The petrogenesis of metamorphosed carbonatites in

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Alkali pyroxenites associated with the carbom- ..... euhedrah, pahitic alkali feldspar c~grstds, and does not ...... Masuda, A., Nakamura, N., and Tanah, T. 1973.
The petrogenesis of metamorphosed carbonatites in the Grenville Province, Ontario David P. Moecher, Eric D. Anderson, Claudia A. Cook, and Klaus Mezger

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Abstract: Veins and dikes of calcite-rich rocks within the Central Metasedimentary Belt boundary zone (CMBbz) in the Grenville Province of Ontario have been interpreted to be true carbonatites or to be pseudocarbonatites derived from interaction of pegmatite melts and regional Grenville marble. The putative carbonatites have been metamorphosed and consist mainly of calcite, biotite, and apatite with lesser amounts of clinopyroxene, magnetite, allanite, zircon, titanite, cerite, celestite, and barite. The rocks have high P and rare earth element (REE) contents, and calcite in carbonatite has elevated Sr, Fe, and Mn contents relative to Grenville Supergroup marble and marble melange. Values of 6M"8,,,, (9.9- 13.3x0) and 6'3C,,, (-4.8 to - 1.9'&,) for calcite are also distinct from those for marble and most marble melange. Titanites extracted from clinopyroxene-calcite-scaplite skarns formed by metasomatic interaction of carbonatites and silicate lithologies yield U-Pb ages of 1085 to 1035 Ma. Zircon from one carbonatite body yields a U -Pb age of 1089 f 5 Ma; zircon ages from two other bodies are 1178 -b 3 and 1143 f 8 Ma, suggesting several carbonatite formation events or remobilization of carbonatite during deformation and metamorphism around 1080 Ma. Values of E~,,(T)are 1.7 -3.2 for carbonatites, - 1.5 - 1.0 for REE-rich granite dikes intruding the CMBbz, and 1.6- 1.7 for marble. The mineralogy and geochemical data are consistent with derivation of the carbonatites from a depleted mantle source. Mixing calculations indicate that interaction of REE-rich pegmatites with regional marbles cannot reproduce selected major and minor element abundances, REE contents, and O and Nd isotope compositions of the carbnatites.

RCsumd : Les veines et les dykes de roches riches en calcite dans la zone frontikre de la Ceinture centrale de mCtasCdiments, de la province de Grenville, en Ontario, ont kt6 interprktks comme Ctant de vraies carbonatites ou des pseudo-carbonatites, dCrivCes de I'interaction de magmas pegmatitiques avec le marbre grenvillien de Ia rCgion. Ces roches pr6sumCes &re des carbonatites furent mitamorphides, elles sont formkes principalement de calcite, biotite et apatite incluant des proportions rnoindres de clinopyroxkne, magnCtite, allanite, zircon, titanite, ckrite, celestine et barite. Les roches posskdent des teneurs ClevCes en P et en terres rares, et la calcite de la carbonatite contiennent de fortes quantitQ de Sr, Fe et Mn comparativement B celles du marbre et melange de marbres du Supergroupe de Grenville. Les valeurs de 6'8C9s,,, (9,9- 13,3%,) et de 6I3C,,, (-4,8 a - 1,9x0) pour la calcite sont Cgalement diffkrentes de celles du marbre et de la plupart du melange de marbres. Les titanites extraites des skarns a clinopyroxkne-calcite-scapolite, qui rCsultent de l'interaction mktasomatique des carbonatites avec les roches silicatCes, ont fourni des 5ges U-Bb de 1085 a 1035 Ma. Le zircon d'un corps de carbonatite a donnt un age U -Pb de 1089 f 5 Ma; les zircons de deux autres corps ont procur6 des gges de 1170 f 3 et 1143 f 8 Ma; ces donnkes gkochronologiques suggkrent I'existence de plusieurs Cvknements de formation de carbonatites ou de remobilisation de carbonatites durant la difsrmation et I'aItCration rnCtamsrphique vers 1080 Ma. Les valeurs de tNd(T)varient de 1,7 B 3,2 pour les carbonatites, - 1'5 B I ,O pour les dykes formes de granite riche en terres rares qui recoupent la zone marginale de la Ceinture centrale de mktasCdiments, et de 1,6 a 1,7 pour le marbre. La composition minCralogique et les donnCes gCochimiques sont compatibles avec des carbonatites derivCes d'une source mantellique appauvrie. Les calculs effectuCs pour les melanges rCvklent que I'interaction des pegrnatites riches en terres rares avec les marbres de la region ne peuvent pas reproduire les concentrations d9C1Cmentsrnajeurs et mineurs choisis, ni les teneurs en terres rares, et ni les compositions isotopiques de 1'0 et du Nd des carbonatites. [Traduit par la rddaction]

lntroduction Calcite-rich rocks, not of an obvious sedimentary origin, are common in the southwestern Grenville Province of southern

B.P. Mwcher,' E.D. Anderson, and C.A. Cook. Department of Geological Sciences, University of Kentucky, Lexington, KY 40506, U.S.A. K. Mezger. Max-Blanck-lnstitut E r Chemie, Postfach 3060, 55020 Mainz, Germany.

I

'

Corresponding author (e-mail: [email protected]).

Can. 6. Earth Sci. 34: 1185- 1201 (1997)

Ontario and adjacent Quebec. These rocks, variously referred to as vein-dikes , carbonatites, and pseudocarbonatites, are concentrated within the Central Metasedimentary Belt boundary zone (CMBbz) and Central Metasedimentary Belt (CMB) of Ontario. The origin of these lithologies has long been debated. Heinrich (1966) recognized t6ese rocks as a type of carbonatite he called vein-dikes. Gittins et al. (1970) questioned whether these rocks were true carbonatites. They proposed that the putative carbonatites were a hybrid lithology produced by physical mixing and chemical interaction of granite pegmatite with what was interpreted to be regional metamorphic marble having mantle Sr isotopic comp&itions. many of the rocks in question exhibit

ow ever,

O I997 NRC Canda

Can. J. Earth Sci. WI. 34, 1997

Fig. 1. Location of study area within the Grenville Province of southern Ontario, along with the major terranes mentioned in the text. CMBbz, Central Metasedimenbry Belt boundary zone.

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79'w

7R4

760

mineralogical features atypical of the regional Grenville supergroup marbles, for example, those of the Bancroft terrane (Dunn and Valley 1996). These unusual calcite-rich lithologies contain abundant fluorapatite (10-20 modal percent), allanite, zircon, and magnetite, with less common rare earth element (REE) silicates, REE fluorocarbonates, barite, celestite, and strontianite (Anderson 1996). More recently, Lumbers et al. (1990) identified some of these rocks as true carbonatites (their carbonatite-fenite igneous suite of the Grenville Province in Ontario). The confirmation of these rocks as carbonatites may have important implications for the tectono-magmatic history of this belt of the Grenville Province. In addition, REE, Mo, U, and Th mineralization is developed in skarns found adjacent to or near the carbonatite-like rocks (Shaw et al. 1963a, 196%; Lentz 1991a). The present study is an attempt to constrain the petrogenesis of the Grenville carhnatites through petrographic study, electron microprobe analysis, stable isotope analysis, REE geochemistry, Nd isotope analysis, and U -Pb geochronology . From herein the rocks in question will be simply referred to as carbonatite, with the understanding that it remains to be demonstrated that they are carbonate-rich igneous rocks of mantle derivation.

Geological setting Outcrops examined for this study occur in the vicinity of Minden and Bancroft, Ontario (Fig. 1). The study area lies mainly in the CMBbz, a 10-20 h wide zone of intense deformation resulting from northwest-directed thrusting of the CMB over the Central Gneiss Belt (Davidson 1986; Hanmer 1988). The lithologies within this zone include various types of orthogneiss tectonites, meta-anorthosite, aluminous paragneisses, and marble mklange (Fig. 2). Ductile deformation associated with thrusting within the CMBbz is manifested in several ways, with striking contrasts in deformational style resulting from differing rheological properties of the affected lithologies. This deformation plays an impor-

tant role in the physical mixing of lithologies and skarn localization. Silicate lithologies tend to be highly attenuated to mylonitic. Kinematic indicators such as rotated K-feldspar porphyroclasts in mylonitic gneisses with shallow southeasterly dips are consistent with top-to-the-northwest sense of shear (Hanmer 1988). Carbonate-rich rocks (marble and carbonatite) are generally unfoliated and responded to deformation by flowing passively, entraining various silicate clasts, which vary in size from millimetres to tens of metres (Hanmer and Ciesielski 1984; Easton 1990; Easton and Davidson 1994). The CMBbz experienced a protracted history of tectonism and metamorphism. The thrusting and deformation described above was initiated at 1.19 - 1.18 Ga and was reactivated at 1.08- 1.05 Ga (Hanmer and McEachern 1992; McEachern and van Breemen 1993; Mezger et al. 1993). Upper amphibolite facies metamorphism at 600 -7Q0°C and 0.5 -0.7 GPa (Hanmer 1988; Anovitz and Essene 1991; Sharp 1991) accompanied at least the latest period of thrusting and deformation. Pre-, syn-, and posttectonic granitic pegmatite dikes are ubiquitous in the CMBbz and the Central Gneiss Belt (CGB) of Ontario and Quebec. Dike intrusion spans the time from ca. 1175 to 1 0 0 Ma. Lentz (1996) discusses the mineralogical and major element variation of pegmatites in the context of mineralized shrns in the CMBbz. Many of the late syntectonic pegmatites are enriched in U and the REE. The few isotopic constraints on the petrogenesis of the dikes suggest an origin by fractionation of melts derived by partial melting of upper amphibolite to granulite facies lower crust (Lentz 1996).

Previous work Previous studies addressed various aspects of the origin of the carhnatites, but few published geochemical constraints bearing on their petrogenesis are available. Numerous early studies discussed the distribution of and mineralization associated with rocks inferred here to be carbonatites (e.g., Adams and Barlow 1910; Satterly 1956; Hewitt 1967a; Lumbers 1982). Gittins et al. (1970) proposed, based on Sr isotope compositions, that the carbonatite-like rocks are hybrid rocks produced by physical mixing of pegmatite and regional Grenville marble. They assumed, b a s 4 mainly on mineralogy, that the calcite-rich rocks they analyzed were typical marbles. The relatively low initial Sr isotopic compositions measured by Gittins et al. (0.702 -0.705) had been proposed by others to be diagnostic values for mantlederived carbonatites (Powell et al. 1962, 1966). The correlation of low Sr isotopic compositions with assumed marble lithologies led Gittins et al. to conclude that Sr isotopes are in fact not diagnostic fingerprints of carbonatites. However, they did not test the assumption that the rocks they analyzed were actually marbles related to those of the Grenville Supergroup, which have stable isotopic compositions similar to those of limestone protoliths (Vdey and O'Neil 1984; Dunn and Valley 1996). We intend to show that some of the marbles in the southwestern CMB and CMBbz have the geochemical characteristics of carbonatites that could not result from mixing of granitic pegmatites and marbles with sedimentary protoliths. The fenite and carbonatite suite of Lumbers et al. (1990) @ 1997 NRC Canada

Fig. 2. Distribution of lithologies and sample localities within the vicinity of Minden, Ontario (modified from Easton 1990).

Central Gneias Belt (CGB) boundary zone (CMBbz)

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Marble melange

Glamorgan Gneiss Complex Dymrdt Gneiss Complex ille Supergroup Metasedimentirry R w b Medium- to coirm-grained calcitk and dolomitic marble Amphibolite schist Siliceous elastic metasedimentary rocks Syenogranitic gneiss Mgmatitie, quartzofeldspathicgneiss

Trend of foliation

is the youngest of several magmatic events in the CMB and CMBbz. Some of the carbonatite masses investigated for the present study are included in this suite (e.g., the Essonville body, locality 4 1). Mungall (1989a, 1989b) presented stable isotope and REE evidence supporting the interpretation that calcite-rich veins in the CMBbz of eastern Ontario are d s o carbonatites. Alkali pyroxenites associated with the carbomtites in this area yield zircon U -Pb ages that fall within the range of ages exhibited by the fenite-carbonatite suite of Lumbers et al. (1990). The similarity between the carbonatites from eastern and southern Ontario will become apparent in the following discussion. Lentz and co-workers (Lentz and Kretz 1989; k n t z 19918, 1991b; Lentz et al. 1991) studied calcite-rich veins in the CMB and CMBbz, in the context of their genetic relationship with U-, Th-, Mo-, and WE-karing granitic pegmatite~and skarns. h n t z proposed that calcite-fluorapatitefluorite veins have a magmatic -hydrothermal origin. Chemically modified hydrothermal fluids, derived from crystallizing U-, Th-, and REE-bearing pegmatites, mixed with components derived from marble, producing a fluidrich vein system that intruded regional metamorphic rocks and locally resulted in skam formation. This is a variation of the hybrid model of Gittins et al. (1970). We have not sampled any of the lithologies studied by Lentz, and our results do not bear directly on his model, although calcite from several pegmatitea has stable isotopic compositions identical

to those in rocks from the present study (Lentz and Kretz 1989; Lentz et al. 1991). Thus, there are two primary hypotheses for the origin of the carbonatite-like rocks of the northwest CMB and CMBbz: true carbonatites of presumably mantle origin (later modified by deformation and metamorphism) versus pseudocarbonatites produced in the deep crust by mixing of U-, Th-, and EE-bearing silicate lithologies with regional metamorphic marbles. The first process should result in rocks with mantle geochemical characteristics, possibly modified by interaction with crustal reservoirs, and the second process should result in mixing trends between marbles of supracrustal origin and granitic rocks of deep crustal origin.

Field relations The rocks sampled for the present study occur within the CMBbz from south of Minden to east of Bancroft (Figs. 1, 2). Outcrops in this zone exhibit a high degree of deformation, expressed in part as mechanical mixing of calcite-rich lithslogies (marble and carbomtite) with silicate lithologies to produce a melange. Any original intrusive, textural, or mineralogical characteristics of the carhnatites have been modified by deformation and metamorphism. The carbonatites occur as 10-m-scale outcrops of mdlmge similar in appearance to marble melange (calcite-rich matrix with silicate xenoliths), or as 10-m-sale outcrops of weakly O 1997 NRC Canada

Can. J. Earth Sei. Vol. 34, 1997

Tabk I. Representative mineral assemblages.

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Sample

Lithology

Cal

M M MM MM MM MM MM

X

C C C

C C C

S CS CSP AP AG

x x x

x x

x x

X

X

-

x

r

t

X X X X X X X -

x

x

X

X X

-

-

t

X X . X X X X X X

7

Cpx x X X

X

x

X

x

Ap

Bt

tr

r

X

X X X X X X X

X X

r

x

-

X

tr tr

-

Aln -

X

-

-

-

H

b

Gr Dol, Gr Ttn, Zrn, Py, Kfs, P1 Gr, Ttn, Zrn, P y , Kfs, P1 Gr, Qtz, P1 Chn, Gr (PI -@ -Scp - CaI -Bt -Ttn in clast) Py SCP Zrn, Aln Zm, Aln, Scp Zrn, Aln, Py Aln, Py Zrn, Mag, Ilm, Py, Cls, baddeleyite, cerite Ttn, Zrn, Aln, Mag, Py, Brt, cerite Ttn, Mo, ME fluorocarbonate, REE silicate Ttn Ttn Qtz, Mag, Ttn Qtz, Ttn

-

-

-

-

-

-

-

-

P

X

X

X X

X X

-

x

-

-

-

l

Other

-

-

-

Zrn

-

Hbl H b l - - H b l - - H b l - - .

X

P1

-

-

X

Kfs

T r - T r - T r - T r - x T r - -

X X X

-

t

Am

x

-

X

x

X

x

-

Notes: Lithology abbreviations: M, marble; MM, marble mdlange; C, carhnatite; S, skarn; CS, calcite-bearing syenite; CSB, calcite syenite pegmatite; AP, allanite pegmatite; AG, allanite granite. Mineral abbreviations from Kretz (1983): AIn, allanite; Am, amphibole; Ap, apatite; Brt, barite; Bt, biotite; Cal, calcite; Chn, chondrdite; Cls, celestite; Cpx, clinopyroxene; Dol, dolomite; Gr, graphite; Hbl, hornbknde; Ilm, ilmenite; Kfs, K-feldspar; Mag, magnetite; B1, plagioclase; Py, pyrite; Qtz, quartz; Scp, scapslite; Tr, tremslite; Ttn, titanite; Zm, zircon. The sample numbering system denotes sample locality and sample number from that Iwlity (e.g., 41.2 = locality 41, sample number 2). x, phase present as major mineral; tr, phase present in trace amounts.

foliated to massive layers parallel to the regional structural trend. At three localities (3, 9, 15) biotite- and fluorapatiterich calcite carhnatite appears to intrude the ductiaely deformed margin of the Allsaw anorthosite, and rounded xenoliths of scapolitized meta-anorthosite are common within the carbonatite. The carbonatite grades into glimmerite at the contact with meta-anorthosite, suggesting metasomatic reactions between the two lithologies. The contact appears to be an intrusive relationship, but owing to the extreme ductility of the carbonatite during deformation, the contact may have resulted from remobilization of the carbonatite during tectonism at ca. 1050 in the CMBbz (see below). A similar reaction relationship is seen with metre-scale inclusions of various silicate inclusions in the mdlange (e.g., metaanorthosite, syenite, and hornblende -plagioclase -titanite gneiss). This type of carbonatite is easily confused in outcrop with tectonic mClange in which the host lithology is marble. Where silicate clasts are very small (millimetres to centimetres in diameter) or absent (e.g., locality 24), there is little basis for entertaining a priori a carbonatite origin for these rocks. It will be demonstrated, however, that mineral assemblages and various geochemical trends support a carbonatite origin for all of these rocks. Clinopyroxene-rich slearns formed by reaction of carbonatite with host rock or silicate inclusions vary from centimetre-scale rinds on rounded silicate clasts in the mklange to massive skarn several metres thick replacing entire lithologic layers. In some outcrops, exposure is limited, s h r n and carbonatite are the only lithologies present, and skarn protolith lithologies cannot be identified. Bre- to posttectonic granite dikes and granitic pegmatite dikes, some of which are calcite-bearing, are common in the

CMBbz. Many of these dikes are rich in allmite and titanite, and are potential sources of BEE (Hewitt 1967b; Lentz 1991b). An allanite granite and allanite pegmatite were sampled in the area south of Minden to assess the possibility that these lithologies are one of the end members for the hybridization model of carbonatite genesis.

Petrography Five lithologies were identified that are important for discriminating the origin of the putative carbonatites. These lithologic distinctions were based on preliminary petrographic observations and geochemical studies. Establishment of trends in mineralogy and geochemistry based on this preliminary work (e.g., abundant allanite and apatite, elevated Sr contents in calcite, low values of 6% and 6I3C for calcite in carbonatite) permitted discrimination and identification of lithologies with less definitive mineralogical and geochemical characteristics as carhnatite or marble mklange. For example, many of the samples that were presumed to be marble or marble melange, based on outcrop relationships and hand sample identification of minerals, were found to have calcite with trace element and stable isotope compositions that were very different from marble derived from a limestone protolith. Reinspection of these samples commonly revealed accessory mineral assemblages similar to those in samples identified as carbonatite. Major and accessory mineral assemblages are compiled in Table 1.

Marble and marble mglange . Marble and its tectonized equivalent (marble melange) consist mainly of light gray to white calcite, tremolite, O 1997 NRC Canada

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Fig, 3. RepresenQtive phcpBomicrograph of various lithslogies from the study area. Morizsntal field s f view in aIE cases is 4 ream; dl abbreviations after Kretz (1983). (a)Marble (sample 20.2) csnuinning primarily tremslite (Tr), phlogopite-rich biotite (Bt), and calcite (Cal). (b) Marble mClange (sample 30.1) showing margin of a granitic clast in h e calcite matrix. (c) Carbowtire (sample $1) with allanite (Ah) rimming biotite, apatite (Ap), magnetite (Mag), zircon (Zrn), and calcite. ( d ) Carbaenatite (sample 151, same as c, with e u h d r d Jlawite as a discrete phase. (e) Carbsnatite with magnetite and apatite (sample 24). (f)Foliated granite pegmatite (sample 53) with dhnite, biotite, plagiaeIase (Pi), and K-feldspar (Kfs; microcline).

elinopyrsxene, phlogopite, and graphite (Pig. 34, Some samples contain dolomite, mainly exsolved from calcite. The on accessory minerals are pyrite and titanite. The marble mClange consists of a marble matrix with variably disaggregated silicate clasts (usualky consisting 0%microcline, plagioclase, quartz, scapolite, biotite, and titanite) t k t range in size from metres to millimetres (Fig. 3b). Complete disaggregation of silicate clasts occurred within marble rnClange, and some silicate minerals within calcite may be xenocqsdc . M e ~ a s m a ~interaction c between carbonate ia m6lmge matrix and silicate minerals in clasts is apparent as wen, indicating that minerds such as clinopyroxene, hornblende, biotite, scaplite, and titanite in marbk rnklange may

not have been present in the marble befare tectonic mixing.

Carbsnatite The major mineralogy of carbonatites is not markedly different from that in marble, although their macroscopic appearance is different. Light gray to pale pink to orange calcite occurs with black biotite and pde blue to apple green to colorless fluorapatite, in some eases imparting a very distinctive appearance in hand sample. Although below detection limits on the electron microprobe, apatite contains sufficient U ar Th to produce pleochroic halos in biotite. Also present in major to minor amounts are ~linopyroxeneor magnetite. Carbsnatjite tectonic melange contziiajins various silicate ebasts 8 8997 NRC Canada

Can. J. Earth Sei. Val. 34, 1997

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Fig. 4. Backseaeered electron image sf atccesssry cerite and celestite in exbonatite sample 4 12 .

abundant acicular inclusions of a REE Wuoroeabomte (Anderson 1996) and coarse flakes of molybdenite.

AlIanite gradtes Two danite- and zircon-beuing dikes co area were investigated. One of the sample hornblende -biotite granite with abundmt dlanite and zircon (Fig. 2; lwdity 61) that either intmdes or is entrained in marble mClmge near Miners Bay, Ontario (Hanmer and CiesielsE 1984; h s t s n 1998). The second is a medium- to cause-gr~ned,w e d y folded (i.e ., late syntecto~c)dkmite titanite -zircon granite dike that intruded porphy roclastic gneiss south of Miners Bay (Fig. 2, locdity 53; photomicrograph in Fig. 3 j ) . This sample is representative of allaniten to the CMB and CMBbz. bearing pgmatites co Calcite-haring graGtfe pegmatite A number of coarse-grained, weakly to undefsrmed granitic pegmatite dikes also contain cdcite and (or) clinopyrsxene m d titanite. The calcite wckars interstitidly to eoase-grained, euhedrah, pahitic alkali feldspar c~grstds, and does not appear to be a metasomatic mineral. These rocks are similar to the fluid-rich granitic melts discussed by Len& (1996) and Len&.et d o(1991). Their igneous nature m d the presence s f calcite make them candidates for consideration in the process of carbonatite fornation. formed of pale green hornblende, light brown scapolite, microcline, plagisclase, and quartz. In addition to the abundance sf apatite, the accessoq minerdogy of carbonatites was used t s distinguish carbonatite from marble in the study area. Allanite and zircon are the most common accessoy phases (Figs. 3c, 3 4 , but magnetite, celestite, barite, strowtianite, and cerite have d s s been identified (Fig. 4). Allmite may oeeur as e u h d r d phenocqsts (Fig. 3 4 or, more esmrnody, as rims on biotite (Fig. 3c), suggestive of late-shge erystdization or remobilizatioea during me&moqhism. Other minerdogie effects in the carbonatite of the latest phase of tectonism m d metarnovhism in the CMBbz include the reaction of clinopyroxene to mphibole (hornblende or tremslite) and reactions between marble matrix and silicate elasts forming scapolite, biotite, and titanite. The s m p l e from locality 24 (Miners Bay, Fig. 2) illustrates the problem with distinguishing marble from carbonatite. The rock appears to be a white, cdcite-rich marble with 5 - 10 modal percent magnetite a d breccia clasts s f glimmerite. However, hand sample and thin section inspection reveals abundant nuorapatite (up to 20 modal percent; Fig. dc) and coarse-grained zircon, more typical of earbowtite. The trace element and isotopic eharxte~sticsare avpicd of marble as well.

Sbm The minerdogy of s k m depends on the silicate host. Granitic clasts in the mC1ange are enclosed by zoned s k r n consisting of biotite f scaplite nearest the silicate clast, grading outward to clinopyroxene + calcite + titanite nearest the m62ange host. S b m formed from meta-anodosite or dioritic gneiss consists of elinopyroxene f scapolite + titmite f Wuorapatite f calcite. Cdcite in a eoase-grained skim near Easonvdle and near sample l o c d i ~41 conhins

Major and trace element contents s f carbonatite, marble, and granite were determined by X M L (Don Mills, Ontaris). Major and some trace elements were determined by X-ray fluorescence spectrometry, and other trace elements were determind by instmmental neutron-activation analysis (Table 2). Wavelena disprsive s p c t r o m t q WDS) electron microprobe analysis of @aqMg, Fe, Mw, and Sr in calcite was carried out on an ARL-SEMQ electron microprobe at the University of Kenme@ using United States National Museum carbonate stmdards (calcite, dolomite, siderite, strontianite), an accelerating potential of 15 kV, and a sample current of 10 nA. Counting times were 330 s for Fe, Mn, Mg, and Sr and 10 s for @a. Andyses in Tables 3 and 4 are averages of at least three points taken on at least three different calcite grains in each thin section. WDS analysis of apatite, biotite, and clinopyroxene in carbonatite and skarn was performed at 15 kV and 18 -26 nA using United States National Museum Wuorapatite and silicate shndards (Table 5). Allanite was andyzed by WDS at 20 kV using a sample current of 20 nA (Table 6). Preliminary WDS scans on allanite and on the syn&etic United States National Museum REE phosphate standards perfiaed se1mtion of appropriate background positions in order to minimize interferences. Oxygen and carbon isotope analyses were made on C 0 2 extracted from calcite by reaction in phosphoric acid at 22 OC for 12 h (McCrea 1950). All samples were run in duplicate, and 6°C. Issyielding a reproducibi%iQof 0.2%, for B180 topic values are presented in the per mil notation relative to standard mean ocean water (SMOW) and Peedee belemnite standard (PDB) (Tables 3 and 4). Rare earth elemeM concentmtions were dekrmined by isotope dilmion (Table 7) (Cook 1997). Analytic& pmce8 1997 NWC Canda

Moeeher et al.

Table 2. Whole-rock chemical analyses.

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Sample: Lithology :

3.1 C

9.1 C

15.8 C

20.2

M

30.1 MM

24 C

41.2 C

53.2 AP

61.1b AG

SiO, (wt.96) Ti02 A1203 F%O3 Cf203 MnO CaO Na20 K20 P2°5

LO1 Totd

Notes: Abbreviations and sample numbering system as in Table 1.

dures are described in Tomascak et al. (1996). The REE concentrations were normalized to the chondritic abundances of Masuda et al. (1973) as modified by Hanson (1980). Coarse-grained (> 1 cm in longest dimension), wedgeshaped, maroon to dark brown titanite crystals were handpicked from clinopyroxene f scapolite f calcite from four localities in the study area. Zircons were obtained from three samples of carbonatite by dissolution of calcite in HCl and subsequent hand picking. Most of the zircon fractions were abraded following the technique of Krogh (1982). Before dissolution the mineral separates were further purified by washing with deionized water (titanite) or HN03 (zircon). All minerals were spiked with a mixed 205Pb/233Utracer before being digested in 3 mL screw-top Teflon PFA@vials inside Pan@bombs at 210°C. The minerals were dissolved in a mixture of concentrated HF and HN03. After evaporation, the material was dissolved in 2M WCl for ion exchange chromatography. The Pb was separated using HC1- HBr chemistry and the U by HCl-HN03 chemistry using BioRad 1-X8 resin (Tilton 1973; Manhks et al. 1984; Mattinson 1986).

For isotope measurements, Pb and U were loaded on a single Re filament using the phosphoric acid - silica gel technique (Cameron et al. 1967). The total procedural blank for Pb was 30 pg and