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Zirconosilicate Phase Relations in the Strange Lake (Lac Brisson)

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Zirconosilicate Phase Relations in the Strange Lake (Lac Brisson) American Mineralogist, Volume 80, pages 1031-1040, 1995 Zirconosilicate phaserelations in the Strange Lake (Lac Brisson) pluton, Quebec-Labrador,Canada SrrrlNo SALvr,* ANrHorw E. Wrlr.HvrsJoNns Department of Earth and Planetary Sciences,McGill University, 3450 University Street, Montreal, Quebec }J3A 247, Canada Ansrucr Petrographicobservations ofsubsolvus granitesat StrangeLake indicate that the sodium zirconosilicate elpidite crystallized under magmatic conditions, but that the calcium zir- conosilicatesarmstrongite and gittinsite are secondary.This interpretation is consistent with the extensive solid solution displayed by elpidite and the restricted compositions of armstrongite and gittinsite. Both calcium zirconosilicate minerals show textural evidence of having replaced elpidite, and in the case of gittinsite, with major volume loss. In the near-surfaceenvironment, gittinsite plus quartz fill the volume formerly occupied by el- pidite. At gfeater depth, gittinsite and armstrongite partially replaced elpidite but are not accompaniedby quartz, and abundant pore spaceis observedwhere gittinsite is the prin- cipal secondaryphase. Below 70 m elpidite is generallyunaltered. Replacementof elpidite by armstrongite is interpreted to have been a result ofthe cation-exchangereaction NarZrSiuO,r.3HrO + Ca2+: CaZrSiuO,r.3HrO + 2Na* elpidite amstrongite in which volume is nearly conserved, and replacement by gittinsite is thought to have resulted from the reaction NarZrSiuO,r.3HrO+ Ca2++ 5HrO: CaZrSirO,+ 2Na+ + 4H4SiO? elpidite gittinsit€ which is accompaniedby a 650/ovolume reductron. An alteration model is proposedin which external Ca-rich, quartz-undersaturatedfluids dissolved elpidite and replacedit with gittinsite, where a"o,ogwas buffered mainly by the fluid (high water-rock ratio), and with armstrongite plus gittinsite, or armstrongite alone, where 4"o.,ogwas buffered to higher values by the rock (low water-rock ratio). The for- mation of gittinsite createdextensive pore spacethat was subsequentlyfilled, in the upper part of the pluton, when the fluid became saturated with quartz as its temperature de- creasedduring the final stagesofalteration. INrnooucrroN curs as one or more of a large family of alkali and al- In most igneous rocks, Zr is an incompatible trace kaline-earth zirconosilicate minerals. The most impor- (NarZrSi.Oe-2H2O), element present in amounts ranging from < 10 to several tant of these are catapleiite dalyite (KrZrSi.O,,), (NarZrSiuO,,.3HrO), hundred parts per million and is accommodated as the elpidite eudialyte (OH,Cl)r], accessorymineral zircon. However, in peralkaline ig- [Nao(Ca,Ce)r(Fe2+,Mn,Y)ZrSirOr, vlasovite (NarZrSioO,,), (KrZrSirOn), neous rocks its concentration is commonly orders of and wadeite any of which principal partic- magnitude higher and may be locally sufficient to con- may be the zirconosilicate mineral in a (Vlasov stitutea potentially exploitabledeposit (e.g., Ilimaussaq: ular setting, e.g., eudialyte at Lovozero et al., 1966), catapleiite at Mont-Saint-Hilaire (Horv6th and Gerasimovsky,1 9 69 ; Lovozero:Kogarko, 1990; Strange (Fleet Lake: Miller, 1986;Kipawa: Allan, 1992;andThorLake: Gault, 1990),and vlasovite on AscensionIsland Trueman et al., 1988).T};.eZr mineralogyof peralkaline and Cann. 1967). rocks is also unusual. Although most peralkaline rocks Little is known about the factors controlling the distri- contain some zircon, the bulk of the Zr commonly oc- bution of zirconosilicate minerals, and there is little doc- umentation of their parageneses.The only reversed ex- perimental study that provides data on their stability at geologically meaningful conditions is that of Currie and * Presentaddress: Laboratoire de G6ochimie-CNRS, Univer- sitePaul Sabatier,38 rue desTrente-Six Ponts, Toulouse F-3 1400, Z,aleskt(1985) on the dehydration reaction ofelpidite to France. vlasovite and quartz. Marr and Wood (1992) constructed 0003-o04x/94l09l 0- I 03I $02.00 103I r032 SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS |TRANGELAKE a + 300km + E + + \ i.;'t - >,fl S----Ir5 ffi E# il1G,l+ I' ++ x@N*J : +++ + -:@ , ,, ++ + - lrsq, +++ ru €'# ,,+++ + '6 E +l +++++ + ELSONIANINTRUSION +l Fl quarlz monzonite +l APHEBIANBASEMENT +- p-1 amphiboliteand | -r metagabbro + + + + + |ll metadiorite f r--al qu2fir.Jeldspathic L-l & graphiticgneiss ffi EZI calc-srrrcategnetss Fig. 1. Location and simplified geologicalmap of the StrangeLake pluton (modified after Salvi and Williams-Jones, 1992). theoretical P-T diagrams for sodium and calcium zir- the calcium zirconosilicate armstrongite, which they conosilicate minerals using the results of Currie and Za- claimed occursboth as phenocrystsand as a replacement leski (1985),molar volume data, and the few constraints of elpidite. available from natural systems.Further advancesin our We reexaminethe genesisof elpidite, armstrongite,and understandingof Zr mineral petrogenesiswill require ba- gittinsite at StrangeLake, in light of textural and mineral sic thermodynamic data for the major phasesand careful chemical data collected from a representativediamond- documentation of field occurrences.The latter is the ob- drill hole locatednear the zone of economicinterest. These jective of the present contribution. data, in conjunction with mineral stability relationships In the Strange Lake pluton, Zr occurs mainly in the and published fluid-inclusion data (Salvi and Williams- form of the zirconosilicateminerals elpidite, armstrongite Jones, 1990),support a model involving low-temperature (CaZrSiuO,r.3HrO),and gittinsite (CaZrSirO,). These metasomaticreplacement of elpidite by armstrongite and minerals vary greatly in their relative proportions, and, gittinsite. locally, any one of them may be the principal or sole Zr phase.The highest concentrationsof Zr are in rocks con- Gnor,ocrct sETTrNc taining only gittinsite. Salvi and Williams-Jones (1990) The StrangeLake pluton (l 189 + 32 Ma, Pillet et al., observed that gittinsite plus quartz form pseudomorphs 1989), located in the easternRae province of the Cana- after an unidentified phase;on the basis of the morphol- dian Shield, is a peralkaline granite that intruded Aphe- ogy of the pseudomorphs and on fluid-inclusion evi- bian metamorphic rocks (1.8 Ga) and an Elsonian quartz dence,they suggestedthat a Ca-bearingfluid at -200 "C monzonite (1.4 Ga) (Fig. l). The intrusion is a subverti- caused the metasomatic transformation of the sodium cal, cylindrical body approximately 6 km in diameter and zirconosilicate elpidite to gittinsite + qvartz. Birkett et representsthe latest tectonic event in the area. Large roof al. (1992) subsequentlyproposed that gittinsite replaced pendantsare commonly observed(Fig. l), suggestingthat SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS r033 the presenterosional surfaceis close to the apical part of the pluton. On the basis of its peralkalinity, mode of em- placement, and age, the pluton may represent the Lab- rador extension ofthe Gardar anorogenic igneous event (Currie, 1985;Pillet er al., 1989). 18.0 Severalconcentric intrusive pulsesof peralkaline gran- ite formed the Strange Lake pluton, representedby hy- persolvusgranite, transsolvusgranite, and more evolved, 16.0 volatile-saturated subsolvusgranites (Nassif and Martin, l99l) (Fig. l). All phasesare F-rich and conrainprimary fluorite. They are mainly leucocratic, fine to medium 14.0 grained, and quartz saturated. Pegmatites are common and concentrated mainly in the subsolvus granite. An outwardly dipping ring fracture, marked by a heteroge- L 12.0 neous breccia in a fluorite-filled matrix, surrounds the N pluton. E The hypersolvusgranite consistslargely of phenocrysts o. 10.0 of quartz and perthite, and interstitial arfvedsonite, o. whereas the subsolvus granite contains two alkali feld- o spars(albite and microcline), and arfvedsonitecommonly o 8.0 occurs as phenocrysts.Transsolvus granite is transitional c) to the two other tlpes of granite, i.e., it contains perthite x phenocrystsin addition to albite and microcline. Arfved- 6.0 sonite in this unit occurs predominantly as fine-grained anhedra,imparting a melanocraticappearance to the rock. The hypersolvusgranite contains up to 50/oof zirconosil- 4.0 icate minerals, and the subsolvusgranite and pegmatites contain up to 300/oof these minerals. Elpidite is the prin- cipal Zr mineral in the hypersolvus granite, whereas el- 2.0 pidite, armstrongite, and gittinsite are the dominant Zr minerals in the subsolvus granite and pegmatites. kss common zirconosilicates at Strange Lake include cata- 0.0 pleiite, calcium catapleiite,dalyite, hilairite (NarZrSi.On. (t) (t) a(nu) 3H,O), vlasovite, and zircon (cf. Birkett et al., 1992). -s !F) ct) cn a Subsolvus granites commonly display hematitic alter- ,-:. A ation and, where altered, have higher Zr contents than & fresh hypersolvus or subsolvus granites (Fig. 2). Hema- Fig.2. A histogram showing the averagewhole-rock content tization was associatedwith the development of second- of Zr in the principal varieties of granite at StrangeLake. Ab- : : : ary high-field-strength element (HFSE) minerals, many breviations: hs hypersolvus,ts transsolvus,f-ss fresh sub- : : of which are Ca-bearing. Some of the most intense he- solvus, a-ss altered subsolvus,and s-ss strongly altered sub- solvus.Standard deviations are I.9, 2.4, 1.3,2.3, and 0.3 x I 000 matization is in the center of the complex, where a sub- ppm, respectively. economic ore zone has been delineated containing some 30 million tonnesgrading 3.25o/oZrOr, 1.30/o REE
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