REND/CONTt 50011,14 UaUana dl MI""rllloglll " P"frologia, JB fO: pp. UJ·2115

Comunladone pre..,ntala .1 Congre-.o SIMP dl CAglllU1 11 16 ottobre HNll

COMPOSITION, LEVEL OF INTRUSIO A 'D AGEOF THE «SERIE DEI LAGHI» ORTHOGNEISSES(NORTHERN ITALY - TICINO, SWITZERLA D)

ATTlLlO BoRIANI, EVELlNA ORIGONI CIOBSI

Ccntro di Srudio per la Srratigrara e Petrograful delle Alpi untrali del C.N.R. - lo.1il.no,Isr. di Mioct1llogia. Petrogt1lfia, Gcochimica e Giacimenti MinCf1lri, Vi, Bouicelli 23, lOt)} Milano

ALOO DEL MORO

IuitulO di GcocfOllOlogia e Gcochimica lsotopiCll del C.N.R., Via Cardinale Maffi 36, 561(1) Piu

RtA5SUNTO. - La «Serie dei Laghi .. ~ un'unitaa medio grado metamorfico del hasamemo Sudalpi.no, che puo essere divisa in due parti: .. Strona·Ceneri .. (per 10 piu metapsammiti) e • Scisti deiLaghi .. (per 10 piu metapeliti) separate in genereda metabasiti. Emrambe comengono interc:alazionidi ortogneiss; questi derivano da imrusivi da Wn.l­litici a gnnitici. Trcdid campioni di ortogncissdefiniscooo un'isocrona di 466:t.' m.a. (rapponoinwale delle Sr = 0,7087). L'eti. del metamomsmo~ intomo a )25 m... (cd. Rb/Sr delle michc) e I'etldel rafftcddamemo dccresa: \-erso I'. Ivrn·Verba·no.. J caratteri chimici indicano un. differcnzia·zione IIUI3ffiItica con miscuglio variabile di cumuluse intcrc:umulus nci diversi stadi di fruienamento.AI momento dcU'intrusione gli • Scisti dei LaghilO(sicuramente gi.li metamodici.) giacevano sotto ladebolmente metamorfica • Stro!Ul-Ccncri ,.. Que.sr'uhima comicne gneiss occhiadini, ehe dCTivanodai prodoni &ll'estremo frnionamenw del magma,nonchC pegmatiti laminate, che alKOra consc:rvano,nelle rocce il'lC1lSS3nti, relini di un metamorfismoill contano.

A8STlt,4,CT. - The. Serie dei Laghi .. is a mediumgrade metamorphic unit of the Southern Alps thatcan be split in two parts: .. Strona·Cenefi .. (mostlymetapsammites) and .. Scisti dei LaghilO (mostlymctapdites) generally separated by metabashes.Both comain ot!nognciss intercalations. Thesc derivefrom tonalite.to-granite intrusives. Thirteen samplesdefine an isochron of 466 ±;; m.y. (initial Srratio = 0.7087). The age of metamorphism is around32' m.y. (Rb/Sr muscovite ages), with cooling aflesdecreasing towards the • Ivrea·Verbano... Theehemical characters indicate a differentiation withvariable mixing of cumulus and intercumulusin the \'lfiOl.ls fraetionation Stages. At the momentof their intrusion the «Scisti dei Laghi,., whichwere already metamorphosed, underl.y • wcaldymetamorphosed «Strona-Ccneri,.. The latter 00tl­

tains aUl:tn-gnei5SCS, that dc:ri.\"C from the extremefractiorUltion productS of thc: intrusions, andsttOO8ly laminated pegrnatite dykes ....·ith relics ofconr:aet metamorphism in their C'OUfllry rocks.

IntroductionThe « Serie dei Laghi,. is a metamorphic

unit of the South-Alpine basement of theCentral and Western Alps; the seriesconsists of probably late Precambrian orearly Palaeozoic sediments that weresubjected to repeated deformational andmetamorphic actions and were the seat ofigneous activity at different times duringthe Paleozoic era. Basically the series wasnot involved in the Alpine orogeny, whosevery marginal effects can be ignored for thepurpose of this paper; the authors intendthis paper to be a contribution to theunderstanding of the palaeozoic history ofthis part of the continental crust.

The area under consideration is thatextending E and \VI of Lago Maggiore wherethe .. Serie dei Laghi. is known also as«Ceneri Zone» (REINHARD, 1953) o'r« Strona·Ceneri» (SCHM1D, 1967).

The «Serie dei Laghi,. was divided byBoRIANI (1970) in twO subunits: «Scistidei Laghi» mostly composed by pelitic andsemipelitic metasediments, «Strona·Ceneri,.mostly composed by metapsammites. Thetwo subunits, W of Lago Maggiore are sepa·rated by a continuous layer of metabasites.

The relatioDships between the variousrock types of the .. Serie dei Laghi,. werelong a matter of uncertainty so that, in hispaper of 1964 (a conclusive work of aperiod of field and peuographic studies thatlasted from 1925) REINHARD wrote ..... of.fenbar keine scharfen Crenzen zwischen denverschiedenen Cneisarten bestehen.; then,

192 A. BORIANI, E. ORIGONl GIOBBI. A. DEL MORO

N

A

•·•... +... +Ir,-p,-

"

.. "o

1 2

o [°0":,,1345

DD~

Fig. 1. - Sketch map of the .. Serie dei Laghi. (Southern Alp$) wa;t and east of Lago M.ggioreand location of the sampla used for age detcrminatiorn. . l} Hornblende.bearing and hornblende-freeofthogneisses; 2) Augengneisses; 3) .. Sdsd dei Laghi,.; 4) .. Strona·Ccneri.; 51 Metabasites; m) Mig.m.tites; 6) Faults and overthruus. i = Insubric line; p = Pogallo line; c = Couato-Mergozzo.Brissago line.

introducing the «Orthogneistypus»: « ...keinegenetische Deulung verbunden scin ».

The Swiss Authors of that period lookedat the variety of metamorphic ..rock types asat a metasedimentary series that suffered ametamorphism with variably deep texturaltransformation and various degrees ofsoakage by feldspathizing palingenetic s0­

lutions. In REINHARD'S (1964) opinion, nolarge scale melting nor mobilization tookplace. In BACHLlN'S (1937) opinion theimportance of mobilization was greater: forexample he saw the explanation of the dif·ferent tectonic style of the «NordlicheInjectionszone » from that of the «Siidliche

Injectionszone » in an invasion of the formerby a true melt that increased its bulkmobility, whilst the latter was only permeatedby granitizing solutions. BoRIANI (1970)and BoRIANI et al. (1977) used the termoFthogneiss in the original Rosenbusch'smeaning, i.e. an igneous rock that was latersubjected to a regional metamorphism; theyrecognized intrusive bodies of granite-to­tonalite composition with an inferred Or­dovician age of intrusion, that later under­went regional metamorphism and becamegneisses. The old intrusive bodies, nowtransformed into elongated intercalationswithin the mctasedirnentary rocks, were

COMPOSITION, LEVEL OF INTRUSION AND AGE ETC. 193

described only in the «Scisti dei Laghi»of the area W of Lago Maggiore.

Field and petrological work has been car­ried out by E. ORIGONI GIOBBI since 1978on the zone E of the Lake in the areabetween Luino and Val Veddasca. Scopesof her investigation were:

a) to check the possibility of extending Eof Lago Maggiore the distinction, recognizedon the western shore, of the «Serie dei Laghi»into two subunites separated by an amphi­bolite horizon;

b) to investigate the relationshipsbetween onhogneisses and country rocks,

however, some 2 Km E of the coast, theamphibolites are abruptly cut off by theVal Dumentina Fault and E of that point,they no longer constitute the divider betweenthe two subunites.

The orthogneiss body of M. Gradiscaseems to pass gradually into the fine grainedparagneisses of Vat Veddasca (the Hornfels­gneisse or Biotithornfelsgneisse of the SwissAuthors) through an undefinable band ofbiotite-plagioclase gneiss.

The orthogneiss retains its sharp contactstowards the micaschists and paragneissesoccurring in Val Dumentina and belonging,

EtJoo

6 0 6 5 70

Fig. 2. - Alkali/silica diagram for the .. Serie dei Laghillo orthogneisses. Circles = hornblende-bearing;triangles = hornblende-free; open squares = augen-gneisses.

since they were described as mostly transi­tional by the Swiss Authors;

c) to improve the knowledge of the struC­tural setting of the zone in comparison withthe already studied onej

d) to acquire systematic petrochemical andpetrographic data on the orthogneisses.

Having achieved all these targets, thenew data compelled us to undertake a newsurvey on the previously investigatedwestern area and to request the collaborationof A. DEL MORO (Istituto di Geocronologiae Geochimica Isotopica del C.N.R. di Pisa).

As one can see from the map of fig. 1,on the eastern shore of the Lake the amphi­bolites are not present as an individualhorizon but are tectonically repeated twicein the sequence near S. Rocco di Campa­gnano. Nevertheless they retain their roleof divider between the rocks belonging tothe «Scisti dei Laghi» and «Strona-Ceneri»j

beyond any doubt, to the «Scisti dei Laghi»_Our idea that «Scisti dei Laghi» and

« Strona-Ceneri» were separated all alongtheir Contact by metabasites was disproved_We had to concede that the Ordovicianintrosives also penetrated the fine-grainedgneisses of « Strona-Ceneri »_

The «Strona-Ceneri », described by Bo­RlANI (1970), BIGIQGGERO and BoRIANI(1975), BoRIANI et al. (1977), consists ofunquestionable metasediments {fine-grainedgneisses and «Cenerigneisses» l. and augen­gneisses of migmatitic appearance and ill­definable biotite-pIagioclase gneisses thatcould nor be attributed with care to anigneous or sedimentary domain on accountof their variable characters. The only rocktype of the «Strona-Ceneri» that clearlydisplays an igneous origin is represented bythe laminated pegmatite dykes within thefine-grained gneisses. We interpreted these

19' A. BORIANI. E. ORIGONI GlOBBI, A. DEL MORO

TABLE I

Chemical analyses and mesonorms (MIELKE & WINKLER, 1979) 0/ the hornblende-bearingorthogneisses 0/ Ib, « Serie dei Laghi »

• • • '" " " " "5102 66.31 69. \0 64.H 67.38 65.24 I>( .10 66.65 6~. 19 63.01 62.02 61.22 61.06 10.35 IO.M

TiO, ." ." ." 0; ." ." ." ." ." ." ." .. ." .'"~lZ03 16.29 14.84 \6.12 16.18 15.9/ 16.30 16.14 15.85 16.S/ 16.37 17.00 16.26 1S.l' 13.43

,eZO) " ." ." 1.54 1.25 ,.~ ... 1.41 1.61 3.51 2.26 1.64 ." .",~ 2.99 , .00 4.17 ,." 3.52 ,.~ ,." 3.14 Z.83 2.14 1 .75 ,. 1.89 2.10- "' .00 .'" .. .. ."' ."' ."' ." ."' .. .00 .00 .00

"~ 2.41 1.15 2.16 1.46 ,."' 2.15 2.34 2.16 2.15 ,.. 2.71 3.02 ." 1.87Co. 4.18 3.22 4.35 3.96 4.34 4.56 ,.~ 3.71 4.05 4.18 4.72 4.4S 2./1 3.31

NIZO 1.2, ,." 2.72 2.72 Z.63 2.65 3.69 2.84 ,." 2.19 Z.71 3.83 3.82 ,."~. L. 2.18 2.57 2.55 Z.5/ <.n 1.85 2.9/ Le9 2.01 L" ,.. 2.1S 2.43

PZOS .~ ."' ."' ."' "' ."' ."' ." ." ." " ."' .. ."",' ." 1.53 1. 16 ." 1.05 US 1.15 1.8/ 2.<3 2.11 ,." 2.02 1.21 L~

TOTI,I. 100.•2 99.42 99.39 100.11 99.29 99.80 99.\.0 99.81 ~9.02 ~9.02 99 .•9 99.89 100.44 99.15

" ."' ."' ."' ."' ."' ."' ."' ." ." " " ." ." ."" 1.11 ." U2 .00 1.83 .00 1. 22 2.05 .00 .00 .00 .00 1.01 ."~ .00 .00 .00 1. 54 .00 2.04 .00 .00 1.63 3.54 2.27 '" .00 .00

" ." ." ." ." .~ ." ." .. ... .~ " ... •• .".. 35.85 33.06 23.18 23.00 22.44 22.49 31.41 24.10 33.36 23.87 23.07 32.48 32.21 n.'l5., ." 6.4) 4.97 1.54 6.21 5.05 2.43 9.01 2.22 3.55 .00 5.24 11.88 12.30,. 2ll.18 16.00 21.58 19.48 21.48 22.53 18.15 17 .72 19.30 20.08 22.16 18.16 11. 2ll 8.56

" 14.87 10.56 )1.03 12 .•1 14.86 15.27 13.75 13.88 14.11 14.05 18.99 14.08 ,.~ 3.16.. ." ." ." ." ." ." ." ." ." .00 " 4.54 .." 11.20

0, 26.10 31.49 29.13 33.31 31. 11 JO.33 30.19 29.11 25.14 29.57 28.41 21,49 32.59 29.'10, .00 ." ... 1.78 L~ 1.05 1. 22 1. 47 '"' 2.26 2.31 .00 L" .00REST. ." 1.12 ." ." .. ." ." 1.32 1.61 2.17 1. 65 1.36 ." ."1) LM 80·2: Pian Nava-Esio (67709230); 2) LM 80-3: Pian Nava-Manegn (67709110); 3) LM 80-8: Pra­d~olo-M. Gndisca, q. 910 (83709912); 4) LM 80·10: Pradecolo-M. Gradisca, q. 1000 (84309900);5) LM 80-12: Runo-Curiglia rood (83009890); 6) LM 80-13: Runo-Curiglia road (83209978); 7) LM 80-15:M. $egletta road (70609506); 8) VLU 45: M. Gradisca (82889924); 9) PC43: M. Morissolo (71709650);to) VLU51: M. Colmegnino (84819740); 11) VLU46: Runo-Curiglia road (82909%2); 12) ELPC82:Pian Nava (from BoRIANI, 1968) (67749100); 13) ELPC 3: M. Morissolo (from BoRIANI, 1968) (71249604);14) PR 1: Luera (from BoRIANI, 1968) (71709650) [IGM ooordinales).

dykes as the outermost limbs of the Ordo­vician plutons that intruded the «Scisti deiLaghi» underlying the «Strona-Ceneri»rocks.

Having shown that the Ordovician in­uusive reached directly the «Strona-Ceneri»we had to explain the apparently transitionalcontact between orthogneisses and fine­grained gneisses.

Bii.CHLlN (1937) and REINHARD (1964)ascribed this circumstance to a metasomaticorigin of the orthogneissic looking rocks;their formation occurred through a modi­fication of their texture and compositionthat also involved, to some extent, theadjoining rocks. We now believe that their

'._.<

interpretation was essentially correct, butwith the difference that we envisage realintrusive bodies that changed the textureof the adjoining fine-grained gneisses in theircontact aureoles; we cannot disregard thepossibility that metasomatical exchangesalso occurred between the intrusive and thevery low grade country rocks.

The reexamination of the zone W of LagoMaggiore proved that it was possible torecognize the orthogneisses in the «Strona­Ceneri» too. For example along the Ca­dorna Road S of M. Bavatione one can seerepeated contacts between otthogneissesand paragneisses. The repetition is due tothe strong deformation of the original

COMPOSITION, LEVEL OF INTRUSION AND AGE ETC. 195

TABLE 2

Chemical analyses and mesonorms (MIELKE & WINKLER, 1979) of Ihe hornblende-freeorthogneisses of Ihe « Serie dei Laghi»

.. • , .. " " " " "~102 /4.~ /UlO /0.3/ 69.09 68./5 68.51 •• 67.97 67.81 6/./7 /0.39 /1.01 68.95 7(.64 67.73

110l .N ." ." .'" " " ." " ." ." .N " ." ." ."AltJ3 12.19 ll.18 15.21 H.(( 15. (2 15.20 1(.9/ 15.1/ 15.41 15.<1 1(.62 1(.82 15.10 \3.65 15.Je

r~103 " ." I. I2 2.52 " ... '.oo ,." ... 1.01 ." ." 1.26 ." 1.56,. 2.11 2.18 ,." 1.10 2.12 1.(( ..~ ." 1.5/ 1.89 1.63 3.39 1.89 ." ,.- ." .oo " • " ." ." ." ." .oo ." ." .. .oo .""" ." ." I. I5 ." ... 1.J5 1. 1/ 1.73 1.11 I .47 ." ." 1.10 " 1.47,.. ." ." 1.03 1.67 ... "0 1.78 1.77 1.11 ." ,.. 1.63 1.19 .N 1.7l..~ 'N 3.16 ,.. 1.75 3.84 3.67 3.03 3.31 3.l4 l.15 1.32 3.I( 3.35 ,." 3.48

',' 3.79 3.29 3.U 5.93 1.59 3./5 3.91 4.51 3.1l l.41 (.21 4.18 3,65 1.01 3.80

PlOI " ." ." ... " ." ." .ro ." ." ." ." ." .00 ."",' 1. 62 1.51 1.28 1. 42 ," LOO 1.47 •• 1. 6/ ,.. ." ." ." .. OO 1.10

TOTA, 100.75 99.71 99.90 99.71 ~." 100.39 ~.. 100.13 99.92 99.01 ~." 100.(3 100.05 99.18 100.35

.. " • " " ." .0 ." ." .. " ." ." ." ." ..• 1 .07 1.16 .00 .. ..00 .. .. .. .. .. 1.10 ." 1.83 " ..• .. .00 1. 1/ 2.13 .00 I. 35 2.05 '.N ,.. .. •• .00 .00 .00 us

" ." ." ." ." ." .N ." ." N " ... ... ." ." .N.. 19.42 27.69 1/.81 14.8/ 32.18 "." 25.85 2/.92 28.31 11.7~ 28.18 li.48 ••• 15.28 29.37

0, 18.33 16.01 15.40 l1.31 ..~ 11.49 17.81 lCI.92 15.70 13 .3/ 19.74 18.01 15.60 28.45 15.36

" ," 2.19 9.49 6.99 I.ll 5.11 1.97 7.(1 7.74 1.72 ... 1.11 10.40 2.41 1.39

" '.N '.00 8.11 6.ll 10.:10 10.85 8.9/ 'N 10.47 11.43 8.11 11 . 1/ 9.85 2.81 11.81., .00 .00 .. .00 .. .00 .00 •00 .00 .00 .00 .00 .. " ••O. Je.34 41.34 30.01 13.28 30.35 30.11 32.45 1/.95 30.81 34.91 l1.15 11.10 30.91 37.32 19.62, 1.37 loll 1.61 '.N •• ,." 1.85 1.07 '" 1.29 1.8l 2.17 1.81 1.•3 '.NREIT. 1.31 l.l9 " 1.17 ,.. 1.51 1.13 ." 1.26 ..~ ." ." " ." 1.23

1) VLU 32: Colmegna.Luino road, Km 30 (80679778);

2) VLU.6: Colmegna-Torretta (80609750); 3) VLU 3):

Pradecolo-M. Gradisca (g3359872); 4) VLU 2:

.• , .•1 .'$

1.lJ 1.7, US

.61 .W .as

11.5" n.80 '!Il,1O1nl 11.10 1.5"

6.'S 8.S'! PI14.S'! IJ.!oII 14.1.11.0! n.14 30.'14." '.ll l.4'1.'1 1.13 I.'S

".14 06.01 '1.$'.10 .61 .a'

1$.110 1$," II.S!

1.f' 1.!IlI 1.80

1.1' 1.46 I.'".04 .01 .01

1.:111 1.10 1.10

1.S< 1... 1.111

/.61 '.30 ,."'.67 '.61 '.M

.111 . I' .It

1.0' '.1'9 1.'1

n.... lOO." ".'8 ".111 100.10

S. ROCC(].Campagnano (80150022); 5) VLU 32: Col·

megna (81109705); 6) GLU 2: Colmegna (80849685);

7) VLU 17: Due Coslani (82839770); 8) ViU 34:

Pradecolo-M. Gradisca (83409875); 9) VLU 39:

Runo (83499672); 10) VLU 29: M. Gradisca

(83109900); 11) LM 80·4: Colmegna-Maccagno road

(80809720); 12) LM 80-5: N of Colmegna

(80759370); 13) LM 80-6: N of Colmegna

(80759730); 14) LM 80-14 Ronehi, near Trarego

(75009830); 15) VLU 30: «usa Venere. W of Mac­

eagno, q. 359 (80409885); 16) VLU 44: Alpone-­

M. Gradisca (84109930); 17) GLU 5: S. Rocco, Dear

Campagnano (80220042); 18) VLU 41: Campagnano

(80660022); 19) VLU 3: S. Rocoo, near Campa­

gnano (80200038); 20) ViU 20: VsI Casmera

(81920046); 21)- GLU4: Colmegna (80849685);

22) VLU 19: Garabiolo (81100000); 23) VLU 21:

Val Casmera (81900056).

"01.14

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Il.e,lO.la

I'.It11.11,..L]]

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11.5"

'.Ol11.41

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"o65.:111 14.51

." .11011.0ll lUO

1.11 Ul

'.M 1.1'

."" .H1." 1.>11. J(i 1.26'.26 U3'.ll US.Il .Il

1. to 1.10

.52 .It1.10 1.<0

.11 .1S26.01 11.J1

lO.as '.1<8.'S 14.lJ

14." 14.1730.1. lU'

3.al 1."

I." 1.11

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.. .1'lOO I.J!11 .nAt> 1'9.15Or 10.",

"" I'."" '.1000 30.1'( 1.'1

_[$'. .'1

5101 11.t'

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.It', 11.0/rot', 1.3'r"" /.11-."'" .",.0 '.11"t' ,...'10 1.1,"01 .Il

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,~... n.»

196 A. BORIANI, E. ORIOONI GIOBBI, A. DEL MORO

TABLE 3Ch~micQl ana/yus ana m~sonorms (MIELKE

& WINKLER, 1979) 01 the augen-gnehuJ01 the .. Strir dei Llghi ,.

SlOt IJ.H 1/.N 1< .,. 74." ... n.MIt°t ." .. ." ." ." ."Alf) 1'.0/ ll." u .• U.1I 14.1D U.tJ

''t', ." ." ." •• ." ..'" 1.16 M 1.1. "0 2.11 1.11~ .. ." .0 .0 .. .0.. .. .. .. .. ." ."'" ." ." .Y .Y ••• .0-, ,.. ,.y t." '.0 "0 J.Ol

't' .." t.U ••• 4." '." .."'A .. ." .. •0> .. ...,. .M .0 ••• .0 1.0/ .0TOUI. M." 100.00 ".0 ".12 M. M."

• • ." ." ." " .""' .Y ." ." .0 • ."" ." •• ." ." • ."- ".. rl." '1.13 ".H H." U~ n." llI.I4 .." •. n n .• H.'"- '.0 .H '.Y '.0 •.n '.0

" l." '.0 J.7' .." '.t, •••• .." 0.' 40.11 ••• 0." 0.", '.Y ,." ,." '.0 ,." 1.SI~.. M .n • .Y ••• .Y

I) LM 8().17: M. Vadl-Cadorna road (67J09920);2) LM 80-19: M. VadA-Cadorna road (672099221;3) LM80-20: VIII Pag.lIo (61009660); 4) LM-80-21:V.I Pogallo (60959662); 5) ill 81·2: Germ,goo .V.lle Strona di Omegn. C526(8220); 6) L087·7:Cireggio . Valle: Suona di Omegn. (526680401,

igneous contact. The grain size. especiallyof the micas, increases in the 6ne-grainedgrx=isses near the contact, in addition thefine-grained gneisses with white feldsparicbands proved to be a contact zone withhighly laminated concordant pegmatite dykes.

The biotite-plagioclase gneiss of theOspedaleuo locality (Cadorna Road) is abiotite-hornblende orthogneiss very similarin composition to that of Piancompra andPremeno; it is in direct contact with theamphibolites of M. Spalaverai the feldspa­thization of the metabasite horizon Sttmsto be due to metasomatic phenomena relatedto the intrusion of the Ordovician cgmnites».(Originally we attributed the feldspathizationto a migmatic episode that involved the« Cenerigneisses ,. (BoRlANI ... GIOBBt MAN­CINI, 1972)).

One of the most important evidences thatwere at the base of the hypothesis (BIGIOG-

GERO'" BoRIANI, 1975; BoRIANI et al., 1977)of the nature of Caledonian Suptastructureof the «Strona-Ceneri It, was the recogni­tion of the contact metamorphic effect ofthe pegmatite dykes on the fine-grainedgneisses in the zone of Cannobio. Thecontact metamorphism that produced chia­stolite porphyroblasts, now transformed bysuccessive regional metamorphism into Al­silicates nodules, confirmed the very lowmetamorphic grade of the fine.grainedgneisses at the moment of the intrusion ofthe Ordovician granites or, at least, of theirpegmatitic extensions.

The reexamination of the contacts betweenthe orthogneiss bodies and the paragneissesof «Scisti dei Laghi» did nOt reveal thepresence of inequivocable mineral or texturalrelics of contact metamorphism, but if weconsider the distribution of the AI-silicatespolymorphs in the whole area, we noticethat sillimanite seems to be present onlynear the orthogneisses or in the thin interca­lations of paragneisses within the orthogneissbodies (Rio Colmegnino, N of Dumenza).The sillimanite is present as 6brolite in­clusions in the muscovite; kyanite and aoda·lusite are also sometimes present in theserocks that probably coincide with the « Giu­mello-Gneis» of REINl-lARD (1964).

In the bed of the Torrente Giona near theoutlet of Val Veddasca (E of Maccagno)the orthogneiss body of Agta is interruptedby an intercalation of country rocks showinga very peculiar aspect. It can be defined asgneissic migmatites that consist of stronglyheterogeneous material with tightly foldedalternating leococralic and melanocraticbands and veins. Similar rocks can beobserved near Carmine di Cannobio, i.e.exactly on the opposite shore of the Lakein frORt of Maccagno (sample KAW 564HUNZIKER '" ZINGG, 1980).

Only very seldom is it possible to observegood exposures of unquestionable xenolithswithin the orthogneisses. Probably the bestone is thar of M. Piancompra where, in aroad cut (Aurano-Piancavallo road), one cansee many lense-shaped inclusions of para­gneiss a few dm to few m long; one of thesexenoliths shows a crenulation cleavage,which proves that the fragment of meta·sediment included by the «Ordovician,.

COMPOSITION, LEVEL 01' INTRUSION AND ACE ETC. 197

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u

"/'"

• '.· •S,O, · 0 0

. .0~.

0 j .0 0 .

0 0, •.. •• 0 0

• 0· 00

• · 0

• 0 .....-,...• .. ~ ~··t ..

.1,010

0 '0.

0 .. ., 0 •'.. . • • ..• 0, ••; ....to • . ·· • f'- 01'.. .. .. ••

o 0 •

~'IO 0 ...'0 .. ~

~.. ... .' ·• o. . '0o. •·

• 0

. • 0

·" ,

•

Fig. J. - unen/oxides diagrams for the .. Serie dc-:i Laghi,. orthogneisses. For Icgmd sce fig. 2.

intrusive was alr(:ady schist~.

Amphioolite xenoliths occur E of LagoMaggiore along the Runo-Pradecolo road(M. Gradiscll). Biotite or quartz concen­trations with ill-defined margins are verycommon all over the area of occurrence of.the~ orthogneisses.

The characters of the xenoliths are similar,if one does nOf consider the deformationand recrystallization due to the successivemetamorphism, to those displayed by thexenoliths of common unmetamorphosedgranites. These is another evidence for thereal igneous origin of these rocks.

In our preceding papers (BoRIANI, 1970;BoRIANI et alii, 1977) we divided the ortha­gneisses in two types:

a) hornblende-bearing gneisses (Preme­no and M. Morissolo);

b) granitic (biotite-) gneisses (Pian­compra).

This division was motivated by the es­sentially uniform composition of the indi·vidual horizons, in the investigated area.

E of the Lake, where the orthogndssesreach their maximum abundance, thl= com­position appears more heterogeneous withinthe individual bodies. A biotite-gneiss, onlylocally with augen texture (Rune, Colme­gna) prevails between the Lake and the ValDumentina fault. E of Val Dumentina theonhogneisses seem to be mostly biotiticwith patches of more malic, hornblende­bearing types.

The mineral assemblage, described indetail in the preceding papers, is: quartz,plagioclase (15·25 % An), K-feldspar, bio­tite, muscovite, epidote, allanite, apatite,zircon, opaques in the biotite gneisses. Inthe more malic types the hornblende can bevery abundant and the plagiodase becomesmore An·rich 05-40 %).

With the exception of the already men­tioned onhogneisses with augen texture,that can be considered varieties of the maintype, no true augen-gneisses, in the senseof BoRIANI (1970), BoIUANI et al. (1977),were found on the eastern shore of LagoMaggiore S of vat Veddasca.

198 A. BORIANI, E. ORlGONI GIOBBl, A. DEL MORO

C Rb1000

,,,'..B

• ,

00

••••,••

G' C Rb100 1000

100

fA

: .'... \. ,• C· '.

...;.:.

". ;,.~,,,,

10

Ba

100

1000

D100

10

Fig. 5. - TIle Ba, Sr and Rb reLnionships in theIhree types of the • Se:rie dei Laghi,. onhogneisse5.The curves (McCAl.TlIY & ROBB, 1978) representthe composition of the solid phase (dashed lines)and of the melt (solid lines) during the crystalli­zalion of a liquid of composition A lying on theplagiodase·quulz cottctic surface of the Q.Or.Ab-Antetrahedron.

.... ..'"•

• •• •

4 N·20

•

3

••

.,•

K20

0

5 ~0

000 .. ......4

"t.... .. .... A ..

3

2

Fig. 4. - The Na.O/KsO relstionships in thehornblende·bearing (cirde5). hornblende-free (trian­gles) and lugen.peisscs (open squIres) of lhe: • Se­lie dei Laghi ».

In the light of the new evidences on thepresence ,of true metaplUlonites in the« Strona·Ceneri _, we also reconsidered thenature and origin of the augengneisses. Forthis purpose we carried out Rh-Sf isotopicdeterminations as well as chemical analyseson a few samples of augengneisses fromM. VadA, Val Pogallo and Valle Suona diOmegna.

Petrochemistry

43 major element analyses, 27 of whichincluded Rb, Sr. Ba, were performed onsamples collected on both sides of LagaMaggiore (Tables I, 2. ).

In the alkali-silica diagram (fig. 2) thesamples plot almost entirely betw~n thetwo curves of Kuno that bound the fractio­nation products of the high-alumina basalts(the use of this diagram is only for reference

purpose a"nd does not imply a genetic inter­pretation),

On the diagram the amphibole·bearingand the amphibole·free suites trace twO fairlywell distinguishable trends with higher andlower alkali content respectively.

The two trends can be individualized inmost of the oxide/Larsen index diagrams(very clearly in the Ti02, Cao and SiO:!:diagrams, _6g. 3).

COMPOSITION, LEVEL OF INTRUSION AND AGE ETC. 199

-­w

and Sr deter­(apostrophe)

-,. .~ .~ ~~

_1__dOl ..u.,.to....

~~, ". - .n·~~. o' 0' .,.~~ •• m m'~~. •• ~ I'"~

l.II ..... lr •• ~,•.

Ul ..... u •• •• ,..UlIlll_U ... •• .,.n, • ~ m

"' n .., *•• 0 •• .,=.. 0 ... mI'I.U ~6 " '" '"." " '"

..,HO.-nbll""" fr.. orttlo9"'h•••

l.II 8O-~ I~" ., lit'

~~o,•. on 11"

~~. Ill' oo lU'flU 11 ,. 1011 '"=" ,. - ,.=. ,. ., .,~. '" '" •Ul llO-l~ m' m ••

""- _to....

Ul .....11 lIS' ~ ".Ul .,..1. "" " ".~~. ",. m "'Ul ....."

~.

'" ••~all_r ,•. 0.' ,,,.La 11·' lU" m w·

TABLE 4

Ba determined by A.A., Rbmined by isotope dilution

and by XRF

...

w-.... ...... ...-", ..............a......

no''.

~11Oft tJA cOleC11C ....ond tJA isothe,ms ""COlIICIIC _Iocu.

-------... ... ....~ If , If_" ..........., . ........ ....._ _ ..-- ...............Fig. 6. - Winkkr's diagrams for 13 representltivesamples of the ,. Scrie dei Laghi ». For imcrpre­ration see tCl't.

In the K~O/Na~O diagram (fig. 4) thethree groups (hornblende-bearing, horn­blende-free and augen-gneissesl are welldefined by their different alkali ratio; thehornblende-bearing gneisses can be split intwO groups at lower and higher Na~O

content. One can see a major rectilineartrend from high Na low K to high K lowNa members that corresponds to a magmaticdifferentiation with different degr«s ofmixing of cumull\te and melt (MCCARTHY &

ROBB, 1978); the minor trend, resultingfrom the alignement of the points repre­senting the low Na group of the hornblende­bearing gne.isses, can be interpreted as dueto concentrations of cumulate homblerKk.

For sample IOClllion see legcnd of rabIes 1·2·.l.

This is probably the explanation for thepresence of the two apparently differenttrends in the above illustrated diagrams.

The Rb/Sr and the Rb/Ba diagrams (fig.5) give the same indications. For comparisonpurposes we have traced the lines, calculatedby MCCARTHY & ROBB (1978), representingthe effect of fractional crystallization of agranitic melt on Ba, Sr and Rb concentrationsin solid and liquid. One can see that mostof the points lie near the line AB, Le, inthe -fractionation interval corresponding tothe crystalliza"tion on the plagioclase-quanzcotectic surface. One of the homblende·£r~

and all the augen gneisses lie in the linerepresenting the liquid during the cotecticcrystallization of plagioclase, quartz, K-feld­'p"'.

200 A. BORIANl, E. ORIGONI GIOBBI, A. DEL MORO

TABLE .5Whole rtXk isotopic ralios for the

«Se,;e dei 1.ogh;.

..... 17lb/'&.s. I1S.'''5,.=.I •

~~, 0." O.Ton:.J~~, D.n a.TOU:.lUI fO-' 1.5. o.nro:.z~~. J.19 o.nu:,lUl 10-. l.IZ a.un:.?~~. I.J' o.n.o.=..Ul to-IO 1.19 0.7177.:.2UlIO-U I.n 0.1110.=.:Ul IO-U I.JZ o.nn:.:Ul '0-10 1.1\ o.nu.:.:Ul 10-IS 0." 0.7011:.:III 10-17 I.n 0.7611.:.2UI 10-19 n.'~ \.OH1.:.2L.Il 10-10 t.t. o.nu:.zUI 10-/1 6.01 0.1490.:.:LO U-l I.U O. ll15.:.31.0 11-' 9,11 0.110t:.:

For sample location see legend of tables 1·2·3.

TABLE 6Data for mineral separates from the

«Se,;e dei Lagh;,. orthogneiJseJ

.., _.• ... .., - 1._,":'" ..,..- olod.. - ~ m ......~ "':'

.. 11>"_.

~ ..• m l.'loOl:.lo4 "'"........ _. - .., .- ,.~ ..1:.1.... olot'" .~ •• - ....:.,.. ..,.1.0 .1·' ....,.. - ~ •• I,UlI:.J' ..,.

For sample 10000fion see legend of lables 1- 2·3.

• Age value of the tWl)-points isochron (mica +whole·rock).

In fig. 6 a few representative analyses(mesonorms according to MIELKE & WIN­KLER, 1979) are ploued in Winkler's dia­grams; it can be seen that only the plonedhornblende-free orthogneisses fall near theplagioclase-quartz cotectic surfaces, whilstthe hornblende-bearing gneisses fall wellwithin the plagioclase volume and theaugengneisses in the quartz volume.

But, for the preceding considerations, itis very difficult to attribute a real liquiduscomposition to our rocks.

Geochronology: methods and results

The radiometric studv \vas carried out on«total rocks.. as well'as on separated mi·nerals by means of the Rb/Sr method.

Rb and Sr con~ntrations were measuredby isotope dilution with 81Rb (98 %) and~Sr (99,9 %) «spikes ... Sr isotope compo­sitions we~ determinated on fractions of thesolutions containing the Sr «spike... Allthe analyses were performed with a VarianMat TH5 mass spectrometer. The leastsquare method was used to calculate agesand relative errors according to YORK (1966)with a 87Rb decay constant of 1.42·1O-1l /y.

Age 0/ intrusion

The analytical results on total rocks arelisted in Tab. 5.

We must notice that the hornblende­bearing and the hornblende-free orthogneis­ses generally show a Rb/Sr ratio lower thanthat of the augengneisses, on account oftheir minor concentration of Rb and a highercontent of Sr. However this difference is notreRected in the distinction of two rockgroups with different isotopic characlers .All of them de6ne an isochron (6g. 7) of466 ± 5 m.y. and a value of initial isotopicSr composition of 0.7087 ± 0.0002. Thesedata were obtained on 13 samples (fullcircles) of hornblende·bearing, hornblende­free and augen gneisses with the exclusionof the samples (open circles) with clearlydifferent chemical and isotopic characters.Samples LM80-2, LM80-J and LM80-15 fallwell below the isochron (hornblende-bearinggneisses) and show low Rb/Sr ratio. If theybehaved like closed system since their origin,their isotopic Sr ratios were 0.7040, 0.7044and 0.7054 respectively at the moment oftheir solidification. These values, definitelynot typical of crustal magmas, indicate thatthe origin of the «granites It of « Serie deiLaghi.. cannot be confined to the highestlevels of the lithosphere. Also the augengneiss LM80·19 was excluded from thecalculation of the isochron, since its Sfisotopic composition is quite different fromthat of the other samples; its use wouldyield an age value of 469 m.y.

COMPOSITION, LEVEL OF INTRUSION AND AGE ETC. 201

However it is possible that even othersamples used for the isochron calculationwere originally characterized by a value ofSr isotopic composition slightly differentfrom 0.7087. This possibility is suggestedby the small dispersion of some pointsaround the isochron line. Slight modifica-

0775

( 1969) on zircons from a «Strona·Ceneri»orthogneisses (400'.450 m.y.). KOPPEL &

GRUNENFELDER (1971) determined an ageof 430-500 m.y. on zircons from the «Strona­Ceneri» rocks and placed at this time the«anatectic granitization» of this zone.

It is important to emphasize the difference. _

, "

0.750

'"0.725

1.10

MO

'" ,

""lM13

~"'"", ,

T=466!5m.a. (16'-)(81Sfr~Sf)i=O.7087!O.0002 (111"")

0.700+----__,-----~----~----__,-::__;::,._-.,...Jo 2.5 5 7.5 10 81Rb/8~Sf 12,5

Fig. 7. - Rb/Se whole.rock isochron for the '" Serie dei Laghi,. orthogneisses. Sample location inthe legend of table 1- 2 - 3.

tions to our calculated age could result fromthe analyses of other surely comagmaticrocks. This would presumably lead us tothe individuation of a system of parallelisochrons with slightly different initial iso­topic Sr ratios, but for the moment our dataindicate unquestionably that the orthogneis­ses of «Serie dei Laghi» belong to thelate-Ordovician magmatic cycle.

Our results are similar, within the limitsof analytical uncertainity, to those obtainedwith the same method on other plutonicbodies of the alpine region connected to the«Caledonian» magmatism, i.e. on the Anter­selva and Casies orthogneisses (BoRSI et aI.,1973) and on those of the Oetztal region(SATIR, 1975; BORSI et al., 1980).

Similar ages were obtained also by meansof the V-Pb method by PIPGEON et al.

between the age obtained for the « Strona.Ceneri» orthogneisses and that recentlydetermined by HUNZIKER & ZINGG (1980)for the peak of the regional metamorphism(478 ± 20 m.y.) in the adjoining «Ivrea­Verbano» Zone. If metamorphism wascoeval to the magma formation and if weadmit that «Serie dei Laghi» and «Ivrea­Verbano» belonged to the same part of thebasement (as also suggested by the incompletedata of HAMET & ALBAREDE (1973) recalcu­lated by HUNZIKER & ZINGG (op. cit.»the difference of 10-15 m.y. could bereal and correspond to the time employedby the magmas to rise from the source zoneup to the level of their emplacement asplutonic bodies.

It is interesting to compare the analyticaldata obtained on the «Serie dei Laghi»

202 A. BORIANt, E. ORlGONI GIOBBI, A. DEL MORO

Rb ppm

>SO

'" o... -if"-

'.p...-----~~-----__,_------_r_- - - - - --,-o 2 3 Rb/S' 16

'00

•

'"

'00

>0,

'"

\~\ '., ,

\ .\\ \

.,~, ,' ......' ....

1'---...0.... -00-...... ---- ---..--- --0-_ --- -~-..,p...-

'~,-------~-------~,--------'~---=..~I;:sl- - - r6

Fig. 8. - Rb and Sr concentrations versU5 Rb/Sr ratio of rh!:: .. Sc:ric dei Laghi .. orthognei$$CS (opencirdet) and of the pangneissn of the .. lvre• .verbano .. zone (full squa~).

COMPOSITION, LEVEL OF INTRUSION AND AGE ETC. 203

Fig. 9. - Evolutionary scheme of the ... Serie deiLaghi "".1) Lowl!f Ordovicion. From the bottom upwardsthe sequence is; S.L. = metapelites of the «Scistidei Laghi,," (probably low grade}; 0 = basic extnJ­sives; S,c. = psammitic rocks of the .Strona-Ce.neri '" (probably very low metamorphic grade).2) Late Ordovician. Diapiric intnJsion of semi·molten magma of granodioridc composition entrai·ning migmatite blocks (m). Fiher·pre5S mechanismsduring emplacement generate differentiation. Theintrusive bodies reach also the S.C., but this unilis mostly affened by extremely ftactionated liquidsthat generate pegmatites (p}, porphyritic granitesand, to some extent, feldspathization of the countryrocks. Comact aureoles are developped around plu­tonic bodies and pegmatite dykes.

onhogneisses with those obtained by HUN­ZIKER &. ZINGC (1981) on the metapelites(<< kinzigites» and «stronalites») of the«Ivrea-Verbano» zone; the twO rock suites,though genetically different, show essential­ly the same initial Sr isotopic composition,but different corresponding amounts of Rband Sr. The onhogneisses of « Serie dei La­ghi » (open circles) show, at the same Rb/Srratio, a higher concentration of both Rh andSr than the paragneisses of the ~ Ivrea­Verbano» zone (full squares), fig. 8. Thesedifferences could be due to an originally dif­ferent chemical composition between differentcrustal levels or could represent the conse­quence of a «degranitization» process(SCHMtD, 1978-79; HUNZIKER &. ZINGC,1980) that caused a depletion of Rb and Sr.

Metamorphic evolutionThe « Caledonian granites» of « Serie dei

Laghi» were metamorphosed during thehercynian cycle; the stratigraphical relationsindicate that in the area under investigation,it occurred before Westfalian B, becausethis is the age of the base of the post­metamorphic sedimentary sequence. Manyradiometric data on the hercynian meta­morphism are available for this sector of theSouthern Alps. GRAESER &. HUNZIKER (1968)determined an age of 329 ± 19 m.y.(recalculated with)" = 1.42. 1O-1l y-t), forthe peak of the hercynian metamorphismwith the Rb/Sr method applied to separatedslabs of a' single «stronalite» sample.Mc DoWELL (1970), on the basis of K-Ardeterminations on micas and hornblendeconcentrates, concluded that in die « Suona­Ceneri » the metamorphism occurred around325 m.y. ago. KOPPEL (1974), determinedthe minimum age of the metamorphism of« Ivrea-Verbano» and «Strona-Ceneri»zones around 295 m.y. with the V.Pb me­thod on monazites and zircons. Furthermore,on the basis of other K-Ar and Rh-Sr dataon mineral concentrates from the «Massic­cio dei Laghi» region (JAEGER .et al., 1967;MAC DoWELL &. SCHMIO, 1968; GRAESER

3) Westphalian. Hercynian folding of both supra·and infrastructure; sraurolite grade metamorphismand generation of the present schistosity in theorthogneisses. Comae! aureoles are strongly deformedand overprinted by regional metamorphism.

204 A. BORIAN1, E. ORIGONI GIOBBI, A. DEL MORO

& HUNZIKER, 1968; Mc DoWELL, 1970;HUNZIKER, 1974), it was ascertained a syste­matic variation of the cooling age values thatdecrease gradually in NW direction iowardsthe «Ivrea-Verbano» zone. From valuesaround 325 m.y. E of Lago Maggiore, theK-Ar and Rh-Sr mineral ages concentratetowards 170 m.y. in the NW part of «Seriedei Laghi» and in the adjoining «Ivrca­Verbano ».

This cooling age pattern was attributedto a posr-hercynian thermal evolution thatinvolved this part of South-Alpine basementin a different way from place to place: theNW part of the « Serie dei Laghi » towardsthe «Ivrea-Verbano », underwent a moreprolonged cooling period in consequence ofa slower combined process of post+(Jtogenicuplift and erosion. Alternatively all the datacould be interpreted as due strictly to mag­matic cooling of the basic-ultrabasic Ivreacomplex.

Our results fit well in the already describedgeochronological frame. The age of themuscovite concentrate of sample LM80-14is very close to the inferred age of the her­cynian metamorphic peak. The biotitesbecome younger, going from the easternshore of Lago Maggiore towards the « Ivrea­Verbano» in an even more marked waysince .they range from 315 m.y. of sampleLM80-2 to 234·244 m.y. of two samplescollected near the « Strona·Ceneri j).« Ivrea­Verbano j) boundary (Table 6).

If we assume the values suggested byPURDY & JAEGER (1976) for the temperatuteof the Rb-Sr system closure, we obtain fromthe age differences of the micas coexistingin sample LM80-19, a local cooling rate ofonly 4° e/m.y.

ConclU8ion8

Orthogneisses with a Rb/Sr age of in­trusion of 466 m.y. are one of the characte­ristic rock types of both « Scisti dei Laghi »and « Strona-Ceneri ». 8'$r/8oJ$r initial ratio(0.7087) favours a crustal origin of the in­trusives that can be considered cogeneticthough not strictly comagmatic; individuallower initial ratios reflect a deeper origin ofpart of the magma.

The chemical characteristics are compa­tible with magmatic fraetionation mostlywithout a real separation of cumulate andmelt, such that each rock type results frommixing of different amounts of both ma­terials, giving a hulk composition rangingfrom tonalite to granite. Only the augen­gneisses show a marked character of re­sidual melt.

In spite of the presence of superimposedregional metamorphism and deformation,one can recognize the igneous nature of theoriginal contacts with the country rocks.

The already proposed structural setting ofthe « Serie dei Laghi j) at the end of Ordo­vician (a weakly metamorphic « Strona-Ce­neri j) overlying a more deeply metamor­phosed « Scisti dei Laghi») is confirmed:the level of intrusion seems to be moreshallow in the « Strona-Ceneri j) than 10

«Scisti dei Laghi j).

The proposed evolutionary scheme issketched in 6g. 9.

Acknowledgments. _ The Authors are grateful toG. PEYRONEL PAGlIANI, A. GREGNANIN, R. SACCHIfor the fruitful discussion and to J.C. HUNZIKER,A. ZINGG, A. MOTTANA and F. AUMENTO forcritically reading the manuscript.

Lavora escguito nell'amblto delle ricerche delCentra di Studi sulla Straligrafia e Petrografia delleAlpi Centrali del CN.R.

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