359

The Mineralogist Vol. 37, pp.359-373 (1999)

KYANITEIN THEWESTERN SUPERIOR PROVINCE OF : IMPLICATIONSFOR ARCHEAN ACCRETIONARY TECTONICS-

YUANMING PAN'

Department of Geological Sciences,University of Saskatchewan,Saskatoon, Saskatchewan S7N 5E2

MICHAEL E. FLEET

Department of Earth Sciences,University of Westem Ontario, London, Ontario N6A 587

ABSTRACT

Thirteen occunences of kyanite in Archean supracrustalrocks of the western Superior Province of Ontario are grouped into three distinct lithotectonic associations:I) metapelites close to subprovince and terrane boundaries, II) metapelitei near faults within greenstonebelts, and III) metamorphosedA1-Si-rich alteration assemblagesassociated with volcanogenic massive sulfide (VMS) mineralization in greenstone belts. Kyanite of groups I and II is commonly associated with staurolite and texturally predatesthe main assemblageof metamorphic minerals (Sil + Grt + Bt + Pl + Qtz), indicating that a medium-P, low-T Barrovian metamorphism occurred before the main, regional low-P/T metamorphism that characterizesthe western Superior Province. This earlier Barrovian metamorphism (670-730 MPa, 500-560"C) was followed by significant unloading (up to 12 km at the Quetico- Wabigoon boundary) before the development of the main penetrative deformation and the regional low-P/T metamorphism The results of geothermobarometryand the restricted locations of group-I and group-Il kyanite are interpreted to reflect crustal thick- ening as part of accretionary tectonic processesfor the collage of individual greenstonebelts and subprovincesfor the forrnation of the western Superior Province. In particular, the three occurrencesof kyanite along the Quetico-Wabigoon boundary supporl the origin of the Quetico Subprovince as an accretionary wedge during norlhward subduction. Group-III kyanite, on the other hand, reflects unusual whole-rock compositions and hydrothermal fluids, and may be a potentially useful exploration tool for VMS deposits

Keywords: kyanite, occunences,Barovian metamorphism, Archean, accretionary tectonics, western Superior Province, Ontario.

Sovu,qrnB

Nous nous proposons de regrouper treize indices de kyanite dans les roches arch6ennesd'origine suplacrustale du secteur ouest de la Province du Sup6rieur en Ontado en trois associationslithotectoniques distinctes: I) mdtap6litesproches de la bordure d'une sous-subprovinceou d'un socle, II) m6tap6lites situ6es prbs de failles dans des ceintures de roches vertes, et III) roches m6tamorphiques enrichies en Al-Si par alt6ration hydrothermale prbs des gisements de sulfures massifs volcanog6niquesdans des ceinturesde roches veftes La kyanite des groupesI et II, gdn6ralementassoci6e d la stauroiite, est ant6rieureselon descritdres texturaux d I'assemblage principal de min6raux m6tamorphiques (Sil + Glt + Bt + Pl + Qtz). Ce fait montre que 1'6pisode de m6tamorphismebarrovien de pressionmoyenne et de faible temp6raturea pr6c6d6l'6pisode principal de mdtamorphismer6gional de fatble pression et de faible temp6ratue qui caract6risele secteur ouest de la Province du Sup6rieur Cet 6pisode barrovien (670-730 MPa, 500-560'C) fut suivi par une 6rosion importante (usqu'd 12 km de soulbvementi I'interface entre les socles de Quetico et Wabigoon) avant le d6veloppementde 1'6pisodeimportant de d6formation r6gionale et de m6tamorphisme de faible temp6ratureet de faible pression Les r6sultats d'une analyse g6othermobarom6triqueet la rdpartition des indices de kyanite des groupesI et II assezreskeint refl6teraientun 6paississementde la cro0te suite aux processustsctoniques accompagnant I'accr6tion d'un collage de ceintures individuelles de roches vertes et de sous-provincespour former le secteur ouest de la Province du Sup6rieur En particulier, trois indices de kyanite provenant de I'interface entre les socles de Quetico et de Wabigoon dtayent I'hypothbse d'une origine de la Sous-province de Quetico comme prisme d'accr6tion lors de la subduction d polarit6 vers le nord En revanche,les indices de kyanite du groupe III signaleraientla pr6sencede roches de composition particulidre due d I'alt6ration hydrothermale, et seruiraient donc de guides utiles dans les programmes d'exploration pour de gisements de sulfures d'origine volcanog6nique (Traduit par la R6daction)

Mots-clds: kyanite, indices, m6tamorphisme barrovien, arch6en,tectonique d'accr6tion, Province du Sup6rieur (secteur ouest), Ontario.

* LITHOPROBE contribution number 1030. I E-mail address: yuanming [email protected] 360 THE CANADIAN MINERALOGIST

INrnooucrroN west-trending granite-greenstoneand metasedimentary belts (Fig. l), is a classic area where models of plate There is a growing consensusamong investigators tectonics for the formation of Archean crust were first (Card 1990, Thurston & Chivers 1990, Williams 1990, introduced (Krogh & Davis 1977,Langford & Morin Williams et al. 1991, Percival et al. 1994) that the 1976, Blackburn 1980). Therefore, knowledge of the Archean Superior Province of North America was as- distribution of metamorphic grade, conditions of peak sembledby accretionaryplate-tectonic processes, analo- metamorphism, and metamorphic pressure - tempera- gous to thoseofthe modem convergent-plateboundaries ture - timing - deformation history in the Westem Su- of the western Pacific (Hamilton 1988, and references perior Province is critical to our understanding of the therein). Plate-tectonic activity in the Archean is sup- tectonic evolution of the Archean Superior Province. ln ported by geochronological, structural, and geochemr- this contribution, we examine the reported occurrences cal evidence, and broad-scalemetamorphic patterns of of kyanite in the Western Superior Province (Table I, the Uchi, English River, Wabigoon, Quetico and Wawa Fig. l) and attempt to provide metamorphic evidence subprovinces of the western Superior Province for and P-T constraints on Archean accretionary tec- (Thurston & Breaks 1978) and the Abitibi subprovince tonics responsible for the collage of individual green- (Jolly 1978). Calvert et al. (1995) provided compelling stone belts and subprovinces in the Western Superior evidencefor subductionbased on seismicreflection pro- Province (Percival & Williams 1989, Williams 1990). files across the Opatica-Abitibi boundary in Quebec. Hamilton (1998), on the other hand, maintained that DescnrprloN oF KyANITEOccunnntvcss plate-tectonicprocesses did not operatein Archean time. The western Superior Province, which is character- Thirteen occuffences of kyanite have been reported ized by a well-defined distribution of alternating, east- in the Archean supracrustalrocks of the Western Supe-

TABLE I SIA4MARY OF KYAMTE OCCI,'RRENCESIN THE WESTERN ST'PERIORPROVINCE

No Iocation LithotectonicAssociation Mineral Assemblage References

Group I: Metapelites nea subprovince bomdaries

I Rainy Lake, Metapelites of the Quetico subprovinm Ky, St, Sil, Grt, Ms, Bt, Pl, Qtz,Ihn Tabor(1988) Minnesota neil the Quetico-Wabigoon boundary

2 Abbess Lake, Metapelites ofthe Quetico subprovince 1 Moore(1940) Atikoka nedtheQuetico-Wabigoonboudary

3 Raith Metapelites ofthe Quetico subprcvince Ky, St, Sil, Gt, Ms, Bq Pl, Qtz, Iln Percivalet al (1985) ne{ the Wawa-Quetico boudary This study

4 North Cdibou metapelites ned the tectonic boundary ofthe Ky, And, Crd, Sl GG Qt4 Pl, Bt Breaksetal(19E4) greenstone belt Norti Cdibou terue ThNton et al (1991)

5? Favourable Lake neil the Bw Head Fault Ayres(1978)

Group II: Metapelitesosociated with faults within greenstonebelts

6 WhiG River-Hmlc metapelites of tlle HemlGHeron Bay greetrtone Ky, St, Sil, tu, Qtz, Pl, Bt,Ihn Pattersonet al (1984),Burk et al Black fuver beltneatheHemloFaultzone (1986),Pil & Fleet(1993; 1995)

Goup III: Al-Si-rich alteration ssemblages md veins dsociated with VMS md Au depositsin greenston€belts

7 Sturg@n Iake quartz veins dsociated with VMS deposits Ky, Qtz, Prl, Ms, And Frmklin et al (1975)

8 Savrut lake Al-Si-rich alteration osociaied with VMS showins Ky, Sil, And, St, Ms Lefebue (1982)

9 Mashall Iake Al-Si-rich alteration osociated with VMS showinss Ky, Qtz, Sil, Om, Crd Ayres(1978), Amukun (1989) l0 Onmm Headway-Coulee mdsive sulfide o@urgnce Ky, Qtz, Ard, Ser Osterberget al (1987)

11 Melchett quartz veins ss@iated with VMS showings Kv, Qtz Breaks(1991)

12 Mmitouwadge Al-Si-rich alteration csociated with VMS deposit Ky, Sil, Ms, Bt, Pl, Qtz Schmdlet al (1995)

13 Heqllo quartz veins at tlle Hemlo gold deposit Ky. Otz. Ms. Tur. Py Burk etal (1986).Kuhs et al (1994)

SeetextfordiscussionontlleFavourableLakeoccunence MineralsymbolsafterKretz(1983) VMS,volcmogenicmdsivesulfidedeposits. KYANITE IN T}IE WESTERN SUPERIOR PROVINCE 361

Frc. 1 Occurrences of kyanite in the Western Superior Province [classification of subprovinces after Card & Ciesielski (1986); except for the2.9-3.0 Ga North Caribou terrane, after Thurston et al. (1991), dashed linesl. Numbers correspond to Table 1. Group I: l-3) Rainy Lake, Abbess Lake (Atikokan) and Raith areas,respectively, of the Quetico Subprovrnce,4) North Caribou greenstone belt near the northem tectonic boundary of the North Caribou terrane, and 5) Favourable Lake greenstonebelt (see text for discussion). Group tI: 6) the Hemlo - Heron Bay greenstonebelt of the Wawa subprovince.Group III: 7-10) SturgeonLake, SavantLake, Marshall Lake and Onaman areas,respectively, of the Wabigoon Subprovince, 11) Melchett greenstonebelt of the English River subprovince, and 12-13) greenstonebelt and Hemlo gold deposit, respectively, of the Wawa Subprovince.

riorProvince (Fig. I, Table 1). Ayres (1978)listedfour Group I: lqtanite in metapelitesnear subprovince and ofthese; three are located in amphibolite-faciesterranes terrane boundaries near subprovince boundaries, and the fourth one is found in quartz veins associated with metamorphosed This group includes three occurrences[Rainy Lake, volcanogenic massive sulfide (VMS) deposits.Table I Abbess Lake (Atikokan), and Raithl in the Quetico shows that eight of the nine additional occurrencesof Subprovince near the Quetico-Wabigoon boundary kyanite belong to thesetwo groups,whereas the remain- (Moore 1940,Percival et al.1985, Tabor 1988; 1-3 in ing one constitutes a new group in which kyanite oc- Fig. 1), and another one in the North Caribou green- curs in metapelitesin close proximity to a major fault in stone belt close to the tectonic boundary of the North greenstonebelt (Table 1, and seebelow). Also, new data Caribou terrane (4 in Fig. 1; Thurston e/ al. 1979,1991, from Amukun (1989) and Thurston et al. (1991) war- Breaks er al. 1984). rant a re-appraisal of two of Ayres's (1978) original The boundary between the metasedimentary Quetico occurrences(1.e., Favourable Lake, 5 in Fig. 1; Marshall Subprovince and the granite-greenstone Wabigoon Lake, 9 in Fig. 1). Subprovincein the Atikokan area(2inFrg. 1) coincides 362 THE CANADIAN MINERALOGIST

;':\ 'j.:1,rIl:.,.l {: t , i" 10.5mm I

FIG. 2 Photomicrographs illustrating textural relationships and associatedminerals in kyanite-bearing metapelites from the Raith (a--e) and Hemlo (f-g) areas of the Western Superior Province. a) Kyanite poikiloblast (Ky); note that the included gamet (Grt), albeit partially surroundedby quartz and plagioclase, is locally in direct contact with kyanite. b) Kyanite inclu- sion associatedwith ilmenite in staurolite porphyroblast (St). c) Kyanite intergrown with staurolite d) Inclusions of kyanite, staurolite (outlined) and biotite in garnet porphyroblast. e) Part of a large garnet porphyroblast (1.2 cm in diameter) illustrat ing a marked discontinuity between an inclusion-rich core (mainly quartz and plagioclase) and an inclusion-bearing rim (mainly magnetite and ilmenite). 1) Staurolite poikiloblast replaced by biotite + sillimanite assemblagein the hanging-wall metapelites.g) Kyanite porphyroblast and associatedstaurolite, discordant with the main foliation (S2; outlined) in the hang- ing-wall metapelites,h) Fibrous sillimanite after kyanite in the hanging-wall metapelites. KYANITE IN THE WESTERNSUPERIOR PROVINCE JOJ with the Quetico Fault (a regional-scale dextral shear- supracrustalassemblages: the 2980 Ma Aguta Arm as- zone; Williams 1991) that swings into the Quetico semblage, the Keeyask assemblage,the McGruer as- Subprovincejust west of Raith (3 in Fig. 1). Moore semblage,and the Eyapamikama assemblage(Thurston (1940) reportedkyanite in metasedimentaryrocks north- et al. l99l). Thurston et al. (1979) reported kyanite rn west of the Abbess Lake in the Atikokan area (2 in metasedimentary rocks from the southern part of the Fig. 1), but did not provide any petrographic descrip- Noth Caribou greenstonebelt at the ForesterLake area. tion or information on the mineral assemblage.Percival Breaks e/ al. (1984) reported kyanite in metapelites of et al. (1985) reported relict kyanite in plagioclase from the Eyapamikama assemblageat the Miskeesik Lake a garnet - sillimanite - biotite - plagioclase - qnaftz area (Table 1). Breaks & Bartlett (1991) observed two schist at Raith. Two of eight metapelitic samplesfrom generationsof kyanite at Miskeesik Lake: kyanite-l as the Raith area collected by the first author contain kya- ragged poikiloblasts intergrown with staurolite in ag- nite and have a similar assemblageof minerals. Kyanite gregatesthat deflect the enclosingphyllosilicate-defined in these two samples occurs not only as fragments in foliation (c/. Figs. 2f, g below), and kyanite-2 truncat- plagioclase,but also as porphyroblasts or poikiloblasts. ing the regional foliation. Breaks & Bartlett (1991) also The kyanite poikiloblasts are discordant with respectto noted the presence of kyanite as an epitactic replace- the main penetrative foliation and contain inclusions of ment after andalusite at the Miskeesik Lake area. It is garnet (Fig. 2a), biotite, plagioclase, quartz, muscovite noteworthy that the Miskeesik Lake occurrenceof kya- and ilmenite. Kyanite also has been found as inclusions nite is also within 0.5 km of a fault boundary between in or intergrown with stauroliteporphyroblasts (Figs. 2b, the Eyapamikama and McGruer assemblages. c), which have an assemblage of mineral inclusions Ayres (1978) classified an occurrence of kyanite at similar to the kyanite poikiloblasts. In addition, kyanite the Favourable Lake greenstonebelt (5 in Fig. 1) as a and staurolite have been found as inclusions in garnet member of Group I, becauseof its location near the Bear porphyroblasts (Fig. 2d). Sillimanite, not in textural Head fault (l.e., the BerensRiver - SachigoSubprovince equilibrium with kyanite (Percival et al. 1985), is in- boundary in Card & Ciesielski 1986; Fig. 1). Thurston variably fibrolitic and is presentmainly in knots aligned et al. (1991) re-interpreted the Berens River Sub- parallel or subparallel to the main penetrative foliation. province to representa2720Ma magmatic welt during Other minerals in apparent textural equilibrium with northward subduction at the Uchi - English River sillimanite include garnet porphyroblasts and biotite, Subprovince boundary. The Bear Head fault is then no plagioclase and quartz in the matrix. Although a few of longer a subprovince boundary, but is a likely ensialic the largest porphyroblasts of garnet (up to 1.5 cm in structurethat was active atabout2.'l Ga within the2.9- diameter) show a textural zonation with a marked dis- 3.0 North Caribou terrane (Thurston et al. 1991). Un- continuity in the internal trails of inclusions (Fig.2e; fortunately, classification of this Favourable Lake indicative of two episodesof gamet growth), most gar- occurrenceofkyanite into Group II or III is not possible net porphyroblastshave intemal trails ofinclusions con- owing to the lack of petrological data. sistent with growth during the development of the penetrativefoliation and, therefore, are part of the main Group II: l

FIc. 3. Details of kyanite occurrence#6 of Figure 1. Kyanite in metapelitesof the Hemlo - Black River assemblagein the Hemlo - Heron Bay greenstonebelt (Group II): 6-1) White River property, 6-2) Hemlo deposit, and 6-3) Padre Resourcesproperty near Black River. Note the close association with the Hemlo Fault Zone.

1994,Pan & Fleet 1995).We considerthese hanging- not show any replacement textures (Burk el al.1986, wall and footwall aluminous rocks at the Hemlo gold Pan & Fleet 1993, Kuhns et al.1994). In this study, ir- deposit to be metapelites because of their association regular grain boundarieshave been observedon kyanite with other metasedimentaryrocks, similarity in stratig- porphyroblasts in direct contact with "fibrolite" knots raphy to metapelites along strike at the White River (Fig. 2h), and may be related to resorption of the former. property and the PadreResources property (Pan & Fleet Kyanite and staurolite from the Hemlo gold deposit 1995), and distinctive geochemicalcharacteristics (Fleet commonly exhibit extensive alteration to muscovite, et al. 1997), although Kuhns et al. (1994) interpreted chlorite and epidote along grain fractures and bound- them to representmetamorphosed rocks that had under- aries, but are less affected by such alteration at the White gone porphyry-style argillic alteration. River property. It is noteworthy that petrographic de- Kyanite-bearing metapelites at the White River prop- scriptions of kyanite and associatedminerals in alumi- erty and Hemlo gold deposit typically contain kyanite, nous rocks (our metapelites) of the Hemlo gold deposit sillimanite, gamet, staurolite,biotite, muscovite, plagio- by Burk et al. (1986) and Kuhns et al. (1994) are simi- clase, and quartz (Burk et al. 1986, Kuhns et al. 1994, lar to those above, although their interpretations differ Pan & Fleet 1993,1995). Sillimanite, mainly "fibrolite", significantly from ours (see below). usually occurs in knots oriented parallel or subparallel to the main penetrative foliation. Kyanite, on the other Group III: lcyanitein Al-Si-rich alteration hand, typically occurs as prismatic porphyroblasts and assemblage s ass ociate d with mineralization poikiloblasts discordant with respect to the main pen- etrative foliation and is commonly associatedwith stau- This group includes sevenoccurences: the Sturgeon rolite poikiloblasts (Figs. 2f, g). Staurolite poikiloblasts Lake (7 in Fig.1; Franklin et al. 1975), the SavantLake in augen are commonly replaced by a sillimanite + bi- (8 in Fig. 1; Lefebvre 1982), the Marshall Lake (9 in otite + muscovite + plagioclase + quartz assemblage Fig. 1; Ayres 1978; Amukun 1989), and the Onaman (Fig.2f). The mineral inclusions in kyanite and stauro- area (10 in Fig. l; Osterberg et al. 1987) of the lite poikiloblasts are similar and include garnet, plagio- Wabigoon Subprovince,the Melchett greenstonebelt of clase,quartz, biotite, ilmenite, zircon, apatiteand pyrite. the English River Subprovince (11 in Fig.l; Breaks Authors ofprevious studies have emphasizedthat kya- 199I ), the Willecho depositin the Manitouwadge green- nite and sillimanite at Hemlo and the White River prop- stone belt of the Wawa Subprovince (12 in Fig. l; erty, where they commonly occur together, generally do Schandl et al. 1995), and the Hemlo gold deposit in the KYANITE IN THE WESTERN SUPERIOR PROVINCE 365

Hemlo - Heron Bay greenstone belt of the Wawa quartz, and ilmenite, and the main one in the matrix in- Subprovince(13 in Fig. 1; Burk et aL.1986, Kuhns el cluding sillimanite + gamet + biotite + plagioclase + al. 1994, Pan & Fleet 1995). The first six occurrences quartz. In this study, four samplesof low-variance, kya- are all associatedwith base-metalvolcanosenic massive nite-bearing assemblages(two from Raith and two from sulfide (VMS) deposits or showings. For exampie, the Hemlo gold deposit)were selectedfor compositional Franklin et al. (1975) noted that kyanite at the Mattabi analysis of minerals (see Pan et al. 1994 for analytical Cu-Zn deposit ofthe SturgeonLake areais restricted to details; Table 2) and geothermobarometry Qable 3). the massive ores or veins in the immediate footwall, Also, geothermobarometricresults from kyanite-bear- whereas andalusiteoccurs as porphyroblasts in all rock ing samples of the Rainy Lake area (Tabor 1988) and types and in veins. Lefebvre (1982) reported the pres- White River property (Pan & Fleet 1993) are included ence of all three polymorphs of Al2SiO5 in Al-Si-rich for comparison. alteration zones in the Handy Lake metavolcanic rocks The mineral inclusions (i.e.,biotite, plagioclase and at the area. Amukun (1989) also docu- gamet) in kyanite and staurolitepoikiloblasts from both mented all three AlzSiOs polymorphs, staurolite and the Raith areaand the Hemlo gold deposit are composi- muscovite in alteration assemblagesin close proximity tionally distinct from their counterparts in the matrix to base-metal sulfide mineralization at the Marshall (Table 2). For example, biotite in kyanite poikiloblasts Lake area At the Manitouwadge greenstone belt, is higher in Mg/(Mg + Fe) values than its isolated coun- Schandl et al. (1995) reported the presence of relict terpart in the matrix, whereas grains of biotite in direct kyanite in sillimanite knots in altered felsic volcanic contact with garnet porphyroblasts are intermediate in rocks from the footwall of the Willecho 3 orebody. The Mg/(Mg + Fe) and apparently have been affected by re- occurrenceofkyanite in quartz veins and its association equilibration during retrogression. Similarly, plagio- with gold mineralization at the Hemlo gold deposit have clasein kyanite and staurolitepoikiloblasts is more sodic beendescribed by a number of authors(Brxk et al. 1986, than its counterpart in the matrix. Chemical zonation has Kuhns er al. 1994, Pan & Fleet 1995). not been detectedin the matrix plagioclase of the Raith samples.However, some isolated grains of plagioclase MrNnnar- CHpursrnv ANDGEoTIDRMoBARoMETRy from Hemlo show an outward increasein the anorthite component (up to 5 molVo An). Also, plagioclase inclu- Textural evidencefrom Group-I and -II occurrences sions in the margin of zoned porphyroblasts of gamet of kyanite at Raith and Hemlo clearly shows two dis- (Fig. 2e) are slightly higher in An than their counter- tinct assemblagesindicative of two episodes of meta- parts in the core. Garnet inclusions in kyanite and stau- morphism (Ml and M2): an earlier one consisting of rolite poikiloblasts from both Raith and Hemlo are kyanite + staurolite + garnet (inclusions in kyanite and enriched in Ca relative to the M2 garnet porphyroblasts, staurolite, Fig. 2a; core of texturally zoned porphyro- whereasthe inclusion-rich core ofzoned porphyroblasts blasts, Fig. 2e) + inclusions of biotite, plagioclase, of gamet (Fig. 2e) has a Ca content similar to that of the

TABLE 2A COMPOSITIONS OF MINEMLS IN KYANITE-BEARING METAPELITE FROM RAITH (SAMPLE RAITH.I)

Incl$ions in Kymite Mircrals in Matix

sio, (s%o) 37 5 635 361 0 381 37E 279 602 355 358 455 Tio, 0 01 0 198 525 002 0 0l 050 0 233 221 0 05 A,O, 2t 2 229 186 0 219 215 533 256 lE9 191 368 FeO 33 5 0 178 466 329 345 tzt 0 191 186 084 McO 3 27 0 117 016 402 245 127 0 t07 113 051 I\4nO 212 0 001 090 I E6 257 028 0 0a 002 0 05 ZnJ nd nd nd 003 nd nd 241 nd nd trd nd CaO 277 390 0 0 t2t tt1 001 7t2 0 0 001 NarO nd 923 022 nd nd nd 0 7I0 014 018 021 &O nd 041 936 nd Dd nd 0 030 91t 921 105 Fnd ndond nd nd 0nd 000 O=I 0 0 000 Tobl 99 8 999 9t 8 1002 1000 1000 976 1003 959 964 945

Si 3011 2 808 5 4r9 0 000 3 022 3 035 8126 26'70 5 355 5 352 6 083 Ti 0 0 0224 0996 0 001 0 001 0ll0 0 0264 024t 0 005 AI 2 006 I 193 3 291 0 000 2047 2033 1t294 l33t 3 360 3 366 5 798 F€ 2245 0 2230 0982 2 t18 2311 2943 0 2 405 2 321 0 094 rytg 0 391 0 26t9 0 006 0 476 0293 0551 0 2401 2520 0 102 Mn o 144 0 0 001 0 019 0.125 0 175 0068 0 0003 0003 0006 Zn 0 001 0 520 Ca 0 187 0 185 00 0 103 0 101 0 0339 0 0 0002 Na 0 192 0 063 0 0610 0 042 0 052 0 055 K 0.023 | 797 0 0017 1 7',71 | 76t | 795 F 0 0 00 o 120 80 220 30 480 80 220 220 220 366 THE CANADIAN MINERALOGIST

TABLE28 COMPOSITIONSOF MINERALS IN KYANITE-BEARINGMETAPELITE FROM I]EMLO (SAMPLEIIEMLO.I)

lnclusions in K)@i& Minerals in Matix

St PI

sio, (fr %) 372 630 356 0 366 37| 279 619 )52 356 45r Tio, 001 0 1 t9 521 0 005 0u 0 r70 181 058 Arq 2t | 233 195 005 212 212 534 241 198 202 368 FeO 3r2 0 189 453 313 3t2 0 193 115 077 Mgo 236 0 955 019 2',75 188 16E 0 901 967 054 MnO 602 0 001 150 5 80 720 035 0 0t2 020 0 ZtA no no nd 001 nd nd 041 nd nd nd nd CaO 243 430 00 r44 140 s39 004 0 001 Naro Dd 935 032 nd 005 0 840 0 41 025 t23 K,O nd 019 956 nd nd nd 003 936 962 916 F nd nd 0nd nd nd nd 004 0 0 0 002 Tohl 1003 100l 95 3 992 993 1000 979 999 950 950 949

Si 2997 2'785 5 403 0 000 2980 3 004 a09t 2746 5 377 s 393 6 013 Ti 0 000 0 000 0 215 0997 0 000 0 003 0 140 0 000 0 196 0206 0 058 2003 l2t4 3 488 0 002 2034 2023 1t247 1260 3 564 3 606 5 182 Fe 2 09t 0 000 2394 0964 2t27 2t09 3 268 0 000 246t 2214 0 086 Mg 0284 0 000 2 tl2 0 007 0334 0227 0 726 0 000 2053 2185 0107 Mn 0 411 0 000 0 001 0032 0 400 0 494 0 086 0 000 0 016 0025 0 000 Zn 0 000 0 089 Ca 0 210 0 204 0 000 0 000 0t26 0122 0 000 0 257 0 006 0 000 0 002 Na 0 801 0 04E 0 00E 0 000 0000 0722 0 t2r 0 073 0 3r1 K 0011 I E6l 0 000 0 002 | 829 1 E63 I 663 F 0 000 0 000 0 019 0 000 0 000 o t20 80 220 3 0 120 480 80 220 220 220

MineralsymbolsafterKrez(19E3)'a'replesenbtheouhportionofM2gmetpo+h)roblatwithmaximmMe/8evalue,'b', 'd' outmost mgin of M2 gmet porph)Tobltrst in direct conact with biotite; 'c', isolated grain of brotite; grair of biotib in direcl con@ with gmd porph,rcblat; w %, weight p€rcmt

garnet inclusions (Table 2). The M2 garnet porphy- roblasts from both Raith and Hemlo exhibit a weak com- positional zonation typical of prograde metamorphism (i.e., increase in Mg and decreasein Fe and Mn) from core to rim, except that the outemost margin (<50 pm wide) in direct contact with biotite or chlorite shows reversetrends in Mg, Fe and Mn (Fig. 4), consistentwith an exchange of these elements during retrogression. However, the Ca profile of these M2 garnet porphyro- G33 blasts does not show any apparentchanges, even where o\ they are in direct contact with plagioclase (Fig. 4). The two textural varieties of staurolite (i.e., inclusion in kya- = nite and porphyroblasts or poikiloblasts in the matrix) a from the Raith area do not show any compositional dif- I.IJ ferences and are characteized by minor amounts of Zn o (rp to 2.4 wt.Vo ZnO; Table 2A). Similarly, minor =3 amounts of Zn (0.4-2.5 wt.% ZnO) are typical of stau- o rolite from Hemlo. The well-calibrated garnet-biotite geothermometer and garnet - Al2SiO5 - qruartz- plagioclase (GASP) geobarometerwere selectedfor temperature and pres- CaO sure calculations to facilitate direct comparison of the # two episodesof metamorphism (Table 3). In addition to the experimental calibrations of Ferry & Spear (1978) for the garnet-biotite geothefinometer and Koziol & Frc 4. Compositional profile for a gamet porphyroblast (8 Newton (1988) for the GASP geobarometer,we have mm in diameter) from kyanite-bearing metapelite of the also used the TWEEQU software package of Berman Raith area.This garnet porphyroblast most likely grew dur- (1991), with the internally consistent thermodynamic ingthe M2episode of metamorphism, becauseits S-shaped dataset of Berman (1988) and solution models of internal trail of inclusions is continuous with the main pen- Berman (1990) for garnet, McMullin et al. (1991) for etrative foliation. KYANITE IN THE WESTERN SUPERIOR PROV]NCE 36'7

TABLE 3, SUMMARY OF METAMORPHIC PRESSURE-TEMPERATURE RESULTS

M1 Metamomhism

Sample Gamet Biotite Pl Temperature("C) Pressure(MPa)

X(Alm) X(Pyr) X(crs) X(Sps) X(Mg) X(Fe) X(AD X(TD X(K) F&S TWQ K&N TWQ

Raith-l 0.751 0132 0.063 0 049 0.453 0.386 0.123 0 039 0.966 18.5 510 560 690 670 Raith-2 0.710 0.101 0.072 0117 0397 0.393 0.169 0.041 0.982 18.1 495 540 730 690 Hemlo-l 0.696 0 095 0.070 0137 0.382 0.423 0157 0.038 0.975 20.7 515 560 690 670

M2 Metamomhism

Biotite Temperature("C) Pressure(MPa)

X(Alm) X(P1r) X(Grs) X(Sps) x(Mg) x(Fe) x(Al) x(rD x(K) An F&S TWQ K&N TWQ

Raith-l 0.756 0 165 0.036 0.043 0.41s 0.415 0 123 0.046 0 917 350 640 645 320 330 Raith-2 0.712 0.125 0.060 0.103 0.3s4 0.449 0.157 0.040 0.961 53.0 650 670 340 450 Hemlo-l 0.7i2 0.112 0 042 0 134 0 363 0.435 0.167 0.03s 0.938 26.2 590 600 430 450 Hemlo-2 0.641 0.167 0 058 0.134 0 477 0.353 0.138 0 033 0.958 35.0 595 620 450 520

Retrogression (M3)

Gamet Biotite Temperature ('C)

X(Alm) X(Pyr) X(crs) X(sps) x(Mg) x(Fe) x(AD X(TD X(K) F&s rwQt

Raith-i 0.802 0.102 0.035 0.061 0 434 0.399 0 r24 0.043 0.971 450 460 Raith-2 0.125 0.092 0.059 0.124 0.431 0.377 0.151 0.041 0.977 435 455 Hemlo-l 0.714 0.077 0.041 0.167 0.390 0.395 0.178 0 037 0.962 430 440 Hemlo-20.661 0.139 0.042 0.158 0.535 0.298 0.135 0.032 0 956 450 455

F&S is the gamet-biotitegeothermometer of Ferry & Spear(1978); K&N is the GASP geobarometerof Koziol & Newton (1988) using the solution modelsofBerman & Koziol (199i) for gametand Fuhrman& Lindsley (1988) for plagioclase;TWQ is calculated ftom the TWEEQU methodof Berman(1991) using the internally-consistentthermodynamic data set of Berman(1988) and solution modelsof Berman (1990) for gamet,McMullin et al (1991) for biotite, and Fuhman & Lindsley (1988) for plagioclase.f, assuminga oressureof 200 MPa.

biotite, and Fuhrman & Lindsley (1988) for plagioclase, A12SiO5polymorph (Table 3). In addition, the garnet- to evaluate the influence of other components (e.g., Ca biotite geothermometerhas been applied to the outer- and Mn in garnet, and A1 and Ti in biotite) on the pres- most margin of gamet porphyroblasts and biotite in sure-temperature estimates. These geothermobarom- direct contact with it to constrain the event of refrogres- eters also were chosen to facilitate comparison with the sion (M3; Table 3). The lack of variation in Ca in garnet resultsofTabor (1988) for the Rainy Lake area,Percival Gig. a) where it occurs in direct contact with plagio- (1989) for the Raith area, and Pan & Fleet (1993) for clase indicates that the net-transfer reaction involving the White River property. For both the Raith area and the breakdown of 3 anorthite to give grossular+ Al2SiO5 the Hemlo gold deposit, pressuresand temperaturesof + quartz was not important during the retrogression.Al- the M1 metamorphismwere calculatedexclusively from ternatively, the retrograde path may have been parallel the compositions of mineral inclusions in kyanite to the GASP isopleth. porphyroblastsor poikiloblasts (Table 3). We calculated Geothermobarometric results from the Raith area the pressuresand temperatures of the M2metamorphism and the Hemlo gold deposit are given in Table 3 and by assuming that the outer portion of the M2 garnet illustrated in Figure 5. These results confirm the tex- porphyroblastswith the maximum Mg/Fe value (Fig. 4) tural relationshipsfor two distinct episodesof metamor- was in equilibrium with the isolated grains of biotite and phism and a reffograde event. Moreover, the Raith area plagioclasein the matrix, where sillimanite is the stable and the Hemlo gold deposit both have undergone an 368

earlier medium P-T (Barrovian-style) metamorphism MPa from the Koziol & Newton GASP geobarometer. and a main low-P/T metamorphism (Table 3, Fig. 5), Similarly, the results of the M2 metamorphism at the and hence are discussedtogether below. Hemlo gold deposit are comparableto those at the adja- Pan & Fleet (1993) reported pressureand tempera- cent White River property (400-500 MPa and 550- ture conditions of 60G-650 MPa and 500'C for the Ml 650oC; Pan & Fleet 1993). The temperature estimates metamorphism at the White River property. The gar- for the retrogressionevent (M3) at Raith and the Hemlo net-biotite geothermometerbased on the calibration of gold deposit are similar and range from 430 to 460"C Ferry & Spear (1978) also yielded 495-515"C for the 0able 3). Ml metamorphism at both the Raith areaand the Hemlo Pan & Fleet (1993) also used mineral-inclusion gold deposit (Table 3). However, the temperatureesti- geothermoba"rometry (St-Onge 1987) to construct a mates from the TWEEQU calculation are higher (540- metamorphic P-T path involving both Ml andM2meta- 560'C; Table 3). Also, pressure estimates of the Ml morphism at the White River property. Unfortunately, metamorphism from the GASP geobarometerof Koziol mineral inclusions in garnet porphyroblasts from the & Newton (1988) are slightly higher than those from Raith and Hemlo samples examined in this study are the TWEEQU calculation (Table 3). It is noteworthy not sufficiently developed to permit use of this method. that Bauer & Tabor (1991) also recognized an earlier Nevertheless, the compositional zonation of the M2 medium-P, low-T metamorphism in the Rainy Lake garnetporphyroblasts (l. e., increasein Mg/Fe from core area. However, the geothermobarometricresults of Ta- to rim except for the outermost margin; Fig. 4) corre- bor (1988) are difficult to evaluate, becausehe did not spondsto a temperatureincrease of about 60"C, assum- differentiate the two episodes of regional metamor- ing a constant composition of the coexisting biotite. phism. However, pressuremost likely remained constant dur- Our pressure and temperature estimates of the M2 ing the growth of the M2 garnetporphyroblasts, because metamorphism at Raith (Table 3) are almost identical the temperature increase is offset by an increase in the to those obtained by Percival (1989): 650oC from the anorthite component of plagioclase inclusions. These Ferry & Spear gamet-biotite geothermometer and 330 results are in agreementwith the calculated metamor-

800 a Raith b Hemlo 700

(u 600 (L 500 = E 4oo 5 o 3oo 2 ., 0 t_ tl tL I +I 2oo + I I ;M3 i tvtg 100

4oo 500 600 700 400 500 600 700 Temperature(oC)

Frc. 5. Summary of metamorphic P-T conditions calculated from kyanite-bearing metapelites for a) the Quetico-Wabigoon boundary at the Rairh area, and b) the Hemlo gold deposit; also shown is the calculated P-T path from the White River Foperty (Pan & Fleet 1993; dashed lines). Boundaries of A12SiO5polymorphs are calculated using the thermodynamic datasetof Berman (1988). Ml is the earlier Barrovian metamorphism, M2 is the regional low-P, medium-T metamorphism, and M3 is the retrogression. KYANITE IN THE WESTERN SUPERIOR PROVINCE 369 phic P-T path from mineral-inclusion geothermo- different from their modern counterpafis and were not barometry at the White River property (Pan & Fleet appropriate for high-P/T metamorphism but, instead, 1993; Fig. 5b). Burk et al. (1986) and Kuhns et al. might have yielded medium-P/T (Barrovian) metamor- (1994), on the other hand, interpreted kyanite as part of phism (including kyanite-staurolite assemblagesin the peak metamorphic assemblageand sillimanite as a metapelites).Therefore, we propose that the Barrovian retrograde phase, although their calculated temperatures metamorphism in the Western Superior Province, as of the kyanite-forming event are noticeably lower than revealedby the kyanite occurrencesin metapelites,may those of the sillimanite-forming event. Sillimanite in be directly related to Archean subduction-accretion tec- many parts of the Western Superior Province is clearly tonlc processes. part of the prograde assemblageof the regional low-P/T metamorphism (Ayres 1978, Thurston & Breaks 1978, Implications for accretionary tectonics in the western Percival 1989, Thurston et al. 1991, Williams et al. Superior Province 1991,Pan & Fleet1993). According to accretionary plate-tectonic models DrscussroN (e.g.,Percival & Williams 1989,Card 1990,Thurston & Chivers 1990, Percival et al.1994), the subprovinces Metamorphic significance of Group-I and -II of the Western Superior Province represent volcanic occurrences of lcyanite arcs (granite-greenstone subprovinces), accretionary complexes (metasedimentarysubprovinces), and conti- Most previous studieshave emphasizedthe regional nent-margin magmatic arcs (plutonic subprovinces), low-P/T metamorphism in the Western Superior Prov- assembled by sequential north-to-south accretion ince (Thurston et al. 1991, Williams 1991, Williams er (Krogh & Davis l97l,Cafi 1990,Percival et al. 1994). al. 1991, and referencestherein). Our textural evidence Specifically, Percival & Williams (1989) interpretedthe from the kyanite occurrencesin metapelitesand quanti- Quetico Subprovinceas an accretionarywedge in which tative geothermobarometric calculations demonstrate sedimentsof submarinefans and abyssalturbidites were the existenceof an early Banovian-style metamorphism first deformed by accretion onto the active Wabigoon (seealso Burket a|.1986, Bauer & Tabor 1991,Pan & arc and later became compressed by docking of the Fleet 1993) before the regional penetrativedeformation Wawa arc (Percival 1989, Williams 1990). Percival & that accompaniedthe main low-P/T metamorphism.The Williams (1989) also suggesteda northward subduction geothermobarometryalso reveals a significant decom- on the basis of the dominantly northward-facing stratig- pression following the Barrovian metamorphism from raphy in the Quetico metasedimentaryrocks. However, approximately 700 MPa to about 300 MPa at the theseauthors cautioned that the regional low-P/T meta- Quetico-Wabigoon boundary (Table 3), corresponding morphism in the Quetico Subprovince is not consistent to an exhumation of up to l2 km. Exhumation of at least with the thermal structure of accretionary wedges. We 6 km (i.e., from 700 MPa to 400-500 MPq Table 3, suggest that the earlier Barrovian metamorphism, and Fig. 5b) occurred also in the Hemlo - Heron Bay green- not the main regional low-P/T metamorphism, formed stone belt along the Hemlo Fault Zone. during this early stage of the development of the Quetico Barrovian metamorphismis common in Phanerozoic Subprovince.Several authors (e.g., Tabor 1988,Percival orogenic belts, and is related to large-scale thickening & Williams 1989, Williams 1990) have documented of the crustfollowed by thermal relaxation (Spear 1993, early thrust- and nappe-like structures at the Quetico- Windley 1995).High-P/T metamorphism(l.e., blueschist Wabigoon boundary. Underplating and thrusting of and eclogite facies) that characterizesPhanerozoic sub- sedimentswould be expected to thicken the accretion- duction complexes (Platt 1986, Hamilton 1988), on the ary wedge (Platt 1986) and hence create conditions for other hand, requires anomalously 1ow geothermal gra- Barrovian metamorphism in Archean subduction com- dients (Ernst 1972, Windley 1995). The absenceof plexes. Platt (1986) also demonstratedthat continued blueschists and eclogites in Archean terranes has long underplating at the base of the wedge must be compen- been attributed to either significant higher geothermal satedby extension above, thus providing a mechanism gradientsin Archean time or to difficulties in preserva- for bringing up high-P/T metamorphic rocks to upper tion (Ernst l912,Windley 1995,andreferences therein). levels at the landward margin of the wedge. This com- There is ample evidence that the Archean mantle was bination of continued underplating and extension pro- considerably hotter than today's mantle (e.g., Bickle vides a plausible explanation for the observeduplift of 1978, Martin 1986, Herzberg 1996); therefore, plate up to 12 km at the Quetico-Wabigoon boundary. production and rate of oceanic lithosphere spreading Ptatt (1986) suggestedthat preferential uplift in ac- may have been greater in the Archean (Bickle 1978, cretionary wedges occurs near the landward margin Martin 1986, Condie 1990), resulting in younger, hot- where high-P/T metamorphic rocks are commonly ter, and thicker oceaniclithospheres in Archean subduc- observed in Phanerozoic accretionary complexes. The tion zones. Consequently, the thermal regimes in restricted occurrences of kyanite in the Quetico Archean subductionzones must have been significantly Subprovince along the northern (Quetico-Wabigoon) 370 THE CANADIAN MINERALOGIST

boundary appearsstrikingly equivalent to the restriction single tectonic environment. However, theselithologies of lawsonite- andjadeite-bearing blueschists to the east- are merely typical of Archean island arcs.In light of the ern side of the accretionary prism of the Franciscan present survey of kyanite occurrences,they could well Complex in California. This provides further evidence have formed in separateisland arcs or indeed could be for northward subduction during formation of the separatefragments of the same island arc system. Fur- Quetico Subprovince (Percival & Williams 1989). On ther high-precision U-Pb zircon geochronological the basis of east-north-east shallowly plunging mineral analysesof volcanic rocks and associatedsedimentary lineations and indicators of dextral shear,Tabor (1988) rocks in the Hemlo - Heron Bay greenstone belt are suggestedsouthward subductionin the western Quetico clearly neededto test whether the Hemlo - Black River subprovince in northern Minnesota and proposed a re- and Heron Bay assemblagesrepresent two related or versal of the polarity of subduction along the strike of unrelated fragments of greenstone. the Quetico Subprovince,similar to the Apine Fault in Thurston et al. (199I) proposedthe formation of the New Zealand. However, the occurrence of kyanite in northwestem Superior Province (i.e., the Berens River the Rainy Lake area suggests that subduction in the and Sachigo subprovinces in Card & Ciesielski 1986) western Quetico Subprovince was northward, as it was by a northward accretion onto the North Caribou ter- throughout the remainder ofthe subprovince. rane. The occurences of kyanite in the North Caribou The timing of the Barrovian metamorphism at the greenstonebelt, near the northern boundary ofthe North Quetico-Wabigoon Subprovinceboundary has not been Caribou terrane, are consistent with crustal thickening directly constrained.Percival & Williams (1989) brack- related to this northward accretion. Altematively, the eted the formation of the proposed accretionary wedge Miskeesik Lake occurrence of kyanite close to a fault fram2750 to 2700 Ma. This was followed by compres- boundary between the Eyapamikama and McGruer as- sion as a result of oblique collision with the Wawa arc semblagesmay be related to accretionaryprocesses that at27U0-2684 Ma (Corfu & Stou 1986, Percival & Wil- assembledthe North Caribou greenstonebelt. Further liams 1989, Percival 1989). The M2 metamorphism in researchof these North Caribou occurrencesof kyanite the Quetico Subprovincehas beenbracketed at approxi- is needed to establish their relationships with the tec- mately 2670 Ma in amphibolite-facies zones (Percival tonic events,especially in view ofthe possiblepresence & Sullivan 1988) and 2666-2650 Ma in a granulite-fa- of multiple generations of kyanite (Breaks & Bartlett cies zone (Pan et al. 1998), and is interpreted to relate 1991). to thermal relaxation following collision with the Wawa arc with additional sources of heat from underplated Commentson the Group-III occurrences of lcyanite mantle-derived magmas (Percival 1989, Pan et al. 1994). The occurrences of kyanite and other aluminosili- It has become clear in recent years that many cates in Al-Si-rich alteration assemblagesassociated Archean greenstone belts may represent a collage of with VMS and Au mineralization clearly reflect the originally unrelated greenstonefragments assembledby unusualwhole-rock compositionsof their host rocks and accretionary tectonic processes(c/. Williams 1990, may representpotential exploration tools for base-metal Thurston et al. l99l). On the basis of two U-Pb zircon mineralization (e.9., Franklin et al. 1975, Amukun age determinations,Corfu & Muir (1989) suggestedthat 1989). However, the relationship of these kyanite oc- the Hemlo - Heron Bay greenstonebelt representsa jux- curences to tectonic processesis difficult to decipher taposition of two originally unrelatedfragments (i.e., the because of the high variance of their mineral assem- 2772 MaHemlo - Black River assemblageand the 2695 blages. In particular, the presence of two or all three Ma Heron Bay assemblage)separated by a major struc- Al2SiO5polymorphs in thesealteration assemblages has tural discontinuity, possibly representedby the Hemlo been suggestedby someto representmetastabllity (e.g., Fault Zone (see also Williams et al. 1991).If this sug- Lefebvre 1982). Franklin et al. (1975) suggestedthat gestionis valid, the restrictedoccurrences ofkyanite and kyanite at the Mattabi deposit formed from the break- inferred earlier Barrovian metamorphism in close spa- down of pyrophyllite and did not require medium-P tial association with the Hemlo Fault Zone may have metamorphic conditions. Similarly, Fleet & Pan (1995) recorded an event of crustal thickening related to small- interpretedkyanite in the quartz veins of the Hemlo gold scale accretion that assembledoriginally unrelated deposit to have formed during a late low-grade alter- slivers of greenstones(e.g., Williams 1990). It is well ation event in a manner equivalent to the formation of known that accretion of allochthonous terranes(bv col- kyanite in veins and segregationsin the Lepontine Alps, lision) in the CoastRanges of British Columbia resulted Switzerland (Kenick 1990). Fleet & Pan (1995) sug- in intense deformation and Barrovian metamomhism gestedthat the P-T-t trajectory locally crossedinto the followedby rapiduplifting te.g., Hollister 1982). stability field of kyanite, and pyrophyllite stability was Pan et al. (1991) had noted that, at the White River suppressedby the high salinity of the fluids. Kyanite in property, the sedimentary and mafic volcanic rocks from Al-Si-rich alteration assemblagesof the Manitouwadge both sidesof the Hemlo Fault Zone areremarkably simi- greenstone belt, on the other hand, occurs as a relic lar; they suggestedthat they most likely formed in a within sillimanite (Schandl et al. 1995).It is notewor- KYANITE IN THE WESTERN SUPERIOR PROVINCE Jtl thy that the Manitouwadge greenstonebelt is situated Avnrs, L.D. (1978): Metamorphism in the Superior Province to crustal de- closely to the Quetico-Wawa boundary, and the occur- of northwestem Ontario and its relationship (J.A. rence of kyanite here may still record crustal thickening velopment. In Metamorphism in the Canadian Shield WW. Heywood, eds.) Geol. Surv. Can., Pap' related to the docking of the Wawa arc onto the Fraser & Quetico 78-10,25-36. accretionarywedge (Percival 1989).Also, Ayres (1978) classified the Marshall Lake kyanite occurrence as a BAUER,R.L. & Teeon, J.R. (1991): Syntectonic, intermediate- member of Group I becauseof its location close to the pressureregional metamorphism along the northem mar- - English River Wabigoon boundary, although petro- gin of the Quetico belt in northeastemMinnesota: contrasts graphic or geothermobarometricevidence for an earlier with adjacent boundary areas.Inst. I'ake Superior GeoI. Barrovian metamorphism is still lacking there. Proc 36,2-4.

CoNcr-usIoNs BERMAN,R G (1988): Intemally-consistent thermodynamic data for minerals in the system Na2O-K2O-CaO-MgO- Kyanite in the supracrustal rocks of the Archean FeO-Fe2o3-Al2o3-SiO2-Tio2-Hzo-C.oz. J. Petrol. 29, Western Superior Province falls into three distinct 445-522. lithotectonic associations: I) metapelites near sub- province and terrane boundaries, II) metapelites near (1990): Mixing properlies of Ca-Mg-Fe-Mn gar- 75. 328-344 faults in greenstonebelts, and III) Al-Si-rich alteration neLs.Am. Mineral. assemblagesassociated with VMS and Au mineraliza- ( 199 1 Thermobarometry using multi-equilibrium tion in greenstonebelts ): calculations: a new technique, with petrological applica- Kyanite in Groups I and II indicates an earlier tions.Can. Mineral.29, 833-855 Barrovian metamorphism (670-730 MPa and 500- 560"C) before the regional penetrativedeformation that &Kono\ A M. (1991): Temary excesspropertres accompaniedthe regional low-P/T metamorphism. The of grossular - pyrope - almandine gamets and their influ- presence of group-I kyanite along the Quetico- ence in geotherm obarometry . Am. Minetal. 76, 1223-1231 Wabigoon boundary and the inferred metamorphic P-T path support the origin of the Quetico Subprovince as BtcKLs, M.J. (1978): Heat loss from the Earth: a consffaint on an accretionary wedge during northward subduction. Archean tectonics from the relation between geothermal Group-II kyanite from the Hemlo - Heron Bay green- gradients and the rate of plate production. Earth Planet' 40,301-315 stone belt may have recorded accretion that assembled Sci.Lett originally unrelated slivers of greenstones.Our results BLACKBURN.C.E (1980): Toward a mobilist tectonic model support the two-stage-accretionmodel of Williams for part of the Archean of northwestem Onraio' Geosct' (1990) for the development of the Western Superior Can. 7, 64-72 Province. Kyanite in Group III reflects unusual host-rock com- Bnnr.rs, F.W (1991): English River Subprovince. In Geology positions and hydrothermal fluids, and may be a poten- of Ontario (P.C. Thurston, H.R. Williams, R.H. Sutcliffe & tially useful tool for exploration of volcanogenic G.M. Stott, eds.).Ontario GeoI Sun., Spec VoL.4,239' massive sulfide deposits. 277

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