite enclosed by cordierite in aluminous greywacke of the Burntwood Group from Mys- tery Lake. The cordierite is rimmed by fine-grained sillimanite along the bottom of the image. (b) Photomicrograph of staurolite inclusions in plagioclase from greywacke of the Burntwood Group from Mystery Lake. (c) Back-scattered electron (BSE) image of fine-grained quartz– cordierite intergrowth rimming garnet in greywacke from the Burntwood Group. The cordierite is locally pinitized which appears very dark grey to black. The sample is from the granulite-facies zone west of Mystery Lake. (d) Photomicrograph of oscillatory-zoned clinopyroxene (Cpx) in a partially amphibolitized Molson dyke at Wintering Lake. (e) Metamorphosed Paleoproterozoic mafic dyke hosted in strongly de- formed Archean gneiss at Paint Lake. Note the preserved chill margin and incipient boudinage wrapped by the S2 foliation. (f) Back-scattered electron image of clinopyroxene with exsolution lamellae of Ca-poor . The sample is from the dyke pictured in Fig. 5e.(g) Photomi- crograph of K-feldspar and orthopyroxene (Opx) replaced by skeletal quartz–biotite intergrowth in iron formation from the Pipe Formation. The sample is from the granulite-facies zone west of Mystery Lake. (h) Massive orthopyroxene-bearing leucosome (L) hosted by ferruginous wacke at Paint Lake. The leucosome cuts across the S2 foliation. Abbreviations as in Figs. 5 and 6. Ap, apatite; Hbl, hornblende; Po, pyrrhotite.

in Grt. The S2 foliation wraps around porphyroblasts of garnet, matitic Kfs + Sil assemblages define upper amphibolite-facies which in turn, contain inclusion trails that parallel S2.Theleu- conditions. The highest grade pelitic assemblages, represen- cosome is typically parallel to, and weakly wrapped by, the S2 tative of granulite-facies conditions, are characterized by foliation. Grains of biotite located within the leucosome may Kfs + Crd + Grt. be randomly oriented. Antiperthite is locally present and occurs most commonly in the leucosome. Cordierite occurs in several Aluminous greywacke bulk compositions forms: as aggregates rimming garnet; as enveloping grains and All occurrences of aluminous greywacke are interpreted to aggregates enclosing sillimanite with or without spinel; and lin- be from the Paleoproterozoic Burntwood Group. Burntwood ing the contacts between melanosome and leucosome (Fig. 5e). Group rocks are present only along the west margin of the Fine-grained intergrowths of quartz + cordierite, plagioclase + TNB. All assemblages contain Qtz + Pl + Bt + Grt, and are biotite, and quartz + biotite are common at the grain bounda- free of Ms. K-feldspar is present only at the highest metamor- ries of garnet. Skeletal quartz–biotite intergrowths are also a phic grades. Common accessory minerals include Gr, Ilm, common replacement of K-feldspar (Fig. 5f). S3 biotite and Po, Ap, Tur, Mnz, and zircon (Zrn). chlorite is associated with pinite alteration in cordierite. A sample of pelite from Paint Lake contains very coarse- Middle amphibolite facies grained garnet porphyroblasts rimmed by leucosome. The Greywacke characterized by the assemblage Bt + Qtz + melanosome is composed dominantly of biotite, cordierite, Pl + Grt + Crd + Sil, with or without St, indicates middle garnet, and sillimanite. The sillimanite occurs as monocrys- amphibolite-facies peak metamorphic conditions. White leu- talline and polycrystalline pseudomorphs after andalusite cocratic, layer parallel segregations containing predominantly (Figs. 5g,5h), and as scattered fine-grained inclusions, paral- quartz with minor plagioclase are wrapped by S2. The leuco-

For personal use only. lel to S2, in cordierite. The sillimanite pseudomorphs are cratic segregations give the rock a migmatitic appearance, but completely enclosed, and partially replaced, by granoblastic they are interpreted as subsolidus segregations (cf. Gordon cordierite. Garnet is characterized by inclusion-free cores, lo- 1984; Sawyer and Robin 1986). Larger segregations locally cally with euhedral outlines, inclusion-rich annuli, and inclu- contain porphyroblasts of cordierite. Staurolite occurs as rare sion-free rims. Inclusions consist dominantly of sillimanite, porphyroblasts, locally enclosed by granoblastic cordierite quartz, staurolite, spinel–sillimanite symplectite, replacing (Fig. 8a), and more commonly as fine-grained inclusions in staurolite, rutile, ilmenite, and pyrrhotite. Both the garnet and garnet, plagioclase, and rarely cordierite (Fig. 8b). Garnet the pseudomorphed andalusite are wrapped by S2. Local S3 porphyroblasts are wrapped by the S2 foliation; however, shears are associated with pinite alteration of cordierite, chlor- both garnet and cordierite commonly contain biotite inclu- ite alteration of biotite, and the presence of myrmekite along sions oriented parallel to S2. Cordierite is common in the K-feldspar grains. The rock’s foliation is oriented parallel to groundmass where it is locally wrapped by shear bands con- the Paleoproterozoic north-northeast D2–D3 fabric of the TNB taining biotite and very fine-grained fibrous sillimanite. The – rather than the regional east west-trending foliation (SA)of shear bands locally merge with, and tighten, the dominant S2 gneisses in the adjacent Superior craton. The presence of foliation. It is unclear if the shear bands developed as part of Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 S2 sillimanite inclusions in cordierite, and presence of pseu- the S2 fabric, or represent a later, subparallel S3 foliation. domorphed andalusite, suggests that the preserved foliation Chlorite replacement of biotite and pinite alteration of cor- and prograde assemblage likely developed as part of the Pa- dierite is commonly present along the bands, suggesting that leoproterozoic Hudsonian metamorphism. they were at the least reactivated during D3. Metamorphic assemblage zones Upper amphibolite facies The pelite and semipelite assemblages described in the Greywacke containing Bt + Qtz + Pl + Grt + Sil + Crd is preceding section define four metamorphic assemblage zones characteristic of the upper amphibolite-facies metamorphism. (Figs. 6, 7). The lowest grade assemblages contain Ms + Two types of leucocratic segregations are present: (i) quartz- St ± And and define a low-grade subzone in the core of the rich segregations (similar to those described in the preceding middle amphibolite-facies zone in the Pipe mine – Ospwagan subsection), and (ii) quartz–plagioclase segregations. The Lake area, and near the city of Thompson. Staurolite-absent, generally seriate texture of the type ii segregations, along Ms ± Sil-bearing assemblages define a higher grade subzone with the increased plagioclase content, suggest that they may of middle amphibolite-facies metamorphism. Variably mig- be the result of partial melting. Both varieties of segregation

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Published by NRC Research Press 1132 Can. J. Earth Sci., Vol. 49, 2012

Fig. 9. Prograde mineral-assemblage map for aluminous greywacke bulk compositions. Solid black lines define indicated assemblage-zone boundaries, dashed lines define inferred boundaries, and dotted lines indicate limits of data. The numbered sample locations are referred to in the text. The unfilled symbol in the Kisseynew Domain is from Growdon (personal communication, 2010) and includes orthopyroxene in the assemblage. Abbreviations as in Figs. 5 and 6. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

are wrapped by S2 biotite and sillimanite. Garnet porphyro- Granulite facies blasts are also wrapped by the S2, but together with cordierite Greywacke containing the assemblage Qtz + Pl + Kfs + typically contain inclusion trails oriented parallel to the folia- Bt + Crd + Grt, with or without Sil, indicate granulite-facies tion. Sillimanite is more abundant than at middle amphibolite peak metamorphic conditions and is the same mineral assem- facies and also occurs as discrete fibrous knots. Staurolite oc- blage observed in pelites at this grade. The leucosome is typi- curs as rare inclusions in garnet and plagioclase. cally plagioclase-rich with subordinate quartz and K-feldspar,

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and the plagioclase is locally antiperthitic. The foliation is Granulite facies typically tightened along leucosome margins. Porphyroblasts Mafic rocks with the assemblage Hbl + Pl + orthopyroxene of cordierite and garnet are typically wrapped by the folia- (Opx) + Cpx + Qtz + Bt, with or without Grt and tion. S2 sillimanite occurs as relict clusters of needles at the (Mag), are characteristic of granulite-facies metamorphism. cores of cordierite porphyroblasts and is locally accompa- Some rocks contain leucocratic segregations of quartz and nied by fine-grained spinel. Fine-grained intergrowths of plagioclase with local antiperthite and orthopyroxene por- quartz + cordierite (Fig. 8c), and plagioclase + biotite with phyroblasts. Metamorphic typically have partial or without quartz, are common at the margins of garnet rims of hornblende with or without wormy intergrowths of grains. Skeletal intergrowths of quartz + biotite are also quartz. common and appear to be a replacement of K-feldspar and Rare weakly deformed diabase dykes (Fig. 8e) with pre- possibly cordierite. S3 shear planes are locally developed served chilled margins are present at Paint Lake. They are and associated with chloritization of biotite, pinitization of medium grained and granoblastic with hornblende and two cordierite, and replacement of plagioclase and cordierite by pyroxenes defining a weak S2 foliation. Grains of plagioclase very fine-grained white mica. and pyroxene are relatively homogeneous chemically, with the exception of diffuse zoning in the plagioclase, and the Metamorphic assemblage zones presence of cryptic exsolution lamellae, visible in back-scatter Three metamorphic assemblage zones can be defined for electron images, in both the plagioclase and clinopyroxene rocks of aluminous greywacke bulk composition (Fig. 9), cor- (Fig. 8f). Diffuse zoning in the plagioclase appears to be re- responding to the three metamorphic facies. The first assem- lated to the presence of adjacent Ca-bearing phases such as blage zone (middle amphibolite facies) is characterized by hornblende, clinopyroxene, and garnet. Garnet occurs as the presence of porphyroblastic staurolite and quartz-rich leu- fine-grained, idioblastic inclusions in plagioclase. The cocratic segregations. With increasing metamorphic grade weakly deformed nature of the dykes in otherwise strongly (upper amphibolite facies), staurolite disappears or is present deformed Archean gneiss suggests that they are Paleoproter- only as small relict inclusions in plagioclase, garnet, and cor- ozoic (likely Molson dykes), or at least, postdate the Ar- dierite; plagioclase-rich leucocratic segregations are observed, chean metamorphic events. Incipient boudinage of one of and sillimanite becomes more abundant. In the highest grade these dykes, which is wrapped by S2, suggests it intruded zone (granulite facies), the greywacke contains Kfs + Crd + prior to, or during, D2. Grt and Kfs-bearing leucocratic segregations as observed in pelitic rocks at this grade. Metamorphic assemblage zones Three metamorphic assemblage zones are recognized for Mafic bulk compositions mafic bulk compositions (Figs. 10, 11). Rocks subjected to Most samples of metamorphosed mafic igneous rocks were middle and upper amphibolite-facies conditions do not contain orthopyroxene. Orthopyroxene first appears in clino-

For personal use only. collected from amphibolitized dykes and sills hosted by Osp- wagan Group supracrustal rocks, or from volcanic rocks of pyroxene-free, Ca-poor mafic rocks, in the transition zone be- the Bah Lake assemblage at the top of the Ospwagan Group tween the upper amphibolite-facies and granulite-facies sequence. Exceptions to this are the mafic rocks sampled at domains. Mafic rocks of the granulite facies contain both Wintering and Paint lakes, which represent amphibolitized clinopyroxene and orthopyroxene. dykes that intruded Archean gneiss. Accessory (Ttn), Ilm, Po, and Ap are common. Iron-formation bulk compositions All iron-formation samples are from the Pipe Formation of Middle and upper amphibolite facies the Ospwagan Group. Mineral assemblages vary broadly ac- Mafic rocks containing a high-variance assemblage of cording to whether the protolith was dominated by iron ox- hornblende (Hbl) + Pl, with or without Bt, Qtz, Grt, cum- ides, carbonates, sulphides, or . For comparative mingtonite (Cum), and clinopyroxene (Cpx), are typical of analysis of metamorphic grade, samples rich in iron silicates middle and upper amphibolite-facies metamorphic conditions. were chosen that contained no carbonate, and minimal At Wintering Lake, situated near the eastern boundary of amounts of iron sulphides and oxides. Accessory phases in-

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 the TNB (Fig. 2), Molson dykes are locally undeformed and clude Ap and Mnz, and rare Pl. only partially affected by Hudsonian metamorphism. The dykes retain an ophitic texture, and plagioclase and relict cli- Middle amphibolite facies nopyroxene and orthopyroxene are characterized by oscilla- The mineral assemblages observed in iron formations vary tory zoning (Fig. 8d). Both clinopyroxene and orthopyroxene considerably from one layer or lamination to the next, but have thick rims of hornblende. Preservation of the primary assemblages containing Qtz + Bt + grunerite (Gru) + ophitic texture and oscillatory zoning of minerals suggest the Hbl + Mag + Po, with or without Grt, are typical of middle dykes are Paleoproterozoic and postdate the Archean granu- amphibolite-facies peak metamorphic conditions (Couëslan lite-facies metamorphic event. et al. 2011). Exsolution lamellae of hornblende within gru- At the transition from the amphibolite facies to the granulite nerite, and grunerite within hornblende are common. Min- facies, several samples of mafic rock contain orthopyroxene eral assemblages of iron formation subjected to middle without clinopyroxene. These rocks may represent low-Ca amphibolite-facies conditions are best observed in the Pipe mafic rocks, where the orthopyroxene formed from a reac- mine area and are described in greater detail in Couëslan et tion involving cummingtonite. al. (2011).

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Fig. 10. Prograde mineral-assemblage map of the north half of the Thompson Nickel Belt for mafic bulk compositions. Solid black lines define indicated assemblage-zone boundaries, dashed lines define inferred boundaries, and dotted lines indicate limits of data. The numbered sample location is referred to in the text. The unfilled symbols in the Kisseynew Domain are from Growdon et al. (2006). Abbreviations as in Figs. 5, 6, and 8. Cum, cummingtonite. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

Upper amphibolite facies occurs as rare porphyroblasts, typically along metachert lam- Assemblages consisting of Qtz + Opx + Bt + Hbl + Po + inations. Retrograde grunerite is common as optically contin- Mag, with or without Grt and Kfs, are characteristic of iron uous overgrowths on hornblende, fine-grained intergrowths formation metamorphosed at upper amphibolite-facies condi- with biotite, pseudomorphs after orthopyroxene, and as ag- tions. In more foliated samples, orthopyroxene and garnet are gregates associated with locally developed S3 foliation elongate parallel to, and wrapped by, S2. Potassium feldspar planes. Prograde hornblende with exsolution lamellae of gru-

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Fig. 11. Prograde mineral-assemblage map of the south half of the Thompson Nickel Belt for mafic bulk compositions. Solid black lines define indicated assemblage-zone boundaries, dashed lines define inferred boundaries, and dotted lines indicate limits of data. The numbered sample location is referred to in the text. The unfilled symbols in the Kisseynew Domain are from Growdon et al. (2006). Abbreviations as in Figs. 5, 6, and 8. Cum, cummingtonite. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

nerite is present; however, grains of retrograde grunerite do spar appear interstitial and are rarely associated with not contain exsolution lamellae of hornblende. myrmekite. In one sample, the myrmekitic plagioclase also forms a fine-grained intergrowth with orthopyroxene. Ortho- Granulite facies pyroxene is locally elongate parallel to S2 and commonly con- Iron formation containing the assemblage Opx + Qtz + tains inclusions of S2 biotite. Grains of orthopyroxene and K- Bt + Grt + Kfs + Po with or without Mag is typical of granu- feldspar are commonly partially replaced by fine-grained, skel- lite-facies metamorphic conditions. Perthitic grains of K-feld- etal intergrowths of quartz + biotite with or without garnet

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Fig. 12. Prograde mineral-assemblage map of the north half of the Thompson Nickel Belt for iron-formation bulk compositions. Solid black lines define indicated assemblage-zone boundaries, dashed lines define inferred boundaries, and dotted lines indicate limits of data. The num- bered sample location is referred to in the text. Abbreviations as in Figs. 5 and 8. Gru, grunerite; Mag, magnetite. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

(Fig. 8g). This texture suggests local back-reaction of K- mation metamorphosed at middle amphibolite-facies conditions. feldspar and orthopyroxene to form biotite + quartz, a com- Orthopyroxene first appears at the transition between middle am- mon retrograde feature of granulite-facies migmatitic rocks phibolite- and upper amphibolite-facies conditions and becomes (Waters 2001; Sawyer 2008). the dominant Fe-silicate in upper amphibolite-facies rocks. At Metamorphic assemblage zones the highest metamorphic grade, the presence of Opx + Kfs, com- Three metamorphic assemblage zones are recognized bined with evidence for the incongruent breakdown of biotite, (Figs. 12, 13). Grunerite is the dominant Fe-silicate in iron for- suggests granulite-facies metamorphic conditions.

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Fig. 13. Prograde mineral-assemblage map of the south half of the Thompson Nickel Belt for iron-formation bulk compositions. Solid black lines define indicated assemblage-zone boundaries, dashed lines define inferred boundaries, and dotted lines indicate limits of data. Abbre- viations as in Figs. 5 and 8. Gru, grunerite; Mag, magnetite. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

Ferruginous wacke bulk compositions ble 3). The ferruginous wacke is part of the metasedimentary The ferruginous wacke differs from iron formation in that sequence of uncertain age. In all outcrops, the bedding and it is likely of clastic, rather than chemical, sedimentary ori- dominant foliation of the wacke parallels the north-northeast gin, and is typically interbedded with psammitic horizons regional S2 foliation of the TNB, rather than the east–west SA rather than chert. Although it is Fe-rich, it does not contain preserved in the adjacent Superior craton. Accessory minerals the extreme Fe-enrichment present in iron formation (Ta- include Ap, Po, Zrn, Mnz, and Ilm.

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Fig. 14. Metamorphic-facies map of the Thompson Nickel Belt with the location of metamorphic assemblages in ferruginous wacke. The numbered sample location is referred to in the text. Abbreviations as in Figs. 5 and 8. Pl, plagioclase. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

Upper amphibolite facies Granulite facies Ferruginous wacke containing the assemblage Qtz + Pl + Ferruginous wacke containing Qtz + Pl + Bt + Opx + Bt + Grt is characteristic of upper amphibolite-facies meta- Grt + Kfs with or without Hbl, Cpx, and Po is characteristic morphic conditions. The rock typically contains discontinu- of granulite-facies metamorphic conditions. The rocks are ous leucocratic segregations parallel to the bedding and S2. typically migmatitic, with orthopyroxene-bearing leucosome. Subidioblastic garnet has locally overgrown S2 biotite, but is The leucosome typically parallels S2 and locally pools in D2 in turn weakly wrapped by S2. Chlorite occurs as a replace- boudin necks; however, the leucosome also locally forms ment of biotite along locally observed S3 . massive discordant pools (Fig. 8h). Antiperthitic plagioclase

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is relatively common in both the matrix and leucosome. Po- mafic rocks, iron formations, and ferruginous wackes. Equili- tassium feldspar is most common in the leucosome, but it oc- brium assemblage diagrams were calculated for each of these curs locally in the matrix where it is associated with both bulk compositions, and of these, 15 were selected which pro- myrmekite and skeletal quartz + biotite intergrowth. Ortho- vided well-constrained P–T conditions for the observed min- pyroxene in the leucosome is randomly oriented and idioblas- eral assemblages present in the rock. tic to subidioblastic, whereas in the matrix it is typically xenomorphic and elongate parallel to S2. Orthopyroxene is Middle amphibolite facies locally replaced by skeletal quartz + biotite intergrowth, gru- Three samples, spanning a distance of ca. 73 km, were se- nerite, or chlorite. Garnet is typically anhedral and weakly lected from the elongate middle amphibolite-facies zone. wrapped by the foliation. Pooling of the leucosome into D2 Aluminous greywacke sample MY-07-03A from the Burnt- structures, and the local occurrence of apparently post-D2 wood Group at Mystery Lake contains the assemblage Bt + leucosome, suggests at least a portion of the orthopyroxene- Qtz + Pl + Grt + Crd + Sil + Ilm + St. The equilibrium- bearing leucosome is Paleoproterozoic. assemblage diagram for this sample is show in Fig. 15a. This assemblage defines a narrow field in P–T space of ca. Metamorphic assemblage zones 610–620 °C and 4.4–5.0 kbar; however, this assemblage A scarcity of recognized ferruginous wacke in the majority may not represent equilibrium. Although porphyroblasts of of the TNB hampers efforts to define assemblage zones and staurolite are present, staurolite more commonly occurs as also limits the usefulness of this bulk composition. Where fine-grained inclusions in garnet, plagioclase, and cordierite, found, the combination of orthopyroxene + K-feldspar leuco- suggesting it may no longer have been stable in the matrix. some, antiperthite, and skeletal quartz + biotite intergrowths, If so, the upper P–T limit could be as high as ca. 680 °C is indicative of granulite-facies metamorphism (Fig. 14). and 5.5 kbar as constrained by the solidus. Pelite sample OP-06–27C from the Pipe Formation at Osp- Phase equilibria modelling wagan Lake contains the prograde assemblage Qtz + Ms + Bt + Pl + St + Grt + Ilm. The phase-diagram section gener- Equilibrium-assemblage diagrams were calculated using ated for this sample is given in Fig. 15b. Although the as- the Theriak-Domino software package (de Capitani and semblage defines an area in P–T space of roughly 550–650 °C Brown 1987; de Capitani and Petrakakis 2010), using the up- and 3.25–6.25 kbar, the presence of andalusite-bearing, syn- dated 2003 ds5.5 thermodynamic dataset of Holland and Po- metamorphic quartz veins suggest P–T conditions within the well (1998). The activity models used are those outlined in field of stable andalusite. This constrains the P–T condi- Tinkham and Ghent (2005), Pattison and Tinkham (2009), tions at ca. 550–585 °C and 3.2–4.1 kbar. and Couëslan et al. (2011). Samples were modelled in the Samples PP-08-08B and PP-06-28C (see next section) are – – – chemical system MnNCKFMASHT (MnO Na2O CaO from the Pipe II mine and were previously presented in K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2), with C and P2O5 Couëslan et al. (2011). The Pipe II mine is situated at the

For personal use only. omitted from the whole-rock analysis, and projected from boundary between the middle amphibolite-facies zone and pyrrhotite. The chemical system was oversaturated with a upper amphibolite-facies zone. Pelite sample PP-08-08B from pure H2O fluid phase below the solidus. For calculating the Pipe Formation contains the middle amphibolite-facies as- melt-bearing systems above the solidus, the H2O content was semblage Qtz + Bt + Ms + Pl +St + And + Sil + Grt. limited to that contained in hydrous phases immediately be- This assemblage defines a short univariant line at ca. 585 °C low the solidus. and 3.9 kbar in the calculated equilibrium assemblage dia- For estimating P–T conditions, mineral assemblages of in- gram (Fig. 15c). Textural evidence suggests that the stauro- dividual bulk compositions commonly provide tight P–T esti- lite and andalusite may be metastable relics, in which case mates. In a number of situations, however, the P–T the metamorphic conditions could have been as high as 585– constraints provided by individual rocks were so broad as to 600 °C and 3.7–3.9 kbar. Overall, there appears to be a north- be of little value. In these situations, a particularly useful ap- east increase in pressure from the Pipe mine – Ospwagan proach was the use of overlapping P–T domains from two or Lake area to the Mystery Lake area within the middle am- more bulk compositions in the same metamorphic domain, phibolite-facies zone. which in many cases provided tight P–T estimates.

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 Upper amphibolite facies Sample selection Seven samples were selected from the upper amphibolite- Samples of various from throughout the TNB facies zone: four from the eastern domain (samples SL-07- were examined under petrographic microscope to look for 29C, PP-06-28C, TM-06-14A2, SP-02-01; Figs. 6, 7) and suitable, low-variance assemblages. The extensive use of three from the western domain (samples MN-07-17A, CC- drillcore for this study often resulted in a limited sample 771, CC-786; Figs. 6, 9). size, not suitable for bulk geochemical analyses. As a result, Arkose sample SL-07-29C from the Grass River Group at samples available for geochemical analyses were restricted to Setting Lake contains the prograde assemblage Qtz + Pl + those collected from outcrops, at mine sites, or from continu- Bt + Kfs + Sil + Ilm. This defines a field in P–T space of ous, relatively homogeneous units from drillcore. Under these ca. 660–680 °C and 3.3–4.5 kbar in the calculated equili- restrictions, samples were selected to cover the largest areal brium-assemblage diagram (Fig. 16a). extent along strike within each metamorphic-facies zone. Semipelite sample PP-06–28C from the Setting Formation Bulk geochemical analyses were obtained for 40 samples at the Pipe II mine site contains the upper amphibolite-facies consisting of pelites, semipelites, aluminous greywackes, assemblage Qtz + Bt + Kfs + Pl + Sil. The outcrop contains

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Fig. 15. Equilibrium-assemblage diagrams for samples from the middle amphibolite-facies zone (Figs. 4, 6, 8). The dashed grey lines indicate the aluminosilicate stability fields in the system Al2O3–SiO2–H2O, as defined by Holland and Powell (1998) based on Pattison (1992). The observed metamorphic assemblage for each sample is circled. (a) Greywacke MY-07-3A, from the Burntwood Group, Mystery Lake. The narrow grey field represents the observed metamorphic assemblage containing staurolite, cordierite, and sillimanite. (b) Pelite OP-06-27C, from the Pipe Formation, Ospwagan Lake. The grey field represents the peak metamorphic conditions as indicated by the observed staurolite- bearing metamorphic assemblage and the presence of andalusite-bearing quartz veins. (c) Pelite PP-08-08B, from the Pipe Formation, Pipe II mine (data from Couëslan et al. 2011). The grey field represents the peak metamorphic conditions as indicated by the observed metamorphic assemblage (see text for details). Whole-rock compositions can be found in Table 2. 1 kbar = 100 MPa. Abbreviations as in Figs. 5 and 8. Chl, chlorite; Cz, clinozoisite; Ilm, ilmenite; Ky, ; Pg, paragonite; Rt, rutile; St, staurolite; Ttn, titanite. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

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Fig. 16. Equilibrium-assemblage diagrams for samples from the upper amphibolite-facies zone (Figs. 4, 6, 7). The grey fields represent the observed metamorphic assemblages (see text for details). The dashed grey lines indicate the aluminosilicate stability fields in the system Al2O3–SiO2–H2O, as defined by Holland and Powell (1998) based on Pattison (1992). (a) Arkose SL-07-29C, from the Grass River Group, Setting Lake. (b) Semipelite PP-06-28C, from the Setting Formation, Pipe II mine (data from Couëslan et al. 2011). The observed meta- morphic assemblage is circled. (c) Pelite TM-06-14A2, from the Pipe Formation, Thompson mine. (d) Semipelite SP-02-01, from the Mana- san Formation, Thompson mine. Whole-rock compositions can be found in Table 2. 1 kbar = 100 MPa. Abbreviations as in Figs. 5–15. Ab, . For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

no leucosome, and the K-feldspar occurs as relatively equant conditions of 640–660 °C and 3.0–3.6 kbar (Fig. 16b; Couë- porphyroblasts distributed throughout the matrix. These tex- slan et al. 2011). tures suggest the K-feldspar crystallized under subsolidus Pelite sample TM-06-14A2 from the Pipe Formation at the conditions. The observed assemblage indicates metamorphic Thompson mine contains the prograde assemblage Bt +

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Fig. 17. Equilibrium-assemblage diagrams for samples from the upper amphibolite-facies zone west of Thompson (Figs. 6, 8). The grey fields represent the observed metamorphic assemblages (see text for details). The dashed grey lines indicate the aluminosilicate stability fields in the system Al2O3–SiO2–H2O, as defined by Holland and Powell (1998) based on Pattison (1992). (a) Greywacke MN-07-17A, from the Burnt- wood Group, north of Ospwagan Lake. (b) Greywacke CC-771, from the Burntwood Group, west of Thompson. (c) Pelite CC-786, from the Pipe Formation, west of Thompson. (d) The overlapping fields of b and c yield a best estimate for the peak metamorphic conditions in the upper amphibolite-facies zone west of Thompson, as indicated by the long dashed lines (see text for details). Whole-rock compositions can be found in Table 2. 1 kbar = 100 MPa. Abbreviations as in Figs. 5–15. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

Qtz + Pl + Sil + Kfs + Grt + Ilm. Significant amounts of calculated equilibrium assemblage diagram (Fig. 16c)of muscovite are present in this sample; however, it occurs as a ∼655–800 °C and 3.5–8.0 kbar. Better pressure and temper- retrograde replacement of sillimanite and as skeletal inter- ature constraints are obtained from semipelite sample SP-02- growths of quartz + muscovite after K-feldspar. The prograde 01 from the Manasan Formation at the Thompson mine assemblage for this sample defines a rather large field in the (Fig. 16d). The semipelite contains the prograde assemblage

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Fig. 18. Equilibrium-assemblage diagrams for samples from the granulite-facies zone at Paint Lake (Figs. 4, 6, 10, 14). (a) Diabase 108-09- 415, the grey field represents the observed metamorphic assemblage. (b) Ferruginous wacke 108-08-226, the dark grey field represents the observed metamorphic assemblage; however, the mode of occurrence of K-feldspar may suggest metamorphic conditions indicated by the light grey field (see text for details). (c) Average Archean pelite (Cameron and Garrels 1980), the grey field represents the observed meta- morphic assemblage in pelite PT-07-18A, Paint Lake (see text for details). The dashed grey lines indicate the aluminosilicate stability fields in the system Al2O3–SiO2–H2O, as defined by Holland and Powell (1998) based on Pattison (1992). (d) The overlapping fields of a–c yield a best estimate for the metamorphic conditions in the granulite-facies zone at Paint Lake, as indicated by the long dashed lines (see text for details). Whole-rock compositions can be found in Table 2. 1 kbar = 100 MPa. Abbreviations as in Figs. 5–15. Act, . For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

Bt + Qtz + Kfs + Pl + Sil + Ilm. The prograde assemblage Several samples of aluminous greywacke from the Burnt- defines a P–T field of roughly 670–710 °C and 3.5–5.5 kbar. wood Group and a sample of pelite from the Pipe Formation These lower pressures are in agreement with the sillimanite were collected in the upper amphibolite-facies zone west of pseudomorphs after andalusite in adjacent parts of the Thompson (Figs. 6, 9). All samples of greywacke contain Thompson structure (Fig. 5d). the same peak metamorphic mineral assemblage of Bt +

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Qtz + Pl + Grt + Crd + Sil + Ilm. Sample MN-07-17A de- Although sillimanite is present in the prograde assemblage, fines a P–T field of 4.4–5.8 kbar and 675–755 °C (Fig. 17a), it is typically partially overprinted by cordierite. Assuming whereas sample CC-771 defines P–T conditions of 5.0– the sillimanite is a relict prograde phase, the assemblage de- 5.9 kbar and 680–760 °C (Fig. 17b). The pelite sample CC- fines a broad P–T field of 2.7–7.0 kbar and 715–840 °C, the 786 contains the mineral assemblage Bt + Pl + Qtz + Sil + upper pressure limit corresponding to the maximum pressure Grt + Kfs + Ilm. This assemblage defines a P–T field of ca. for the assemblage Kfs + Crd + Ilm. This ∼7.0 kbar limit is 5.3–7.5 kbar and 720–790 °C (Fig. 17c). A better estimate remarkably consistent amongst the pelite compositions mod- for the P–T conditions of the upper amphibolite-facies zone elled in this study. west of Thompson can be made by overlaying phase equili- The P–T conditions of peak metamorphism in the Paint bria diagrams for the greywacke and pelite (Fig. 17d). The Lake area can be better constrained by overlapping the three P–T region of overlap is 5.3–5.9 kbar, 730–755 °C. phase equilibria diagrams (Fig. 18d). The minimum tempera- Similar to the middle amphibolite-facies zone, there ap- ture and pressure is constrained by the diabase assemblage at pears to be a northeast increase in pressure across the upper 780 °C and 6.5 kbar, the maximum temperature and pressure amphibolite-facies zones. The upper amphibolite-facies rocks by the pelite assemblage at 830 °C and 7.0 kbar, giving rise west of the city of Thompson appear to have been subjected to a combined estimate of 6.5–7.0 kbar, 780–830 °C. to higher pressure than those east of the city. One sample of aluminous greywacke from the Burntwood Group (HY-06-22A), and one sample each of pelite (CC-601) Granulite facies and iron formation (CC-602) from the Pipe Formation were Six samples were selected from the granulite-facies zone: selected from the granulite-facies zone west of Mystery three from the eastern belt at Paint Lake (samples 108-09- Lake. The greywacke sample contains the peak metamorphic 415, 108-08-226, PT-07-18A; Figs. 6, 10, 14) and three mineral assemblage Qtz + Pl + Kfs + Bt + Crd + Grt + Ilm. from the western belt (samples HY-06-22A, CC-601, CC- This assemblage defines a P–T field of ca. 2.1–7.0 kbar and 602; Figs. 6, 9, 12). 720–825 °C in the calculated equilibrium-assemblage dia- The metadiabase sample (108-09-415) contains the assem- gram (Fig. 19a). blage Hbl + Pl + Opx + Cpx + Qtz + Bt + Ilm + Grt. Local The pelite and iron-formation samples were collected from wormy intergrowths of quartz in hornblende, and the ubiqui- a single diamond drillhole. The pelite contains the assem- tous rimming of pyroxene by hornblende, suggests that a por- blage Qtz + Kfs + Pl + Bt + Crd + Grt + Sil + Ilm. The tion of the observed hornblende is the result of retrograde limited sample size available from the drillcore, together metamorphism; however, assuming at least a portion of the with the heterogeneous, migmatitic nature of the rock pre- hornblende is part of the prograde assemblage suggests P–T cluded obtaining a representative whole-rock geochemical conditions of at least 6.5–9.9 kbar and 775–825 °C (Fig. 18a). and modal analysis. An equilibrium assemblage diagram was The ferruginous wacke sample (108-08-226) contains the therefore calculated using an average Pipe Formation pelite assemblage Pl + Qtz + Bt + Opx + Kfs + Grt + Ilm. The composition (n = 16, supplementary Table S61, Fig. 19b). For personal use only. assemblage defines a large area in P–T space from ca. 4 to The observed assemblage plots in a narrow field extending 9.5 kbar and from 810 to 870 °C (Fig. 18b). However, this from 660 to 800 °C and from 3.5 to 6.9 kbar. likely represents a maximum set of P–T conditions. Most K- The iron-formation sample CC-602 contains the prograde feldspar occurs in the leucosome and is only locally present in metamorphic assemblage Opx + Qtz + Bt + Grt + Kfs + the groundmass. This suggests that most, and possibly all, of Ilm, with ubiquitous skeletal quartz–biotite intergrowth over- the K-feldspar crystallized out of the coexisting silicate melt. printed on K-feldspar and orthopyroxene. Potassium feldspar This could expand the area defined by the equilibrium assem- is not a discrete phase in any of the predicted phase assemb- blage to lower temperatures and pressures (725–870 °C, <2– lages, but it likely represents a component of the crystallized 9.5 kbar). coexisting melt. The K-feldspar and quartz–biotite inter- The pelite sample (PT-07–18A) contains the prograde as- growth suggests that incongruent melting of biotite had taken semblage Bt + Qtz + Grt + Crd + Pl + Sil + Kfs + Ilm. place. The upper stability of biotite from the assemblage de- The heterogeneous, migmatitic, and coarse-grained texture of scribed in the preceding text would be the maximum condi- the rock precluded obtaining meaningful whole-rock geo- tions for peak metamorphism at 830–875 °C and >5.5 kbar chemistry or modal analysis. Therefore, the composite Ar- (Fig. 19c); however, the incongruent melting of biotite takes Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 chean pelite composition of Cameron and Garrels (1980) place as a continuous reaction, and it is likely that at least a was used to calculate an equilibrium-assemblage diagram for portion of the biotite present in the observed assemblage is comparison with the observed mineral assemblage in the part of the prograde assemblage. Approximately 25% of all Paint Lake pelite (Fig. 18c). This composite was compiled biotite in the rock appears to be retrograde (occurring as skel- from 406 samples of Archean shale collected from the Supe- etal intergrowth with quartz). Assuming an equal proportion rior craton, which were variably, but weakly, metamorphosed. of biotite had originally reacted to melt, a minimum temper- Isopleths calculated for Ca in plagioclase and garnet, Mg# ature can be estimated of 780 °C (Fig. 19c). (Mg/(Mg + Fe)) of cordierite, biotite, and garnet, and Mn in Overlapping the three-phase equilibria diagrams of sam- garnet show a close correspondence between the composition ples from the granulite zone west of Mystery Lake provides of minerals analyzed in sample PT-07-18A and the predicted a better constraint on the peak metamorphic conditions mineral compositions from the equilibrium-assemblage dia- (Fig. 19d). The maximum temperature and pressure are gram, recognizing that this diagram is suitable only for a gen- defined by the pelite assemblage at 800 °C and 6.9 kbar, eral comparison between the observed mineral assemblage in and the minimum temperature and pressure are defined by sample PT-07-18A and the calculated assemblage diagram. the iron formation at 775 °C and 5 kbar, giving rise to a

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Fig. 19. Equilibrium-assemblage diagrams for samples from the granulite-facies zone west of Mystery Lake. (a) Greywacke HY-06-22A, from the Burntwood Group. The grey field represents the observed metamorphic assemblage. The dashed grey lines indicate the aluminosilicate stability fields in the system Al2O3–SiO2–H2O, as defined by Holland and Powell (1998) based on Pattison (1992). (b) Average Pipe Forma- tion pelite (sample size n = 16, Table S61), the grey field indicates the observed metamorphic assemblage in pelite CC-601 of the Pipe For- mation. The dashed grey lines indicate the aluminosilicate stability fields in the system Al2O3–SiO2–H2O. (c) Iron formation CC-602, from the Pipe Formation. Dashed grey lines indicate the percentage of biotite that has reacted to form melt. The grey field represents the observed metamorphic assemblage (see text for details). (d) The overlapping fields of a–c yield a best estimate for the metamorphic conditions in the granulite-facies zone west of Mystery Lake, as indicated by the long dashed lines (see text for details). Whole-rock compositions can be found in Table 2. 1 kbar = 100 MPa. Abbreviations as in Figs. 5–15 and Table 3. Act, actinolite. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

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combined estimate of 5.0–6.9 kbar, 775–800 °C. These find- for mafic bulk compositions (Figs. 10, 11); and the ings are similar to the 750 ± 50 °C and 5.5 ± 1 kbar esti- isograd corresponding to the incongruent melting of biotite mate of Gordon (1989), and ca. 775 °C and 6.8 kbar in iron-formation bulk compositions (Figs. 12, 13). The estimate of Growdon et al. (2006) for the adjacent Kisseynew idealized reactions for the isograds are, respectively: Domain, but are lower than the ca. 850 °C and 8 kbar esti- ½5 Bt þ Sil þ QtzPl ¼ Grt þ Crd þ meltKfs mate of Growdon (2010).

Discussion ½6 Cum ¼ Opx þ Qtz þ H2O ðSpear 1995Þ Mineral isograds or The idealized reactions that correspond to each mineral ½7 Cum þ Hbl þ Grt þ Qtz isograd are summarized in Spear (1995) for amphibolite-facies ¼ Opx þ Pl þ H O ðHollocher 1991Þ reactions and Pattison et al. (2003) for granulite-facies reac- 2 tions, unless otherwise indicated. Areas of St-bearing pelite and aluminous greywacke, the ½8 Hbl þ QtzGrt ¼ Opx þ Cpx þ meltPl lowest grade rocks of the TNB, are located in the core of the middle amphibolite-facies zone (Figs. 6, 9). The boundary of ½9 Bt þ QtzPl ¼ Opx þ meltGrtKfs these areas defines the St-out isograd for pelite and grey- wacke, which separates the middle amphibolite-facies domain These isograds were used individually or collectively to de- into lower and upper subzones. The disappearance of stauro- fine the lower limit of the granulite-facies zone. lite in pelitic rocks is likely the result of the reaction: Bathozones ½1 Ms þ St þ Qtz ¼ Bt þ Al2SiO5Grt þ H2O A combination of diagnostic peak-metamorphic mineral The St-out reaction for rocks of pelitic composition is not the assemblages and inferred peak metamorphic P–T conditions same as for rocks of greywacke composition. The absence of were used to identify domains of different metamorphic pres- coexisting muscovite leads to a different idealized staurolite- sure, or “bathozones” (cf. Carmichael 1978). The lowest consuming reaction in the greywacke in which garnet and pressure bathozone (3–4 kbar) is interpreted to stretch from cordierite are produced instead of sillimanite and garnet: Ospwagan Lake in the north to Setting Lake in the south (Fig. 20). Pelite assemblages from this zone are characterized ½2 St þ Qtz ¼ Grt þ Crd by andalusite at middle amphibolite-facies conditions and sub- þ H2O ðBailes and McRitchie 1978Þ solidus K-feldspar + sillimanite at upper amphibolite-facies conditions (see Fig. 21a). Staurolite-bearing assemblages Reaction [2] is predicted to occur at slightly higher tempera- containing sillimanite pseudomorphs after andalusite at the tures than reaction [1], ∼20 °C higher at 4.5 kbar (cf. For personal use only. Figs. 16c,17b). city of Thompson suggest that this zone could extend far- ther north. The boundary between zones of Qtz + Ms-bearing pelite An intermediate pressure bathozone (4–5 kbar) occurs to and Kfs + Sil-being pelite defines the Kfs + Sil-in isograd (Figs. 6, 7), represented by the idealized reaction: the north and east of the low-pressure bathozone (Fig. 20). Semipelites from this bathozone are migmatitic and typically ½3 Ms þ Qtz ¼ Kfs þ Sil þ H2O ðbelow ca: 3:5 kbarÞ contain sillimanite and K-feldspar but not garnet (Figs. 16b, or melt ðabove ca: 3:5 kbarÞ 16d), indicating pressures typically in the range of 3.5– 5.5 kbar. Middle amphibolite-facies aluminous greywackes in Where possible, this isograd was used to define the boundary the Mystery Lake area contain assemblages with coexisting between the middle amphibolite-facies and upper amphibo- staurolite, cordierite, and sillimanite that limit pressure to ca. lite-facies zones (Fig. 4). 4–5 kbar (Fig. 21b). Pelites do not contain a diagnostic min- There is a close correlation between the location of the eral assemblage for this bathozone. Kfs + Sil-in isograd for pelite and the Opx-in isograd for The highest pressure bathozones occur northwest of the in- iron formation (Figs. 12, 13). The Opx-in isograd is located termediate pressure bathozone and in the Paint Lake area

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 between the zones of Gru-bearing and Opx-bearing iron for- (Fig. 20). Pressures of 5–6 kbar occur west of the city of mation, giving rise to the idealized reaction: Thompson – Mystery Lake area where migmatitic greywacke assemblages of the upper amphibolite-facies domain contain ½4 Gru ¼ Opx þ Qtz þ H2O coexisting cordierite, sillimanite, and garnet (Fig. 21b). Pres- Couëslan et al. (2011) found the orthopyroxene-in isograd to sures slightly farther west in the granulite-facies domain are occur roughly 20 °C below the Kfs + Sil isograd at 3–4 kbar. limited to <7 kbar by the presence of K-feldspar, cordierite, This suggests that at low to moderate pressures and in the and ilmenite in pelite and greywacke bulk compositions presence of a H2O-rich fluid, the orthopyroxene-in isograd can (Fig. 21), and to pressure >5 kbar by the presence of garnet be used to define the beginning of upper amphibolite-facies and relict sillimanite needles included in cordierite porphyro- metamorphism for iron-formation bulk compositions. blasts in aluminous greywacke and coexisting garnet, silli- At the boundary between upper amphibolite facies and manite, and cordierite in pelites (Fig. 21). Similar pressures granulite facies, three isograds occur in close proximity: the of 5–7 kbar are inferred for the granulite zone at Paint Lake Kfs + Crd + Grt-in isograd for pelite and greywacke bulk based on pelite assemblages containing K-feldspar, garnet, compositions (Figs. 6, 7, 9); the Opx-in and Cpx + Opx-in cordierite, and ilmenite, and phase equilibria calculations

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Fig. 20. Bathozone map of the Thompson Nickel Belt. Solid lines define indicated boundaries, the dashed lines define inferred boundaries, and dotted lines indicate limits of data. The bathozones correspond to the pressure bands in the phase diagrams in Fig. 21. 1 kbar = 100 MPa. Abbreviations as in Figs. 5, 6, and 15. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

(Fig. 18d). Although sillimanite pseudomorphs after andalu- with the expected increase in metamorphic grade with in- site at Paint Lake and the city of Thompson are not indica- creasing pressure. However, the two do not always corre- tors of peak pressure, they do imply a relatively shallow P–T spond. For example, the ca. 4 kbar boundary between the path for these rocks (see the following text). low-pressure and intermediate-pressure bathozones cuts There is a broad correlation between the metamorphic-facies across the middle amphibolite-facies zone in the Ospwagan domains (Fig. 4) and the bathozones (Fig. 20), consistent Lake – city of Thompson area, with higher pressures to the

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Fig. 21. Equilibrium-assemblage diagrams for (a) average pelite of the Pipe Formation (sample size n = 16, Table S61) and (b) average alu- minous greywacke of the Burntwood Group (Bw; n = 11, Table S71), summarizing the metamorphic conditions and P–T paths for pelite and aluminous greywacke, respectively. The bathozones in Fig. 20 correspond to the shaded bands in this figure. The long dashed lines indicate the pressure range of key mineral assemblages used to define bathozones. Refer to the text for explanation on the defining of the bathozones. Abbreviations as in Figs. 5–15 and Table 3. KfsSS, subsolidus K-feldspar; P, pressure. 1 kbar = 100 MPa. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

northeast. This pattern implies greater degrees of postmeta- A second example of the noncoincidence of bathozones morphic uplift of middle and upper amphibolite-facies rocks and metamorphic facies is at the Pipe II mine site, where in the northern part of the TNB compared with the south. both middle and upper amphibolite-facies rocks occur in

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the low-pressure, 3–4 kbar bathozone. This pattern implies This textural relationship may indicate decompression near temperature gradients across the same relatively shallow peak metamorphic conditions, implying a clockwise P–T path. crustal level, such as may occur due to advection of heat The interpreted P–T paths for the majority of samples are associated with or fluids. This situation is seen in characterized by a segment of increasing pressure and tem- other localities in Figs. 4 and 20. perature (Fig. 21). Samples from the granulite-facies zones imply an interval of relatively high-temperature decompres- Metamorphic P–T paths sion (cordierite after garnet) after peak metamorphism. Mineral microtextures allow inferences to be made about Although individual P–T paths may not be representative of the P–T path of metamorphic rocks of the TNB. The lowest the P–T path of the whole belt, the overall pattern seems to grade, middle amphibolite-facies pelite from the Pipe II mine be a broadly clockwise P–T path. In contrast, the samples contains staurolite, andalusite, and sillimanite, in which staur- from Pipe mine in the middle amphibolite-facies zone are olite is locally observed as inclusions in andalusite and pris- suggestive of an isobaric path. matic sillimanite is rarely observed as an overgrowth on andalusite (Couëslan et al. 2011). Fibrous sillimanite is also Geothermal gradients prevalent in the matrix. A sensibly isobaric metamorphic P– Geothermal gradients were calculated for eight locations in T path can be inferred beginning in the field of stable stauro- the TNB from Setting Lake in the south to Mystery Lake in lite, passing into the field of stable andalusite, and finally the north (Fig. 4). Calculations were made using a crustal into the field of stable sillimanite (Fig. 21a). Although a P– density of 2.77 g/cm3 (White et al. 2005) and assuming a lin- T path of increasing pressure is also possible, the nearby oc- ear gradient from the surface. The calculated gradients corre- currence of subsolidus K-feldspar + sillimanite-bearing as- spond to the instantaneous geothermal gradient at the time of semblages, formed at a similar pressure of 3–4 kbar peak metamorphic P–T conditions. The highest geothermal (Couëslan et al. 2011), favours the former. A similar path gradients were recorded by the isobarically metamorphosed could be proposed for middle amphibolite-facies pelites in rocks at Pipe mine where middle amphibolite-facies rocks in- the vicinity of the city of Thompson where staurolite is typi- dicate an average gradient of ca. 47 °C/km, and upper am- cally rimmed by muscovite, and fibrous sillimanite has pseu- phibolite-facies rocks an average gradient of ca. 51 °C/km. A domorphously replaced andalusite. slightly lower geothermal gradient of ca. 44 °C/km is sug- Middle amphibolite-facies aluminous greywacke from the gested by the middle amphibolite-facies rocks at Ospwagan Mystery Lake area locally contains staurolite porphyroblasts Lake. The geothermal gradient continued to decrease within enclosed by cordierite with fibrous sillimanite present in the the middle amphibolite-facies zone towards the north: at matrix. This implies a P–T path that passed through the field Mystery Lake, the geothermal gradient at the time of peak of stability for staurolite, into the field of coexisting staurolite metamorphism was ca. 36 °C/km. and cordierite, and reached peak metamorphic conditions in There is also a general decrease in the recorded geothermal (or possibly slightly beyond) the field of stable St + Crd + gradient towards the north and west in upper amphibolite-facies

For personal use only. Sil (ca. 4.5 kbar, 600–650 °C; Fig. 21b). rocks. At Setting Lake, the rocks indicate a gradient of ca. Sillimanite pseudomorphs after andalusite are observed in 45 °C/km. Although the gradient was higher at Pipe Mine upper amphibolite-facies semipelite from the Thompson (ca. 51 °C/km), it was lower at Thompson Mine (ca. 42 °C/km), mine area, and relict staurolite was reported as inclusions in and lower still in the upper amphibolite-facies zone west of plagioclase at the Thompson mine (Bleeker 1990). Estimated Thompson (ca. 37 °C/km). peak metamorphic pressures >4 kbar, based on the stability The lowest geothermal gradients are indicated by rocks range of Kfs + Sil + Grt in upper amphibolite-facies pelite from the granulite-facies zones. Rocks from the Paint Lake from Thompson mine, favours a P–T path of increasing pres- area indicate a geothermal gradient of ca. 32 °C/km, and sure (Fig. 21a). Greywacke from the upper amphibolite-facies rocks from the granulite-facies zone west of Mystery Lake zone west of Thompson may have followed a similar P–T indicate a gradient of ca. 36 °C/km. The presence of andalu- path as at Mystery Lake, as indicated by the presence of site pseudomorphs in pelite at Paint Lake suggests an initially small, round inclusions of staurolite in both plagioclase and steeper geothermal gradient. garnet, finishing in the field of stable Grt + Crd + Sil + Geothermal gradients in the TNB are generally consistent melt (Fig. 21b). with andalusite-to-sillimanite P–T paths and imply an exter- Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 In granulite-facies migmatitic pelites from Paint Lake, nal input of heat such as magmatism (e.g., England and staurolite inclusions in garnet and pseudomorphous replace- Thompson 1984). There are few recognized intrusions of sig- ment of andalusite by sillimanite + cordierite suggests a P–T nificant size that correlate with the timing of peak metamor- path that began in stability fields of staurolite and andalusite phism (Zwanzig et al. 2003; Percival et al. 2004, 2005; before attaining a peak metamorphic assemblage of Kfs + Burnham et al. 2009; Machado et al. 2011a, 2011b). How- – Crd + Grt (Fig. 21a). It is possible that the P T path is ever, metre- to kilometre-scale, syn-D2 granitoid sheets and more complicated that the simple partial loop in Fig. 21a. dykes occur in abundance throughout the TNB and may Relict fibrous sillimanite at the core of cordierite porphyro- have advected heat from zones of crustal melting at greater blasts in greywacke, from the granulite-facies zone west of depths. Metre-scale, syn-D3 to post-D3 granitoid dykes are Mystery Lake, suggests a path continuing out of the field of also relatively common. An analogous mechanism has been stable sillimanite to a peak metamorphic assemblage in the proposed for the central Kisseynew Domain where crustal field of stable Kfs + Crd + Grt (Fig. 21b). Garnet rimmed by melts from the base of the Burntwood Group sedimentary cordierite is a common feature in rocks of pelite and aluminous pile migrated upwards, advecting heat to shallower crustal greywacke bulk composition from the granulite-facies zones. levels (White et al. 2005).

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Fig. 22. A schematic model of the major tectonic phases in the Thompson Nickel Belt. The metamorphic-facies zones of Fig. 4 correspond to the shaded zones in this figure. (a) Thrusting of the Burntwood Group rocks onto the Superior craton margin along early D2 faults. (b) Crea- tion of a thrust and nappe stack, which consists of reworked Archean gneiss and Ospwagan Group rocks with intercalations of Burntwood Group rocks, during the main D2 phase. (c) The metamorphic-facies zones were likely established prior to D3 and were deformed by upright F3–F4 folding accompanied by steeply east-dipping mylonite zones that record vertical stretching and sinistral movement. (d) Continued dif- ferential uplift along D3–D4 structures lead to the present-day distribution of metamorphic-facies zones as presented in this schematic cross section along line X–X′ on Fig. 4. For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12

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Timing of metamorphism with respect to deformation and the Grass River lineament coincides with the west boun- The majority of prograde metamorphic minerals in the dary of the Paint Lake granulite domain. TNB are either synchronous with or wrapped by the domi- In the southern exposed part of the TNB, there appears to nant S2 foliation. Garnet and cordierite commonly contain in- be less variation in metamorphic grade and fewer isograds. – clusion trails of minerals oriented parallel to S2, which Either the vertical deformation that accompanied the D3 D4 implies growth during the D2 event. In the granulite-facies event was less intense, or there may have been a change in zones, cordierite locally overgrows S2, suggesting protracted the metamorphic history of this portion of the belt in which growth that outlasted D2. Leucosome and leucocratic segre- high-grade metamorphism continued during the D3 event gations are typically parallel to and wrapped by S2; however, (Zwanzig 1998). The later D4 event of Zwanzig (1998) may in the granulite-facies zone at Paint Lake, leucosome locally not have produced enough vertical movement to juxtapose zones of significantly varying metamorphic grade. Another in-fills S2 boudin necks, and irregular pools have also been possibility is that occurrences of Ospwagan Group rocks and noted that crosscut S2 (Fig. 8h). In the upper amphibolite-facies zones, randomly oriented muscovite has locally replaced other lithologies that develop useful metamorphic mineral as- both K-feldspar and sillimanite, and this unoriented musco- semblages are less abundant in the southern part of the belt, so that the apparent lack of metamorphic variation could be vite is in turn crosscut by S3 muscovite, biotite, and chlor- ite. These observations indicate that prograde metamorphism an artifact of insufficient data. Although all current tectonic models for the TNB include occurred during and possibly outlasted D2, and was fol- lowed by a period of quiescence and some cooling, prior a period of transpression (Bleeker 1990; White et al. 1999; Burnham et al. 2009), the duration of this transpression event to D3. Greenschist-facies assemblages associated with S3 and later fabrics have been identified in rocks of most bulk remains in question. The long-lived transpression model of compositions throughout the TNB. Machado et al. (2011a) predicts that variations in metamor- Local exceptions to this general pattern are rare occurrences phic grade will not follow a systematic distribution across of fibrous sillimanite reoriented parallel to S at Pipe II mine the belt. In contrast, this study shows that there is a system- 3 atic variation in metamorphic grade, with a correlation be- (Couëslan et al. 2011) and at Setting Lake (Zwanzig 1998). tween the distribution of metamorphic zones and D –D This texture could be interpreted to indicate that peak metamor- 3 4 structures of the nappe-tectonics model (Bleeker 1990). phic conditions prevailed into D , although mechanical reorien- 3 Evolution of the nappe-tectonics model has led to the sug- tation of fine-grained sillimanite in D is also a possibility. 3 gestion that northwest- or west-northwest-directed conver- gence between the Superior craton and the Reindeer Zone Tectonic implications was greatest at the Thompson promontory (Fig. 1; White et The parallel spatial relationship between the upright, doubly al. 1999, 2002; Kuiper et al. 2011). Greater convergence to- – plunging F3 F4 structures and shear zones, and the metamor- wards the Thompson promontory would explain the apparent phic domains, isograds, and bathozones (Figs. 3, 4, 20) sug- plunge of the TNB towards the north, as manifested by the

For personal use only. gests that the isograd surfaces were largely established prior to closure of the 3–4 kbar bathozone towards the north, and in- the D3 event. The metamorphic domains were then deformed creasing metamorphic grades towards the north. into their current positions during the combined folding and faulting of D3–D4. There is locally a strong correlation be- Acknowledgements – tween the metamorphic domains and F3 F4 fold structures. All fieldwork in this study was supported by the Manitoba For example, the middle amphibolite-facies zones appear to Geological Survey, which also provided the majority of thin correlate with synclines or structural basins north of Setting sections and geochemical data. The project would not have Lake, and stretching from the Pipe mine to beyond Mystery possible without the access to diamond drillcore and mine Lake. In areas such as the upper amphibolite-facies zone be- sites provided by Inco Limited (now Vale Limited), Crow- tween Ospwagan and Paint lakes, however, the metamorphic flight Minerals Inc. (now CaNickel Mining Limited), and field gradient appears to be developed at a significantly longer Anglo American Exploration Limited (Canada). Sincere – wavelength than finely corrugated F3 F4 folding. This may im- thanks to J. Macek for sharing his knowledge of the geology – ply that major D3 D4 shear zones had a stronger influence on of the TNB. Thanks also to H. Zwanzig for enlightening dis- the distribution of metamorphic domains (Fig. 22). cussions on the Trans-Hudson Orogen. R. Marr (UCLEMA) Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 11/23/12 Mylonite zones with subvertical stretching lineations paral- is thanked for his assistance with the electron microprobe, – lel the limbs of F3 F4 fold structures and indicate substantial and D. Tinkham (Laurentian University) is thanked for help vertical movement (Bleeker 1990). Some of the more signifi- with aspects of the phase equilibrium modelling. This manu- cant shear zones also correlate with the boundaries between script was improved by the constructive comments of two both metamorphic zones and bathozones (Figs. 3, 4, 20, 22). anonymous reviewers and Associate Editor K. Bethune. The Superior Boundary Fault along the west shore of Ospwa- gan Lake, and an unnamed mylonite zone along the east References shore, correlate with the boundary between the middle am- Ansdell, K.M. 2005. Tectonic evolution of the Manitoba–Saskatchewan phibolite-facies and upper amphibolite-facies zones as well segment of the Paleoproterozoic Trans-Hudson Orogen, Canada. as the 4 kbar boundary between the low-pressure and inter- Canadian Journal of Earth Sciences, 42(4): 741–759. doi:10. mediate-pressure bathozones. The Burntwood mylonite zone 1139/e05-035. coincides with the boundary between middle amphibolite-fa- Ansdell, K.M., Lucas, S.B., Connors, K., and Stern, R.A. 1995. cies rocks in the city of Thompson – Mystery Lake area, and Kisseynew metasedimentary gneiss belt, Trans-Hudson orogen upper amphibolite-facies rocks of the Thompson structure, (Canada): back-arc origin and collisional inversion. Geology,

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23(11): 1039–1043. doi:10.1130/0091-7613(1995)023<1039: Fueten, F., and Robin, P.-Y.F. 1989. Structural petrology along a KMGBTH>2.3.CO;2. transect across the Thompson Belt, Manitoba: dip slip at the Bailes, A.H., and McRitchie, W.D. 1978. The transition from low- to western Churchill–Superior boundary. Canadian Journal of Earth high-grade metamorphism in the Kisseynew sedimentary gneiss Sciences, 26(10): 1976–1989. doi:10.1139/e89-167. belt, Manitoba. In Metamorphism in the Canadian , Fueten, F., Robin, P.-Y.F., and Pickering, M.E. 1986. Deformation in Geological Survey of Canada, Paper 78-10, pp. 155–178. the Thompson Belt, central Manitoba: a progress report. In Bell, C.K. 1971. History of the Superior – Churchill boundary in Current Research, Part B. Geological Survey of Canada, Paper 86- Manitoba. In Geoscience Studies in Manitoba. Edited by A.C. 1B, pp. 797–809. Turnock. Geological Association of Canada, Special Paper 9, Gapais, D., Potrel, A., Machado, N., and Hallot, E. 2005. Kinematics pp. 5–10. of long-lasting Paleoproterozoic transpression within the Thomp- Bleeker, W. 1990. Evolution of the Thompson Nickel Belt and its son Nickel Belt, Manitoba, Canada. Tectonics, 24(3): 1–16. Nickel Deposits, Manitoba, Canada. Ph.D. thesis, University of doi:10.1029/2004TC001700. New Brunswick, Fredericton, New Brunswick. Gordon, T.M. 1984. Metamorphic processes in the Kisseynew Bleeker, W., and Hamilton, M.A. 2001. New SHRIMP U-Pb ages for sedimentary gneiss belt: 1. Initial stages of formation the Ospwagan Group: implications for the SE margin of the Trans- in the Noble Lake area, Manitoba. In Current Research, Part A. Hudson Orogen. Geological Association of Canada – Miner- Geological Survey of Canada, Paper 84-1A, pp. 307–312. alogical Association of Canada. Program with Abstracts (Geolo- Gordon, T.M. 1989. Thermal evolution of the Kisseynew sedimentary gical Association of Canada), 26:15 gneiss belt, Manitoba: metamorphism at an early Proterozoic Böhm, C.O. 2005. Bedrock geology of northern and central accretionary margin. In Evolution of Metamorphic Belts. Edited Wintering Lake, Manitoba. In Report of activities 2005. Manitoba by J.S. Daly, R.A. Cliff, and B.W.D. Yardley. Geological Society Industry, Economic Development and Mines, Manitoba Geologi- of London, Special Publication 43, pp. 233–243. cal Survey, pp. 32–39. Growdon, M.L. 2010. Crustal development and deformation of Böhm, C.O., Zwanzig, H.V., and Creaser, R.A. 2007. Sm-Nd isotope Laurentia during the Trans-Hudson and Alleghenian . technique as an exploration tool: delineating the northern extension Ph.D. thesis, Indiana University, Bloomington, Indiana. of the Thompson Nickel Belt, Manitoba, Canada. Economic Growdon, M.L., Percival, J.A., Rayner, N., and Murphy, L. 2006. Geology and the Bulletin of the Society of Economic Geologists, Regional granulite-facies metamorphism in the northeastern 102(7): 1217–1231. doi:10.2113/gsecongeo.102.7.1217. Kisseynew Domain, Manitoba. In Report of activities 2006. Burnham, O.M., Halden, N., Layton-Matthews, D., Lesher, C.M., Manitoba Science, Technology, Energy and Mines, Manitoba Liwang, J., Heaman, L., Hulbert, L., Machado, N., Michalak, D., Geological Survey, pp. 104–115. Pacey, M., Peck, D.C., Potrel, A., Theyer, P., Toope, K., and Hajnal, Z., Ansdell, K.M., and Ashton, K.E. 2005. Introduction to Zwanzig, H. 2009. CAMIRO Project 97E-02, Thompson Nickel special issue of Canadian Journal of Earth Sciences: The Trans- Belt: final report March 2002, revised and updated 2003. Hudson Orogen Transect of Lithoprobe. Canadian Journal of Earth Manitoba Science, Technology, Energy and Mines, Manitoba Sciences, 42(4): 379–383. doi:10.1139/e05-053. Geological Survey, Open File OF2008-11, digital report on CD. Heaman, L.M., Peck, D., and Toope, K. 2009. Timing and Cameron, E.M., and Garrels, R.M. 1980. Geochemical compositions geochemistry of 1.88 Ga Molson Igneous Events, Manitoba: For personal use only. of some precambrian shales from the Canadian Shield. Chemical insights into the formation of a craton-scale magmatic and Geology, 28: 181–197. doi:10.1016/0009-2541(80)90046-7. metallogenic province. Precambrian Research, 172(1–2): 143– Carmichael, D.M. 1978. Metamorphic bathozones and bathograds: a 162. doi:10.1016/j.precamres.2009.03.015. measure of the depth of post-metamorphic uplift and erosion on Heaman, L.M., Böhm, Ch.O., Machado, N., Krogh, T.E., Weber, W., the regional scale. American Journal of Science, 278(6): 769–797. and Corkery, M.T. 2011. The Pikwitonei Granulite Domain, doi:10.2475/ajs.278.6.769. Manitoba: a giant Neoarchean high-grade terrane in the northwest Corkery, M.T., Lauhn-Jensen, D.M., Franceschet, T., and Hopper, D.J. Superior Province. Canadian Journal of Earth Sciences, 48(2): 1994. Manitoba geological highway map, 2nd ed. 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