Hydrothermally altered volcanic rocks metamorphosed at ...€¦ · Abstract: A 1.2 Ga association of aluminous gneisses, garnetites, and white felsic gneisses of andesitic composition

ARTICLE

Hydrothermally altered volcanic rocks metamorphosed atgranulite-facies conditions: an example from the GrenvilleProvinceMarisa Hindemith, Aphrodite Indares, and Stephen Piercey

Abstract: A 1.2 Ga association of aluminous gneisses, garnetites, and white felsic gneisses of andesitic composition in thesouthern Manicouagan area (central Grenville Province) provides evidence consistent with protolith formation and hy-drothermal alteration in a submarine volcanic environment. In addition to field relations, potential relics of quartz phenocrystsin the aluminous gneisses, revealed by SEM–MLA (scanning electron microscope with a mineral liberation analysis software)imaging, are consistent with a volcanic precursor. Moreover, in these rocks, aluminous nodules and seams of sillimanite areconsidered to represent metamorphosed hydrothermal mineral assemblages and to reflect former pathways of hydrothermalfluid. These features are preserved despite the Grenvillian granulite-facies metamorphic overprint and evidence of partialmelting. In addition, the garnetites are inferred to represent hydrothermally altered products of the white gneisses, based on thegradational contacts between the two rock types. The compositional ranges of minerals are generally similar to those ofgranulite-facies metapelites, but moderately elevated contents of Mn in garnet from the garnetites, and Zn in spinel from thealuminous gneisses, are consistent with hydrothermal addition of these elements to the protolith. The most prominent altera-tion trends are an increase in Fe–Mg–Mn from the white gneisses to the aluminous gneisses and the garnetites, and a trend ofincreasing alumina index in some white gneisses, suggesting mild argillic alteration. The new findings highlight the preserva-tion of early hydrothermal alteration in high-grade metamorphic belts in the Grenville Province, and these altered rocks arepotential targets for exploration.

Résumé : Une association de 1,2 Ga de gneiss alumineux, grenatites et gneiss felsiques blancs de composition andésitique dansla partie sud de la région de Manicouagan (province de Grenville centrale) fournit de l’information qui appuie l’interprétation dela formation des protolites et de leur altération hydrothermale dans un milieu volcanique sous-marin. En plus des relations deterrain, des reliques possibles de phénocristaux de quartz dans les gneiss alumineux, révélés par imagerie MEB–ALM (micros-copie électronique a balayage combinée a un logiciel d’analyse des taux de libération des minéraux) appuient l’interprétationd’un précurseur volcanique. Dans ces roches, des nodules alumineux et des veines de sillimanite sont en outre considérés commereprésentant des assemblages minéraux hydrothermaux métamorphosés qui reflètent d’anciennes voies de circulation defluides hydrothermaux. Ces éléments sont préservés malgré le métamorphisme grenvillien au faciès des granulites surimposé etdes indices de fusion partielle. De plus, il est inféré que les grenatites représentent des produits d’altération hydrothermale desgneiss blancs a la lumière de la nature graduelle des contacts entre les deux types de roches. Les fourchettes de composition desminéraux sont généralement semblables a celles de métapélites du faciès des granulites, mais des teneurs modérément élevéesen Mn dans les grenats des grenatites et en Zn dans les spinelles des gneiss alumineux appuient l’interprétation de l’ajouthydrothermal de ces éléments au protolite. Les tendances d’altération les plus importantes sont des augmentations de Fe–Mg–Mn des gneiss blancs aux gneiss alumineux, puis au grenatites, ainsi qu’une tendance a l’accroissement de l’indiced’aluminium dans certains gneiss blancs, qui indiquerait une légère argilisation. Ces nouvelles données font ressortir lapréservation d’une altération hydrothermale précoce dans des ceintures de haute intensité de métamorphisme dans la provincede Grenville, et ces roches constituent des cibles d’exploration potentielles. [Traduit par la Rédaction]

IntroductionSubmarine volcanic environments and associated hydrothermal

activity commonly contain characteristic alteration patterns andgeochemical signatures, with the latter hydrothermally altered ar-eas being prime targets for mineral exploration (Large et al. 2001).These features are well established in pristine volcanic belts, buttheir identification is less obvious if they are reworked by a subse-quent orogeny. In such cases, tectonic processes result in variableloss of the original coherence and modification of the relations be-tween rock units. In addition, if metamorphosed under high-grade

conditions, the precursor rock types are transformed into gneisscomplexes where deformation, metamorphic recrystallization, andunder granulite-facies conditions, partial melting, lead to substantialoverprint of their original characteristics.

In the first place, distinction between unaltered felsic volcanicrocks or tuffs from granite is not obvious when these rocks aretransformed into gneisses. In addition, common types of hydrothermalalteration (e.g., argillic, sericitic, and ferromagnesian) in submarineenvironments lead to chemically modified rocks, the metamorphicproducts of which are commonly misinterpreted as metapelites

Received 16 August 2016. Accepted 29 January 2017.

Paper handled by Associate Editor Toby Rivers.

M. Hindemith, A. Indares, and S. Piercey. Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, NL A1B 3X5, Canada.Corresponding author: Aphrodite Indares (email: [email protected]).Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink.

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Can. J. Earth Sci. 00: 1–17 (0000) dx.doi.org/10.1139/cjes-2016-0146 Published at www.nrcresearchpress.com/cjes on 7 February 2017.

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(e.g., Hodges and Manojlovic 1993; Froese 1998). However, it hasbeen shown that in high-grade metamorphic belts, diagnostic fea-tures of volcanic protoliths and their hydrothermally alteredproducts may still be revealed by careful examination of rockassociations, petrography, and geochemistry (Gifkins et al. 2005;Bonnet and Corriveau 2007). For example, recrystallized fiamme,lapilli and bomb fragments have been recognized in amphibolite-to granulite-facies rock units of the Grenville Province (Bonnetand Corriveau 2007), and quartz xenocrysts have been identifiedin amphibolite- to granulite-facies gneisses of the Broken Hill area(Stevens and Barron 2002). In terms of mineralogy and bulk rockchemistry, features indicative of hydrothermal alteration in felsicgneisses include an unusual abundance of aluminous and (or)mafic minerals, the presence of aluminous nodules, and the pres-ence of atypical minerals in rock types where major elements arein excess or depleted with respect to the unaltered protoliths (seeBonnet and Corriveau 2007). Metamorphosed alteration zones aremostly documented in gneissic belts containing mineral deposits(e.g., Zaleski et al. 1991; Hodges and Manojlovic 1993; McFarlaneet al. 2007); however, in the absence of reported mineralization,they tend to be overlooked.

The Grenville Province, a Mesoproterozoic continental collisionorogen built from 1.09 to 0.98 Ga, upon the southeastern activemargin of Laurentia (Rivers et al. 2012), is a prime example of alarge hot orogen (Beaumont et al. 2006) that overprinted multiplearc systems. During the Grenvillian Orogeny, large tracts of theorogenic hinterland were metamorphosed to the granulite facies(Rivers 2008) and are presently exposed as gneissic belts. Mostinformation about the pre-Grenvillian evolution of the Lauren-tian margin comes from (meta) plutonic rock units. Recognizablevestiges of volcanic belts are mainly reported from low-grade,southern portions of the orogen (e.g., Montauban, Bernier andMacLean 1993; Composite Arc Belt, see review in Carr et al. 2000).Examples from high-grade metamorphic domains, which makethe vast majority of the province, are rare and less clear (e.g., SandBay gneiss association, Culshaw and Dostal 1997; Bondy Complex,Blein et al. 2003), but there are also cases with well-preservedfeatures of the protoliths and with a remarkable record of pre-metamorphic hydrothermal alterations along a transition fromthe amphibolite to the granulite facies (Wakeham Group and LaRomaine Belt, Bonnet et al. 2005; Corriveau and Bonnet 2005).

In the Manicouagan area of the hinterland in the central Gren-ville Province, a recently identified �1.25 Ga association of felsicand aluminous rocks within a layered felsic-dominated bimodalsequence (LBS) is inferred to represent hydrothermal alterationzones in an ancient volcanic belt, evidence of which is preserveddespite a Grenvillian mid-P (P, pressure) granulite-facies overprint(Indares and Moukhsil 2013). In contrast to the Wakeham – La Ro-maine case (Bonnet et al. 2005; Corriveau and Bonnet 2005), thissequence is enclosed in a granulite-facies domain, and there is notransition to lower grade metamorphic equivalents for comparison.

The aim of this contribution is to document felsic and alumi-nous rocks that may represent metamorphosed equivalents ofhydrothermally altered volcanic units of the LBS and to infer po-tential types of protoliths and tectonic environment of formation,the degree and type of alteration(s), and the effects of the meta-morphic overprint. This is accomplished by means of a detailedpetrological and geochemical investigation involving examina-tion of microstructures at a range of scales, mineral chemistry,and bulk rock geochemistry.

Geological settingThe Grenvillian hinterland in the Manicouagan area mainly

consists of Paleoproterozoic to Mesoproterozoic, 1.7–1.4 Ga litho-logical associations generally younging to the southeast, and also1.25 Ga rocks exposed in between the older units in the southernpart of the area (e.g., LBS and Banded Complex), as well as Gren-

villian age anorthosites and granite (Fig. 1; Indares and Dunning2004; Dunning and Indares 2010; Valverde Cardenas et al. 2012;Indares and Mouksil 2013). During the Grenvillian Orogeny, mostof these hinterland units were metamorphosed at granulite-faciesconditions, between �1.08 and 1.04 Ga (Dunning and Indares 2010;Lasalle and Indares 2014; Lasalle et al. 2014).

The layered bimodal sequence (LBS)The LBS, first described by Indares and Moukhsil (2013), is part

of the Canyon domain (Hynes et al. 2000; Dunning and Indares2010). Exposures were mainly reported from several locations alongthe southern branch of the Manicouagan Reservoir, amongst a maficvein complex and layered quartzofeldspathic rocks both of un-known affiliation and age (Fig. 1; Indares and Moukhsil 2013). Thisentire lithologic association ends to the south against a �1.5 Gametasedimentary sequence of quartzofeldspathic gneiss, metapelite,quartzite, marble and calc-silicate rocks known as the Complexede la Plus Value (PLV; Lasalle et al. 2013; Moukhsil et al. 2012), and�1.4 Ga mafic rocks (Dunning and Indares 2010).

The LBS primarily consists of felsic (dominant) and mafic layersup to a few meters thick. The mafic layers are heterogeneous,commonly preserve internal centimeter-scale compositional lay-ering, and are pervasively recrystallized into plagioclase, garnet,orthopyroxene, clinopyroxene, and hornblende-bearing mineralassemblages. The felsic layers occur as pink massive rocks, and(or) as white gneisses. The pink felsic layers (locations B–D; Fig. 1)have no ferromagnesian minerals (Fig. 2a) or contain millimeter-to centimeter-long biotite-rich elongated specks (Fig. 2b). Thewhite gneisses (locations A–E) have a “bleached” appearance. Theyare generally leucocratic with traces of garnet and biotite, butlocally contain thin concordant garnet–biotite–sillimanite–bearing(aluminous) layers (Fig. 2c). In addition, they locally grade intoaluminous gneisses (Fig. 2d), some varieties of which are nodular(Figs. 2e, 2f), and (or) into garnetite (Figs. 2g, 2h). Both the whiteand aluminous gneisses commonly contain concordant quartzveins and the nodular varieties have in addition sillimanite seamsand veins, up to 1 cm thick (Fig. 2e). Quartz veins are most wide-spread in location D.

Based on the overall good preservation of the LBS in the inves-tigated localities, it is inferred that they represent low-strain do-mains compared with much of the interior Grenville Province. Inaddition, at the outcrop scale, some aluminous gneisses containpods and layers rich in aluminous minerals alternating with lay-ers or domains rich in quartz and feldspar (Fig. 2d), but clearlydefined evidence of leucosome is lacking. This implies that theserocks would have been rheologically stronger than the perva-sively migmatitic metapelites of the PLV a few kilometers to thesouth (see fig. 2a in Lasalle and Indares 2014). However, aluminousgneisses of both the PLV and LBS have similar metamorphic as-semblages and ages (Lasalle and Indares 2014; Lasalle et al. 2014).

The protolith age of a nodular aluminous gneiss from the LBS(location B; Fig. 1) yielded a unique population of 1238 ± 13 Maslender igneous zircon prisms (U–Pb by laser ablation – inductivelycoupled plasma – mass spectrometry; Lasalle et al. 2013), whichsuggests a volcanic origin for this rock. On the basis of lithologicassociation, age, and correlation with another 1238–1202 Mafelsic–mafic layered sequence, dominated by pink felsic gneisses,exposed a few tens of kilometers to the northeast (Banded Com-plex; Dunning and Indares 2010; Fig. 1), the LBS is interpreted torepresent a metamorphosed supracrustal sequence composed ofGeon 12 volcanic rocks emplaced in an extensional crustal setting(Indares and Moukhsil 2013). In addition, the presence of garnetiteand nodular aluminous gneisses with igneous zircon is suggestiveof hydrothermal alteration prior to metamorphism (for similarcases elsewhere, see Bonnet and Corriveau 2007). An alternativeinterpretation that such rocks may be derived from paleosols (e.g.,Gall 1992) is not favored because of the good preservation of igne-ous zircons in the aluminous gneisses and the dominance of igne-


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ous rocks in the LBS. This contribution focuses on the felsic andaluminous rocks from the hydrothermally altered zones of the LBSand also discusses geochemical data from felsic rocks of the BandedComplex, for comparison.

PetrographyThe mineralogy and textures from a selection of aluminous

gneisses, felsic rocks (white and pink varieties), and garnetiteswere documented by, in addition to optical microscopy, false colourmineralogical maps produced by a scanning electron microscopeequipped with a mineral liberation analysis (SEM–MLA) software.This method allows for imaging the distribution of phases andmicrostructures at the thin-section scale (Fig. 3), and also estima-tion of modal percentages (Table 1). The images were acquiredwith a FEI Quanta 600 ESEM bearing a Roentec (now Bruker-AXS)XFlash 3001 SDD energy-dispersive X-ray (EDX) detector, and con-trolled by a JKTech MLA image analysis software (Gu 2003) at theBruneau Centre, Memorial University of Newfoundland. Analyti-cal conditions included an accelerating voltage of 25 kV, a beam

current of 10 nA, a 2 mm frame size (or horizontal field width,HFW), dwell time of 10 ms, and a step size of 50 �m.

Microstructures of key rock typesThe different types of rocks are petrographically distinguished

by the mineralogy of the felsic groundmass (quartzofeldspathicversus K-feldspar dominated) and by the proportions, type, andmicrostructural arrangements of aluminous minerals (garnet, bi-otite, sillimanite, and in some cases, spinel and corundum), wherepresent.

Felsic rocksThese rocks are dominantly medium- to fine-grained, and con-

sist of perthitic K-feldspar (with some microcline), quartz, andplagioclase. The “pink varieties” are massive, and contain rareresorbed relics of large antiperthite and (or) quartz. They have traceamounts of interstitial biotite, and in some cases, millimeter-scale elon-gate clusters of biotite–ilmenite ± garnet (Fig. 4a) that define afabric. The “white varieties” (white gneiss) are compositionally

Fig. 1. Geological framework of the Manicouagan area, central Grenville Province (Quebec), and location of the sampled sites. Modified afterDunning and Indares (2010) and Indares and Moukhsil (2013). BC, Banded Complex; CD, Canyon domain; HJT, Hart Jaune terrane; LBS, layeredbimodal sequence; PLV, Complexe de la Plus Value; QFU, layered quartzofeldspathic unit; VC, vein complex; A–E, locations discussed in thiscontribution. Inset, location of the Manicouagan area in the Grenville Province. [Colour online.]

Parautochthonous beltArchean/Paleoproterozoic

& 1.30

mangerite

PLV

Ga mafic sills

CD

1.7–

1.6

1.5–

1.4

1.2

5 BC

Anorthosite

1.17

granite

LBS

age (Ga)

AB

C D

E

C

mk 01

& 1.22

1.06 –1.02

VC & QFU

HJT

(a)

Hinterland

ParautochthonousComposite Arc belt

Ma

Belt

1.16–1.00 Ga Anorthosite


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layered, and contain minor garnet (some with sillimanite inclu-sions, and rarely, associated with fine-grained sillimanite clus-ters), as well as traces of large (up to 1 mm) rutile and monazite(Table 1). In addition, these rocks may display several millimeter-long, resorbed, and partly recrystallized quartz of lenticular shape(Figs. 3a, 4b), and (or) perthitic K-feldspar. The latter has inclusionsof idiomorphic quartz and is locally rimmed by myrmekite. Gar-net forms euhedral grains clustered or scattered in the ground-mass, and is locally replaced by biotite (Fig. 3a).

Aluminous gneissesMost complex microstructures are observed in the aluminous

gneisses. In these rocks, aluminous minerals (�10%–30%; garnet +

sillimanite + biotite; Table 1) are concentrated in nodules, seams,or in diffuse pods and layers, within a K-feldspar-rich (quartzo) feld-spathic groundmass along with millimetric-sized rutile. Some char-acteristic examples are described in the following subsections.

Gneisses with diffuse aluminous pods or layersThese rocks (e.g., Fig. 2d) consist of the following: elongate,

millimeter-scale clusters of biotite and (or) garnet porphyroblastsvariably overgrowing sillimanite, and mantled by plagioclase; par-tially recrystallized and resorbed quartz domains, some of whichhave an angular shape (Fig. 3b); and, locally, large resorbed perthiticK-feldspar, in a relatively fine-grained K-feldspar-rich groundmass

Fig. 2. Main rock types in the hydrothermally altered zones of the LBS: (a) pink massive felsic rock; (b) pink felsic rock with elongate clusters ofbiotite; (c) white felsic gneiss with discrete aluminous layers (darker colour) and nodules; (d) gneiss with diffuse aluminous layers; (e, f) aluminousgneiss with garnet nodules, concordant quartz veins (red arrows), and sillimanite seams (yellow arrow); (g, h) garnetite. [Colour online.]




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(locally microcline) with minor disseminated sulfides (mostly py-rite). The angular quartz domains are readily obvious in SEM–MLAimages, where they stand out from the K-feldspar-dominated ground-mass, but not under the optical microscope. In the latter, thesimilar colour, birefringence, and relief of quartz and feldspars,masks this phase distribution.

Aluminous gneisses with composite nodulesIn these rocks, the nodules are aligned along (millimeter-width), con-

cordant aluminous veins (Fig. 3c), or are isolated and elongatedparallel to the foliation (Fig. 3d). Generally, they show a zoneddistribution of minerals, with sillimanite ± spinel ± corundumenclosed in garnet variably overgrown by biotite (Figs. 3c, 3d, 4c,

Fig. 3. SEM–MLA false colour mineral maps of thin sections: (a) white gneiss with lenticular quartz (yellow arrow); (b) gneiss with diffusealuminous layers and an angular quartz domain outlined in white; (c, d) gneiss with composite aluminous nodules (white arrows point toareas of analyzed spinel); (e, f) gneiss with garnet nodules; the white patches associated with sillimanite seams in (f) represent holes in thethin section, and the light blue domains are sericitized feldspar. Scale: the horizontal bar on each image is 1 cm long.


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4d). The nodules are mantled by plagioclase, which becomes morealbitic towards the groundmass (Figs. 3c, 3d). Away from the nod-ules, the groundmass consists of relatively fine-grained perthiticK-feldspar, quartz, and minor plagioclase, randomly oriented in-terstitial biotite, as well as millimeter-sized euhedral garnet. Insome cases, quartz forms elongate, partly recrystallized and re-sorbed ribbons and lenses that are several millimeters long, ex-ceeding by far the average grain sizes in the groundmass. Theserocks also contain sulfides (mainly pyrite), which are in most casesrestricted to the nodules.

Aluminous gneisses with nodules of garnetThese rocks (e.g., Figs. 2e, 2f) are characterized by the following:

centimeter-sized garnet, ranging from skeletal clusters of fram-boidal grains (Figs. 3e, 4e) to coherent porphyroblasts (Fig. 3e);sillimanite seams (Figs. 3f, 4f); several millimeter-long partly re-crystallized and resorbed quartz ribbons (attenuated veins?), aswell as lenticular quartz domains (Figs. 3e, 3f); and large resorbedK-feldspar, in a fine- to medium-grained groundmass. Parts of thegarnet porphyroblasts are flanked by sillimanite seams with mi-nor biotite, and (or) thin albite rims (Fig. 3f). In addition, “patchy”clusters of garnet locally overgrow sillimanite (Figs. 3e, 4f). Thegroundmass mainly consists of perthitic K-feldspar, locally rimmed bymyrmekite, with subordinate plagioclase, minor quartz, intersti-tial biotite, and traces of pyrite.

In the aluminous gneisses, garnet porphyroblasts have inclu-sions (or embayments) filled with quartz (and less commonly sil-limanite, biotite, plagioclase) in a pool of K-feldspar (Fig. 4g), andare variably resorbed by biotite with quartz fingers (Fig. 4h). Inaddition, there are rare films of feldspar in the groundmass, andin between garnets. These features are mostly observed in thegneisses with diffuse aluminous pods, and also in the groundmassof those with composite aluminous nodules.

GarnetitesLocation A displays the best examples of “garnetization” of the

white gneisses (Figs. 2g, 2h, 5a–5d). The rocks classified as garnetitesrange from �20% to 71% garnet (Table 1), and are compositionallylayered with variable proportions of a quartz ± K-feldspar ± plagio-clase groundmass (Figs. 5a–5d). They also contain minor graphite,rutile, rare biotite, and those with the highest proportions ofgarnet, several millimeter-size monazite grains (Fig. 5d). Garnetoccurs as tightly packed euhedral grains with tiny inclusions ofquartz and plagioclase.

Interpretation of the microstructuresThe microstructures documented earlier in the text are a result

of a high-grade metamorphic overprint on earlier features, someof which are still recognizable.

Metamorphic microstructuresThe aluminous gneisses preserve the most complex and diag-

nostic mineral associations. In these rocks, the dominant mineralassemblage, garnet–sillimanite–biotite–quartz–K-feldspar–plagioclase,is typical of mid-P granulite-facies metamorphism. This assemblage isstable in the P–T (T, temperature) field of the vapour-absent meltingof biotite by the continuous reaction: biotite + sillimanite + quartz¡garnet + melt + K-feldspar (field II in Fig. 6; Spear et al. 1999). Evidenceof this reaction is recorded in the following form: (a) resorbedquartz ± sillimanite ± biotite (reactants) in films or pools of feld-spar, which are inferred to represent former melt, best preservedas inclusions and embayments in some garnet porphyroblasts(e.g., Fig. 4g; for this interpretation, see also Holness et al. 2011 andLasalle and Indares 2014); and (b) garnet (product) that overgrew sil-limanite (e.g., Fig. 4f), a feature present in all the aluminous rocks. Inaddition, local replacement of garnet by biotite ± quartz or feldspar(Fig. 4h) is consistent with a reversal of this reaction, during meltcrystallization.

The presence of sillimanite inclusions in garnet suggests thata large part of the prograde evolution of these rocks occurred inthe sillimanite stability field. This, together with the absence ofretrograde cordierite during melt crystallization is consistentwith the generalized P–T path shown in Fig. 6. This P–T path isbroadly similar to those predicted by P–T pseudosections calcu-lated for other aluminous rocks from the same area (Lasalle andIndares 2014).

Pre-metamorphic featuresDespite the granulite-facies metamorphic overprint, relict mi-

crostructures potentially diagnostic of earlier features are locallypreserved. For instance, the partly resorbed angular quartz do-mains observed in aluminous gneisses of location B (e.g., Fig. 3b)have a broadly bipyramidal shape, suggestive of quartz phenocrysts infelsic volcanic rocks (see Gifkins et al. 2005). In the same outcrop,an aluminous gneiss with garnet nodules was inferred to be ofvolcanic origin by Lasalle et al. (2013), based on zircon morphol-ogy; and the fine, elongate clusters of biotite–ilmenite ± garnetobserved in a pink massive felsic rock (Figs. 2b, 4a) are similar to

Table 1. Phase proportions (in %) of selected samples.

Type:

Aluminous gneiss White gneiss Garnetite

ACN AGN AD

Sample: 354-2-04 10-A1-76 339-11 333x-03 331E2-11 331-3-03 355a-11 355-b1-11 206a-04 216-d2-11 355-2a-04 355-c1-11 355-c2-11 355-c3-11 355-cx-11

Quartz 18.74 21.73 12.10 5.81 13.39 28.56 25.85 32.98 31.18 38.59 40.50 36.70 30.40 21.12 53.53K-feldspar 27.26 45.46 25.34 35.72 35.41 28.55 55.68 33.61 26.91 26.29 23.10 18.89 35.46 0.00 0.36Plagioclase 30.98 16.52 16.83 23.25 19.93 10.79 15.13 24.50 25.67 18.45 2.53 3.55 9.63 0.00 10.85Sum QF 76.98 83.71 54.27 64.78 68.73 67.90 96.66 91.09 83.76 83.33 66.13 59.14 75.49 21.12 64.74

Garnet 6.09 4.44 22.97 19.05 3.28 4.50 0.70 3.91 10.71 9.00 28.56 32.40 22.62 69.49 22.51Biotite 10.97 4.86 5.65 1.91 17.11 12.41 0.12 0.10 0.27 1.40 0.17 0.13 0.12Sillimanite 0.48 2.66 1.16 0.53 0.32 0.03Spinel 0.16 0.23Corundum 0.14Sum Al 17.84 9.53 31.28 22.12 20.92 17.23 0.85 4.01 10.98 10.40 28.73 32.40 22.62 69.62 22.63

Sulfide 0.06 0.15 0.07 0.08 0.54 0.44 0.00 0.01 0.03 0.05 0.06 0.03 0.02 0.14 0.23Graphite 0.68 2.89 0.04 0.12 2.72Ilmenite 0.15 0.15 1.63 0.40 0.34 0.61 0.03 0.19 1.09 0.19 0.61 1.49 0.15 3.38 0.94Rutile 0.10 0.04 0.16 0.39 0.02 0.14 0.01 0.27 0.07 0.25 0.21 0.16 0.26 0.17 0.05Monazite 0.02 0.01 0.02 0.02 0.01 0.03 0.00 0.03 0.02 0.01 0.01 0.02 0.02 0.45 0.00Magnetite 0.01 0.01 0.32 0.00 0.05 0.03 0.00 0.01 0.01 0.03 0.06 0.16 0.02 0.49 0.14Apatite 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.03 0.64 0.01 0.02 0.14 0.13 0.01

Note: ACN, gneiss with composite aluminous nodules; AGN, gneiss with garnet nodules; AD, gneiss with diffuse aluminous pods–layers; Sum QF, sum ofquartzofeldspathic phases; Sum Al, sum of aluminous minerals.




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the biotite aggregates interpreted by Stevens and Barron (2002) aspotentially derived from altered fiamme or lapilli in the HoresGneiss of the Broken Hill district. Finally, flattened angular, orlenticular quartz domains, as well as large K-feldspar relics locally

present in white gneisses in all locations, may also represent for-mer phenocrysts in a volcanic host.

A characteristic feature of the aluminous gneisses is the nodu-lar or veinlike distribution of the aluminous minerals. The best

Fig. 4. Photomicrographs of key microstructures: (a) biotite–ilmenite–garnet clusters in the pink felsic unit (plane polarized light); (b) largelenticular quartz, partially recrystallized and resorbed, in a white gneiss (cross-polarized light); (c–h) aluminous gneisses: (c) sillimanite, spinel,and corundum in the center of a composite nodule (cross-polarized light); (d) garnet nodule with spinel inclusions (plane polarized light);(e) clusters of framboidal garnet overgrowing a K-feldspar groundmass (plane polarized light); (f) sillimanite seam partly overgrown by garnet(cross-polarized light); (g, h) garnet porphyroblasts with polymineralic inclusions of sillimanite, biotite, plagioclase, and quartz in pools offeldspar (cross-polarized light). bi, biotite; crn, corundum; grt, garnet; ilm, imenite; Kfs, K-feldspar; qtz, quartz; sil, sillimanite; spl, spinel.[Colour online.]


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examples are the gneisses with garnet nodules and sillimaniteseams (e.g., Fig. 2e), which also have abnormally high concentra-tions of K-feldspar in the groundmass (e.g., Fig. 3f), and the gneisseswith composite nodules, which display a strong mineralogicalzonation from core to rim (e.g., Fig. 3c). The most aluminousphases (spinel and corundum) present at the centers of these nod-ules are not considered as part of the metamorphic assemblage,but are attributed to a chemical potential gradient of Al. The samegradient also accounts for the zonation in the quartzofeldspathicgroundmass of these samples, with plagioclase mantling the nod-ules and becoming more albitic (e.g., less aluminous) towards thegroundmass.

Collectively, these microstructures are suggestive of local Al,Fe–Mg, and K enrichments, which are consistent with signaturesseen in submarine hydrothermal environments due to hydrothermalfluid – rock reaction (Hannington et al. 2003; Bonnet et al. 2005;Bonnet and Corriveau 2007; Galley et al. 2007). In addition, ahydrothermal input of Fe–Mg–Mn is inferred for the gradual gar-netization of the white gneisses observed in several localities(Figs. 2g, 2h).

Mineral chemistryThe compositional ranges of garnet, biotite, spinel, and feldspars

from selected samples of aluminous gneisses, white gneisses, andgarnetites of the LBS are listed in Tables 2–6, and key parameters

are graphically represented in Fig. 7. In addition, representativecompositions of these minerals are provided in supplementaryTables S1–S41. The data were acquired with a JEOL JXA-8200 EPMAat the Department of Geosciences, University of Calgary, and witha JEOL JXA 8230 EPMA at Memorial University, both in wavelengthdispersive mode, with an accelerating voltage of 15 kV, a beamcurrent at 20 nA, beam diameter of 1 �m, and using natural stan-dards.

Garnet is Fe rich with generally high Mg and variable Mn, and Cacontents (Table 2; Fig. 7a). In individual samples, garnet is chem-ically homogeneous, except for an increase in almandine and adecrease in pyrope at rims against biotite (Fig. 7b). However, thereare compositional variations in garnet both within and betweenthe different rock types. In the aluminous and the white gneisses,spessartine content of garnet is inversely correlated with theproportion of garnet in the rock. In contrast, garnet in the garne-tites show distinctively higher spessartine contents than in therest of the rocks (4%–16% versus 1%–4%), and one sample has mark-edly high Ca (grossular and andradite up to 13%).

Biotite (included in garnet, adjacent to garnet, and scattered inthe groundmass) was analyzed in the aluminous and white gneissesonly (Table 3). In individual samples, biotite in the groundmass hasgenerally highest Ti contents and lowest XMg (Fig. 7c), but there arenot distinctive differences in the composition of biotite betweenthe different rock types. Spinel (only present in the cores of com-posite aluminous nodules; Figs. 3c, 3d) is hercynite rich with lesser

1Supplementary data are available with the article through the journal Web site at http://nrcresearchpress.com/doi/suppl/10.1139/cjes-2016-0146.

Fig. 5. SEM–MLA false colour mineral maps illustrating the increasing proportion of garnet in the white gneisses and the transition togarnetite: (a) white gneiss with minor garnet; (b) white gneiss with garnet-rich layers; (c, d) garnetites; green, monazite. Pl, plagioclase.Horizontal bar: 1 cm.

Grt

Qtz

K-fs

Pl

(b)

(c)

(d)

white gneiss

garnetite

(a)




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http://nrcresearchpress.com/doi/suppl/10.1139/cjes-2016-0146

proportions of spinel and gahnite (Table 4; Fig. 7d). In specificnodules, the analyzed spinel has uniform chemistry. However, inone sample, the composition of spinel displays a marked differ-ence between nodules, with gahnite proportions as high as 20%where it occurs with sillimanite and corundum, versus �10% else-where (Fig. 3c, nodule 1 and 2, respectively).

The composition of K-feldspar in the different rock types over-laps (Table 5; Fig. 7e). In addition, it includes detectable Ba con-tents that are highest in the aluminous gneisses and garnetites(BaO up to 0.47–0.88 wt.% versus 0.32–0.58 wt.% in the whitegneisses). Plagioclase is albite rich, with highest albite concentra-tions in the aluminous gneisses (68%–90% versus 64%–75% in therest), and shows a systematic increase in anorthite towards garnet.This pattern is most marked towards the aluminous nodules, as alsoillustrated in some SEM–MLA mineral maps (see Figs. 3b, 3c).

Interpretation of mineral chemistryIn metamorphic rocks, the composition of solid solution min-

erals depends upon both the P–T conditions of metamorphismand the bulk rock composition. Generally, the composition ranges ofthe analyzed minerals in terms of major components, as well asthe observed compositional variations of garnet and biotite at thesample scale, are typical of those in granulite-facies aluminousrocks. For instance, the observed chemical zoning of garnet (Fig. 7b)is indicative of diffusional homogenization under high-T meta-morphic conditions followed by partial re-equilibration of the rimsby Fe–Mg exchange with biotite during retrogression. Along thesame lines, the variations in the composition of biotite (Fig. 7c) areconsistent with more extensive retrograde resetting of biotite as-sociated with garnet, relative to that in the groundmass (Spear

et al. 1999). However, the relatively high Mn contents in garnetfrom the garnetites point to some contribution of hydrothermalfluid in the bulk rock chemistry. The same may hold for the Zncontent of spinel, although this is less conclusive, as the compo-sitions of the analyzed spinel fall in the wide field of unaltered andhydrothermally altered Fe–Al supracrustal rocks (compare Fig. 7dwith fig. 6 in Heimann et al. (2005). Finally, the Ba concentrationsin K-feldspar may also provide an indication of hydrothermal alter-ation, as Ba-enrichment is common during seafloor hydrothermalfluid – rock interaction.

Geochemistry

Field observations and petrography suggest that (a) the aluminousgneisses and the garnetites of the LBS represent hydrothermally al-tered equivalents of the white gneisses, and (b) the pink rocks are theleast-altered felsic components of this sequence, although theirrelationship to the white gneisses is unclear. This section focuseson the geochemical signatures of the felsic and aluminous rocksof the LBS, excluding those with composite nodules owing todifficulty in selecting representative rock volumes for analysis. Inaddition, data from the pink felsic layers of the Banded Complex(Fig. 1) are included for comparison. These are outside the zones ofhydrothermal alteration and have the same protolith age as thenodular gneisses of the LBS (Lasalle et al. 2013); therefore, theirgeochemistry may best represent the original environment ofdeposition.

Bulk rock lithogeochemical data are listed in Table 6. The datawere acquired by Actlabs (Ancaster, Ontario) using the combinationpackage Code 4B. Most aluminous gneisses and garnetites arecharacterized by low SiO2 contents relative to those of the felsicrocks (�48–65 wt.% versus 66–82 wt.%). Overall, Al2O3 (�10–21 wt.%)

Fig. 6. Schematic P–T diagram showing reactions involving melt in the Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O (NKFMASH) system (modifiedafter Spear et al. 1999) and inferred P–T path for the LBS. II, P–T field of the continuous reaction: biotite + sillimanite + quartz ¡ garnet +melt + K-feldspar (for rocks with bulk compositions on the left side of the AFM (alumina index – FeO – MgO) diagram associated with thisfield). And, andalusite; As, aluminosilicate; Bt, biotite; Crd, cordierite; Grt, garnet; Kfs, K-feldspar; L, liquid; Ms, muscovite; Opx, orthopyroxene;Qtz, quartz; Sil, sillimanite; V, vapour. 1 kbar = 100 MPa. [Colour online.]

NKFMASH

Bt G

rtO

px A

s L

Bt G

rtOp

x Cr

d L

Bt As

Grt Crd

L

Ms V

As L

Ms

Kfs

As L

Ky

+Qtz+Kfs +L

As

Sil

Sil

And

Grt

Crd

Bt

A

F M

II

P (k

bar)

T ( C)o

12

10

6

4

2

500 600 1000900800700


Hindemith et al. 9


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and Fe2O3T (total iron as Fe2O3, �0.5–27 wt.%) are inversely corre-

lated with SiO2, having highest values in the garnetites and the nod-ular aluminous gneisses. Aluminous gneisses from all locationshave the highest Cr concentrations (90–270 ppm) and those fromlocation C are the richest in K2O, TiO2, and Zn. The garnetites havedistinctively high MnO, up to �3 wt.% in the samples with thehighest proportions of garnet. In addition, the most garnet-richsample has the highest concentrations of CaO, Y, Sc, and Zn. Onthe Na2O versus Al2O3/Na2O diagram (Spitz and Darling 1978),most samples fall on the fresh to weakly altered field except forthe garnetites, which show a marked Al2O3 enrichment and Na2Odepletion (Fig. 8a).

Potential protoliths of the investigated rocks may be inferred bytheir immobile trace-element signatures. In terms of Nb/Y versusZr/TiO2 ratios, the large majority of the LBS rocks are classified asandesite, in contrast to the felsic layers of the Banded Complex,which have distinctively higher Zr/TiO2 and fall in the rhyolitefield (Fig. 8b). In most cases, the Zr/Y ratios are consistent with acalc-alkaline to transitional character (Fig. 8d). In tectonic discrim-ination diagrams, all rock types are clustered in the volcanic arc(and syncollisional) fields straddling the within-plate fields (Fig. 8c),

which, given the geological setting of these rocks on the active mar-gin of Laurentia, is consistent with formation during extension ofarc-related crust (see also Dunning and Indares 2010; Indares andMoukhsil 2013).

Chondrite-normalized rare-earth element (REE) plots are shownfor specific rock types or associations and specific locations inFig. 9. The pink felsic gneisses, white gneisses, garnetites, andaluminous gneisses all exhibit distinctive REE patterns. The pinkfelsic gneisses from both the Banded Complex and the LBS showdifferent degrees of enrichment in light REEs, negative Eu anom-alies, and overall slopes of the REE patterns consistent with crust-derived melts (Figs. 9a, 9b). The white gneisses have a cumulatesignature characterized by positive Eu anomalies (Figs. 9c, 9d), while thegarnetites display negative Eu anomalies and variable enrichmentin heavy REEs (Fig. 9c), which is directly correlated with the pro-portions of garnet in these rocks. Except for Eu, the garnetiteshave higher REE contents than their precursor white gneisses.Finally, the aluminous gneisses have no Eu anomalies and similarto higher contents of the other REEs compared with those of thewhite gneisses (Figs. 9d, 9e).

Alteration trendsFigure 10 shows ternary plots commonly used to assess chemi-

cal variation trends potentially linked to alteration. The white

Table 2. Compositional ranges of garnet.

Sample Typea Alm Prp Sps Grs + And XFe

354-2-04 ACN06 70–77 15–25 2–3 3–4 0.74–0.8210-AI-76 ACN05 64–67 23–25 4–5 4–6 0.72–0.75339x-11 AGN23 56–65 31–40 1 2–7 0.58–0.67333x-03 AGN19 62–65 33–36 1 1–2 0.63–0.66331E2-11 AD03 57–70 23–36 1–3 3–4 0.60–0.75355a-11 WG01 59–63 30–33 3–4 3–4 0.64–0.68206a-04 WG11 66–70 21–25 1–2 6–8 0.73–0.77355-2a-04 G29 66–68 20–22 6–7 5–6 0.75–0.77355c2-11 G23 52–54 26–28 15–16 4–5 0.65–0.67355c3-11 G23 62–65 14–16 9–10 11–13 0.78–0.81355cx-11 G70 56–59 32–34 4–5 4–6 0.63–0.65

Note: Alm, almandine; Prp, pyrope; Sps, spessartine; Grs, grossular; And,andradite; XFe = Fe2+/(Fe2+ + Mg).

aRock types: ACN, gneiss with composite aluminous nodules; AGN, gneisswith garnet nodules; AD, gneiss with diffuse aluminous pods–layers; WG, whitegneiss; G, garnetite. Number following rock type refers to the modal; % of garnet,from Table 1.

Table 3. Compositional ranges of biotite.

Sample Type XMg Ti Alvi

354-2-04 ACNIG 0.61–0.66 0.29–0.41 0.36–0.60AG 0.57–0.60 0.34–0.43 0.48–0.66M 0.51–0.53 0.55–0.65 0.28–0.34

10-A1-76 ACNIG 0.52–0.61 0.57–0.68 0.26–0.34AG 0.55–0.61 0.49–0.59 0.25–0.40M 0.50–0.60 0.55–0.68 0.21–0.34

339x AGN 0.50–0.79 0.20–0.36 0.40–0.58333x-03 AGN 0.50–0.62 0.44–0.52 0.41–0.53331E2-11 AD

IG 0.55–0.57 0.37–0.42 0.48–0.60AG 0.54–0.64 0.23–0.44 0.40–0.64M 0.55–0.60 0.33–0.45 0.37–0.75

216d2-11 WGAG 0.59–0.70 0.33–0.56 0.22–0.33M 0.65–0.72 0.39–0.56 0.21–0.42

Note: XMg = Mg/(Fe + Mg). Ti and Alvi are on the basis of22 oxygens. ACN, gneiss with composite aluminous nod-ules; AGN, gneiss with garnet nodules; AD, gneiss withdiffuse aluminous pods–layers; WG, white gneiss; IG, in-cluded in garnet; AG, adjacent to garnet; M, matrix(groundmass).

Table 4. Compositional ranges of spinel in specific alu-minous nodules.

Sample Nodule Spl Hc Gah Mag

354-2-04 1 23–24 55–57 20–22 12 24–25 62–64 10–12 1

10-AI-76 1 28–31 60–62 8 1–22 30–31 60–2 6–7 2

Note: Hercynite (Hc) = [Fe2+ − (Fe3+/2)]100; Gah, gahnite; Mag =(Fe3+/2)100; Fe3+ was calculated from stoichiometry.

Table 5. Compositional ranges of feldspars.

Sample Type

Plagioclase K-feldspar

Ab Or Or Ab BaO (wt.%)

354-2-04 ACNAG 71–86 1–2M 81–87 1 77–91 9–24 0.20–0.47

10-A1-76 ACNAG 73–76 1–2M 75–77 1–2 76–92 8–19 0.30–0.54

339x AGN 80–98 1 85–90 9–15 0.45–0.71333x-03 AGN 88–93 1–2 93–94 5–7 0.25–0.34331E2-11 AD

AG 80–3 1M 79–90 1 80–91 8–20 0.30–0.74

355A-11 WGAG 72–76 1M 78–79 1 80–86 11–14 0.20–0.32

206a-04 WGAG 67–70 1M 69 1 82–94 5–18 0.42–0.58

2163d2-11 WG 73–74 1 91–93 8–9 0.29–0.43355c2-11 G

AG 69–75 1M 69–74 1 80–89 11–20 0.35–0.61

355cx-11 G 69–73 2–3355-2a-04 G 83–92 6–18 0.25–0.33

Note: ACN, gneiss with composite aluminous nodules; AGN,gneiss with garnet nodules; AD, gneiss with diffuse aluminous pods–layers; WG, white gneiss; G, garnetite. For plagioclase only: AG, adja-cent to aluminous domains; M, matrix. Ab, albite; Or, orthoclase. Badetection limit: 360 ppm.




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gneisses, aluminous gneisses, and garnetites are clearly separatedon the Al2O3–CNK–F= diagram (Fig. 10a), where their distributionforms a well-defined trend from feldspar towards the F= apex. Thisimplies Fe + Mg + Mn enrichment in the altered rocks, and is mostprominent in the garnetites. The A–K2O–F=diagram (Fig. 10b) shows asimilar trend, and in addition, a separate group defined by thesillimanite-bearing white gneisses. These plot closer to the A–K2Oside of the triangle and towards the K2O apex, consistent withsericitic to argillic types of alteration.

Finally, on the alumina index – FeO – MgO (AFM) diagram(Fig. 10c), all the data plot on the FeO side. The aluminous gneisses,the garnetites, and some of the white gneisses cluster away from

the A apex, around the compositional field of garnet, with thegarnetites and the aluminous gneisses with garnet nodules be-ing more Fe rich than the rest. In contrast, the white gneissesdefine a vector towards the A apex, with the sillimanite-bearingsamples plotting closest to A, consistent with an argillic-typealtered protolith. In addition, this type of alteration may havebeen concealed in the aluminous nodular gneisses because ofsampling issues: the analyzed portions of the gneisses withgarnet nodules excluded large sillimanite seams (e.g., Fig. 2e),and there are no data for the aluminous gneisses with compos-ite nodules, due to difficulties choosing a representative rockvolume.

Table 6. Bulk rock chemistry.

Aluminous gneiss White gneiss

Type: AGN AGN AD AD AD AD AD AD WG-AD WG-AD

Sample: 331-1 331-2 331E2-11 216z-04 333-1 331-E1 216 216-3D 216-d1A 216-d1B 361A-11 355A-11 355b1-11 355-B2 355cxB-11 2163d2-11 334a-04

SiO2 (wt.%) 62.91 57.91 57.03 69.62 59.34 55.89 61.08 68.11 76.22 71.69 70.74 72.82 72.26 75.09 66.44 68.26 73.42TiO2 0.80 0.87 0.95 0.72 1.13 0.87 0.86 0.76 0.64 0.62 0.32 0.03 0.63 0.08 0.45 0.80 0.21Al2O3 18.52 19.83 17.74 13.96 17.91 20.94 18.49 14.11 10.85 14.31 14.70 14.74 14.03 14.17 18.07 14.39 13.34Fe2O3

T 6.46 9.91 7.96 6.06 9.12 7.78 7.07 6.69 4.48 4.16 1.82 0.58 3.95 0.97 3.83 6.15 1.37MnO 0.07 0.10 0.07 0.05 0.16 0.07 0.08 0.07 0.06 0.05 0.02 0.02 0.08 0.03 0.21 0.06 0.05MgO 1.76 2.50 3.24 2.22 3.20 3.55 2.94 2.41 1.74 1.76 0.62 0.11 0.68 0.21 0.86 2.31 0.26CaO 0.81 0.50 0.78 1.00 1.59 1.08 1.17 1.04 1.63 1.89 1.85 1.26 1.27 0.89 1.65 1.06 1.14Na2O 4.38 3.58 2.85 2.06 3.84 2.94 2.83 2.03 2.02 3.26 3.09 3.01 2.49 2.23 3.21 2.00 3.18K2O 4.61 5.16 6.40 4.25 3.86 6.69 3.50 3.81 1.34 2.06 4.67 5.69 4.34 6.02 5.63 4.43 5.21P2O5 0.05 0.05 0.02 0.10 0.03 0.05 0.07 0.09 0.09 0.03 0.04 0.06 0.14 0.05 0.06 0.10 0.06LOI 0.01 0.19 1.29 0.84 0.22 0.68 1.68 0.57 0.21 0.69 0.31 0.16 0.15 0.25 0.28 0.87 0.42Total 100.40 100.60 98.35 100.90 100.40 100.60 99.77 99.69 99.27 100.50 98.18 98.47 100.00 99.97 100.70 100.40 98.66

Co (ppm) 142 100 99 256 199 74 171 166 216 163 105 97 87 143 85 110 543Cr 120 240 150 90 150 160 230 270 90 100 10 10 10 120 10 80 10Cu 5 20 60 30 10 5 40 20 5 10 10 5 5 5 5 20 5Ni 10 20 70 70 30 50 40 70 20 40 10 10 10 10 10 50 10Sc 15 23 21 15 33 16 18 17 9 10 6 2 9 3 10 15 5Be 0.5 0.5 0.5 0.5 0.5 1.0 1.0 1.0 1.0 2.0 1.0 1.0 0.5 0.5 1.0 0.5 1.0V 108 134 165 110 172 182 134 112 80 80 37 6 33 6 39 117 10Zn 40 50 120 40 70 110 90 40 40 50 15 15 30 15 50 40 15Ba 992 1040 1117 639 982 1410 462 658 270 355 1288 693 803 709 1773 687 1360Ga 24 25 24 18 24 31 24 18 16 17 15 13 15 15 17 17 15Ge 2 3 2 1 2 2 2 1 1 1 1 1 1 1 2 1 1As 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5Ag 0.6 0.5 1.2 0.25 1.2 0.5 0.5 0.5 0.25 0.25 0.5 0.25 1.6 0.25 0.6 0.25 0.25In 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Sn 1.0 0.5 0.5 1.0 0.5 0.5 1.0 3.0 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1.0 3.0Sb 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 2.10Cs 0.25 0.25 1.3 1.0 0.25 2.6 1.1 0.9 0.8 1.0 0.25 0.25 0.25 0.25 0.25 1.00 0.25Rb 98 124 177 101 122 213 120 110 59 84 103 115 90 154 109 103 102Sr 115 68 230 76 316 153 67 85 94 146 250 144 119 108 291 78 248Th 15.2 17.9 10.1 12.6 9.3 24.7 15.8 11.8 8.1 0.9 17.4 0.05 33.7 2.1 4.9 11.2 34.5U 1.8 2.2 1.3 2.9 0.9 2.5 2.4 2.8 1.8 0.4 1.6 0.2 1.4 0.4 0.8 2.8 2.2Zr 253 233 290 259 502 213 228 243 201 148 192 27 478 22 220 238 170Hf 5.9 5.3 7.8 6.4 12.2 4.8 5.2 5.5 4.3 3.0 5.0 0.9 11.6 0.6 6.2 6.2 5.4Nb 16 14 14 11 22 13 10 9 7 6 10 0.5 18 2 22 11 7Mo 3 2 1 1 14 8 2 5 1 3 1 1 1 1 1 1 1Ta 3.6 2.5 1.8 2.5 2.6 1.7 3.6 3.5 3.6 2.6 2.8 1.9 2.2 3.4 2.7 2.3 0.6Tl 0.4 0.5 0.8 0.6 0.6 1.3 0.6 0.6 0.3 0.4 0.7 0.7 0.6 0.8 0.7 0.6 0.5Pb 21 13 52 10 21 26 10 9 8 13 26 36 26 35 35 8 61Bi 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2Y 29 40 31 23 57 15 28 29 21 15 26 11 44 23 32 20 48La 64.0 71.1 45.4 39.4 53.8 84.5 56.7 47.0 32.9 7.6 67.0 10.0 151.0 10.5 37.0 37.7 80.6Ce 126 141 75.7 78 98 158 110 94.1 68.4 13.4 130 17.8 319 19.9 63.5 77 154Pr 14.1 16 8 9.22 10.3 18.4 12.4 10.9 7.49 1.42 14.6 1.96 36.4 2.31 6.46 9.07 15.8Nd 49.7 56.8 28.8 36 35 64.2 44.4 38.9 27.2 5.1 54.5 7.1 131 8.5 22.4 34.3 52.7Sm 8.5 9.9 5.0 6.6 6.1 10.3 7.4 7.1 5.0 0.9 9.8 1.3 19.7 2.0 4.3 6.2 8.1Eu 1.75 1.65 2.1 1.33 2.26 1.89 1.54 1.43 1.12 1.23 2.73 1.76 1.59 1.45 2.79 1.27 1.2Gd 6.3 7.8 5.2 5.2 6 6.8 5.7 5.7 4.1 0.7 7.2 0.9 12.1 2.1 5 4.6 5.8Tb 0.9 1.2 0.9 0.8 1.2 0.8 0.9 0.9 0.6 0.2 1.0 0.2 1.6 0.4 0.9 0.7 1.0Dy 5.5 7.6 5.6 4.5 9.2 3.8 5.1 5.4 3.8 1.8 4.9 1.4 8.2 3.1 5.4 3.6 6.5Ho 1.0 1.5 1.2 0.9 2.3 0.6 1.1 1.1 0.7 0.5 0.9 0.4 1.5 0.7 1.1 0.8 1.6Er 3.2 4.6 3.6 2.5 8.2 1.7 3.2 3.3 2.2 1.6 2.4 1.2 4.4 2.1 3.2 2.4 6.0Tm 0.48 0.71 0.58 0.38 1.48 0.23 0.51 0.49 0.33 0.27 0.31 0.21 0.65 0.36 0.51 0.37 1.15Yb 3.2 4.6 3.9 2.5 12.2 1.5 3.4 3.3 2.1 2.0 1.9 1.4 4.1 2.6 3.4 2.7 8.1Lu 0.51 0.70 0.65 0.4 2.25 0.24 0.59 0.52 0.34 0.30 0.31 0.22 0.64 0.4 0.57 0.43 1.24

Note: Fe2O3T, total iron as Fe2O3; LOI, loss on ignition.


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It is worth mentioning that because the investigated rocks haveplagioclase, the AFM diagram in Fig. 10c differs from those pub-lished elsewhere that are valid for plagioclase-free rocks (e.g.,Bonnet and Corriveau 2007). The alumina index A for Fig. 10c iscalculated as Al2O3 – (K2O + Na2O + CaO) instead of A = Al2O3 – K2Oto exclude the Al2O3 content of plagioclase (which cannot berepresented on the AFM space) from the “effective” bulk Al2O3.Without this correction, plagioclase-bearing rocks would plot ar-tificially close to the A apex along trends partly controlled by theproportions of plagioclase, which could be mistaken for variabledegrees of argillic alteration.

Summary and discussionIn the central Grenville Province, 1.2 Ga old layered felsic-

(dominated) – mafic lithologic associations known as the LBSand the Banded Complex are intermittently exposed amongolder units (Fig. 1), and are inferred to have formed in an exten-sional setting (Dunning and Indares 2010; Indares and Mouksil2013). In the LBS, felsic rocks are pink and white gneisses withrespective geochemical signatures of rhyolite and andesite. Thelatter rocks locally grade into aluminous gneisses or garnetites.The gradual transition between these rock types suggests that the

White gneiss Garnetite Pink gneiss Pink gneiss, Banded Complex

333 334 354 10-A1-87B 355cxA-11 355cxC-11 355c2-11 355c3-11 339b1-11 339-B2 2161b-11 216e-11 08-03-G1 08-03-G2 08-03-G3 08-03-G4 08-03-G5 08-03-G6

78.09 73.95 69.37 76.14 64.74 77.07 61.93 47.86 77.63 78.43 81.67 71.53 77.46 73.94 79.88 72.88 71.48 70.830.19 0.24 1.00 0.01 0.35 0.27 0.55 0.34 0.15 0.15 0.10 0.54 0.15 0.23 0.12 0.50 0.59 0.5910.72 13.50 14.13 13.85 13.46 11.32 15.53 17.20 11.36 11.35 9.69 13.78 11.70 11.99 10.75 12.81 13.23 12.411.69 1.86 5.37 0.28 14.56 5.42 10.85 27.07 1.29 1.10 1.43 3.58 2.11 2.81 1.75 3.42 3.92 4.490.05 0.04 0.07 0.01 0.80 0.29 2.47 3.15 0.02 0.01 0.01 0.09 0.05 0.09 0.05 0.07 0.07 0.070.54 0.35 0.90 0.06 3.26 1.22 2.42 2.96 0.13 0.14 0.09 0.76 0.13 0.18 0.11 0.17 1.21 0.251.48 1.21 2.30 1.92 1.22 1.63 0.97 3.36 0.54 0.66 0.45 1.65 0.56 0.64 0.53 1.34 2.25 1.482.16 2.89 1.91 2.85 1.05 2.01 1.22 0.02 3.16 3.46 1.78 3.10 2.98 2.65 2.88 3.50 4.15 3.523.52 5.38 4.56 4.72 0.63 0.37 4.03 0.06 4.46 3.99 4.74 4.70 5.54 5.59 4.39 5.40 2.93 4.790.01 0.05 0.26 0.07 0.05 0.05 0.15 0.10 0.01 0.01 0.01 0.16 0.02 0.04 0.03 0.05 0.09 0.080.29 0.19 −0.24 0.22 −0.03 0.09 −0.45 −1.49 0.16 0.11 0.21 0.22 0.14 0.21 −0.15 0.56 −0.39 0.0398.72 99.66 99.65 100.10 100.10 99.73 99.66 100.60 98.92 99.39 100.20 100.10 100.80 98.36 100.30 100.70 99.53 98.55

150 135 105 285 191 142 164 199 122 160 128 91 0.5 0.5 0.5 0.5 8.0 0.590 40 10 10 10 10 20 10 10 10 10 10 10 10 10 10 10 105 5 5 5 10 5 5 5 5 5 5 5 40 30 20 5 20 3010 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 20 108 4 10 2 33 8 16 84 2 2 0.5 7 2 6 2 10 6 110.5 2.0 1.0 1.0 0.5 1.0 0.5 0.5 2.0 1.0 0.5 1.0 0.5 0.5 0.5 0.5 2.0 0.59 19 53 0.5 91 34 66 66 8 8 11 12 8 17 9 6 49 1315 15 60 15 130 50 110 140 15 15 15 70 15 15 15 50 80 701817 1240 1291 2296 183 71 1214 39 191 481 1335 1477 225 273 286 1168 345 116710 16 20 11 10 12 15 16 13 14 10 18 17 16 14 21 20 221 1 1 0.5 5 3 4 6 1 1 1 2 1 0.5 0.5 2 0.5 12.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 15.0 2.5 2.50.25 0.25 1.5 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.80 1.40 0.25 0.25 0.25 0.25 0.25 0.250.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.10.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.50.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 1.5 0.25 0.8 0.7 1.00.25 1.1 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.2598 212 150 71 12 6 98 2 125 122 89 106 107 105 84 57 75 58191 236 183 471 62 107 201 1 29 107 304 123 57 74 58 81 121 971.2 31.7 4.3 0.4 8.2 18.5 11.2 32.9 20.8 18.1 6.7 9.5 5.4 5.4 2.8 10.0 6.3 12.40.3 2 0.7 0.3 1.1 1.6 1.3 2.7 1.6 1.0 0.7 0.6 0.9 0.7 0.4 0.9 0.5 1.2138 178 615 52 166 120 183 53 141 108 248 453 328 636 293 993 328 11503.4 4.5 13.6 1.3 4.4 3.2 4.8 1.7 4.8 3.5 7.7 12.0 11.3 16.9 9.5 21.9 9.2 25.02 6 19 2 15 9 11 12 13 6 1 11 6 6 2 6 11 161 5 1 1 2 1 1 1 1 1 1 1 0.1 0.1 0.1 0.1 0.1 3.02.1 2.1 2.7 7.5 3.0 2.8 2.3 2.6 2.7 3.5 2.2 1.6 0.2 0.1 0.5 0.5 0.4 0.40.5 0.9 0.7 0.5 0.05 0.05 0.7 0.05 0.6 0.4 0.4 0.6 0.4 0.5 0.3 0.2 0.4 0.215 29 19 38 6 7 28 2.5 16 16 16 20 14 18 11 8 14 60.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.235 33 38 4 108 34 39 414 27 15 8 45 15 59 45 32 27 3721.6 88.2 63.8 6.3 37.8 74.7 52.4 94.5 44.7 51.1 30.1 89.3 48.8 66.3 34.3 192.0 53.0 207.038.1 169 138 10 77.5 159 114 188 93.8 104 64.4 187 94.9 131 71.4 391 104 4093.34 17.7 17.1 1.02 9.09 18.7 12.7 21.7 10.4 11.3 7.16 21.6 8.67 12.4 8.79 47.3 10.0 42.010.7 56.7 66.2 3.7 35.9 71.0 49.1 82.2 37.2 37.4 26.8 83.0 30.6 45.9 26.9 149 37.9 161.01.5 9.0 12.5 0.7 10.1 12.5 9.5 19.3 6.8 6.9 5.6 14.9 4.8 7.4 5.3 21.9 6.8 24.81.09 1.14 2.41 2.24 0.81 1.06 1.68 0.92 0.31 0.71 0.68 2.73 0.61 0.5 0.33 2.95 0.8 2.991.7 6.3 9.7 0.6 15.5 8.6 7.6 27.3 5 4.6 4.0 10.9 3.4 6 4.6 13.6 5.2 15.70.4 0.9 1.3 0.05 2.8 1.2 1.2 6.5 0.8 0.6 0.5 1.5 0.5 1.2 1.0 1.9 0.9 2.04.4 5.3 7.6 0.6 18.2 5.9 6.7 54.6 4.4 3.0 2.1 8.5 2.8 7.9 7.0 8.7 4.8 8.91.2 1.2 1.4 0.1 3.6 1.1 1.4 13.8 0.9 0.5 0.3 1.8 0.6 2 1.7 1.5 1.0 1.54.5 3.8 3.9 0.5 10.5 3.0 4.3 42 3.1 1.0 0.7 5.6 2.5 7.5 6 4.4 3.1 4.60.8 0.63 0.54 0.09 1.66 0.43 0.66 6.36 0.55 0.11 0.10 0.87 0.52 1.3 1.06 0.64 0.5 0.85.6 4.6 3.4 0.7 10.9 2.8 4.6 40.3 3.9 0.6 0.7 5.7 3.3 8.3 6.1 3.5 2.9 3.90.93 0.79 0.53 0.1 1.68 0.47 0.75 6.09 0.64 0.1 0.14 0.97 0.54 1.28 0.88 0.54 0.42 0.58




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garnetites and the aluminous gneisses are hydrothermal altera-tion products of the white gneisses.

The rocks were metamorphosed to the mid-P granulite faciesduring the Grenvillian Orogeny, with the mineral assemblagegarnet–sillimanite–biotite–quartz–K-feldspar–plagioclase in thealuminous gneisses. These gneisses also provide evidence for par-tial melting based on their mineral assemblage and microstruc-tures involving former melt. In this context, the lack of clearlydefined leucosomes at the outcrop scale suggests either that alarge portion of the melt escaped during anatexis and was redis-tributed at a larger scale, or that the degree of melting was low.Melt escape is commonly accepted for partial melting of rocksmetamorphosed in orogenic environments (e.g., Brown 2007). Onthe other hand, a low degree of melting would imply that thealuminous rocks had no muscovite prior to metamorphism. Thismay seem at odds with a hydrothermal alteration origin of their

aluminous character, but may better explain both the overallpreservation of the LBS, and the preservation of inherited micro-structures. As noted, a few kilometers to the south, metapelitesfrom the PLV, which are inferred to be of the same metamorphicgrade as the LBS, display evidence of melt loss at the sample scaleand are thoroughly migmatitic, with abundant leucosome in out-crop (Lasalle et al. 2014).

The aluminous gneisses of the LBS have been inferred to repre-sent hydrothermally altered volcanic protoliths, based on theidentification of a single population of thin igneous zircon prismsin an aluminous gneiss with nodules of garnet (Lasalle et al. 2013).In this context, the variably resorbed angular quartz domainswith a broadly bipyramidal shape identified by SEM–MLA mineralmapping in some aluminous gneisses (e.g., Fig. 3) are possiblyderived from quartz phenocrysts (e.g., Gifkins et al. 2005). In high-grade metamorphic settings, relict quartz phenocrysts have also

Fig. 7. Mineral chemistry plots: (a) garnet average compositions; (b) example of garnet zoning; (c) example of XMg versus Ti (per formula unit)in biotite from two textural settings (AG, adjacent to garnet; IG, included in garnet; M, matrix (groundmass)); (d) spinel average compositions;(e) plagioclase and K-feldspar (average compositions). ACN, aluminous gneiss with composite nodules; AGN, aluminous gneiss with garnetnodules; AD, gneiss with diffuse aluminous pods–layers; WG, white gneiss; G, garnetite. [Colour online.]

Fe Mn

Mg

Ca Mn+Fe

355c3-11 355cx-11

354-2-04

333-2-03 10-A1-76

331E2-11333-03

355A-11206a-04 2163d2-11

355-2a-04 355c2-11

339x 333x-03

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hc gah

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been reported from the amphibolite- to granulite-facies HoresGneiss near the Broken Hill Pb–Zn–Ag deposit (Stevens and Barron2002).

By far, the most widespread microstructures are those relatedto hydrothermal alteration, such as the aluminous nodules, seamsof sillimanite, and locally a K-feldspar-rich groundmass in thealuminous rocks of the LBS. Similar features were reported byBonnet et al. (2005) in amphibolite- to granulite-facies aluminousgneisses associated with 1.50 Ga felsic-dominated volcanic rocksin the La Romaine Supracrustal Belt of the southern GrenvilleProvince and were inferred to be fingerprints of metamorphosedvolcanogenic massive sulfide environments (Bonnet and Corriveau2007). Aluminous nodules in amphibolite- to granulite-facies hy-drothermal alteration zones have been mostly described as dom-inated by sillimanite with lesser quartz ± muscovite ± Fe oxide(Bonnet et al. 2005; Spry et al. 2010; Steadman and Spry 2015).However, in the aluminous rocks of the LBS, they show a morecomplex mineralogy, also including spinel and corundum (but nomuscovite) largely overgrown by garnet, likely as a result of par-tial melting at granulite-facies conditions.

In addition, the garnetites are inferred to represent hydrothermalalteration products of a volcanic precursor (white gneisses), based

on the gradational transition between these two rock units. Spryand Wonder (1989) and Spry et al. (2007) have described granulite-facies garnetites in the Broken Hill district, which, however, wereinterpreted as meta-exhalites.

Chemical trends of alteration include (a) feldspar dissolution cou-pled with progressive enrichment of Fe–Mg–Mn from the whitegneisses to the aluminous gneisses and to the garnetites, and (b) anargillic-type alteration in some white gneisses. In particular, the ele-vated Mn contents in garnet from the garnetites, combined with thepresence of graphite in these rocks, are consistent with Mn and Fecontribution from a high-T and CO2-bearing fluid (see Spry andWonder 1989; Large 1992; Heimann et al. 2011). Variable degree ofplagioclase dissolution by the hydrothermal fluid is also suggestedby the differences in Eu between the precursor rocks (white gneiss:positive Eu anomalies), the aluminous gneisses (no anomalies), andthe garnetites (negative Eu anomalies). In addition, the presence orabsence of these negative Eu anomalies in these latter two rock typesmay reflect differences in the composition, oxygen fugacity (fO2), pH,and temperature of the hydrothermal fluid (see Heimann et al. 2009).

In summary, the relict microstructures and the chemical trendsof hydrothermal alteration preserved in the LBS are consistentwith development in a submarine hydrothermal (i.e., volcanogenic

Fig. 8. Whole-rock chemistry plots: (a) Na2O versus Al2O3/Na2O (wt.%) diagram after Spitz and Darling (1978); (b) inferred protoliths based onNb/Y (ppm) – Zr/TiO2 (ppm/oxide wt.%) ratios (revised after Winchester and Floyd 1977; Pearce 1996); (c) Nb versus Y (ppm) geotectonic settingdiagram of granitic rocks (Pearce et al. 1984); WPG, within plate; VAG, volcanic arc; COLG, collision; ORG, orogenic; (d) Y versus Zr (ppm)diagram. AGN, aluminous gneiss with garnet nodules; AD, gneiss with diffuse aluminous layers; WG, white gneiss; PFL, pink felsic layers;G, garnetite; BC, felsic layers in Banded Complex; A–E, locations (Fig. 1). [Colour online.]

0.01 0.1 1 10

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iO2

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Fig. 9. Chondrite-normalized patterns of rare-earth elements (REE) from (a) pink felsic layers of the Banded Complex, (b) pink felsic layers ofthe LBS (locations B and D), (c) white gneisses and garnetites of location A, (d) white and aluminous gneisses of location D, and (e) aluminousgneisses of location C. Normalization after Sun and McDonough (1989). For abbreviations of rock types, see Fig. 8. [Colour online.]

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La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu

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WG-AD-D

AD_D

WG_D

(b)

(c) (d)

(e)

Fig. 10. Ternary plots in molecular proportions showing potential alteration trends and location of some common metamorphic minerals:(a) Al2O3–F=–CNK; (b) A–K2O–F=; (c) A–MgO–FeOT. A = Al2O3 – (K2O + Na2O + CaO); F = FeOT; M = MgO; F= = FeOT + MgO + MnO; CNK = CaO + Na2O +K2O; FeOT, total Fe as Fe2+. Bt, biotite; Chl, chlorite; Crd, cordierite; Fel, feldspar; Grt, garnet; Mu, muscovite; Sil, sillimanite; Sm, smectite. NASC, NorthAtlantic shale composite (from Gromet et al. 1984); PG, pink gneiss. For abbreviations of other rock types, see Fig. 8. [Colour online.]

FeOT MgO

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SilAGN-C

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G_A BCNASC

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Fel Sm

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massive sulfide type) environment (see Bonnet and Corriveau 2007).Although the LBS is only exposed as discontinuous strips of anancient hydrothermally altered volcanic belt, precluding a globalassessment of the geometry and pattern of alteration, the datamake a compelling case for further exploration of granulite-faciesterranes for such a setting in the Grenville Province.

AcknowledgementsThis contribution is derived from the M.Sc. thesis of M.H. and

was funded by the Natural Sciences and Engineering ResearchCouncil of Canada (NSERC) discovery grant of A.I. The authorsthank the Journal reviewers Paul Spry and David Lentz for insight-ful comments as well as the Associate Editor for handling themanuscript. M. Shaffer (CREAIT, MUN) is thanked for his assis-tance with the SEM–MLA mapping of thin sections, and RobertMarr (University of Calgary) and Wanda Aylward (CREAIT, MUN)for their assistance with the microprobe analyses.

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Hydrothermally altered volcanic rocks metamorphosed at ...€¦ · Abstract: A 1.2 Ga association of aluminous gneisses, garnetites, and white felsic gneisses of andesitic composition