335
TheCanadian Mineralogist Vol 37,pp. 335-357 (1999)
HIGH.PRESSUREAND HIGH.TEMPERATUREMETAMORPHISM OF THE MAFIC AND ULTRAMAFICLAC ESPADONSUITE, MANICOUAGAN IMBRICATE ZONE, EASTERNGRENVILLE PROVINCE. QUEBEC-
RICHARD COXS aNo APHRODITE INDARES
Depat'tmentof Earth Sciences,Memorial University of Newfoundland,St John's,Newfoundland AIB 3X5
ABSTRACT
The Lac Espadon suite (LES) of the Manicouagan Imbricate Zone (Quebec) is comprised of layered mafic and ulffamahc rocks, Labradorian in age (ca. 1650-7630 Ma) that were variably deformed and metamorphosedunder high-pressureand high- temperature(high-PT) conditions during the Grenvillian Orogeny. Maximum P-T conditions of780-930"C at 16-19 kbar (high- T eclogite facies) are recorded in massive coronitic troctolite and hornblendite from the westem part of the LES. In theserocks, metamorphic coronas of orthopyroxene, clinopyroxene and garnet have grown at the expenseof igneous olivine and plagioclase Relict plagioclase contains inclusions ofkyanite and corundum, and garnet coronas locally preserve growth zoning. Deformed margins of the mafic rocks have granoblastic hydrous assemblagesthat are interpreted to have equilibrated during exhumation at ctt 70O"C ar I{r_-I2 kbar and t}len down to ca. 600'C at 5 kbar (amphibolite-facies conditions), suggesting a steep retrograde P-Tpath OlivinegabbrofromtheeastempartoftheLESrecordspeakconditionsofTT5-870'Cat145-1625kbar.Granoblastic areas in the rock are partially hydrated and give conditions of 760-820"C at 12-14 kbar, suggesting a near-isothermal P-T trajectory The P-T paths are compatible with structural evidence suggestingtectonic exhumation of theserocks by norlhwesterly thrusting, with extension on top of the pile. The high-PT conditions and steep decompressionpaths recorded by the LES are similar to those in severaladjacent and nearby terranes,suggesting widespread tectonic exhumation of the lower crust in this area of the Grenville Province.
Keywords: Grenville Province, mafic rocks, coronas, eclogite facies, amphibolite facies, Manicouagan, Quebec.
Soulrarne
La suite du lac Espadon,membre de la zone imbriqu6e de Manicouagan (Province du Grenville, Qu6bec), contient des roches mafiques et ultramafiques stratiformes d'Age labradorien (environ 1650-1630 Ma) qui ont 6td plus ou moins d6form6es et m6tamorphis6es) pression et d tempdrature6lev6es au cours de l'orogendse grenvillienne. Les conditions du m6tamorphisme ont atteint 780-930"C d 16-19 kbar (facids dclogite de hauts temp6ratue), comme en t6moignent la troctolite et la homblendite coronitiques massives du secteur ouest de cette unit6 Dans ces roches, ies textures coronitiques impliquant orthopyroxdne, clinopyroxbne et grenat se sont ddveloppdes aux d6pens de I'olivine et du plagioclase primaires Les reliques de plagioclase contiennentdes inclusions de kyanite et de corindon, et les couronnesde grenatconservent ici et ld une zonation due d la croissance. Les bordures d6formdes des massifs mafiques contiennent des assemblagesgranoblastiques de min6raux hydrat6s qui tdmoigneraient d'un r6-6quilibrage au cours d'une exhumation sur I'intervalle 700'C et 10-12 kbar jusqu'd environ 600'C A 5 kbar (conditions du facids amphibolite), indication d'une dvolution P-T r6trograde abrupte Le gabbro d olivine du secteur oriental de cette unit6 a subi un m6tamorphismejusqu'i 775-870'C et 14.5-1 6.25 kbar Les portions dLtexture granoblastiqueont dt6 partiellement hydrat6es) 760-820"C et 12-14 kbar, ce qui fait penser d une trajectoire P-T quasiment isotherme. Les tracos P-T sont compatibles avec l'6vidence structurale d'une exhumation tectonique de ces roches au cours d'un chevauchementvers le nord-ouest,accompagn6 d'une extension despafties sup6rieuresde I'empilement Les conditions de pressionet de temp6rature 6lev6eset la d6compressionabrupte sont semblablesaux conditions d6duites dans les socles adjacentsou voisins, ce qui indique une exhumation rdpandue de la cro0te inf6rieure dans cette r6gion de la Province du Grenville
Mots-clts: Province du Grenville, roches mafiques, couronnes,facibs 6clogite, facids amphibolite, Manicouagan, Qu6bec.
LITHOPROBE contribution number 1049 E -mail addres s : rcox@ soarkv2.esd.munca
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InrnooucrroN a|.1987, Rivers & Mengel 1988,Indares 1993, Indares & Rivers I 995). In conhast, their deformed equivalents High-grade metamorphic terranes constitute petro- commonly display granoblastic textures due to en- logical windows through which processesin the deep hanced chemical diffusion and fluid infiltration from crust in orogenic belts can be studied. In such terranes, country rocks (e.9., Heinrich 1982, Rubie 1986, Koons rocks in which metamorphic reactionshave not gone to et al. 1987).The result is that coronitic rocks at the cores completion may allow segmentsof their pressure and of igneous bodies commonly record near-peak condi- temperature (P-T) history to be recovered. Rocks with tions of metamorphism better than their recrystallized corona texturesare especially suitable for studiesof this margins, as the latter are also more readily retrograde, type, and may be regarded as displaying local or again due to the presenceof fluid that allows the devel- domainal equilibrium (Rubie 1990). The best examples opment of hydrous phasesat amphibolite-facies condi- of such textures are commonly found in coarse-grained, tions. Application of thermobarometry to coronitic rocks dry and relatively undeformed, mafic (meta-igneous) is particularly difficult becausethe presenceof coronas rocks (e.9.,Mgrk 1985,1986, Pognante 1985, Koons el suggests global disequilibrium. However, a detailed
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PARAUTOCHTHONOUS BELT 3fi) km
m MIZ ManicouaganImbricate Zone Paleoproteruzoic ffi sedimentary rocks ffi ALLocHTHoNoUSBELT Labrador Thans terrane ffi Batholith HJT Hart Jaune MLT Molson Lake terrane GT Gagnon terrane
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petrographic study may allow identification of domains GBor-oclcar SBrrtNc in which local (domainal) equilibrium can be assumed, and which can be used for determinations of P and T, The Manicouagan Imbricate Zone is a 2000 km2 Coronitic gabbros in the southern part of the stack ofhigh-P crustal rocks exposed along the shores parautochthonousbelt ofthe easternGrenville Province of the Manicouagan Reservoir in eastern Quebec. It have been shown to record P-T conditions up to eclogite occurs at the same structural level as the Molson Lake facies (e.9., Indares 1993, Indares& Rivers 1995). It terrane (Fig. 1a), which has previously been reported to seemslikely, therefore, that coronitic textures have re- contain eclogitized metagabbro (Rivers et al. 1989, sulted from incomplete transformation at high-pressure Indares & Rivers 1995). To the north, both the and high-temperature (high-P-T) conditions. Evidence Manicouagan Imbricate Zone and the Molson Lake ter- for high-P-T conditions is only preserved where fluid rane tectonically overlie a Grenvillian fold-thrust and infiltration is limited or absentduring retrogressionand nappe belt (the Gagnon terrane) along a thrust contact. exhumation. The Manicoua ganlmbicate Zone, eastern To the south, the Manicouagan Imbricate Zone is over- Grenville Province, contains a variety of mafic units lain by the Hart Jaune terrane along an extensional with coronitic rocks (e.9., Indares et al. 1994). Among shear-zone(Fig. 1a).The Hart Jauneterrane experienced them, the Lac Espadon suite (LES) in the Boundary zone medium-P metamorphism during the Grenvillian orog- consists of lenses of ca. 1650-1630 Ma (Labradorian) eny. coronitic mafic and ultramafic rocks along with am- The Manicouagan Imbricate Zone consists of two phibolites that were metamorphosed under high-P-T fault-bounded lithotectonic packages.The lower pack- conditions during the Grenvillian Orogeny (ca. 1050- age, known as the Lelukuau terrane (Fig. 1b), is a stack 1000 Ma; Cox et al. 1998). Our aim in this study is to of thrust slices largely composed of Labradorian (ca. describe the textures of the mafic and ultramafic rocks 1650 Ma) rocks that are considered to represent an of the LES, and to determine the P-T conditions under igneous AMCG (anorthosite-mangerite-charnockite- which thev developed. granite) suite. The terrane has experienced high-P-T
llili:tLelukuau terrane
I5henulmtish terrane i \ Stretching IIJTIJTIIBaieduNord segurent ,K Thnrstboundarie, lineation
EiEtrvtiddte slice LT-2 Shetching lg Extensional bomdaries \ lineation f :l Upperslice LT-3
Frc. 1. Maps showing a) location of the Manicouagan Imbricate Zone in the eastern Grenville Province, and b) simplified lithotectonic framework of the Manicouasan Imbricate Zone. Note: the tectonic divisions are from Indares et al. (1998).
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metamorphism(16-18 kbar and 850-900"C; Indares (LES). To the south, the Boundary zone is bounded by 1997) which was coeval with the emplacementof mafic the Hart Jaune shear zone that separatesit from the Hart dykes between 1050 and 1000 Ma (Gale et al. 1994, Jaune terrane and truncates all structural features in the Indares et al. 1998). The tectonically overlying Tshenukutish terrane. The Harl Jaune shear zone dis- Tshenukutish Terrane (Fig. 1b) comprises two litho- plays evidencefor top-to-the-southeasttransport (stage- tectonic segments,the structurally lower Baie du Nord 2 extension: Indares et al. 1998). segment and the overlying Boundary zone. Both units The LES can be divided into two parts, exposed are transectedand bounded by extensional shear-zones along the westem and easternshores ofthe Hart Jaune (Fig. 1b), with evidence for top-to-the-southeastor top- Arm, and separated by an extensional shear-zone to-the-southwest transport (stage-l extension: Indares (Fig. 2). The western part comprises ultramafic rocks et al. 1998). (troctolite, dunite and hornblendite) and amphibolite The Baie du Nord segment is mainly composed of that occur locally as tectonic enclaves in a sheared ca. l45O Ma megacrystic diorite intruded by ca. ll70 orthogneiss, and an apatite-rich, Fe-Ti mafic intrusion Ma Fe-Ti gabbros. The Fe-Ti gabbros show a transr- (nelsonite) to the north (Fig. 2). The easternpart ofthe tion from coronite (770-810'C and,l2-I4 kbar) in the LES comprises a massive body of mesocratic olivine southwest to eclogite (720-825"C and 14-17 kbar) in gabbro. U-Pb geochronology and geochemical data the northeast(Cox & Indares 1999).The Boundary zone suggest that the ultramafic rocks and the olivine gabbro (Fig. 2) consistsof the Hart Jaunegranite (101'7+ 2Ma), represent parts of a ca.1650-1630 Ma (Labradorian) the Brien anorthosite(1169 + 3 Ma; Scott & Hynes mafic to ultramafic complex similar to the AMCG rocks 1994) and subordinatemetasedimentary rocks. In addi- of the Lelukuau terrane. The nelsonite from the north- tion, the Boundary zone contains Labradorian mafic and ern part of the LES contains metamorphic zircon that ultramafic rocks, referred to as the Lac Espadon suite gives an upper intercept also suggestinga Labradorian
Boundaryzone
l+Fl Hart Jaunegranite -l Erren anorthosrte ffi mainlymetapelite Lac Espadon suite Fffi ofivine gabbro Knelsonite W troctolite, hornblendite sample localities O ;Y r+l-*r- ;Y +++++4 .' Hart Jauneshear zone 1 stoetching | (stage2 extension) V li
sla3e I extensional / * stretching / shearzones \ nn.
\ thrust related stretching lineation Ilart Jaune terrane -.--- trace of S-fabrics in major shearzones
Ftc.2. Map showing the main lithologies, structuresand sample localities of the Lac Espadon complex in the Boundary zone
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age of emplacement.U-Pb agesof metamorphic zircon minor magnetite and chromite and metamorphic and titanite (ca. 1040-1000 Ma) suggestthat metamor- onhoamphiboleand serpentine. phism in the LES was contemporaneouswith the rest of Hornblendite (sample 3b): Small pods of coarse- the Manicouagan Imbricate Zone (Cox et al. 7998). grained, poikilitic hornblendite are found in close asso- ciation with dunite and ffoctolite (Fig. 2). In theserocks, Pgtnocnapnv amphibole forms oikocrysts up to 10 cm in diameter. The oikocrysts in association with phlogopite and Sample selection and petrographic study clinopyroxene megacrystsenclose chadacrystsof oliv- ine, plagioclase and minor proportions of opaque min- The sampleschosen for detailed petrographic study were collected from the massive parts of the igneous bodies and lenses, and from their deformed margins. The massive parts preserverelics of igneous phases overprinted by high-P metamorphic minerals that dis- play textures ranging from coronas to pervasive granoblastic assemblages.The margins comprise foli- ated granoblastic, amphibole-rich assemblages(am- phibolite), indicating extensive infiltration of fluid at some stage of their metamorphic evolution. In order to assesspossible differences in metamorphic conditions recordedby the dry interiors and hydrous margins, both were sampled from single outcrops where possible. From the western LES, a troctolite (sample 5a) and its amphibolite margin (sample 6am, olivine garnet am- phibolite), a hornblendite (sample 3b) and a gamet am- phibolite (sample2b) were selectedalong with a sample of nelsonite (sample 9c). From the easternLES, an oli- vine gabbro(sample 35b) with granoblasticand coronitic varieties was studied. Sample localities are shown in Figure 2. The detailed petrographic study was complemented by cathodoluminescence(CL) microscopy. Sections were examined using a Nuclide Luminoscope unit with doubly magnified objectives,giving magnification from 20x to 640x. Operatingconditions varied, but in gen- eral an accelerating voltage of 10-15 kV and a beam current of 0.1-2 mA were used. CL images were used to document fine-grained inclusions in relict igneous plagioclase and growth textures of secondary plagio- clase in both corona and amphibolite assemblages. Frcs. 3a-j. Photomicrographs Troctolite (sample 5a): a) amphibole-free corona assemblageand b) amphibole- Textures bearing corona assemblage.Both images are 0.75 mm wide. Homblendite (sample 3b): c) general texture with large oikocrysts of amphibole, relict phlogopite and Troctolite (sample 5a): Troctolite and associated chadacrystsof plagioclase with coronas of garnet, d) clino- rocks from the western LES preserverelict igneous as- pyroxene corona around relict olivine chadacryst. Both semblagesrepresented by olivine, plagioclase, and mi- images are 1.5 mm wide. Olivine gamet amphibolite (Sam- nor ilmenite. Olivine and plagioclase are separatedby ple 6am): e) gamet porphyroblast and slightly foliated, triple coronasoforthopyroxene, clinopyroxene and gar- granoblastic matrix assemblage of amphibole, orthopy- net (Figs. 3a, b). Grains of plagioclase contain numer- roxene and relict olivine. Note the amphibole and plagio- ous oriented inclusions ofkyanite, which shows a bright clase inclusions in the garnet. Width of image is 1 5 mm. with red CL emission (Fig. 4a), and non-luminescentcorun- Gamet amphibolite (sample 2b): f) general texture of plagioclase, amphibole and quartz, dum, which forms small rounded grains. In addition, granoblastic matrix with minor ilmenite and biotite, and partial resorption of amphibole and biotite coronas surround ilmenite. Am- some garnet porphyroblasts Width of image is 1.5 mm. grow- phibole also occurs locally in the triple coronas Nelsonite (sample 9c): g) generaltexture showing partially ing at the expenseof clinopyroxene and garnet, Where preservedcorona of garnet adjacentto amphibole, ilmenite amphiboleis more extensivelydeveloped, clinopyroxene and plagioclase, which contains fine-grained spinel and is also partially replaced by orthopyroxene (Fig. 3b). corundum inclusions, h) corona of garnet around ilmenite Troctolite gradesinto coarse-graineddunite containing with abundant biotite inclusions and spinel in plagioclase.
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Widthsol-iutages at'e | -5and 0 75 nm. respectir.'elyOlivinc gabbro(satrple 3-5b): i) garnetcorona between plagioclase (whiclrcontains ntlmerous inclusions of kyaniteancl corunriurr) and tcnonrasncsianareas. j) granoblasticarea with plagioclase.garnet. arnphibole and rcliclclinopyrorene (which contuins rutile inclusions) Both itnagesare I -5nrn wide
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Ftcs 4a-f Cathodoluminesccnce(CL) irr.ragesa) CL itnage showingkyanite inclusions (red) in relictplagioclase (blue) fiorn sarnple5a (coronitictroctolite) b) CL imageshowing kyanitcinclusion (rcd) in relictplagioclase (blue.1 in sam- ple 3b Nore thc presenceof kyanitein the non-luntines- centgarnet corona c) C[- imageshowingbltLe-luminescent plagioclaseancl dull rcd-lutrinescentquartz both in thc matrixand in thc garnet d) CL inage showingthe deve,- oprnentof secondary(yellow-greenl luminescent plagto- clasc, which appearsto be overgrowing the garnet fronl sarr.rple2b (girrnetamphibolite) Corr.rparethe apparentlack ol quartzinclusions in thesubhedral. resorbed garnet ctys tal e) CL imageshowrng tlte extensivecollar of plagio- clase.as well as therelict igneousplagioclase (both blue) and n.regacrystsof apatite(bright yellow) in sample9c (nelsonite)f) CL imageshowing kyanite inclLtstons (red) in relict plagioclase(blue) frorn sarrple35b (olivine -sabbro).Note rhepresence of kyanite in the nott-lumines- ceul grxnoblasticgarnet
separated by orthopyroxene coronas (Figs 3c, d). In contrast to the troctolite, both the garnet and plagioclase ir-rthe hornblendite contain numerolls inclusions of kya- nite. which is readily distinguished in CL (Fig 4b), and corundurn. both of which are oriented parallel to the cleavagesin the plagioclase Olivine gttrnet (filphibolite (sample 6an): Deformed eralsand apatite,and representthe relict igneousassem- margins of troctolite are transformed to granoblastic blage.Plagioclase and amphiboleare separatedby gar- olivine garnet amphibolite that consists of abundant net coronas.whereas olivine and amphibole are relict olivine, amphibole and orthopyroxene and ls
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Iargely plagioclase-free.The igneous protoliths of these Gamet occurs as discontinuous coronas between clino- rocks probably contained more olivine and less plagio- pyroxene and plagioclase, and locally as granoblastic clase than in the original troctolite. Locally, the olivine porphyroblasts. Amphibole is present throughout the garnet amphibolite displays large porphyroblastsofgar- olivine gabbros, around original euhedral grains of net with inclusions of amphibole and small quantities biotite and ilmenite, and also around clinopyroxene of plagioclase (Fig. 3e). The deformed margins of where it is associatedwith granoblastic garnet and in- hornblendite, in contrast, do not develop obvious retro- clusion-poor plagioclase (Fig. 3j). In these areas,relict grade assemblages. This may be in part due to the crystals of clinopyroxene also show a reduction in abun- hydrous (amphibole-rich) nature of the protolith. dance of rutile inclusions adjacent to the gamet - am- Garnet amphibolite (sample 2b): Granoblastic gar- phibole - plagioclase contacts.Commonly, both garnet net amphibolite outcrops adjacent to the hornblendite and recrystallized plagioclase cores contain kyanite in- (Fig. 2). It is composed of subhedralto anhedral gamet clusions, which are evident in CL images (Fig. 40. porphyroblasts with equigranular plagioclase, quartz, amphibole and lesser amounts of biotite, rutile, titanite, Mnqpner,CspursrnY and ilmenite (Fig. 3f). CL images reveal two genera- tions of plagioclase growth. Plagioclase with a bright Mineral analysis blue CL emission appea.rsas inclusions in subhedral porphyroblasts of gamet in association with dull, red- Garnet, olivine, orthopyroxene, clinopyroxene, am- luminescent quartz, and it is also uniformly distributed phibole, plagioclase and spinel were analyzed with a in the matrix (Fig. 4c). The blue CL emission of the CAMECA SX-50 electron microprobe, using a LINK plagioclase is identical to that in the undeformed energy-dispersionspectroscopy (EDS) X-ray analyzer. troctolite and hornblendite (and olivine gabbros, see Back-scatteredelectron (BSE) images were used to ex- below). However, anhedralporphyroblasts of garnet are amine fine-scale features prior to analysis. The EDS rimmed by plagioclase with a yellow CL emission analyzer was calibrated using a Costandard and com- Gig. ad). The luminescence colors in plagioclase are pared with several mineral standards before and after causedby small amounts of Ti giving blue and Mn giv- each run. Conditions for analysis were set at 15 kV ac- ing yellow CL colors (Marshall 1988). Thus, the break- celerating voltage, a beam current of 20 nA and a beam down of the Mn-bearing garnet may be responsible for diameter of I pm, except for plagioclase which was the development of the yellowJuminescent plagioclase analyzedusing a 10 nA current and a beam diameter of in these areas. Titanite also is found in these textural 3 pm to avoid Na loss during analysis. Count times settings growing at the expenseof rutile. varied from 50 secondsfor garnet to 100 secondsfor Nelsonite (sample 9c): The nelsonite is dominated pyroxene and amphibole. Results were corrected using by large (1-5 cm) crystals of apatite and ilmenite. The the ZAF software. Typically 50-100 analysesof each original plagioclasehas beenrecrystallized and contains mineral in each section were done, along with zoning minute inclusions of spinel with rare inclusions of fine- profiles where appropriate. Representative results of grained corundum (Fig. 3g). Gamet forms irregular co- electron-microprobe analysesof mineral compositions ronas between areas rich in mafic phases (ilmenite, are shown in Table 1. The structural formulae for gar- amphibole, biotite) and plagioclase. Biotite also occurs net, orthopyroxene, clinopyroxene, amphibole and pla- as coronas around ilmenite, commonly in association gioclase were calculated using THEBA v. 6.0 (J. with gamet and also as inclusions in the latter (Fig. 3h). Martignole er a/., unpublished). The Fer+ content of the Secondary plagioclase commonly forms between gar- ferromagnesian minerals was estimated using the net and amphibole, showing up as well-defined blue method of Droop (1986). All mineral abbreviationsare collars in CL adjacentto the bright yellow apatite.These from Kretz (1983) and Spear(1993). phasescontrast with the nonluminescent gamet, biotite, amphibole and ilmenite (Fig. 4e). Plagioclasecollars are Gamet compositions and zoning interpreted to have grown during decompression (Indares1993). Representativecompositions of the garnet are shown Olivine gabbro (sample 35b):In the olivine gabbro in Figure 5. Garnet compositions from the most Mg- from the eastem LES, the original igneous assemblage rich rocks, i.e., troctolite (in contactwith clinopyroxene), consists of plagioclase, olivine and clinopyroxene, the hornblendite, olivine gabbro and also the olivine gamet latter being rich in rutile inclusions. Large crystals of amphibolite all have high Mg contents (>Prp5e).In con- apatite are common, and minor ilmenite also is present. trast, the nelsonite and gamet amphibolite are more Fe- The olivine gabbro shows more pervasive development rich, and the gamet has a correspondingly lower Mg of metamorphic textures, ranging from coronitic to contents (ca. Prp2s4d. Thus there is a correlation be- granoblastic (Figs. 3i, j). Plagioclase domains preserve tween the Mg content in garnet and the bulk-rock com- an original igneous shapebut are extensively recrystal- position. On the other hand, the troctolite, hornblendite lized. Olivine has been replaced by aggregates of and olivine gabbro contain mineral assemblagesindica- orthopyroxene rimmed by coronas of clinopyroxene. tive of high-P metamorphism, whereas the garnet am-
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Ftc.5. Garnet compositions plotted as Grs, Alm+Sps and Pqp for selected samplesffom the Lac Espadon suite.
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TABIE 1 REPRESENTATIVE COMPOSITIONS OF GARNET. LAC ESPADON SI]ITE
Sample )a 6am 3b 35b 2b 9c
retrograde nm core nnul cores nm
cp* Pl Amp Cpx Opx PI Pl Amp Pl cp* Amp PI
SiO, wtolo 40.02 3965 40.22 3925 4024 397t 39.69 4075 4tI6 40.75 40.96 41.33 4lO3 4139 38.08 3771 3775 ao, 2276 2207 2274 22.572267 2251 2t85 2243 2338 2279 234t 2299 22.942305 2062 21.1721 1l Tio, 000 0.M 009 0.00 003 0.00 000 002 010 000 000 000 000 002 000 0,05 009 FeO(total) 20.90 2090 1978 2213 20.98 2138 2457 2312 1863 19 57 1594 1634 1624 1.496 26 57 33.90 33.8s Mgo 1101 761 10.381074 1102 817 10.771194 1331 12.381206 13.421433 r20r 2.89 352 356 MnO 050 050 039 053 044 037 090 052 043 074 0.37 040 043 0.26 2.42 179 1.89 CaO 5E7 10.25 792 582 643 957 309 334 578 563 912 725 579 10.5610.79 408 4.23
Total l0l 09 l0l 06 l0l.s6 10111 101 87 t0t76 100.95102 t2t0282 r0192 101.E8101 78 1007910230 101 54 l023Zr025o
siqfu 2.98 300 299 295 2.98 298 300 301 298 300 298 300 3.OO 2.99 2.99 297 2.97 AI 2.OO 197 196 200 198 199 195 1.95 2.OO 197 20L 197 1.98 1.96 l.9l 196 196 Ti 000 0.01 004 000 000 000 000 000 005 000 0.00 000 000 000 000 000 0.00 Fel 129 129 1.16 134 127 132 I 50 1 38 I 1l I l7 0.97 095 097 0.85 r65 217 217 Fe3* 0.01 003 005 005 003 003 005 004 002 003 001 003 0.02 0 04 0.10 0.06 0.06 MC 1.22 0.86 109 120 1.22 091 127 132 1.44 136 131 153 156 1.28 034 041 042 Ca 047 083 075 047 051 07',1 0.25 026 050 0.44 071 050 045 086 091 034 035 Mn 0 03 0.03 0 03 0.03 0 03 0 02 0 06 0.03 0 03 0.05 0 02 0.o2 0 03 0 03 0.16 0 12 0.ll
Total E.ol E.Ol 803 805 E02 802 8.02 799 8.02 E02 E01 801 8,01 8 02 E 05 8.05 E 04
xe 0.428 0.429 0.401 0440 0419 0435 0.497 0.462 0.367 0.3E80.321 0320 0.322 0.292 0.530 0.712 0.705 xuP 0.4060.285 0381 0.3950403 0302 0402 0.4390.476 0449 0435 04E4 0.5190431 012E 0136 0137 xh ot49 0.2600.197 0.130 0151 0241 0057 0065 0'.t360.132 0.234 0173 0139 0257 0.2300.081 007E xt" 0010 0011 0.0080.0il 0009 000E 0019 00ll 0009 0015 000E 0.0080009 0005 0051 0039 0041
Number of ions expressedin atoms per formula unit (aplfa).Nature ofthe samples: 5a troctolite, 6am oliyine garnet anphibolite, 3b hornblendite, 35b olivine gabbro,2b garnet amphibolite, 9c nelsonite. The lst lin€ ofcolumn headings indicates the mineral with which the garnet is in contact.
phibolite has amphibolite-facies assemblages.The oh- phasesinstead ofconcentrically outward, zoning is con- vine gamet amphibolite containsevidence for both high- trolled by the chemical gradient between the minerals P metamorphism,e.9., coexisting garnet and olivine at the growth interfaces. In other words, the garnet co- (Harley & Carswell 1990) and re-equilibration,e.9., rona has higher Ca toward the plagioclase and higher orthopyroxene and amphibole. Thus there may also be Fe and Mg toward the clinopyroxene. In the coronas a correlation between the type of mineral assemblage where orthopyroxene and amphibole replace clinopy- (high-P versus retrograde) and whole-rock chemical roxene, garnet zoning displays a similar trend, but in composition. addition there is a reversal at the gamet rims. In such Gamet zoning profiles are shown in Figures 6a-h. cases,the Ca content drops from a maximum of Grsa6 The garnet coronas in the troctolite (sample 5a) are to Grs35at the garnet-plagioclasecontact. Mg decreases strongly zoned.In amphibole-freecoronas, Ca increases to Prp37, and Fe increases to Alma5 @ig. 6b) at the toward the plagioclase,with a maximum Grsa3adjacent clinopyroxene - orthopyroxene- pargasite(triple- junc- to the contact. This is compensatedby decreasing Fe tion) contact. The reversal in zoning is interpreted to be and Mg from clinopyroxene (Prp:gAlmaz)to plagioclase the result of retrograde resetting. The amphibole-bear- (Prp1eAlm32)contacts (Fig. 6a). The XFe l(i.e., Fe/ ing garnet coronas also show less strong XFe zonation, (Fe+Mg)] in these garnet coronasincreases in conjunc- probably due to the effects ofchemical re-equilibration. tion with Ca content. The above patterns are typical of Gamet coronas in the homblendite (sample 3b) are growth zoning in coronitic garnet (Indares & Rivers weakly zoned (Fig. 6c), with compositionsranging from 1995, Indares & Dunning 1997). Since coronitic garnet Alm:gPrp+sGrs13(amphibole contact) to Alm35Prpae grows from the inner to outer rim between reactant Grs13(plagioclase contact), probably due to homogeni-
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TABLE I (continued) REPRESENTATM COMPOSITIONS TABLE I (contirued) REPRESENTATM CoMPOSITIONS OF PLAGIOCLASE. LAC ESPADON SUITE OF CLINOPYRO)GNE, ORTHOPYRO)GNE, OLIVINE AND SPINEL, LAC ESPADON SUTTE
Smple 5a 6m 3b 35b 2b 9o dict dist incl reliot relict relict mtrix relict Smple Getrognde) (retrogade)
Cpx Cpx Cpx Cpx Cpx Op* Opx Ol spl SiO,wt% 58 29 60 05 6439 6021 59 87 59 98 6027 6420 @rom itrcl @rona@rcm gmo corom mtrix mtrix iDcl ALo. 2665 2463 23 tt 2389 25 t3 25 49 2493 2324 ft-@n rstro retro ft-@n G-@n FeO (total) 024 069 031 017 012 012 043 0 19 CaO 772 616 4t3 662 683 715 668 424 '775 Nqo 735 760 EE8 7.03 776 758 872 Siorwt% 5406 5375 5423 5322 5339 5453 5329 3722 017 rLo 026 020 012 003 024 0 19 026 0l2 At"o, 3 92 476 3 oo 5 06 5 18 171 259 000 6048 Tio, 018 027 009 027 018 003 009 000 000 Total 10051 99 33 1010l 9995 99 95 1005 r0032 1007l Feo(total) 513 522 466 331 364 158l lE 14 30 12 27 lO MgO l42l 1369 15.21 1478 1459 2752 2553 3251 921 siqfu 269 266 28t 266 267 266 264 2Al MnO 007 oo2 0 12 009 000 0ll 020 033 008 AI 130 133 119 t37 t32 l 33 131 120 Cr2O3 0 17 000 027 016 034 000 Fd. 003 001 001 001 000 000 002 0 0l MO 012 0M 000 000 013 014 Ca 030 033 019 032 0 33 034 032 020 ZoO 137 Na 066 067 075 061 067 0 65 06'1 074 CaO 2l3l 2145 2260 2323 2285 023 00't 007 K 001 002 001 000 001 001 001 001 NqO 153 159 097 106 r32
Total 499 502 491 496 501 500 496 Total 10072 100E0 l0l 20 lol 23 lol 74 10000 99 91 10025 99 15
v 0 362 0 306 0 210 0 342 0323 0339 0 318 0 210 Siqf" I 96 I 95 I 96 1 91 I 91 196 194 I 00 0021 v 0 624 0 683 0 790 0 656 0 664 0 650 0 668 0 783 Al 017 020 0r3 021 022 008 011 000 1933 0 015 0 012 0 000 0 002 0 014 0 011 0 015 0 007 Ti 001 001 000 001 001 000 001 000 0000 Fd- 015 016 013 008 007 047 055 068 0621 te 001 000 001 002 004 000 001 000 0000 Nahre of the wples: 5s troc'tolite, 6m olivire garnet mphibolite, 3b homblsditg MS o77 074 082 079 078 1 4'l 138 131, 03'16 35b olivin€ gabbro, 2b gmet mphibol.ite, 9c nelsonite MI 000 000 000 000 000 000 001 002 0002 Cr 001 000 002 001 001 0000 Ni 002 oo1 000 000 001 0003 Zt 0 028 Ca 083 083 087 089 088 002 000 000 Na 0ll 0 1l 007 007 009
Total 400 400 400 401 401 400 400 300 2983
Xfu 0 161 0 l7s 0 135 0 095 0 087 0 r39 0 825 0 865 0 905 0 913 TABLE I (continued) REPRESENTATTVECOMPOSITIONS xb 000E 0001 0013 0017 0035 OF AMPHIBOLE. LAC ESPADON S{.]ITE 0 100 0 ll1 0055 0057 0057
Suple 5a 6m 3b 35b 2b 9c Neture of the wpls: 5a boctolite, 6u olivine gmet mphibolite, 35b olivine retlo matrix oikocryst retrc mtrix @rona gabbro,9c nelsonite
Siorwt% 4249 4O3l 44 15 43 49 43 08 423r Alro, 15 57 1233 1546 1578 t2't9 1294 Tio, 0 93 1,76 0ll 063 050 060 zation at elevatedtemperatures. XFe also increaseswith FeO(total) 6'10 18 45 1595 5 53 1E,16 1903 Fe content. Large porphyroblasts of garnet from the Ivfno 007 016 000 003 021 00E MgO 1534 t 64 ll 6s 1590 972 9 52 garnet olivine amphibolite (sample 6am) show a weak Nio 009 008 047 015 000 000 increasein Fe content and decreasein Mg content (Fig. CaO 119l ll 63 3 36 1234 l0 E4 l0 48 NarO 288 lE9 601 230 175 183 6d) from the core (Alm4sPrpso)to the rim (Alm56Prpa7), KrO ll0 l8l 001 112 053 067 along with an increasein XFe. Theseporphyroblasts are
Total 97 19 9'l 10 9729 9752 9749 9753 unzoned with respect to Ca, and indeed Grs contents are low (Grse5).The lack ofzoning displayed in the core Siqpfu 6 15 622 628 622 646 640 of the gamet porphyroblastssuggests high-T homogeni- Al 266 224 259 266 263 231 Ti 0 l0 020 001 007 005 007 zation. The increase in XFe toward the rim (Fig. 6d) Fd- 069 224 060 056 172 172 indicates that Fe-Mg exchangecontinued between gar- Fd. Ot2 0r4 o25 010 060 069 IvIn 001 002 000 000 003 001 net rims and adjacent phasesduring cooling. Mg 3 31 199 338 339 217 215 Both subhedral and resorbed garnet porphyroblasts N 001 001 005 002 000 000 in the gamet amphibolite (sample2b) arehomogeneous. Ca 185 192 178 1E9 t74 1 70 Na 0E1 056 093 064 051 054 The subhedral grains (Fig.6e) locally display a slight K 020 036 0 00 020 010 013 decreasein Mg (Prpa5-as)and increase in Fe and Mn Total 1592 1592 1596 1579 1602 t5 7l toward the rims (Alm+s-soSpsoz-os),along with an in- crease in XFe. The increase in XFe toward the gamet
Natrc ofthe wples: 5a boctolite,6m olivitregmet mphibolite, 3b hombl@dite, rim suggestsretrograde resetting. Garnet coronas from 35b olivire gabbro,2b gmet mpNbolite, 9c nelsonite the nelsonite (sample 9c) also are weakly zoned (Fig.
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a) o.7 b) o.7 )(Fe o.6 o.6 .E o's Grs o.5 (, E o.o o.4 Alm FIi F o.: k o.3 o.2 Prp 0.1 o.t o o 0 80 160 240 Distance (microrn) Distance(microns) cpx <------> Pl cp*
c) Prp XFe ^ 0.4 E Alm 'E 0.3 k
Grs 0.1
Sps 0 115 230 345 Distance (microns) Amp PI
d) 0.6
0.5
E o.q C) t 0.3
0.1
380 Distance(microns) Core Rim
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e) XFe
d 5 0.6 Alm
q 0.4 Grs o.2 Prp sps
520 1040 Distance (microns) Rim <-______-_____Core <_,____------Rim
f)
H XFe
Xt Alm E 0.6 \ //' Biotite inclusions I 0.4
Pm Gn Sps 40 80 120 180 Distance (microns) PI Ilm
c) 0.6 h) 0.6 Prp 0.5 0.5 Prp '5 XFe '5 U.u+ XFe U.4 q) I Alm FI Alm ri o.l fi o.t F € Gn = nt = nt Grs 0.I 0.1 Ollvine gabbro Sps 0 sps 0 60 120 240 Distance(microns) cpx PI Rim <----> Rim
FIcs 6a-h. Gamet zoning profiles for a) sample 5a (coronitic troctolite), amphibole-free corona, and b) amphibole-bearing corona, c) sample 3b (hornblendite), kyanite-bearing gamet corona around a plagioclase chadacryst,d) sample 6am (olivine amphibolite), zoning traverse across a garnet porphyroblast, e) sample 2b (garnet amphibolite), traverse across a subhedral porphyroblast, f) sample 9c (nelsonite), gamet corona with biotite inclusions, g) sample 35b (olivine gabbro), large gamet corona around a relict igneousclinopyroxene, and h) small kyanite-bearing garnet.Symbols on all profiles: square:spessartine, cross: grossular, circle: pyrope, triangle: almandine and dash: Fe/(Fe + Mg).
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6f), with Ca increasing slightly and Fe decreasingto- and the zoning may be a relict growth-related feature. ward the plagioclase (AlmTsGrsleto A1m65Grs15).XFe However, the XFe content of the garnet porphyroblasts increasestoward biotite inclusions in the corona. In ad- is homogeneous,presumably because the garnet dition, the garnet compositions are higher in Mn con- porphyroblastswere partially homogenizedduring high- tent (Sps63)than in the other samples.Thus, the garnet T metamorphism. zoning profiles in the nelsonite show clear evidence of homogenization and retrograde Fe-Mg exchange with Clinopy roxene,plagioclas e biotite inclusions. and amphibole compositions The olivine gabbro from the eastern LES (sample 35b) displays areas with both coronitic and porphyro- In contrast to the garnet, clinopyroxene, plagioclase blastic garnets.In coronitic examples, garnet composi- and amphibole display more restricted compositional tions generally increasein Ca (Grs27-16)and decreasein ranges.The clinopyroxene compositions from the both Mg (Prp+s-sz)toward the plagioclasecontacts (Fig. 69). the troctolite and olivine gabbro (Fig. 7) are all sodian XFe also increasesalong with Ca. Garnetporphyroblasts augite, (Di + Hd)6s-e5(Jd+ Ae)s-zo. (Fig. 6h) also are zoned, with (kyanite-bearing) cores Plagioclase compositions for the troctolite and richer in Ca (Alm2sPrpa2Grs27)and rims richer in Fe and homblendite lie in the range Ana6-36Ab6e-76@ig. 8). Mg (Alm32Prp5sGrs15).This finding indicates that de- Plagioclasefrom the garnetamphibolite (sample2b) and spite their granoblastic texture, these garnet porphyro- plagioclase inclusions in garnet poikiloblasts from the blasts may have replaced original plagioclase domains, olivine garnet amphibolite (sample 6am) are slightly
Na-aug
o
Aug
Ftc. 7. Compositional diagrams for clinopyroxene for the foctolite and olivine gabbro in the Lac Espadon suite.
Or 20
oO o o o o
OMe gmd mphlbdb
Plcolh PlElicr
Ab An 50
FIc. 8. Plagioclase compositions for samplesin the Lac Espadon suite
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more sodic (Anzz-zzAbea7s). Plagioclase compositions pyroxene and garnet. For garnet-amphibole thermom- in the olivine gabbro lie in the range An32-36Ab6s-66.In etry, version 1.01 of the program was used (Berman, the nelsonite, the plagioclase with spinel inclusions pers. commun.) with the activity model of Miider & ranges in composition from An3sAb62to An36Ab70, Berman (1992) tor amphibole and Berman (1990) for whereas the secondary collars are more sodic (Anzs-rs garnet. The results were compared with the Fe-Mg ex- Abrs-sz). change thermometers of Ellis & Green (1979) and In all samples, the amphiboles lie in the composi- Krogh (1988) for garnet-clinopyroxene, Carswell & tional range pargasite - ferropargasite - ferroedenite Harley (1990) for garnet-orthopyroxene, O'Neill & (Fig. 9). Samples with the best-preservedtextural evi- Wood (1980) for garnet-olivine, and Graham & Powell dence for high-P metamorphism (troctolite, hornblendite (1984) for gamet-amphibole becausethese have been and olivine gabbro) also contain the most magnesian extensively used in the literature. The P-T conditions compositions of pargasite. This finding is consistent with the interpretation that the pargasitic amphibole grew as a replacement of high-P (Mg-rich) gamet and TABLE 2. CONDITIONS OF PRESSUREAND TEMPERATURE presence RECORDEDBY THE MAFIC SAMPLBS clinopyroxene in the offluid shortly after peak OF TTIELAC ESPADONSTIITE conditions. The amphibole-rich rocks, olivine garnet amphibolite, garnet amphibolite and nelsonite, contain Thmometry (T mg€ in prsue irtwal ft@ 0 to 20 16e) the most Fe-rich pargasitic amphiboles. Amphibole compositions in the olivine garnet amphibolite are the TWEEQU E&G79 K88 c&H90 0&wr0 G&PE4 most Na-rich, lying close to the pargasite-edenite compositionalboundary (Fig. 9). The more Fe-rich com- I 7EG€70 720-El0 670-765 positions t 100-775 650-715 600-460 in thesesamples may be the result of re-equili- 3 79G-a90 730-E30 695-790 bration with garnet or other Fe-rich phases during 4 6t5-790 750-865 retrogression. 5 745--E10 760-825 650-860 7lo-915 7 59H70 685-730 INtenpnsrarroN oF TEXTURESlwo THsRMonARoMETRy 8 605-700 6t0-720 640-750 l0 800-95 825-950 General approach ll 620-795 630-105 550-690 485-560 75N60 760-8E0 715J25 765-8E0 For each sample, the interpretation of textures and t4 775-470 790-aA5 745-A4o of mineral compositions was followed by selection of appropriate microdomains for P-T determinations. Bqometry (atl TWEEQU; P mge for t@pdature intflal shom above) These are domains in each sample for which local equi- librium may be inferred, and pressure-and temperature- Grt-Pl-Ky-{m GRAIL+TIn ft-Pt-Spl-Cm dependentreactions can be written. Temperatureswere I 155-18 5 calculated using garnet-clinopyroxene, garnet-ortho- 2 135-15 75 pyroxene, garnet-olivine and garnet-amphibole Fe-Mg 145-r7 10 14'15-19 exchangereactions. Pressures were calculatedusing the 9 75-1175 garnet - plagioclase - kyanite - corundum reaction t2 (Indares & Rivers 1995) for the troctolite, homblendite 13 l2-14 t4 t4 5-16.25 and olivine gabbro.The assemblagegarnet - plagioclase - qnartz - rutile - titanite was used as a variation of the List of rock typ€s ud aswblsges: I S@pl€ 54 corcaitic troctolite, Arlp-fi@ GRAIL barometer (e.9., Ghent & Stout 1984) in the oromumblege Grt{px-Pt-Ky-Cm (@ttacts) 2 SEryle58, coronitic troctolite, garnet amphibolite. The garnet - plagioclase - spinel - Amp-bediry orcm mmrblage, Grt-4px-Pl-Ky-{m (mtacts) 3 Smple 54 corundum reaction after Indares & Dunning (1997) was corcdtic troctoliie, Amp-bwing orcm ownblege, Grt-Cpx (incl ) 4 Supl€ 54, coronitictrwtolit€,Amp-bqiry@rcmssrblsg€, Gd-OPx(@ntacts) 5 Ssmpl€ used for estimates of P in the nelsonite. Owing to the 54 rcronitic trcctolit€, Amp-bqing corom ssmblagq Grt-Amp (@ntacts) 6 lack of contactsbetween granoblastic domains and pla- Seple 6as! olivine gmEt mphibolite, giln€t pdlltytoblsst @re ed rir4 Grt-Ol (@res),7 S@ple 6ao, olivinegmst mpbibott!, gimetporphyrcblast@reed dn' gioclase inclusions in the olivine garnet amphibolite, Grt-Ol (@rltacts).8 Smple 6ar4 olivire gametmpbibolite, gm€t Poryhyrcblast independent pressurescould not be calculated for this m md risr, Gtt-Opx (@d8rts) 9 Smpl€ 68ll! olivire 88met@phibottg gsmet porplryrctrlut m mdri4 Grt-Atr4 (cod!cts). 10 Smplc 3b,homblf,dite, @rom sample. Given the errors associatedwith the estimation smblage, Grt-Amp-Pl-Ky{m (@!trcts). I I Sampl€2b, gmet enpbibolite Grt of the Fe3* content, all temperatures were calculated rims md @trirq @-Amp-Pl-Qtz-TtrRt (@rtracts).12 Smple 9c, oelpaite, assuming mm sffiblege, Grt-Amp-Pl-Spl-Cm (mtads). l3 Supl€ 35b,oliviregabbrc' the Fe to be Fe2*. gftmblartic dmblsg€" Gt-cpx-Pf-Ky{m (ritr). 14smple 35b,olivine gsbbrc' Temperatures and pressureswere calculated using gmoblastic umblage, Grt-Cpx-Pl-Ky-Cm (rc*aag appropriate reactions with the program TWEEQU Abbryiatios for the mtional thmmetqs: E&G79: Eilis & Grm (1979)' K88: Krogh ( l98t) for gmet-dinopyrcxene, C&tDo: Cffiwel & Harlery(1990) for v. 2.02 (Bernan 1991) that usesan internally consistent g@et{rthopyrq@, O&WEo:O'NeiU & Wod (1980)forgemet{liviDe,G&PE4: database.Activity models used are those of Fuhrman & Gnhm&Powell(1984)forg8met-mPhibole. BqmetricmctimsretEkmftom Indres & Riffi (1995) for gmet - plagislaw - kyuite - cmdurn' Ghmt & Lindsley (1988) for plagioclase, andBerman et al. Stout (1984) for gmet - plagioclas - rutile - quanz - titrDite (GRAIL + Tfii (1995) and Berman & Aranovich (1996) for olivine, vuiut), md Indrcs & Duing (1997)6r garnd - plagioclss - spirel - @rundm
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o + nsg "'z s
0.0 7.O 6 5 6.0 5.5 5.0 7 0 6.5 6.0 5.5 5.0 Si si
Olivine gemct mphibolite magneslo- €dflite sadmagaite o +u 0.5> u
7.O 65 6.0 55 7.0 6.5 6.0 5.5 5.0 Si si
l0
magn€slo- sadmagaite o o tr fr+ b0 N 0.5a 0.5a bo m
sadmagaite
7.O 6.5 6.0 5.5 7.O 6.5 6.0 5.5 5.0 Si Si
Ftc. 9 Ca-amphibole compositions for samples from the Lac Espadon suite. Diagram parametersare Cas ) 1.50; (K+Na)a 2 0.50 and Ti < 0.50 (after Leake et aL 1997).
calculatedusing TWEEQU are shown in Table 2 and rn roxene - clinopyroxene - garnet is consistent with Figure 10. higher concentration of A1 in the plagioclase domains and higher concentration of Fe and Mg in the olivine Troctolite (sample 5a) domains. Therefore, high Ca and high Mg and Fe in the outer and inner rims of the garnet corona are a direct In the troctolite, the development of a triple corona result of growth in contact with plagioclase and of orthopyroxene, clinopyroxene and gamet between clinopyroxene. The presenceof corundum and kyanite olivine and plagioclase involves Fe and Mg diffusion inclusions in the plagioclase, on the other hand, is from the olivine and Ca and Al diffusion from the pla- causedby the garnet acting as a buffer for Al (e.9., Yund gioclase. For instance, the corona sequenceorthopy- 1986) leading to an excessof Al in the plagioclase site.
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a Trcctolite 18 (Arnp-fteecorom) ffi (Anp-beuingcoromCpx-Inclusions)i t6 -l (Amp-bearingcorona contact) KY i l -14 tt Homblerdite I 512 Gametanphibolrte N g\u ffi -8
6 sil
And 0 200 300 400 500 600 700 800 900 1000 Tbmperature(oC)
Olivine gabbro 18 b (coronacontacts) W rirns; Ll 16 Ky d iGranoblastic ^ 14
312 I I dro P Fi^ a 6 sil
2 And 0 200 300 400 500 600 700 800 900 1000 Temperaturc(oC)
Frc. 10. P-T diagrams showing the maximum and minimum metamorphic conditions calculated pa(ly using the results of analyses in Table 1 for troctolite, hornblendite, garnrt amphibolite and nelsonite (a) and olivine gabbro (b). The range shown is based 6pically on 5-20 compositions in total, as well as those in Table 1. All calculations were carried out as independent reactions using the program TWEEQU version 2.02 (Berman 1991), except for gamet-amphibole, which uses version 1.01. P-T estimates quoted are to the nearest5 oC and 0.25 kbars. The results of all P-T calculations are also shown in Table 2.
Thus the development of the triple corona assemblage in a quartz-absent system, along with the presence of corun- dum and kyanite in the plagioclase, can be represented by the general reactions:
[R1] 3CaAl2Si2Os + 3[Mg,Fe]zSiOa <+ 3[Mg,Fe] SiO: + Ca:AlzSi:Orz + [Mg,Fe]:AlzSi3Op+ 47203 An Fo,Fa En,Fs Grs Prp,Alm Crn
in Pl Ol Opx corona in Grt corona incl. in Pl
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[R2] 4[Mg,Fe]SiO3+ 4CaAl2SizOe+ NaAlSi3Os+ 2Al2O3e En,Fs An Ab Cm Opx corona in Pl incl. in Pl
Ca[Mg,Fe]Si2O6+ NaAlSi2O6+ Ca3A12Si3O12+ [Mg,Fe]3Al2Si3O12 + 5Al2SiO5
Di,Hd Jd Grs Prp,Alm Ky
in Cpx corona in Grt corona incl. in Pl
In this textural setting, the garnet - plagioclase - achieved during peak-T conditions and are likely pre- kyanite - corundum barometer (Indares& Rivers 1995) served (Indares & Rivers 1995). Therefore, calculating can be applied to garnet-plagioclase contacts.The dis- P-conditionswithgarnetcompositions attheplagioclase tribution of Fe and Mg between the garnet and rims and T-conditions at the garnet-clinopyroxene con- clinopyroxene rims can be used to consftain the meta- tact should give peak conditions of metamorphism. morphic temperatures.The absenceofretrograde reset- The breakdown of the corona assemblagelocally to ting at both rims ofthe garnet corona suggeststhat the give amphibole and orthopyroxene can be represented equilibrium compositions of the adjacent phases were by the general reactions:
lR3l 5[Mg,Fe]:AlzSi:On+ Ca3AhSi3Or2 + l8NaAlSi2O6 + 9Ca[Mg,Fe]Si2O6 + 6H2O <+ 12NaAlSi3O8 + 6NaCa2[Mg,Fe]4Al3Si6O2lOH)2
Alm,Prp Grs Jd Di,Hd Water Ab Prg,Feprg
in Grt coronain Cpx corona retrograde coronas
lR4l 8[Mg,Fe]:AlzSi:Orz+ 1OCa[Mg,Fe]SizO6 + 2NaAlSi3Os+ 2H20 <+ 26[Mg,Fe]SiO3 + 6CaAl2Si2O3 + 2NaCa2[Mg,Fe]dl:SirOzz(OH)z
Alm,Prp Di,Hd Ab Water En,Fs An Prg,Feprg
in Grt corona in Cpxcorona in Pl retrograde coronas
Reactions [R3] and [R4] require plagioclase among point on the retrograde P-T path. However, the results both the reactantsand products. Plagioclaseis not adja- should be viewed with caution. In addition, compara- cent to both the garnet and clinopyroxene in the corona tive estimates of temperature were made using assemblage.This fact suggeststhat some open-system clinopyroxene inclusions in the gamet in the amphib- behavior involving fluid is likely responsible for retro- ole-bearing corona assemblages. gressionin the amphibole-bearingcorona assemblages. Peak P-T conditions obtained in amphibole-free In these coronas,the partial resetting of Ca, Fe and Mg coronas with garnet-clinopyroxene corona contacts are is shown in the zoning profiles ofthe garnet(Fig.6b), in the range780-870"C at 15.5-18.5kbar (Fig. l0). In which is indicative of retrogression.Therefore, the re- amphibole-bearingcoronas, the temperatureconditions equilibration of Fe and Mg between the garnet and calculated using garnet-
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ventional exchange themometers (Table 2). The gar- between690-800'C (assumingpressures of l0-12 kbar) net-clinopyroxene thermometers of Ellis & Green and 740-860 oC (assuming pressuresof 16-18 kbar). (1979) and Krogh (1988) gave ranges of temperature Rangesrecorded by the garnet rims and adjacentmahix overlapping with the TWEEQU approach,although the phasesare much narrower; they range from 590-670'C Krogh (1988) calibration gave slightly lower estimates. for garnet-olivine, and 605-700oC for garnet- The temperaturescalculated using orthopyroxene-gar- orthopyroxene, to 610-720"C for garnet-amphibole net (Carswell & Harley 1990) and garnet-amphiboie exchange (Table 2). The garnet-orthopyroxene ther- (Graham & Powell 1984) also are in general agreement mometer of Carswell & Harley (1990) and garnet- with TWEEQU results. amphibole thermometer of Graham & Powell (1984) give almost identical ranges to those inferred from Homblendite (sample 3b) TWEEQU. However, the estimates of temperature made using the garnet-olivine exchange thermometer of Sample 3b displays well-preserved igneous textures O'Neill & Wood (1980) are about 60oC higher. Over- such as amphibole oikocrysts and olivine and plagio- all, the temperaturesfrom the cores are similar to those clase chadacrysts,with garnet coronas around the pla- for the retrograde (amphibole-bearing) coronas in the gioclase and orthopyroxenecoronas around the olivine. troctolite, whereas the temperatures recorded by the Unfortunately, it is not possible to describe the forma- garnet rims and matrix phasesare lower. tion of eachtype of corona with balancedreactions. Tne abundanceof hydrous phases,together with the lack of Garnet amphibolite (sample 2b) zoning in the gamet coronas(Fig. 6c), suggestdiffusion with some open-system behavior. However, the pres- Like the olivine garnet amphibolite (sample 6am), ence of the garnet - plagioclase - kyanite - corundum sample 2b displays a granoblastic texture; it is thus not assemblagein the plagioclasedomains allows pressures possible to suggestreactions for the development of the to be calculated. Given the evidence oflarse-scale dif- main metamorphic assemblage.The texture revealedby fusion in the sample, it is reasonableto coinbine these CL images (Figs. 4c, d) suggeststhe breakdown of the estimates of pressurewith the temperaturescalculated garnet and quartz along with rutile (1.e.,high-P assem- at the adjacent garnet-amphibole contacts and assume blage) to form titanite and secondary plagioclase (1.e., that both domains achieved equilibrium at the same ther- low-P assemblage).Garnet zoning also is indicative of mal conditions, 1.e.,close to the metamorphic peak or at retrogression. Given these observations, reactions in- some early stage of cooling. The P-T conditions re- volving the garnet - amphibole - plagioclase - rutile - corded by the homblendite are 800-93OoC at 14.j5- titanite - quartz assemblage can be used for calculation 19 kbar (Fig. 10). The garnet-amphibole thermometer of retrograde P-T conditions using garnet rims and ad- of Graham & Powell (1984) also gives ranges of tem- jacent matrix minerals. perature that overlap with the TWEEQU results. The Fe-Mg exchange reaction between garnet and pargasite and a variant of the GRAIL barometer (e.9., Olivine gamet amphibolite (sample 6am) Ghent & Stout 1984) were used to estimate the retro- grade P-T conditions. The P-T conditions inferred Sample 6am is granoblastic; given the extent of using TWEEQU are estimated at 620-'795"C at9.75- metamorphic overprinting, it is not possible to suggest I l 75 kbar. The range of temperaturegiven by the Gra- reactions for the development of the main assemblage. ham & Powell (1984) garnet-amphibole thermometer The widespread presenie of amphibole and orthofy- is almost identical (630-705'C). roxene in the matrix does, however, suggestpervasive retrogression,whereas olivine is clearly a relict igneous Nelsonite (sample 9c) phase.On the other hand, the presenceofabundant in- clusions of amphibole in the gamet poikiloblasts sug- The nelsonite is markedly higher in Fe and lower in gests that fluid infiltration must have occurred prior to, Si (Cox et al. 1998) than the other samplesand displays or during ganet growth. In this rock, garnet cores and distinctive textures. Despite the presence of coronitic olivine compositions were chosento estimateminimum textures, high-P minerals other than gamet are absent peak conditions oftemperature. In addition, areaswith from the sample.In addition, plagioclasedisplays spinel coarse-grained assemblagesand straight contacts be- inclusions instead of kyanite, probably owing to exten- tween the rims of garnet, olivine and orthopyroxene sive diffusion of Fe in the plagioclase structure. Garnet were chosento determine the temperaturesof retrogres- compositions are extensively modifred by homogeniza- sion or re-equilibration. However, the absenceof pla- tion and partial retrograde resetting. Diffusion rates in gioclase in textural equilibrium with the garnet and plagioclase have been shown to be greatly increasedin matrix minerals prevents an independent estimate of the presenceof F (Snow & Kidman 1991). Thus, the pressureto be calculated for the sample. high amounts of apatite (try to llvo) and presumably Conditions of temperaturerecorded by gamet cores the high contents of F in the nelsonite may also have using garnet-olivine thermometry cover a wide range contributed to enhancedre-equilibration during cooling.
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P-T conditions were calculated using the garnet - pla- in the adjacent hornblendite (800-930'c at 14.75- gioclase- spinel - corundum barometer,which hasbeen 19 kbar) in the westem LES (Fig. loa). P-T conditions recently applied in coronitic rocks (Indares & Dunning defined by the clinopyroxene inclusions and plagio- 1997), and combined with garnet-amphibole exchange clase-garnet contactsin the amphibole-bearingcoronas themometry to give the P-T conditions recordedby the overlap those in the amphibole-free coronas and the sample: 550-690"C and 4-5.5 kbar (Fig. 10). The Gra- hornblendite(780-870'C at 15.5-18.5kbar). The pres- ham & Powell (1984) calibration gives lower tempera- ervation of growth zoning in garnet in the amphibole- tures (485-560'C). TheseP-T estimatesare interpreted free coronas of the troctolite suggeststhat the P-T con- to represent re-equilibration of the textures at some stage ditions listed above are those close to the peak on the retrograde path. conditions of metamorphism.P-T conditions calculated for the garnet amphibolite (620-:795"C at 9.75-11.75 Olivine gabbro (sample 35b) kbar) and the nelsonite (550-690'C at 4-5.5 kbar), along with the type of textures and pattern of zoning in The olivine gabbro displays both coronitic and garnet, suggest that only retrograde conditions are re- granoblasticareas, both ofwhich are locally amphibole- corded by thesesamples. The range in temperaturesug- bearing. The presenceof granoblastic garnet with pre- gested by the garnet contacts (with amphibole, served growth-induced zoning in contact with plagio- orthopyroxene and clinopyroxene) in the amphibole- clase, both of which contain kyanite and corundum bearing coronas in the troctolite (710-800"C) is very inclusions (Fig. 4e), suggeststhat in places the grano- similar to that in the gamet amphibolite, whereas the blastic domains escapedretrogression. In these areas, apparentpressures are clearly higher. This pattem sug- and also in the coronitic domains, reactions [R1] and geststhat the net-transfer(P-sensitive) reactions involv- [R2] can be inferred. In such domains,both granoblastic ing Ca at the gamet-plagioclasecontact closedat higher cores and coronas are likely to preserve high-P condi- temperatures than the Fe-Mg exchange at the garnet - tions. However, only in the coronas where both pressures clinopyroxene - orthopyroxene - clinopyroxene con- and temperaturescan be measuredcan thermobarometry tacts. Thus, the resultant retrograde P-T point may be be applied to calculate peak P-T conditions. In the erroneousand cannot be used to constrain the P-T path. granoblasticareas, the garnetrims will tend to re-equili- The interval of temperatures recorded by the olivine brate. Retrograde modifications indicated by the pres- gamet amphibolite is more difficult to assess,as no in- ence of amphibole replacing clinopyroxene may be rep- dependentpressure could be calculated. However, the resentedby the generalreactions [R3] and [R4]. Indeed, highest temperaturesrecorded by the gamet cores (740- the presenceof both clear plagioclase and pargasite in 860"C assumingI 8 kbar) may representpeak conditions the granoblastic rims suggeststhat these assemblages or cooling during the early stagesof exhumation. The represent those reactions better than in the troctolite. garnet rim - matrix range of temperature(590-720'C) Thus, the retrograde P-T conditions in this sample can is clearly lower and probably indicates re-equilibration be estimatedif we assumethat equilibrium was achieved at amphibolite-facies conditions, as in the case of the at some point during retrogression. The compositions garnet amphibolite or nelsonite samples. ofplagioclase, garnet and clinopyroxene were measured In the easternLES, textural evidence together with where all were in contact. However, contactsinvolving local preservation of growth zoning in garnet suggest kyanite and corundum in the plagioclase and garnet are that part of the assemblage should record conditions sparse,and the effects of secondaryreactions involving close to the peak of metamorphism. The peak P-T con- pargasite and clinopyroxene on the P-T estimates for ditions obtained (750-870"C at14.5-16.25 kbar) over- retrogression in this sample are not known. Thus, the lap those in the western LES. The retrogradeconditions P-T estimatesfor the conditions of retrogression must calculated(750-850'C at 12-14 kbar) overlap those be treated with caution. recorded by the amphibole-rich rocks in the westem P-T estimatesare 77 5 -87 0" C at | 4.5-16.25 kbar for LES, although the range in pressureis slightly higher. the corona assemblagesand 750-850'C at l2-l4kbar Given the evidence from the troctolite that the Ca ex- for the retrograded granoblastic assemblages(Fig. l0). change and Fe-Mg exchange seem to have closed at Rangesof temperaturecalculated using TWEEQU and different times, the samemay be the casefor the olivine Ellis & Green (1979) closely overlap and are only mar- gabbro. However, the retrograde conditions inferred ginally higher than the Krogh (1988) estimates,and the from this sample are recorded by granoblastic rather Graham & Powell (1984) estimatesfor garnet-amphib- than coronitic domains. In addition, the sampleis not as ole are identical (765-880'C, T able 2). extensively retrograded as the amphibole-rich samples (garnet amphibolite and nelsonite) from the western DrscussroNAND CoNcLUSIoNs LES. Therefore, it is likely that the retrogradeddomains in the olivine gabbro simply preserveinformation from The highest P and T conditions are recorded by the a different stage of the P-T Path. amphibole-free coronas in the troctolite (780-870"C at The P-T paths are difficult to compare, as there are 15.5-18.5 kbar) and the kyanite-bearing garnet coronas insufficient data from the easternLES to fully constrain
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the P-T conditions during exhumation. However, the for detailed and constructive comments, which in- available data suggestthat both P-T paths are steep,and creasedour understanding of the rocks and ultimately compatible with evidence for tectonic exhumation improved the final version of the manuscript. Bob Mar- throughout the area (Indares et al. 1998). The most ex- tin also is thanked for his thorough editorial handling of treme conditions recorded in the LES overlap the con- the paper. R Cox was supported by NSERC and ditions of metamorphism recorded by the highest Lithoprobe grants during this study. structurallevels in the underlying Lelukuau tenane (e.g., Indares 1997). Results of U-Pb geochronologycon- Rnrr,nrNcrs straining the agesof both protoliths and metamorphism show that both units are Labradorian in age and that Ausrnnrru, H., EnArvtssnr,M & ENGVIK,AK (1997): peak Grenvillian metamorphism occurred in both Processingof crustin theroot of theCaledonian continen- tal collisionzone: the role of eclogitization.Tectonophys. between ca. lO50 and 1040 Ma (Gale et al. 1994. Cox 273,129-t53. et al. 7998, Indareset al. 1998).The above data support the hypothesis that the LES is a dismembered part of & GRIFFIN,W.L. (1985): Shear deformation and the Lelukuau terrane. Similar P-T conditions (720- eclogite formation within granulite-faciesanorthosites of 825"C at 14-17 kbar) are also recorded by eclogites rn the Bergen Arcs, western Norway Chem. Geol. 50,26'7- the adjacent Baie du Nord segment (Cox & Indares 28r. 1999). P-T conditions in the samerange and steepP-T paths also characterizeeclogite, coronites and amphibo- BERMAN,R G. (1991): Thermobarometry using multi-equilib- lites in the nearby Molson Lake and Gagnon terranes rium ca.lculations:a new technique, with petrological ap- (Rivers & Mengel 1985, Indares 1993, Cornelly et al. plications. Can Mineral. 29, 833-855. 1995,Indares & Rivers 1995).These studies,along with (1996): the datafrom the LES, indicate a widespreadexposure of & AneNovrcu, L.Y. Optimized standard state and solution properties of minerals. I. Model calibra- deep crust in this area of the eastem Grenville Province. tion for olivine, orthopyroxene, cordierite, garnet and Interestingly, although these rocks record P-T con- ilmenite in the systemFeO-MgO-{aO-Al2O3-TiO2-SiO2. ditions in the eclogite facies field, they have not been Contrib. Mineral. Petrol. 126. l-24. transformed into "true" eclogites (Carswell 1990). It is possiblethat in massiveand dry igneousrocks with large C.cRswEU-,D.A. (1990): Eclogites and the eclogite facies: detr- grain-sizes, such as the interiors of the LES mafic and nitions and classification. InEclogite Facies Rocks (D.A. ultramafic bodies, reaction rates are slow, resulting in Carswell, ed.) Blackie and Son Ltd., Glasgow, U.K. (1-13). the formation of metamorphic coronas and the preser- vation of igneous relics (Rubie 1990). Deformed and & Hanr-Bv,S.L (1990): Mineral barometry and ther- hydrated rocks, however, allow metamorphic reactions mometry. InEclogrIe Facies Rocks (D.A. Carswell, ed.). (83-110). to go to completion during the peak (Austrheim & Grif- Blackie and Son Ltd , Glasgow,U.K. fin 1985,Rubie 1986,Koons et al.1987, Austrheim el (1992): al. 1997) but are also more prone to retrogression Cnarneeonrv, S. & GaNcur-v, J. Cation diffusion in aluminosilicate garnets: experimental determination rn (Heinrich 1982). However, high-P coronites metamor- spessartine-almandinediffusion couples, evaluation of ef- phosed at eclogite facies commonly display omphacitic fective binary coefficients, and applications. Contib. Min- clinopyroxene(e.9., MOrk 1985, 1986, Indares 1993, eral. P etrol ll1, 74-86. Zhang &LioLl 1997, Cox & Indares 1999) suggesting "partial" eclogitization. In contrast, the high-P samples CoNNeu-v, JN., Ful'rns, T. & Jevns, D.T. (1995): Thermo- in the LES contain clinopyroxene coronas which, al- tectonic evolution of the Grenville Province of western though Na-bearing, are not omphacitic. The most likely Labrador Tectonics 14, 202-217. reason for this is the nature of the protoliths, which are parts of a mafic and ultramafic suite and thus, have low Cox, R., DusNrNc, G.R. & INDARES,A (1998): Pehology and bulk Na-contents. U-Pb geochronology of mahc, high-pressule, metamor- phic coronites from the Tshenukutish domain, eastern AcrNowr-Bocnvewrs Grenville Province. Precambrian Res.90, 59-83
(1999): This study forms part of the Ph.D. thesis of R. Cox. & INoAnes. A Tranformation of Fe-Ti gabbro to coronite, eclogite and amphibolite in the Baie du The authorsthank Maggie Piranian, Pam King, and Jeff Nord segment, Tshenukutish terrane, eastern Grenville Saundersfor assistanceduring the microprobe analysis Province. J. Metamorphic Geol (in press). of the samples.Roger Mason and Toby Rivers are grate- fully acknowledged for help with the CL microscopy CvcrN, R.T. & LASAGA,A.C. (1985): Self-diftusion of magne- and in revision of the final version of the manuscript. sium in gamet at 750' to 9O0"C.Am. J. Sci. 285, 328-350 Greg Dunning is thanked for his supervision of R. Cox during the long haul. The authors also gratefully ac- Dnoop, G.T.R. (1986): A general equation for estimating Fe3+ knowledge Rob Berman and one anonymous reviewer concentrationsin ferromagnesiansilicates and oxides from
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