RENDlCONTl OELLA SQCIETA ITAUANA DI MINERllLOGIA E PETROLOGIA, 1988, Vol. 43·2, pp. 367·384

Geological and petrological studies on the hercynian plutonismof Serie dei Laghi - geological map of its occurrence between Valsesia

and Lago Maggiore (N-Italy)

AITIUO BORlANI, LmGI BURUNI, VALERlA CAIRONI,EVEUNA GlOBBl ORlGONI, ANGEUCA SASSI, ELENA SESANA

Dipartimento di Sdenze della Terra, Universitl, Via Bonicclli 23, 20133 Milano

ABSTRACT. - The plutonic rocks are represented bymany small intrusive bodies of calcalkaline basic tointermediate rocks (appinites) and a few large graniteplutons; all of them were intruded parallel to theboundary between Serie dei Laghi and lvrea·Verbano,a boundary which is represented by a set of faults (CMBand Pogallo-Lago d'Orta Lines) of late·Paleozoic age,but with more recent reactivation.

Most of the calcalkaline basic imrusives are generallyolder {around 29.'5 Mal than the granites, as demonstratedby the deeper environment of their intrusion (partialmelting induced in their country rocks).

The granites came later, in the Lower Permian,ascending slowly in the crust with acauldron subsidencemechanism.

Volcanic rocks also occur; rhyolitk ignimbrites coverthe basement in the southernmost part of this area, nearBorgosesia and Arona.

The age of intrusion of the granites and ofemplacement of ignimbrites has been established byvarious radiometric methods to be around 275 Ma.

The host rocks of the plutonites are metasedimenlSand metagranites belonging to the two subunils (Stronll·Ceneri: metabasites, metaarenites, metagranites; SciJti deiLag/Ji: metapelites) of the Serie dei Laghi, whose mutualrelationships are complicated by the «schlingenbaulOstructure near the CMB Line. The appinjtes are mostlyintruded into Strona-Ceneri rocks, whilst the granitesare contained in both subunits.

All the magmatic rocks show calcalkaline affinity withfeatures that suggest variable amounts of cruStalcontamination. Pure anau:ctic proclocts are present onlyin the migmatites that fotm a narrow belt along theCMB.

Key words: Late-Hercynian magmatism, granites,appinites, Setie dei Laghi, Southern Alps.

Introduction

At the end of the Hercynian orogeny somevery important magmatic activity took placein most of the basement rocks of the SouthernAlps (and not only there). As far as one cantell from the present occurrences, the productsof this activity were mostly acidic epiplutonicand volcanic rocks of Lower Permian age(about 275 Ma). Mafic to intermediate rocksare generally older, and are mostly representedby intrusive rocks emplaced at a deep or evenvery deep crustallevel, as one can see fromtheir relations to the country rocks. Their ageof intrusion cannot generally be determinedwith certainty because the Rb/Sr method doesnot give reliable results on account of thegenerally low Rh content and Rb/Sr ratio, aswell as of their isotopic inhomogeneity dueto variable crusta! contamination.

In the Massiccio dei Laghi (N-Italy) a hugelayered mafic body is present in the Ivrea·Verbano Zone (Fig. 1). It is intruded intohigh-grade metasediments; its deepest partsare dearly reequilibrated at the granulite faciesconditions, whilst the upper part still preservesigneous microstructures. The age of the maficlayered body as well as its relationships withthe metamorphism are still the matter of avery lively discussion (see BORIANI and

368 BORIANI A., BURLlNl L., CAIRONI v.. GIOBBt ORIGONI E., SASSI A_. SESANA E.

RIVALENTI, 1984; ZINGG, 1983). Thegeological evidence (and now more and moreV/Ph and Sm/Nd data: PIN, 1986; PIN andSILLS, 1986; VOSHAGE et al., 1987) suggestsa late-Hercynian age, since the body wasinvolved in the last folding phase which canbe attributed to that orogeny, and because theheat transported by the mafic intrusioncontributed to the regional metamorphismand to its own subsolidus equilibration.

Mafic-to·intermediate igneous rock bodies(appinites) occur in a narrow zone along theCMB (Cossata·Mergozzo.Brissago) Line, animportant tectonic discontinuity thatseparates the Ivrea-Verbano Zone from theSerie dei Laghi (BORlANI and SACCHI, 1973,1974, 1985; BoRIANI et aL, 1974; ORIGONIGIOBBI et al., 1975).

Mafic igneous rocks also occur as finegrained dykes in all the units of the Massicciodei Laghi and particularly in the zone N ofVerbania.

The acidic rocks build up the large graniticpiutons (Graniti dei Laghi) that are the mainobject of this article.

Country rocks: the Serie dei Laghi

The Serie dei Laghi is a geological unitbelonging to the intermediate crust, which istectonically juxtaposed to the lower crustalIvrea-Verbano Zone (Fig. 1). It consists of twOmain lithological subunits, i.e. Strona-CeneriZone (SCHMID, 1967) - formerly calledCeneri Zone (REINHARD, 1953) - and Sdsddei Laghi (BoRlANI, 1970). The contactbetween the subunits is marked with thepresence of a continuous, rather thick, layerof amphibolites.

In both submits orthogneisses (Ordovician

metagranites, BoRIAM et al., 1982-83; BoRIANIand GIOBBI ORIGONI, 1984) are also present.

The metamorphism is certainly pre­Permian, but there is no agreement about itsage, despite the many radiometricdeterminations performed during the last twodecades (JAEGER et al., 1967; McDoWELL andSCHMID, 1968; McDoWELL, 1970; I'IDGEONet aI., 1970; KOEPPEL and GRUENENFELDER,1971; HAMETand AusAREoE, 1973; Au.EGREet al., 1974; HUNZIKER, 1974; KOEPPEL, 1974;KOEPPEL and GRUENENFEWER, 1978-79;HUNZIKER and ZINGG, 1980; BORIANI et aI.,1982-8).

From the available radiometric data itseems that the metamorphic transformationsof the metasediments of Serie dei Laghi beganaround 450 Ma ago. In particular, HUNZIKERand 2INGG (1980) consider the Rb/Sr WRisochron of 473 ± 20 Ma, defined with 4samples of metasediments of Serie dei taghi,as representing the age of the thermal peakof the Caledonian regional metamorphism.BORlAM et al. (1985b) and BoRlAM and GIOBBIORrGONI (1984), based on petrographic andgeologic evidence, consider this isochron asgiving the age either of a very weak regionalmetamorphism or even of the Ordovidanmagmadsm. The temperature rise, thecirculation of fluids and the mobility ofpotassium connected with the intrusions,could well be responsible for thehomogeneization of the Rb-Sr system in themetasediments. It must be kept in mind thatthe meaning of the Rb/Sr WR age onmetapelites is generally that of the diagenesisor of the first weak metamorphism.

The lower intercept U/pb ages of thediscordant zircons of the paragneisses analysedby PIDGEON et al (1970), KOEPPEL and

Fig. I. - Geological sketch-map of the Massiccio dei Laghi (after BORI ...Nl and S"'COll, 197J, modifiedl.1 . Pliocene and Quaternary. 2 _Volcanic and sedimentary Permo·Mesozoic cover. 3 - Late·Hercynian granites(<<Graniti dei Laghh,/. 4 . Calcalkaline mafic stocks and dykes: full squares. «Appinites,,; full triangles. maficdykes. VAL COLLA ZONE: 5· Schists, phyllonites, epidote-amphibolites, «Gneiss Chiari". Serie dei Laghi: 6- Strona·Ceneri Zone (a: paragneisses, b; melabasites and subordinate u1tramafitesl. 7 . Sdsti dei Laghi (micaschists,paragneissesl. g . M. Riga and Gambarogno Zone: mainly Strona·Ceneri rocks with complex deformation. 9 .Orlhogneisses.lVREA.VERBANO, 10· Basic rocks, mainly in granulite fades, including some u1tramafites andsubordinate mctascdimcnts. 11 . Kinzigites (pclilic and semi.pelitic, high grade melasediments with minor marbleand amphibolile intercalations). 12 . ALPINE DOMAIN. IJ: Faults: CN. Canavese; TC. Tonale·Centovalli;CMB. Cossaro.Mcrgozzo-Brissago; LGQ. Vat Lessa, Germagno, Quarna; GR. Grottacdo; po. Pogallo-Lagod'Orla; CR. Cremosina; D .. Val Dumentina; VC. Val Colla. 14: Overthrusts; RC .. Riale di Cannero;IT .. Indemini·Montc Tamaro.

GEOLOGICAL AND PETROLOGICAL STUDIES ON nlE HERCYNIAN PUTI'ONISM OF SERlE DEI ETC. 369

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370 BORlANI A.. BURUNl L.. CAIRONI V. GIOBBI ORIGONl E_. SASSI A.. SESANA E.

GRUENEFELDER 097l), HAMET andALBAREDE (1973), KOEPPEL (1974) andKOEPI'EI. and GRUENENFELDER 0978·79),probably have the same meanin.li\.

Probably the best geological evidence thatat least part of the Serie dei Laghi was notmetamorphosed (or very weaklymetamorphosed) ac che moment of theintrusion of the Ordovician granites is thepresence of AI-silicate rich nodules in the fine­grained gneisses of the Strona-Ceneri nearmany metapegmatite dykes presumablyconnected with those granites. The nodulesare interpreted by BAECHUN (1937) andBIGIOGGERO and BoRIANI (l975) as the PrOOuetof the regional metamorphism of originalchiastolite poikiloblasts.

Younger, Hercynian or e~n more recentages were found using the KJAr method onbiotite (200-250 Ma), muscovite (250·290 Ma)and hornblende (325 Ma) (McDoWEll, 1970;HUNZIKER, 1974), the Rb/Sr method onbiocite (250 Ma) and muscovite (310-320 Ma)OAEGER et al., 1967; HUNZlKER, 1974;BORlANI et al., 1982-83) and the U/pb methodon monazite (275 Ma) (KOEPPEL, 1974;KOEPPEL and GRUENENFElDER, 1978·79).McDoWEll (1970) noted that the ages foundbecome younger towards the NW, i.e.towards the Ivrea-Verbano Zone.

The Serie dei Laghi is tectonically situatedbetween the Ivrea·Verbano Zone, from whichit is separated by the CMB Line, and the VaIColla Zone (a medium-grade, locallyreuograded, metapelicic complex). Thecontact with this unit is represented in theeascern part by the VaI Colla Line, an UpperCarboniferous discontinuity, and in chewestern part by the Cremosina Line, which,despite its AIpine rejuvenescence, is probablya continuation of the VaI Colla Line (BoRlANIand COLOMBO, 1979). The Serie dei Laghi isfurther dissected by more recent faults andoverthrusts into various blocks, as shown inthe maps of REmHARD (964), BoRlANI et al.(1977), in the geological sketch of GIOBSIORIco},,'l et al. (1982-83), and in the pre~ntwork.

The metamorphic evolution in most of theSerie dei Laghi is characterized by a firstamphiboIite facies event (ky + St) and by a

second, very local, retrogressi~, grec=nschistfacies event that can be observed only alongshear planes. This event is responsible for thepartial replacement of biotite and garnet byMg-Fe chlorite as well as for the ~ricitization

of AI-silicates and, to a minor extent, ofplagioclase.

Sc.isti dei Laghi

This subunit is mostly made up bymicaschists and paragneisses with rareamphibolite inliers, interpreted as an originallyLate Precambrian or Lower Paleozoic peliticsequence (BoRlANl et al., 1977). The mostwidespread mineral association is: quartz,plagioclase (2096 An), white mica, biotite andporphyroblasts of garnet. kyanite andstaurolite which are syn-postkinematic inrespect to the main foliation. Garnet showsalso synkinematie rotational textures in itsinner parts (BORIANI, 1970). Accessoryminerals are apatite, zircon, opaques andtourmaline.

From the structural point of view (exceptin the «schlingen~zone, see below), at leasttwo phases of deformation can be recognized.The first one, which shows an isoclinalgeometry and transpositive character, isresponsible for the regional foliation. Thepre~Dt strike of this foliation is N 50o_70oEnear the Mottarone-Baveno pluton and e3stand south of the Alzo-Roccapietnl pluton andN 45°E north of Verbania; the dip is generallysubvertical.

The second phase of deformation induceda crenulation cleavage and seems to beconnected with a retrogressive episode. Nearthe CMB Line the presence of another phaseof deformation, intermediate between the twOabove mentioned phases, is connected withthe «schlingenbau» structure developed inthat part of the Serie dei Laghi. North ofPasse delIa Colma (Omegna-Varallo road),where the Scisti dei Laghi occur as thenorthern limb of the large fold with verticalaxis of the zone of Cesara, the foliation isblurred by the partial melting of the rock inproximity of the appinitic OOdies.

GEOl,(X;ICAL AND PETRQl,(X;ICAL STUDIES ON THE HERCYNIAN PwroNISM OF SERlE DEI ETC. 371

Strona·Ceneri Zone

The amphibolite horizon that marks thecontact between Scisti dei Laghi and Strona­Cene~ ~ consic:bed the metamorphic productof onglnal, more or less reworked mafiesediments, belonging to the basal par~ of theStrona-Ceneri Zone (GlOBBI ORrGONl et al1982·83). In the sector W of Lago Maggior'~and o~ th~ western shore of Lago d'Orta theamphlbohtes ohen contain large K·feldsparporphyroclasts (BoRlANI and GIOBBI MANCINI1972; GIOBBI ORIGONI et al., 1982-83). Ou;present opinion is that they result fromgranitization of still permeable clastic materialinduced by a water-rich residual melt comingfrom the Ordovician granites. The mineralasse~blage of the amphibolite is: hornblende,plaglocl~se 05% An), biotite, garnet andquartz; In the granitized varieties, beside K·feldspar, also biotite is very abundant.

1?e outcrops of the main amphibolitehonzo~ west ~f Lago d'Orta form the largefold with vertical axis near Cesara.

Along the horizon not only maficmetatuffites are present: small scarcediscontinuous, but very significan;occurrences of metabasalts, metagabbros andrnetaultrarna.!ites can~ found in many places,such as CeJ.lio (Valsesla), Oira (Lago d'Orta)and Sarangl.o (E shore of Lago Maggiore) andfurther E In Val Colla (Ticino). Eclogiticamphibolitcs occur near the Alpe MoreUoperidotite body (northern part of our map),although they are mostly attributed to theIvrea-Verbano zone (BORIANl and PE;VRONELPAGUANI, 1968).

The Strona-Ceneti Zone is the product ofthe metamorphism of a fine- to coarse-grainedarenaceous sequence. The two main rock typesare: paragneisses with detrital phenoclasts(Cen~rign~iss). and Hne-grained gneisses(Gneiss Minun). Both rocks contain small Ca·silicate bearing nodules (BoRlANl and CLERICIRISARI, 1970).

In BAECHUN'S (1937) opinion the GneissMin':lti (Bi.otithornfe1sgneise) represent the:straugraphically highest and Cenerigneiss thelowermost part of the sequence (sec: alsoBOmANI et al., 1977). The abundance of thearenaceous, S?metimes very coarse, materialand the scarclry of pelites suggest a shallow

water, epicontinental, sedimentaryenvironment. The frequent and repeatedintercalations of one type present in the otherwere intepreted as both the evidence of anheteropical relationship between the twosediments and the result of a tectonic processsuch as a transpositive defonnation phase(GIOBBJ ORJGONI et al., 1982-83).

In . the Lago d'Orta area it is onlyoccasJOnally possible to distinguish theCenerigneisses from the fine-grained gneisses?ue to their widespread mobilization probablymduced by the intrusion of the appinitiem~gma .along th~ CMB Line. The resultingm.1gmatltes conSist of leucosomes of graniticcomposition and of restitic meIanosomes veryrich in a1umosilicates (sillimanite, andalusite)and cordierite. Near the CMB the «granitisedamphibolites» also underwent partial melting.

The structural setting of the Strona-CeneriZone is very complicated and still littleknown. The best approach seems to be thatsuggested by BAECHLIN (1937), who publisheda detailed L50,OOO map of the area NE ofLago Maggiore. In his view, the Strona·Cenerishould represent the core of a synform withaxis steeply dipping SW, whilst the Scisti deiLaghi represent the flanks. The northern limbof this structure is affected by folds withvertical axes {cSchlingenbau»}: this featurewas interpreted by BAECHUN (1937) as theevidence of a high plasticity of this part dueto abundant granitization duringmetamorphism, and also as the effect oftranscurrent movements along a paleo­Insubric Line. BORJANI and SACCHI (1985)attributed the «schlingenbau» structure to thetranscurrent movements along the CMB. Anice example of schlingen structure is thealready mentioned fold with nearly verticalaxis occurring near Cesara, W of Lago d'Orta.It should be noted, however, that this foldseems rather open in comparison with the«schlingen» folds of the northern LagoMaggiore area.

Orthogneilses

The metagranite and metagranodioritelenses i!1tercalated in the Serie dei Laghiparadenvates were considered by the Swiss

372 BORIANI A., BURLlNI L.. CAIRONI V., GIOBBI ORIGONI E" SASSI A.. SESANA E.

geologists who mapped the Souoceneri (seeREINHARD. 1964) the product of a more or lessintense synmetamorphic granitisation ofmetasediments. The original intrusive natureof the rocks was assessed by BoRlANI (1970);a petrochemical, petrographical andgeochronological study of the orthogneissesof the ugo Maggiore area was performed byBORlANI et al. (1982-83).

The orthogneisses are mainly representedby hornblende.bearing and hornblende-free.biotite-plagiocIase-quartz gneisses with veryvariable amounts of K·feldspar and with atypical medium-grained gneissic texture thatbecomes augen or £laser in the moreleucocratic types. Muscovite-bearing varietiesalso occur. Many igneous textures arepreserved both at the micro- and at themesoscopic level, such as idiom:>rphic crystals,

.00

1 ---- 3 -_.- 5 -0- 7 - .....•2--- 4- 6-·-··· 6··--

Fig. 2. _ Mean poims of zircon ~atiolU from theSe:ric dei Laghi orthogneisses in the typologic evolutiondiagram. Dala after CAlllONt, 1986. Fields after PuPl.N,1988. Full triangles: biotite + hornblende and biotitcorthogncincs. Full squares: biotite + muscovileorthogncisscs. 1: .luminous, A1-siliatc{s) • bearingleucognnilcs; 2: aluminous and peraluminousgranooiorilcs - moozograniles with cordieritc nodules;}: porphyritic grmodiorites - monzogrmites, sometimescomainiog automorpbic oordieritc; 4: calca1bIinc andK-nkaIkalinc (cMg-K») series; ,: high K-c.aJcalkalinc(eFe·K») series; 6: .rkaliDc and pcraIbline granites; 7:continental tholeiitic granites; 8: ocnnic tholeiiticgraniles (plagiogranites).

mirmekitic intergrowths. xenoliths. aplitic­pegmatitic dykes.

Another confirmation of the originalintrusive nature of these rocks was suppliedby the typological study of their zirconpopulations (method by PUPIN, 1976)performed by CAmoNI (1986). The biotite andbiotite + hornblende samples. which representthe main type. fall in the calcalkaline field ofthe typological diagram. whilst the ra~

muscovite-bearing samples plot in the typicalarea of a1uminous and peraJuminous suites(Fig. 2).

The age of intrusion was determined witha very good Rb/Sr WR isochron (BORIANI etaI., 1982-83) yielding 466 ± 5 Ma with ani.r. of .7087 ± .0002; it is in agreement withthe concordant UfPb zircon ages of about 450Ma determined by KOEPPEL andGRUENENFELDER (1971) and KOEPPEL (1974).

In the migmatite belt along the CMB Linethe orthogneis.ses were involved in the late­Hercynian anatectic remobilization.

Calcalkaline basic intrusives

a) Appiniks

The most widespread intrusives are nearlyvertical sheet-like bodies of variable thicknessor stocks of remarkable size that occurdiscontinuously in an area elongated NE-SWalong the Ivrea·Verbano/Strona-Ceneriboundary (Borgosesia. Quarna. MonteCerano, Brissago, Monte Ceneri), i.e. alongthe CMB Line.

It should be noted that the appinites arelacking along the Pogallo Line and in the lowerVaI Strona di Omegna between the Val LessaFault and Omegna. The Val Lessa Fault, aswell as many other minor faults orthogonalto the CMB (Quarna, Grottaccio), played animportant role in the emplacement of theappinites. A possible interpretation is thatthey acted as transform faults, dividing«sp~ading.centers along the CMB; anotherpossibility is that these faults were boundariesof pull-apart extensional zones.

The appinito vary in composition fromgabbro to diorite and granCKiiorite even in the

GEOLOGICAl. AND PETROLOGICAl. STllDlES ON THE HERCYNIAN PurroNISM OF SERIE DEI ETC. 373

same dyke, often with a pseudobrecciatedstructure; the more Jeucocratic products arerepresented by aplitic and pegmatitic dykesor by parts of multiple dykes.

1beir mineralogical composition is variable:plagioclase, hanblende, biotite are ubiquitousminerals, whilst clinopyroxene is rarer; quartzand K-feldspar are present only in the moredifferentiated parts. Orthopyroxene andgarnet, partly or completely replaced bycummingtonite and biotite, are widespread inthe lower Valsesia stocks (ORlGONI GIOBBI et.1., 1975).

The texture is highly inhomogeneous onaccount of the presence of chilled marginsand/or multiple intrusions. Many dykes arebreccias with fragments of foliatedgabbtodiorite cemented by a more leucocratic,non-foliated matrix.

All these features and their chemicalcomposition make these igneous rocks verysimilar to appinites of Scotland and Ireland(BORIANI and SACCHl, 1973; BORIANI et al.,1974).

The age of intrusion of the appinites cannotbe determined by means of a Rb/Sr isochronon account of their initial inhomogeneousisotopic composition (probably due to variousdegrees of hybridisation, see PINARELU et al.,1988). The mineral ages are highly variableand therefore can be interpreted as mixed agesdue to the more or less intense re-opening oftheir isotopic system, induced by a repeatedtectonic reactivarion of the eMB Line. Thebest age determination is probably the V/Pbage of 285 :l: 5 Ma on monazite from anappinitic dyke near Mergozzo (KOEPPEL andGRUENENFELDER, 1978-79; CUMMING et al.,1987).

From geological evidence, appinites predatethe uplift and erosion of the Hercynian beltand the emplacement of the granite pIu tons.

Effects ofthe appinilic inln4siom on the countryrocks

Field and petrographic evidence suggestthat this basic magma was emplaced into thecountry rocks in a rather deep-seatedenvironment and that the heat carried by theintrusion caused the partial melting of the

country rocks.The anatectic phenomena can be observed

in the Serie dei Laghi from the intersectionbetween CMB and Pogallo Lines N ofMergozzo till the zone of Borgosesia (the onlyexception being a short stretch between ValI.essa and Omegna in the lower Valle Strona,where the appinites are also absent). Thewidth of the mobilized belt is about 3 km SEof the CMB Lne. In the lower Val d'Ossolaand in Val Sesia, the mobilization producesdramatic effects on the rocks that are mostlytransformed into an agmatitic complex.

The best section through the mobilizedzone can be observed orthogonaIly to theCMB in the Cesara- Nonio zone, W of Lagod'Orta, between Riale Acqualba and RialeFiumetta (BURUNI and CAIRONI, 1988). Thefirst appearance of thin leucocratic discotdantveins is accompanied, on a microscopic scale,with the appearance of mirmekite-likemuscovite.quartz intergrowths associated withrelic sillimanite. This texture can easily beexplained with an incipient dehydrationmelting of muscovite and a subsequentrecrystallisation in a water poor environment.With increasing mobilization of the rocks themirmekite-like intergrowths appear also inbiotite, implying a temperature increasetowards the CME. Then follows a zone ofvery interne migmatization of all the Serie deiLaghi rock types included the K·feldspar­bearing metabasites.

The age of the anatexis can be estimatedas about 295 Ma from the concotdant V/pbage of a monazite from a migmatite nearGermagno, detetmined by KOEPPEl. andGRUENENFEl.DER (974). This age is in fairlygood agreement with the measured andinferred age of the emplacement of theappinites.

Outside the area covered by the geologicalmap, in the Val Cannobina-Brissago zone (seeGeological Map in BORIANl et al., 1977) thecountry rocks of the appinitic dykes areapparently not migmatitic, although they showa static lower pressure overprint on theregional metamorphic parageneses. BetweenVal d'Ossola and Valsesia the CMB Line ismarked by the presence of a discontinuoushorizon, a few meters thick, of

374 BORIANI A.. BURLlNI L.. CAIRONI V. GIOBBI ORlGONI E., $ASSl A., SESANA E.

blaslomylonites. Here the blastomylonites,near the appinitic bodies, show staticrecrystallization or are involved in themobilization (BoRlAI'.'1 and SACOU, 1973).

b) Mafic dyk"

The other group of mafic rocks consists ofseveral dyke swarms intruded in themetamorphic rocks of Serie dd Laghi. Theyare very abundant W of Lago Maggiore in theScisti dei Laghi N of Verbania (see GeologicalMap in BORIANI et al., 1977), very rare E ofthe lake and almost absent in the Strona­Ceneri rocks.

In the area of Verbania they are subverticalwith a NS strike in Valle Imrasca, where theyare connected with the NS fault system,approximately ENE·WSW in the Valle di SanBernarclino, where they are aligned with thegranitic plulons (BORIANI et al., 1985c).

Their thickness varies from 0..5 to 1 m andtheir maximum visible length is 40-.50 m.They are very dark-eoloured, with small darkphenocrysts of hornblende, pyroxene andolivine in a very fine groundmass.

As far as the mechanism of emplacementis cona:me:l, contrasting indications are givenby the two sets of dykes. The dykes of ValleS. Bernardino show angular contacts andchilled margins; they were emplacedpermissively along preexisting fractures in anextensional regime. The dykes of ValleIntrasca show undulating contacts thatindicate a more forceful mechanism(CARMINE, 1987).

The mafic fine-grained dykes were injectedbefore, during and after the emplacement ofthe granites.

The chemical composition of both appinitesand mafic dykes reveals a general calcalkalinecharacter (BORIANI et al., 1974; GlOBBlORlGONI et al., 1988).

Granites

The granites of the Serie dei Laghi buildup a batholith, elongated in NE-SW direction,comJX>Sed of many plutons occurring from thezone of BieUa to the western shore of LagoMaggiore. E of the lake the granites are

absent, but granophyres and graniteporphyries occur in the Lago di Lugano area,indicating the presence of the underlyingbatholith.

Extensive studies of the granites occurringSW of Lago Maggiore were carried out byGAJ..UI'Fl..U, during the tronies and the forties(19,7, 19,8, 1941, 194,); VENIALE, 1961;GANDOLFI and PAGANELll, 1974; BALCONl andZEZZA, 1964, 1976; ZEZZA, 1977; ZEZZA etal., 1984; BORIANI et al., 1985a, d. A reviewcan be found in GIOBBI OJuGONI (1987).Deuiled geological, petrological andgeochemical researches are now being carriedout by BORIANI, BURLlNI, CAIRONI, DELMORO, GIOBSI OJuGONI, ODDONE, PiNARELU,SASSl, SESANA and VANNUCCI.

The Montorfano and Mottarone-Bavenogranites are the best known and will bedescribed in detail. Research on the othergranites is still under way and we will give onlysome preliminary new data, besides what canbe found in existing literature.

The westemmost part of the batholith isthe Valsessera - Biellese pluton, which doesnot appear in our Geological Map.

The Biellese pJuton is intruded into theSerie dei Laghi metamorphites; in the lowerValsessera it is in contact with the overlyingPermian rhyolites (PiGORlNI and VENIALE,1962; BALeONI, 1963; FRIZ and GOVI, 1963;BORTOLAMI, 1964).

This granite piu ton is characterized by thepresence of many aplite, pegmatite, graniteporphyry dykes and quartz veins. ZEZZA(1977) gave a detailed petrographic accountof the main body and distinguished, ontextural grounds, three main varieties: fine­to medium-grained, sometimes porphyritic,granites; pseudo-porphyritic granites;medium-grained granites with hypidiomorphictexture. Rare patches of granodiorite,feldspathic granite and quartz-syenite arescattered in the main granite. Marginal microgranular fades, probably chilled margins, arefound in contact with the volcanites. Thecolour of the granite varies from white­yellowish (0 pink (La Colma, Rive Rosse). 'Themineral assemblage includes: plagioclase(22-35% An), microcline and/or onhoclase,quartz, biotite, muscovite; accessory minerals

GEOLOGICAL AND PETROLOGICAL STUDIES ON THE HERCYNIAN PUfI'ONISM OF SERlE DEI ETC. 375

are apatite, zircon, magnetite, orthite,fluorite, tourmaline, garnet and hematite. Themineralogy varies from west to east: themonzogranites dominate in the western partand the syenogranites in the eastern part. Inthe monzogranites the main components aremicrocline and plagioclase with fractured,altered and corroded cores; they wereinterpreted as relics of a partially meltedprocolith. In the syenogranites orthodase andplagioclase do not show any relic core.

The petrochemical data indicate that theBiellese granite belongs to a caIcalkaline series;the west-east variation corresponds to anevolution from a yosemite-granitic magma toan aplite-granitic magma (ZEZZA, 1977).

Alzo-Roccapietra granite

This pluton is exposed between the lowerValsesia and Lago d'Orta. Its northwesternpart is mostly intruded in the Strona-Cenerigneisses, whilst the host rocks of its southernpart are Scisti dei Laghi metapelites (see alsoTOLOMIERJ, 1985).

Mineralogical and petrochemical data onthis granite can be found in .ARllNl and MELZl(1900) and in GALUTELU (1941, 1943).

GALUTELU described the textural andmineralogical variations between the centraland the peripheral zones of the granite body:near Alzo and La Colma the granite is whiteand contains little biotite; plagioclase ismarkedly zoned. Other varieties are richer inbiotite, others show porphyritic textures.Microgranite dykes are very frequent in thegranite itself and in the country rocks nearArola.

The granodiorites, tonalites and maficdykes that occur in the lower VaIsesia betweenRoccapietra and Agnona were interpreted byGALUTELLI (1941) as mafic differentiates ofthe granite. More recent studies by BORIANIand SACCHI (1973), ORIGONl GIOBBT et al.(1975) have shown their similarity to theappinites; they actually belong to thepregranitic magmatic cycle (see calcalkalinebasic intrusive).

A new geologital mapping, carried out byTOLOMIERI (1985), revealed that the mostwidespread facies of the Alzo·Roccapietra

piuton is a white, medium-grained biotiticgranite; pink varieties are reported in the NWpart of the plueon.

A hornblende-bearing granodiorite occursin the Roccapietra quarry; its composition is:plagioclase, K-feldspar, biotite, hornblende,quartz. The granite is cut by a microgranitedyke rich in veins and cockades of tourmaline.

FONTANA (1976) carried out studies on thegranite of the Roccapietra quarry; on the basisof geochemical data she estimated the T andP conditions of crystallization at about600·7000 ( and P H 20 = 4-5 kbat.

A low pressure contact metamorphicaureole surrounds the granite pluton, but itcan be seen and studied only in a few sectionssince the northern and western host rocksconsist of migmatitic rocks with high-grade- low-pressure restites and since most of thesouthern area has very poor exposures.

The pluton has apparently preserved itsoriginal position (i.e. it was not significantlytilted later). Fragments of contactmetamorphosed country rocks, associatedwith the arkose due [0 a pre-Quaternary deepweathering, can be found crossing the plutonin N-S direction; this means that the plutonis barely unroofed by the erosion. Otherwisethe scarcity of country rocks xenolithssuggests a cauldron subsidence mechanism ofemplacement in a hors[ and graben tectonicregime. The N directed granite tongue of Pellawas considered by BORIAM and SACCHI (1973)as being the possible expression of a draggingof the intruding granite along the Pogallo ­Lago d'Orta Fault (interpreted as syngranitic).Actually the peculiar shape of that outcropcan be better understood if we consider theright angle between the tongue and the restof the pluton as the boundary of a subsidingand/or tilting block that guided the intrusionof the granite.

Quarna granite

Most of the exposed part of the Quarnagranite is deeply arenitized, since it was notcleaned by the Quaternary glacial erosion. Thepluton shows a roughly elliptical outline, witha maximum diameter of about 4 km.

The granite intruded into migmatitic

376 BORIANI A., BURLlNI L., CAIRONl V., GIOBBI ORIGONl E., SASSI A., SESANA E.

Strona-Ceneri gneisses, except in itsnorthwestern part where the host rock is agabbrodiorite belonging to the appinite suite.This small pluton is therefore characterisedby the presence of more basic rocks thatprobably represent the only evidence of a closeassociation of appinites and granites. Thefollowing fades have been recognized (fromSE to NW): granite (main fades),granodiorite, quartz-diorite arx:I gabbrodiorite.

The field evidence shows that the moreleucocratic fades intrude the basic ones. Themineral composition of the granite is:plagiodase (22·30% An), quartz, microcline,biotite ± hombJ.~e. Accessory minerals are:allanite, sphene, zircon, apatite, ilmenite.Plagioclase shows very calcic cores; it appearsas clots together with decussate biotite flakes.

The contact aureole cannot be clearly seensince the country rocks are migmatites withhigh-T restites; only very sddom is it possibleto recognize neoformation of biotite wherea pregranitic chloritization took place.

The mechanism of intrusion Sttms again tobe of the cauldron subsidence type. Thepresence of roof pendents in Valle Fiumettasuggests once more that this pluton is also stillmore or less in its original position and thatit was only unroofed by the erosion.

A detailed account of the Quarna graniteis given by BURUNI (1986) and BURUNl andCAIRONI (1988).

Data on the chemical composition of thevarious fades of the Quarna pluton were alsogiven by BURANl (1961).

Montorfano and Mottarone-Baveno plutons

The Moncorfano and Mottarone·Bavenoplutons are composite intrusive bodies, alignedin N-S direction, exposed over an area ofabout 30 sqkm.

Both Montorfano and Mottarone showclear marks of the morphogenetic action ofthe Pleistocene glaciers; the ice entirelycovered the Montorfano, whilst on Monaroneit reached an altitude of about 1000 m.

In all the granite types the presence ofordered microfractures in quartz favours thesplitting of the rock along a series of rathersmooth parallel planes called «pioda », which

are utilized for quarrying.In both plutons it is possible to recognize

a sequence of intrusion on the basis of theircomposition and field relations (GALUTELLI,1937,1938; GANDOLFl and PAGANELU, 1974;SASSl and SESANA, 1986).

1) Mottarone-Bmmlo granites

The pluton of Mottarone-Baveno consistsof two main rock varieties: the pink granite(Granito Rosa di Baveno), which isextensively quarried, and the white granitewhich constitutes the bulk of the pluton.Other rather important varieties are: thexenolithic granodiorite at the top ofMonarone and a porphyrjtic fades(GAllII'ELU, 1937, 1938; GANDOLFt andPAGANEl.J..J, 1974; SASSJ and SESANA, 1986).

The pink granite is a medium-grained rockwhich owes its colour to the K·feldspar; thisgranite is known all over the world for itsmiarolitic cavities (literal1y: cavities of granite;«miarolo. is an ancient local name for granite)lined with magnificent crystals of more than60 mineral species. This granite type isexposed over a narrow area elongated in NNE- SSW direction at an altitude of 1100-1400m on the southern flank of the mountain andbetween 600 and 900 m in the northern part.

The minerals of the pink granite are: K·feldspar, quartz, oligoclase, biotite, fluorite,white mica, zeo1ites, epidote, alIanite, zircon,fayalite and magnetite (in nodules).

K-feldspar, which is the most abundantmineral, is present mostly as ratherxenomorphic crystals, bearing inclusions ofauromorphic plagioclase and biotite crystals.K-feldspar shows all the possible transitionfrom tiny perthite veins to an almost completereplacement by albite.

Quartz is also abundant as xenomorphicgrains; sometimes aggregates of a fewautomorphic grains are present.

Oligoclase (25% An) is less abundant thanthe previously described minerals; it formsautomorphic, or subautomorphic crystals,often with a more calcic, altered core,sometimes replaced by f1uorite. A thin, clearrim of albite surrounds many plagioclaseprisms, as well as the K·feldspar crystals.

GEOUX;ICAL AND PETROLOGICAL STUDIES ON THE HERCYNIAN PWTONISM OF SERIE DEI En:. 377

Biotite is present as small isolated orgrouped autOloorphic lamellae, which are veryoften altered in chlorite and/or replaced byepidote, zeolites, fluorite and white mica; theunaltered crystals show a strong pleochroism,from pale yellow to dark greenish brown.

The order of crystallization cannot alwaysbe recognized very easily, although biotite andplagioclase are often automorphic; we mustconclude that the crystallization interval wasvery small. Miarolitic cavities are also presentat the microscopic scale: the cavities arealmost completely filled with newly-formedchlorite, white mica and/or fluorite_ The pinkcolour of the rock is probably caused by thealteration of Fe} + replacing some AI in theK-feldspar. The colour is actuallyinhomogeneous in both hue and saturation;at the transition to the white granite the pinkcolour of the feldspar is patchy or even absent,though the rock is petrochemically identicalto the pink granite. Sometimes oligoclase canalso be pink or reddish.

The pink granite was evidently the resultof the crystallization of a magma that reachedthe water oversaturation very early, wirh theformation of vapour bubbles. This implies anincrease in volume that was possible only ifthe intrusion took place at shallow depth,pushing upwards the roof rocks. In thebubbles the same minerals of the rock (plusmany others) crystallized in the presence ofvapour, giving rise to the celebrated crystalsof Baveno. The subsolidus transformationswere also very deep due to an abundance offluids either in the pneumatolithic or in thehydrothermal stage of the evolution withformation of £luorite, Li-micas, albitization,kaolinization etc. (PAGUANI, 1936, 1941;PE,YRONEL PAGUANI, 1948).

Coarse·grained pockets, often rimmed byfine-grained, aplitic fades, occur besides thetypical miarolitic cavities. BALCO"''l and ZEZZA(1967) suggested that the abundance of theBaveno-Manebach associations in K-feldsparshould be due to the fluid influence duringcrystallization.

The white granite occupies most of thevolume of the exposed piuton; contacts withthe pink variety can be individuated in thefield only because of the complete

disappearance of the pink hue of the feldspar.This is essentially a medium-grained

granite, with very rare coarse-grained parts,composed of quartz, K-feldspar (oftenshowing the microcline grid), plagioclase,biotite, zircon, allanite, sphene and fluorite.

The presence of clots of zoned plagioclasecrystals is peculiar to this fades of the pluton.

In the western part of the piuton (in anarrow zone elongated in N - S direction) thegnmite is muscovite-cordierite-bearing.

The general sequence of crystallization canbe established very clearly from the texturalevidence as follows: biotite, plagioclase,quartz, K-feldspar.

The porphyritic variety, with phenocrystsof K-feldspar, quartz, plagioclase and biotitein a sometimes granophyric ground mass, ispresent in a small, late, crosscutting body inthe northern part of the pluton (SASSI andSESANA, 1986)_ Other fine-grained,porphyritic varieties appear as marginal fadesof the white granite towards the countryrocks.

The topmost part of Mt Mottarone consistsof a small body of granodiorite showing knife­sharp contacts either with the pink granite orwith the schistose country rocks withoutchilled margins.

The Mottarone granodiorite is a biotite­rkh, medium- to fine-grained rock withzoned, automorphic phenocrysrs of plagioclaseand sometimes pink K-feldspar megacryststhat give the rock a marked heterogranulartexture. But the most striking feature of thisrock is the presence of countless small,schistose xenoliths without any preferredorientation and very homogeneouslydistributed. The xenoliths show the evidenceof high grade thermal metamorphism sincebiotite is almost completely replaced by anaggregate of cordierite, spinel and corundum.

The compositional zoning and the internalstructure of the Mottarone-Baveno plutonimply that this intrusive body is still more orless in its original attitude.

lI) Montor/ano granites

Most of Mt Momorfano, an isolated moumat the confluence of ValIe Strona and vat

378 BORIAN! A., BURLlNI L.. CAIRONI V.• GIOBBI ORIGONI E., $ASS! A" SESANA E.

d'Ossola, consists of a white, medium-grainedgranite. The country rocks (medium grademetapelires of the Scisti dei Laghi formation)are exposed only in small outcrops on thenorthwestern flank of the mount. Even thegrain-size variations are very rare and confinedto small pockets and veins.

The mineral assemblage is: plagioclase,quartz, K-feldspar, biotite, apatite, zircon.

The plagioclase is always auromorphic, withregular or rythmic zoning, with andesine oreven more calcic cores, that appear corrodedas if the crystal were in disequilibrium withthe coexisting melt at a given stage ofcrystallization. Clots of plagiodase crystals,connected to each other through the cores,are very frequent. The clots also often containbiotite indicating early crystallization of bothminerals.

The K-feldspar shows both Baveno andCarlsbad twins; it forms sub-automorphic orxenomorphic interstitial grains sometimes inmicrographic intergrowths with quartz.

The quartz forms rather coarse,xenomorphic grains with rare inclusions; thin,parallel fracture lines are often visible.

The biotite is dark honey brown; it includestiny apatite, zircon and, more rarely, allanitecrystals.

Green hornblende is only present in arestricted zone in the northwestern part ofthe granite stock (GALUTEllI, 1938; SASSIand SESANA, 1986). In the amphibolic granite,allanite is abundant in large, automorphiccrystals.

In the easternmost part of the Montorfanostock (Cava Donna) the granite is full ofenclaves of a greyish heterogranular fades, inturn containing mafic, fine-grained enclaves.The mineral assemblage of the grey graniteis the same as that of the white granite: theminerals are present in two distinctgenerations. The margin towards the whitegranite is sharp, without any particular contacttextures or compositional variations.

Microgranular, compact, dark enclaves arewidespread in the Montorfano stock, whilstthey are rare in the Mottarone-Baveno pluton.The maximum concentration is within theheterogtanular fades of «(Cava Donna>,; theirsize ranges from a few millimeters to some

dedmeters (up to 1-2 m). They probablyrepresent syngranitic dykes, disrupted andpartly granitized by a slowly convectingmagma. The enclaves show an intersertaltexture of biotite, hornblende and plagioclase;new biotite is formed from hornblende. K·feldspar and quartz, introduced bygranitization, are present as integranularmaterial and patches.

A small, distinct granite body, crowdedwith schistose xenoliths, occurs in thenorthern part of the stock. It is composed ofwhite medium- to fine-grained rocks,sometimes slightly porphyritic in texture, withpale pink K·fddspar phenocrysts; it does notcontain plagioclase clots of earlycrystallization. The size of the xenolithsranges from microscopic to more than 1 macross. They preserve the original schistosityand are oriented at random in the enclosinggranite; their mineral assemblage includessillimanite, andalusite, spind, corundum,cordierite.

A dyke of granite porphyry cuts across thenorthern contact of the granite and penetratesinto the country rocks.

Addic differentiates are represented bymany aplitic dykes and very few pegmatiticpockets.

The «green granite» of Mergozzo occursin the northern part of the stock as anarrow band near the contact with themetapelites. This is clearly the result ofepisyenitization, i.e. subsolidus, low­temperature transformation of a preexistingintrusive rock, since, despite its igneousappearance, it is composed of albite, chlorite(analyses in MORTEN and ROSSI, 1971), quartz,seridte, sphene, carbonates. Within this rockmicrogranular enclaves are present, similar insize and texture to those of the white graniteand showing the same, low-! paragenesis.

Field relations and petrographic evidencesuggest (SASSI, 1985) a derivation of the«green granite» from the common, whiteMontorfano granite (on geochemical groundsBORIANI et al., 1988, also consider a possibleaffinity with the appinites). The contactbetween the white and the green granite isgradual and occurs through a transition zonea few metres thick or, more frequently,

GEOLOGICAL AND PETROLOGICAL STUDIES ON THE HERCYNIAN PLUTONISM OF SERJE DEI ETC. 379

through a thick zone characterized by thepresence of pale pink granite.

A typical sequence is:white granite;granite with biotite and pale pink feldspar;granite with chlorite and some pinkfeldspar;green granite of Mergozzo with chloriteand albite.

The transformation occurs throughincreasing albitisation and chloritisation inprogressive steps.

At the beginning K-feldspar is replaced bychessboard albite, while plagioclase remainsunaltered, then the chessboard albiterecrystallizes in polycrystalline aggregates andplagioclase is transformed into medium­grained albite. A further step is therecrystallisation of the aggregates into coarsercrystals. During albitisation, the albite itselfbecomes partially replaced by seridte, namely,the fine-grained aggregates and the chessboardalbite. Quartz also is progressively replacedby albite (desilicationl.

The first appearance of chlorite is due toa common alteration of the biotite of theigneous rock; at a second step, newly formedchlorite in fine-grained aggregates replacesboth the chlorite due to alteration of thebiotite and the sericite that had formerlyreplaced part of the albite.

The great abundance of chlorite seemstherefore due to a deep metasomaticcompositional change and not to a derivationfrom a rock richer than granite in maficminerals.

The intensity of both albitisation andchloritisation is very unequal from place toplace. These hydrothermal transformationsalso occur in the schists near the contact overa width of a few metres.

Ill) Zircon typology

The different fades of the Mottarone·Baveno and Montorfano plutons showdistinctive typologic characters of their zirconpopulations (CAlRONI, 1985a, b). On thetypologic evolution diagram (see Fig. 1 inBORlANI et al., 1988) three typologic trendscan be defined. The first one is given by the

white Mottarone granite and by itsporphyritic variety (the zircons of the latterdiffer from those of the main granite only insome secondary characters, suggesting a morerapid cooling).

The second trend is given by theMoncorfano white granite and by its greyishheterogranular facies (enclaves at CavaDonna); in the latter the zircons show featuresindicating a crystallization in the samechemical environment at higher temperature.

Both trends fall within the field of typicalcalcalkaline granites. The zircon populationof the Mottarone granocl.iorite could be relatedeither to the Montorfano or, better, to theMottarone trend.

The third group is formed with the typicalpink Baveno granite and the white granite ofthe transition zone. Their zircon populationsshow all the features of a crystallization fromaIuminous alkaline granitic magma. Themagma should have had an alkaline characterfrom the very beginning of the zirconcrystallization (about 850°C). The high fluidpressure extended the crystallization of thezircon towards low temperatures (about600°C).

IV) Contact metamorphism

The Mottarone-Baveno and theMontorfano plutons show subvertical, sharpand discordant contacts; the regional attitudeof the schistose country rocks appears almostcompletely undisturbed.

In the main granite varieties the presenceof country rocks xenoliths is restricted tozones immediately near the contacts.

The granite does not show any primarypreferred orientation of the minerals. Themain granite varieties show chilled margins(maximum 10 metres thick) against the wallrocks. Marginal apophyses ace scarce, shortand thin.

The contact metamorphism produced acomplete static recrystallisation in a rathernarrow aureole; there we find newly formedbiotite, andalusite, cordierite, spinel,corundum (the last two only at Montorfano),and late-metamorphic muscovite. In the outerrim of the aureole, biotite grows at the

380 BORIANI A.. IlURllNI L., CAIRONI V.• CI06Bl ORICONI E., SASSI A., SESANA E,

expense of retrograde chlorite, indicating thatthe country rocks had already cooled down,with local retrograde metamorphism, beforethe intrusion of the granite.

Through a detailed study of the aureole,and using the conductive model of JAEGER(1957) a minimum temperature of 720°C forthe intrusion of the Monrorfano granite at adepth of 4-5 km, and a slightly lowertemperature for the Mottarone can be inferred(SASSI and SESANA, 1986).

The contact aureole can still be recognizedeven in episyenitised country rocks near the_green granite» (p~nce of ~lic corundum).

V) PetroehmJistry

The pink Baveno granite stands out clearlybecause of its different chemical characters,such as: very restricted range of composition,high SiO", low FeO tot., Ti02, CaO andAI20). Unly the granite porphyry ofMonrorfano shares these same characters.

The white granite of Mottarone (and itsporphyritic variety) can be easily distinguishedfrom the Montorfano granite; the latter isgenerally more basic and has a broadestvariation range.

The comparatively low Si02 and highAl20 and Ti02 of the granodiorite of thetop orMt Mottarone can be attributed to thepresence of abundanr xenolithic material.

The «green granite» shows a lower SiO..;content and a higher A120" Na20 and MgUcontent compared to the white Montorfanogranite.

The modal compositIOn of the granitesvaries from monzogranitic in the Montorfanoto monzo- and syenogranitic in theMottarone-Baveno (GANOOLfl and PAGANELLI,1974).

If we consider the major element chemistryand the geological-petrographical data, thecompositional variations can be compatiblewith a process of fractional crystallisationoccurring in a reservoir at depth, before the·intrusions.

An account of the geochemical features ofthe granites can be found in BORIANl et al.,1988.

VD Mechanism of emplacement

The field relations as well as thepetrographic characters suggest that most ofthese granites intruded by means of apermissive cauldron subsidence mechanism,as suggested mainly by the absence ofxenoliths in the internal parts of the plutonsand by the steep, discordant contacts with thecountry rocks.

The subsidence was probably triggered bythe initial forcdul intrusion along preexistingtectonic discontinuities of the xenolith·richvarieties, i.e. the Mottarone granodiorire and(perhaps) the white granite with schistosexenoliths of the northern part of Monrorfano.

The order of emplacement of the variousgranite types cannot be precisely defined. Apossible sequence could be:_ forceful and turbulent intrusion of the

Mottarone granodiorite and of thexenoJith-bearing granite of Montorfano;passive intrusion of the Mont?rfanogranite with a cauldron subSidencemechanism and contemporaneousinjection of mafic dykes in the viscousgranitic magma; the priority of theMontorfano granite is geologicallysuggested by the presence of a graniteporphyry dyke similar in composition. tothe pink granite of Baveno crosscuttingthe Montorfano granite;emplacement, with the same mechanism,of the Mottarone-Baveno pluron. Thepink granite magma was the upper, lighterpart of the intruding magma column. ~n

other words, the intrusion of the twO mamvarieties of the Mottarone-Baveno plutonseems to have been a continuous processoccurring at pace with the progressivesinking of the subsiding block.

Conclusions

The Hercynian plutonism of the Serie deiLaghi must be considered in the context ofthe huge magmatic event that followed t~eHercynian orogeny. Its activity occurred mthe time span between about 300 and 275 Ma,during the uplift of the Hercynian. belt ~ndthe subsequent extensional tectOOlC regimethat characterized the lower Permian. At the

GEOLOGICAL AND PETROLOGICAL STUDIES ON THE HERCYNIAN PLUTONISM OF SERI[ DEI ETC. }81

~ginning. only mafic and intermediatemagmas could ascend as small bodies in thecrust; the basically compressive (or, hener.transpressive) tectonics of that early phasefavoured the assimilation of crustal materialas well as fractional crystaIlization. Later. theenormous volwnes of slowly prrouced gramticmagma could either reach directly to thesurface (Permian volcanism) or formcomposite plutonic bodies at a rather shallowdepth.

From our field and petrological data itseems very unlikely that the plutons underconsideration were volcanic reservoir. in otherwords that they vented at some stage of theirevolution. 1beir internal heterogeneity seemsto be the consequence of multiple intrusionof magma fractions that were differentiatingat depth.

The tectonic lineaments of the zone andnamely the CMB. Pogallo and Paleocremosinalines guided the ascent of the granitic magmasproviding ideal gra~n boundaries thatfavoured the cauldron subsidence. The CMBalone is responsible for the emplacement ofthe older appinitic dykes and stocks and SttffiS

therefore the deepest structure of the zone.Recent moods for the CMB and Pogallo

Lines (see SoiMm et al., 1987 for details)imply an original role of low angle normalfaults of Triassic-Jurassic age. and a later(Alpine?) tilting up to 60°. TIle results of ourfield and petrological research do not agreewith such an intepretation.

AcknowWgtmmts. -' This work was fin.nci.llysupported by MPI 40%.

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