tt37 Thz Cana-dian M iner alng i st Vol. 35, pp. 1137-1160(1997)

METAMORPHISMIN THEPALEOPROTEROZOIC TORNGAT OROGEN. LABRADOR: PETROLOGYAND P-T-t PATHSOF AMPHIBOLITE. AND GRANULITE.FACIESROCKS ACROSSTHE KOMAI(TORVIK SHEAR ZONEg

FLEMMINGMENGEL'

Danish Lithasphere Centre, Ostervoldgade 10, DK-1350 Copenhagen K, Dewnark

TOBYRTVERS

Deparxnent of Earth Sciences, Memorinl University of Newfound.Iand., St. John's, Newfoandland AIB 3X5

ASSTRACT

The Paleoproterozoic Torngat Orogen in northern l,abrador developed as a result ofcollision between the Archean Nain and Southeast Rae cratons. Early phases involved contractional deformation and crustal thickening, whereas later phases were dominated by transcurrent and subsequent near-vertical displacements. The axial region ofthe orogen is occupied by two major structures, the Abloviak (ASZ) and Komattorvik (KSZ) shear zones. The gneisses of the Nain Province east of the KSZ are characterized byArchean granulite-facies assemblages; these become progressively overprinted westward by static amphibolite- to granulite-facies assemblages of Paleoproterozoic age toward the KSZ. P-T determinations on the Avayalik dykes and Archean mafic gneisses define a P-T array from ca. I I kbar - 750'C to ca. 6kbar - 600'C. The KSZ is undeilain by variably reworked Archean gneisses, mafic and pelitic supracrustal rocks, the Avayalik dykes, anorthosite, and members of the Paleoproterozoic diorite - tonalite - granodiorite (DTG) suite. The peak grade of metamorphism is upper amphibolite to granulite facies; howeveq P-T determinations from vadous microstructural settings record a substaatial part of the post-peat history. In a range ofrock types, an array is defined fuom ca.1l.7 kbar - 720-800'C to ca. 6.5 kbar - 540'C. P-T data from west of the KSZ define an alray from ca. 10 kbar - 786'C to ca 4 kbar - SOOoC.ln any of the geographic zones above, P-T arrays from each rock type are parallel to fle composite array defined by data from all zones. Furthermore, vectors connecting P-T deterninations from cores to rims of coexisting minerals are parallel to the overall trend. The data strongly suggest thtt the Avayalik dykes are most resistant to resetti-ng durilg cooting, and that mafic orthogneisses, mafic supracruital iocks and pelitic supracrustal rocks are progressively less robust. Available U-Pb geochronological data suggest that the KSZ and its bordering segments followed different P-T-t paths. From west to east, there is an overall younging in ages of metamorphic zircon, and almssg $Q milliqn ysars of tectonic and metamorphic activity along the KSZ is recorded by iges determined on titanite. The geographic variarion in metamorphic ages is interpreted to result from migration of zones whire active deformation (and availability of fluid) allowed continued equilibration and gowth of metamorphic mineral assemblages.

Keyvords: Paleoproterozoic, Tomgat Orogen, shear zone, decompression reactions, geothermobarometry, P-T array, geocbronology, Labrador.

Sonruenn

La ceinture orogdnique pal6oprotdrozo'ique de Torngat, dans le nord du Labrador, rdsulte de la collision entre le socle cratonique archden de Nain et la partie sud-est du craton de Rae. Les phases prdcoces ont impliqu6 un 6pisode de ddformation d0 i la contraction et a l'6paississement de la cro0te, tendis que des mouvements horizontaux e! plus tard des mouvements presque verticaux ont domin6 les phases tardives. La partie axiale de cette region comprend deux structures, les zones de cisaillement d'Abloviak (ZCA) et de Komaktorvik (ZCK). ks roches gneissiques de Ia kovince de Nain i I'est de ZCK sont des roches arch6enes poss6dant des assemblages typiques du facids granulite. Ceux-ci se voient progressivement recristrtlis6s vers I'ouest, en direction delaZCK par des assemblagesde m6tsmorphisme statique dans le faciBs amphibolite ou granulite d'6ge pal6oprotdrozo'ique. Des d6terminations de P et de T faites sur les filons de Avayalik et sur les gneiss nafiques arch6ens d6finissent des conditions allant d'environ 1l kbar - 750"C i environ 6 kbar - 6@'C. I^a ZCK est associ6e d un socle de peiss et des roches lch6eqs supracnrstales mafiques i pdlitiques plus ou moins recycl6es, des filons de la suite Avayalik, en plus d'anorthosite et de membres d'une suite pal6oprot6rozoique i diorite - tonalite - granodiorite (DTG). Le m6trmorphisme a atteint le faciEs amphibolite supdrieur i granulite. Toutefois, les d6terminations de P et de T provenant de plusieurs milieux

! LITHOPROBEcontribution number 867. 1E-mnil address:[email protected]

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microstructuraux t6moignent des conditions largement post6riewes au paroxysme m6trmorphique. Dans une grande vari6t6 de roches, les conditions s'dtalent enffe environ 11.7 kbar - 720-800oC et environ 6.5 kbar - 5zt0"C. [.es d6terminafions de P et de T l l'ouest de la ZCK d6finissent des conditions entre environ 10 kbar - 786"C et environ 4 kbar - 500'C. Dans chacune des zones g6ographiques cit6es, les fac6s P-T pour chaque rype de roche sont paraleles au trac6 composite comprenant toutes nos donndes pour chacune des zones. De plus, les vecteum joiglant les d6termilations du coeur vers la bordure de min6raux coexistants sont paralldles au trac6 composite. D'aprds ces donn6es,les filons de la suite Avayalik semblent les plus r6sistants au r6-dquilibrage pendant le reftoidissement, tandis que les orthogneiss mafiques, sinsi qus ls5 roches supracrustales mafiques et p6litiques, sont progressivement moins robustes. [,es donn6es g6ochronologiques U-Pb disponibles font penser qte IaZCK et les secteurs qui la longent ont subi des 6volutions P-T-t diff6rentes. D'ouest en est, il y a un rajeunissement progressif de l'6ge du zircon m6tamorphique, et presque 80 millions d'arut6esd'activit6 tectonique et m6tamolphique le long de la ZCK sont indiqu6es par les iges ddterminds sur la titanite. La variation g6ographique des iges m6tamorphiques r6sulterait de la nigration des zones de d6formation active (et de disponibilit6 de la phase fluide) qui ont permis un r6-6quilibrage et la croissance des assemblages de min6raux m6tamorphiques. (Traduit par la R€daction)

Mots-clds: pal6oprot6rozoique, ceinture orog6nique de Torngat, zone de cisaillement, r6actions de d6compression, g6othermobarom6trie, conditions de P et de T, g6ochronologie, Labrador.

INTRoDUc"uoN characterizedby two crustal-scale,ductile transcurrent shear zones that developedfollowing a period of The estimationof metamorphicconditions places crustalthickening (e.g., Korstgird et al. 1987, Mengel powerful constraintson the tectonic evolution of & Rivers 1990, Ryan 1990, Van Kranendonk & orogenicbelts, especiallywhen usedin conjunction Ermanovics1990, Bertrand et al. 1993,Nverset aI. with structuraland geochronologicalstudies. Since the 1996, Yan Kranendonk 1996, Yan Kranendonk & mid-1970s,when thermal modeling of tle orogenic Wardle L996).Itis the aim of this studyto investigate processwas flrst attempted,there has beena viable the P-T-t relationshipsof tle youngerKomaklorvik theoreticalframework and reasonable estimates of the shear zone (KSZ) in the northern part of Torngat principal variablesttrat control the disfribution of heat Orogen(Fig. 2); the P-T regime of Abloviak shear in an activeorogen, including thermal conductivities, zonewill be the subjectof a later study. mantleheat-flow, crustal thickening, uplift andexhu- mation(e.9., England & Richardson1977,England & P-T DsrERrm{ATroNSIN l[cn-Gnaos TERRANES Thompson1984, Thompson & England1984, Peacock 1992,Spear 1992, L994). This theoryhas been success- One of the major potentialdffiiculties associated fully exploited to interpret the tectonic context of with the estimationof P andT in granulite- and upper- pressure(P) - temperature(T) data in many studies amphibolite-facies terranes is that the closure of Paleozoicand younger orogenic belts and in a temperafureof a geothermometeror of a geobarometer smallernumber of studiesof older orogenicbelts. In may be less than ttre peak temperatureof metamor- the caseof deeplyeroded Proterozoic orogens, where phism, with tle result that the minerals may protoliths of many units are uncertain,mineral assem- re-equilibrateon the retrogradepath and record a P-T blagesand P-T datamay provide essentialevidence of point that is lessthan the peakconditions attained by many aspectsof ttre tectonic history. the rock (e.g.,Frost & Chacko1989, Harley 1989, Most orogenspreserve evidence of a predominant Spear& Florence1992, Pattison & B6gin 1994).This thrust senseof movement,implying an important early problem has been referred to as tle "granulite stageof crustalthickening. Many orogensalso preserve uncertainfyprinciple" by Frost & Chacko(1989). In evidenceof a subsequentperiod of crustalextension addition, if there is non-synchronous closure correlatedwith orogeniccollapse of the overthickened ("mismatching":Harley 1989)of the thermometerand crust(e.8., Dewey 1988).In deeplyeroded orogens, barometer,the estimatedP-T wilt be an erroneous P-T determinationscan provide critical evidenceto point that doesnot lie on the true P-T-t path (e.g., distinguish imbricate stacksformed by thrusting from Selverstone& Chamberlain1990, Frost & Tracy 1991). thoseformed by extension,and in favorablecases, the There areno clear-cutways aroundttrese problems, calculatedP-T differencesbetween adjacent blocks or as discussedextensively by the authorsnoted above, imbricateslices can be utilised to reconstructdetails of but there are severalways that their effects can be the orogenichistory (e.9., Hodges& Royden 1984, assessedand mitigated. Spearet aI.1984, l99O). 1) It is essentialthat the petrographiccontext Deeply eroded orogens exhibiting evidence of of the mineralsused for P-T determinationsbe well crustal-scaletranscutent motion havereceived rela- established,and the natureand compositionsof the tively little attention.The TorngatOrogen (Fig. 1) is neighboringphases be known.

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Ftc. l. Overview geological map of the Torngat Orogen, northem Labradoq showing the main lithotectonic elements and geographic locations mentioned in the text (modified from Taylor 1979, Korstg6rd et al. 1987, Van Kranendonk & Ermanovics 1990, Wardle er al. 1993, Wardle & Van Kranendonk 1990. "Bwwell masmatic arc" furcludes the DTG a:rd Killinek charnockitic suites.

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2) The significanceofreaction texturesshould be unconformably overlain by the Paleoproterozoic evaluatedin the contextof constraintsavailable from Ramah and Lake Harbour groups, respectively petrogeneticgrids. Textures typical of refrogradereactions (Jackson& Taylor L972, Knigbt & Morgan 1981, associatedwith quasi-isobariccooling (IBC) and Taylor 1979).The westernmargin of the Nain Craton quasi-isothermaldecompression QTD) in granulite-facies was strongly reworked in the Torngat Orogen, and rocks have been summarizedby Harley (1989) and tlere is a steepstructural and metamorphicgradient provide the flrst cluesto the extentand natureof the easfwardtoward the Nain foreland,which consistsof retrogradepath. Although Harley's study was restricted unreworkedArchean gneisses with a little-deformed to granulite-faciesrocks, equivalent textures must be RamahGroup cover (Mengel 1988,Van Kranendonk soughtin amphibolite-faciesterraneso in addition to 1996).In contrast,all of the SoutheastRae Craton, its petrographicevidence for the progradeor retrograde Lake Harbour Group cover and interlayered natureof the amphibolite-faciesmetamorphism. Paleoproterozoicgranitic rocks of the Lac Lomier 3) There is empirical evidence tlat different complex@rmanovics & Van Kranendonk1990) were samplesfrom a single areaand,-in some cases, even stronglyreworked during the Torngatand New Qu6bec different textural locations from within a single orogenies@ertrand et al. L993,Van Kranendonketal. sample,may record different parts of the P-T path. 1993b). Such"domainal equilibrium' is the resultof variable The axial region of the TorngatOrogen is underlain reaction-progress(controlled by for example,compo- by a linear belt of strongly deformed, high-grade sition and 4mount of fluid, bulk composition, Paleoproterozoicmetasedimentary rocks known as deformation) and can conspicuouslyincrease the Tasiuyakgneiss (Wardle 1983) that has been inter- amountof P-T informationfrom metamorphicrocks preted as an accretionaryprism covering the suture if interpretedcorrecfly. between the two marginal cratons (e.9., Scott & 4) Progradeand retrograde reactions, as inferred Campbell1993, Van Kranendonk et al. 1994,Rivers et from texturesand constraintsfrom a petrogeneticgrid, al.1996). must be usedas qualitative constraintson the slope of The easternmargin of northern Torngat Orogen is the progradeand retrogradeportion of the P-T-t path intruded by a suite of Paleoproterozoicplutons of (e.9.,Mengel & Rivers1991). diorite-tonalite-granite (DTG) composition,whose 5) Ideally, several geobarometersshould be age,location and composition have been used to infer employed,so tlat tle slopeof the P-T-t path can be that the subductionzone was east-dipping,under the constrainedby various equilibria. Nain Cratonoimmediately prior to the TorngatOrogeny 6) Equilibria that are known to close at low (Van Kranendonk& Scott 1992, Scott & Campbell temperaturesa1s nnlikely to yield realistic estimates 1993,Rivers et aI. 1996,Van Kranendonk& Wardle of peakP-T conditions.As evidenceis accumulated 1996). on intracrystallinerates of diffusion of ionic species The northwesternpart of the Torngat Orogen, in a variety of phases,it has becomeapparent that known asBurwell domain(Korstgird et al. I978,Yan net-transferreactions (typical geobarometers)are gener- Kranendonket al. L993c),is largely underlain by ally morerefractory than exchangereactions (typical the weakly deformed PaleoproterozoicKillinek geothermomelers),such that this consfaint is particularly charnockiticsuite. These sals-alkaline plutonic rocks, applicableto the latter (e.9.,Pattison &Begin 1994). which are broadly of the sameage and compositionas In this study, we have worked within these theDGT suite(e.g., Scott 1995b), have been interpeted constraintsboth to provide the best estimatesof the as representingthe root of a continental-margin high-grademetamorphic conditions recordedin rocks magmaticarc emplaced into theNain craton(e.g.,Yan ofthe KSZ andto define the (progradeand) retrograde Kranendonk& Wardle 1994, 1996\. part of the P-T path.The limitations of the resultant A detailed cbronology has been erectedfor the data-setare discussed below. northernTorngat Orogen based on integratedfield and geochronologicalstudies (most recently summarized ToRNGATOnocnN in, for example,Van Kranendonk& Wardle 1996,rn press).Arc magmatismat ca. l9lO-L885 Ma was Overview followed by collision and crustal 1fuis[sning[D,*1i following Van Kranendonket aI. (1993a),all Archean The tectonicsetting ofthe PaleoproterozoicTorngat deformation is assigned to DJ and associated Orogen(Frg. 1) hasbeen describedby several authors magmatismat 1870-1860Ma (agedata from Bertrand (e.9., Korstgtrd et aI. 1987, Hoffman 1989, Van et aI. 1993, Scott & Machado 1995, Scott 1995a). Kranendonket al. 1993a"bo Rivers et aI. I996,Wardle Subsequently,the TorngatOrogen was dissectedby an & Van Kranendonk 1996). The orogen occupiesa anastomosingnetwork of crustal-scale,ductile to brittle narrow north-trendinglinear belt betweenthe Archean transcurrentshear-zones up to 15 km wide, ofwhich Nain and Southeast Rae provinces, which are the largest are the ductile Abloviak and Komaktorvik

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. \t . a \ \.1 2 - \ \'- .--'Killlnek.'lsland 0 skm Lr . / r-r r - Komaktorvik shearzone

\ Labrador Sea

ir FourPeaks 1\ domaln tl

1 '.if". \:n

,' , 1 firy ["ft""''M;*' tffi*"'"

64"45' 64'30'

Paleoproterozolc Paleoproterozolcand/or Archean E DTG-suite(diorites, tonalites, granodiorites) Wl Metagabbro @ Aurwelldomaln (Opx-bearlng dlorltes El Metapelltlcsedlments tonalltes,granodiorltes; enclaves of Arche- an gneisses with Proterozoicdykes) Archean(Naln Provlnce) Paleoproterozolcdykes E Archeangneisses (s.1.) lffi Avayallkdykes (shownschemailcally) I Huttonanorthosite fl Ultramaficrocks (s.1.)

Ftc. 2. Simplifred geology ofthe study area in northern Torngat Orogen, showing the main lithological units and the location of the samples investigated. The position of the Komaktorvik shear zone ii taken from Wardle et al. (1993); see Van Kranendonk & Wardle (1996) for more detaits. Modified from Wardle et at. (1993).

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shearzones (Frg. l). TheAbloviak shearzone (ASZ), a lineationsare down-dip,and occur in ultramylonites sinistral, transpressionalDo*2 strucfure formed at associatedwith later west-side-upmovements (Van 1845-L822Ma (Bertrandet al. 1993),is principally Kranendonket aI.1993a, l994,Yan Kranendonk& developedin Tasiuyakgneiss and reworkedgneisses of Wardle 1994).The earlierstructures have been inter- the SoutheastRae craton. The youngerKomalrtorvik pretedto indicatean overall scissormovement on the shearzone (KSZ) is a Do*3structure that was mainly Komaktorvikshear zone, with increasedeast-side-up activeca. 1798-1780Ma (Bertrandet al. l993,Yan displacementtoward the north (Van Kranendonk& Kranendonket al. 1,994,Scott & Machado 1995, Wardle1996, in press). Van Kranendonk& Wardle L996,in press)and that Most rocks in the Komaktorvik shearzone are of affectedreworked gneissesof the Nain Craton and amphibolite-faciesgrade, but there is widespread the PaleoproterozoicDTG suite.In the northernpart evidencein mafic orthogneissesand supracrustalrocks of the orogen,the two shearzones are separatedby for earliergranulite-facies metamorphism (Grt{px-Pl the Burwell domain, whereas farther south, the and Grt-Opx-Pl assemblages).Since it occursin the Komaktorvik zone (as defined in Van Kranendonket DTG suiteand Avayalik dykes,this episodeof meta- al. I993c) diesout atca.59" latitude(Ftg. 1). morphismmust be Paleoproterozoicin age.Note that mineral abbreviations,unless otherwise noted, are from Northem TorngatOrogen Kretz(1983). T\e Burwell domain, west of the Komaktorvik The geologyof the northernpart of TorngatOrogen shearzone, is dominatedby the Killinek charnockitic adjacentto the Komaktorvik shearzone is shownin batholith, witl some pelitic rocks (some of which more detail in Figure 2 (from Wardle et aI. 1993).For are similar to the Tasiuyakgneiss) occurring around the purposesof this study,the areahas been subdivided the marginsand structurallyon top of the batholith into threenorth-south-trending segments centered on (VanKranendonk & Wardle 1996).The intrusive rocks the Komaktorvik shearzone. The main featuresof the are variably foliated and lineated, and contain geologyin eachzone in the studiedcross-section ate as granulite-faciesmetamorphic mineral assemblages. follows. In the eastern part of the batholith, adjacent to T)heFour Peaksdomain, east of the Komaktorvik the Komaktorvik shear zone, fabrics are oriented shearzone, is underlain principally by an Archean north-soutl. Farther west, theseDo*1 foliations are basementgneiss complex consistingof tonalitic to folded into NW-plunging Do*2 structures (Van granodioritic gneisswith rafts of Archean pelitic and Kranendonket aI. L994).Throughout its ourcroparea, basicsupracrustal rocks, cross-cut by the undeformed the Killinek charnockitecontains numerous enclaves PaleoproterozoicAvayalik dykes(Wardle et aI. 1992, (m to km scale) of folded Archean gneisseswith 1993).This areais part of the structuralforeland of the discordantmafic dykes. These enclaveshave been TorngatOrogen" but both dykesand gneisses have been tentatively linked to the Four Peaksdomain by Van affected by a static Paleoproterozoic,granulite- to Kranendonket aI. (1994) and Connelly & Mengel amphibolite-faciesmetamorphic overprint (Morgan & (1995), whereas reconnaissancePb-isotopic data Taylor 1972,Mengel & RiversI994,Yan Kranendonk (Campbell et aI. 1995) show greatersimilarities to et al. 1,994,Wardle et al. 1994,Connelly & Mengel gneissesof the RaeProvince. Thus their a.ffiliationis 1995,Van Kranendonk& Wardle1996) (see below). presentlyuncertain. The Komaktorvik shear zone is underlain by reworked Archean gneisses,the isoclinally folded, MetevonpnsM nI ToRNGATORocEN late Archean(?)Hutton anorthositicsuite (definedin Van Kranendonket al. I993c) andby Paleoproterozoic Previousstudies ofmctamarphism in the Torngat dykes,metasediments and DTG suiteintrusions. The Orogen pronouncedstructural grain of the zoneis a result of penetrative,broadly north-striking, steeplydipping LS Therehave been several studies of the metamorphic fabrics that are mylonitic to locally ultramylonitic developmentofTorngat Orogenin the last few years. in character(Van Kranendonket al. 1993a).Most hevious work by theauthors (Mengel 1988, Mengel & high-strainzones are nillrow discontinuousfeatures, Rivers1990, L99I,1992,1994) was concerned witl suggestingthat strain was localizedin progressively the metamorphicdevelopment of Torngat Orogen narrower zonesoand that overall displacementwas along an E-W transectat the latitude of SaglekFiord, limited (e.g., Yan Kranendonk & Wardle 1996). extendingfrom the foreland (RamahGroup) to the Stretchinglineations in the shearfabric, defined by interior(ASZ). Van Kranendonk (1,992,1996) reported amphibolite-faciesmineral assemblages,are of two results from a parallel transectat the latitude of Okak dominantgenerations. The oldergeneration is broadly Bay (Fig. l). south-plungingand associatedwith oblique,sinistral, The metamorphicsignature along Saglek Fiord east-side-upmovements (i.e., Do*r).The youngerDo*a (Frg.1) wasfirst examinedby Mengel& Rivers(1990,

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65. 63" N \<@ allE-rurn\, I Ell\rrz.lJlt l-. I Nainptutonic suite 3" PALEOPROTEROZOIC ll=lil Burwetlmagmatic arc

l.'..ril Tasiuyaksneiss o) o 4:t,+ ARCHEAN ; | | Variablyreworked Archean 7 / gneissesof the Nainand / ,' , Bae Provinces i.'r, || Huttonanorthosite 'r', Shearzones SOUTHEAST . FI\il RAE PROVINCE ','r19"9"xrio'a

z-r$1t z .

METAMORPHICZONES

k \ Granulitefacies HebronFiord Ls-t 76,/..,/ *\ffi)',1 (07 t t, nAmphibotiteracies \ /6 facies )r4' mGreenschist / / 1t z I or"rn"y n[:[:n,"o"amPhiborite Qa"; trI Cpx-Grtsranutite tacies 6 61.

Frc. 3. Schematic extent of Paleoproterozoic metamolphic zones in the Tomgat Orogen, including data from Wardle el a/. (1994), Wardle & Van Kranendonk (1996), Van Kranendonk & Wardle (1996,1997) and Van Iftanendonk & Mengel (1996). The base map is identical to that in Figure 1. The patterns for the Lake Harbour, Ramah and Mugford Groups-have been omitted for clarity.

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1991),who subdividedthe ASZ into distinctgranulite- exhibit variably retrograde granulite-facies mineral ald upper-nmphibolite facies slices that were assemblages,and reworked gneisses ofthe Southeast juxtaposedduring late (i.e., post-transcurrentshearing) Rae hovince basement, south and west of the Abloviak east-directed thrusting [D3 of Van Kranendonk & shear zone, similsrly show widespread evidence of an Ermanovics(1990), Stage 3 ofBertrandet aI. (L993)1. amphibolite-facies retrogression of earlier granulite- Using samplesof mafic rocks from both tlese slices facies assemblagesofunknown age. (their nmphibolite-and granulite-faciesblocks, respec- tively), Mengel& Rivers(1990, 1991)defined a P-T Age of mctamnrphism path constrainedby concordantP-T vectorsfrom both individual samplesand the samplepopulation as a Available geochronological datz on Paleopro- whole, indicatinga quasi-ITDpath in this part of the terozoic metamorphism in and adjacent to the ASZ from ca. 10 kbar - 800'C to ca. 5 kbar - 650oC. Komaktorvik shear zone ftom the northern part of the There havebeen several interim reportsof ongoing orogen ate summarized in Table I and Figure 4. The metamorphic studies in and adjacent to the age determinations have been grouped in the three Komaktorvik shear zone in the northern part of the segments defined above, i.e., (1) Four Peaks domain orogen(e.g., Mengel & Rivers1992,l993,Parny L994, east of the I

relationshipto transcurrentshearing. It is our aim in TABLE I. PALEOPROTEROZOICMETAMORPHISM this paperto summarizethese results, present some IN THE TORNGAT OROGEN,NORTHERN I.ABRADOR new data, and integrate the resultant data-set with SELECTEDGE@HRONOLOGICAL DATA availablegeochronological and structuralconstraints. Mns"l R@k type Regional distribution of metamorphicrocks B!ryell Domah

Figure 3 shows tle regional distribution of 1745rZMa Ttl greisictols.lite (r) Orogen. 1767+ 4 Ttn diorite (1) metamorphic grade in tle northern Torngat t776*2 Ttu torslite (l) In general, metamorphic grade increasestoward I 814 MM felsic gnois (2) Iil4 Zra felsicgneiss a) the axial region of the orogen, where it attains the 1843+3 zrn chm@kite (r) uppermostemphibolite to granulitefacies. Metamorphic ru3 Zn mfic dyke (2) l87l f 3 ka cbmwkito 0) orthopyroxeneis widespreadin iron-rich tonalites and 1877 zm mdcgreis (2) basiclithologies, whereas the highest-gradeassemblages in pelitesdisplay the associationGrt - Bt - Sil - Pl - KoEsktoMk Shq Zole Kfs - Qtz. 1710* 4 M4 Tt! diqite (l) r'120 Tt! (2) At the latitude of SaglekFiord, the across-strike r?1< + I Ttn dioritic dylc (D distributionof metamorphicgrade is well displayed. t74513 Tm h2.EcatoDalite (r) 1748+ 3 ka gpbbrc @rthGite (r) Here,tlere is a progressiveincrease in metamorphic t75t L4 Tt! tonslile o) grade westward from the greenschistfacies at the 177413 Tttr gmitic vein (l) imoboudia pegnatite (r) (defined 1780t2 zm ndME TorngatFront as the easternmostPaleoprote- 1780+ 3 z[a Hrtton morlhositic $ite (t) rozoic thrust, e.g., Mengel 1988, Van Kranendonk 1781+2 zn Hutton mrthositic sire (r) l7E9* I zm synkio@dic p€gmatit€ (r) 1990,Van Kranendonket aI. 1993c)toward upper- I'tgt L2 zn gl@ite (r) amphibolite to granulite-faciesassemblages in the t799 zra Eaflc greis (2) (2) central part of the orogen,resulting in the familiar lE@ ha nafic gneiss telescopedpattern of "inverted" metamorphiczones Natn Pmvire

seenin manyorogens with a predominantthrust-sense I'l19*2Ma 7n syDti!@dic p€Bode (l) of displacement.However, in the northernpart of the t74O+EI4 Zn Avayalikdyke (l) 1743 zm Artyalftdyke (2) orogen,tle structuraland metamorphicfronts of the ta34 +71-3 Zn Avayalikdyke (l) orogendiverge, and, as noted above,there is a static upper-amphibolite-to granulite-faciesmetamorphic Southm Tomgat Omgfl CIO) overprint well to the eastof the Komaktorvik shear 1730-1740Me }IbI tu-fu d'F ftom rcuthsn To (3) 1750 t190 Itbl tu-Ar ddr from suth@ TO (4) zone (Morgan & Taylor 1972), wbtch on structural t740-1795 TnardMM D, uplift alog ASZ (4) groundsmarks the easternboundary ofttre orogenat taz-tu5 Zrt D, defomltion - mermorphim (1,4) - (1,4) this latitude (Van Kranendonk& Mengel 1996,Wardle 1866-1860 7fr Dr dofomdiotr metaoorphim & Van Kranendonk1996). Mllml symbolsre thore of IGetz (1983).SuB of .ldq (l): Sott & Mebado West of the Komaktorvik shear zone, foliated (1995), S@tt (l95ab), (2): Corcly & Mogel (199s, 19,,6), (3): M@spl e, aI intrusive rocks of the Killinek charnockitic batholitb (l99l), (4): Bstud et ql (1993).

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1850

1800

1750

1700

(d

o o)

BD KSZ FPD

FIc. 4. Slmmary of metamorphic age datz (in Ma) from the northern Torngat Orogen, compiled from Scott (1995a b), Scon & Machado (1995), Connelly & Mengel (1995, 1996). Gray sha.ling in the Komaktorvik shear zone (KSZ) covers the range of metamorphic and synkinematic ages obtained by Scott & Machado (1995). Samples from the western KSZ are shown schematically on the left side of the KSZ field, and data for the eastern KSZ are on the right side. Samples within the Burwsll ismain (BD) and the Four Peaks d66ni1s (FPD) are not arranged geographically. Sources of data are given in Table l. Bars labeled Do*p D,.2 and D"4 represent the main leslsnis phases recognized in the northern Torngat Orogen (references above and in text).

The data show several groups of ages based the KSZ, andat l7l9 Ma in the Four Peaksdomain. on metamorphiczircon: ca. 1875Ma, 1843Ma and ,{ similslo although less pronouncedvariation is 1816Ma in Burwell domain,ca. 1795Ma, 1775Ma shownby the agesbased on titanite, which rangefrom and 1750Ma in theKSZ, and 1834Ma, ca. l740Ma t775 to L745 Ma in the Burwell domain and from andl7l9 Ma in the FourPeaks domain. The youngest ca. 1775to 1710Ma in theKSZ. generationof metamorphiczircon closedto Pb loss The availableAr/Ar cooling agesfor hornblende at 1816 Ma in the Burwell domain. at 1748 Ma in from the areabetween Saglek Fiord and Okak Bay

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TABLE 2. SUMMARY OF R@K TYPES AND MINERAL ASSEMBLAGES TABLE 2 (@mimed)

Sample Rek tlpe t23456 Rmk q/pe t23456

Four Poks domaln (@st of Komaktorvlk shw rcne) W€tem trfumaLtoFft rhq ane

Zla Grt-rich nafic gmulite opq 4a ft-Ilbl-bering n"ffc $pIlmsta.l r@k x x oPq 2sb Grt{px-rich layer in spla squmw opq l3b cn-b@ringmst8pelite , opq 3l Pdyke,Grt-banfug xxxxx Cal Kfs lft Grt-bwingmaficwpmta.lmk Cal, Ttn 2tb clt-cpx mafic gnei$ 39 Grt-bwing layed mefic grmulite x x x Ap, om ta. Grt-Bt-rich lays in $pffi sequq@ x Zta lma Grt{px mafic grutite 34 Anonbositeftom Eafic layercdoErplq opq f2,4d Grt-Bt-Sil Eehpelite x x r39b P dyke, Grt-rich ZnL Opq, Eastem Komahorik rhtr atre CaL Ep r47f Grt-Hbl-b@riq gpbbro Ap 52a Cnt-Ilbl goeis l47g P dyke"Gd-b€rhg dyke orttirg l47f Cal Opq 524 P dykg Grt-bsring r47h P dyke, fr-baing dyke otting l47f 90a Iscosdic cfi amph:bolit€ /3d crt-b€ringmyloniticmetapelite 90b crt-Cpx mphibolite 139 Pdyke,ft ampbibolite Cal, Opq x x x xZm 90c cat-Hbl-Pl lays in mphibolite " opq ,f23 MyloniticGrt-Bt-bqd4m@pelite 9Ib Grt nPb$olite (mdlgabbrc) x x x Cal I 2k Cat-Br-sil metapeliticsbist x x Cbl, Kfs 9tg Gt-Sil-bwing netapolite x x x Wzr\ ,f24l Grt*Bt-Hbl rnfic sbist x Tt! Ms opq, Ep /37 P dyke, Pl-phyric, Grt-b@irg x x x x cal Ep, elj cn-Hbl-b€rhg merrsedinmt Rc EP, Cal, Cbl, Ap Cum, Opq 56b l,ys€d cbmckirc * mafic tolalite xAp Bwe[ domin (s6t of Komaldovik sher zone) 9td Mylonhe with Grt-Pl clasts ,$ Shw€d gabbroic r@k x x x opq Cdtral Komaktonik 8hw zore (Eutton morthosltic sufte) 197 Grt-Bt Sil metapelite, spffi sslus@ " OPC,zm,Kfi 20oa Grt mphibotite, $pwustal sluae x x x tun,N 9Ia Grt mylodte tays in Anorthosite 234b Grt-Cpx gmlite iq Opx gteiss x x x Ilm 94b crt-Cpx-bsitrg mafic zupmsa.l rek TM 82 Chtrcckji.evm ldo Ap, Opq, Zm l07a Grt-Cpxmaficgnei$ Ttn, [r, Mag 176 Chu@lste*N lato x Opq, Kfs 108 Grt-Cpxn trcgneis Ep, Cat [E lTSachmwhewldo xxxx Kfs l47b Gt-Opx-IIbl layq i! eofthositic gneis Cal [n "f93 Tolslfuegmodiqite opq' Ap 91c GrtOpxgnei$ftomUMsequ@@ Ap 206 Tonrlitq€modiarite Cal, nn I 4 7a Cit-lftl$simg morthositic gneis Ep, Rt 225, Tooalib€modiorite xxxx opq, Ap

qcept Colum h@dings:I g!fiet, 2 dthopyrqag 3 clircpyrmre" 4 homblmde, 5 biotite, 6 sillimmite. The mineml symbolsused re those of Kretz (1983), that 'spmpf Opq reprmts the opague ninca6. eUUreviatiom: = $prumta! I-lM = rntrma-fig P = ProErozoic. Smple numbss in bold-fuc€ fype: umples wce usei in ttrmobrometiic calcul*ions. Samplenumbas shom initalis: sampls woe not usd for thmobmmety. Note thal all umples omain guartz ud plagioclrc.

(Frg. 1) arein the rangeL79Fl730Ma@Iergel er al. Paleoproterozoicmetamorphic grade along the length 1991,Bertrand et aI. 1993)and overlapwith titanite of this part of the shear zone, (3) to determineif agesin the KSZ from the northernpart of the orogen metamorphic grades of narrow, structurally later (L774-I7lO Ma, Fig.4, Table1), whichimplies that high-strainzones differ from the main part of the KSZ, amphibolite-faciesfabrics in the orogendeveloped over and (4) to provide a metamorphicevaluation of the a periodof ca. 80 M.y. hypothesis,defined on structural grounds,ttrat the shearzone underwenteast-side-up movement (Van Sampleselection Kranendonk& Scott I992,YanKranendonk & Wardle t994,1996). For the present study, sampleswere chosen to representthe full east-westwidth of the northern The majority of the samplesof Archean mafic Komaktorvik shearzone and to extendinto the adjacent supracrustalrocks andProterozoic mafic dykescontain Four Peaksand Burwell domains(Fig.2, Table 2). one or more of the following assemblages:Grt - Hbl - - - Inasmuchas permittedby the distribution of lithol- PI - Qtz, Grt- Cpx - Pl - Qtz, Grt - Opx Pl Qtz.A ogies,an attemptwas madeto sampleArchean mafic few sampleslack garnet,but containthe assemblage gneisses,mafic and pelitic supracrustalrocks, and Opx - Cpx - Hbl - Pl - Qtz. Samplesof the pelitic Proterozoicmafic dykes acrosstle areaof interest. supracrustalrocks containthe assemblageGrt - Bt - Pl Locally, membersof the ProterozoicDTG suite and - Sil - Qtz - (t Rt - Iln). Table3 showsselected com- some bodies of gabbro also were sampledin order positionsof the mineralsused for thermobarometry. to completethe sampledistribution. Sampling was The completeset of analyticaldata is availablefrom designed(l) to estimatethe P-T conditionsduring the first author and from the Depositoryof Unpub- shearingalong the Komaktorvik shearzone, (2) to lished Data. CISTI, National Research Council, test whether there is a systematic variation of Ottawa"Ontario, Canada KlA 0S2.

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TABLE 3. REPRESENTATTVEMINERAL COMPOSMONS

Smple 31 5?4 9la Minenl Grt Hbl PI Gn Hbl Pl cn Hbl Pl Gn HbI PI Gn Hbl Pl Code IYZ 194 193 86 t5 84 t05 lfi 106 208 26 201 34 35 36

SiO2wtqo 37.4141.a7 59.s2 38.1540.19 60.98 3A.OS4t.42 57.69 37.9742.36 6.30 [email protected] 56.t7 Iio2 0.01 I.09 0.0r 0.01 0.?6 0.01 0.01 0.98 0.0r 0.0r r.53 0.01 0.0t 2.12 0.01 Al2D3 21.15t3,41 U.l4 21.s514.a224.24 2t.53 14.3925.s1 2ns2 12.92 25.53 21.311231 n.70 c(x)3 0.04 0.01 o.fi 0.09 0.01 0.01 0.09 0.04 0.01 0.0t 0.o2 0.02 0.01 0.04 0.04 FeO 29.ti t5.41 O.4l 26.89t9.t5 O.24 26.3413.59 0.01 29.14t1.14 O.0l 24.6911.70 0.16 MrO 1.24 0.01 0.01 2.31 0.20 0.01 0.72 0.0t 0.01 L87 0.2i O.2l 1.05 0.0t 0.01 MgO 3.31 9.n 0.16 2.29 7.43 O.O1 6.6310.38 0.01 3.88 8.80 0.01 3.s8 8.41 0.01 CaO 7.1711.34 6.14 10.3611.33 5.29 5.1411.28 6.98 6.7\ r.37 6.y2 1.49r.39 9.16 Na2O O.35 t.r2 E22 0.05 2.00 8.7E o.ot 1.69 7.77 o.M 2.04 9.t2 o.o1 1.94 9.O K2D 0.01 r.84 0.21 0.t3 1.35 0.01 0.0t 1.il 0.10 0.0? t.36 0.01 0.01 1.58 0.0? Sum 9.82 96.7398.89 101.8397.24 99.58 98.5394.89 98.10 t01.2498.38 IO2.t3 99.7 96.88103.60

s*pl" l3c 25b 9lg Miner8.l Gn Cpx Pl Cfl Cpx Pl Gft Opx PI Gn Bt Pl Gn Bt PI Code 123 124 t22 t l24 t?5 132 t33 135 l 13 114 l 15 to7 106 104

SiO2wqo 38.22s1.49 50.32 3't.96s0.U2 51.86 38.5651;15 47.43 3E.0336.53 6r.55 37.7835.13 59.t0 'tio2 0.01 0.24 0.01 0.0t 0.47 0.0t 0.01 0.01 0.01 0.01 1.90 0.01 0.01 3.0s 0.13 A'1203 21.72 1.8230.74 21.t5 3.4829.76 21.6 t.7s 31.38 21.91 t7.C1 23.82 2t.42 18.24 25.31 C"2O3 0.02 0.04 0.01 0.10 0.01 0.02 0.01 0.07 0.01 0.10 0.29 0.01 0.01 0.09 0.01 Feo 26.3s10.38 0.01 24.n 8.11 0.01 5.18 21.69 o.8 30.7012.39 0.0r 33.81)7.96 0.20 MnO 0.t 0.28 0.01 0.t2 0.01 0.01 0.83 0.19 0.01 0.86 0.01 0.01 t.61 0.0t 0.01 MgO 3.53ll.7l 0.01 6.3812.73 0.01 6.1621.9 0.01 5.5?15.34 0.01 3.7? 9.90 0.01 Cao 10.362i.25 13.2.4 6.6422.11 12.10 6.89 0.46 15.33 3.69 0.0r 5.4i1 2.35 0.01 7.15 Na2O 0.01 0.39 3.99 0.01 0.58 4.42 0.01 0.01 2.70 o,uz 0.24 4.96 0.0r 0.16 7.57 K20 0.0r 0.01 0.01 0.0t 0.0t 0.13 0.01 0.01 0.01 0.07 9.r9 0.01 0.lt 9.7? 0.t3 Sm f 01.2099.61 98.31 97.3597.59 9833 9a.72n.% n.l'l t00.9693.E7 99.E7 [email protected] 94.36 9.62

Mneral symbols a0er Kretz (1983). All Fe is as$med to be in the ferrous state.

PsrRocRAPIilcOssERvATroNs & Wardle,in press)in theseveins suggeststhat preser- vation of tle Grt - Cpx assemblagemay in part have Four Pealcsdomnin beendue to thepresence of a CO2-richfluid phase. A similar ProterozoicGrt - Cpx - Pl overprinton Metabasitesin this area,including both Archean the Avayalik dykes and Late Archean(Scott 1995a) supracrustal rocks and Proterozoic dykes, are Opx - Cpx - Pl granulite-faciesassemblages occurs in characterizedby the assemblagesGrt - Cpx - Pl - Qtz gneissesin the SevenIslands Bay areafarther south t Opx or Hbl - Pl - Qtz t Grt (Fig. 5a),both of which (Fig. l; Wardleet aI. 1994,Van Kranendonk& Wardle are interpreted to have developed during Do*, L997). Ia that area, grains of orthopyroxeneare deformation(coeval with the formation of the ASZ: rimmed by a progradecorona of Cpx - Grt, without Van Kranendonket aI. 1,994).In a few samplesof the involvementof Hbl. Subsequentdecompression the mafic dykes,there is no Hbl associatedwith the is shown by a variably developedrim of Pl on Grt. In Grt-Cpx subassemblageobut the majority show evi- the Four Peaksdomain, the progradereactions took denceof Hbl replacingGrt or Cpx (or both), suggesting place during Do*2 transcurrent shearing along the that there was variable progressof the generalized ASZ, when the area was buried obliquely below decompression-hydrationreaction Grt + Cpx + H2O = the Burwell domain.Decompression features, on the Hbl + Pl after the main Do*, event. In llbl-free or otherhan{ developedas a resultofDo*3 shearing and Hbl-poor samples,incipient formation of Opx-Pl exhumationalong the easternKSZ (VanKranendonk symplectiteat Grt - Cpx interfacesattests to initiation & Wardle 1996, t997, Van Kranendonk& Mengel of the generali2edanhydrous decompression reaction 1996). Grt + Cpx - Opx + Pl. Someof the Proterozoicdykes g6atainthin veins(0.5 cm) with thesubassemblage Grt In summary,there is evidencefor a static meta- - Cpx, implying preservationof the progradeproducts morphic event (Do*2)that led to formation of the of the generalizeddehydration reaction Hbl + Pl = Grt assemblageGrt - Cpx, which variably overprinted + Cpx + H2O.The presence of calcite(Fig. 5b; Mengel Archean granulite-facies assemblages.This was & Rivers1992) znd,locally, scapolite (Van Kranendonk followed by decompressionor hydration (or both),

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Ftc. 5. Selected examples of microstructural and paragenetic features described in the text (all photomicrographs in plane-polarized light; w: width of photomicrograph). (a) Grt-Cpx-Opx-PI-Qtz assemblage from mafic layer in supracrustal sequence. Sample 25b, w = 3.5 mm. (b) Calcite and biotite in Grt-Cpx vein (lower left to upper right) in an Avayalik dyke. Note small laths of Pl outside vein. Sample 31, w = 3.5 mm. (c) Ribbon mylonite developed in Grt-Bt-Sil-bearing metapelite. Note shear bands. Sample 13d, w = 7 mm. (d) Deformed Avayalik dyke. Hbl and Grt porphyroclasts in Pl-Qtz matrix. Sample 13g, w = 7 mm.

leading to the formation of Opx - Pl and Hbl - Pl Metabasic supracrustalrocks typically display the assemblagesduring the D!*3event. Some rocks went assemblageGrt - Hbl - Pl - Qtz (Fig. 5e).The assem- directly from Grt - Cpx to Hbl - Pl, whereasotlers blage Grt - Cpx - Pl - Hbl - Qtz occurslocally and went through an intermediateOpx - Pl stage,the always with petrographicevidence of hornblende variation presumablybeing due to the availability replacingclinopyroxene, implying operationof the of H2o. generalizedreaction Grt + Cpx + H2O= Hbl + Pl. Proterozoicdykes and gabbroare also composedof The Komnlaonik shear zone the assemblageGrt - Hbl - Pl - Qtz t Cpx (Fig. 5f), with the amount of garnet being quite variable. In Rocks from within the Komaktorvik shearzone both lithologies,hornblende with typicalbrown-green showa variableintensity of fabric developmentdue to pleochroism is locally rimmed by blue-green the anastomosingnature of the zonesof strain.Highly amFhibole(actinolite), and epidote occursin a few strainedsamples are mylonites and ultramylonites (e.g., samples,Subareas ofthin sectionswith thesefeatures Fig. 5c), whereasthe least-strainedrocks arefoliated were avoidedfor thermobarometry. or augen gneisses(Fig. 5d). The highest strains Pelitic rocks in the Komaktorvik shearzone contain are observedin discontinu6gs,unlinked, 1-5 km the assemblageGrt - Bt - Sil - Pl - Qtz - Kfs + Rt. wide strands,which can be traced for up to 30 krn Sillimanite occursas both prismatic grains and as late along strike (e.g.,Yan Kranendonk& Wardle 1996). fibrolite on grain boundaries,and grains of garnet

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Ftc. 5 (continued).(e) PI partially rimmed by Grt and Hbl. Sample4a, w = 1.4 mm. (f) crt{px-Hbl-pl assemblagein recrystallizedAvayalik dyke. Sarnple 1479, w = 3.5 mm. (g) Chamockitewith the assemblageOpx-Cpx-Ilbl-pl. Sample 1784 w = 7 --. (h) Metapelitewith Grt-Bt-Sil-Rt assemblage.Sample 197, w = 3.5 mm.

commonlypreserve trails of inclusionsof sillimanite orthopyroxene and clinopyroxene in many thin defining a former folded foliation. The K-feldsparis sections. Pelitic rocks west of the KSZ contain tle pethitic microcline. Although not present in our assemblageGfl - Bt - SiI - Pl - Qtz - Kfs + Rt + Tlm samples,relict kyanite also has been observed,and (Fie. 5h). tlere is petrographic evidence that it preceded sillimanite (Patey L994, 1995, Van Kranendonk& Mnenet- ConposrrroNs Wardle1996). The chemical composition of selected minerals TheBurwell damain (Tables 2,3) was determined with an electron micro- probe. Operating conditions are given in the Appendix. Both prograde and retrograde transitions are observed in the variably foliated and lineated Hornblende amphibolite-to granulite-faciesintrusive rocks of the Burwell domain(Van Kranendonk & Wardle,in press). The compositional differences among grains of Conditionsof peak metamorphismare recordedby amphibole ("hornblende" in the following) from the assemblageOpx - Hbl - Pl - Qtz t (Cpx, Grt) the study area were found to be minor, regardless (Fig. 5g). The amountofhornblende is quite variable, of lithology and geographic position. Hence all with petrographicevidence of hornblendereplacing amphiboles are considered tosetler below. No

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/5/1137/3435643/1137.pdf by guest on 26 September 2021 I 150 0.40

0.30

0.20

0.10

0.00

0.30 - matlc gnolsses -----. maflcsuD€crustal r@ks -.-.-- Avayallk dykes 0.20 -----. pelltlc supracrustal coro --+ rlm

0.10

0.00

0.08

0.06

0.04

0.02

0.00 0.5 0.6 o.7 0.8 XAlm

FIc. 6. Patterns e1 2ening in selected grains 6f gilrs1 from the main lithological groups. Each arrow points from core to rim compositions. Open arrowheads and bold text pertain to gamet grains showing "prograde 2qning" (Xa. decrease toward the rim), whereas fiIled arrows are used for the remainder, all showing rimward increase in X^6. Full lines: mafic gpeisses, stippled lines: mafic supracrustal rocks, dash-dot lines: Avayalik dykes, dotted lines: pelitic supracrustal rocks. Sample numbers in italics pertain to garnet showing "deviating" patterns of zoning (see text). Note different scale along ordinate in Xoyoversus Xs*.

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0.8 B 123 o.7

0.6 tooaoo'rooooo .;'larrr. aooooooa 0.5

o.4 OXAIm f XSm AXPm XXGrs

0.3

o.2 xxx iSUtoo,follt& !IrIrx Ahjllfp$^ 0.1 I fryh tttatr^ 0.0 +10mm -+ +2.7 mm+ +7mm+

0.8 13d ) 124e D 'ac4t51 o.7 ooaaoloaa

0.6 aooooaaaao 0.5

o.4 O XAlm I XSps A XPrp X XGrs

0.3 ^^^^^^^^^ XxxxxxxXxx o.2 1t1a1attt1 0.1 rllltrrllr 0.0 + 7mm J> +1.4 mm+

F)c. 7. Compositional profiles in garnet from the western part of the KSZ. (A)-(C): 39, 123 and 124f are mafic supracrustalrocks, (D), (E): l3d and 124e arepelitic supracrustalrocks, (F): 137is anAvayalik dyke.

compositional zoning was detected in any of the Garnet grains analyzed, which range from edenite, through ferro-edenite and pargasite, to ferropargasite for those Garnet occurs in all the rock fypes investigated. with (Na + K)a > 0.5, and from magnesiohornblende In mafic orthogneisses, the compositional range is to ferrohornblende for those with (Na + K)o < 0.5 Almso{sPrp11-2eGrs13-3eSpsr-6,very similal to that in (classification ofleake et al. 1997),. mafic supracrustalgneisses (Ahn5r-65Prpe-33Grs t6-3rSps6r).

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Garnet from the mafic ProterozoicAvayalik dykes An""-s5.However, there are also significant differences haveslightly lower Prp (Alms6-64Prpz-17Grs15-36Sps6*).within single lithological groupsthat can be related to Garnet from pelitic metasedimentaryrocks is more textural setting. For example, in mafic gneisses, Alm-rich and Grs-poor,in the rangeAlms5-r6P4r16-ru plagioclasethat is the product (with orthopyroxene)of Grs3-2Spss-a(Alm55-7sPrp12-ruGrsr,,Spssa if one the decompressionreaction between garnet and Ca-richrock is excluded).Mthin individual samples, clinopyroxeneis up to Anz.r.. Elsewhere,where the the compositionalvariation rarely exceeds5 mol.Voof compositionof both recrystallizedmatrix plagioclase individual end-members,and largely stems from and fine-grainedsymplectitic plagioclase are known, compositionalzoning. the range may be quite large (e.g., A\s to An6ofor Core-to-rim compositionalvariations in selected sample107a), but within most samples,the rangeis grains of garnet from the study area ate shown in less tlan LUVoAl.. Both normal and reversezoning Figure 6. The majority of the samplesconform broadly occur in plagioclase,but no systematicrelationships to the samepatterns; however, grains of garnetfrom with respect to mineral assemblages,lithology or pelitesstand out with respectto both their overall com- geographicposition were observed. position and their patternssf 26ning. Thesegrains show a rimward increaseof Xo6 andXr*, and decrease Pyroxenes of X*. Xo*, however, shows very little changeo consistentwith retrogradecooling (in equilibrium with Orthopyroxcnewas found in eachofthe rock types; Sil-Pl-Qtz: Martignole & Nantel 1982). Grains of X* rangesfrom 0.42 to 0.68,with the highestvalues in garnet in mafic supracrustalrocks display a fairly mafic andfelsic supracrustalrocks. Xy, zoningpatterns systematicrimward increaseof Xo6, decreaseof X*, are generallyflat, and Al, which rangesfrom 0.6 to whereasXs* and Xo^ trends are less clear. The garnet 2 wtVo Al2O3,does not display systematic2oning. ftom mafic gneissesgenerally shows the srme patterns Clinopyroxene occurs in mafic gneisses, mafic of zoning as those in mafic supracrustalrocks. The supracrustalrocks and dykes.X"1, ranges from 0.55 to patternsorf 26ning described above are interpretedto O.74,with the highest values in mafic supracrustal be the result of continuedre-equilibration after peak rocks and anorthositic gneisses(Table 3). The Al conditions of metamorphism. contentranges from I to 3 wtEoN2O3, and is generally In contrast,"prograde" growth zoning(1e., rimward highest in the core of the grains. decreasein Xofi hasonly beenobserved in garnetfrom gys65amples (open arrows in Fig. 6). Both arefrom the eastern,highly strainedpart ofthe KSZ, andpreserve Biotite evidenceof garnetreplacing earlier hydrous minerals. Sample52c, an Avayalik dyke, showsgarnet replacing Biotite occurs in most rock types and samples. hornblende,whereas clear Grt in sample 9lg, a 11is unzoned,with Xo4uranging from 0.44 to 0.76, metapelite,replaces biotite, Similar patternsof zoning and showsthe highestvalues in biotite from pelitic and have beenreported from farther south in the KSZ (20 mafic supracrustalrock types. Among the pelitic and 80 km southofthe presentstudy-area), and inter- supracrustalrocks, Xyu falls into groups of around0.5 pretedto reflect progradegrowth during Do*3burial of and0.7 , apparentlydue to bulk-compositioncontrols, the easternKSZ (VanKranendonk & Wardle1997). as tle Xyu of the coexisting garnet shows similar Figure 7 shows typical compositionaltraverses maxfuna. acrossgrains of garnet from mafic supracrustalrocks (A{), pelitic supracrustalrocks (D, E) and an Rrsrrl-rs oN Trnnuon aRoMETRY Avayalik dyke (F). Only D and E show systematic variations(core-to-rim increase in Xo6 anddecrease of Xnn), whereasthe others show variably "flat" central Presentation of P-T data portions combinedwith post-peakretrogression and re-equilibration recordedin gain margins(increase of The activity models and assemblages used for X*, Xrp"). thermobarometry are described in the Appendix. Results are displayed graphically in Figure 8, in which Plagioclase symbols identify both the rock type and the assemblage used. In order to minimize the "non-synchronous First-order differencesin averagecompositions of closureo'problem discussed earlier, Figure 8 only plagioclaseare largely a function of lithology. For shows peak conditions of P-T inferred for each sample instence,in the metapelites,Avayalik dykes and mafic and the range of additional determinations. The firll gneisses,most plagioclaseis An36-ae,whereas in the set of P-T data is available from the first author Hutton anorthositicsuite, it is Anao-, andin someof upon request, and from the Depository of Unpublished the metagabbroicrocks, ttre range may be as large as Data.

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iq't,.l.j (n = 50) l4a (n=27) 013b a 52a tl 13c (n = 14) L 52c 439 @I 90a tr 94b a147t +90b aO 107a a147h tr 90c a 108 O100a oa 9lb a147b osls o124d | 91a 4139b T.C T'C | 91i 200 400 600 800 0 200 400 600 800 200 400 600 800

Burwelldomain Four Peaks domain

L (U cl -o Legend -o l< # 10 J TL 10 Maflc orihognolsasB oo@".-f"4i"""""""""C[L Maflc supractuslala rtr@ ili Pellilc aupractustalo oe /n Proterozolc dykes A t"il DTG/Burwell V i9i i,l ,t (n = 221 1...: = Y43 (n 10) e 1e7 a21a I 200a tr@25b tG234b Toc Ar.31 T"C 0 200 400 600 800 0 200 400 600 800

Ftc. 8. P-T data from the Four Peaks domain, Komaktorvik shear zone [eastern, central (= Hutton anorthositic suite) and western partl and Burwell domzin. For simplicity, only peak determinations for each sample are shown, and the range is indicated by a line. Stippled lines enclose all determinations (n). Small, horizontal tics are used to distinguish samples with similal s)nnbols. In the legend symbol-filI is linked to type of mineral assemblage used for thermobarometry (open, fille4 gray, dot" cross), whereas symbol shape is keyed to lithological types (circle, square, diamond, triangle). "DTGlBurwell" comprises rocks of the DTG and Killinek chamockitic suite.sin the Burwell domain. For geothermobarometric assemblages, the abbreviations are: G Garnet, H Hornblende, C Clinopyroxene, O Orthopyroxene, B Biotite, AAl-Silicate, R Rutile, IL rlmenite, P Plagioclase, and Q Quartz. See Appendix for details about the thermobarometric methods employed.

P-T resultsfrom the Four Peaksdomain Avayalik dyke (sample31, Table3) yields the highest (eastof thc KSZ) P-T estimates,with P-Tqop being higher than the single P-Too (note that the acronymsare defined in P-T data for all rock types from the Four Peaks the caption to Fig. 8). Data from mafic supracrustal domain, where overprintedby the static Paleopro- rocks form a more restrictedcluster at the lower end of terozoic metamorphism,form a linear array from the array, in the range 6-8 kbars - 600-700"C. For ca. Il kbar-750"C to ca 6kbar - 6O0'C(Fig. 8). The theseestimates, it is noteworthythat valuesof P*. are

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lower than valuesof Po.ro,in accordwith petrogaphrc DISCUSSION observationsof the generalizeddecompression reaction Grt + Cpx = Opx + Pl (whereP-Tc*ewas determined P-T paths in textural settings with no evidence for Opx-Pl formation from Grt{px)- The P-T determi{ations come from a transect acrossthe ftrll width of ttre KSZ and well into the P-T resultsfrom the KSZ adjacentunsheared wallrock domains. In any one domain,P-T arraysdefined by eachofthe four rock Data from the KSZ were subdividedinto three typestaken individually areparallel to the composite groups in order to test for systematicdifferences with array defined by the whole population. Arrays respectto locationin the shearzone. For eachgroup, consistingonly of the highestP-T determinationsftom the samplesform a subconcordantlinear array (Frg. 8). eachsample (Fig. 8) alsoare parallel to the composite East of the Hutton anorthositic suite (Fig. 2, arrays.Furthermore, where reliable P-T determinations "EasternKSZ" in Fig. 8), the highestpressures come could be obtainedfrom both core andrim of coexisting from tle Avayalik dykes (11.7 kbar - 720"C with minerals,P-T vectors are again parallel to the overall GHPQ,Table 3), whereasthe lower end of the array P-T array.These observations, together with ttre fact (ca. 6kbar) is recordedin mafic supracrustalrocks. that the P-T arrayshave significantly steeperslopes The centerof the array is well defined, and includes a thanisopleths of thebarometric equilibri4 supportthe core-to-rimvariation in the GCPQ assemblagefrom suggestionthat individual samplesrecord different 8.2kbar - 620"Cto 6.5kbar - 540"C(sample 90c, Fig. parts of the P-T path. 8). For individualsamples, P-To*p is generallysimilar The resultsindicate that the degreeofresetting of to P-To."q, but may be higher by up to 2 kbar. tle geothermometersand geobarometersduring Data from the Hutton anonhositic suite @ig. 8) cooling was a function of rock type. Comparisonsof comefrom gneissicmafic layerswithin the anorthosite, P-T determinationsin the Avayalik dykes, mafic including a Grt-Hbl-Pl-Qtz porphyroclasticmylonitic orthogneissesand ma.fic supracrustal rocks showthat gneissalong the easternmargin (11.6 kbar - 815'C the highest P-T conditions were most commonly with GHPQ,Table 3). The remainder6f 5amplesfrom preservedin the Avayalik dykes,indicating that this this unit yields a tight array from 9.5 to 7 kbar at group of rocks was the most resistantto resetting ca.7OO"C(GIPQ andGCPQ). during cooling, followed by ttre mafic orthogneisses $amples from the KSZ west of the Hunon and the mafic supracrustalrocks, respectively. anorthositicsuite ("WestemKSZ" in Fig. 8) yield a Comparisonsamong the P-T arraysfor the tlree similar pattern.High pressuresare recordedin the domainsindicate negligible differences,laking into Avayalik dykes(Table 3), and the lowestP-T results accountthe errors inherent in geothermobarometry come from pelitic supracrustalrocks, with all data (t50oc, tl kbar). This result is compatiblewith the points defining a steepP-T array. Values of P-To6n., predominantlytranscurrent nature of the KSZ and are equal to or slightly higher than valuesof P-T6to, suggeststhat the late myloniteswith down-dipstretching whereasvalues of P-Tcnerqfrom pelitic lithologies lineationswere not the loci of major displacements. recordlower P-T conditions,presnmably established The degree to which peak P-T conditions are during later stagesof re-equilibration. preserved is uncertain. Considering the sample P-T datafrom the threezones across the KSZ show population as a wholeoP-T*rpq is generally similar to the samepeak P-T conditions,and the arrayshave the or greaterthan P-T66pq,as noted above,but this could sameoverall orientation (Fig. 8). The degreeto which be interpretedin severalways: (1) variableaccess of lower-gradeconditions are recorded varies according to H2O-rich fluid before, during and after attainment rock types (metapelitesin the WesternKSZ record the of peak P:T, (2) incorrect calibration of ttre lowest-gradeconditions), but it is also inferred that geothermobarometers(i.e., overestimationof P-T with heterogeneous,domainal re-equilibration is responsible GHPQ or underestimationof P-T with GCPQ, or for someof therecorded variation (see Discussion). both), or (3) an effect of relative rates of diffirsion during retrogression,indicating that diffusion is slower P-T resultsfrom the Burwell domnin(west of the KSZ) in hornblendethan in clinopyroxene.Several samples containing both hornblende and clinopyroxene Samplesfrom this areacomprise various rock types illustrate that P-T61gais generallysimilar to P-Tccpq; belongingto the DTG and Killinek charnockitic suites this finding rules out (2) as the main sourceof the as well as to adjacentpelitic supracrustalrocks. The discrepancy,but we are unableto distinguishbetween P-T data (Fig. 8) define an extended array from (1) and (3) with the presentdata-set. ca. lO kbar - 780"C (mafic granulitesfrom the DTG The near-IID signaturesdisplayed by the P-T suite, Table 3) down to ca. 4 kbar - 500'C (pelitic arraysin eachdomaino together with the widespread supracrustalrocks). preservation of high-pressure assemblages(and

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local preservationof progradefeatures), suggest that minimal.If post-peakgrowth of zircondue to dynamic exhumationof all threedomains was sufficiently rapid recrystallizationwas widespread,the age of peak that higher-gradeassemblages did not thoroughly metamorphismcannot be unambiguouslyinterpreted re-equilibrate.The degree to which sampleshave from the data, as even the oldest determinedage may recordedretrograde conditions is inferred to be a result representpost-peak growth of zircon. of variable reaction-progressduring cooling, which These arguments can be extended to the itself is a complexfunction of theminelal assemblage, interpretation of the U-Pb zircon data for the age of strain and the availability and compositionof fluids metamorphismin KSZ. In this case,the measured during retrogression. rangein agesis 1795-1750Ma, superficiallyimplying that metamorphismwas initiated >30 M.y. later than in Relationshipb etw een deformntion and the adjacentBurwell andFour Peaksdomains. However, metamorphism our preferredinterpretation is that the peneftativeDo*3 shearingin the KSZ resultedin the almostuniversal As noted earlier, 5amples from tle KSZ are recrystallizationof Do*2metamorphic zircon, with the moderatelyto strongly lineated,with hornblendein result that evidencefor metamorphicgrowth during mafic rocks defining the gently plunging Do*, Do*2in the KSZ has so far eludeddetection. In support stretching lineation. Combined with the P-T data of this hypothesis,Figure 4 showsthat there is a 30 presentedabove, this observationsuggests tlat bottr M.y. overlap of zircon and titanite ages(benveen 1775 the Do*3shearing and the reffogradereactions that led and 1745Ma), during which both zircon and titanite to the formation of hornblendein the mafic rocks took closedto Pb diffusion in different samples,a pattern placeover a rangeof P-T conditions,including at or that is incompatiblewith a simple model basedon near the peak of metamorphism. closuretemperature. The concordanceof es.'matesof the maximumP-T Comparisonof the datafor the Burwell domainand and the slopesof P-T paths from acrossthe entire the KSZ indicatesthat at 1800-1780Ma, rocksin the transectcan be explainedby two end-membermodels: KSZ wereat 11.7kbar - 800"Cand undergoing strain (1) the KSZ andneighboring domains went tbrough the in a transcurrentshear-zone, whereas those in the P-T-t history synchronously,and the late mylonitesdid Burwell domain had alreadyexperienced their peakof not accommodatemuch vertical displacementafter metamorphismand were undergoingexhumation and closureof the thermobarometricsystems; (2) the KSZ isothermaldecompression. This patternis compatible and neighboringdomains were exhumedat different with an overall youngingof the exhumationprocess fimes,but, as shownby the geometryof the P-T paths, from the hinterlandtoward the foreland of the Torngat the cumulative rates and amounts of uplift were Orogen. broadlythe samein all domains.Considering only the In summary,the Paleoproterozoicgranulite-facies Burwell and Four Peaksdomains initially, the U-Pb eventaffecting the Archeangranulite-facies rocks in data superficiallyappear to supportthe latter model, the westernmargin of the Four Peaksdomain was i.e., therange in agesof metamorphismas definedby coeval with Da*21oD. the basis of interpretationsof U-Pb zircondata is 1875-1816Ma in the Burwell U-Pb ages in the metamorphosedAvayalik dykes domain comparedto 1834-17L9Ma in the Four Peaks (1843Ma in theBurwell domain,1834 Ma in the Four domain (Fig. ). However,the wide spreadof U-Pb Peaksdomain: Van Kranendonk& Wardle 1996, 1997:, zirconages (59 M.y. in Burwell domainand 115M.y. Fig. 4). Do*2involved oblique tectonicburial of the in Four Peaks domain) in such n:urow domains westernmargin of the Four Peaksdomain beneath suggeststhat many of the agesdo not date the closure the Burwell domain syncbronouslywith the formation temperatures,but probably representmetamorphic of the Abloviak shear zone in the central Torngat growth of zircon below its closuretemperature due Orogenat L845-1.822Ma @ertrandet al. 1933, Scott to other factors, such as dynamic recrystallization & Machado1995, Van Kranendonk& Wardle 1996, during shearingor the influx of metamorphicfluids. In 1997).Penetrative sinistral fianscurrentshearing in the supportof this hypothesis,we note that the highest KSZ occurredca. 1800-1780Ma duringDo*3, in asso- determinationsof metamorphictemperature in all three ciation with east-westcontraction and folding of the domainsare in the range800-750'C (Fig. 8), below the thickenedcrust. Zircon is inferred to havedynamically closwe temperatureof Pb diffrrsion in zircon (>800'C: recrystallizedthroughout the KSZ at this time. Do*3 Heaman & Parrish 1991). The role of dynamic shearingin the KSZ was coevalwith the early stages recrystallizationin particular as a factor facilitating of exhumationand unroofing in adjacentBurwell and growth of zircon below its closure temperatureis Four Peak domains.Later deformationin the KSZ gupportedby ttre observationttrat the oldest age in andFour Peaksdomain, recorded by the narrow,Do* the Four Peaks domain comes from a statically lower-amphibolite-faciesmylonites with down-dip recrystallizedAvayalik dyke, in which the effectsof stretchinglineations and by the intrusion of syntectonic dynamic recrystallizationmight be expectedto be granitic pegmatitesand sheetsof granite,took place

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episodicallybetween l79l and 1710Ma (Scott & ACKNOWLEDGEMENTS Machado 1995) and was associatedwith the later stagesofunroofing andexhumation (Van Kranendonk We are grateful to tle leaders of the Torngat & Wardle1996,1997). mapping project and ttreir associatedorganizafions (Dick Wardle,Newfoundland Department of Mines CoNCLUSIONS and Energy, and Martin Van Kranendonk,then at the GeologicalSurvey of Canada)for help andsupport in ln conclusion,the study of the metamorphicevolution the field. FM thanks the Daaish Nafural Sciences in the northern Torngat Orogen, focused on the ResearchCouncil for fellowships,and TR acknowledges Komaktorvik shear zone and its adjacent blocks, has tundingfromNSERC andLITHOPROBE. FM thanks shown that: J@rnR6nsbo and John Flpng for maintainingan excellent 1) Gneisses and supracrustal rocks ofthe Four Peaks electron-microprobefacility in Copenhagen.Dick domain east of the KSZ experienced a static Wardleand Dave Scottcommented constructively on Paleoproterozoic metamorphic overprint during D"-z as earlier versionsof the manuscript.Guest Editor Rob a result of oblique burial beneath the Burwell domain Berman and reviewers Ed Ghent and Martin Van at ca. 1.845-1822 Ma. This thermal event is best Kranendonkprovided exhaustive,yet thoroughand monitored by the Avayalik dykes, which record peak constructivereviews. conditions of.ca. Il kbar - 750'C. 2) Peak conditions of metamorphism determined RSFERENcES within the KSZ were ca. 1,1.7kbar - 800'C, and were reached during Do*, ca. at 1800--1780Ma. BERMAN,R.G. (1988): Internally-consistent thermodynamic 3) Peak conditions of metamorphism in the Burwell data for minslals in the system NarO-KrG{aO-MgO- domain west of the KSZ were ca. lOkbar -790'C, and FeO-Fe2O3-AI2O3-SiO2-TiO2-H2O{O2. J. Petrol. 29, were reached during Do*2at ca. l84O-I835 Ma. 445-522. 4) In each ofthe tbree d66ains, post-peakretrogression and re-equilibration are shown by decompression or - (1990): Mixhg properties of Ca-Mg-Fe-Mn hydration reactions (or both). E taets.Am, Mineral. 75, 328-3M. 5) The P-T:urays in all three domains are similar ald (1991): Thermobarometry using multi-equilibrium record isothermal decompression - QTD). calculations: a new technique, with petrological applica- 6) In each of the three domainso P-T arrays from trolns. Can. M ine ral. 29, 833 -855. individual samples and rock types overlap with the composite array for the whole population. Bpnrnexo, J.-M., Ropolcx, J.C., VAN KRANENDoNT,M.J. & 7) The maximum P-T recorded by the assemblagesis a ERMANovIcs,I. (1993): U-Pb geocbronology ofdeforma- function of both the rock type and the assemblage used tion a:rd metamorphism across a central transect of the for geothermobarometry. Highest values of pressure and Early Proterozoic Tomgat Orogen, North River map area, 0emperaturewere derived from theAvayelik dykes, with Labrador. Can. J. Earth Sci. 30, 1470-1489. the mafic orthogneisses, mafic pelitic and supracrustal Cawnmr, L.M., Bnncwaren, D. & FARMER,L. (1995): A rocks recording progressively lower values. comparison of Proterozoic crustal formation along 8) Although the P-T arrays for each domain are the Torngat - Nagssugtoqidian - Lapland collision belts: similar, geochronological data suggest that zircon in preliminary constraints from Nd and Pb isotopic studies in the KSZ cryst4llized some 40 M.y. after zircon in northern Labrador. In Eastern Canadian Onshore-Offshore adjacent domains, Furthermore, within each domain, Tramect (ECSOOT), Transect Meeting (November 28-29, determinations of the age of metamorphism by a single 1994; R.J. Wardle & J. Hall, compilers). IJTHOPROBE method (e.9., U-Pb zircon) show a span of >50 M.y., Rep.45,22-36. and U-Pb zircon and titanite ages overlap. These data Comvnnx, J. & MnNcsL, F. (1995): keliminary U-Pb data are interpreted to indicate new growth of zircon and from Archean gneisses in a transect across the titanite below their closure temperature as a result of Komaltorvik shear zone, northem Torngat Orogen. In dynamic recrystsllization during shearing or the ingress Eastern Canadiaa Onshore-Offshore Transect (ECSOOT), of meramorphic fluids (or both). Transect Meeting (November 28-29,1994; R.J. Wardle 9) Transcurrent movement on the KSZ was coeval with & J. Hall, compilers). UTHOPROBE Rep. 45,72-82. the early stages of exhumation in the adjacent domains. & _ (1996): The Archean backdrop to l0) This study has demonstratedthe importance of (1) Paleoproterozoictectonism il tle Nagssugtoqidianand analyzing peak mineral both and post-peak assemblages Torngat Orogens: new constraints from U-Pb in a range of rock types to obtain the P-T path, nd (2) geochronology.In E,asteraCanadian Onshore-Offshore linking structural and metamorphic observations with Transect(ECSOOT), Transect Meeting (March 14-15, geocbronological data from well-constrained carefully 1996;R.J. Wardle & J. Hall, compilers).LITHOPROBE chosen samples. Rep.57,15-22.

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Dnwev, J.F. (1988): Extensional collapse of orogens. Knerz, R. ( 1983): Symbols for rock-formin g minerals. Am. Tect onic s 7, 1123-11 40. Mineral. 68,277-279.

ENcr-a.ro, P.C. & RrcHARDsoN,S.W. (1977): The influence of LEAKE, B.E. and 21 others (1997): Nomenclature of erosion upon the mineral facies of rocks from different amphiboles: report of the subcommittee on arnphiboles of metarnorphic environments. J. GeoI. Soc. London L34, the International Mineralogical Association, Commissiel 201-213. on New Minerals and Mineral Names. Can. Mineral.35. 21.9-246. & THoIvpsoN,A.B. (1984): Pressure-emperature- time paths of regional metamorphism.I. Heat transfer Iv{.rben, U.K., Prncwnr, J.A. & BERMAN, R.G. (1994): during the evolutionof regionsof thickenedcontilental Thermobarometry of gamet - clinopyroxene - homblende crust.J. Petrol. 25. 894-928. granulites from the Kapuskasing structural zone. Can. J. Earth. Sci.31, 1134-1145. EnnaaNovrcs,I.E. & VAN KneNrxooNr, M.J. (1990): The Torngat Orogen in the North River - Nutak transect MARTrcNor-u,J. & NANTEL,S. (1982): Geothermobarometry areaof Nain and Churchill provi_nces.Geosci. Can. L7, 279-283. of cordierite-bearing metapelites near the Morin anorthosite complex, Grenville Province, Quebec. Can. FRosT,B.R. & CHAcKo,T. (1989):The granuliteuncertainty Mineral.20.307-318. principle: limitations on thermometryin granulites."/. Geol.97,435450. McMulr-w, D.W., BmueN, R.G. & Gnm.moop, H.J. (1991): Calibration of the SGAM thermobarometer for pelitic & Tnacv, R.J. (1991):P-T pathsfrom zoned rocks using data from phase-equilibrium experiments and gamets:some minimum citeria. fun J. Sci.291, 917 -939. natural assemblages. Can. Mineral. 29, 889-908.

Funlr,taN,M.L. & Lrosrry, D.H. (1988):Ternary-feldspar Mswcnr, F. (1988): Thermotectonic Evolution of the modefling andthermometry. Am- Mineral.73,Z0|-Zl5. Proterozoic-Archaean Boundary in the Saglek Area, Nonhcm Labrador. Ph.D. thesis, Memorial University of HARLEv,S.L. (1989):The origin ofgranuliles:a me[amorphic Newfoundland, St. Jobn's, Newfoundland. perspective.Geol. Mag. 126,215-247. & RrvBRs,T. (1990):The synmetamorphicP-T-t HSAMAN,L. & Pennrsn,R. (1991):U-Pb geochronologyof path of granulite facies gteisses from Torngat orogen, accessoryminerals. In Applicationsof RadiogenicIsotope and its bearing on their tectonic history. Geosci. Can. 17, Systemsto Problemsin Geology (L. Heaman& J.N. 288-293. Ludden, ed,s.).Mineral. Assoc. Can., Shon Course Handbook19,59-102. & _ (1991):Decompression reactions and P-T-t pathsfrom individual samplesin high graderocks, Hoocrs, K.V. &. RoyDEN, L. (1984): Geologic northernLabrador: implications for Early Proterozoic thermobarometryof retrogradedmetamorphic rocks: an tectonicevolution. J. Petrol. 32. 1.39-167. indication of the uplift trajectory of a portion of the (1993): northernScandinaviaa Caledonides. J. Geophys.Res. 89, & - 3-Dimensionalarchitecture 7077-7090. and thermal structue of a deepcrustal-scale transcurrent shear zone. Report 2: Preliminary results from the HoFFMAN,P.F. (1989): hecambrian geology and tectonic Komaktorvikshear zone. 1n Eastern Canadian Onshore- (ECSOOT), history of North America. In T\e Geology of North Offshore Transect Transect Meeting America - An Overview (A.W. Bally & A.R. Palmer, @ecember4-5, 1992;R.J. Wardle & J. Hall, compilers). eds.). The Geological Society of America, Boulder, UTHOP ROB E Rep.32, 64-73. Colorado(A: M7-512). & - (1,994):3-D architectureand thermal structure of a crustal-scale transcurrent shear zone. Report JAcKsoN,G.D. & Tanon,F,C. (1972):Correlation of majel 4: P-T results from the northern Torngat transect. In Aphebianrock units in the northeastemCanadian Shield. Eastem Canadian Onshore-Offshore Transect Can.J. Earth Sci.9,1659-1669. @CSOOT), Transect Meetirg (December 10-ll, 1993; R.J. Wardle & J. Hall, compilers). IITHOPROBE Rep.36,163-17O. KtucHr, I. & MoRGAN,W.C. (1981):The AphebianRamah Group, northern Labrador. In Proterozoic Basins of & Coxxu-ly, J. (1996): Metsmolphic Canada(F.H. Campbell, pap. ed.), Geol. Surv. Can., signatures i-n deep cnrstal shear zones: Abloviak and EL-10,313-330. Komaktorviok shear zones in the Paleoproterozoic Torngat Orogen, northem Labrador. GeoI. Assoc. Can. - KonsrcAno,J.A., RvAN,A.B. & Wenor-s,R.J. (1987):The Mineral. Assoc. Can., Program Abstr.2L, A-65. boundary between Proterozoic and Archean blocks in central West Greenland and northern Labrador. In & RsyNor-Ds,P. (1991): Lithotectonic Evolution of the Lewisian and ComparablePrecambrian elements and tectonic evolution of Tomgat orogen, High GradeTerrains (R.G. Park & J. Tarney, eds.).Geol. SaglekFiord, northernLabrador. Can J. Earth Sci.28, Soc.,Spec. PabI. 27, 247-259. MA1-t423.

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MoRGAN. W.C. & Tnvlon, F.C. (1972): Granulite facies SBr-vaRstoNE,J. & CHaNmsRLAtrI,C.P. (1990): Apparent iso- metamorphosed basic dykes of the Torngat Mountains, baric cooling paths from granulites: two counterexamples Labrador. Mineral. Mag. 38, 666-669. from British Columbia and New Hampshire. Geology lE, 307-310. PATBY, B.P. (1994): The metamorphic history of the (1992): determination of metamorphic Proterozoic Komaktorvik Shear Zone: preliminary results. SpsAR, F.S. Perologic - - ttme paths. In Metamorphic Iz Eastern Canadian Onshore-Offshore Transect pressrue temperatule - - Time Paths (F.S. Spear & S.M. (ECSOOT), Tfansect Meeting (December 10-11, 1993; Pressure Temperatue Geophys. Union, Short Course in R.J. Wardle & J. Hall, compilers) . UffiOPROBE Rep. K, Peacock, eds.). Am. I 85-187. Geology, l-55.

- (1994): Metamorphic Phase Eqailibria and (1995): Metamorphichistory of the Proterozoic Pressure - TemDerature - Tlnz Paths. Mineral. Soc. Am.' preliminary Komaktorvit Shear Zone: thermobarometric Monogt l. results. In Eastern Canadian Onshore-Offshore Tralsect (ECSOOT), Transect Meeting (November 28-29, 1994: & FLoRENcE,F.P. (1992): Thermobarometry in R.J. Wrdle & J. Hall, compilers). UffiOPROBE Rep.45, granulites: pifalls and new approaches. Precambrian Res. 200-205. ss,209-241.

PATnsoN, D. & BEcnr, N.J. (1994): Zoning patterns in Kornr.M.J., Flonmcq F. & MB{ARD,T. (1990):A orthopyroxene and garnet in granulites: implications for model for garnet and plagioclase growth in pelitic schists: geothermometry. J. Metamorphic Geol. 12, 411428. implications for thermobarometry and P-T path determinations. J. Metamorphic GeoI. 8, 683-696. PEAcocK, S.M. (1992): Thermal modeling of metomorphic pressrue-temperafure-time paths. /n Metamorphic SElvnnstoNs, J., flrcxvorr, D., Cnowuv, P. & hessure - Temperature - Time Patbs (F.S. Spear & S.M. Hopcns, K.V. (1984): P-T paths from garnet 2sning:. L Peacock, eds.). An. Geophys. Union, Short Coarse in osw rcshnique for deciphering tectonic processes in Geology,5T-1O2. crystalline terranes. Geology t2, 87 -90.

Rrvens, T., MENcn, F., Scorr, D.J., CeMpss[, L.M. & Tanon, F.C. (1979): Reconnaissance geology of a part of GouLEr, N. (1996): Torngat Orogen - a Paleoproterozoic the Precambrian shield, northeastern Quebec, northern syample of a doubly vergent compressional orogen. In Labrador, and Northwest Territories. GeoI. Surv. Can., Precambrian Crustal Evolution in the North Atlantic Mem.393. Regions (T.S. Brewer, ed.). Geol. Soc., Spec. PabI. lL2, ll7-136. Tgori/psoN, A.B. & ENcmnt, P.C. (1984): Pressure - tem- peratue - time paths of regional metamorphism. tr. Their RveN, B. (1990): Basement--cover relationships and met2mor- inference and interpretation using mineral assemblages in phic patterns in the foreland of the Torngat Orogen in the metemolphic rocks. J. Petrol. 25, 929-955, Saglek-Hebron area, Labrador. Geosci. Can. fi,n6-n9. VAN KRANENDONK,M.J. (1990): Structural history and (1996): Comrnentary on the location of the Nain- geotectonic evolution of the eastem Torngat Orogen in the Churchill boundary in the Nain area. Newfoundland Dep. North River map axea, Labrador. Geol. Surv. Can., Pap. Natural Resources, Geol. Sum., Rep.96-1, lO9-129. 90-lc,81-96.

Scorr, D.J. (1995a): U-Pb geochronology of a (7992): Geological Evolurton of thc Archean Nain Paleoproterozoic conti-nental magmatic arc on ttre westem Province and the Earb Proterozoic orogen as Seen along margin of the Archean Nail craton, northern Labrador, a kansect in thz North River - Nutak Map Areq Northzm Canada. Can. J. Earth Sci. 32. 1870-1882. Iabrador Canada. Ph.D. thesis, Queen's University, Kingston, Ontario. (1995b): U-Pb geochronology of the Nain craton on the eastern margin of the Torngat Orogen, Labrador. - (1996): Tectonic evolution of the Paleoproterozoic Can. J. Eanh Sci.32,1859-1869. Torngat Orogen: evidence from pressure - temperature - time - deformation paths in the North River map area' & CAMlBRrr, L.M. (1993): Evolution of the Labrador. Tectonics 15. 843-869. Paleoproterozoic Torngat Orogen, Labrador, Canada: recent advances using U-Pb geochronology and Nd & ERMANovIcs,I.E. (1990): Structural evolution of isotope systematics. GeoI. Soc. Am., Abstn Program25, the Hudsonian Torngat orogen in the North River map A-237. area, Labrador: evidence for east-west Fanspressive collision of Nain and Rae continental blocks. Geosci. Can & MAcHADo, N. (1995): U-Pb geochronology of r7.283-288. the northern Torngat Orogen, Labrador, Canada: a record of Paleoproterozoic magmatism and deformation. -, GoDD{,L., Mwcn, F.C.,Scorr, D.J.,Wenom' Precambrian Res.70. 169-190. R.J., CAMrBEE, L.M. & BRTDGwATER,D. (1993a):

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/5/1137/3435643/1137.pdf by guest on 26 September 2021 METAMORPHISM IN T}IE TORNGAT OROGEN. LABRADOR TT59

Geology and structural development of the Archaean to Northwest Territories. GeoI. Surv. Can., Current Res. Paleoproterozoic Burwell domain, northern Torngat 1994-C,32t-332. Orogen, Labrador and Quebec. GeoI. Surv. Can., Pap. 93-LC.329-340. RyeN, B. & Rrwns, T. (1993c): Lithotectonic divisions of the northern part of the & MENGBL, F. (1996): Meramorphism in the Tomgat Orogen, Labrador, Quebec ald N.W.T. Iz Eastern Paleoproterozoic Torngat orogen, northem Labrador: Canadian Onshore-Offshore Transect (ECSOOT), structural and geocbronological constraints, GeoI. Assoc. Transect Meeting (December 4-5, 1992; R.J. Wardle Can. - Mineral. Assoc. Can., ProgramAbstr 21, A-98. & J. Hall, compilers).LTTHOPROBE Rep.32,2l-31.

Sr-ONcn,M.R. & Hrxopnsox,J.R. (1993b): WARDI-E,R.J. (1983): Nain{hurchill province cross-section, Paleoproterozoic tectonic assembly of Northeast Laurentia Nachvak fiord, northern Labrador. Newfoundland Dep. of through multiple indentations. Precambrian Res. 63, Mines anl Energy, Current Research, Rep. E3-1,68-89. 325-347. BRDGwATER,D., MnNcu-, F.C., Cnrwnnrr, L.M., & Scorr, D.J. (1992):Preliminary report on rhe Ven KnarnlDoxK, M.J., HauueN, A., CHuRcril-L, R. & geology and structural evolution of the Komaktorvik Zone Rsp, L. (1994): Mapping in the Tomgat Orogen, of the Early hoterozoic Tomgat Orogen, Eclipse Harbour northernmost Labrador. Report 3. The Nain Craton area, nortlern Labrador. GeoI. Sun. Can., Pap.92-lC, (including a note on ultramafic dyke occurrences in s9-68. northernmost Labrador). Newfoundland Dep. of Mines and Energy, GeoI. Sarv. Branch, Rep. 94-L,399407 . & Wannw, R.J.(1994): Geological synrhesis and musingson possiblesubduction-accretion models in the & Ver IkalnNDoNK, M.J. (1996): The eastern formafion of the northem Torngat Orogen.In Eastern Churchill Province of Labrador-Qu6bec, Canada: Canadian Onshore-Offshore Transect (ECSOOT), Orogenic development as a consequence of oblique TransectMeeting (December l0-11, 1993;R.J. Wardle convergence and collision. Iz hecambrian Crustal & J. Hall, compilers).UffiOPROBE Rep.36,62-80. Evolution in the North Atlantic Region (LS. Brewer, ed.). GeoI. Soc.,Spec. Publ. lI2,137-154. & _ (1996): Burrpell domain of the Paleoproterozoic Torngat Orogen: tilted cross-section of a MENcEL,F. & Scorr, D. (1992): Geo- magmatic arc caught befween a rock aad ahardplace. In logical mapping in the Tomgat Orogen, northernmost Precambrian Crustal Evolution in the North Atlantic Labrador: preliminary results. Newfoundland Dep. oJ Region (T.S. Brewer, ed.). GeoI. Soc., Spec. Publ. ll2, Mines and Energy, GeoI. Surv. Branch, Rep. 92-L, 9I-tt6. 413429.

- & (1997): Crustal-scaleflexural slip Scnwanz, S. & folding during late tectonic amplification of a^norogenic Rver, B. (1993): Geological mapping in the Torngat boundaryperturbation in the PaleoproterozoicTorngat Orogen, northernmost Labrador. Report 2. Newfoundland Orogen,NE Canada.Can. J, Earth. Sci.34 (in press). Dep. of Mines and Encrgy, Geol. Surv. Branch, Rep.93-1, 77-89. MENGEL, F.C., CAI\,PBELL.L.M. & REto, L. (1994): New results and summary of the Archean and Paleoproterozoic geology of the Burwell Received Janwry 15, 1997, revised manwcript accepted July domain, northern Torngat Orogen, Labrador, Quebec and 17. 1997.

AppsNDx Albertsen, Geological Institute. Natural and synthetic minerals were employed as standardso and were Analytical details analyzed before, during and after each session. Grains chosen for analysis in each sample were in All analyseswere carriedout with the JEOL 733 contact or near contact (two grain diameters away). Superprobe housed at the Geological Institute, Large grains of garnet and clinopyroxene were University of Copenhagen.The instrument was analyzed for compositional homogeneity along operatedat an acceleratingvoltage of 15 kV with a traverses from rim to core. Rims of ferromagnesian beam current of 15 nA. Counting time was 20 s on minerals separated by quartz or plagioclase were peaksand 2 s on background.X-ray intensitieswere generally used for determinations of "peako' P-T. Core correctedon-line by the ZAF method,and the raw data compositions of coexisting minerals were only used in were subsequentlycorrected for drift using prrocedures cases where reliable correlation could be made from and programsdeveloped by JAra Rlnsbo and Tue textural observations. Where possible, several micro-

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structuralsettings were chosenin eachthin section In the thermodynamic database employed in order to test for variablereaction-progress and to (JLIN92.GSC),six amphiboleend-members are presently evaluateany effects due to domainalequilibrium. available:Tremolite (Tr), Aln min otschermakite (Ts), Pargasite(Pg) andtheir Fe-e4uivalentsferro-actinolite, Thermobarometry s.lumins-fsrrolschermakiteand ferropargasite(nomen- clatureofleake et al. l997).Thermochemicaldata and In this study,thermobarometry was performedusing mixing models for these end membershave been the TWEEQU (ver. 1.02)progmm of Berman(1991). by Mhder et al. (1994). These authors In all calculations, thermodynamic data for end determined end-membersin all members were taken from Berman (1988, 1990). recommendedusing Fe and Mg Garnetwas modeledas a mixture betweenPrp, Alm Grt-Hbl exchangereactions, in conjunctionwith Mg and Grs, with mixing propertiesdescribed by Berman end-membersin the net-transferreactions. Through a (1990).Plagioclase was modeledas a mixture of An numberof tests,we obtainedsimilar results(within 0.3 andAb, with mixing propertiesdescribed by Fubrman kbar and 25'C) by usingTs, Tr andtheir Fe-equivalents & Lindsley (1988).The two typesof pyroxenewere in both exchangeand net-transferreactions. Conse- treatedas ideal solutions.Biotite solid-solutionswere quently, only these amphibole end-memberswere modeledas end membersbetween Ann andPhl, with included in thermobarometriccalculations witl the the mixing modeldescribed by McMullin et al. (1991). GIIPQ assemblagein this study.

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