119

The C anadian M ine ralo gis tVol. 35, pp. I 19-133 (1997)

MANGANOAN.FAYALITE.BEARING GRANITIC PEGMATITE FROM OUIRRA,SARDINIA: RELATION TO HOST PLUTONIC ROCKS AND TECTONIC AFFILIATION

ELISABETTAPANI

C.N.R, Centro Studi Geominzrart e MineralurgicL c/o DIGITL Univenitd. di Cagliari, Piazzn d.'Armi, 09123 Cagliari, Italy

ROBERTO RTZZO

Dipanimento di Geoingegneria e Temolagie Anbientali, Universitd di Cagliari, Pia.aa d.'Armi 09123 Cagliari' haly

MATIRATIDSEPP

Department of Eanh and Ocean Sciences, 63i9 Stores Roa{ The University of British Columbia. Vancoaver,B ritish C olwnbia V6T I 7A

ABSTRACT

Manganoan-fayalite-bearing pegrnatites occur at the northern margin of the Mt. Perdosu granitic pluton in the Quirra region,southeast Sardinia, Italy, in the apical part of a more extensive granitic complex. The granitic massif comprises post-orogenicA-type subsolvus biotite and biotite-muscovite leucogranites and minor monzogranites of the aluminous trend emplaced at ashallow depth at the end of the Hercynian orogeny. In the study area, pegmatites occur as lenses, veinlets, and pockes. All of thepepatitas are hosted in granitic rocks close to their contact with metamoryhic rocks, and are characterized by locally abundantaltered and unaltered manganoan fayalite, together with quartz and alkali feldspar. Fayalite occurs as partly altered large crystalsup to several tens of cm in size, as smaller, severely altered crystalline aggregates, and as nicrometer-scalg largely unaltereddrop-like inclusions in quarE. Generally, the altered crystals offayalite are enclosed in alkali feldspar, against which rims ofbiotite, cblorite and magnetite have developed. Subhedral to euhedral magnetite, manganoan grunerite, greenalite, ferripyrophyl-lite and laihunite are the products of subsolidus alteration of the fayalite. On the basis of geological, mineralogical andgeochemical data thase pegmatites me considered to be products of crystallization of aresidual liquid from a parent magm4 thecomposition of which can be represented by the least evolved granitic rocks outcropping in the area

Keywords: granitic pegmatite, manganoau fayalite, A-type granite, Sardinia ltaly.

Sotvrnrans

Nous d6crivons des exemples de pegmatite granitique i fayalite prBs de la bordure nord du pluton granitique du mont Perdosu,dans la r€gion de Quirr4 du sud-est de la Sardaigne, en ltalie. 11 s'agit de la partie apicale d'un complexe granitique plus 6tendu.Le massif granitique est fait de leucogranites post-orog6niques subsolvus de type A, i biotite ou i biotite + muscovite, et devenues monzogranitiques mineures, le tout d6finissant une lign6e hyperalumineuse; ces roches ont 6t6 mises en place i faibleprofondeur au terme de I'orogen0se hercynienne. Dans la rdgion 6tudi6e, les pep.alites se pr6sentent sous forme de lentilles, deveinules, et de poches, toutes encaiss6es dans des roches granitiques prds du contact avec des rcches m6tarnorphiques. Danschaque cas, la pegmatite contient de la fayalite manganifBre, localement abondante et alt6r6e, avec quartz et feldspath alcalin. Lafayalite se prdsente en gtos cristaux at[eignant plusieurs dizaines de centimdtres de taille, tous partiellenent al€r6s, en agr€gtsplus petits, dont 1'alt6ration estplus avanc6e, et en "goutelettes" g6ndralement non alt6r6es dans le quartz. En g6n6ral, les cristauxalt6r6s sont encaiss6s dans le feldspath atealin, le long duquel on peut voir un liser€ de biotite, chlorite et magn6tite. Unassemblage de magndtite sub-idiomorphe i idiomorphe, grunedte manganif re, greenalite, et laihunite r6sultede l'altfration subsolidus de la fayalite. A la lumibre des donn6es g6ologiques, min6ralogiques et g6ochimiques, ces pegnatitesrepr6senteraient des produits de cristallisalion d'un liquide r6siduel issu d'un magma parental dont la composition peut Ctrerepr6sent6e par les granites les moins 6voluds qui affleurent dans cette r6glon.

(Traduit par la R6daction)

Mots-cl6s: pegmatite granitique, fayalite manganif0re, granite de type A, Sardaigne, Italie.

r20 TIIE CANADIAN MINERALOGIST

Flc. 1. General geology of Sardinia- Geological base nodified after Bralia et aI. (1982). l: Metamorphic rocks. 2: T?o-micaleucogranite and cordierite-bearing gmnite. 3: Granite, granodiorite aud gneissic tonalite. 4: Tonalitic-gabbroic rtuases.5: Tonalite and tonalitic granodiorite. 6: Equigranular monzogranitic biotite-bearing granodiorite. 7: Biotite- and biotite-amphibole-bearing monzogranite and granodiorite. 8: Pink biotite-bearing monzogranite. 9: Equigranular biotite-bearingmonzogranite. 1 0: Pink biotic-bearing leucogranite. 1 I : Mesozoic, Cenozoic and Quaternary sequences of sedimentary andvolcanic rocks. 12: Faults. Box outlines the area ofFisure 2.

Moddoleno l . N

A

El . . ! {|.,'.g

ffi

f . ' ' 1

mm

mrFTil r

Nr

AXIAL ZONE

1 2

1 1

1 0

lrjzoN

lrlo-(L

z.

t!zoN

J

zE,lrJFxlrJ

0 2Okm

INTT.ODUCIIoN

The occurrence of fayalite in granitic pegmatites isfairly unusual. More commonlyo fayalite occurs insilica oversaturated to undersaturated felsic rocks (Frostet al. 1988). Fayalite is associaled with quara inhighlyevolved siliceous volcanic rocks ffills & Rose (1991),and references therein]; phenocrysts of manganoanfayalite occur for instance in pumice fragments of high-silica rhyolite from the Timber Mountain Ttrff, in Ne-vada. Fayalite also is typical of metamorphosed ironand manganese orebodies.

In Sardini4 Mona et al. (1994) re*owlzed fayaliteas a primary phase in peralkaline volcanic rocks atSulcis, southwestern Sardini4 but the only occurrencefound so far from a plutonic environment is withinpocket pegmatites in leucograniies nem Villacidro,southwestem Sardinia (Lovisato 1900). In this paper,we describe manganoan-fayalite-bearing granitic peg-mafites in the Quirra region, southeastem Sardinia inwhich fayalite @curs as partly altered large crystals,as severely altered crystalline aggtegates, and asmicrometer-scale largely unaltered drop-like inclusionsin quartz. [n addition, we describe the relation of thepegmatites to their plutonic host-rocks, post-orogenicHercynian subsolvus biotite leucogranites of theMt. Perdosu massif. These granitic rocls characteristic-ally contain fayalite and biotite, and are of a chemicalcomposition that is similar to A-t,?e post-orogenicalkaline granitic suite.s of the aluminous trend @onin1990). These rocks also have chemical characteristicssimilax to topaz-bearing rhyolites @once et aL 1993). Adetailed account of the crystallization history of thefayalite, and its subsequent hydrothermal alteration andweathering, is described elsewhere @an et al. L994,Raudsepp et al., tnptep.).

Grorocrcer, SsI]TlIc or SARD[,[A

The geology of Sardinia is characterized by varioussedimentary and magmatic cycles ranging ftom eadyPaleozoic to Recent in age @ig. 1). The Paleozoic crys-talline basement gives its fundamental stucture to theHercynian orogenic cycle (Carmignani etal. L992),which developed by intense deformation, by synkine-matic metamorphism, and by an important postcolli-sional, mainly infrusive magmatic cycle. The Sardiniancrystalline basement represents a segment of the south-European Hercynian Chain.

The structural fabric of the crystalline basement ischaracterized by three belts, referred to as ExternalZone, Nappe Zone, and ArtaI Znne, with a NW-.SEtrend, and a SW to NE zoning withrespectto metamor-phic grade. These belts represent a t€ctonometamorphiczoning developed from a low-grade sub-greenschistfacies to the greenschist facies (moving from the Exter-nal to the Nappe zone). The highest grade meta:norphicsequence, intuded by lafe Hercynian magmas, is in the

FAYALITE.BEARING GRANMC PEGMATITE, SARDINIA

1.78

0.06

9163.00

Axial Zone, where the metamorphic grade rapidlyincreases from greenschist to amphibolite facies withmigmatites. Here, the high thermal gradients that werepresent during the post-collisional event caused wide-spread anatexis and the emplacement of several bodiesof syn-tectonic peraluminous gfanitic magma.

The Sardinian batholith of Hercyniat age (Ghezzo &Orsini 1982) is a typical composite sale-alkaline andsubordinately peraluminous batlolith that intrudedthe metarnorphic complex produced by the regionalHercynian metamorphism (Fig. 1).Most of this batho-lith was emplaced at a relatively shallow deptft it ischaracterized by two cycles of intrusive events. Thefhst cycle yielded syn-and late-tectonic tonaliles,granodiorites, monzogranites and leucocratic mon-zogranite infiusions @ralia et aI. 1982); the secondcycle is chamcteized by post-tectonic biotite-bearingleucogranites (Guaspariet aL 1984a). According to theclassification of Chappell & White (1974),Gbezzo andOrsini (1982) proposed that the Sardinian batholith ischaracterized mainly by 'of'-Op", and, subordinately,'S"-type plutonic rocks. Also, these features distin-guish the Sardinian batholith from the other EuropeanHercynian batholiths, which seem to be predominanilyof o'S"-g4re

@tcher 1979), although the abundance ofthe I-type granitic bodies is quite considerable @inger& Steyrer 1990). However, in our study area, theMt Perdosu granitic massif exhibits chemical charact€r-istics more like those of A-type post-orogenic alkalinegranitic suites of the aluminous nend @onin 1990).

EpsRtruB.trAL ME"ffIoDs

Electron-probe micro-analyses of fayalite (Iable l)were done on a fully automaJed CAMECA SX-50 mi-croprobe, operating in the wavelengfh-dispersionmode, with the following operating conditions: excita-tion voltage 15 kV, beam current 20 nA, peak count

TABLB 1. SELECIED RBSTJLTS OFELBCTRON.PROBE MICROANALYSBSOFMANGANOAN FAYALITB, QUTRRA SARDINIA

SiO2 wt.7o 29.83 29.M 29.58 29,59 3057

Feo 63.17 &.49 61.80 52.14 57,23

Mgo 1.15. 1.23 0.63 O,U o23

MnO 5:16 537 7J9 18.50 ll.4'l

Torsl 9.91 1m.13 99.80 l@.47 99.50

Nuber of iw oa the bsis of 4 O

1.00 0.98 l.m 1.00

121

siFgl

Mg

Mn

Total

t.82

0.06

0.15

3.m

t.75

0.03i ,

3.@

t.47

0.01

3.00

1.03

l .6 t

0.0r

0J32.57

l. Fsyalite ftom ualtsred @rc of largc rcned oystsl,2. Fayalite ftm ualter€d pan of sra[q @!ed cry$81.3. Drctrlike fayaliE froo @Epcite agigregFle.4. DrotrliLe fayalite ftom @nposite agigregple.5. Droplike feyalite ftom NiarDd leu@gmite.

122 TTIE CANADIAN MINERAL@IST

QIIIRRA FAYALIIE.BEARING CRANTIC ROCKSquartz, orthocla$e, and albite (Hfut 1991). Bucla (1993)described fayalite from coarse-grained pegmatiticohests" 15-25 cm in diameter occurring with basicenclaves in a monzogranitic intrusion ofHercynian age;here the fayalite is found as anhedral to euhedral pris-matic crystals measuring 4 X 6 mm. Palache (1950)summarized the occurrence of fayalite at four localitiesat Cape Ann, Massachusetts. Although all of the fay-alite is found in various granitic pegmatites, the evi-dence presented suggests that the pegmatites are notrelatred to the Rockport granite, but are xenoliths prob-ably related to an earlier fayalite-bearing quartz-bearingsyenite.

In southeastern Sardinina bodies of fayalite-bearinggranitic pegmatite occur at the northwestem margin ofthe Mt. Perdosu massif @g. 2); the largest and mostdifferentiated are best exposed in an erosional window@igs. 2, 3) cutting through the overlying Ordovician-Silurian metasedimentary rocks into which theMt. Perdosu massif had been intruded. Smaller bodiesof pegmatite also occur scatfered near the northerncontact between the metasedimentary rocks and themain exposure of the massif (Figs. 2, 3). The absoluteage of the pegmatites has not been determined, butfield, minsplogical and geochemical data suggestthat they are post-orogenic and closely related to theHercynian leucogranites.

The pegmatites occur as lenses, veinlets and pocket-shaped bodies. All are hosted by tle granite close to thecontact with the metasedimentary rocks, which sug-gests that the pegmatites crystallized in the upper partof the cupola of the host pluton (Fig. 3). Two types ofpegmatite bodies are exposed: (i) a large lens-shapedbody with partly altered crystals of manganoan fayalite("main" pegmatile, Fig. 3, Unit 8); (ii) numerous small,variously shaped bodies with or without fayalite, and asubhorizontal pegmatitic facies, a grain-size variationof the granitic pluton (ooothet'o pegmatites, Fig. 3,Units 5, 6, 7). Selected results of electron-probe micro-analyses of manganoan fayalite from the pegmatitesand host granitic rocks are given in Table 1.

Mainpegmatiu

The lens-shaped main pegmatite (Fig. 3, Unit 8) isfound in the fine-grained leucogranite (Frg. 3, Unit 3),tends northwest-southeast, measures 20 X 60 m, andcontains abundant large crystals and crystallineaggegates ef manganoan fayalite. The contact with thewallrock is generally staight and sharp, but is locallycomplexly contorted and diffrrse. Also present in themain pegmatite is microcline, albite, quartz, and biotite.Muscoviie occurs as aggegate,s of fine crystals or fillsfractures, mainly in quartz and alkali feldspar.

Abundant occlurences of manganoan fayalite arefound along the upper part of the pegmatite in tlreetypical habits: (i) spectacular large anhedral to subhe-dral single crystals or composite groups of a few

Q l l L7 Q%B Q18SiO2 wtqo 75,69TiO2 0.11Alror 13.11FezQ* 1.46MtrO O.O1MgO 0.16cao O.nNazO 3.56KrO 4.58PzOr 0.03L.O.l. 0.56Total 100.10

75.36 76.730.04 0.05

t3.71 t2,701.10 1.400.04 0.040.01 0.01o 0.v3.60 3.205.01 4.860.01 o.g20.66 0.71

9.89 100.08

35.19 38.752,41 1.5'7

29.7t 8.8230.46 27.U0.00 t,at.2t t. l4

?< a t

0.06t2.68t.69o,l20.030.653.054.E2o.a2o.v2

1m.05

n30o.g2

n,n0,970.04

<d"1.o.2l4.184.Mo.v2

100.20

0.03

1.060.06

< d"l.o233.685.000.010.48

l@.19

35.231.44

29.6831.t4

1.081.12

CI"P.W. omstive miseBk

Cm O.Yz l.l3Or n.l9 2924Ab 30.t2 25.81Ao 3J3 3A"Sr t.gl 1.10

3?.051.26.@

35.37o.94l . l 1

A.S.I. = almina satmtim index (Alrqy(CaolNa2olK2o)i 3!ot8l irotr I Fe2o3.

time 20 s, background count time 10 s, beam diameter1 trrm. Data reduction was done with the "PAP" Q(p!method @ouchou & Pichoir 1985). For the elementssoughg the following standards, X-ny lines and crystalswere used: synthetic Mg2SiOa, MgKcr, TAP; syntheticFe2SiOa, SiKtr, TAP, FeKoc, LIF: synthetic MnSiO3,MnKoq LIF. The following elements were sought butnot detected: Na, A1, C4 Cr, Ni.

Whole-rock major-element analyses (Table 2) weredone by X-ray fluorescence spectromety usirig thefusion method. FeO was determined using ammoniummetavanadate titation Trace-element analyses (Iable 3)were done by fusion-mode coupled plasma- mass specfrometry; the concentation of Li was estab-Iished using total digestion.

TsxlrRAL AND CovposmoNer DETArr,sCoNcsRNtrtc fifi FAvALmE AND rrs

ALTtsRATToN h.oDucrs

The man ganoan-fayalite-bearing granitic pegmatitesconsidered here are similar in mineralogical aspects tothose previously described in the literature. Janeczek(1989) outlined in detail the multi-stage hydrothermalalteration of anhedral fayalite (up to 9 cm in diameter)from unzoned schlieren pegmatites up to 0.5 m wide inbiotite-hornblende Hercynian granite near Sfizegom,Lower Silesia. The fayalite there is surrounded bymicrocline, has a manganoan greenalite - magnetiterim, and a manganoan grunerite - magnetite - man-ganoan minnesotaite outer zone in a stilpnomelane- or$eenalite-rich groundmass. In the fayalite-bearingpegmatites from ttre Sawtooth bathotth, central ldaho,fayalite occurs as euhedral crystals intergrown with

FAYALITE.BEARING GRAN]TIC PEGMATITE SARDINIA t23

individuals up to 15 X 30 cm in size enclosed bymicrocline and variously altered by subsolidus, hydro-thermal and weathering processes @gs. 4.A', B); (ii)smaller, more or less completely altered irregularaggregates of several crystals up to a few cm in sizeenclosed by microcline (Fig. 4C); (iii) micrometer-scale drop-like inclusions in quartz in composite aggre-gates consisting of biotite, chlorite, quartz, and raremuscovite (Fig. 4D). Notably, such dropJike crystals

of fayalite also occur in the quartz of the host leu-cogranite adjacent to the pegpatite bodies.

Although some of the large crystals of fayalite seemto be aggregates, the best example of this type is opti-cally continuous in all ofits unaltered parts and appearsto have been a single subhedral crystal imbedded inmicrocline (Frg. A). The degree of alteration of thelarger crystals (15 x 30 cm range) is controlled by thesize of the crystalo its cleavage and by fractures. The

TABLE 3. TRACE.ELEMENT CHEMICAL COMPOSMON OF SELECTED QTJIRRAFAYALIE.BEARING GRANMC ROCKS

L7 Q23t3 Q10 Q18 QzeQ l l

Ni ppm 5.0Pb 44.07^ 57.0Bi < d.l.Sn 7.0w 1.4Mo 23Rb n7.2Cs 4.3Ba 2@2Sr 38.8Tl t.7Ga 19.0Li 151.0Be 7.0Ta 2.9l\Ib 1?.9IIf 3.8Zt 88.6Th 23.2u 6.5Y 65.5IA 23.5Ce 53.0Pr 6.3Nd 26.6Sm 8.8Eu 0.40Gd 8.8Tb 2.0Dy lO.2Ho 2.2Er 7.1Tm 1.2Yb 7.4Lu l.l'REE 158.6

'-RBE 118.6

XIREE 4O.O(Irsm)n 1.7(GtVLu)1 1.0(Lallu)n 2.2Nby'Ta 4.2Ga/Al 2.7

8.0 2l.o65.0 80.039.0 146.04.7 0.59.2 ZL.92.8 2.22.2 2.5

309.6 341.52.8 3.6

30.9 53.89.4 l1. l1.8 3.0

24.0 23.028.0 32.010.0 3.06.5 4.8

23.2 19.23.6 4.5

n.2 68.421.8 25.57.4 11.1

103.8 81.915.1 13.935.8 34.05.3 3.6

25.t 20.r11.8 7.30.03 0.10

14.5 9.33.4 1.9

17.8 t3.43.8 2.8

t2.3 8.72.0 1.4

13.6 9.81.9 1.6

to.4 rn.g93.1 79.0

t2.066.0

131.0o.4

15.92.73.3

410.94.3

467.925.53.4

22.O120.0

14.34.0

80.129.45.6

83.217.441.84.4

25.48.40.1 89.21.9

12.82.66.J

t .49.31.5

144.897.5i 1 a

t .30.7t.26.0J . J

13.032.094.0

< d.l.7.40.9

245.42.8

96.631.52.1

2t .o36.03.01.07.92.1

37.118.04.0

24.8t2.o26.12.7

l ) . J

4.30.324.50.84.50.92.80.42.70.4

77.760.7

1.81.33.07.92.9

2t .o40.0

t14.00.13.8l . J

4.0240.0

3.4105.416.92.0

2r.03'1.02.0

15.13.4

63.929.612.74.613.231 .0

5.2

18.35.90.207.21.5

10.22.16.61 .16.8l . l

108.471.836.61.40.81.26.33.0

69.30.81.00.83.63.5

48.91.2o.70.94.0J . J

n.d. = not determined; d.l. = detection Iimit.

rz4

//: //: /,

rM

0 1 k m

oocn

oq)

++

++

++

++

++

+

T

+

+

+

+

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++

++

++.+

-r

/|r s\ \ \

++

+

l4\l .l/t i7t i\ ,*. i

ltiltil\.)tiiNlittitlq/lI'(i//t///)iiiitith

++

N

,tI

zffifv v v'll v v It-v-.,!--JJ

affi

Em

Ftc. 2. Geological sketch map of Mt Perdosu granitic massif and the fayalite-bearing pegmatites. 1. Ordovician-Silurianmetasedimentary rocks. 2. Granitic rocks. 3. Greisen. 4. Pegmatites. 5. Basic subvolcanic dikes. 6. Hydrothermal lodes.7. Faults. 8. Recent alluvial deposits. Box outlines fhe area ofFigure 3.

largest crystal exhibits three primary zones: core, inter-mediate zone, and rim (Fig. 4B). The core is clear,colorless to pale yellow fayalite, invariably opticallycontinuous, and with sparsely scattered enclosed aggre-gates ofgrunerite + magnetite t greenalite that are leastabundant at the core. Along well-developed cleavagesand near fractures, the fayalite is of a darker yellow topale reddish brown color due to oxidation. Locally,there are veins filled with greenalite, calcite and Fe-richsphalerite. The Mn content of the unaltered core, havingthe struc0ral formula (Fer"8lvlgb.06Mrb.ro)Sir.mOa, variesbetween 5 and 6 wt.7o MnO (Table l, No. 1).

The rim consists mainly of various phyllosilicateminerals, of which a:rhedral green to pale brown-greento dark brown-green pleochroic biotite [Fe/@e + Mg) =0.851 is the most abundant. At the outermost edge, the

biotite is deformed and mixed with very fine-grained,Fe-rich red-brown to opaque mica-like material. At theboundary with, and in some cases extending into theintermediale zone, ferripyrophyllite occurs as anhedraldark yellow, nonpleocbroic grains. Subhedral maguet-ite and anhedral quartz grains also occur scatteredthroughout the rim.

The intermediate zollie comprises the major part ofthe crystal, and is generally concenffic about the core(Fig. 4B). However, because of a major fracture,the intermediate zone cuts tle core into two parts(Ftg. 4B). The intermediate zone consists essentially ofmicrometer-scale intergrowths of laihunite, iron oxidesand hydroxides. In some previous studies (e.9.,Janeczek 1989), this material has been described as'1ddingsite". We have positively identified the bulk of

I

7

6

5

4

3

2

N

t

+ ++ +

+ \+-d+++ + +

+ + ++ + +

+ + +

+ + + ++ + + +

+ + + ++ + + +

+ + + +

++ O +

++ +

+

++

+

++

++

++

Q29Q18

FAYALITE.BEARING GRANMC PEGMATITE. SARDINIA

200 m

Ftc. 3. Detailed geology of the pegmatite locality, showing sample locations. Host rocks: 1. Ordovician-Silurian metasedimentaryrocks. 2. Medium-grained leucogranite. 3. Fine-grained leucogranite. 4, Greiser! pegmatites, 5. "Othet'' euhedral quartz,alkali feldspar pepatite facies. 6. 'Othet'' 6 X 11 m lens-shaped pegmatite. 7, 'Ottret'' altered fayalite-bearing pegmatites.8.'Main" fayalite-bearing pegmatite.

125

8Nlol

l \ l

ffi|...!...!..!...!g

f+ +lL***J

f-----11

t l

7

6

5

4

3

2

the alteration product to be laihunite from X-ray dif-fraction and electron-probe micro-analysis. The pres-ence of significant Fd* in fayalite from a variety ofrocks (including granitic pegmatite) due to finely inter-grown laihunite has been documented by others (e.9.,Schaefer 1983, 1985).

Near the rin the intermediate zone contains abun-dant magnetite with small grains of quartz, and large,irregular patches of brown-stained but generally color-less grunerite. Further away from the rinc, grunerite ispresent only as isolated acicular and prismatic grains,generally with magnetite, miasl quartz and raregreenalite. The contact with the core is very iregular,but shows clearly that the alteration of the fayalite wascontrolled by the fractures and its cleavage. The Mn

content of unaltered parts of the intermediate zone issimilar to that of the core, varying between 5 and6wt.%oMnO.

Inegular aggregates of fayalite occur also only inmicrocline, and are probably smaller equivalents of thelarge crystals described above @g. 4C). Their mor-phology and mineralogy are different by virtue ofsevere to complete alteration of a smaller volume ofprimary malerial. These grey to dark green to brownaggregates are also chwactoized by a rim of biotite,mixed with chlorite and magnetite. Because of the smallsize ofthe aggegates, the alteration assemblages are notzoned as in the large crystals, but consist of complexmixtures of some relatively unaltered fayalite (rare),laihunite, iron oxides and hydroxides not yet defined

126

magnetite, gunerite, greenalite, plus phases not relaledto fayalite, such as kaolinite and unidentified clays.Typical bulk Mn contents vary between 3 ard 9 wt.VoMnO.

Mcrometer-scale drop-like inclusions of fayalite inquartz are found in composite aggregates consistingessentially of biotite, chlorite, quartz, and rare musco-vite @ig. 4D). The MnO content of individual inclu-sions of fayalite, in round to amoeboid grains, variasbetween 7 and l9Vo, i.e., much more and more variablethan the fayalite described above (fable 1). Most ofthese drop-like inclusions of fayalite are monominer-alic; some, however, have magnetite and greenalite atthe fayalito-quartz boundary Gie. 5A). These drop-likeinclusions provide the clearest evidence of magmaticcrystallization of the fayalite. Drop-like crystals of fay-alite in quaftz in the associated granitic rocls also havehighMn content, aboutl2wt.Vo MnO (fable l, No.5).

The Mg content of drop-like fayalite in both the aggre-gates and granitic rocks is significantly lower (about0.24.6 wt.Vo MgO) than those of the larger crystals(about 1.2 wt.VoMgO).

Other pegmatites

Numerous small veins, pockets and lens-shapedbodies (1-11 m in length) of pegmatite are foundwithin about 600 m of tle main pegmafile. Some occuradjacent to the main pegmatite and are exposed by theerosional window in the metamorphic rocks; others areat the contact between the massif and the metamorphicrocks to the south @igs. 2, 3).

Near the greisen facies at the northwest margin of themain pegmatite (Fig. 3), there is a subhorizontal peg-matitic facies, which is developed as a grain-size vari-ation of the granitic pluton; it comprises massive quartz

Fro. 4. A. Typical large crystals of fayalite in potassium feldspar. Scale card marked in cm. B. Photograph of cut surface throughthe center of a typical large crystal of fayalite (30 cm long) showing the unaltered core (pale grey), partly to completely alteredintermediate zone (dark grey to black), and micaceous rim (mostly missing). Note the fracture-control of the degree ofalteration, and veins of calcite, greenalite and sphalerite (pale grey to white). C. Altered irregular aggregate of fayalite witha rim of dark mica- Coin is about 2 cm in diameter. D. Back-scattered electron image of drop-like fayalite inclusions (white)in quartz (grey) from composite quafiz - biotite - chlorite aggegate. Scale bar: 100 pm.

FAYALITE-BEARING GRANMC PEGMATITE. SARDINIA IN

FrG. 5. A. Back-scattered electron image of drop-like fayalite 0ight gey) with magnetite (white) and greenalite (medium grey)included in quartz (dark grey). Scale bac 1@ Um. B. Back-scattered electron image of magnetite (white) + quartz (mediumgey) and secondary iron oxides and hydroxidgs (hght greg in aggregate from 6 X 1l m'bthey''pegmatite. Scale bar:2@rtm.

and large euhedral crystals (>10 cm), microcline, andirregular aggregates (up to 10 cm) of alteration prod-ucts after fayalite. The.se aggregates are complex inter-growths of magnetite + grunerite + qtJartz scatteredtbroughout a groundmass that has a composition iden-tical to tlat of fayalite. In the pegmatite, the quartz ispresent both as euhedral crystals (>10 cm) perpendicu-lar to the projected contact rvith t}te overlying metasedi-mentary rocks and as anhedral masses.

The largest of the "othet'' pegmatites is a 6 X 11 mlens-shaped body at the contact befween metamorphicrocks and fine-grained leucogranite @ig. 3). The majorminerals are massive microcline and euhedral quartz upto 10 cm in diarneter. Scattered fhroughout the micro-cline are aggregates of intergrown magnetite + quartz(4-5 cm in size). The magnetite occurs with two habits:skeletal intergrowths with porous quartz, and anhedralto subhedral crystals embedded in larger, massivequafi (Fig. 5B). The quartz-magnetite intergrowthscontain abundant secondary iron oxides in fractures andfilling cavities. The magnetite-quartz aggregates in thispegmatite contain no material that appears to be alteredfayalite.

The other small veins, pockets and lens-shaped bod-ies of pegmatite up to 3 m long are similar in mineral-ogy. Quartz and feldspar are the main constituents, withscattered magnetite + quartz and magnetite + quartz +grunerite aggregates up to 5 cm long and similar tothose described above. Locally, unidentified mica-likeminerals (probably clays and ferripyrophyllite) andmixtures of iron oxides and hydroxides are abundantproducts of weathering. Some of the aggregates are richin fayalite, now altered to a mixture of laihunite andvarious oxides and hydroxides. Notably, some of the

magnetite + grunerite intergrowths also contain fine-grained grunerite + magnetite symplectite.

GSOI-OGY AND GEOCffiVflSlRY OF T{E

Assocrarno Gnermc Rocrs

In general, the Quirra granites belong to the Mo-Wspecialized Sardinian leucogranite swts (Ghezzo et al.1981, Guasparri et al. 1984b). The Mt. Perdosu massifcomprises plutons of post-orogenic monzogfanites andleucogranites, which were emplaced at a shallow levelin the crust in a distensive regime at the end of theHercynian orogeny @iste 1979, BraJia et al. 1982,Guasparri et al. 7984a). All of these granitic rocks andrelated pegmatites inftuded the Ordovician-Silurianmetasedimentary rocks belonging to the Gerrei tectonicunits (Camignani et al. 1986); they are now generallymetamorphosed to hornfelses and skam-type facies.

The portion of the Mt. Perdosu massif exposed in theerosional window has a well-defined vertical zoning intexture and mineralogy (Fig. 3). The most internalleucogranite is typically medium- to coarse-gminedsubsolvus biotite leucogranite (Fig. 3, Unit 2), whichgrades into a fine-grained facies toward the externalpart of the pluton (Fig. 3, Unit 3), together with agreisen-type facies @ig. 3, Unit 4) where muscovitepredominates over biotite @iste 1979). Upward, thesame fine-grained leucogranite grades into a subhori-zontal pegmatitic facies characteized by euhedralquartz crystals (>10 cm) perpendicular to the contact,together with massive microcline (Fig. 3, Unit 5). Thelast stage of consolidation of the gfanitic magma isrepresented by the emplacement of the pegmatitebodies @g. 3, Units 6,7, 8).

A

t ' .rg\+rt \

!

t28 TIIE CANADIAN MINERALOGIST

36

go,Y+o.Uz=ro-3

* 1 5oYt t o(!z

Metaluminous Peraluminous B

-/Peralkaline

0.4 0.8 1.2 1.6 2.0Al2Os/(CaO+Na.O+KrO) (molar)

Ftc. 6. Chemical characteristics of Quira granites using A. Total alkah versus silica plot (IAS) (Middlemost 1994) (.medium-grained leucogranite, r fine-grained leucogranite, I medium grained leucogranite assosiated with greisen" Vfine-grained leucogranite associated with main pegmatite, + fine-grained leucogranite associated with euhedral quartz, alkalifeldspar pepatite facies). B. alumina saturarion diagram (Maniar & Piccoli 1989). Symbols as for Figure 6A. C. himitive-mantle-normalization diagram (Sun 1982) (. Ql1, + Q10, r Q29, tr Q18, I L7,f Q2313). D. Chondrite-normalized REEpatferns [normalization values after Evensen et al. (1978)]. Symbols as for Figure 6C. E. Ocean-ridge-granite-normalize<lelement pafiems @euce et al. 1984). Symbols as for Figure 6C.

uJFt,ozoIoPotf

IIJJFz

P

c r l

Cs Ba U Nb Ce Nd Zr EuGd Y YbRbTh K La Sr Hf SmTi Dy Er Lu

LaCe Pr Nd Sm Eu GdTb Dy Ho Er Tm Yb Lu

K2O Rb Ba Th Ta Nb Ce t4 Zr Sm Y Yb

FAYALITE.BEARING GRANMC PEGMATITE. SARDINIA r29

The mineralogy (compositions determined byelecfron-probe micro-analyses) of the granites associ-ated with the pegmatites consists mainly of quartz,microcline (Orsr-ee) with or without an obvious perthi-tic texture, plagioclase (Anrs-t, biotite [Fe/(Fe + Mg)in the range 0.81-0.95; A1.r in the range 3.2-5.3(n qj and minor muscovite. Accessory minerals in-clude fayalite, zircon, apatite, fluorite, rarely sphaleriteand other sulfides, niobian rutile, xenotime-(lO andmonazite-(Ce). In the total alkali versus si\ca diagram(Fig. 6A), the Quirra suite plots within the mon-zogranite field. Di $imFlicio et al. (1974),Bralia et al.(L982), and Guasparri et al. (1984a) considered theserocks to be leucogranitic owing to their low content ofmafic minerals (modal content of biotile <5 vol.Vo).

Resulr of major- and tace-element whole-rockanalyses, and nomative mineralogy of representativesamples of each granitic facies, are given in Tables2 and 3. These rocks are characterized by highSiO2contents (>75 wt.Vo),low Ca (<0.81 wt.7o CaO),N(<L3.77 ttrt.Vo AJ2O) and high FeO6/MgO (8-140)and high alkali abundances (Na2O + K2O in the range7.72-4.68 fi.7o). Moreover, ttrey are peraluminous, asshown by their position on the alumina saturationdiagram of Maniar & Piccoli (1989) @ig. 6B), togetherwith normative corundum contents (0.71-2.52 tttt.Vo,Table 2).

Trace-element concentrations in these granites arecharacleristic of a generally high degree of fractiona-tion. Selected samples representative of the graniticfacies are plotted on spider diagrams (Frg. 6). Ba and Srcontents are very low @ig. 6C, Table 3), except for themedium-grained leucogranite facies (Ql1, Fig. 3, Unit2) and for the associated greisen facies (L7, Fig. 3, Unit4). This is to be expected, as they are probably derivedfrom an eadier, less differentiated facies. Furthermore,Cs (<12 ppm), Rb (<456 ppm), and U (<14 ppm)(Fig. 6C, Table 3), and Be (<10 ppm) and Li(<181 ppm) (Iable 3) are relatively high.

In spite ofboth the highly fractionated and peralumi-nous character of the magmas from which these leu-cogranites crystallized, concentrations of the rare-earthelements (REE) are low (total REE it the range33-190 ppm). All the samples are slightly enriched inthe light rare-earth elements (L,REE) comparison to theheavy rare-earth elements (HPEE) Cfable 3). In a plotof chondrite-normalized abundances (Fig. 6D), theREE fractionation among the selected samples of thegranitic facies is evident; the more depleted frend isshown by the granite sample intimately associ*ed withthe main pegmatite; also, the LREE ard HREE frac-tionation is not pronounced, as well indicated by(LalSm)o, (Gd/Lu)" and (Lall-u)o values (0.8-1.7,0.7-1.3, and 0.8-3.0, respectively: Table 3). The flatREE pattems indicate that there was little or no frac-tionation of REE-bearing minerals, andthatLREE- andIlREE-selective phases are both present @izzo et al.1996). Furthermore, all samples show a pronounced

negative Eu anomaly, likely due to fractionation ofmainly feldspar or Eu-bearing accessory minerals [e.9.,monazite-(Ce)1.

In the tectonic discrimination plot (Fig. 6E) onwhich compositions are normalized to ORG @earceet al. L984),the Quirraleucogranites show ageochemi-cal affinity with granites belonging to collisionalgeodynamic environments. However, there is a signifi-cative shift of some elements from the general pattem.In particular, Ba (especially for samples Q18 and Q29)is relatively depletd Sm, Y and Yb are enriched, andTa and Nb show slight enricbment. These shifts can beexplained by the fact that crystal fractionation wasprobably an important mechanism in the genesis offhese granites, during which Ba depletion was causedby the fractionation of feldspar, and the Sm, Y and Ybenrichment was a result of incorporation into mineralsrich in these elements [e.g., xenotime-(Y)]. Ta and Nbenrichments can be explained by the lack of fractiona-tion of a phase that hosts these elements (e.9., niobianrutile).

Most of the samples display geochemical features of'\vithin-plate" granites (Figs. 7A, B), and in general allof them plot in the field of the "post-collisional' gran-ites. The observed depletion ofhigh field-strength ele-ments (IIFSE) in samples from the granitic facies

associated with the main and subhorizontalpegmatites (Fig. 3), sffis their positions to the VAG +syn-COLG field. This sffi could be attributed to a'dilution" of these elements by a "pegmatite effect"from a possible loss of a volatile phase, as also pro-posed by Parce et al. (1984). Evidence of this effect isfound in the presence of fluorite crystals enclosed byplagioclase and quartz in most of the granite samples,and by the occurrence of fluorite with sulfides (e.9.,sphalerite, pyrite, chalcopyrite, galena) in the main peg-matite samples. The "within plate' features are typicalof A-type ganites (Loiselle & Wones lpl!, Q6llinset al. 1982, Whalen et al. 1987, Eby 1990, 1992). Totest whether our granitic rocls belong to the A-typesuite, compositions were plotted on discrimination dia-grams @igs. 7C,D,E: Fig. 8) tlat make use of certaincombinations of major and fiace elements (e.9., Whalenet al. 1987, Eby 1990). According to Eby (1990), theeffective dlscriminant between A-type and I- and S-type granitic rocks is the FeOr/MgO versz.s SiO2dia-gram. In our samples, the FeO./IvIgO value is in therange 8-140, and they plot in the A-type field on Fig-ure 7C. Eby (1992) distinguished two sub-groups ofA-type granites on the basis of Y/l'{b ratio. The Y/I'{bratio in these samples is in the rarge 1.44.7, and theyplot in the Az-t1rpe field (Figs. 7D, E). Furthennore,according to the Fl versusF2 major-element discrimi-nant functions of Ponce et al. (L993), these leu-cogranites are derived from parent magmas that can berelated to those of topaz-bearing rhyolites (Fig. 7D.

An examination of the behavior of Ga/Al, Zr, Nb,Ce, Y and various major-element ratios (Figs. 8A-L)

130 THE CANADIAN MINERALOGIST

1 000

100Eo.o-

z

c 100o.g-ocr

10

$ roo

oo

LL

oz-otr

1 0

10 't 00Y (ppm)

0F1

60 65 70 75 80si02

1000

1

YINb

FIc. 7. A, B. Rb varszs Y + Nb and Nb versas Y tectonic discrimination diagrams @earce etal. 1984) (VAG, volcanic arcgranites; syn-COLG, syn-collisional granites; WPG, within-plate granites; ORG, ocean-ridge granites). C. FeO*/1VIgO rersasSio2diagram @by 1990). The dashed line represents the field of the I- and S-type granites. D. Y-Nb-4a*3 diagram @by1992) (4l-fype, anorogenic rifting-phase tectonic settings; Az-typ, postorogenic tectonic settings). E. Rbn{b yersas YAllbdiagram @by 1992) (Al-fype, anorogenic rifting-phase tectonic settings; A2-type, postorogenic tectonic seftings). F.Fl versusF}major-elementdiscriminantfunctions @onceet a\.1993), showing fields forcalc-alkaline, peralkaline andtopazrhyolites. Ellipses enclose areas in which 957o ofthe type samples plot. Symbols: as for Figure 6A.

BW P G . . .

aa

.'i.VAG +

syn-coLG

I

ORG

A-type

lL,

*Y Y -V t

l r V

. . " ' t i ' -

l- and S-typet-t \

Az-tyoe rll. tr * ,_- ]. -

E

V

, - - - - * ' -, ' .t.#i

tl t'- - l I - I

a t I '

,' t. A2'lYPe -.'

r A 1 - t y p e . r - - - - - . 't a

tr F, 'r Topaz Rhyolites

Peralkaline tRhyo l i tes \ . - ; - .

( ''i.#i' .

. ! I\ - - , , I

, l tI t a

. ', \ . 'r' catcafaline Rhyolites

I

FAYALITE.BMRING GRANITIC PEGMATITE. SARDINIA

shows that the chemical compositions of these granites lower content of some trace elemenls (especially Zr andare consistent with the compositions of the subsolvus Ce) than the A-type granites of the peralkaline tendpostorogenic A-t)"e granites of the aluminous tend, (Bonin 1988, 1990).although the Quira granites ue chwactpized by a

1,31

Broo3o't ro

1000o@of roodtz+

d 1 05

10

oEooIl

xo

;E 1

3. roo6z+ 1 0oY

t(,ooo

100 1000 10000Z r + N b + C e + Y

I 0000.Ga/Al

1 0000.Ga/Al

-.{ '

aa t'

1 L1o 100 1000 10000

Z r + N b + C e + Y

1

10@0

1000

N 1oo

't0

1

1000

I 0000,Ga/Al

1 0000.Ga/Al

F G t

1

1000

100

10

1

1000

100

10

1

0.1

z oo

1 0000'Ga/Al

10000.Ga./Al100 1000 10000

Z r + N b + C e + Y

Frc. 8. A. Zr + Nb + Ce + Y yersas FeO*/lVIgO discriminant diagram of A-type granites (Whalen er a/. 1987) (FG, fractionaredfelsicgranite,s;OGT,unfractionatedM-,I-andS-typegranites).8.Zr+Nb+Ce+Yversrar(K2O+Na2O/CaOdiscriminantdiagrarn of A-type granites (Whalen et al. 1987) (FG fractionated felsic granites; OGT, unfractionated M-, I- and S-typegranites. C-L: 1@00*Ga/Al rerszs FeO*/IVlgO, (KrO + NaO/CaO, agpaitic index, Z, Nb, Ce, Y, Zn discriminant diagramof A-type granites (Whalen e/ a/. 1987), and Zr + Nb + Ce + Y versus 10000*GalAl discriminant diagram of A-type granites(Whalen et al. L987) (FG, fractionated felsic granites; OGT, unfractionated M-, I- and S-type granites). Symbols as forFigure 6A.

* r

Jt.EFG

OGT

t { .

v t a

A

a

OGT

FG

c

{v a r

l.'II

I 0000,Ga/Al

D

'iij'

G H

't+,.

1 0000.Ga/Al

ff$;I

132 TIIE CANADIAN MINERALOGIST

Coucr-usroNs

The presence of manganoan-fayalite-bearing grani-tic pegmatite and of associated fayalite-bearing graniticrocks at Quirra constitutes a new aspect of the regionalgeology of Sardinia- These pegmatites differ in compo-sition from Sardinian pegmatites so far known @ani1994). The high Mn content of the fayalite can bejustified by the high silica character of the Quipa leu-cogranitic magmas from which the pegmatites origi-nated. Watson (L977) experimentally verified thatMn-6uouiq' increases in favor of the olivine as the SilOatomic ratio of the melt increases.

On the whole, the geochemical character of the leu-cogranites indicates an origrn of a specialized kind,comparable to that ofpost-orogenic A-fype graniles offts alumin6us trend. The parageneses of the leu-cogranites from Quirra suggest low-.(Oz) crystal-lization processes. However, thefiO) conditions seemto have changed during the last stages of consolidationof the granite-pegmatite complex, as shown by thelow-tempemture, subsolidus alteration of the fayalite tolaihunite, magnetite, grunerite, greenalite and ferripyro-phyllite.

On the basis of geological, mineralogical and geo-chemical data the association of quartz + fayalite +microcline + biotite in the Quira pegmatites is consid-ered to indicate that the pegmatite itself represents aproduct of crystallization of a residual liquid issuedfrom a parent magma with a composition similar tosample Q1l (Fig. 3, Unit 2), the least-evolved graniticfacies in the area. Jls manganiferous character ofthefayalite in the pegmatites, as well as that of some of thehost granites, also argues indirectly for the derivation ofthe pegmatites and the host granites from the sarnemagma.

AcKI.IOIVI-BDGElvm]{Ts

Financial support for this work was provided by theConsiglio Nazionale delle Ricerche (8.P. & R.R.), aS.I.M.P. havel award to E.P.. and a Natural Sciencesand Engineering Research Council ofCanada operatinggmnt to MR. We axe grateful to R.F. Martin, B. Boninand an anonymous reviewer for suggestions that sub-stantially improved the manuscript.

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Received April 26, 1996, revised m,enuscript accepted Decern-ber 17. 1996.