Tectonothermal Evolution of the Northern Minto Block, Superior Province, Quebec, Canada

345

The Canadian Mineralogist Vol. 38, pp. 345-378 (2000)

TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK, SUPERIOR PROVINCE, QUEBEC, CANADA

JOHN A. PERCIVAL§ AND THOMAS SKULSKI§ Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada

ABSTRACT

Previously considered one of Earth’s largest granulite terranes, the Minto block, in northern Quebec, consists dominantly of granitic rocks with sparse supracrustal remnants that record low-pressure, high-temperature metamorphism to the amphibolite and granulite facies at ca. 2.7 Ga. Much of the region is underlain by plutonic rocks, including igneous charnockitic bodies, that provide little evidence of metamorphism. In contrast, pelitic assemblages in low-grade greenstone belts and their higher-grade shoulders define four metamorphic zones: 1) sub-staurolite-grade slates, found only in the Duquet belt, 2) staurolite – andalusite – sillimanite – garnet-zone schists, present in all belts, 3) marginal garnet – sillimanite ± kyanite migmatites, and 4) garnet – sillimanite ± cordierite and garnet – orthopyroxene granulites, as enclaves within plutonic units. A prograde low-pressure, high- temperature regional facies-series is defined by andalusite – sillimanite assemblages, thermobarometric analyses that indicate peak conditions ranging from 3 kbar, 560°C to 8.4 kbar, 840°C, and short segments of anticlockwise P–T trajectories preserved in two of the lowest-grade areas. Kyanite occurs in migmatites in the vicinity of a ca. 2.81 Ga shear zone bounding two distinct supracrustal assemblages and marks an earlier Barrovian event. All belts are polydeformed, with a common, broadly synmetamorphic, NNW-striking, steep foliation and down-dip lineation related to regional-scale F2 folds. Distributed late-metamorphic dip-slip shear appears responsible in some areas for cross-strike juxtaposition of low- and high-grade metamorphic zones. Older, generally poorly preserved D1 deformation, recognized in several belts, may represent accretionary structures. Variably developed younger deformation includes NNW dextral transcurrent brittle–ductile shear zones (S3), variably developed east-trending F4 folds, and WNW-striking pseudotachylite zones. U–Pb monazite ages of 2702 Ma date high-grade metamorphism of a pelitic granulite, crystallization of two diatexites, and staurolite-zone metamorphism in one schist. Together with similar zircon ages of crust-derived granites, these data point to a major collisional orogeny at this time. Older titanite (2748–2765 Ma) indicates areas that escaped >650°C metamorphism during the 2.7 Ga event. Younger monazite occurs in a migmatite-grade rock (2688, 2672 Ma) and staurolite-zone schists (2679–2642, 2648, 2637, and 2642–2628 Ma), similar to titanite ages in greenstones (2643–2550 Ma) and 40Ar/39Ar hornblende ages (2.64–2.56 Ga). These post-peak metamorphic ages in the greenstone belts based on monazite and titanite are attributed to hydrothermal growth from fluids channeled along permeable zones; they may reflect late reheating events observed throughout the Superior Province as leucogranitic magmatism, metamorphism in the deep crust, and mineral growth in shear zones.

Keywords: greenstone belts, granulite, amphibolite, Archean, U–Pb ages, monazite, zircon, P–T conditions, geothermobarometry, tectonic models, Superior Province, Quebec.

SOMMAIRE

Le socle de Minto, dans le nord du Québec, considéré antérieurement comme un des plus vastes terrains de roches transformées aux conditions du faciès granulite sur terre, est surtout un collage de roches granitiques avec des restes épars de roches supracrustales qui témoignent d’un métamorphisme à faible pression et à température élevée, aux conditions du faciès amphibolite et granulite il y a environ 2.7 Ga. La plupart de la région expose des roches plutoniques, y compris des charnockites ignées, qui ne manifestent que très peu de signes d’une recristallisation métamorphique. En revanche, les assemblages pélitiques des ceintures de roches vertes, métamorphisées à faible température, et leurs bordures à un degré de métamorphisme plus élevé, définissent quatre zones métamorphiques: 1) ardoises, degré de métamorphisme inférieur au sous-faciès à staurolite, limitées à la ceinture de Duquet, 2) schistes à staurolite – andalusite – sillimanite – grenat, présents dans chaque ceinture, 3) migmatites à grenat – sillimanite ± kyanite, en bordure, et 4) granulites à grenat – sillimanite ± cordierite et grenat – orthopyroxène, en enclaves dans les unités plutoniques. L’identification d’une séquence de faciès typique de faible pression et de température élevée repose sur la présence d’assemblages à andalusite – sillimanite, les analyses thermobarométriques indiquant des conditions maximales allant de 3 kbar, 560°C à 8.4 kbar, 840°C, et de courts intervalles de tracés P–T contre le sens de l’horloge qui sont préservés dans deux des régions où le degré de métamorphisme est le plus faible. On trouve la kyanite près d’une zone de cisaillement datée à environ 2.81 Ga entre deux assemblages supracrustaux distincts; ce serait le vestige d’un événement barrovien antérieur. Toutes les

§ E-mail addresses: [email protected], [email protected]

345 38#2-avril00-2137-05 345 12/07/00, 8:43 346 THE CANADIAN MINERALOGIST

ceintures sont polydéformées, ayant en commun une foliation généralement synmétamorphique, orientée vers le NNO et à pendage abrupt, avec linéations parallèles au pendage liées au plis F2, développés à l’échelle régionale. Un cisaillement tardif parallèle au pendage semble responsable de la juxtaposition transversale et locale de zones métamorphique de faible et de forte intensité. Nous avons trouvé des exemples d’une déformation D1, généralement mal préservée, dans plusieurs ceintures, et il pourrait s’agir de structures d’accrétion. Parmi les signes d’une déformation plus tardive, inégalement développée, seraient les zones de cisaillement dextres NNO à caractère cassant ou ductile (S3), des plis F4 variablement développés et orientés vers l’est, des zones de pseudotachylite à orientation ONO. Des âges U–Pb obtenues sur monazite, 2702 Ma, définissent l’épisode de métamorphisme d’une granulite pélitique, la cristallisation de deux diatexites, et le métamorphisme d’un schiste dans la zone de la staurolite. Avec les âges semblables déterminés sur zircon des granites dérivés dans la croûte, ces données indiquent une orogenèse majeure liée à une collision dans cet intervalle. Les âges plus anciens déterminés sur la titanite (2748–2765 Ma) caractérisent les endroits ayant échappé au métamorphisme à >650°C au cours de l’événement de 2.7 Ga. Des âges plus jeunes déterminés sur la monazite caractérisent une roche migmatitique (2688, 2672 Ma) et des schistes à staurolite (2679–2642, 2648, 2637, et 2642–2628 Ma), et ressemblent aux âges déterminés sur la titanite des roches vertes (2643–2550 Ma) et aux âges 40Ar/39Ar sur la hornblende (2.64– 2.56 Ga). Ces dates postérieures à la culmination métamorphique, établies sur la monazite et la titanite des ceintures de roches vertes, seraient dues à une croissance hydrothermale de ces accessoires à partir d’une phase fluide migrant le long de zones perméables. Cette croissance pourrait résulter d’événements causant un réchauffement tardif observé partout dans la province du Supérieur, par exemple la mise en place de leucogranites, un métamorphisme de la croûte inférieure, et une croissance de minéraux accessoires dans des zones de cisaillement.

(Traduit par la Rédaction)

Mots-clés: ceintures de roches vertes, granulite, amphibolite, archéen, âges U–Pb, monazite, zircon, conditions P–T, géothermobarométrie, modèles tectoniques, province du Supérieur, Québec.

INTRODUCTION high-grade region (Stevenson 1968, Percival et al. 1992). Our aim is to provide a tectonometamorphic Metamorphic rocks house a wealth of information, framework for the evolution of the region within the concerning for example minerals and structures devel- broad-scale context of the Superior Province. Charac- oped during burial and exhumation, as well as ages of terized by low-pressure, high-temperature facies series deposition, crystallization, metamorphism and cooling. at a range of exposure levels, the Minto block provides Resolving these events may be particularly challenging insight into the driving mechanisms of metamorphism in polydeformed regions where strain markers, meta- and the relationship between metamorphism and morphic assemblages, barometers, thermometers and magmatism, key elements in tectonic models of orogens chronometers may record non-coincident points on P– of all ages. T–t paths. Valid interpretations of the tectonic significance of regional metamorphism depend on multiple GEOLOGICAL SETTING field- and laboratory-derived constraints. Models for the tectonic evolution of Precambrian Located in the northeastern Superior Province of terranes have relied to a large degree on Phanerozoic Quebec, the Minto block (Fig. 1) represents part of a analogues. That uniformitarian principles can be applied Mesoarchean (3.0–2.8 Ga) protocraton that was as- indiscriminately to crust formed as long ago as the sembled and reworked by magmatic and tectonic pro- Archean has been challenged on the basis of dissimi- cesses between 2.8 and 2.68 Ga (Percival et al. 1994, larities in the geological record (e.g., Goodwin 1996, Stern et al. 1994). It is characterized by a prominent Hamilton 1998), and has rejuvenated an alternative tec- NNW structural grain that contrasts with the east–west tonic model for the Archean involving vertical additions alignment of subprovinces and structural fabrics in the to the crust and associated diapirism. It is useful in the southern Superior Province. The gradational, 200-km- context of deciphering the Earth’s early evolution to test wide transition zone occurs within the pluton-dominated these competing models of the Archean tectonic regime Bienville Subprovince, suggesting that the structural with observations from Archean cratons. transition is an Archean feature, intruded by the domi- As a natural laboratory, the Archean Superior Prov- nant 2.73–2.69 Ga suites of granitic rocks of the Bienville ince of the Canadian Shield offers the benefits of large Subprovince (Skulski et al. 1998) and Minto block. size, in which the scale of tectonic features may be as- Whereas the northern Superior protocraton, includ- sessed, a range of exposure levels and metamorphic ing the North Caribou Terrane of Ontario (Fig. 1; grade, and a rapidly expanding geoscience database Thurston et al. 1991) has Mesoarchean ancestry, many (e.g., Card 1990, Williams et al. 1992, Stott 1997). In of the subprovinces of the southern Superior represent this paper, we contribute quantitative P–T–t informa- juvenile Neoarchean crust (Machado et al. 1986, Henry tion on the Minto block of northeastern Superior Prov- et al. 1998). In current tectonic models, the northern ince, previously known only as a large (2.5 ϫ 106 km2) protocraton is considered a continental nucleus onto

345 38#2-avril00-2137-05 346 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 347

FIG. 1. Regional domains of the Minto block, compiled from Percival & Card (1994), and Percival et al. (1995b, 1996b, 1997b) and extrapolations from aeromagnetic maps. Inset map shows location of the Minto block in the Superior Province.

which juvenile plateau and arc terranes were accreted at (Percival 1991, 1994). The main regional deformation 2.71–2.69 Ga (Card 1990, Williams et al. 1992, Percival in the Abitibi belt occurred prior to 2.69 Ga (Mortensen et al. 1994, Stott 1997). Prominent metasedimentary 1993, Corfu 1993), whereas the rocks experienced their belts represent collisional flysch basins fed by tectonic greenschist-facies peak of metamorphism between highlands shedding arc-type detritus mixed with older 2.675 and 2.645 Ga (Powell et al. 1995a, b). Several sources (Davis et al. 1989, 1990, Corfu et al. 1995, conclusions may be drawn from these data: 1) individual Davis 1996). subprovinces have distinct tectonothermal histories, 2) Where it has been accurately dated, metamorphism deformation and metamorphism proceeded in a general in the middle to upper crust of the southern belts fol- way from north to south, and 3) metamorphism occurred lowed the main deformation by 10–30 m.y. For ex- shortly after deformation in the southern Superior Prov- ample, major deformation in the English River ince belts. Subprovince occurred at ca. 2.70 Ga, whereas high- Northwesterly aeromagnetic trends in the Minto grade metamorphism affected the rocks in the range block of northern Quebec reflect domains of distinct 2.69–2.68 Ga (Corfu et al. 1995). Deformation of simi- geological and geophysical character. In general, the lar age in the Wabigoon – Winnipeg River region was broad positive aeromagnetic anomalies correspond to followed by greenschist- to amphibolite-facies meta- relatively magnetic pyroxene-bearing granitic massifs, morphism ca. 2.69 Ga (Menard et al. 1997). In the whereas the lows are occupied by belts of less magnetic Quetico belt, polyphase deformation occurred prior to supracrustal rocks and associated biotite- and horn- 2.68 Ga, and medium- to high-grade metamorphism blende-bearing granitic rocks (Pilkington & Percival took place ca. 2.67 Ga (Percival 1989), similar to con- 1999). Magnetic models indicate that the belts of straints provided by the on-strike Ashuanipi complex supracrustal rocks have V-shaped keels up to 10 km

345 38#2-avril00-2137-05 347 12/07/00, 8:43 348 THE CANADIAN MINERALOGIST

thick. Gravity anomalies mimic the long-wavelength ANALYTICAL METHODS magnetic pattern, with highs over granitic and granu- litic massifs, and lows associated with supracrustal belts. Electron-microprobe analyses were done on a Geological characteristics of the domains have been Camebax instrument equipped with wavelength-disper- explored through recent mapping and geochronology sion spectrometers. We used an accelerating voltage of (Fig. 1; Percival & Card 1994, Skulski et al. 1996, 15 kV, with specimen current varying from 10 to 30 Percival et al. 1997a), augmented by previous helicop- nA, depending on the mineral species. Counting times ter-based reconnaissance mapping (Stevenson 1968) varied from 10 to 30 seconds per element, for total count and reconnaissance SHRIMP dating (Percival et al. durations of ~100 s per spot. Standards included a vari- 1998). These studies provide an image of the Minto ety of minerals, oxides and metals, and data reduction block as the complex end-product of a series of orogenic was performed on routines provided by Pouchou & events between 2.8 and 2.7 Ga (Percival & Skulski Pichoir (1984). Analytical reproducibility is on the or- 1997). Scattered remnants of ancient (2.8–3 Ga) crust der of ±5% for most elements. Structural formulae were occur in the east, in the Goudalie and Douglas Harbour calculated using programs developed by G.J. Pringle. domains (Fig. 1), possibly representing an orogenic The extent of zoning was assessed through step foreland during subsequent tectonic events. Supracrustal traverses. Garnet with significant zoning occurs in rocks, including associated ultramafic units and quartz- sample 1139 and was mapped in one crystal on a 5 ␮m ites in the Faribault and Leridon belts (Percival et al. grid using 3-s count times. 1997a), may represent a platform rift-cover sequence Accessory minerals were extracted for geochronol- similar to that described from the northwestern Supe- ogy using conventional crushing, heavy liquid and rior Province (Thurston & Chivers 1990). Volcanic and Frantz purification techniques. Zircon and monazite associated rocks in the Qalluviartuuq domain of the were hand-picked using an optical microscope, and the north-central Minto block range in age from 2.84 to 2.77 zircon was air-abraded following the method of Krogh Ga and have juvenile Nd isotopic signatures (Skulski et (1982) prior to dissolution and analysis. U–Pb analyti- al. 1996) which, with pillows and other evidence of sub- cal procedures were summarized by Parrish et al. marine deposition, suggest ocean-floor and arc settings. (1987), and the treatment of errors follows Roddick Intra-oceanic D1 accretionary structures and associated (1987), with regression analysis modified after York metamorphism, recognized locally, are described below. (1969). Isotopic data have been corrected for the mea- Development of a successor arc at ca. 2.775 Ga was sured U and Pb blanks, 0.3 pg and 2–31 pg, respectively. followed by deposition of an unconformable sequence Hand-picked hornblende separates intended for 40Ar/ of conglomerate, greywacke and iron formation after 39Ar dating were loaded into aluminum foil packets and 2748 Ma (Percival & Skulski 1998), possibly in a rifted arranged in an aluminum can with flux monitor PP–20 arc setting. These sedimentary units are exposed mainly hornblende, collected from the same quarry as (and as high-grade schists and gneisses in the Lake Minto identical to) HB3gr, with an apparent age of 1072 Ma domain (Fig. 1). Calc-alkaline magmatism of 2730– (Roddick 1983). Samples were irradiated at the research 2720 Ma age is widespread throughout the Minto block reactor of McMaster University, with a resulting J-fac- (Percival, Stern & Skulski, unpubl. data) and represents tor of approximately 0.0195 to 0.0198, with variation voluminous continental arc magmatism (Stern et al. corrected by interpolation along the length of the can. 1994). In the east (Utsalik domain), Nd isotopic signa- Irradiated samples were split into 2–4 aliquots and tures of granodiorites indicate significant involvement heated separately using a 45 W Weck CO2 surgical laser of older crust, whereas in the west (Lake Minto domain), with a beam ca. 200 ␮m in diameter optically attenu- rocks of similar age and composition are relatively ju- ated by 20ϫ, and analyzed with a VG3600 gas-source venile isotopically. mass spectrometer; data-collection protocols follow The western Minto block was reworked during a Villeneuve & MacIntyre (1997), and data reduction of second major orogenic event at ca. 2.70 Ga. Effects in- individual aliquots follows Roddick (1988). Some of the cluded burial of ca. 2.72 Ga supracrustal rocks of the data show the effects of “hot spots” that disrupt small Vizien belt (Percival et al. 1993, Skulski & Percival gas-fractions derived from the higher-temperature parts 1996, Lin et al. 1996), low-P, high-T metamorphism of the gas-release spectra, likely the result of slightly (Bégin & Pattison 1994), formation of upright folds and heating previously unheated regions of the grain. Ana- shear zones, and emplacement of crust-derived granites lytical results are available from Depository of Unpub- and diatexites (Percival et al. 1992, Stern et al. 1994). lished Data, CISTI, National Research Council, Ottawa, Most of the information in this paper relates to the ca. Ontario K1A 0S2, Canada. The data plots differ from 2.7 Ga event. The wide extent of high-grade metamor- conventional 40Ar/39Ar spectra in that data from indi- phic rocks exhumed from mid-crust levels supports a vidual aliquots are displayed in adjacent, alternately model of crustal thickening and metamorphism fol- shaded regions of the gas-release spectra. Once repro- lowed by isostatic rebound. Late east–west-trending ducibility of individual spectra and plateau regions is cross-folds are responsible for some structural relief. established between aliquots, the data were combined

345 38#2-avril00-2137-05 348 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 349

by integrating plateau portions. The error in J-factor plutons. The relationship between plutonism and meta- (±0.5%, 1␴) is then applied to the final age quoted. morphism is discussed in more detail below. Four prograde regional metamorphic zones have TECHNIQUES OF GEOTHERMOBAROMETRY been identified in the present study, primarily on the basis of assemblages developed in aluminous meta- In general, analyses were done on proximal garnet, sedimentary rocks (Fig. 2). The lowest grade, in the biotite, pyroxene, cordierite and plagioclase grains sepa- upper greenschist facies, occurs in a 5 ϫ 10 km patch in rated by non-ferromagnesian minerals, in order to avoid the northern Duquet belt. There, delicate sedimentary potentially retrograde zones. Mineral compositions were structures are preserved in slaty rocks characterized by used to calculate P–T conditions using a variety of equi- the assemblage chlorite – muscovite – biotite (Fig. 3). libria and TWQ software (Berman 1988, 1991) updated At higher grade, equivalent units contain spectacular to TWQ, version 2.02. Thermochemical parameters are porphyroblasts of andalusite and staurolite, suggesting taken from the internally consistent database of Berman grade control rather than unfavorable composition for & Aranovich (1996), which contains modifications rel- the lower-grade assemblages. The amphibolite facies, evant to this study for biotite, orthopyroxene and cordi- defined by pelitic assemblages including staurolite and erite. For pelitic rocks, the garnet – biotite thermometer andalusite (Fig. 4), characterizes the main belts of (Grt–Bt; abbreviations from Kretz 1983) and the gros- supracrustal rock. On their flanks, in regions dominated sular – anorthite – sillimanite – quartz barometer pro- by plutonic rocks, are upper-amphibolite-facies zones vide the most common P–T intersections. in which garnet- and sillimanite-bearing pelites are Several samples contain the assemblage Grt–Bt–Pl– migmatitic. Large areas within the granulite facies con- Qtz–Sil–Crd, allowing application and comparison of sist largely of charnockitic plutonic units with sparse the Grt–Bt–Pl–Qtz–Sil and Grt–Crd–Sil–Qtz thermo- screens of migmatitic supracrustal rocks containing gar- barometers. For rocks of the granulite facies, mineral- net – sillimanite ± cordierite or orthopyroxene-bearing core analyses yield similar P–T values using both assemblages (Percival et al. 1990). systems, whereas for rims, Bt–Grt–Pl–Sil–Qtz gives Assemblages of the middle-amphibolite facies in pressures lower by 1–1.7 kbar than Grt–Crd–Sil–Qtz. rocks of pelitic (Table 1), mafic, ultramafic and altered In lower-grade rocks, both cores and rims yield lower compositions define low-P, high-T metamorphic con- (0.1–1.3 kbar) Grt–Crd–Sil–Qtz pressures and generally ditions. P–T conditions are constrained on petrogenetic lower (up to 135°C) Grt–Crd temperatures. grids (Fig. 5) by textural relations in all belts for the An estimate of P–T errors for individual samples, prograde reactions: taking account of the reproducibility of the electron- microprobe analyses and uncertainty in thermochemi- andalusite = sillimanite [1], cal parameters, is generally taken to be on the order of ±50°C, 1 kbar (Kohn & Spear 1991). Additional uncer- and tainty is due to the possible effects of retrograde equilibration and other natural factors. This uncertainty may staurolite = garnet + aluminosilicate be assumed to be larger in the granulite facies than in + quartz + H2O [2] lower-grade rocks owing to increased probability of retrograde Fe–Mg exchange at high temperature (Frost & In the FMASH system, appropriate for these generally Chacko 1989, Bégin & Pattison 1994). muscovite-free assemblages, these equilibria cross at approximately 3 kbar, 550°C (Carmichael 1978, DISTRIBUTION AND GENERAL CHARACTER Davidson et al. 1990). OF METAMORPHIC ZONES Metamorphosed altered rocks yield cordierite– anthophyllite-bearing assemblages that also are sensi- Herd (1978) divided the Minto block into amphibo- tive monitors of grade in the amphibolite facies. lite, granulite and retrograde granulite facies on the ba- Assemblages define P–T conditions on Spear’s (1993) sis of a re-examination of Stevenson’s (1968) collection. grid in the range 2.5–4 kbar, 550–600°C. Ultramafic However, much of the Minto block consists of plutonic units also are useful indicators of temperature in the rocks, which are not sensitive monitors of metamorphic amphibolite facies. grade. Hence definition of metamorphic zones depends on the sporadic presence of units with informative as- BELTS OF AMPHIBOLITE-FACIES SUPRACRUSTAL ROCKS semblages. Plutons include hornblende- and biotite- bearing and charnockitic (pyroxene-bearing) types, here Coherent packages of supracrustal rocks occur in referred to for convenience as “wet” and “dry” plutons, isolated belts (greenstone belts), separated by along- or respectively. There is a spatial association between across-strike granitic massifs. Although there are com- amphibolite-facies schists and “wet” plutons, and be- mon stratigraphic, structural and metamorphic elements tween granulite-facies gneisses and “dry” (charnockitic) among the belts, they are first described individually,

345 38#2-avril00-2137-05 349 12/07/00, 8:43 350 THE CANADIAN MINERALOGIST

followed by a synthesis of the regional tectonometa- Vizien belt morphic history. Precise correlation of lithological units, structural and metamorphic events is tenuous owing to The 40 by 10 km Vizien belt (Figs. 1, 2) consists of the inability to date critical events. Thus correlations are middle-amphibolite-facies rocks of volcanic and sedi- based on common sequences of events and geometrical mentary origin. It has been divided into six fault- similarity. bounded lithotectonic panels with distinct internal

610 Nantais L.

greenschist facies Belt Klotz L.

Duquet PHILPOT DOMAIN

Belt Nantais Becard QALLUVIARTUUQ L. 600 DOMAIN Pelican L.

Qalluviartuuq LEPELLE DOMAIN L. LAKE MINTO DOMAIN Payne L.

Qalluviartuuq- Pelican- Payne L. Belt Orthopyroxene- bearing rocks Pavy L. kyanite

andalusite 0 sillimanite 59 L. Vernon kyanite- sillimanite andalusite- 020km sillimanite Granitic supracrustal rocks UTSALIK GOUDALIE DOMAIN Middle-amphibolite- DOMAIN facies supracrustal rocks Kogaluc Belt Vizien Upper-amphibolite- facies supracrustal rocks LAKE MINTO DOMAIN Belt 580 760 750 740 730 720

FIG. 2. Metamorphic zones and major supracrustal belts of the north-central Minto block.

345 38#2-avril00-2137-05 350 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 351

FIG. 3. Slaty cleavage in greenschist-facies metagreywacke, northern Duquet belt.

FIG. 4. Staurolite porphyroblasts in middle-amphibolite-facies pelitic schist, Payne Lake belt.

stratigraphy (Percival & Card 1992, Percival et al. 1993, comprises mafic through felsic compositions. The basal Skulski et al. 1994, Lin et al. 1995). unit consists of andesite and basalt with lenses of peri- The oldest supracrustal rocks comprise a sheet of dotite, iron formation and metamorphosed alteration mafic–ultramafic composition with basal serpentinite zones (cordierite – anthophyllite – cummingtonite – schist, overlain by peridotite, gabbro and basalt. A gab- tourmaline rocks). Overlying these are porphyritic bro sill yielded concordant zircon with a 207Pb/206Pb age andesite, dacite and rhyolite with a U/Pb zircon age of of 2786 ± 1 Ma (Skulski & Percival 1996). The sheet is 2724 ± 1 Ma (Percival et al. 1992). 2) A second volca- in tectonic contact (Lin et al. 1996) with three distinct nic sequence consists of bimodal, probably subaerial, sequences: 1) A calc-alkaline volcanic arc sequence tholeiitic lavas and rare clastic sedimentary rocks

345 38#2-avril00-2137-05 351 12/07/00, 8:43 352 THE CANADIAN MINERALOGIST

(Percival et al. 1993, Skulski et al. 1994). The estimated al. 1993). Above the conglomerate are psammitic and +15 age of a rhyolite is 2722 –8 Ma; inherited grains up to pelitic schists, locally intruded by gabbro sills (2700 ± 2793 ± 8 Ma are present. 3) A basement-cover sequence 3 Ma) and overlain by basaltic andesite and siliceous consists of tonalitic basement (2940 ± 5 Ma) and an high-magnesium basalt (Skulski & Percival 1996). The unconformable coarse clastic sedimentary sequence. uppermost unit is a mélange containing angular and Boulder conglomerates contain mainly tonalitic clasts; rounded fragments of variably deformed gabbro, one graphic granite cobble provided single-crystal (zir- serpentinite and rare chert in a serpentinitic matrix con) 207Pb/206Pb ages of 2718 to >3000 Ma, thus indi- (Percival et al. 1993, Skulski & Percival 1996). cating sediment deposition after 2718 Ma (Percival et

345 38#2-avril00-2137-05 352 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 353

core rim Granulite facies 9 Upper amphibolite facies Middle amphibolite facies Opx-Sil-Qz 255

P-T paths Grt-Crd Ms 8 Retrograde Kfs-Als-L Prograde 131b 310

691 Ms Als-L 639

MIDDLE AMPHIBOLITE 7 UPPER 310

Kyanite AMPHIBOLITE

St-Ms 428

Grt-Als-Bt 281 Sillimanite 639 133 6 337 618 GREENSCHIST

LOWER 1517 AMPHIBOLITE 618 401 281 337 69 1352 Grt-Chl

St-Bt Opx-Kfs-L 5 428 Bt-Pl-Qtz GRANULITE 756 2009 495 4 1433

Pressure, kbar Sillimanite516 3 1139 Andalusite Ms Kfs-Als-V 1538 Grt-Crd-Sil Spl-Qtz 2 500 600 700 800 Temperature,o C

FIG. 5. P–T petrogenetic grid for pelites showing the position of equilibria relevant to rocks of the amphibolite and granulite facies in the Minto block (modified after Davidson et al. 1990, Carrington 1995). P–T points are those reported in Table 1.

The Vizien belt records a complex structural history reaction [3], which occurs at approximately 550°C in (Percival & Card 1992, Percival et al. 1993, Skulski et the 3–4 kbar pressure range (Evans 1977): al. 1994, Lin et al. 1996), including five phases of ductile deformation and late brittle faulting (Lin et al. 1995). serpentine (antigorite) = forsterite + talc + H2O [3] The oldest (D1) involved formation of a mélange as the mafic–ultramafic sheet was thrust onto the younger Assemblages in altered bulk-compositions include basement-cover sequence. The D2 event imposed a pen- anthophyllite, cordierite, cummingtonite, magnesian etrative shear fabric (S2) during amalgamation of the clinochlore, and rare staurolite (Schwarz 1992); they discrete lithotectonic packages. D3 resulted in open to define metamorphic conditions in the range 550–580°C, tight folds of S2 and panel boundaries, and was followed 3–4 kbar on the basis of Spear’s (1993) grid. In pelite by the development of open D4 cross folds, and later from the basement-cover sequence (2009), relict stau- (D5) dextral transcurrent tectonic mixing in an eastern rolite and andalusite occur in association with the peak boundary zone. D1 deformation postdates deposition of assemblage garnet – biotite – plagioclase – sillimanite, the cover sequence (<2718 Ma), and S2 is cut by peralu- suggesting operation of the reaction: minous granitic pegmatite with 207Pb/206Pb monazite ages of 2620 to 2693 ± 1 Ma (T. Skulski, unpubl. data). staurolite + quartz = garnet The grade of metamorphism is expressed in a wide + aluminosilicate + H2O [4]. variety of bulk-rock compositions that consistently indicate amphibolite-facies conditions. Metaperidotites In detail, determination of whether the reaction com- contain relict igneous forsterite (Fo80–87; 0.5 wt% NiO) menced in the andalusite field and continued in the sil- and spinel, as well as metamorphic minerals defining limanite field, or took place in the presence of metastable

345 38#2-avril00-2137-05 353 12/07/00, 8:43 354 THE CANADIAN MINERALOGIST

345 38#2-avril00-2137-05 354 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 355

andalusite in the sillimanite field, is prevented by the absence of sub-sillimanite-grade rocks. Mineral com- cipal penetrative fabric S2 is defined by the alignment positions (Tables 2–7) from the peak assemblage of biotite and sillimanite and is overgrown by randomly (Table 1) yield core P–T estimates (Grt–Bt thermom- oriented sillimanite. D1 deformation may be recorded eter; GASP barometer) of 4.6 kbar at 635°C, whereas in trains of aligned inclusions within staurolite and rims record 3.8 kbar, 565°C (Table 1, Fig. 6). The lack andalusite porphyroblasts. The youngest (F3, F4) map- of significant core-to- rim Ca zoning in garnet (Table 2) scale folds have minor macro- or microscopic expres- suggests that the apparent change in pressure may be an sion. effect of declining temperature on the barometer. Peak metamorphism in the Vizien belt occurred after Relationships between structural fabrics and meta- 2718 Ma, following deposition of the cover sequence, morphic minerals are best expressed in pelites. The prin- D1 and D2 structural events. It may have coincided

345 38#2-avril00-2137-05 355 12/07/00, 8:43 356 THE CANADIAN MINERALOGIST

Dominated by sedimentary protoliths, the generally highly strained Kogaluc belt also contains thin calc- alkaline volcanic units including pillowed basaltic andesite, rhyolite and concordant quartz-feldspar porphyry, interpreted as a continent-margin association (Skulski et al. 1996). The felsic rocks, with U–Pb zircon ages of 2759 ± 3 and 2760 ± 5 Ma, respectively (Skulski et al. 1996), are considered to represent the age of the Kogaluc strata. Where stratigraphic relations are preserved, the volcanic rocks underlie a prominent unit of banded quartz–magnetite iron formation up to 100 m thick, which is associated with quartz-rich arenites and overlain by lithic arenite and greywacke with minor pelite. The belt is characterized by pervasive north-striking D2 fabrics, including steep S2 foliation and down-dip L2 mineral lineation, with rare and conflicting indica- tions of kinematic sense (Percival et al. 1995a, Lin et al. 1995); F1 folds are rarely preserved. Folds with “Z” and sheath geometry (Lin et al. 1995) have been variably interpreted as part of a progressive D2 sequence (Lin et al. 1995), or as a separate D3 event (Chapdelaine et al. 1997). A younger set of north-striking brittle-ductile shear zones indicates late dextral strike-slip. Peak metamorphic minerals define the D2 structures. The S2 foliation is marked by biotite alignment, and the L2 lineation is formed by hornblende in mafic rocks and sillimanite in pelites. Garnet, andalusite and staurolite are wrapped by the S2 fabric, defined by biotite and sillimanite. Throughout most of the belt, the peak assemblage in broadly with emplacement of peraluminous granitic pelites is garnet – sillimanite – biotite – plagioclase – pegmatites, which also postdate D2. However, there is quartz. Near its northern extremity, the belt contains evidence in some isotopic systems for later resetting. pelites with relics of staurolite and andalusite, and one Titanite concentrates from several conglomerate clasts sample contains cordierite with garnet and sillimanite. show a range of U–Pb ages from 2650 to 2550 Ma (J.K Three samples from this region provide quantitative P– Mortensen, unpubl. data, 1993). At 635°C, metamor- T estimates. The temperatures derived from the garnet– phic recrystallization could have produced some lead biotite thermometer range from 660–710°C at GASP loss in titanite (Tc ≈ 650°C; Scott & St-Onge 1995), barometer pressures of 4.8–6.5 kbar for cores, to 605– however, consistent cooling ages would be expected if 660°C, 4.4–5.1 kbar for grain margins. Core and rim this were the case. The spread of ages suggests a conditions estimated from cordierite – garnet – silliman- younger disturbance of variable intensity. Similarly, a ite – quartz are 705°C, 5.3 kbar, and 660°C, 4.9 kbar, 2444 Ma Ar/Ar age for muscovite in a granitic pegma- respectively (Table 1). The 760°C temperature estimate tite that elsewhere contains monazite of 2620–2693 Ma, for specimen 428 places it above the stability field for appears anomalously young with respect to the ca. 2.7 muscovite and quartz, in conflict with the presence of Ga metamorphism that occurred at relatively shallow these minerals in the rock. It is possible that the coarse, (8 km) levels in the crust. Thus, a younger disturbance randomly oriented muscovite is of retrograde origin. appears likely. Payne–Qalluviartuuq belt Kogaluc belt Three distinct lithotectonic assemblages are present The Kogaluc belt is one of several narrow belts of in the Payne–Qalluviartuuq lakes area (Fig. 2). 1) The amphibolite-facies supracrustal rocks of the Lake Minto Qalluviartuuq sequence is a volcanic-rock-dominated domain (Fig. 2). It has a strike length exceeding 100 package of pillow basalts, associated gabbro and an- km, a maximum width of a few kilometers (Percival et orthosite (2833 Ma; Skulski et al. 1996) with depleted al. 1995b), and is bordered by granitic massifs gener- light rare-earth-element (LREE) contents and positive ally of 2.78–2.69 Ga (Percival et al. 1992, Skulski et al. values of ␧Nd. The volcanic rocks are cut by andesite 1996) containing small remnants of high-grade dykes, including one dated at 2842 Ma (T. Skulski, supracrustal rocks (Percival et al. 1992, 1995b). unpubl. data), and overlain by a dominantly andesitic

345 38#2-avril00-2137-05 356 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 357

Nantais L. 610 3.7 1538

greenschist facies Belt

Klotz L. 4.3 1433 Duquet PHILPOT DOMAIN

Belt Nantais

5.6 1352 Becard L. QALLUVIARTUUQ 600 DOMAIN Pelican L.

Qalluviartuuq LEPELLE DOMAIN L. 756 4.9 LAKE MINTO 5.8 1517 DOMAIN 6.8 5.8 618 5.7 Payne L. 7.3 4.4 1139

6.0 7.4 Qalluviartuuq- Pelican- 639 691 Payne L. Belt 495 5.1 Opx-bearing rocks 401 5.7 428 6.5 5.3 kyanite 5.2 5.8 5.7 8.4 255 andalusite 590 sillimanite 5.6 6.2 020km kyanite- 6.3 5.2 069 281 sillimanite 6.2 131 6.2 133 andalusite- TWQ Pressure 7.8 sillimanite 5.2 Grt-Px-Pl-Qtz-Hbl 7.0 UTSALIK 6.1 Grt-Als-Pl-Qtz-Bt310 GOUDALIE DOMAIN DOMAIN 7.0 Grt-Als-Crd-Qtz Kogaluc 310 sample number Belt Vizien Granitic supracrustal rocks LAKE MINTO Middle amphibolite facies supracrustal rocks Belt Upper amphibolite 2009 4.6 DOMAIN facies supracrustal rocks 580 760 750 740 730 720

FIG. 6. Regional map showing distribution of maximum recorded pressures in supracrustal units of the Minto block, based on TWQ results from this study (Table 1) and a few additional samples from Stevenson’s (1968) collection (Percival & Berman 1996).

volcanic section with diorite and minor peridotite, all 1996). This assemblage is separated by a tectonic con- characterized by fractionated REE patterns and positive tact from a second, undated sequence (Payne Lake values of ␧Nd. The supracrustal rocks are cut by a high- gneisses: Percival et al. 1995a) that occurs mainly at level body of granodiorite (2832 Ma; Skulski et al. high metamorphic grade in Lake Minto domain to its

345 38#2-avril00-2137-05 357 12/07/00, 8:43 358 THE CANADIAN MINERALOGIST

west. 2) The Payne Lake sequence is dominated by erates contain granodiorite clasts dated at 2768 Ma pelitic metasediments, with silicate-facies iron forma- (Skulski et al. 1996). These rocks and their basement tions, andesites and diorites. Where exposed in the west- carry a prominent steep S2 foliation and associated ern Payne Lake area, the contact with the Qalluviartuuq down-dip lineation, locally cut by sheets of late syntec- sequence is a D1 shear zone overprinted by regional F2 tonic granite, and warped by open, north-trending F3 folds and overgrown by middle-amphibolite-facies min- folds. Sparse kinematic indicators associated with the eral assemblages. Both the Qalluviartuuq sequence and L2 stretching lineations suggest west-side-up motion Payne Lake gneisses are cut by calc-alkaline Hbl – Bt ± (Percival et al. 1995a). Px granodiorites with ages of 2770–2730 Ma (Skulski D1 shear zone boundary. The boundary between the et al. 1996). 3) The third, metasedimentary assemblage Qalluviartuuq sequence and Payne Lake gneisses is a occurs sporadically as small, unconformity-bound in- 15-m-wide shear zone that can be traced around F2 folds liers on units of the Qalluviartuuq sequence and plutons in middle-amphibolite-facies rocks of the western Payne as young as 2775 Ma. It includes basal conglomerate, Lake area (Fig. 7). Mafic rocks of the Qalluviartuuq as well as overlying sandstone, pelite and cordierite– sequence are progressively transformed from pillow orthoamphibole rocks; a sandstone contains detrital basalts to mafic schists over a distance of 5–10 m, and grains of zircon with ages of 2935, 2850, 2836, 2822 bedded metapelites of the Payne Lake sequence become and 2748 Ma (T. Skulski, unpubl. data), and conglom- homogeneous schists adjacent to the contact. The

050 Payne 020 cm Lake cm

~ ~ ~ ~ Qalluviartuuq sequence

~ Payne Lake sequence

Legend Qalluviartuuq Sequence pillowed flow

basalt plagioclase phenocrysts 050 carbonate cm amygdules Payne Lake Sequence 2809 Ma metagreywacke, pelite ~ ~ ~ ~ D shear zone 1 050 D shear fabric 1 02550 cm 0 tonalite m 59 28’55” 740 30’” 50

FIG. 7. Field relationships in the western Payne Lake area. The sketch map shows the Qalluviartuuq and Payne Lake sequences juxtaposed along a D1 shear zone; blowups show details of relationships among the shear zone, tonalite dyke and host rocks.

345 38#2-avril00-2137-05 358 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 359

FIG. 8. Photomicrographs of textures illustrating structural-metamorphic relationships in the vicinity of the D1 shear zone: a) staurolite porphyroblasts in schists with preserved bedding contain randomly oriented inclusions of quartz; b) subhedral staurolite porphyroblasts within meters of the shear zone overgrow an intense tectonic fabric.

premetamorphic nature of the shear zone is well illus- to 2814 Ma that reflect small amounts of inheritance trated by porphyroblast–matrix relationships (Fig. 8). (Fig. 10). This date provides a minimum age for the Where bedding is preserved, staurolite contains ran- undated Payne Lake supracrustal sequence and coin- domly oriented inclusions of quartz (Fig. 8a), whereas cides with D1 shear-zone deformation. The tectonic sig- in the shear zone, staurolite porphyroblasts overgrow a nificance of this sheared boundary ranges from a minor strong tectonic fabric (Fig. 8b). A foliated tonalite dyke fault within a coherent Qalluviartuuq – Payne Lake occurs within the shear zone and is inferred to be stratigraphic pile, to an important accretionary bound- syntectonic on the basis of apophyses of tonalite that ary between two unrelated sequences. cut mafic schists at a high angle but are themselves Younger events in the Payne–Qalluviartuuq region. folded with a weak axial planar foliation concordant to Regional middle-amphibolite-facies metamorphism is the schistosity in the host. The main body of tonalite recorded in all units including the <2748 Ma sedimen- carries the characteristic strong foliation and prominent tary rocks. The lowest-grade rocks occur in western stretching lineation of the host schists (Fig. 8). This dyke Payne Lake, where staurolite and andalusite porphy- was dated to constrain the age of D1 tectonism. roblasts are well preserved, and sillimanite is locally The 2-m-wide tonalite dyke (Fig. 9) yielded three scarce. Two types of andalusite are present in this area: zircon fractions of small, pale, euhedral stubby prisms grey, inclusion-riddled porphyroblasts, and pink, inclu- (single #4, 6a and multigrain #5; sample PBAS96–154, sion-free rims and discrete grains. The pink andalusite Table 8). These define a U/Pb crystallization age of could represent a product of incipient breakdown of +2 2809 –1 Ma (Fig. 10) with a MSWD = 0.05 and ca. 0 staurolite in the andalusite field, which would indicate Ma lower intercept. Four zircon fractions (single #1 and bathozone 2 of Carmichael (1978) [<3.3 kbar; see dis- multigrain 3a, 3b and 6b, Table 8) of larger prisms and cussion in Pattison & Tracy (1991)]. Minerals show sig- prism fragments have slightly older 207Pb/206Pb ages up nificant compositional zoning. A detailed compositional

345 38#2-avril00-2137-05 359 12/07/00, 8:43 360 THE CANADIAN MINERALOGIST

345 38#2-avril00-2137-05 360 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 361

FIG. 9. Strongly foliated tonalite dyke in the core of the D1 shear zone, western Payne Lake.

0.56 Payne Lake tonalite dyke PBAS95-154

2820 0.55 5

2800 6B 0.54 2780 6A 1 3A U 2760 3B 238

0.53 Pb/

2O6 2809 +1.6/-1.4 Ma

0.52

0.51 14.0 14.214.8 15.2 2O7 235 Pb/ U

FIG. 10. U–Pb concordia diagram showing analyses of zircon grains in syn-D1 tonalite dyke from the southwest shore of Payne Lake (PBAS–96–154). The U/Pb age of +2 2809 –1 Ma is based on a regression through fractions 4, 5 and 6a.

map of a single grain of garnet in sample 1139 shows a represents part of a rarely preserved prograde anticlock- core rich in Mn and Fe, which indicates P–T conditions wise P–T path. Higher P–T conditions are indicated of 3 kbar, 530°C, in the andalusite field (Table 1, Fig. 5). along strike 40 km to the north, where core and rim Rims yield higher values (4.4 kbar, 585°C), consistent compositions of minerals in pelite 1517 suggest a de- with final growth in the sillimanite field. This zonation crease from 5.8 kbar, 710°C, to 5 kbar, 630°C.

345 38#2-avril00-2137-05 361 12/07/00, 8:43 362 THE CANADIAN MINERALOGIST

For the most part, the peak metamorphic assem- Duquet belt blages define the prominent S2 foliation and L2 lineation. Constraints on the age of D2 are offered by a body Two lithological sequences are recognized in the of late syntectonic granite in the northern Qalluviartuuq Duquet volcanic-sedimentary belt (Percival et al. 1996a, Lake area. The concordant sheet of granite cuts strongly b, 1997a, b): 1) a dominantly basaltic assemblage, and foliated (S2) mafic schists and carries a weaker, concor- 2) an unconformable sedimentary sequence including dant foliation. Apophyses of granite cutting mafic schist iron formation. The older sequence consists primarily are folded, with axial-planar fabric. This body was dated of massive and pillowed basalt and gabbro, with pla- to define the age of late D2 deformation. It contains gioclase-phyric andesite and thin sedimentary layers, in- abundant prismatic zircon and some cores, which were cluding greywacke, pelite and silicate-facies iron avoided during picking. Regression through a discor- formation. The calc-alkaline basalts have flat to slightly dant (4.8%) prism tip (#4, PBAS95–1064; Table 8) and LREE-enriched patterns, with Nd isotopic signatures a near-concordant, euhedral prism yields a U/Pb crys- consistent with some interaction with evolved continen- +4 tallization age of 2675 –3 Ma (Fig. 11). Fraction 3 is a tal crust. The volcanic rocks are cut by a hypabyssal prism tip that is 6.4% discordant and has a lower 207Pb/ tonalite pluton dated (U–Pb zircon) at 2775 ± 1 Ma 206Pb age (2645 Ma), interpreted to reflect complex loss (Percival et al. 1996b), coeval with a large pluton of +5 of lead, including ancient and more recent disturbances. hornblende–biotite granodiorite to the east (2775 –2 The 2675 Ma age is interpreted to mark a late stage of Ma; T. Skulski, unpubl. data). To the west, hornblende– D2 deformation in the Qalluviartuuq belt and also rep- biotite granitic rocks of the Philpott domain include resents a minimum age for the syntectonic metamorphic tonalitic gneiss (2755 Ma; Percival et al. 1997b) and peak. granodiorite (2722 Ma; T. Skulski, unpubl. data). The strong D2 fabrics may represent sheared limbs The younger sequence, which rests unconformably of regional-scale, NW-trending F2 folds developed in on a 2775 Ma pluton as well as units as old as 2830 Ma the high-grade parts of the region. An east–west crenu- (T. Skulski, unpubl. data, 1998), has a lower coarse clas- lation cleavage (S4), present locally in fissile NW-strik- tic unit, intermediate amphibolite unit and upper fine ing zones, probably relates to cross-folds in adjacent clastic unit including pelite, silicate- and oxide-facies high-grade zones. iron formations. Deformed conglomerate clasts of 2764

FIG. 11. U–Pb concordia diagram showing analyses of zircon grains in late syn-D2 granite from north of Qalluviartuuq Lake (PBA–95–1064). The U/Pb age is based on regres- +4 sion through fractions 4 and 9 and yields an upper intercept age of 2675 –3 Ma, interpreted as the age of crystallization of the granite.

345 38#2-avril00-2137-05 362 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 363

Ma (T. Skulski, unpubl. data) provide a maximum age of K-feldspar-megacrystic granodiorite (2719 ± 1 Ma; of deposition for the sequence. Metamorphic grade var- Percival et al. 1997b) that carries the S1 foliation and is ies from greenschist facies in well-preserved slaty rocks folded around an F2 closure in the Becard Lake (Fig. 2) in the north, to middle-amphibolite facies throughout area. Deformation in this belt is relatively young (<2719 most of the belt. The low-grade rocks are characterized Ma) with respect to that in belts to the west (>2730 Ma), by the non-diagnostic mineral assemblages muscovite but is of similar age to that in the Vizien belt (<2718 – biotite – plagioclase – quartz and chlorite – biotite – Ma). plagioclase – quartz. For the most part, pelites carry Quantitative metamorphic information is limited to relict staurolite and andalusite in the peak assemblage pelite sample 1538 from the northern reaches of the belt garnet – sillimanite – biotite – plagioclase – quartz in the Nantais Lake area. Porphyroblasts in the stauro- (Table 1). Relict low-grade almandine is preserved lo- lite – garnet – andalusite – sillimanite – biotite – mus- cally as inclusions in staurolite. A specimen from the covite schist are wrapped by the strong foliation in the central part of the belt returned core and rim Grt–Bt/ matrix. Biotite is partly replaced by chlorite on the 5– GASP estimates of 4.3 kbar, 600°C, declining to 3.3 20 ␮m scale, and is relatively poor in K (Table 3). Core kbar, 535°C (Table 1, Fig. 5). garnet, plagioclase and altered biotite yield P–T esti- The structural chronology of the Duquet belt is com- mates of 2.7 kbar, 530°C, in the andalusite field. Rim plex. Structures of D1 age are recognized only as foli- compositions indicate higher conditions (3.7 kbar, ated (S1) clasts in conglomerate of the younger 550°C), in the sillimanite field, suggesting that the rock sequence. An east–west mylonitic S2 fabric is recog- records a segment of a prograde anticlockwise P–T path nized locally and is overprinted by the penetrative re- (Fig. 5), although there is considerable uncertainty in gional north-striking S3 foliation including high-strain the quantitative estimates because of biotite alteration. zones indicating west-over-east movement. The S3 fabrics are overgrown by minerals formed during the peak UPPER-AMPHIBOLITE-FACIES ROCKS of metamorphism, which are deformed in narrow, north- striking D4 shear zones. An east–west S5 crenulation Supracrustal rocks in the upper amphibolite facies cleavage overprints the sheared rocks and forms the occur on the flanks of the middle-amphibolite-facies dominant axial-planar slaty cleavage in a F5 fold occu- belts (Fig. 2). Pelites at this grade consist of the peak pied by the lowest-grade rocks. metamorphic assemblage garnet – sillimanite – biotite – plagioclase – quartz – granitic leucosome ± cordierite. Pelican–Nantais belt The migmatites indicate that temperatures exceeded the water-saturated granodiorite solidus (ca. 660°C; The <2-km-wide by 220 km long Pelican–Nantais Patiño-Douce & Beard 1995) but not equilibria [5] or belt (Fig. 2) of dominantly mafic rocks is present 75– [6], with estimated temperatures in the 725°C range: 100 km east of the main chain of supracrustal belts (Percival et al. 1997a; Fig. 2) and represents the only biotite + sillimanite = garnet preserved belt of supracrustal rocks of Utsalik domain. + cordierite + K-feldspar + H2O [5] These fine- to medium-grained amphibolites contain minor lenses and layers of metagreywacke, rhyolite biotite + quartz = orthopyroxene (2742 ± 1 Ma; Percival et al. 1997b), quartz–feldspar + K-feldspar + H2O [6]. porphyry and rare pods of peridotite. Metamorphic grade varies from local middle-amphibolite facies (stau- Kyanite occurs sporadically as a relict phase in pelites rolite – andalusite – sillimanite – muscovite) near the of the Payne Lake sequence. Its distribution (Fig. 2) northern and southern ends, to granulite facies (ortho- parallels the Qalluviartuuq – Payne Lake boundary over pyroxene – clinopyroxene – hornblende) near Klotz 70 km of strike length between Qalluviartuuq Lake and Lake (Fig. 2). Metasedimentary rocks occur in associa- Pavy Lake (Fig. 2). Kyanite occurs as polygonal tion with volcanic units and as discrete migmatitic belts porphyroblasts overgrown by sillimanite (Fig. 12) and up to 3 km wide. Both hornblende–biotite granodiorite as isolated inclusions in plagioclase. Its significance is (2720 ± 2 Ma; Percival et al. 1997b) and pyroxene-bear- discussed further below (see P–T paths). ing granodiorite (2723 Ma; Percival, Stern & Skulski, Peak assemblages provide a range of P–T conditions, unpubl. data) occur in proximity to the belt, with pyrox- broadly intermediate between the middle-amphibolite ene-bearing plutons more common in the south. and granulite facies (Table 1, Fig. 5). Values based on Metamorphic assemblages and migmatitic layering Grt–Bt and GASP core compositions range from 5.6 to in the supracrustal rocks define a prominent steep, north- 7.4 kbar and 710–820°C. Pressures based on Grt–Crd– striking S1 foliation that is variably expressed in adja- Sil–Qtz are similar (5.7–6.0 kbar), although tempera- cent granitic units. A down-dip mineral and stretching tures are lower (675–690°C). P–T estimates for diatexite lineation is developed in the hinge regions of tight to 516 cannot be explained. This 100-m-wide sill of mas- isoclinal folds presumed to be of D2 age. An upper sive, medium-grained garnet – sillimanite – biotite gra- bracket on the age of deformation is provided by a sheet nodiorite cuts migmatitic paragneiss of the northern

345 38#2-avril00-2137-05 363 12/07/00, 8:43 364 THE CANADIAN MINERALOGIST

Kogaluc belt. Its compositional similarity to paragneiss grade equivalents of the Payne Lake sequence. With the suggests derivation from metasedimentary sources and exception of the iron formations, the supracrustal units synmetamorphic emplacement. However, core and rim carry a steep S1 migmatitic layering concordant to the compositions indicate that pressure increased during enclave margins. In some areas, screens of supracrustal garnet growth, from 3.3 kbar to within the range of rock, granitic hosts and S1 foliation are folded into northern Kogaluc pelites, 5.25 kbar; temperatures of prominent, map-scale, north-trending, upright, close to 625–630°C are lower than those inferred for the sur- tight, moderately doubly-plunging F2 folds (Percival et rounding migmatites (>650°C). These observations are al. 1995a, b, 1996b, Lin et al. 1995). Plunge reversals difficult to reconcile with evidence for regional tempera- of F2 hinges can be related to open, upright, east–west- tures above those required for garnet homogenization trending F4 folds (Percival et al. 1995b). (ca. 600°C; Spear 1991) and remain enigmatic. It is Peak metamorphic minerals define the S1 foliation possible that fluids evolved during melt crystallization in migmatite paleosomes. In metapelites, peak assem- promoted extensive re-equilibration. blages include garnet – sillimanite – biotite – plagioclase – quartz, garnet – sillimanite – cordierite – biotite GRANULITE-FACIES REGIONS – plagioclase – quartz, and garnet – sillimanite – cordierite – K-feldspar – plagioclase – quartz. Psammitic High-grade supracrustal rocks are abundant only in compositions have garnet – orthopyroxene assemblages, Lake Minto domain (Figs. 1, 2), where they occur as mafic gneisses orthopyroxene – clinopyroxene – horn- narrow (<500 m), elongate (up to 20 km strike-length) blende – plagioclase ± quartz ± garnet, and silicate- screens within granitic massifs made up mainly of gra- facies iron formations, garnet – orthopyroxene – quartz nodiorite ranging in age from ca. 2770–2700 Ma ± grunerite. The minimum temperature for the granulite (Percival et al. 1992, Stern et al. 1994, Skulski et al. facies estimated from these assemblages is 725°C. The 1996). Rock types include mafic gneiss, paragneiss, presence of garnet – cordierite rather than ortho- pelitic migmatite, and both silicate- and oxide-facies pyroxene – sillimanite limits pressure to <8 kbar. iron formation. The affinities of most units are uncer- P–T estimates for pelitic granulites based on core tain owing to migmatization and deformation. However, compositions of garnet, biotite and plagioclase fall oxide-facies iron formation is a distinctive unit present within the range 5.3–8.4 kbar, 700–840°C, with similar throughout the western Minto block that could corre- values provided by cordierite-bearing rocks (5.8–7.0 late with the young, unconformity-bound sequences of kbar; 690–815°C). Rim compositions yield significantly the Kogaluc (<2755 Ma) and Duquet (<2764 Ma) belts. lower temperatures (560–725°C) and correspondingly Plutonic rocks as old as 2770 Ma also are present in the lower pressures, indicating substantial retrograde equili- eastern Lake Minto domain, suggesting that older com- bration in some rocks. Core compositions of two ponents could exist in this region. These could be high- orthopyroxene-bearing rocks gave estimates in the range

FIG. 12. Photomicrograph showing polygonal kyanite overgrown by sillimanite in upper- amphibolite-facies pelitic schist. Also present are garnet, biotite, plagioclase and quartz.

345 38#2-avril00-2137-05 364 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 365

5.2–6.2 kbar, 630–730°C using the Fe–Mg Grt–Opx limbs of regional folds. At the northern end of the belt, thermometer (Table 1). Even the generally robust Al- pressures in amphibolite and adjacent higher-grade in-orthopyroxene thermometer (Aranovich & Berman rocks are similar (ca. 6 kbar), whereas to the south, the 1997) yields modest temperatures. These values fall in pressure differential between middle-amphibolite and the retrograde range defined by the pelitic assemblages granulite-facies regions is as large as 3 kbar, implying and may indicate thorough re-equilibration during cool- considerable along-strike structural relief. ing from high-grade conditions. The degree to which Similar relationships are evident at the eastern mar- cores also were reset during cooling (cf. Pattison & gin of Lake Minto domain in the Payne Lake area. Bégin 1994) is difficult to assess, although temperatures There, charnockitic plutonic rocks with granulite-facies much in excess of 840°C would have produced spinel – enclaves grade across strike to the east into “wet” quartz instead of the observed garnet – cordierite – plutons with screens of upper-amphibolite-facies supra- sillimanite assemblages. crustal rocks, and further east into the middle-amphibo- In the Lake Minto area to the southwest (Figs. 1, 2), lite-facies Qalluviartuuq – Payne Lake belt. The S1 spinel and quartz inclusions in cordierite were attrib- foliation is folded and transposed within this transition. uted to peak temperatures locally in the 900–1000°C P–T conditions based on minerals defining the S2 fab- range (Bégin & Pattison 1994), followed by retrograde rics decrease from the range 6.0–7.4 kbar, 700–805°C growth of cordierite. There, core GASP pressures and in high-grade gneisses to 4.4–5.8 kbar, 585–710°C in Grt–Bt temperatures attained the range 5–6 kbar, 700– the amphibolite-facies schists. Some parts of the transi- 800°C, similar to those of pelitic granulites in the present tion zone show a minor cross-strike pressure differen- area. Orthopyroxene-bearing assemblages from south- tial, whereas as much as a 3 kbar differential exists ern Lake Minto domain give significantly higher esti- elsewhere. It is apparent that the variable amounts of mates of temperature (950–1000°C) using Harley’s vertical offset were accommodated late during meta- (1984) Al-in-Opx thermometer. A recalculation of this morphism, along the steep, ductile foliation and linea- equilibrium (Aranovich & Berman 1997) yields tem- tion, consistent with observations of rare west-side-up peratures only slightly higher than garnet–biotite esti- kinematic indicators (Percival et al. 1995a). The along- mates. Together, these observations suggest that peak strike structural relief within the amphibolite-facies belt temperatures were probably not significantly higher than cannot be attributed to F4 cross-folds, which would yield those recorded by the core compositions. sympathetic variations in pressure within the adjacent high-grade regions. Rather, this relief must have devel- SUMMARY OF STRUCTURAL – oped during formation of earlier structures, and prob- METAMORPHIC – AGE RELATIONSHIPS ably reflects plunge variations during F2 folding, such as in the New England Appalachians (cf. Kohn et al. Relationships among belts of supracrustal rocks, 1992). granitic plutons and high-grade equivalents Further north, the Qalluviartuuq and Duquet belts are bordered by “wet” plutonic massifs with sparse Several types of field relationships are evident at supracrustal remnants. Owing to a lack of appropriate contacts between belts of supracrustal rocks and plu- assemblages, the metamorphic level of these plutonic tonic bodies. In the Vizien belt, the supracrustal rocks domains is unknown, and hence constraints on vertical are both in unconformable and fault contact with older displacement between greenstones and granites are not tonalites of the Goudalie domain; cross-cutting plutonic available. However, the structural style and chronology rocks are restricted to dykes of peraluminous granitic in this area are similar to those in areas further south. pegmatite. The Goudalie tonalites are transected to the Taken together, the association of schistose rocks east and west by charnockitic plutons (ca. 2725 Ma; and low metamorphic grade could support an interpre- Percival et al. 1992) of the Utsalik and Lake Minto do- tation that the greenstone belts represent retrograded mains, respectively, which impose contact aureoles on high-grade rocks. However, several observations con- the older rocks (Percival et al. 1991, Ross 1991). flict with this interpretation: 1) primary sedimentary and To the north, the Kogaluc belt within Lake Minto volcanic structures are preserved in the amphibolite-fa- domain is bounded by variably foliated “wet” plutonic cies belts and obliterated by coarse metamorphic min- rocks containing amphibolite-facies supracrustal en- erals at higher grade, 2) assemblages in the lower-grade claves. These give way east and west to charnockitic rocks indicate prograde breakdown of staurolite and plutonic rocks carrying granulite-facies supracrustal andalusite to produce peak-metamorphic garnet and sil- enclaves. Peak metamorphic minerals in the Kogaluc limanite, and 3) retrograded migmatitic layering is not belt define a pressure in the interval 4–6 kbar, a strong present in the amphibolite-facies rocks. S2 foliation and down-dip L2 lineations. From field re- Diapirism has been suggested to be an important lationships and geobarometry, variable amounts of late mechanism of juxtaposing granites and greenstones in synmetamorphic, cross-strike vertical displacement many areas. It is important to note that in the Minto have been accommodated by S2 structures within the block, upright F2 folds, outlined by supracrustal en- Kogaluc belt and its shoulders, possibly shear zones on claves, are recognized throughout the granite–granulite

345 38#2-avril00-2137-05 365 12/07/00, 8:43 366 THE CANADIAN MINERALOGIST

regions. This style of fold is inconsistent with diapiric leading to loss of lead and uranium (de Wolf et al. 1993, rise of the granite-dominated massifs. Furthermore, the Lanzirotti & Hanson 1995, Hawkins & Bowring 1997, high-grade regions are characterized by gravity highs, 1999). Monazite has been shown to be a useful chro- indicating that these regions have higher bulk density nometer in estimating the age of prograde amphibolite- than the greenstone belts. As the driving force for facies conditions and initial cooling within the granulite diapirism is buoyancy, and the granitic rocks crystal- facies (Schaltegger et al. 1999, Hawkins & Bowring lized long before the deformation, the gravity data do 1999). not support gravity-driven tectonics. Monazite from five rocks in the middle-amphibolite facies, two in the upper-amphibolite facies and two in Structural – metamorphic – age relationships the granulite facies was analyzed in an attempt to directly constrain the age of regional metamorphism in The D1 shear zone bounding the Qalluviartuuq and the Minto block. In the staurolite-bearing schists, mona- Payne Lake sequences, dated at 2809 Ma, is premeta- zite occurs as sparse, fine (20–70 ␮m) subhedral ovoid morphic with respect to the main low-pressure, high- crystals with abundant quartz and fluid inclusions. The temperature event. Ages of deposition of the youngest grains have a characteristic patchy surface coating that rocks affected by the NNW foliation include <2768 Ma was not removed in a warm nitric acid bath, and is most (Duquet belt), <2765 Ma (Qalluviartuuq belt), 2755 Ma likely graphite. Owing to its small size and unknown U (Kogaluc belt), <2748 Ma (Payne Lake belt), 2745 Ma content, monazite was generally analyzed as small (3– (Pelican–Nantais belt), and <2718 Ma (Vizien belt). 10 grains) multigrain fractions. Even so, handling was Lower bounds on ages of deformation are more diffi- difficult, and several fractions were lost in transfer. In cult to establish owing to the scarcity of cross-cutting migmatitic rocks and diatexites, the monazite selected plutons and the high uranium contents and discordant for analysis occurs as abundant large (<300 ␮m), nature of their zircon. However, a granitic pegmatite subhedral, inclusion-free crystals, which were run as cutting D2 fabrics in the Vizien belt has a monazite age single-grain fractions. Monazite in the five staurolite- of 2693 Ma, and the late syn-D2 granite in the zone samples has variable U contents (307–1672 ppm) Qalluviartuuq belt, a zircon age of 2675 Ma. and Th/U values (3.8–19.5), whereas that from the high- Available geochronological results support the inter- grade rocks and diatexites has higher U contents (985– pretation of the prominent NNW-striking foliation as an 6442 ppm U; Table 8). S1 gneissosity in migmatitic rocks, folded and over- Four rocks yielded identical U–Pb monazite ages: printed by regional D2 structures. Subsequent NNW- 2702 ± 2 Ma (Table 8; Figs. 13, 14). These include all striking brittle-ductile dextral transcurrent shear-zones of the high-grade and diatexite samples, as well as a are locally developed in the Kogaluc and Duquet belts. sample of staurolite schist from the northern Nantais Still later, east–west-trending cross-folds and crenula- belt. A previous monazite age of 2704 Ma was reported tion cleavage represent a regional D4 event. Younger, from the western Utsalik domain (Stern, 1992, Fig. 14). brittle northwest-trending shear-zones overprint part of A single-grain fraction from the Pelican diatexite the Vizien belt (D5; Lin et al. 1995, 1996). The young- (sample 149) has an age of 2719 Ma and may have been est regional structures are zones of pseudotachylite inherited from surrounding units with similar age. More breccia that occupy discordant, WNW-trending aero- complex results characterize the remaining staurolite- magnetic lows. The similar style and orientation of D2– zone rocks, which generally yield a range of monazite D5 structures, along with their systematic relative ages. Quartzite sample 274 from the Vizien belt has chronology from belt to belt, imply a regional extent of three fractions with indistinguishable ages of 2635, 2637 these structural events. In the following section, we re- and 2638 Ma. To the north, sample 495 from the north- port U–Pb and other ages that record a spectrum of ern Kogaluc belt contains at least two ages of monazite, metamorphic and younger events. as multigrain fractions gave 2643 and 2628 Ma. Simi- larly, in pelite sample 1734 from the Duquet belt, two MONAZITE, TITANITE, RUTILE multigrain fractions gave 2679 and 2672, and a single AND HORNBLENDE GEOCHRONOLOGY grain, 2641 Ma. One multigrain fraction from the pelite sample 1477, Payne Lake yielded 2648 Ma. Upper- Monazite amphibolite sample 618 gave single-grain ages of 2688, 2673 and 2674 Ma. Metamorphic monazite generally begins to crystal- lize near the staurolite isograd (Smith & Barreiro 1990, Titanite Lanzirotti & Hanson 1995, 1997) and is stable throughout the granulite facies (Parrish 1990). Estimates of its Titanite is a common accessory mineral in calc- closure temperature are in excess of 700°C (Copeland alkaline compositions and has a U–Pb closure tempera- et al. 1988, Suzuki et al. 1994, Smith & Giletti 1997), ture in the range 660–700°C (Scott & St-Onge 1995). It and possibly as high as 800°C (Parrish 1990), although therefore provides useful constraints on the early cool- it is also subject to lower-temperature recrystallization, ing history of igneous and metamorphic rocks. Titanite

345 38#2-avril00-2137-05 366 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 367

was extracted and analyzed from several samples of morphic peak by a few tens of million years (Corfu tonalite and granodiorite in the course of the zircon geo- 1988, 1996, Mezger et al. 1989). chronology program (Skulski et al. 1996). A bimodal age population is evident in data for the Ar/Ar geochronology five rocks presented in Table 8. Two rocks yielded ages near their 2775 Ma age of zircon crystallization (2765, Hornblende, with a closure temperature for argon in 2748 Ma), significantly older than the age of regional the 550°C range, was analyzed from three rocks from metamorphism. These plutonic rocks (samples 1461, both low- and high-grade environments, and the results 1739) escaped heating beyond ~650°C during this event. are summarized in Figure 15. The complete dataset is The second age population is younger than the ca. available from the Depository of Unpublished Data, 2.7 Ga high-grade event, in the 2686–2606 Ma range. It CISTI, National Research Council, Ottawa, Ontario derives from rocks in low-grade belts (samples 1418, K1A 0S2, Canada. Hornblende from leucogabbro 1421, 1738), as well as one pyroxene-bearing sample sample QAN92 of the northern Qalluviartuuq belt gave (290). Granodiorite sample 1738 has distinct ages for 2569 ± 14 Ma, whereas that from quartz diorite sample multigrain fractions: 2686 and 2654 Ma. This scattered 1739 from Lake Ikirtuuq, adjacent to the Duquet belt, is distribution of ages is difficult to interpret as a regional 2643 ± 15 Ma. Mafic granulite sample 132 from the cooling phenomenon. Rather, it may reflect late, hydro- Lake Vernon area, east of the Kogaluc belt, has similar- thermal growth or recrystallization of titanite. aged hornblende, at 2641 ± 14 Ma.

Rutile Discussion of geochronological results

A single fraction of rutile was analyzed from pelitic A correlation is evident between metamorphic grade granulite sample 255. This mineral has a closure tem- and monazite ages (Fig. 13). High-grade rocks and perature in the 400°C range (Mezger et al. 1989) and diatexites have homogeneous populations of monazite thus provides information on the advanced stages of ages, 2701–2704 Ma, whereas lower-grade rocks have regional cooling. The age of 2495 ± 13 Ma suggests variably younger ages. Exceptions are the staurolite protracted cooling or late hydrothermal growth or re- schist from the Nantais belt (2702 Ma monazite) and setting. For comparison, rutile in high-grade metamor- arenite from the Vizien belt, with a single population of phic zones of the southwestern Superior Province ages (2635–2638 Ma). Three explanations for these pat- commonly yields ages younger than that of the meta- terns of metamorphic ages can be considered: 1) the

0.530 U-Pb monaziteages diatexite 149-4 149-1,2,3 2720 (inh.) granulite 255-1 2705 0.522 1538-1,2,3 516a-1,2,3 2702 618-4 upperamphibolite 516a-4 618-1, 2 618-3 2703 2701-02

U 2688

1734-2 238 mid-amphibolite1734-3 2679 1734-3 0.518 495-2 274-1274-3 1477 495-1 274-2 2672-74

Pb/ 2648 2672 206 2643 2641 0.514 2635-38 2628

0.510 13.0 13.25 13.50 13.75 14.0

207 235 Pb/ U

FIG. 13. U–Pb concordia diagram showing data for monazite from eight rocks.

345 38#2-avril00-2137-05 367 12/07/00, 8:43 368 THE CANADIAN MINERALOGIST

monazite ages reflect the time of peak prograde meta- belts, which have stratigraphic equivalents at higher morphism. Although the intent of the monazite-dating grade, could have escaped the effects of the regional program was to define metamorphic ages, this interpre- high-temperature event at 2702 Ma. It is similarly chal- tation appears unlikely for the greenstone belts for sev- lenging to postulate a mechanism by which the younger eral reasons. First, the age of metamorphism would be metamorphic events were not recorded in the monazite different in each belt, and multiple ages from single systematics of the deeper-level rocks. Third, migmatitic rocks could not be explained. Second, it is difficult to paragneisses of the northern Kogaluc belt are cut by conceive of a mechanism whereby the lower-grade massive diatexite with 2702 Ma monazite, providing a

610 Nantais L. 2702 1538 greenschist facies Belt 2686, 2654 Klotz 1738 L. 2765 1739 2679 h 2672 2643 2641 Duquet PHILPOT DOMAIN 1734

Belt Nantais Becard QALLUVIARTUUQ L. 600 2642 DOMAIN 2641 1421 2749 Pelican LEPELLE DOMAIN 2633 1461 L. 2668, 2625 290 Qalluviartuuq 63A L. 2702 LAKE MINTO 149 DOMAIN 2688, 2673 1418 618 2608 Payne L. 2648 1477

Qalluviartuuq- Pelican- Payne L. Belt Orthopyroxene- 2643, 2628 bearing rocks 495 Pavy L. kyanite 2702 andalusite 516 2702 590 r 2495 sillimanite 020km 255 h kyanite- 2641 sillimanite U-Pb ages andalusite- 2702 monazite sillimanite 2608 titanite UTSALIK r rutile GOUDALIE 39 40 DOMAIN h Ar/ Ar hornblende DOMAIN 255 sample number Kogaluc Granitic Belt Vizien supracrustal rocks 2600-2552 Middle amphibolite LAKE MINTO 2704 facies supracrustal rocks 2637 Upper amphibolite 274 facies supracrustal rocks DOMAIN Belt 580 760 750 740 730 720

FIG. 14. Map showing regional distribution of U–Pb monazite, titanite and rutile ages, as well as 40Ar/39Ar plateau ages for hornblende.

345 38#2-avril00-2137-05 368 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 369

minimum age for the metamorphism in this part of the belt. Thirty kilometers to the northeast, staurolite- bearing schist sample 495 contains monazite of 2643 and 2628 Ma, yielding a direct contradiction of the hypothesis. As a second possible explanation, the age of metamorphism is recorded in the homogeneous populations of monazite in the high-grade rocks and diatexites; the younger ages in lower-grade belts reflect late structural events. In terms of relative chronology, the northwest- trending brittle-ductile shear zones and east–west folds and cleavage are post-peak metamorphism, and therefore candidates for new growth of monazite. Simple resetting is unlikely, given the closure temperature of monazite above 700°C and the modest conditions of peak metamorphism in the greenstone belts. This hypothesis does not explain how a second generation of monazite could grow to dominate the population at post- peak temperatures in a closed system (cf. Pan 1997). As a third possible explanation, the peak of metamorphism was ca. 2702 Ma in both low- and high-grade regions, but late hydrothermal fluids were channeled along permeable structures, and crystallized monazite in structural traps. This interpretation would require that small quantities of monazite grew in the schists during the 2702 Ma event, and that new monazite overgrew old grains or crystallized in new settings at 2645–2628 Ma (cf. Lanzirotti & Hansen 1996, Crowley & Parrish 1999). The multigrain fractions that gave intermediate ages are thus either physical mixtures of these two age components, or contain composite grains (cf. Ayers et al. 1999). The Vizien arenite may not have contained “primary” metamorphic monazite owing to its lack of reactants during the regional metamorphism. Given the small quantities of monazite present in the low-grade pelites and the minor retrogression in most of the rocks, it is unlikely that the proposed hydrothermal event involved large volumes of fluid. Evidence for distinct textural settings for monazite is provided by Duquet pelite sample 1734. Observations in scanning electron micros- copy (SEM) show monazite as both inclusions in garnet, biotite and andalusite porphyroblasts, and along grain boundaries (Fig. 16). These may represent examples of synmetamorphic and late growth, respectively. Further resolution of this question would require in situ electron microprobe (cf. Crowley & Ghent 1999, Williams et al. 1999) or SHRIMP (Stern and Sanborn FIG. 15. Hornblende gas-release spectra for three samples 1998) analyses of potentially complex monazite grains from the Minto block. Alternating backgrounds indicate in different structural settings. different aliquots. Steps used in age calculation are indicated by lines with arrowheads. Lake Ikirtuuq tonalite DISCUSSION +5 (PBAS–95–1739; zircon U/Pb age of 2775 –2 Ma) has a plateau age of 2643 ± 15 Ma. Mafic granulite from high- P–T paths grade gneisses east of the Kogaluc greenstone belt (PBAS– 94–132; likely age is >2725 Ma) has a plateau age of 2641 The linear distribution of quantitative P–T points on ± 14 Ma. Qalluviartuuq Lake leucogabbro (QAN92; zircon minimum age of 2832 ± 2 Ma) has a plateau age of 2569 ± Figure 5 defines a low-pressure, high-temperature facies 14 Ma. Raw data can be found in the Depository of Unpub- series, which is supported by andalusite–sillimanite lished Data (see text). relationships in middle-amphibolite-facies rocks on the

345 38#2-avril00-2137-05 369 12/07/00, 8:43 370 THE CANADIAN MINERALOGIST

FIG. 16. Textural settings of monazite in Duquet pelite sample 1734. The SEM images show monazite as inclusions in peak- metamorphic minerals such as garnet (a), as well as along grain boundaries (b), suggesting that two ages of monazite growth may be present in the sample.

regional scale. Additional information on the pressure– >2748 Ma titanite ages in western Qalluviartuuq do- temperature history of the region can be derived from two main, which indicate that this area was not heated above datasets: 1) disequilibrium textures preserved in middle- ~650°C during the ca. 2.7 Ga metamorphism. and upper-amphibolite-facies rocks, and 2) chemical Neither hypothesis accounts explicitly for the pres- zonation in pressure- and temperature-sensitive minerals. ence of kyanite in upper amphibolite rocks and its ab- Aluminosilicates. The regional distribution of alumi- sence in lower-grade rocks. Consideration of the kinetics nosilicate polymorphs provides an indication of a more of kyanite dissolution may offer a possible explanation. complex early metamorphic history than that suggested Kyanite grains below a certain threshold in size may by the trend on Figure 5. The occurrences of relict kya- have recrystallized more readily during the high-tem- nite in upper-amphibolite-facies rocks (e.g., Figs. 2, 12) perature phase than larger porphyroblasts, which could are isolated from the equilibrium assemblage garnet – have been sufficiently refractory to account for the ob- sillimanite – biotite – plagioclase – quartz that indicates served patchy preservation. Similar observations of ca. 700°C, 5–6 kbar conditions. relict kyanite in high-temperature, low-pressure Although sporadic, the kyanite occurrences are sig- migmatites of the Quetico belt in the southern Superior nificant in that they cannot be reconciled with a P–T Province support this general explanation (Percival path that produces an andalusite–sillimanite facies se- 1989, Tabor & Hudleston 1991). ries regionally. Two possibilities can be considered. 1) There may be a polymetamorphic history. A ca. 2809 Paths of chemical zoning Ma Barrovian metamorphic event, preserved only in rocks older than 2809 Ma and associated with an early Single specimens from the Nantais (1538) and Payne collisional event between the Qalluviartuuq and Payne (1139) belts show compositional profiles in garnet con- Lake sequences, was overprinted by low-pressure, high- sistent with preserved growth-induced zoning (Fig. 5). temperature metamorphism at 2702 Ma. Key evidence Both have cores rich in Mn and Fe, and a detailed com- favoring this hypothesis includes the spatial association positional map of a garnet grain in sample 1139 shows between the kyanite occurrences and Qalluviartuuq – concentric zonation and a bell-shaped profile. If we as- Payne Lake boundary, and the absence of kyanite in sume that aluminosilicate was present during garnet rocks known to be younger than ca. 2760 Ma (Kogaluc growth, P–T paths calculated on the basis of composi- belt and correlative rocks in the Duquet belt). In the tions of core and rim garnet, biotite and plagioclase sug- “polymetamorphic history” hypothesis, the ca. 2702 Ma gest moderate core-to-rim increases of pressure and high-temperature event destroyed most of the evidence temperature. Both rocks move from the andalusite to of the earlier higher-pressure metamorphism. 2) There sillimanite field (curve of Holdaway 1971), matching was a single ca. 2.7 Ga regional metamorphic event with textural relationships. The maximum recorded tempera- regional variations in geothermal gradient. Early kyan- tures, 580°C (1139) and 550°C (1538), are consistent ite grew in a relatively high P/T regime along the west- with observations from many areas that garnet compo- ern margin of the >2775 Ma Qalluviartuuq domain, sitions remain unhomogenized up to temperatures in the which represented an older, relatively cooler environ- 600°C range (Spear 1991). These rare preserved paths ment than the Lake Minto domain to the west. An un- are significant in indicating an anticlockwise segment derpinning of this hypothesis is the observation of of the prograde P–T trajectory.

345 38#2-avril00-2137-05 370 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 371

A) ca. 2720 Ma W s.l. E Lake Minto Utsalik Douglas Harbour

ca. 3.1 Ga crust Goudalie

B) W ca. 2710 Ma <2718 Ma Vizien E 0 Tikkerutuk cover sequence Inukjuak Lake Minto Utsalik km Douglas Harbour Goudalie

ca. 2700 Ma 2710-2702 Ma arc C) <2718 Ma Vizien 0 Tikkerutuk cover sequence ? Lake Minto Utsalik Tikkerutuk km Douglas Harbour Goudalie

0km 300

FIG. 17. Schematic tectonic evolution diagram for ca. 2.7 Ga tectonothermal events in the Minto block. A) Cessation of 2730–2720 Ma magmatism in the Utsalik and Lake Minto arcs. B) Westward transfer of the locus of magmatism to the Tikkerutuk arc (2710– 2702 Ma). C) Eastward thrusting of the hot Tikkerutuk arc superstructure onto the Lake Minto domain, causing burial and rapid heating, followed by thermal relaxation.

The majority of the samples analyzed show compo- or andalusite in sillimanite schists. This observation sitional zonation typical of retrograde P–T paths. Granu- could indicate relatively rapid cooling, but conflicts with lite-facies rocks appear to be most affected, with rim the evidence for significant retrograde diffusion-con- temperatures an average of 100°C lower than those trolled re-equilibration in most rocks analyzed. This is recorded by cores. Temperatures derived from rims in a common phenomenon in most high-grade metamor- rocks of all grades occupy the 560–720°C range from phic terranes, and extended discussion is beyond the both garnet–biotite and garnet–cordierite Fe–Mg e- scope of this paper. xchange thermometers. There is unlikely much significance in the lower apparent pressures for the rim, which Low-P, high-T metamorphism and magmatism may simply be attributed to lower-temperature intersections of the GASP equilibrium curve (Bohlen 1987, Plutonic rocks constitute large areas of the Minto Frost & Chacko 1989). block and have a wide spectrum of ages. Given the inti- Declining temperatures did not produce significant mate link between metamorphism and magmatism in new growth of minerals, such as coronas in granulites many areas (Bohlen 1987, 1991, Clemens 1990,

345 38#2-avril00-2137-05 371 12/07/00, 8:43 372 THE CANADIAN MINERALOGIST

Lucassen & Franz 1996), it is interesting to consider thermal pulse 30–40 m.y. later. Part of the mainly whether mantle-derived plutonic rocks could have sup- eroded overthrust block may be preserved in the Vizien plied heat for the metamorphism, or whether regional belt. There juvenile mafic and ultramafic rocks, which metamorphism caused crustal melting and granite pro- crystallized at 2786 Ma and are interpreted as an oce- duction. In light of the comprehensive geochronologi- anic plateau sequence (Skulski et al. 1994, Skulski & cal constraints on magmatism and metamorphism in the Percival 1996), were emplaced tectonically on <2718 Minto block, a minimal number of assumptions is re- Ma sediments (Percival et al. 1993, Lin et al. 1996). quired. These rocks could represent part of the basement on Both metamorphic zircon and monazite in high- which the Tikkerutuk arc was built. grade metamorphic rocks and diatexites cluster in the The hypothesis of a western thrust-load is consistent age range 2704–2690 Ma, with monazite ages in lower- with the inferred cross-strike geometry of ca. 2.7 Ga grade or altered rocks trailing off to ca. 2630 Ma (Table metamorphic effects in the western Minto block 8, Fig. 13). Older plutonic suites cannot have contrib- (Fig. 17). On the regional scale, metamorphic pressure uted directly to the metamorphism, although they could and temperature are highest (up to 8 kbar, 850°C) and have produced high ambient temperatures. In particu- accompanied by abundant granite and diatexite of lar, ca. 2725 Ma granodiorites represent a widespread 2.7–2.68 Ga (Percival et al. 1992, 1998) in granulite- plutonic event in the Minto block (Percival et al. 1992, facies rocks of Lake Minto domain. Peak temperature 1994, 1998, Stern et al. 1994). These large batholithic and extent of erosion decline eastward through the am- bodies (Percival & Card 1994) commonly have pyrox- phibolite-facies rocks of the Goudalie and Qalluviartuuq ene-bearing phases attributed to high-temperature crys- domains. Evidence of 2.7 Ga metamorphism is weak in tallization of H2O-deficient magmas (Percival et al. the Utsalik domain, although appropriate assemblages 1992, Stern et al. 1994). Evidence supporting an igne- are sparse. ous rather than metamorphic origin for the pyroxenes A second manifestation of the ca. 2.7 Ga tectonism includes: 1) coarse-grained, massive homogeneous tex- involved introduction of an ancient crustal component. tures (Shore 1991; cf. Eggins & Hensen 1987), rather Stern et al. (1994) noted the relatively evolved chemi- than migmatitically layered rocks, 2) the high-tempera- cal and Nd isotopic character of 2.70–2.68 Ga diatexites 2.7 ture assemblage orthopyroxene – clinopyroxene – and granites of Lake Minto domain ( ␧Nd = –1 to –3), biotite (cf. Naney 1983, Weiss & Troll 1989, Troll & in contrast to the relatively juvenile nature of its 2725 2.7 Weiss 1991, Kilpatrick & Ellis 1992) followed in the Ma granodiorites ( ␧Nd = +1.5 to +2.5). These obser- order of crystallization by hornblende of igneous ap- vations suggest the introduction of older, geochemically pearance, 3) geochemical trends indicating increasing evolved sources into the western Minto block between fractionation with more hydrous compositions (cf. 2725 and 2700 Ma. Insight into the nature of the under- Petersen 1980), and 4) simple morphology and U–Pb thrust crust may be provided by the age of a tonalitic systematics of zircon (Percival et al. 1992). Apparently, enclave in 2.69 Ga diatexite from Lake Minto domain. the ca. 2700 Ma metamorphism had minor effect on This meter-scale gneissic inclusion, presumably derived these large bodies of relatively anhydrous character. from a deep source in the crust, has an age of 3125 Ma, Tectonic causes of the low-pressure, high-tempera- with 3500 Ma inheritance (Percival et al. 1992), both ture event may be sought in regional constraints. Ob- ages older than any rock dated in the exposed Minto servations from the Vizien belt require burial of rocks block. In summary, both overthrust and underthrust as young as 2718 Ma to depths of ca. 12 km, followed plates are necessary to explain relationships in the west- rapidly by the peak of regional high-temperature, low- ern Minto block, and the overall geometry may resemble pressure metamorphism at ca. 2700 Ma. Heating to peak the schematic cross-section of Figure 17c. temperatures within <20 m.y. is inconsistent with the In the southern Superior Province, a major tectono- 30–40 m.y. time scales typical for thermal relaxation thermal event at ca. 2700 Ma has been recognized in following tectonic loading (cf. England & Thompson the Wabigoon and Winnipeg River subprovinces. Inter- 1984). Rapid transfer of heat could have been accom- preted as docking of an oceanic Wabigoon and conti- plished by emplacement of a hot thrust load (Kohn et nental Winnipeg River block (Davis & Smith 1990, al. 1992), which would have provided an instantaneous Corfu & Davis 1992, Corfu 1996), the collision pro- thermal pulse (Fig. 17). A probable candidate is the duced widespread deformation and metamorphism to Tikkerutuk arc, the roots of which are exposed west of grades ranging from greenschist to granulite facies Lake Minto domain. These granodiorites and granites, (Corfu et al. 1995). The similarity in age to the 2.7 Ga with ages of crystallization in the 2710–2704 Ma range tectonothermal events in the Minto block may be coin- (Percival & Card 1994), are autochthonous with respect cidental, or the two areas could represent segments of a to the adjacent domains, on the basis of cross-cutting continuous orogenic boundary, although the extrapola- relationships and zircon-inheritance observations. East- tion is long beneath Phanerozoic cover. The nature of ward thrusting of the decapitated arc’s superstructure such a boundary between granite-dominated regions would have led to rapid burial and heating, followed by could be difficult to recognize through the screen of thermal relaxation that would have produced a second younger collisional magmatism.

345 38#2-avril00-2137-05 372 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 373

Late metamorphic events in Superior Province CONCLUSIONS

Late to post-metamorphic monazite ages have been The Minto block is a complex orogenic belt formed observed in several greenstone belts of the southern through a series of 2.8–2.7 Ga tectonomagmatic events, Superior Province, including Hemlo (Corfu & Muir some of which are recognized elsewhere in the Supe- 1989), Manitouwadge, Winston Lake and Abitibi (Davis rior Province. The oldest rocks (2.9–3 Ga tonalites) are et al. 1994). Krogh (1993) drew attention to these and time-correlative with similar tonalites in the North Cari- other young ages in mineral deposits, particularly gold bou Terrane (Thurston et al. 1991). Juxtaposition of deposits, in the Superior Province, and proposed a tholeiitic and calc-alkaline volcanic sequences in the model in which fluids liberated by 2660–2620 Ma meta- Payne Lake area at 2809 Ma represents an intraoceanic morphism of the deep crust (8–11 kbar), such as ob- accretion event, was associated with Barrovian meta- served in the Kapuskasing structure (Percival & Krogh morphism, and was followed by successor arc mag- 1983, Krogh & Moser 1994, Moser et al. 1996), were matism. An arc–rift sequence of conglomerate, channeled along fault zones and crystallized accessory greywacke and iron formation, widespread in the west- minerals upon cooling at high (2–4 kbar) structural lev- ern Minto block, was engulfed by widespread continen- els. Although similar deep (8 kbar) and shallow (3–6 tal arc magmatism at 2730–2720 Ma. kbar) levels are juxtaposed in the Minto block, there is Warm ambient crust may have contributed to high- no evidence that the high-grade rocks were metamor- temperature, low-pressure metamorphism during a sec- phically active at the time of monazite crystallization in ond orogeny in the western Minto block at ca. 2.7 Ga. the lower-grade belts. Rather, it is apparent from struc- Rapid burial of rocks as young as 2718 Ma was induced tural-metamorphic relationships that the juxtaposition of by eastward thrusting of the hot superstructure of the structural levels occurred late during the metamorphism. 2710–2704 Ma Tikkerutuk arc, which produced peak Therefore, the exposed high-grade rocks were not the conditions at 2.7 Ga, followed by the prolonged effects source of fluids for the 2645–2628 Ma events. of thermal relaxation (2690–2660 Ma). An older Events in the 2660–2620 Ma range occurred (>3.1 Ga) continental block may also have been thrust throughout the Superior Province. Some are cryptic, under the western Minto during the same tectonic event. such as the 2630–2660 Ma monazite of the Ashuanipi Continued or renewed convergence late in the metamor- complex (Percival et al. 1992). In addition to monazite phism (ca. 2675 Ma) produced upright, map-scale F2 ages, Algoman granites (ca. 2650 Ma; Heather et al. folds and shear zones that account for the local preser- 1996) and granitic pegmatites (2640 Ma; Corkery et al. vation of middle-amphibolite-facies rocks. Further 1992) are widespread, and high-grade metamorphism structural relief developed during formation of east– accompanied by deformation is evident in deep-crust west-trending cross-folds, possibly far-field effects of settings, including Kapuskasing and Pikwitonei collisional deformation in the southern Superior Prov- (Heaman et al. 1986). These events are synchronous at ince. Greenstone belts acted as channelways for late- to the scale of the Superior Province (2000 ϫ 1600 km), postmetamorphic fluids, resulting in postmetamorphic implying a process not linked to a margin. Plumes have crystallization of monazite between 2688 and 2628 Ma diameters of appropriate dimension, but generally pro- in low-grade rocks. Post-tectonic activity of this age is duce mafic and ultramafic magmas, which are not ob- common throughout the Superior Province and may re- served in the Superior Province at this time. Large-scale flect large-scale lithosphere delamination. delamination, in which mantle lithosphere, thickened during final assembly of the Superior Province, became ACKNOWLEDGEMENTS gravitationally unstable, sank and was replaced by hot- ter asthenosphere, could account for late metamorphism We thank field colleagues and assistants from the of the deep crust, production of leucogranites, and asso- 1994–1996 Minto campaigns, particularly Ken Card, ciated release of hydrothermal fluids. Moser et al. Léopold Nadeau, Shoufa Lin and Katherine Venance, (1996) proposed a similar model of delamination, driven for discussions and input. Olga Ijewliw performed the by slab roll-back at the southern Superior Province electron-microprobe analyses. Personnel of the GSC margin, but this is difficult to extend north to the Minto Geochronology Laboratory are thanked for U–Pb analy- block. Skulski et al. (1997) suggested that mantle-de- ses; in particular, Mike Villeneuve was helpful in col- rived carbonatite (2660 Ma) and nepheline syenite (2653 lecting and interpreting the Ar–Ar data. Additional Ma) in the Minto block are also related to slab delami- discussions with Rob Berman and Cees van Staal on nation. It may be difficult to distinguish simultaneous thermobarometry and regional tectonics have been help- slab roll-back at several margins from a process of ful. Reviews by Rob Berman, Fernando Corfu, Bill wholesale delamination. Davis and Simon Harley improved both the presenta- tion and interpretation. Bob Martin is thanked for his tireless attention to detail.

345 38#2-avril00-2137-05 373 12/07/00, 8:43 374 THE CANADIAN MINERALOGIST

REFERENCES COPELAND, P., PARRISH, R.R. & HARRISON, T.M. (1988): Iden- tification of inherited radiogenic lead in monazite and its ARANOVICH, L.Y. & BERMAN, R.G. (1997): A new garnet– implications for U–Pb systematics. Nature 333, 760-763. orthopyroxene thermometer based on reversed Al2O3 solubility in FeO–Al2O3–SiO2 orthopyroxene. Am. Mineral. 82, CORFU, F. (1988): Differential response of U–Pb systems in 345-353. coexisting accessory minerals, Winnipeg River Sub- province, Canadian Shield: implications for Archean AYERS, J.C., MILLER, C., GORISCH, B. & MILLEMAN, J. (1999): crustal growth and stabilization. Contrib. Mineral. Petrol. Textural development of monazite during high-grade meta- 98, 312-325. morphism: hydrothermal growth kinetics, with implications for U,Th–Pb geochronology. Am. Mineral. 84, 1766- ______(1993): The evolution of the southern Abitibi 1780. greenstone belt in light of precise U–Pb geochronology. Econ. Geol. 88, 1323-1340. BÉGIN, N.J. & PATTISON, D.R.M. (1994): Metamorphic evolution of granulites in the Minto block, northern Québec: ex- ______(1996). Multistage zircon and titanite growth and traction of peak P–T conditions taking account of late Fe– inheritance in an Archean gneiss complex, Winnipeg River Mg exchange. J. Metamorph. Geol. 12, 411-428. Subprovince, Ontario. Earth Planet. Sci. Lett. 141, 175- 186. BERMAN, R.G. (1988): Internally-consistent thermodynamic data for stoichiometric minerals in the system Na2O–K2O– ______& DAVIS, D.W. (1992): A U–Pb geochronological CaO–MgO–FeO–Fe2O3–Al2O3–SiO2–TiO2–H2O–CO2. J. Petrol. 29, 445-522. framework for the western Superior Province, Ontario. In Geology of Ontario (P.C. Thurston, H.R. Williams, R.H. ______(1991): Thermobarometry using multi-equilibrium Sutcliffe & G.M. Stott, eds.). Ont. Geol. Surv., Spec. Vol. calculations: a new technique, with petrological applica- 4(2), 1335-1346. tions. Can. Mineral. 29, 833-855. ______& MUIR, T. (1989): The Hemlo – Heron Bay ______& ARANOVICH, L.Y. (1996): Optimized standard greenstone belt and Hemlo Au–Mo deposit, Superior Prov- state and solution properties of minerals. I. Model calibra- ince, Ontario, Canada. 2. Timing of metamorphism, alteration for olivine, orthopyroxene, cordierite, garnet, and tion and Au mineralization from titanite, rutile, and ilmenite in the system FeO–MgO–CaO–Al2O3–TiO2– monazite U–Pb geochronology. Chem. Geol. 79, 201-223. SiO2. Contrib. Mineral. Petrol. 126, 1-24. ______, STOTT, G.M. & BREAKS, F.W. (1995): U–Pb BOHLEN, S.R. (1987): Pressure – temperature – time paths and geochronology and evolution of the English River a tectonic model for the evolution of granulites. J. Geol. Subprovince, an Archean low P – high T metasedimentary 95, 617-632. belt in the Superior Province. Tectonics 14, 1220-1233.

______(1991): On the formation of granulites. J. CORKERY, M.T., DAVIS, D.W. & LENTON, P.G. (1992): Metamorph. Geol. 9, 223-229. Geochronological constraints on the development of the Cross Lake greenstone belt, northwest Superior Province, CARD, K.D. (1990): A review of the Superior Province of the Manitoba. Can. J. Earth Sci. 29, 2171-2185. Canadian Shield, a product of Archean accretion. Precamb. Res. 48, 99-156. CROWLEY, J.L. & GHENT, E.D. (1999): An electron microprobe study of the U–Th–Pb systematics of metamorphosed CARMICHAEL, D.M. (1978): Metamorphic bathozones and monazite: the role of Pb diffusion versus overgrowth and bathograds: a measure of the depth of post-metamorphic recrystallization. Chem. Geol. 157, 285-302. uplift and erosion on the regional scale. Am. J. Sci. 278, 769-797. ______& PARRISH, R.R. (1999): U–Pb isotopic constraints on diachronous metamorphism in the northern Monashee CARRINGTON, D.P. (1995): The relative stability of garnet– cordierite and orthopyroxene – sillimanite – quartz assem- complex, southern Canadian Cordillera. J. Metamorph. blages in metapelitic granulites: experimental data. Eur. J. Geol. 17, 483-502. Mineral. 7, 949-960. DAVIDSON, A., CARMICHAEL, D.M. & PATTISON, D.R.M. CHAPDELAINE, M., DAIGNEAULT, R., ARCHER, P. & BOUCHARD, (1990): Field guide to the metamorphism and geodynamics M. (1997): Structural controls of gold mineralization in an in the southwestern Grenville Province, Ontario. Int. Geol. Archean banded iron formation, Kogaluk property, north- Correlation Program, Project 235/304, Field Trip Guide 1. eastern Superior Province, Quebec. Geol. Assoc. Can. – Mineral. Assoc. Can., Program Abstr. 22, A25. DAVIS, D.W. (1996): Provenance and depositional age constraints on sedimentation in the western Superior transect CLEMENS, J. (1990): The granulite–granite connexion. In area from U–Pb ages of zircons. In Western Superior Granulites and Crustal Evolution (D. Vielzeuf & P. Vidal, Transect, Second Annual Workshop (R.M. Harrap & H. eds.). D. Reidel, Dordrecht, The Netherlands (25-36). Helmstaedt, eds.). Lithoprobe Rep. 53, 18-23.

345 38#2-avril00-2137-05 374 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 375

______, PEZZUTTO, F. & OJAKANGAS, R.W. (1990): The age HEAMAN, L.M., MACHADO, N., KROGH, T.E. & WEBER, W. and provenance of metasedimentary rocks in the Quetico (1986): Preliminary U–Pb zircon results from the subprovince, Ontario, from single zircon analyses: impli- Pikwitonei granulite terrain, Manitoba. Geol. Assoc. Can. cations for Archean sedimentation and tectonics in the – Mineral. Assoc. Can., Program Abstr. 11, 79. Superior Province. Earth Planet. Sci. Lett. 99, 195-205. HEATHER, K.B., SHORE, G.T. & VAN BREEMEN, O. (1996): ______, POULSEN, K.H. & KAMO, S.L. (1989): New insights Geological investigations in the Swayze greenstone belt, into Archean crustal development from geochronology in southern Superior Province, Ontario: a final field update. the Rainy Lake area, Superior Province, Canada. J. Geol. Curr. Res., Geol. Surv. Can., Pap. 1996-C, 125-136. 97, 379-398. HENRY, P., STEVENSON, R.K. & GARIÉPY, C. (1998): Late ______, SCHANDL, E.S. & WASTENEYS, H.A. (1994): U–Pb Archean mantle composition and crustal growth in the dating of minerals in alteration halos of Superior Province western Superior Province of Canada: neodymium and lead massive sulphide deposits: syngenesis versus metamor- isotopic evidence from the Wawa, Quetico and Wabigoon phism. Contrib. Mineral. Petrol. 115, 427-437. subprovinces. Geochim. Cosmochim. Acta 62, 143-157.

______& SMITH, P.M. (1990): Archean gold mineralization HERD, R.K. (1978): Notes on metamorphism in New Quebec. In in the Wabigoon subprovince, a product of crustal accre- Metamorphism in the Canadian Shield (J.A. Fraser & W.W. tion: evidence from U–Pb geochronology in the Lake of Heywood, eds.). Geol. Surv. Can., Pap. 78-10, 78-83. the Woods area, Superior Province, Canada. J. Geol. 99, 337-353. HOLDAWAY, M.J. (1971): Stability of andalusite and sillimanite and the aluminosilicate phase diagram. Am. J. Sci. 271, DE WOLF, C.P., BELSHAW, N. & O’NIONS, R.K. (1993): A meta- 97-131. morphic history from micron-scale 207Pb/206Pb chronometry of Archean monazite. Earth Planet. Sci. Lett. 120, 207-220. KILPATRICK, J.A. & ELLIS, D.J. (1992): C-type magmas: igneous charnockites and their extrusive equivalents. Trans. R. Soc. Edinburgh 83, 155-164. EGGINS, S. & HENSEN, B.J. (1987): Evolution of mantle-derived, augite–hypersthene granodiorites by crystal–liquid KOHN, M.J., ORANGE, D.L., SPEAR, F.S., RUMBLE, D., III & fractionation: Barrington Tops Batholith, eastern Australia. HARRISON, T.M. (1992): Pressure, temperature, and struc- Lithos 20, 295-310. tural evolution of west-central New Hampshire: hot thrusts over cold basement. J. Petrol. 33, 521-556. ENGLAND, P. & THOMPSON, A.B. (1984): Pressure – temperature – time paths of regional metamorphism. I. Heat trans- ______& SPEAR, F.S. (1991): Error propagation for barom- fer during the evolution of regions of thickened continental eters. 2. Application to rocks. Am. Mineral. 76, 138-147. crust. J. Petrol. 25, 894-928. KRETZ, R. (1983): Symbols for rock-forming minerals. Am. EVANS, B.W. (1977): Metamorphism of Alpine peridotite and Mineral. 68, 277-279. serpentinite. Annu. Rev. Earth Planet. Sci. 5, 397-447. KROGH, T.E. (1982): Improved accuracy of U–Pb ages by the FROST, B.R. & CHACKO, T. (1989): The granulite uncertainty creation of more concordant systems using an air abrasion principle: limitations on thermobarometry in granulites. J. technique. Geochim. Cosmochim. Acta 46, 637-649. Geol. 97, 435-450. ______(1993): High precision U–Pb ages for granulite GOODWIN, A.M. (1996): Principles of Precambrian Geology. metamorphism and deformation in the Archean Kapus- Academic Press, London, U.K. kasing structural zone, Ontario: implications for structure and development of the lower crust. Earth Planet. Sci. Lett. HAMILTON, W. (1998): Archean magmatism and deformation. 119, 1-18. Int. Geol. Rev. 40, 1-33. ______& MOSER, D.M. (1994): U–Pb zircon and monazite HARLEY, S.L. (1984): The solubility of alumina in ortho- ages from the Kapuskasing uplift: age constraints on defor- pyroxene coexisting with garnet in FeO–MgO–Al2O3–SiO2 mation within the Ivanhoe Lake fault zone. Can. J. Earth and CaO–FeO–MgO–Al2O3–SiO2. J. Petrol. 25, 665-696. Sci. 31, 1096-1103.

HAWKINS, D.P. & BOWRING, S.A. (1997): U–Pb systematics of LANZIROTTI, A. & HANSON, G.N. (1995): U–Pb dating of major monazite and xenotime: case studies from the Paleo- and accessory minerals formed during metamorphism and proterozoic of the Grand Canyon, Arizona. Contrib. Min- deformation of metapelites. Geochim. Cosmochim. Acta eral. Petrol. 127, 87-103. 59, 2513-2526.

______& ______(1999): U–Pb monazite, xenotime and ______& ______(1996): Geochronology and geo- titanite geochronological constraints on the prograde to chemistry of multiple generations of monazite from the post-peak metamorphic thermal history of Paleoprote- Wepawaug Schist, Connecticut, USA: implications for rozoic migmatites from the Grand Canyon, Arizona. monazite stability in metamorphic rocks. Contrib. Mineral. Contrib. Mineral. Petrol. 134, 150-169. Petrol. 125, 332-340.

345 38#2-avril00-2137-05 375 12/07/00, 8:43 376 THE CANADIAN MINERALOGIST

______& ______(1997): An assessment of the utility of ______, RODDICK, J.C., LOVERIDGE, W.D. & SULLIVAN, staurolite in U–Pb dating of metamorphism. Contrib. Min- R.W. (1987): Uranium–lead analytical techniques at the eral. Petrol. 129, 352-365. Geochronology Laboratory, Geological Survey of Canada. In Radiogenic Age and Isotopic Studies, Report 1. Geol. LIN, SHOUFA, PERCIVAL, J.A. & SKULSKI, T. (1996): Structural Surv. Can., Pap. 87-2, 3-7. constraints on the tectonic evolution of a late Archean greenstone belt in the northeastern Superior Province, PATIÑO DOUCE, A.E. & BEARD, J. S. (1995): Dehydration-melt- northern Quebec (Canada). Tectonophys. 265, 151-167. ing of biotite gneiss and quartz amphibolite from 3 to 15 kbar. J. Petrol. 36, 707-738. ______, ______, WINSKY, P., SKULSKI, T. & CARD, K.D. (1995): Strucural evolution of the Vizien and Kogaluc PATTISON, D.R. M. & BÉGIN, N.J., (1994): Zoning patterns in greenstone belts in Minto block, northeastern Superior orthopyroxene and garnet in granulites: implications for Province, northern Quebec. Curr. Res., Geol. Surv. Can., geothermobarometry. J. Metamorph. Geol. 12, 387-410. Pap. 1995-C, 121-130. ______& TRACY, R.J. (1991): Phase equilibria and LUCASSEN, F. & FRANZ, G. (1996): Magmatic arc metamor- thermobarometry of metapelites. In Contact Metamor- phism: petrology and temperature history of metabasic phism (D.M. Kerrick, ed.). Rev. Mineral. 26, 105-206. rocks in the Coastal Cordillera of northern Chile. J. Metamorph. Geol. 14, 249-265. PERCIVAL, J.A. (1989): A regional perspective of the Quetico metasedimentary belt, Superior Province, Canada. Can. J. MACHADO, N., BROOKS, C. & HART, S.R. (1986): Determina- Earth Sci. 26, 677-693. tion of initial 87Sr/86Sr and 143Nd/144Nd in primary minerals from mafic and ultramafic rocks: experimental proce- ______(1991): Granulite-facies metamorphism and crustal dure and implications for the isotopic characteristics of the magmatism in the Ashuanipi complex, Quebec – Labrador, Archean mantle under the Abitibi greenstone belt, Canada. Canada. J. Petrol. 32, 1261-1297. Geochim. Cosmochim. Acta 50, 2335-2348. ______(1994): High grade metamorphism. In Archean MENARD, T., CRUDEN, A.R., DAVIS, D. & ROBIN, P.-Y.F. Crustal Evolution (K.C. Condie, ed.). Elsevier, Amsterdam, (1997): Himalayan-style metamorphism and tectonics The Netherlands (35-410). between 2700 and 2650 Ma in the Winnipeg River and Wabigoon subprovinces near Dryden. In Western Superior ______& BERMAN, R.G. (1996): Minto block: metamor- Transect, Second Annual Workshop (R.M. Harrap & H. phic-plutonic hinterland in northeastern Superior Province. Helmstaedt, eds.). Lithoprobe Rep. 63, 46-54. Geol. Assoc. Can. – Mineral. Assoc. Can., Program Abstr. 21, A74. MEZGER, K., HANSON, G.N. & BOHLEN, S.R. (1989): High- precision U–Pb ages of metamorphic rutile: applications to ______& CARD, K.D. (1992): Geology of the Vizien the cooling history of high-grade terranes. Earth Planet. greenstone belt. Geol. Surv. Can., Open File 2495 (scale Sci. Lett. 96, 106-118. 1:50,000).

MORTENSEN, J.K. (1993): U–Pb geochronology of the eastern ______& ______(1994): Geology, Lac Minto – Rivière Abitibi Subprovince. I. Chibougamau – Matagami – Joutel aux Feuilles, Quebec. Geol. Surv. Can., Map 1854A (scale region. Can. J. Earth Sci. 30, 11-28. 1:500,000).

MOSER, D.E. (1994): The geology and structure of the mid- ______, ______& MORTENSEN, J.K. (1993): Archean crustal Wawa gneiss domain: a key to understanding tec- unconformity in the Vizien greenstone belt, Ungava Penin- tonic variation with depth and time in the late Archean sula, Quebec. Geol. Surv. Can., Pap. 93-1C, 319-328. Abitibi–Wawa orogen. Can. J. Earth Sci. 31, 1064-1080. ______, ______, STERN, R.A. & BÉGIN, N.J. (1990): A ______, HEAMAN, L.M., KROGH, T.E. & HANES, J.A. (1996): geological transect of northeastern Superior Province, Intracrustal extension of an Archean orogen revealed using Ungava Peninsula, Quebec: the Lake Minto area. Geol. single-grain U–Pb zircon geochronology. Tectonics 15, Surv. Can., Pap. 90-1C, 133-141. 1093-1109. ______, ______, ______& ______(1991): A geo- NANEY, M.T. (1983): Phase equilibria of rock-forming logical transect of the Leaf River area, northeastern Supe- ferromagnesian silicates in granitic systems. Am. J. Sci. rior Province, Ungava Peninsula, Quebec. Geol. Surv. 283, 993-1033. Can., Pap. 91-1C, 55-63.

PAN, YUANMING (1997): Zircon- and monazite-forming meta- ______& KROGH, T.E. (1983): U–Pb zircon geochronology morphic reactions at Manitouwadge, Ontario. Can. Min- of the Kapuskasing structural zone and vicinity in the eral. 35, 105-118. Chapleau–Foleyet area, Ontario. Can. J. Earth Sci. 20, 830-843. PARRISH, R.R. (1990): U–Pb dating of monazite and its application to geological problems. Can. J. Earth Sci. 27, 1431- ______, MORTENSEN, J.K., STERN, R.A., CARD, K.D. & 1450. BÉGIN, N.J. (1992): Giant granulite terranes of northeast-

345 38#2-avril00-2137-05 376 12/07/00, 8:43 TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK 377

ern Superior Province: the Ashuanipi complex and Minto Abitibi greenstone belt, Quebec. Can. J. Earth Sci. 32, block. Can. J. Earth Sci. 29, 2287-2308. 787-805.

______& SKULSKI, T. (1997): Multiple tectonothermal ______, HODGSON, C.J., HANES, J.A., CARMICHAEL, D.M., events in the Minto block, northeastern Superior Province, MCBRIDE, S. & FARRAR, E. (1995b): 40Ar–39Ar geochro- Canada. Geol. Assoc. Can. – Mineral. Assoc. Can., Pro- nological evidence for multiple postmetamorphic hydro- gram Abstr. 22, A115. thermal events focused along faults in the southern Abitibi greenstone belt. Can. J. Earth Sci. 32, 768-786. ______& ______(1998): Tectonic history of the Minto block from 3.1–2.6 Ga. Geol. Assoc. Can. – Mineral. Assoc. RODDICK, J.C. (1983): High precision intercalibration of 40Ar– Can., Program Abstr. 23, A143-144. 39Ar standards. Geochim. Cosmochim. Acta 47, 887-898.

______, ______, CARD, K.D. & LIN, SHOUFA (1995b): ______(1987): Generalized numerical error analysis with Rivière Kogaluc – Lac Qalluviartuuq region (parts of 34J applications to geochronology and thermodynamics. and 34O), Quebec. Geol. Surv. Can., Open File Map 3112 Geochim. Cosmochim. Acta 51, 2129-2135. (scale 1:250 000). ______(1988): The assessment of errors in 40Ar/39Ar dat- ______, ______, LIN, SHOUFA & CARD, K.D. (1995a): ing. In Radiogenic Age and Isotopic Studies; Report 2. Granite–greenstone terranes of the northern Goudalie do- Geol. Surv. Can., Pap. 88-2, 3-8. main, northeastern Superior Province, Quebec. Curr. Res., Geol. Surv. Can., Pap. 1995-C, 141-150. ROSS, A. (1991): Geological Interpretation of Reaction Selvages in the Ungava Peninsula, Northern Quebec, ______, ______& NADEAU, L. (1996a): Granite– Canada. B.Sc. thesis, Univ. of Alberta, Edmonton, Alberta. greenstone terranes of the northern Minto block, northeastern Superior Province, Quebec. Curr. Res., Geol. Surv. SCHALTEGGER, U., FANNING, C.M., GUNTER, D., MUARIN, J.C., Can., Pap. 1996–C, 157-167. SCHULMANN, K. & GEBAUER, D. (1999): Growth, anneal- ing and recrystallization of zircon and preservation of ______, ______& ______(1996b): Geology, Lac Cou- monazite in high-grade metamorphism: conventional and ture, Quebec. Geol. Surv. Can., Open File Map 3315 (scale in-situ U–Pb isotope, cathodoluminescence and micro- 1:250 000). chemical evidence. Contrib. Mineral. Petrol. 134, 186-201.

______, ______& ______(1997a): Granite–green- SCHWARZ, S.H. (1992): Metamorphism of Cordierite– stone terranes of the northern Minto block, northeastern Anthophyllite and Associated Pelitic Rocks in the Vizien Quebec: Pelican–Nantais, Faribault–Leridon and Duquet Greenstone Belt, Northeastern Superior Province, Quebec. belts. Curr. Res., Geol. Surv. Can., Pap. 1997–C, 211-221. B.Sc. thesis, Carleton Univ., Ottawa, Ontario.

______, ______& ______(1997b): Reconnaissance SCOTT, D.J. & ST-ONGE, M.R. (1995): Constraints on Pb clo- geology of the Pelican–Nantais belt, northeastern Superior sure temperature in titanite based on rocks from the Ungava Province, Quebec. Geol. Surv. Can., Open File Map 3525 orogen, Canada: implications for U–Pb geochronology and (scale 1:250,000). P–T–t path determinations. Geology 23, 1123-1126.

______, STERN, R.A. & SKULSKI, T. (1998): Geochro- SHORE, G.T. (1991): On the Nature and Origin of Micro- nological reconnaissance of the Minto block from SHRIMP granitoid Enclaves in the Leaf River Area, Ungava Penin- II U–Pb zircon studies. Geol. Assoc. Can. – Mineral. Assoc. sula, Superior Province, Quebec. B.Sc. thesis, Univ. of Can., Program Abstr. 23, A144. Toronto, Toronto, Ontario.

______, ______, ______, CARD, K.D., MORTENSEN, SKULSKI, T., ORR, P. & TAYLOR, B. (1997): Archean carbonatite J.K. & BÉGIN, N.J. (1994): Minto block, Superior Province: in the Minto block, NE Superior Province. Geol. Assoc. Can. missing link in deciphering assembly of the craton at 2.7 – Mineral. Assoc. Can., Program Abstr. 22, A138-A139. Ga. Geology 22, 839- 842. ______& PERCIVAL, J.A. (1996): Allochthonous 2.8 Ga oce- PETERSEN, J.S. (1980): The zoned Kleivan granite – an end anic plateau slivers in a 2.72 Ga continental arc sequence: member of the anorthosite suite in southwest Norway. Vizien greenstone belt, northeastern Superior Province, Lithos 13, 79-95. Canada. Lithos 37, 163-179.

PILKINGTON, M. & PERCIVAL, J.A. (1999): Crustal magnetiza- ______, ______& STERN, R.A. (1994): Oceanic tion and long-wavelength aeromagnetic anomalies of the allochthons in an Archean continental margin sequence, Minto block, Quebec. J. Geophys. Res. 104, 7513-7526. Vizien grenstone belt, northern Quebec. Curr. Res., Geol. Surv. Can., Pap. 1994–C, 311-320. POUCHOU, J.L. & PICHOIR, F. (1984): A new model for quantitative X-ray microanalysis. La Rech. Aérosp. 3, 167-192. ______, ______& ______(1996): Archean crustal evolution in the central Minto block, northern Quebec. In POWELL, W.G., CARMICHAEL, D.M. & HODGSON, C.J. (1995a): Radiogenic Age and Isotopic Studies: Report 9. Curr. Res., Conditions and timing of metamorphism in the southern Geol. Surv. Can., Pap. 1995–F, 17-31.

345 38#2-avril00-2137-05 377 12/07/00, 8:43 378 THE CANADIAN MINERALOGIST

______, STERN, R.A. & CIESIELSKI, A. (1998): Timing and THURSTON, P.C. & CHIVERS, K.M. (1990): Secular variation in sources of granitoid magmatism, Bienville subprovince, greenstone sequence development emphasising Superior northern Quebec. Geol. Assoc. Can. – Mineral. Assoc. Province, Canada. Precamb. Res. 46, 21-58. Can., Program Abstr. 23, A174-A175. ______, OSMANI, I.A. & STONE, D. (1991): Northwestern SMITH, H.A. & BARREIRO, B. (1990): Monazite U–Pb dating of Superior Province: review and terrane analysis. In Geol- staurolite grade metamorphism in pelitic schists. Contrib. ogy of Ontario (P.C. Thurston, H.R. Williams, R.H. Mineral. Petrol. 105, 602-615. Sutcliffe & G.M. Stott, eds.). Ont. Geol. Surv., Spec. Vol. 4(1), 81-142. ______& GILLETTI, B. (1997): Lead diffusion in monazite. Geochim. Cosmochim. Acta 61, 1047- 1055. TROLL, G. & WEISS, S. (1991): Structure, petrography and emplacement of plutonic rocks. In Equilibrium and Kinet- SPEAR, F.S. (1991): On the interpretation of peak metamorphic ics in Contact Metamorphism: the Ballachulish Igneous temperatures in light of garnet diffusion during cooling. J. Complex and its Aureole (G. Voll, J. Töpel, D.R.M. Metamorph. Geol. 9, 379-388. Pattison & F. Seifert, eds.). Springer-Verlag, Heidelberg, Germany (39-66). ______(1993): Metamorphic Phase Equilibria and Pressure – Temperature – Time Paths. Mineralogical Society of VILLENEUVE, M.E. & MACINTYRE, D. (1997): Laser 40Ar/39Ar America, Washington, D.C. ages of the Babine porphyries and Newman Volcanics, Fulton Lake map area, west-central British Columbia. In STERN, R.A. (1992): Nd and Sr-isotope studies of monazite, Radiogenic Age and Isotopic Studies; Report 10. Curr. zircon and other minerals in granitoids: examples from Res., Geol. Surv. Can., Pap. 1997-F, 131-139. northeastern Superior Province, Quebec and Mount Ever- est, Tibet. In Radiogenic Age and Isotopic Studies: Report WEISS, S. & TROLL, G. (1989): The Ballachulish Igneous Com- 5. Geol. Surv. Can., Pap. 91-2, 43-50. plex, Scotland: petrography, mineral chemistry, and order of crystallization in the monzodiorite – quartz diorite suite ______, PERCIVAL, J.A. & MORTENSEN, J.K. (1994): and in the granite. J. Petrol. 30, 1069-1115. Geochemical evolution of the Minto block: a 2.7 Ga continental magmatic arc built on the Superior proto-craton. WILLIAMS, H.R., STOTT, G.M., THURSTON, P.C., SUTCLIFFE, Precamb. Res. 65, 115-153. R.H., BENNETT, G., EASTON, R.M. & ARMSTRONG, D.K. (1992): Tectonic evolution of Ontario: summary and syn- ______& SANBORN, N. (1998): Monazite U–Pb and Th–Pb thesis. In Geology of Ontario (P.C. Thurston, H.R. geochronology by high-resolution secondary ion mass Williams, R.H. Sutcliffe & G.M. Stott, eds.). Ont. Geol. spectrometry. Curr. Res., Geol. Surv. Can., Pap. 1998-F, Surv., Spec. Vol. 4(2), 1255-1332. 1-18. WILLIAMS, M.L, JERCINOVIC, M.J. & TERRY, M.P. (1999): Age STEVENSON, I.M. (1968): A geological reconnaissance of Leaf mapping and dating of monazite on the electron micro- River map-area, New Quebec and Northwest Territories. probe: deconvoluting multistage tectonic histories. Geol- Geol. Surv. Can., Mem. 356. ogy 27, 1023-1026.

STOTT, G.M. (1997): The Superior Province, Canada. In YORK, D. (1969): Least squares fitting of a straight line with Greenstone Belts (M.J. de Wit & L.D. Ashwal, eds.). correlated errors. Earth Planet. Sci. Lett. 5, 320-324. Clarendon, Oxford, U.K. (480-507).

SUZUKI, K., ADACHI, M. & KAJIZUKA, I. (1994): Electron microprobe observations of Pb diffusion in metamorphosed detrital monazites. Earth Planet. Sci. Lett. 128, 391-405.

TABOR, J.R. & HUDLESTON, P.J. (1991): Deformation at an Archean subprovince boundary, northern Minnesota. Can. Received May 30, 1998, revised manuscript accepted January J. Earth Sci. 28, 292-307. 6, 2000.

345 38#2-avril00-2137-05 378 12/07/00, 8:43