Canodian MineralogistVol. 28, pp. 77-86 (1990)

THE FORMATION OF ALLANITE-(Ce) lN CALCIG GRANOFELSES,NAMAOUALAND, SOUTH AFRICA

JOHN M. MOORE AND JONATHAN H. McSTAYDepartment of Mineralogy ond Geology, University of Cape Town, Private Bag, Rondebosch 7700, South Africa

ABSTRACT

Significant concentrations of allanite-(Ce) are reportedfrom two calcic granofelses in the granulite facies of thewestern Namaqualand Metamorphic Complex, SouthAfrica. The host rocks have plagioclase-garnet-clinopyroxene-quartz-titanite parageneses and areassociated with mid-Proterozoic supracrustal quartzofeld-spathic gneisses and quartz-biotite-sillimanite schisrs.Allanite-(Ce) occurs in trro nonmetamict forms: firstly ina plagioclase-quartz gneiss as bands of prismatic grains withrelatively"high REE contents and simple zoning to a Ca-,Al-rich rim, and secondly with lower R.gE contents as coaxsexenoblasts and fine inclusions in sieve-textued poikiloblastsin massive plagioclase-garnet granofelses. The coarsexenoblasts display reverse zoning to a Ca-, Al-rich core.The a[anite-(Ce) concentrations in the banded plagioclase-quartz gneiss are considered to represent premetamorphicdetrital accumulations derived from the weathering of felsicvolcanic rocks. .R.8E concentrations in the plagioclase-garnet granofelses represent metamorphic mobilisates intoadjacent impure carbonate layers, that were initially accom-modated in epidote at low metamorphic grades and subse-quently in residual allanite-(Ce) enclaves when P-Tcondi-tions exceeded the stability range of epidote.

Keywords: allanite-(Ce), mineral composition, metamorphicreaction, calcic granofels, granulite facies, Nama-qualand, South Africa.

Sotvttrletnn

Des concentrations importantes d'allanite-(Ce) ont dtddecouvertes dans deux granofels calciques du socle m€ta-morphique (facies granulite) du Namaqualand occidental,en Afrique du Sud. Les roches encaissantes, qui contien-nent l'assemblage plagioclase - grenat - clinopyroxine -quaftz - titanite, sont associ€es i une sequence supracrus-tale d'dge proterozoique moyen contenant des gneissquartzo-feldspathiques et des schistes i quartz + biotite+ sillimanite. L'allanite-(Ce) s'y trouve sous deux formesnon m6tamictes. Dans un gneiss i plagioclase + quartz,elle forme des passdes de grains prismatiques ayant desteneurs 61ev6es en terres rares et une zonation simple versune bordure enrichie en Ca et Al. Une allanite moins enri-chie en terres rares est pr6sente sous forme de xdnoblastesgrossiers et d'inclusions fines dans des poeciloblastes d tex-ture de tamis dans des granofels massifs d plagioclase +grenat. Les x6noblastes grossiers montrent une zonationinverse, vers un coeur enrichi en Ca et Al. Les concentra-tions d'allanite-(Ce) dans les gneiss d plagioclase + quartzrepr€senteraient des accumulations ddtritiques, et donc pr€-m6tamorphiques, d6riv6es d'un socle volcanique felsique

lessiv6. Les concentrations de terres rares dans les grano-fels i plagioclase + grenat r€sulteraient de leur mobilisa-tion metamorphique dans des niveaux carbonat€s voisins;les terres rares, situees dans l'dpidote i un faible degr€ dem6tamorphisme, ont par la suite 6t6 concentr6es dans desenclaves d'allanite-(Ce) quand les conditions de P et de 7ont depasse le champ de stabilitd de l'6pidote.

(Traduit par la R6daction)

Mots-clds: allanite-(Ce), composition chimique, r€actionm€tamorphique, granofels calcique, facies granulite,Namaqualand, Afrique du Sud.

INTRODUCTION

Allanite-(Ce) is reported as a minor to trace con-stituent of metamorphosed carbonate rocks (Deer etaL 1986), commonly from skarn assemblages at con-tacts with felsic igneous rocks (Papunen & Lindsjd1972, Pavelescu & Pavelesqr L972, Derick 1977,Gier6 1986), and more rarely from regionallymetamorphosed calc-silicate assemblages (Sargent1964). At the majority of these localities, allanite-(Ce) occurs in association with mineral paragenesesthat include calcic plagioclase, clinopyroxene, gran-dite garnet, scapolite, calcic amphiboles and titanite.

The chemical relationship between allanite andepidote is governed by the coupled replacementof C*+ + Fe3+ (epidote) for REE3+ + Fd+ (allan-ite). In the epidote structure, Ca occupies two cry$-tallographic sites of different sizes; the light rare-eafih elements show a preference for the largerl0-fold coordinated/2 site (Dollase 1971). Allaniteby definition contains in excess of 5090 rare-eafihelements (R.EE) in the A2 site, equivatentto a REEoxide content of about 15 w.90. In ntture, a com-plete range of REE contents is found from epidoteto allanite (Maaskant et al.1980, Sakai el al. 1984,Deer et al. 1986, Gier6 1986).

Epidote and clinozoisite are common constituentsof impure carbonate metasediments at low amphibo-lite grades of metamorphism. With increase in meta-morphic grades to temperatures between 600 and700"C in the pressure range 3 - 5 kbars, epidotebreaks down to form mineral assemblages thatinclude anorthite, grandite garnet and magnetite(Holdaway 1972, Liou 1973). Epidote in meta-sedimentary rocks may contain significant amountsof the REE (Maaskant et al. 1980), and detrital

77

78 THE CANADIAN MINERALOGIST

allanite-(Ce) is reported from certain metapeliticrocks, rimmed by REE-bearing metamorphic epidote(Sakai er al.1984).

Epidote is widely distributed in the western Nama-qualand Metamorphic Complex (NMC), where it isparticularly prevalent in amphibolites and calc-silicate rocks in the amphibolite-facies terrane(Coetzee 1941, Moore 1977) and as a minor retro-grade constituent of mafic granulites and plagioclase-rich gneisses in the granulite-facies terrane (Joubertl97l). Allanite-(Ce) occurs in local stratabound con-centrations in association with tourmaline, apatite,monazite, zircon and magnetite in leucocratic

quartzofeldspathic gneisses in the Kakamas area ofthe western NMC (Hugo 196l). Minor metamictallanite-(Ce) also is described from simple pegma-tites in the Onseepkans-Kenhardt pegmatite belt(Hugo 1970) and from alaskitic leucogranitic bodiesin the Springbok area (Robb 1986).

We report here the presence of significant concen-trations of allanite-(Ce) (hereafter referred to asallanite) in certain calc-silicate rocks in the granulite-facies portion of the western NMC. These occur-rences provide textural and chemical evidence thathas a direct bearing on the formation of allanite incalc-silicate rocks, and leads to proposals concerning

Ftc. l. Regional.map of portion of the western Namaqualand Metamorphic Complex, showing the distribution ofsupracrustal rocks (black), granitic gneisses (unadorned) and overlying Pan-African metasediments (hatching). Theregional metamorphic zonation includes amphibolite facies (l), cordierite-garnet subzone of the granulite facies (2),spinel-quartz subzone of the granutte facies (3) of the Namaqua event, and overprinted Pan-African staurolite zone(4). Localities are Smorgen Schaduwe (SS) and Rietfontein (RFN), and towns are Garies (GAR), Springbok (SPR),Aggeneys (AG) and Steinkopf (SK). Inset shows location of map area.

ALLANITE IN CALCIC GRANOFELSES, NAMAQUALAND 79

the origins of high concentrations of light-rare-earthminerals in certain mid-Proterozoic metamorphosedcarbonate rocks.

GENERAL Gnolocv

The allanite occurrences are located on the farmsSmorgen Schaduwe and Rietfontein, respectively45 km east and 40 km southeast of Springbok,Namaqualand, South Africa (Fig. l). The calc-silicate lenses that host the allanite concentrationsoccur within remnants of a supracrustal volcano-sedimentary sequence in a terrane dominated byintrusive granitic and charnockitic augen gneisses(Joubert 1971, Moore 1980. At Rietfontein, thesupracrustal sequence comprises quartzofeldspathicbiotite gneisses and leucogneisses, with minor calc-silicate, basic granulite and amphibolite bands.

These lithologies are typical of the basal portionsof the supracrustal tectonostratigraphic sequence inwestern Namaqualand (Moore 198O. In contrast, thesequence at Smorgen Schaduwe is dominatgd byquartz-biotite-sillimanite schists and quartzites, withminor garnetiferous iron formations, typical of the

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upper stratigraphy of the supracrustal sequence. *'rY*.5-:!!!P".-ldrj?-Amphibolites in this portion of the stratigraphy ai nAggeneys, 100 km to the east, have yielded a Sm- op@ttoq lc€ced,€ atrd @dttl@ e rr,@ (!ss5)'

Nd isochron age of 1650 Ma (Reid et ol. 1987).Metamorphii zonation in the western NMC (Fig. At Smorgen Schaduwe, allanite-bearing rocks

l) is chiefly defined by mineral assemblages in occur locally at the contact between quartzofeld-metapelitic iocks. The paragenesis biotite + sil- spathic gneisses and an overlying sequence oflimanite + quartz typifies the upper amphibolite quartz-biotite-sillimanite schists and quartzites. Thefacies, with P-Iestimates in the region of 650'C at allanite-rich rocks consist of a lower unit ( a I meter5 kbars; cordierite + garnet + K-feldspar + quartz thick) of plagioclase-quartz gneiss containing dis-typifies the lower-Ipbrtion of the granulite facies crete bands of clinopyroxene, allanite, titanite and(ZSO"C at 5 kbars), and hercynite + quartz, the magnetite, and overlying pods (20-30 cm thick) ofhigh-I portion of the granulite facies (>800'C at more massive garnet-clinopyroxene-allanite-5 kbars) (Waters 1989). The two occurrences of plagioclaserock,similartothoseatRietfontein.Theallanite fall within the low-Zgranulite-facies terrane allanite-rich calc-silicate rocks are directly overlain(Fig.l). by clinopyroxene-hornblende-quartz granofelses

At the Bobbejaanpoort locality on the farm Riet- that become increasingly quartzitic away from thefontein, allanite-rich rocks occur as boudinaged pods allanite band.of dark brown granofels in an impersistent band, The bulk chemical compositions of the allanite-10-20 cm thick ind 100 m long, enclosed within a rich calc-silicate rocks reveal that the rocks are gener-

10-20-m-thick calc-silicate unit. the allanite-rich ally calcic (20 wt.9o CaO), magnesium-poor (<0.50pods consist of a thin (l-5 cm), finer-grained wt.9o MgO), peraluminous Q0 wt,alo Alro) andplagioclase-allanite-magnetite-titanite layer, directly iron-rich (10 wt.9o FeO), containing approximatelybvJrhin by a thicker (tO-tS cm), coarser-grained 2 wt.tlo REE oxides (Table 1). The plagioclase-plagioclase-garnet-clinopyroxene-allanite layer that quartz gneiss at Smorgen Schaduwe is, in addition,gtul.t ptogi"ssively into an allanite-free gianofels highly sodic (5 wt.9o Na2O). Relatively high levels

of otnerwisi similar composition. The encloiing calc- of strontium (up to 1500 ppm) are present in all rocksilicate assemblages are 6oth garnet granofelses and types. Enrichment in the REEis not associated with

clinopyroxene-bJaring rocks, and are themselves significant concentrations in other incompatible ele-enclosid by mabnJtite-rich quartzofeldspathic ments such as K, Rb, Zr,Th, \! and P. Orerlyinggneisses. Several other calc-silicaie units are plesent clinopyroxene-quaftz granofelses at Smorgenivithin the quartzofeldspathic gneisses on Rietfon- Schaduwe are more magnesium-rich, sodic (2.5 wt.9otein, but none of these have yielded allanite NarO) and have dramatically lower REE contentsconcentrations. (< 500 ppm, Table 1)'

80 THE CANADIAN MINERALOGIST

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trast to plagioclase and garnet, clinopyroxenecontains few inclusions of allanite.

Allanite (10-33 vol.9o) shows several textural var-iations within the calcic granofelses. In the allanite-rich pods, it commonly occurs as coarse (l mm),brown to red-brown, xOnoblastic grains showingsimple twinning. The allanite is nonmetamict, andclusters of grains show mutual l20o contacts (Fig. 2).A common additional texture observed in the allanitepods is a sieve-textured distribution of fine allanitegrains in garnet and plagioclase (Fig. 3).

In the plagioclase-quartz gneiss at SmorgenSchaduwe and the thin plagioclase-allanite-magnetite-titanite layer at Rietfontein, allaniteoccurs in distinct bands as prismatic to tabular idio-blastic grains generally elongated along the gneissicfabric (Fig. 4). Rarely, a complex intergrowth orsymplectite between allanite and titanite is observed.

Allanite grains in the plagioclase-quartz gneiss atSmorgen Schaduwe show color zonation, with adarker core and distinct narrow Q5 pm) pale rim(Fig. 4), corresponding to an increase in the epidotecomponent (Fig. 5a). In the allanite-rich pods, thezoning is more complex, with large grains having alighter core that grades progressively into a darkermantle which, in turn, reverts more abruptly to animpersistent light rim (Fig. 2). Both the paler coreand rim are relatively enriched in Ca and Al (Fig. 5b).

The coupled substitution Ca2+ + Fe3+ = REE*+ Fe2+ is inadequate to account for chargebalances in the zoned grains. It is apparent fromTable 3 that the exchange mechanism requires com-plex substitutions in the M3 site, whereby both Fd+and Al3* are replaced by Fd* and Mg2+ to main-tain charge balance. Substitutions such as Mg2* +F- = {13+ + 02- (Peacor & Dunn 1988) are notapplicable, as F concentrations are below detectionlimit (0.24 wt.9o) in the NMC allanite.

Allanite from the Namaqualand calcic granofelseshas a low total REE content (0 . 50 to 0 .7 6 in the A2site) (Table 3). The rim of certain grains consists ofallanitic epidote rather than allanite. The highestR.EE contents are recorded from the plagioclase-quartz gneiss at Smorgen Schaduwe, and the lowest,from the overlying allanite-rich pods. Allanite grainsin the plagioclase-quartz gneiss contain higher Mgand Mn contents, whereas those from the allanite-rich pods are more Al-rich. The absence of detec-table levels of Th and U acco.unts for the non-metamict nature of the allanite.

Titanite is a minor constituent in all the allanite-bearing lithologies and is most common in theplagioclase-quartz gneiss at Smorgen Schaduwe,where it occurs as anhedral grains in layers withmagnetite, diopside and allanite. The titanite showsconsiderable departure from the pure end-member,with complex substitutions involving Al and Fe forTi, light REE and Y for Ca, and F for O (Table 2).

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PETRoGRAPHy aNo MNsnAL CHEMISTRY

Plagioclase is the major constituent (38-73 vol. Vo)of all the allanite-bearing lithologies and commonlyoccurs in granoblastic mosaics of equidimensionalgrains. It is mostly unzoned and generally containsnumerous inclusions of quartz, allanite, apatite andzircon. Plagioclase shows considerable compositionalvariation, from Anro_r, at Rietfontein to An77 inthe allanite-rich pods and Anru in the plagioclase-quartz gneiss at Smorgen Schaduwe. Scapolite(Mqr) is associated with plagioclase in the allanite-rich pods at Smorgen Schaduwe.

Garnet occurs in all the allanite-bearing rocks (upto 22 vol.u/o), although it is a rare constituent in theplagioclase-quartz gneiss. It is generally present aslarge dendritic poikiloblasts enclosing allanite andplagioclase. Garnet locally forms a corona enclosingclinopyroxene and also occurs in complex symplec-tite with magnetite and quartz, replacing clino-pyroxene. Garnet compositions from the variouslithologies have restricted ranges: andradite*r,grossularrs almandineq_, (Table 2).

Clinopyroxene is present as dark green polygonalgrains (2-13 vol.9o) containing, in places, fine inclu-sions of rutile. It has a composition in the heden-bergite field and a low Na content (Table 2). In con-

ALLANITE IN CALCIC GRANOMLSES, NAMAQUALAND 8 l

Quartz is present in the calcic granofelses eitheras disseminated xenoblastic grains or, at Rietfontein,as coarse veinlets rimmed by clinopyroxene that isbeing replaced by garnet-magnetite-quartz symplec-tite. Magnetite occrus as rounded anhedral grains,altering to a hematite rim, in the plagioclase-quartzgneiss at Smorgen Schaduwe and as arcuate platesin garnet symplectite in the allanite-rich pods. Smallrounded zircon grains are observed in cefiain allanitebands in the plagioclase-qtJafiz gneiss.

FoRMATToN oF ALLANTTE-(Ce)

Based on their bulk chemistry and local associa-tion with marble lenses, the calcic granofelses arebelieved to represent thin impure Fe-, Al-rich car-bonate sedimentary layers associated with rhyoliticto rhyodacitic extrusive volcanic rocks Qeucogneissesand biotite gneisses: Moore 1986) and overlyingpelitic and siliceous sediments (quartz-biotite-sillimanite schists and quartzites). At amphibolitemetamorphic grades, the carbonate sediments arerepresented by epidote-rich lithologies (Coetz@ 1941,Moore 1977).

Epidote has a relatively wide field of stability inthe.temperature range 300 to 700'C at the pressuresexperienced in the western NMC (5 kbars) (Liou1973, Liou et ol. 1983). Grandite garnet has an evenwider range in temperature at these pressures,forming inilialy in discontinuous reactions resultingin the breakdown of prehnite at temperaturesbetween 350 and 4ffi'C (Liou et al. 1983). Dependingon epidote composition and prevailingflO) condi-tions, epidote breakdown occurs at temperaturesbetween 600 and 700oC, forming garnet and calcicplagioclase (Liou 1973):

36Ca2FeAl2Si3Or2(OH) : 36CaAlzSizOs +epidote anorthite

l2Ca3FqSi3Os * 4Fe3Oa + 02 + 18H2O (1)andradite magnetite

In the same temperature range, epidote will reactwith ilmenite to form titanite (thermodynamic cal-culations based on Powell & Holland 1988) in reac-tions such as:

4Ca2FeAl2SirOrz(OH) + 3FeTiO3 * SiO2 =epidote ilmenite quartz

4CaAl2Si2O8 + CaFeSirOu +anorthite hedenbergite

2FqOa + 3CaTiSiO5 + 2H2O (2)magnetite titanite

Reactions 1 and 2 contain all the major constituentsof the calcic granofelses of the western NMC, otherthan allanite.

Frc. 2. Three grains of nonmetamict allanite-(Ce) (A)showing mutual l20o contacts and complex colorzoning, with a paler core, a darker mantle and an imper-sistent pale rim. Also present are plagioclase (colorlass),clinopyroxene @) and grandite gamet (C). Sample SS-4,Smorgen Schaduwe. Photo length 2.5 mm.

Ftc. 3. Large grain of sieve-textured garnet poikiloblast-ically incorporating allanite-(Ce). Sample SS-4,Smorgen Schaduwe. Photo length 2.5 mm.

Frc. 4. Prismatic to tabular grains of allanite-(Ce) (A),each with a dark core and a thin pale rim, aligned inthe fabric and associated with clinopyroxene @), titanite(l) and plagioclase (colorless). Sample SS-18, SmorgenSchaduwe. Photo length 2.5 mm.

82 THE CANADIAN MINERALOGIST

progressively consumed in the breakdown reaction,r8 ^ imall enclaves of increasingly REE tich epidote and16 X ultimatelv allanite became isolated within growing'o ; plagioclase and garnet. This process of allanite for-,, i mation in addition accounts for the relatively low-

levels of the REE within allanite, as the R-AE-concentrating process would continue only untilsufficient concentrations of the R.EE were accom-modated within the epidote-group mineral to stabi-lizeitatthe prevailing P-Tconditions of the granu-lite facies (-750'C, 5 kbars).

*.\:Rt

Frc. 5. a. Simple compositional zoning across a prismaticgrain of allanite-(Ce), illustrating the Ce-poor' Ca-richrim. Sample SS-18, Smorgen Schaduwe. b, Complexcompositional zoning across large grain of allanite-(Ce),illustrating the Ce-poor, Ca-rich rim and core, and theCe-rich, Ca-poor mantle. Sample SS-4, SmorgenSchaduwe.

In contrast to epidote, allanite is a stable mineralphase in the low-Tportion of the granulite facies inthe western NMC (-750'C). The upper stabilitylimits of allanite in metamorphic terranes have yetto be reported, although critical saturation temper-atures of approximately 800oC are reported forallanite crystallization in silicic magmas (Chesner &Ettlinger 1989). Assuming that the calcic granofelsescontained REE-bearing epidote, then, on commence-ment of epidote breakdown during progrademetamorphi$m, the REE could not be accommo-dated by newly formed plagioclase, garnet andclinopyroxene. Instead they would be progessivelyconcentrated in residual allanite enclaves. In themajority of calc-silicate rocks, only minor REIE'con-centrations may be expected in epidote, resulting inthe formation of allanite as an occasional traceconstituent.

In the case ofthe calcic granofelses in.the westernNMC, however, significant REE concentrations (atleast 2 wt.9o REf oxides) must have been presentin epidote to account for the large amount of allanite.The above mode of formation of allanite wouldexplain its characteristic sieve-texture withinplagioclase and garnet (Fig. 3). As epidote was

The narrow Ca-, Al-rich rim displayed by virtu-ally all allanite grains formed in response to a post-peak cooling episode. The more complex pattern ofzonation in large grains from the allanite pods (Fig.2), however, represents a combination ofthe aboveprocess superimposed on an earlier reverse zonation.The latter zoning to a more Ca-, Al-rich core is aconsequence of diffusion of the REE into theenclaves of residual allanite during epidote break-down. Similar zoning to a Ca-, Al-rich core has beenreported from metamorphic allanite in biotite-plagioclase-quartz hornfelses (Black 1970) and isassociated with migation of the REE duringmetamorphism. Zonation in allanite of igneousorigin is generally of the opposite sense' from RE'E-rich core to a Ca- and Al-rich rim Morin 1977,Harding et al. 1982, Gromet & Silver 1983).

R.EE ENntcuusNr

The primary, but localized, concentrations of theREE in calcic granofelses of the western NMC aresomewhat enigmatic, and several possible geneticmodels may be considered. The allanite-bearinggranofelses generally show at least 1000 x enhance-ments in light REE contents compared to similargarnet-plagioclase granofelses in the western NMC(Moore 1986) and immediately adjacent clino-pyroxene granofelses (Table l). Three differentprocesses of enrichment, both pre- and syn-metamorphic, are considered below:

Detrital concentrutions of REE-bearing minerals

The banded nature of the basal plagioclase-quartzgneiss at Smorgen Schaduwe and the plagioclase-allanite-magnetite-titanite layer at Rietfontein sug-gests that processes of sedimentary sorting may havebeen responsible for the concentration of detritalREE-bearing minerals as layers of heavy mineralsin association with impure carbonate beds. If thelayers do represent heavy-mineral sands, however,they must have been of an unusual composition, asmodern-day REE placer deposits are dominated bythe gangue minerals ilmenite, rutile and zircon andcontain monazite as the principal REE ore (Neary& Highley 1984). Magnetite-, ilmenite-, and zircon-

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83

bearing heavy-mineral layers are reported from leu-cogneisses and quartzites elsewhere in the westernNMC @rick & Wheelock 1983, Lipson et al. 1986,Moore 1986).

Epidote occurs as one of the dominant heavy-mineral constituents in the Miocene diamondiferousgravels of the Orange River (van Wyk & Pienaar1986). It is conceivable that allanite may be selec-tively concentrated during the weathering of felsicvolcanic rocks. Allanite commonly occurs as a minorconstituent of felsic to intermediate igneous rocks(Ghent 1972, Morin 1977, Brooks et al. 1981,Harding et al, 1982, Gromet & Silver 1983,Mitropoulos 1987, Chesner & Ettlinger 1989).Allanite in the plagioclase-quartz gneiss has thehighest R.EE content (Table 3) and consists ofanhedral grains that lack both the sieve texture andcomplex reverse zonation patterns of the allanite-richpods. Titanite (after ilmenite, see reaction 2),magnetite and zircon (350 ppm Zr) have their highestconcentrations in this gneiss (Table l).

Metamorphic mobilization of the REE

Under certain conditions, the REE are highlymobile at,elevated temperatures, particularly in car-bonatite, hydrothermal and low-Z metamorphicsystems (Wood et ol. 1976, Mclennan & Taylor1979, Hellman et al. 1979, Andersen 1986, Gier61986). Mobility of the REE is largely dependenr onthe nature of the original mineral phases containingthe REE, composition of the fluid phase, and abilityof minerals formed during the reactions to accom-modate the REE from the fluid (Humphris 1984).Fluorine-R.EE complexes in the fluid phase are con-sidered to play a significant role in the migration ofthe REE (Alderton et al. 1980, Humphris 1984,Andersen 1986, Gier6 1986). Light-REE ftactiona-tion may occur during fluid transport owing to selec-tive mobility of the R.EE (Hellman et sl. 1979,Andersen 1986) or by selective accommodation innewly formed minerals.

During the initial low-Z phase of the major pro-grade Namaqua metamorphism, it is conceivable thatthe REE were mobilized by leaching from minormineral phases in the metavolcanic rocks and selec-tively accommodated in epidote that was forming inadjacent impure carbonate horizons. Minor amountsof F in titanite in the calcic granofelses suggest thatF-REE complexes may have been present. If theREE-bearing fluids were not pervasive but restrictedor channeled, this may explain the localized natureof the allanite occurrences.

COyassociated REE concentrqtions

The granulite-facies metamorphic conditions in thewestern NMC are closely associated with the intru-

aBBr,a 3. @&lIe]L co,tP(Exncsl 08 ALLAifTxt-(ae) rRo{ glcRGgNs@lIInB

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*fe$ ettqea afts Drp@ (f987). od - b€Ifr det*ttoo ltFlt.f. Allel,ts-(ce) @E, sEple ss-4, s@(go sctl8dre.2. tlantle of @ qnia. 3. Rlo of €@ SEtn.a. AlIsttF(cE) @E, rople 9sF18, socg@ schadm.5. Rlo of, g@ SEin.

l@fyBla by el@trqr Dl@pEe (ca@r a@I@tt!q Fot@tlal25 kV, @ryIe qret 40 dl) aqalEt BttBI ard sfdtlEtlcEirel 6tardaEd6. REEe aDlirsed Elq DEko & sblll (1972)daDdads, Ll.F st€tal,, Ld peks fd 16, Oar Y attd LP psksf@ lild, lt, 8a. PAP @stl@ facl@ (Pordor & PlctDlr19S4) @ amued.

sion of suites of charnockitic and anofihositic tonoritic rocks (vanZyl1978, Mclver et al. 1983, NbaI1984, Andreoli & Hart 1987). Fractional crystalli-zation of the anorthositic-noritic suite led to theresidual concentration of silica-poor melts enrichedin oxides (magnetite, ilnenite), sulfides (chalcopyrite,bornite, pyrite) and phosphates (apatite, monazite)(vanZyl1978, Andreoli & Hart 1987). REEenrich-ment is apparent in these late-stage melts, as a resultof partitioning of the REE rollto the residual meltsand an associated CO2 vapor phase (Wendlandt &Harrison 1979). Charnockitic veinlets enriched inlithophile elements, including the REE, are presentin certain portions of the granulite facies of thewestern NMC, particularly in association with theanorthositic-noritic suite. These are believed to haveformed at the peak of the granulite-facies metamor-phism, when CO2-rich vapors containing high con-centrations of lithophile elements were released fromthe crystallizing H2O-poor magmas and penetratedthe country rocks (Andreoli & Hart 1987).

It is conceivable that such a vapor phase couldhave reacted locally with the calcic granofelses toform the allanite concentrations. The allanite-bearingrocks, however, do not contain the full complementof large-ion lithophile elements anticipated, with K,

97.4t 97.62

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ALLANITE IN CALCIC CRANOFELSES, NAMAQUALAND

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84 THE CANADIAN MINERALOGIST

Th and P noticeably absent. Certain textural features(REE zonation pattern, sieve textures, lack ofmetamictization) indicate the presence of a precursorREE-bearing epidote mineral prior to the peak ofmetamorphism,. when the deep-seated CO2-richvapor phase would not have been present.

Although the source of the R.EE in the calcicgranofelses is by no means resolved, a modelinvolving detrital concentration and localized low-Imetamorphic remobilization is currently favored.Allanite may well have been a minor phase in therhyolitic-rhyodacitic volcanic rocks, with localheavy-mineral bands forming during sedimentaryreworking. With the onset of regional metamor-phism, the R.EE were mobilized to a limited degreeand selectively reconcentrated in adjacent epidote-rich calc-silicate layers. Increasing grades ofmetamorphism resulted in epidote breakdown andthe formation of allanite of distinctive composition,zoning and texture in the calcic granofelses.

AcKNowLEDGEMENTS

JMM acknowledges partial research funding fromthe CSIR under the auspices of the CooperativeScientific Programme. Dick Rickard is thanked forhelp in unraveling the complexities of REEmicroprobe analysis; Chandra Mehl and NadimaEbrahim assisted with the bulk-rock analyses. Wethank two anonymous reviewers for providing inci-sive comments that led to improvements in the text.

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Received July 12, 1989, revised manuscript acceptedJanuary 4, 1990.