RENDlCONTI DELLA SOCIETA ITALlANA Dl MINERALOGL" E PETROLOGIA, 1985, Vol. 10, pp. 341·352

Characterization of different granitic faciesin the Baveno·lUottarone pluton

by meaDS of the typologic study of zircon populations

VALERIA C.URONIDiparlimento di S<:ienze della Terra, Sez. Mineulogia, Petrografi.a, Grochimica e

Giadmenti Minerari, Universid, degli Studi, Via Bottieclli 23, 20133 Milano

ABSTRACT. - Zirron populations taken from samplesof the Baveno-Mottarone pluton, studied with thetypology method, can be clearly divided into twogroups. Zircons from group 1 have the typicalcharacte'1. of populations from caIealkaline granites;thdr if, T points in the typologic evolution diagramare distributed along a differentiation rrend startingfrom the: granodiorite. A sample with hererogranulartexture snows zircons similar to those of volcanicrocks, suggesting a high remperature crystallizationdue to npid cooling. Zircons from group 2 rca.l1the typical popularions of a1uminous alkaline gr:anitcsin their rypologic distributions, in rheir positionsin the typologic e.,'Olution diagram, in rhe typOlogicevolutionary trend of every single sample, as wellas in the typologic diffenntiarlon trend of the groupand in the serondary dta~. Studies on mine­ralogy, petrography and chemistry of these granitcsarc: still in POOSrc:ss; first data Sttrrt to Igttt withthe remlrs of the typologic study of zirconpopulations.

Kty wo~Js; zircon, rypology method, calcv.lkalinc:granodiorite, calcalkaline granite, aluminous alkalinesubsolvus granite.

RIASSUNTO. - Le popolazioni di zirconi prove·niemi da akuni campioni del plUlone di Baveno­Monarone, studiate con il metodo tipo[ogico, pos.sono essere divise in due gruppi. GB zirconi delgruppo I possiedono i caratteri lipid del1~ papa­[azioni dei graniti calcoalcalini; i toro pund medi,if, ..., ne! diagramma di evoluzione tipologica sidimibuiscono lungo un trend di differen~;azione

avente alia base: la granodiorite. Un campione asttuttura eterogranularc: prc:senta zirconi simili aquelli di rocce vulamiche, il che suggcrisce unaa1ta tcmpenllura di cristallizzazione pmvocata daun raflrcdd.mcnto rapido. Gli zirconi del g:uppo 2sono motto simili alk tipiche papolazioni dei granitialluminosi fllcalini per quaDtO riguarda le distri.buzioni tipoqiche, k posizioni cotm il diagtllmmadi cvohwone tipologica, le tendenze di evoluzionc:ripoqia di ogni singolo campione, come pure iltrend di dilferc:n%iazionl!' globaJe del gruppo e i

canned secondari. Sono in corso studi sui carat·tcri mineralogici, pt:trogralici e chimici di que5tigraniri; i pdmi dati scmbrano in buon at'cordocon'i risuh~li ottcnuti con 10 studio tip<'llogicodelle popolazioni di zirconi.

PQ~o{e chill~: zircone, melodo tipologico, grana­diorite calcoalcalina, granito calcoalcalioo, granito1II11uminoso lII1calioo subsolvus.

I. InlrodudioD

The Baveno-Mouarone massif, elonguedfrom SSW to NNE, is formed by a maingranitic body and a minor granodioriticintrusion (M. Monarone). It intrudes the... Scisti dei Laghi .., that is the metapcliticpart of the ... Serie dei Laghi .. metamorphicunit, in the Southern Alps (fig. 1). Thispluton, though well known for its quarriessince many centuries, has been little studiedin detail; the work of GALLITELLI (1937),based on chemical, optical and field obser­vations, can be mentioned, as well as thegeologic.petrographic study of GANDOLFI '"PACANELLI (1974), where a detailed de·scription of the geological setting of thepiuton can be found. These studies haveshown the presence of different graniticfacies: red, pink, white granite; micro- andheterogranular granite; granodiorite. Thefinal goal of the present work is to establishthe similarities and mutual relations amongthese fades by means of a relatively newmethod: the typologic study of zirconpopulatjons, firn proposed by PuPIN &

TURco in 1972 and further develo~

(PUPIN & TURCO, 1972 a, b, c; 1975; PuPIN,1976, 1980).

342 V. CAIRONI

Group J: it includes the white hetero­granular granite G 54, two white granites

In fig. 2 the A, T points of the samplesin the I. A vs I. T diagram are comparedwirh the petrogenetic classification proposedby PUPIN (1980). The samples can be dearlydivided into two groups, the former fallingin the calcalkaline, the second in the alkalinefield; both groups show typical features intheir zircon populations.

pink K-feldsparpink K.feldsparyellowish K-feldspar

F 74G 1F 59

3. Results

The following samples have been examined:MO 5: granodiorite

G 54 : Iwhite granite with heterogranulartexturewhite granitewhite granitewhite granitegranite with red K-feldspar and pinkplagioclasegranite withgranite withgranite with

F 39,F 54G 56G2

perature = {lOO} > {110} and Vice versafor Iow temperature); aluminium and alkilicontent controls the relative development ofpyramids (aluminium favours the {211}pyramid, alkalies the {101} and the {301}pyramids). On this basis information can beobtained on the physieo<hemical conditionsof the medium during the sometimes longcrystallization period of zitcon. The se­quence of appearance of different crystaltypes in each population can be representedby a calculated «Typologic EvolutionaryTrend» (T.E.T.) (PUPIN, 1976). In corn·posite cogenetic bodies a differentiationtrend in the zircon populations from the lessto the mo~ differentiated terms can bedetermined (PUPIN, 1976).

Secondary chaNlcters as colour, transpa­~ncy, presence of overgrowths and wRing,kind of inclusions... are also taken intoaccoun~ in order to give a more detailedrep~sentation of the genetic conditions ofeach population (PUPTN, 1976).

This method has given some interestingresults on the Baveno-Mouarone pluton, too.

2. The typology method

This method consists of a morphologic·statistic study of zitcon crystals, based on atypologic classification (PUPIN & TURco,1972 a), which takes into account thepresence and relative development of theprincipal cristallographic faces: pyramids{2Il}, {IOI} and {JOI} and prisms {lOO}and {llO}. Two indices, A and T, reRectthe rdative development of pyramids andprisms respectively i every ~pulation can berepresented by a point of If, T, coordinateson a A index vs. T index diagram (PUPIN &

TURco, 1972 b). The main factors controllingthe typologic variations a~ now known(PUPIN, 1976; PUPIN et al., 1978): thecrystallization tempeNlture determines therelative development of prisms (high tern·

FiG. 1. - Geological sketch map of the Baveno·Mottarone pluton. Mapping after SASSI, 198'; SE­SANA, 198'; unpublished data.. iv = Ivrea.verhanozone; se = Suona·Ceneri zone; si = Sdsti dei La·ahi zone; It = :unphibolites; os = orlhogneisscs;black = appinitk dybs; cmbi = Cossato-Mergozzo­Bri$$ago line; pt = Pogallo line. G,anitts; q =Quama; p = Pedemome; m = Montorfano; I = Ba·ver»Mottarone, group I (ClIICl1lkaline); 2 = Bavcno­Mottaronc. group 2 (alkaline); gd =. Mottaronegranodiorile.

CHARACTERIZATION OF DIFFERENT GRANITIC FACIES ETC. 343

" ~ 21h101 21t>'kl1 21h1()1211 <101 21'k101 '" '"I.A "" "" ;00 <00 ;00 ;00 '"" 000

I.' PRm ,

'"

f~"",

'"',

100<110_~7 i I, ,4 /'. I i

, " i;00 , /" ,I 100<"0

I ,-" p

1 '4b' 'JP.2 ""

", 3 '/,'.'" S 100:110(/ ", ;;00

,- ,100"10, « ,,

i

"'" i i H 100>110, i ,

", m

Fig. 2. - if, f points of the analyzed samples inthe typologic evolution diagram of ziccon popu­lations, compaced with typical distribution areasfoc diffeunt magmatic suites (after the petrogeneticclassification of PUPIN, 1980). Arrows repusenttypologic differentiation trends. - 1 to 3 = aluminousand hyperaluminous granites and granodiorites;4=calcalkaline series; 5=high K monzogranilcs andgranites (<< subalkaline sede .. ); 6 = alkaline andhyperalkaline granites (H = hypersolvus; T= trans­solvus; 5 = subsolvus).• = group 1: sequenceM05, F39, F54, (G54);'" = group 2: sequenceG56, G2, F74, G I, F59.

with normal texture, F 39 and F 54, andthe granodiorite MO 5.

The three granites have wide typologicdistributions (29-30 types; fig. 3). Since thecrystallization temperature of zircon deter­mines the relative development of the twomain prisms, and since this development isexpressed by the T index, then the widedistribution along this index for sampl..:sF 39 and F 54 indicates a large range in thecrystallization temperature.

Looking at fig. 4 it is possible to realizethat the process was more intense in therange 8000 C - 7000 C (60-70 % of crystalswith T index between 600 and 400; fig. 9,IQ; temperature calibration after PUPIN &

TURCO, 1972 C).In granitic magmas the crystallization

period of zircon is limited to high tem­peratures only if the fluid pressure is low,but it extends towards lower temperatureswith increasing Pt (PUPIN et al., 1978). Thefrequency distribution along the temperaturerange in these two samples (fig. 4) shows

that Pt was not high. This is confirmed bythe characters of late stage crystals: theyare mostly faintly coloured and transparentand their overgrowths of hydrozircon arescarcely developed (fig. 11).

Sample G 54 has almost the same rangeof variation along the T index as samplesF 39 and F 54, but it has a higher contentof high temperature types (6g. 3 and fig. 4).Many characters of the crystals recall thoseof zircons from volcanic rocks (PUPIN, 1976):absence of both colour and overgrowths;transparency; typical inclusions (6g. 12, 13);they indicate therefore that high temperaturecrystallization is probably due to rapidcooling.

This result, together with the hetero­granular (sometimes even pseudoporphyric)texture and with the field relationships, isin accordance with the interpretation of thisgranite as a late stage intrusion, emplacedin a rather cooled environment. The chemicalcomposition of this granite is very doseto the two white «normal» granites, a1suggested by the similar values of the Aindices of the three samples: G 54 = 536;F 54 = 550; F 39 ~ 522.

The typologic evolutionary trend T.E.T.,describing the chemical variations of themedium during the decrease of temperature,is characteristically curved for all threesamples (fig. 5), suggesting a first periodduring crystallization with rather constantrelative amounts of aluminium and alkalies,followed by a late stage alkalinization. Thisis the typical T.E.T. pattern in calcalkalinerocks (see for comparison the Argenteragranite A 1 in fig. 5, after PUPIN, 1976).

The typologic distribution of the grana­diorite population (fig. 3) is rather typicalfor calcalkaline rocks containing amphibo1e:it shows a high frequency of types with{211} pyramid ~ {101} (high aluminium)and prism {lOO} ~ {1l0} (high tempera­ture) (6g. 3 and 6g. 4). The T.E.T. indicatesrather constant relative amounts of alumi­nium and alkalies during crystallization, andis similar to the T.E.T. of the Argenteraamphibole monzogranite A 2 (fig . .5; A 2after PUPIN. 1976).

The if, T points of the samples of this~roup describe a differentiation trend movinf!from the granodiorite to the granites, parallelto the 4 b line of cablkaline series in the

344 V. CAIRONI

.... --- --- ---•••• 4_4 _

550-447

__ 4 _

-_. --- ---

F 54

636- 515G56

F

412 - 6

536 - 497

MO

G 54

F 74 674 - 444 GI 687 - 380 F 59 673 - 362

D m ill • ~. .

---0-2'1, 2-5 'I, 5-10 'I, 10-20 'I, 20-40 'I,

rig. J. - Typologic distribution diaJ;rams for the: analYlw samplC$.

petrogenetic classification of PUPIN (1980)(6g. 2). A typical example of this line isgiven by the Argentera massif, very similar,as already seen, to the samples analyzed inthis paper.

Group 2; it includes all the granites withcoloured feldspan and one while granite(G 56). For all samples the typologic di·stribution is considerably reduced (14-19

types; fig. J). The crystallization temperatureranges from about 850" C to about 600° C,but the percentage of late stage crystalsclearly increases following the order: G 56,G 2, F 74, G I, F 59 (fig. 4). In the lasttwo samples the high frequency of late stagetypes and the great development of sometypical secondary chatacten (dark colours,turbidity, zoned overgrowths of hydrozircon,metamictization; fig. 14, 15, 16) indicate a

CHARACTERIZATION OF DIFFERENT GRANITIC FACIES ETC. 345

F59Gl

II

I

10 20 3040 50

M0510 20 30 10 20 30

"

10 20

F 5410 20 30

lI

II

IJ

F74

10 20

F 39

10 20

l

I

IG26

G54

1020

~

IJ G5

TO I.T % 10 20 30<550 100

600 200

650 3001-----'-,

7004001------'--,

750 5001----,---'

8006001---,-----'

850 700

>900800

<550 100

600200

650 300

700400

750500

800 600

850 700

>900800

Fig. 4. - Histograms of frequency of zircon types related to their T index in the analy.;:ed samples.Temperature calibration after PUPIN 6< TURCO (1972 c).

high fluid pressure in the magma and a longcrystallization petiod for zircon. These se­condary characters connected with the highfluid content were already observed in thepink Baveno granite by PIGORINJ et al.(1964) in their study on zircon with themethod of dimensional statistics and « re­duced major axis ». This 'method wasproposed by POLDERVAART (1950,1956) andlater criticized by PUPIN (1976, on pages314-317). By the way, it is worth notingthat the pink Baveno granite is famousfor its richness in miarolitic cavities, con­taining more than 40 different mineral species(GALLITELLl, 1937).

The typologic distributions of all thesamples in this group are narrow along theA index (fig. 3) remarking only very littlechemical variations in the magma. In par­ticular, the {101} pyramid, that is alwaysmorc developed than the {211}, suggestsan alkaline composition (6g. 15). This .isconfirmed by the subvcrtical T.E.T. of allthese samples, very typical for alkalinegranites, where the alkaline character ispresent from the very beginning of crystal­lization (fig. 5; for comparison: K = Ka­genfels alkaline subsolvus granite, Vosges;I = lIe Rousse alkaline subsolvus granite,Corsicd; data after RftVE, 1985 and PUPIN,personal communication, respectively).

346

IT

100

200 •

300

/400

500

600

700

V. CAIRQNI

,..:W:::D---..::1OO:.-.:3:::00_400.::;:....::500::.-.:6IlI::--:.:'00::...-.:8:::.,1lI1.1 1.1/~ 100

" 11, " 100, ", I '

::: 300, ', '",t, JRD.s 11 "tl, ...-a I

: ... :l soo, "r----I'"Jr T" GODt--- _l~

, "\H:: 100, ,,

" I IL- '_"'_'---l8DD

Fig. 6. _ Position of the analy-"e(\ samples ofgroup 2 in the distribution area of zircon popu·lations from alkaline granites (TESSIEI, 1979).utt = .Iuminous alkaline granites; right = hypo­aluminous hyperalkalinc graniles.

Fig. ,. _ Typologic ('V(lb,lliooary trcncb (T.E.T.)of each anaI~ population. - I = G '4; 2 "" F J9;J=F54; 4=MO,; 5=G56; 6=G2; 7=F74; 8 = G 1; 9 = F 59. For wmpari50n: A 1=Argc:nlenl granite; A 2 = Argcmera amphibolcmonzosranilc (PuPIN, 1976). I = lie Rous.se (Cor.,ia) alkaline subsolvus granite (PuPIN, personalcommunication). K = K,.gcnfds (Vosges) alkalinesubsolvu$ &nni!c (REVE, 1985).

The A, f points of these samples aredistributed along a subvcrtical differentiationtrend (fig. 2), such a slope indicates thatthe different levels of differentiation are dueto an increase of fluid content, while thealkalinization is weak (since the magma isalkaline already in its less diffe~ntiated

product G 56). The differentiation t~nd ofthis group falls in the distribution field ofalkaline subsolvus granites (6g. 2); this isin good agreement with the observed mi­neralogical composition; presence of albiteand ouhodase.perthite.

so

lA 300 400

,

500

, .600 700

4. Diseull8ion

The different granite facies of the Ba­veno-Monarone pluton were considered inprevious works (GALLlTELLI, 1937; GAN­DOLFI &: PAGANELLI, 1974) as differentaspects of a single granite type. the colourof fe1dspars being considered essentially apostmagmatic feature. On the contrary, thetypologic study of zircon populations insamples from this pluton has revealed thepresence of twO different granite types,respectively with a calcalkaline character andwith a distinct alkaline composition. Whereasthe first group includes only white granites,the second one comprises all samples withcoloured fe1dspars as well as one sampl~

of white granite (G .56).This fact proves that the colour of

feIdspars is not a valid criterion to distinguishthe two groups, as it could be thought atfirst.

Since only the old chemical analysis ofGALLITELLl (937) were available, newanalysis have been performed (SAS SI, 198.5;SESANA., 198.5) on 27 samples.

At 6rst sight, these data do not showgreat differences between the two groups,which could be: considered a single groupwith rather large range of variations. But

CHARACTERIZATION OF DIH'ERENT GRANITIC FACIES ETC. 347

TAB. 1Main chemical dif!erenuJ be/wun group 1

and group 2 graniteJ• •• • IIorO • l:D Al/(H•• k·C./l) H.o/(A\·k)

-",- I.§O • I.n 1.tS • 1.]1 0.'0 • D."......... '-. '-. 0.6:

•'-'----;;;;;,------;;;:;;--~;:v. I. A500 600 700

a more careful inspection 'of the same datashows that in almost all oxides vs. SiO:diagrams the samples can be divided intotwo groups, which correspond very wellto the indications given by the typologic

0./1

0.1(1· D.1l

D.U - .....

1. t,I.t]

study of zircon populations (diagrams inSASSI, 198.5; SESANA, 198.5). The main dif.ferences concern alkali content, Al/(Na+K+Ca/21 index and agpaitic coefficientNa/(AI-K) (ZLOBIN, 19.59) (tab. 1).

However, these differences are not asgreat as could be expected from the zircontypologies. In p:micular, the granites of

-,.) ..IU _ttn

...... I.n· I.W 1.lt· 1.1/

b) "ut ......ttll ",,1_ fold.po,..

'0119" 1.'10· '.~] LIt· 1.1'

••

Na/(AJ·KJ vs. A index correlationfor some of the Inalyzcd sampl~.

05

06

Fig. 7.diagram

NoAI_K

OB

REElchondr.200

-Gr.2

100

10

-- --Gr.l

",.""",,,

•,,,, ,•~

'''.-?//

,\ ..--

lo Ce Nd Srn Eu Gd Tb Dy Yb lu

Fig. 8. - REE distribution panern for some samples of the Ba,-cno-Monlrone plulon.

.H8 V. CAIRONI

Fig. 9. - G[QI,Ip l. Type P.: P1 (101);pr {lOO} ~ (110). High tallpenturc: crystal withtypial inclusions of necdle-shaped tplltile.

group 2, though definitely more alkalinethan those of group I, do not show theclassical values for alkaline granites. Since«postmagmatic geochemical variations in­duced locally by deuteric processes arewithout any effect on zircon populations ..(PUPIN, 1980), it must be deduced that thecharacter of this magma at the time ofzircon crystallization was much more alkalinethan it now appears from the chemicalanalysis. This is probably all effect of relativeloss of alkalies and gain of aluminiumthrough intense late-postmagmatic alteration,which is largely widespread in the analyzedsamples.

The typologic characters of zircon popu­lations in the samples of group 2 are quiteclose to the characters of populations ofalkaline aluminous granites. In fact, as de­fined by BoNIN (1982), alkaline suites canbe divided into two «lines.. : hypoaluminoushyperalkaline and aluminous alkaline. h hasbeen observed by several authors (TESSIER,

Fig. 10. - G[QI,Ip I. Type S" (a\OC'XlgC' of So and5..): P1 {211} ~ (IOI); pr {lOO} = (110).Medium temper:uurC' t)'PC' indicatin£ high alumi­nium coment (gresl developmem of (211». Highrelief longprismatic or [Ql,looed inclusions.

1979; BoNIN. 1982) that the distributionfield of A, i points of zircon populationsfrom alkaline suites on the I. A vs. I. Tdiagram can be divided into two main areas(fig. 6): the well defined zone on the right ofthe field is typical for hypoaluminous alkaline­hyperalkaline suites, whereas the area on theleft is typical for aluminous alkaline suites.As shown in fig. 6, the differentiation trendfor the samples of alkaline granite of theBaveno-Mouarone pluton falls almost entirelyin the characteristic zone of aluminous alkali­ne granites.

The A index of zircon populations ge­nerally increases with the alkali content anddecreases with the aluminium content of themagma; this causes the previously describeddistribution. The influence of relativeamounts of aluminium and alkalies on thedevelopmem of pyramids is clearly shown by

CHARACTERIZATION OF DIFFERENT GRANITIC FACJES ETC.

••

349

Fit!. 11. - Group I. Type G,: P1 {IOI}; pT (1I0).Low temperature lype indicating low P. in themagma C'\Im in the I,te Slllges: lhe: crystltl is dearand colourless and shows 00 O\'crgrowths of hydro­zircon.

the general correlation between the if indexof zircon populations and the agpaitie coef­ficient Na/(AI-K) of ZLOBIN (1959)(PUPIN, 1976). This condition is wellrealized for the analyzed samples of theBaveno-Mottarone piu Ion, even if for thealkaline granite samples the agpairic coef­ficient, reflecting the present day chemicalcomposition, is not so high as their zircontypologics would indicate (6g. 7).

Another character of zircon populationsfound in the granites of group 2 is the highfrequency of xenotime outgrowths (fig. 16)on zircon crystals (already observed byPIGORINI et al., 1964); after PUPIN (1980)«xenotime is more likely to be found asoutgrowths on zircons in rocks with analkaline trend •.

Ikside the zircon characters, the granitesof group 2 can be distinguished from those

Fig. 12. - Group 1 (s.ample G '4). Type V..:n {211} "" {101} and POl}; pr {lOO} > {HO}.High temperature: Cf)'5tal showing the typical darkrounded (glass?) or irregular inclusions of ziroonsfrom volcanic rocn.

of group I by other features, visible in thinsections (SESANA, 1985). The most evidentone is the presence of a deep green, stronglypleochroic biotite (deep green/greenishblack), sometimes zoned with a greenishbrown core and a deep green rim; thiskind of biotite has been referred to as«siderophyllite» by GALLITELLI (1937). Itappears to be formed as a gradual transfor­mation of «normal» brown biotite. Thistype of biotite is considered by BoNIN (1982)as typical for aluminous alkaline granites.After this author, biotite in alkalin~ andhyperalkaline granites crystallizes first witha composition near the annite end member,then evolving either towards the lepido­melane composition, in presence of AI-poor,Fe-bearing fluids (in hypoaluminous hyper­alkaline granites), or towards the siderophyl­lite composition, in presen~ of AI-rich,

V. CAtRONI

Fig. D. - Group I (sample G 54). Type 5,,:pr {211} > {IOI}; pr {lOO} = {JIO). Medium­hIgh temperature ct)"Slal showing almost symmetricrounded inclusions and an irKgular ClIvity filledwith lilllC' crystab and glass; ail these' characlenare typical of zircons from \''Olcanic rocks.

F-bearing fluids (in aluminous alkaline gra­nites). During late stage hydrothermal pr<:lttS­ses these siderophyllites can further reactwith F-Li bearing fluids, giving terms of thesiderophyllite-polilithionite serie. Tn thesamples of group 2, Li·bearing micllS arewidespread in miarolitic cavities, (zinnwalditeand protolithionite, GALLlTELLI, 1937), buta mica whose optical characters seem tocorrespond to those of zinnwaldite has beenseen also in thin sections (SESANA, 1985).

Another very typical character of thesamples of group 2 is the widespread occu­rence of Buorite, interstitial or associated withchlorirized biotite.

Studies on minor elements and REEcontents are still in progress; ficst resultson REE (fig. 8) show different distributionpatterns between the two granite types:the granites of group 2 have a more

fig. 14. - Group 2. Type 5,.: py {211} < {IOI};pr {lOO} > {11O}. High tC'mperature type. Darkcolour, turbidity and ronC'd o\'C'rgrowth of hydro­zircon indicatC' high P. in the magma.

pronounced negative Eu-anomaly and arerelatively enriched in medium to heavy REE.as indicated by the presence of minerils suchas xenotime, orthite and gadolinile too (GAL­

L1TELLI, 1937).These characters are considered by several

authors as related to an alkaline composition(BUMA et al., 1971; BOWDEN & WHITLEY,1974), but more detailed studies and a grealernumber of analysis are slill necessary toget reliable informalion on the REEdistribution pattern in the Baveno-Moltaro­ne plulon,

5. Conduaiona

The typologic study of zircon populationsin some samples of the Baveno-MottaroneplulOn has revealed the existence' of twodistincl grnnite types. The well defined zircontypologies of the two types suggest their

CHARACTERIZATION OF DIFFERENT GRANITIC FACIES ETC. 351

Fig. U. - Group 2. Type l,.: P1 {211} « (101);1" (110). Low ternpaature type, with inlerlaizoning showing • grat ck\-dopment of the {101}pyramid from the \-et}' beginning of zircon crystal·lization (high alkali COfltem).

attribution, respectively, to calcalkaline andaluminous alkaline granites. Several chemicaland petrographic characters seem to agreewith Ihis interpretation.

The main problem arising from these dataconcerns the relationships between thesetwO granites, which represent differentgeological environments.

Considering that the whole complex hasclearly intruded in a post-uplift period, apossible solution would be an earlier originof the calcalkaline magma with respect [0

the alkaline one. The calcalkaline magmacould be the last magma originated in thelate otogenic phases. while the alkaline onecould be: the 6rst formed in the post-uplihperiod; it would have enriched in Auidsduring its rise through the wet postorogeniccrust.

However, the emplacement of the twomagmas is probably very close in time:

Fig. 16. - Group 2. Type C,: P1 {101}; pr {IIO}.Low tempenture type with characteristic xenotimeoutgrOWlns; panially included in K·fddspar. Se­condary charal;ters indic:ale high P. in !he: magma.

no dear limits can be seen on the fieldbetween the two gt'1l.nites and no timerelations can be: obtained on the hasis offidd observations; it is only possible toestablish that the pink alkaline facies isyounger than the granodiorite (SASSI, 1985;SESANA, 1985).

The age determination of 276± 5 m.y.(Rb/Sr whole rock isochron) given hv HUN­ZIKER & ZINGG (1980) for these granitescannot help in their interpretation, becausethe different fades have not been consideredseparately.

Another hypothesis on the genesis ofthese magmas cannot be: excluded: it ispossible that they have formed by separationfrom a single magma at deeper levels andhave then intruded as indipendent pulses.However it must be considered th:lt thetypologic characters of zircon populations

352 V. CAIRONl

of the analyzed samples can be better explai­ned by the first hypothesis. In fact, zircontypologies like those of group 2 have alwaysbeen found in granites belonging to alkalinesuites and not in granites derived fromcalcalkaline parents, even if with an alkalinecomposition (PUPIN, 1976, 1980; TESSIER,1979).

Studies on the Baveno-Mottarone plutonwill be continued, since much more work

is still necessary for a better understandingof this multiple intrusion.

AcknowledgementJ. _ Many lhanks are Jue toProL ).P. PUPIN, who teached me the typologymethod and controlled the results of my sludy,and 10 Prof. A. BoRIANI for useful suggestions andcritical discussion of the paper.

Thanks also 10 Dr. M. OPOONE (Istitulo di Chi­mica Generate, Universitii di Pavia), who GIrdedout the determination of minor elements and REEcontenlS.

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