REN/)ICONTI Socletll /taUanG df Mlneralogla e PetrolGgia. 38 (JJ: pp. 27-J3

Comunleazlone preMntata al CongreMO SIMP dl C&gllar1 11 16 ottobre 11181

OCCURRENCE OF LATE-HERCYNIAPERALUMINOUS GRANITESIN THE SOUTHERN ALPS

CLAUDIO D'AMlco, ALEsSANDRO ROTTURA

Istituto di Mineralogia e Petrognlofia dell'Univ., P.ua di Porta San Oonato I, 40127 Bologna (Italy)

RIASSUNTO. - Dopo i graniti pcraluminosi a cor­dierite + muscovite ne! massiccio di Bressanone(VISONA, 1980), altri 5-graniti sono stali ritrovalinelle Alpi Meridionali orie:ntali. Ndla parte: orien·tale di Gma d'Asta vi e un piccolissimo corpo digunito muscovirico con psc:udomorfi da cordierite.AI Monte: Sabion vi c una masserella e minori corpifiloniani di porfidi gnlonitici a muscovite, con rdinidi andalusiu: in via di musoovitizzazione e congranati.

Queste nUO\'C conoscrnze definisoono piu cornpiu­tamerlle la caf1luerizzazione ercinotipll dei complcssiplulOnici mdalpini, aranerittati in osni modo ciaI-granifi :::t> 5-granili.

Esse pongortO problemi petrologici rel.tivi all.pf1ltica Itabiliti dell" muscovite in condizioni in­uusive di piccola profondita, qu.1i 5Ono certamente.su base gco\ogica e petrogntfiro-tcssituf1lte, le condi·zioni dei plutoni considerati. 11 problema non cnuovo e porta. non utilizzare in modo qu.mitativoil ben nota sistema semplice Ms + Qz (muscoviteideale) alle condizioni compositive complesse di unfuso 5-gf1lnitico in risalita e cristallizzazione, ma diconsidef1lre un probabile allargamento del campo distabilita della muscovite in fusi peuilicici versopiu basse P elo piu alte 1.

ABSTRACT. - Following the finding of the cordie­rile + muscovite pc:raluminous glltnites in the Bres-.SllnOne Massif (VtSONA, 1980), other few S-glltniteswere found in the eaSlern Southern Alps. In theeastern area of the Gma d'Asta basement thereis a tiny body of muscovite gf3nite containing cor·dierite pseudomorphics. In the Monte Sabion areathere is one small mass and some: dykes of musco­vite-bearing granite porphyries with minor garnetand andalusite.

1lJese nnding1 confirm the Hercynotype charxterof the Southern.alpine plutonia, the main feature,howcyer, being I·granites ::3> S-gr:anites.

Petro1ogica! problems arise with reladon to thepractical stabilily of muscovite in high.kve! plutonicconditions, which are defined on a gcolosical .ndpet«>te:xtural basis. This is not • new problem;it suggests that the well·known Ms + Qz simple

system may not be employed in a quantitative wayin a rising and crystallizing S-granitic melt. It couldbe considered that the muscovite stability fieldshould be extended towards higher temperature andlower pressure.

Introduction

In the pre.Permian basement of theSouthern Alps there are granitoid massifs(Cima d'Asta, Bressanone, Ivigna. MonteCroce, Monte Sabion) of the late-Hercynianage (290·275 m.y.), which are described byD'AMlco (1974) and BARGOSSt et al. (1979;see references therein). The granitoidrypology is I.type, according to CHAPPELL.WlIITE (1974); in fact the association isgiven by subaluminous granites + grana­diorites and metaluminous granooiotites +tOnalites + quarrzdiorites. Only ra::ently asmall body of cordierite and muscovitebearing peraluminous granite has beendescribed (VISONA, 1980).

In this paper the finding of otherperaluminous granites in the Southern Alps(Cima d'Asta and Monte Sabion areas) isdescribed and some of the problems arisingare discussed.

Petrography and chemistryPeraJuminous granite in the Vanoi Valley(Cima d'Asta)

This forms a small outcrop. of a fewmeters, within the phyllites, at the begin.

28 C. O'AMICO, A. ROTTURA

ning of the road for the Broccon Pass. I t isa very deuterized medium-nne grained,muscovite syenogranite (table 1), withpegmatitic patches and with probable cor­dierite pseudomorphies. Muscovite is abun­dant in the pegmatitic patches; it appearsto be mainly primary in the texture and isin isolated flakes or, morc frequently, inter­leaved with biotite.

The shape of the pseudomorphics israther Stumpy, irregular or rounded, witha lot of scrieite and fewer chlorite. Nomineral relics arc present. The pseudo.morphics can, by probability, be attributed toa previous cordierite origin, but a possibleandalusite origin cannOt be completelyexcluded. Around the pseudomorphics,

TABLE 1Modal composition 0/ the

peraluminous granites

Peraluminous leuco-monzogranitic porphyriesand microgranites 0/ Monte Sabion

Near Monte Sabion (Pinzolo, Val Rende­na) a complex association of granitoid rocks,described by OGNIBEN (1952), outcrops overan area of a few kilometers, within meta·pelitic hornfelses. They are sub-melaluminous

aMonle S<I.t>ion •

Ci ....... d·Asl...

f-,----,--,----,-\

.,.,'..'

O{ 11, (Ill number of samples.

flakes of intergrowing muscovite and biotitemay be found; this indicates of course anearly genesis of the cordierite.

The medium-grained texture and thepegmatitic flecks, within such a small mass,suggest a crystallization in the presence ofa substantial quantity of water.

The small mass belong to an epiplutoniccomplex, certainly intruded to a depth of afew km.

max 0,5

1,6 pr

classification of Monteperaluminous granites.

Fig. 1. - Q-A-P modalSabion and Cima d'Asta

granites-granodiorites (I-granitoids). To date,pcraluminous heterogranular microgranitesand leucogranitic porphyries have not beendescribed.

These form a small mass which outcropsdiscontinuosly over an area of approximately1/2 km~ on the Western slopes of M. Sa­bion (Malga Cioca area), as well as smallermasses and/or maybe large dykes, whichnever clearly outcrop. They are porphyriticor heterogranular rocks, with a PJ. ave­rage = 18; the groundmass is medium-fineto very fine and also granophyric. The compo­sition is monzogranitic (fig. 1). There is asignificant presence of muscovite togetherwith a minor quantity of garnet and anda­lusite. Pseudomorphics possibly attributableto cordierite are rare.

This paper deals only with the peralu­minous character of these rocks, which isnOt very strong (table 2). A more detailedregional study is under way.

The features to be underlined here, incontrast with the main associated I-grani­toids, are:

1) the constant presence of muscovite(100 % of the samples);

2) the frequent presence of andalusite

max 1,07

Val Vanoi tob'lte Sabion(Cima d'Asta)

(1) {11 1

38,2% 34,4%

39,0 30,1

18, 1 29,0

1,7 3,0

, , 4 3 , ,

Cdt, ps.

And

MS

Gt

B1

Kf

p

OCCURRENCE OF LATE·HERCYNIAN PERALUMINOUS GRANITES ETC. 29

TABLE 2Chemical composition 0/ pera!uminous

granitic porphyries 0/ Monte Sabion

""anitic"""""'""'tic~ -X"(n-4) so (n -')

wt .•

51°275.50 0.77 73.43

T102 0.0\ 0.00

60.20

A12

03

13.45 0.18 13.78

Fc2

03

0.62 0.03 0.74

FeD 0.44 0.24 1. 18

MnD 0.04 0.01 0.04

"gO 0.13 0.14 0.44

CaD 0.51 0.16 1. 23

H1I2

O 3.88 0.29 3.30

X,D 4.30 0.48 4.60

P20 5 0.06 0.01 0.10

L.O.I. 0.99 0.35 0.96

N1I2

O/1l:2

O 0.91 0.18 0.72

C 1. 62 0.57 1. 38

PP"

Ce '0 6 35

La " • 24

Ba " " 2S6

S, 17 8 lOO

RI> '16 7 16'Ll 14 5 32

, 34 , 26

Z, 42 7 54

X = average; 11 = number of samples; SD = stan·dard deviation; C=CIPW normative corundum.

relics reacting towatds muscovite (65 % ofthe samples);

J) the relatively frequent presence ofgarnets (40 % of samples) as small, irr'~ular,

cribrous grains in the aplitic groundmass;

4) the rare and un~rtain presen~ ofpseudomorphics from cordierite.

Petrogenctic prohlems oC theperalllmino1l8 minerals

On the basis of their textural relations,the few garnets found in the Sabion rocksappear to have crystallized late in thegroundmass. A similar crystallization isrelatively fr«luent, for Fe-Mn-garnets, inaplitcs (e. g. CAWTHORN-BROWN, 1976;CLARKE, 1981; ABBOT, 1981; ALLEN·CURKE, 1981). The garnets are compatiblewith a pernluminous composition, but theyare nOt a conclusive proof of a primaryperaluminolls character; moreover, theycannot supply critical information about thecrystallization pressure (e. g. CLEMENS­WALL, 1981).

The cordierite, although only inferredfrom some pseudomorphics, clearly indicates,on the other hand, a primary peraluminosity.The shape of the grains is compatible witha magmatic crystallization (e.g. GREEN,1976; SPEER, 1981; PHllLIPS et al., 1981;CLEMENS-WALL, 1981); however, a residualorigin cannot be excluded; it appears onlyless probable because the cordierite is inisolated grains, while metamorphic aggre­gates are lacking. Low pressure conditions,which cannot be better defined, may beinferred from the presence of the cordieritepseudomorphics.

The anda!usit~ also indicates a primaryperaluminous composition and a low pres­sure, not quantifiable because of the un~r­

tainty in the system AbSiO~. Andalusiteis only found as small, irregular relicsenveloped by muscovite. The original shapesare sometimes idiomorphic, but are also ir­regular when andalusite is associated withquartz. The early formation of both of Iheseshapes is suggested by the fact that they canbe found enclosed within subidiomorphicplagioclases.

The idiomorphic shapes are attributed toa magmatic origin; a similar interpretationmay be found in D'AMlco et al. (1981),made on the basis of much more significanttextures.

The irregular shapes associated withquartz may be, however, anatectic relics;this would imply anatexis at a shallow depth,a problem which can only be discussed aftera more detailed regional study.

30 C. O'AMICO, A. ROTTURA

The te:xlUrnl relations.hips between anda­lusite and muscovite are always univocal asa crystallization succession (first andalusite.then muscovite), but correspond 10 twO verydifferent types (fig. 2). The first type (a infig. 2) can be attributed 10 a discontinuousmagmatic reaction. similar to the pyroxene -+

horneblende reaction (se(: also CURKE et

feldspars and quartz. It is found in coarsergrained aggregates within the heterogranularmicrogranites, and in the fine groundmassof porphyritic rocks (fig. 3), and also in thegranophyric groundmass. The homogeneityof the distribution, the sizes equal to thoseof the associated grains. [he contact witheach one of the other minerals, the not

Fig. 2. - Corroded rdio of andalusilc surrounded by muscovite. Two different textural relationshipsmay be recognized: a) subidiomorphic muscovite lamellae enclosing corroded andalusite, which may bealtributed to a discontinuous magmatic reaction; b) ir[('gular dusters of muscovitic flakes, which may beattributed to a subsolidus reaetion. Monte Sabion; one Polar, 8' x.

al., 1976); the second type (b in fig. 2) canbe attributed to a subsolidus reaction, corn­pat'3ble to the wellknown pyroxene --0 ura­lite reaction. The b type aggregates are easilyrecognized even after complete transfor­mation.

MUJcov;t~ is found not only as a procluctof the andalusite reaction, but more fre­quently as Rakes associated with biotite,

unusual interleaving with biotite, the sub­hedral and anhedral shapes, the absence ofreaction structures are all dear indicationsthat the muscovite crystallized together withthe other components. That is, muscovitecrystallized from the melt.

1be crystallization of muscovite fromthe melt is generally accepted (e.g. HA.lUUS,1974; LUTH, 1976; THOMPSON-ALGOR,

OCCURRENCE OF LATE-HERCYNIAN PERALUMINOUS GRANITES ETC. 31

1977; WYLLIE, 1977; THOMPSON-TRACY,1979; MILLER et al., 1981 etc.) and doesnOl, of itself, present any problems.However, to our knowledge, the possiblemagmatic origin of muscovite in fast coolingporphyritic high level plutonics, has neverbeen discussed: a situation in apparentcontradiction with the experimental know·ledge of the muscovite stability in persilicicrocks.

According to generally accepted experi.

few hundred meters above the plutonicmasses;

c) a possible comagmatic relationshipbetween plutonics and volcanics (O'AMlCO,1964);

J) massive hornfelses presence, givingindications of intrusion into a previous cold(and therefore high) crust;

t') fine-grained and porphyritic textureswithin the smallest masses, also indicatinga cold crust.

Fig. J. - Subhcdral to anhcdral muscovite crystals are distributed hornoseneously in lhe groondmllSS oft~ porphyritic S-granite5 of Monte: Sabion. Crossed Polars, 42 x (.), 10' x (b).

mental data (EVANS, 1965; ALTHAUS et al.,1970), the muscovite crystallization frompersilicic magma would have required apressure of at least 35·4 kb, that is a depthof 13·15 km or more. A similar depth isin complete contrast to all the geological,petrological and textural data and cannot beassumed. The data and some implicationsare briefy discussed.

The intrusions of all the Southern Alpsplutonics are considered to be high·level,on geological grounds, for the followingreasons:

a) intrusion into the basement almostup to the Permian covers;

b) radiometric age very dose to that ofthe Permian volcanics (O'AMICO et aI.,1980), which lie stratigraphically only a

O'AMICO (1965) estimated a maximumintrusion depth of approximately 2 km forthe Roncegno pluton (Cima d'Asta region).

The plutonic association of Monte Sa·bion is of exactly the same type. Theperaluminous granites present petrographicevidence of rapid cooling after emplacement(porphyritic, heterogranular, granophyrictextures) and must obviously be consideredhigh.level intrusions.

For all the above reasons, one cannotaccept the deduction of a crystallization depthof 13-15 km, based on the presence ofmagmatic muscovite and on well·knownexperimental data.

An important overpres.sure of water inthe mass cannOt even be assu~, due tothe porphyritic texture of the peraluminousrocks of Monte Sabion. A water overpres.

32 C. D'AMICO, A. ROTTURA

sure could only eventually be claimed forthe small mass in the Vanoi Valley, Cimacl'Asla.

So the question must be asked: does themuscovite stability . in the peraluminousgranilic melts extend lowards lower pres­sures? A similar question has been discussedin various papers (SVLVESTER et al., 1978;SWANSON el aI., 1978; KEITlI el al., 1980;MILLER et al., 1981) and is implicit inothers.

This problem has been discussed exten­sively by D'AMlco el al. (1981). Thereare two nOI ahernarive. but complementaryways to explain the enlargement of themuscovite stability field towards the lowerP and higher T:

1) the complexity of the muscovile solidsolution (MILLER Cl al., 1981; ANDERSON­ROWLEY, 1981);

2) the water undersaturation of the melt(which can be deduffi:! from WYLLIE, 1977,pg. 47-.51, fig. 9 a).

We feel that the second way is the mostimportant, because it lead to tm: sameconstraint for tm: ascent of the graniticmagmas into the crust: that is the waterundersaturation.

The case of Monte Sabion has beenparticularly discussed from a petrographicpoint of view because it is felt 10 be,amongst all those discussed in literature, thedearest case of magmatic crystallization ofmuscovite at relatively shallow depth.

It invites a revision of the applicability,of the most usual Ms-out experimentalcurves (but not all; see discussion inD'AMICO et al., 1981), to the natural si­tuations, assuming the expansion of themuscovite + quartz Si ability field towards

higher temperalures and lower pressuresthan those usually accepted. The OH·forming and cation exchange reaclion, inthe solution of H~O in aluminosilicate melts,as discussed by BURNU.... M (197.5), mayperhaps provide a due of solution.

Here)'llotype features of theplutonic associatiOn

The presence of peraluminous granites isnormal in the HercynOlype frame (PITCHER,1979 a, b) of the Southern Alps. Theprevalence of I-type over S-type granitoidsin the plutons of Ihe Southern Alps appears10 be overwhelming. The ratio is aboul 10: 1

on Monte Sabion, 100: 1 on Bressanone,1000: I, or more, on Cima cl'Asta. This isan extreme case of tm: Hercynian t}'pology,where the S-type granites usually have amore important role (e.g. Calabria, FrenchMassif Central, Moldanubikum etc.).

This moderate hercynotypology, with veryfew or no S-granites, is, however, prevalentin the Hercynian plutons of the Alps (O'AMI­co, 1974; BARGOSSI et al., 1979) as wellas in Sardinia (01 SIMPLICIO et al., 197.5).It is probably not fortuitous that, in thesesame areas, an important Permian volcanismoccurs, on the contrary to other Hercynianareas.

These few considerations reveal thatHercynotype characterization has an internal,very articulate gradualness, and, for thisreason, wider and deeper examination isnecessary.

Aknowledgemen/s. - This research was supportedby the Italian Consiglio Nazionale delle Ricerehe,er 783)0246.0.5. IGCP Project no. .5.

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