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 AGE OF THE «SERIE DEI LAGHI» ORTHOGNEISSES (NORTHERN - , 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'unita Introduction a medio grado metamorfico del hasamemo Sudalpi. no, che puo essere divisa in due parti: .. Strona· The « Serie dei Laghi,. is a metamorphic Ceneri .. (per 10 piu metapsammiti) e • Scisti dei unit of the South-Alpine basement of the Laghi .. (per 10 piu metapeliti) separate in genere Central and Western Alps; the series da metabasiti. Emrambe comengono interc:alazioni consists of probably late Precambrian or di ortogneiss; questi derivano da imrusivi da Wn.l­ litici a gnnitici. Trcdid campioni di ortognciss early Palaeozoic sediments that were definiscooo un'isocrona di 466:t.' m.a. (rappono subjected to repeated deformational and inwale delle Sr = 0,7087). L'eti. del metamomsmo metamorphic actions and were the seat of ~ intomo a )25 m... (cd. Rb/Sr delle michc) e I'etl igneous activity at different times during del rafftcddamemo dccresa: \-erso I'. Ivrn·Verba· no.. J caratteri chimici indicano un. differcnzia· the Paleozoic era. Basically the series was zione IIUI3ffiItica con miscuglio variabile di cumulus not involved in the Alpine orogeny, whose e intcrc:umulus nci diversi stadi di fruienamento. very marginal effects can be ignored for the AI momento dcU'intrusione gli • Scisti dei LaghilO (sicuramente gi.li metamodici.) giacevano sotto la purpose of this paper; the authors intend debolmente metamorfica • Stro!Ul-Ccncri ,.. Que. this paper to be a contribution to the sr'uhima comicne gneiss occhiadini, ehe dCTivano understanding of the palaeozoic history of dai prodoni &ll'estremo frnionamenw del magma, this part of the continental crust. nonchC pegmatiti laminate, che alKOra consc:rvano, nelle rocce il'lC1lSS3nti, relini di un metamorfismo The area under consideration is that ill contano. extending E and \VI of Lago Maggiore where the .. Serie dei Laghi. is known also as A8STlt,4,CT. - The. Serie dei Laghi .. is a medium «Ceneri Zone» (REINHARD, 1953) o'r grade metamorphic unit of the Southern Alps that « Strona·Ceneri» (SCHM1D, 1967). can be split in two parts: .. Strona·Cenefi .. (mostly The «Serie dei Laghi,. was divided by metapsammites) and .. Scisti dei LaghilO (mostly mctapdites) generally separated by metabashes. BoRIANI (1970) in twO subunits: «Scisti Both comain ot!nognciss intercalations. Thesc derive dei Laghi» mostly composed by pelitic and from tonalite.to-granite intrusives. Thirteen samples semipelitic metasediments, «Strona·Ceneri,. define an isochron of 466 ±;; m.y. (initial Sr mostly composed by metapsammites. The ratio = 0.7087). The age of metamorphism is around 32' m.y. (Rb/Sr muscovite ages), with cooling afles two subunits, W of Lago Maggiore are sepa· decreasing towards the • Ivrea·Verbano... The rated by a continuous layer of metabasites. ehemical characters indicate a differentiation with The relatioDships between the various variable mixing of cumulus and intercumulus rock types of the .. Serie dei Laghi,. were in the \'lfiOl.ls fraetionation Stages. At the moment of their intrusion the «Scisti dei Laghi,., which long a matter of uncertainty so that, in his were already metamorphosed, underl.y • wcaldy paper of 1964 (a conclusive work of a metamorphosed «Strona-Ccneri,.. The latter 00tl­ period of field and peuographic studies that tains aUl:tn-gnei5SCS, that dc:ri.\"C from the extreme lasted from 1925) REINHARD wrote ..... of. fractiorUltion productS of thc: intrusions, and sttOO8ly laminated pegrnatite dykes ....·ith relics of fenbar keine scharfen Crenzen zwischen den conr:aet metamorphism in their C'OUfllry rocks. verschiedenen Cneisarten bestehen.; then, 192 A. BORIANI, E. ORIGONl GIOBBI. A. DEL MORO • N ·•...+...,+Ir p,-- A "

o .. "

1 2 345 o [°0":,,1 DD~

Fig. 1. - Sketch map of the .. Serie dei Laghi. (Southern Alp$) wa;t and east of Lago M.ggiore and location of the sampla used for age detcrminatiorn. . l} Hornblende.bearing and hornblende-free ofthogneisses; 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. line.

introducing the «Orthogneistypus»: « ...keine Injectionszone » in an invasion of the former genetische Deulung verbunden scin ». by a true melt that increased its bulk The Swiss Authors of that period looked mobility, whilst the latter was only permeated at the variety of metamorphic ..rock types as by granitizing solutions. BoRIANI (1970) at a metasedimentary series that suffered a and BoRIANI et al. (1977) used the term metamorphism with variably deep textural oFthogneiss in the original Rosenbusch's transformation and various degrees of meaning, i.e. an igneous rock that was later soakage by feldspathizing palingenetic s0­ subjected to a regional metamorphism; they lutions. In REINHARD'S (1964) opinion, no recognized intrusive bodies of granite-to­ large scale melting nor mobilization took tonalite composition with an inferred Or­ place. In BACHLlN'S (1937) opinion the dovician age of intrusion, that later under­ importance of mobilization was greater: for went regional metamorphism and became example he saw the explanation of the dif· gneisses. The old intrusive bodies, now ferent tectonic style of the «Nordliche transformed into elongated intercalations Injectionszone » from that of the «Siidliche within the mctasedirnentary rocks, were COMPOSITION, LEVEL OF INTRUSION AND AGE ETC. 193 described only in the «Scisti dei Laghi» however, some 2 Km E of the coast, the of the area W of Lago Maggiore. amphibolites are abruptly cut off by the Field and petrological work has been car­ Val Dumentina Fault and E of that point, ried out by E. ORIGONI GIOBBI since 1978 they no longer constitute the divider between on the zone E of the Lake in the area the two subunites. between and Val . Scopes The orthogneiss body of M. Gradisca of her investigation were: seems to pass gradually into the fine grained a) to check the possibility of extending E paragneisses of Vat Veddasca (the Hornfels­ of Lago Maggiore the distinction, recognized gneisse or Biotithornfelsgneisse of the Swiss on the western shore, of the «Serie dei Laghi» Authors) through an undefinable band of into two subunites separated by an amphi­ biotite-plagioclase gneiss. bolite horizon; The orthogneiss retains its sharp contacts b) to investigate the relationships towards the micaschists and paragneisses between onhogneisses and country rocks, occurring 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 transi­ beyond any doubt, to the «Scisti dei Laghi»_ tional by the Swiss Authors; Our idea that «Scisti dei Laghi» and c) to improve the knowledge of the struC­ « Strona-Ceneri» were separated all along tural setting of the zone in comparison with their Contact by metabasites was disproved_ the already studied onej We had to concede that the Ordovician d) to acquire systematic petrochemical and introsives also penetrated the fine-grained petrographic data on the orthogneisses. gneisses of « Strona-Ceneri »_ Having achieved all these targets, the The «Strona-Ceneri », described by Bo­ new data compelled us to undertake a new RlANI (1970), BIGIQGGERO and BoRIANI survey on the previously investigated (1975), BoRIANI et al. (1977), consists of western area and to request the collaboration unquestionable metasediments {fine-grained of A. DEL MORO (Istituto di Geocronologia gneisses and «Cenerigneisses»l. and augen­ e Geochimica Isotopica del C.N.R. di Pisa). gneisses of migmatitic appearance and ill­ As one can see from the map of fig. 1, definable biotite-pIagioclase gneisses that on the eastern shore of the Lake the amphi­ could nor be attributed with care to an bolites are not present as an individual igneous or sedimentary domain on account horizon but are tectonically repeated twice of their variable characters. The only rock in the sequence near S. Rocco di Campa­ type of the «Strona-Ceneri» that clearly gnano. Nevertheless they retain their role displays an igneous origin is represented by of divider between the rocks belonging to the laminated pegmatite dykes within the the «Scisti dei Laghi» and «Strona-Ceneri»j fine-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-bearing orthogneisses 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 1.87 "~ ,."' ,.. ." - 4.18 ,.~ 4.4S Co. 3.22 4.35 3.96 4.34 4.56 3.71 4.05 4.18 4.72 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 1.15 2.<3." 2.11." " 2.02 1.21 L~ ",' ." ." "' US 1.8/ ,." 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 " 4.54 11.20 .. ." ." ." ." ." ." ." ." ." .00 .." 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" .00 REST. 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: Pra­ d~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 Ordo­ interpretation was essentially correct, but vician plutons that intruded the «Scisti dei with the difference that we envisage real Laghi» underlying the «Strona-Ceneri» intrusive bodies that changed the texture rocks. of the adjoining fine-grained gneisses in their Having shown that the Ordovician in­ contact aureoles; we cannot disregard the uusive reached directly the «Strona-Ceneri» possibility that metasomatical exchanges we had to explain the apparently transitional also occurred between the intrusive and the contact between orthogneisses and fine­ very low grade country rocks. grained gneisses. The reexamination of the zone W of Lago Bii.CHLlN (1937) and REINHARD (1964) Maggiore proved that it was possible to ascribed this circumstance to a metasomatic recognize the orthogneisses in the «Strona­ origin of the orthogneissic looking rocks; Ceneri» too. For example along the Ca­ their formation occurred through a modi­ dorna Road S of M. Bavatione one can see fication of their texture and composition repeated contacts between otthogneisses that also involved, to some extent, the and paragneisses. The repetition is due to adjoining rocks. We now believe that their the 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-free orthogneisses 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" 110 .N •• .N l ." ." .'" ." ." ." ." ." ." 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 ','PlO .ro .00 I ." ." ... ." ." ." ." ." ." ." ." ",' 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 , 2.17 1.81 1.•3 1.37 loll 1.61 '.N •• ,." 1.85 1.07 1.29 1.8l '.N REIT. 1.31 l.l9 " 1.17 ,.. 1.51 1.13 ." '"1.26 .. ~ ." ." " ." 1.23

" " " " o " " " S. ROCC(].Campagnano (80150022); 5) VLU 32: Col· 5101 11.t' ".14 06.01 '1.$' 65.:111 14.51 10.4' 01.14 megna (81109705); 6) GLU 2: Colmegna (80849685); Tl01 .11 .10 .61 .a' ." .110 ." ." .It', 11.0/ 1$.110 1$," II.S! 11.0ll lUO 14.11 17.13 7) VLU 17: Due Coslani (82839770); 8) ViU 34: rot', 1.3' 1.f' 1.!IlI 1.80 1.11 Ul 1.'1 I.S1 r"" /.11 1.1' 1.46 I.'" '.M 1.1' ,.. l.ll Pradecolo-M. Gradisca (83409875); 9) VLU 39: .04 .01 .01 ."" .H ." ." -.." 1.:111 1.10 1.10 1." 1.>1 ,. 1." Runo (83499672); 10) VLU 29: M. Gradisca ,.0"'" '.11 1.S< 1... 1.111 1.J(i 1.26 1.0! ,. "t' ,... /.61 '.30 ,." '.26 U3 1.'1 '.'1 (83109900); 11) LM 80·4: Colmegna- road '10 1.1, '.67 '.61 '.M '.ll US US 3.!l "01 .Il .111 . I' .It .Il .Il .0 (80809720); 12) LM 80-5: N of Colmegna '10 1./S 1.0' '.1'9 1.'1 1.to 1.10 l."" (80759370); 13) LM 80-6: N of Colmegna ,~... n.» n.... lOO." ".'8 ".111 100.10 ".Il'. (80759730); 14) LM 80-14 Ronehi, near Trarego .. .1' .•, .•1 .'$ .52 .It ." lOO I.J! 1.lJ 1.7, US 1.10 1.<0 '." ,.." (75009830); 15) VLU 30: «usa Venere. W of Mac­ 11 .n .61 .W .as .11 .1S At> 1'9.15 11.5" n.80 '!Il,1O " " 26.01 11.J1 11.lf •• eagno, q. 359 (80409885); 16) VLU 44: Alpone-­ Or 10.", 1nl 11.10 1.5" lO.as '.1< 11.5" Il.e, "" I'." 6.'S 8.S'! PI 8.'S 14.lJ '.Ol lO.la M. Gradisca (84109930); 17) GLU 5: S. Rocco, Dear " '.10 14.S'! IJ.!oII 14.1. 14." 14.17 11.41 I'.It 00 30.1' 11.0! n.14 30.'1 30.1. lU' ••• 11.11 Campagnano (80220042); 18) VLU 41: Campagnano ( 1.'1 4." '.ll l.4' 3.al 1." ,. ,.. _[$'. .'1 1.'1 1.13 I.'S I." 1.11 T.., L]] (80660022); 19) VLU 3: S. Rocoo, near Campa­ gnano (80200038); 20) ViU 20: VsI Casmera 1) VLU 32: Colmegna.Luino road, Km 30 (80679778); (81920046); 21)- GLU4: Colmegna (80849685); 2) VLU.6: Colmegna-Torretta (80609750); 3) VLU 3): 22) VLU 19: Garabiolo (81100000); 23) VLU 21: Pradecolo-M. Gradisca (g3359872); 4) VLU 2: Val Casmera (81900056). 196 A. BORIANI, E. ORIOONI GIOBBI, A. DEL MORO

TABLE 3 GERO'" BoRIANI, 1975; BoRIANI et al., 1977) Ch~micQl ana/yus ana m~sonorms (MIELKE of the nature of Caledonian Suptastructure & WINKLER, 1979) 01 the augen-gnehuJ of the «Strona-Ceneri It, was the recogni­ 01 the .. Strir dei Llghi ,. tion of the contact metamorphic effect of the pegmatite dykes on the fine-grained gneisses in the zone of . The contact metamorphism that produced chia­ SlOt IJ.H 1/.N 1< .,. 74." n.M It°t ." .. ." ." ...." ." stolite porphyroblasts, now transformed by Alf) 1'.0/ ll." u .• U.1I 14.1D U.tJ successive regional metamorphism into Al­ ''t', ." 1.16 ." ." •• ." .. silicates nodules, confirmed the very low M 1.1. "0 2.11 1.11 ~'" .0 metamorphic grade of the fine.grained .. ." .0 .. .0 ...... " ." gneisses at the moment of the intrusion of .Y .Y .0 ,." ,.y." ••• the Ordovician granites or, at least, of their -,'" .. t." '.0 "0 J.Ol 't' .." t.U ••• 4." '." .." pegmatitic extensions. 'A .. ." .. •0> .. . The reexamination of the contacts between .M .0 . .,. .0 ••• .0 1.0/ the orthogneiss bodies and the paragneisses TOUI. M." 100.00 ".0 M. M." ".12 of «Scisti dei Laghi» did nOt reveal the • • ." ." ." ." presence of inequivocable mineral or textural .Y ." ." .0 " ." relics of contact metamorphism, but if we "' ." •• ." ." • ." " " rl." '1.13 ".H H."• U consider the distribution of the AI-silicates ~ .. - n." llI.I4 " •.n n .• H.'" polymorphs in the whole area, we notice '.0 .H .. '.Y '.0 •.n '.0 that sillimanite seems to be present only l." '.0 J.7' - .." '.t, ••• near the orthogneisses or in the thin interca­ ,•" .." 0.' 40.11 ••• 0." 0." '.Y ,." ,." '.0 1.SI lations of paragneisses within the orthogneiss ~ ,." .. M .n • .Y ••• .Y bodies (Rio Colmegnino, N of ). The sillimanite is present as 6brolite in­ I) LM 8().17: M. Vadl-Cadorna road (67J09920); clusions in the muscovite; kyanite and aoda· 2) LM 80-19: M. VadA-Cadorna road (672099221; lusite are also sometimes present in these 3) LM80-20: VIII Pag.lIo (61009660); 4) LM-80-21: rocks that probably coincide with the « Giu­ V.I Pogallo (60959662); 5) ill81·2: Germ,goo . mello-Gneis» of REINl-lARD (1964). V.lle Strona di Omegn. C526(8220); 6) L087·7: Cireggio . Valle: Suona di Omegn. (526680401, In the bed of the Torrente Giona near the outlet of Val Veddasca (E of Maccagno) igneous contact. The grain size. especially the orthogneiss body of Agta is interrupted of the micas, increases in the 6ne-grained by an intercalation of country rocks showing grx=isses near the contact, in addition the a very peculiar aspect. It can be defined as fine-grained gneisses with white feldsparic gneissic migmatites that consist of strongly bands proved to be a contact zone with heterogeneous material with tightly folded highly laminated concordant pegmatite dykes. alternating leococralic and melanocratic The biotite-plagioclase gneiss of the bands and veins. Similar rocks can be Ospedaleuo locality (Cadorna Road) is a observed near Carmine di Cannobio, i.e. biotite-hornblende orthogneiss very similar exactly on the opposite shore of the Lake in composition to that of Piancompra and in frORt of Maccagno (sample KAW 564 Premeno; it is in direct contact with the HUNZIKER '" ZINGG, 1980). amphibolites of M. Spalaverai the feldspa­ Only very seldom is it possible to observe thization of the metabasite horizon Sttms good exposures of unquestionable xenoliths to be due to metasomatic phenomena related within the orthogneisses. Probably the best to the intrusion of the Ordovician cgmnites». one is thar of M. Piancompra where, in a (Originally we attributed the feldspathization road cut (Aurano-Piancavallo road), one can to a migmatic episode that involved the see many lense-shaped inclusions of para­ « Cenerigneisses ,. (BoRlANI ... GIOBBt MAN­ gneiss a few dm to few m long; one of these CINI, 1972)). xenoliths shows a crenulation cleavage, One of the most important evidences that which proves that the fragment of meta· were at the base of the hypothesis (BIGIOG- sediment included by the «Ordovician,. COMPOSITION, LEVEL 01' INTRUSION AND ACE ETC. 197

• • • '. .. I •• ". S,O, · 0 0 : ~;. · ...... 0 ...• o . ~.. 0 • o 0 j . 0 0 . , ..0 0 • • 0 0• • 0 0 0 · ·.• t· .~. • • 0 .. . 0 -,... t.o ..... ~.;.~ • · ..... ~ ~··t .. .. •... eo. o • .. 0 '0. 0 .1,01 0 .. . u .. , 0 • . • 0 ... ' . ..0 , .. ••; ....to • • • • · ': ...... , f'- 01'...... •• .· o • • o 0 • • .o. · o ~'IO o 0 . ~ .. '0 ~ .. .. '0 • o...... ' · o. • . 0 .. ...J.....",. · • 0 • ...... " ....,.r.... o , · ~ .. " oo. " " /' " " - " " .. • Fig. J. - unen/oxides diagrams for the .. Serie dc-:i Laghi,. orthogneisses. For Icgmd sce fig. 2. intrusive was alr(:ady schist~. E of the Lake, where the orthogndsses Amphioolite xenoliths occur E of Lago reach their maximum abundance, thl= com­ Maggiore along the Runo-Pradecolo road position appears more heterogeneous within (M. Gradiscll). Biotite or quartz concen­ the individual bodies. A biotite-gneiss, only trations with ill-defined margins are very locally with augen texture (Rune, Colme­ common all over the area of occurrence of. gna) prevails between the Lake and the Val the~ orthogneisses. Dumentina fault. E of Val Dumentina the The characters of the xenoliths are similar, onhogneisses seem to be mostly biotitic if one does nOf consider the deformation with patches of more malic, hornblende­ and recrystallization due to the successive bearing types. metamorphism, to those displayed by the The mineral assemblage, described in xenoliths of common unmetamorphosed detail in the preceding papers, is: quartz, granites. These is another evidence for the plagioclase (15·25 % An), K-feldspar, bio­ real igneous origin of these rocks. tite, muscovite, epidote, allanite, apatite, In our preceding papers (BoRIANI, 1970; zircon, opaques in the biotite gneisses. In BoRIANI et alii, 1977) we divided the ortha­ the more malic types the hornblende can be gneisses in two types: very abundant and the plagiodase becomes more An·rich 05-40 %). a) hornblende-bearing gneisses (Preme­ With the exception of the already men­ no and M. Morissolo); tioned onhogneisses with augen texture, b) granitic (biotite-) gneisses (Pian­ that can be considered varieties of the main compra). type, no true augen-gneisses, in the sense This division was motivated by the es­ of BoRIANI (1970), BoIUANI et al. (1977), sentially uniform composition of the indi· were found on the eastern shore of Lago vidual horizons, in the investigated area. Maggiore S of vat Veddasca. 198 A. BORIANI, E. ORlGONI GIOBBl, A. DEL MORO

K 0 2 f A 0 : .' 5 ~ ... \ 0 . , 000 .. • C· '. ...;.:...... 100 4 ";,.~ "t . , ...... ,, .. A .. , 3 • .. .. C Rb ., .. 10 100 1000 • '" 2 • • Ba ,, • • • , • 1000 '. . B • • ,

N· 0 3 4 2 0 0 • D • Fig. 4. - The Na.O/KsO relstionships in the • hornblende·bearing (cirde5). hornblende-free (trian­ 100 •, gles) and lugen.peisscs (open squIres) of lhe: • Se­ • lie dei Laghi ». • G' C Rb In the light of the new evidences on the 10 100 1000 presence ,of true metaplUlonites in the « Strona·Ceneri _, we also reconsidered the Fig. 5. - TIle Ba, Sr and Rb reLnionships in the nature and origin of the augengneisses. For Ihree types of the • Se:rie dei Laghi,. onhogneisse5. this purpose we carried out Rh-Sf isotopic The curves (McCAl.TlIY & ROBB, 1978) represent determinations as well as chemical analyses the composition of the solid phase (dashed lines) on a few samples of augengneisses from and of the melt (solid lines) during the crystalli­ zalion of a liquid of composition A lying on the M. VadA, Val Pogallo and Valle Suona di plagiodase·quulz cottctic surface of the Q.Or.Ab-An Omegna. tetrahedron.

Petrochemistry purpose a"nd does not imply a genetic inter­ 43 major element analyses, 27 of which pretation), included Rb, Sr. Ba, were performed on On the diagram the amphibole·bearing samples collected on both sides of Laga and the amphibole·free suites trace twO fairly Maggiore (Tables I, 2. ). well distinguishable trends with higher and In the alkali-silica diagram (fig. 2) the lower alkali content respectively. samples plot almost entirely betw~n the The two trends can be individualized in two curves of Kuno that bound the fractio­ most of the oxide/Larsen index diagrams nation products of the high-alumina basalts (very clearly in the Ti02, Cao and SiO:!: (the use of this diagram is only for reference diagrams, _6g. 3). COMPOSITION, LEVEL OF INTRUSION AND AGE ETC. 199

TABLE 4 Ba determined by A.A., Rb and Sr deter­

~11Oft tJA cOleC11C .... mined by isotope dilution (apostrophe) ond tJA isothe,ms "" and by XRF COlIICIIC _Iocu. -,. .~ .~ ~~ _1__dOl ..u.,.to.... ~~, ". .n· ~~. o' 0' .,. ~~ - •• m m' ~~. •• ~ I'"~ l.II ..... lr •• ~ ,•. Ul ..... u •• •• ,.. UlIlll_U ... .,. n, •• • ~ m "' n .., * •• 0 •• ., ... =.. 0 ... m no' I'I.U ~6 " '" '", '. ." " '" .. HO.-nbll""" fr.. orttlo9"'h••• l.II 8O-~ I~" ., lit' ~~o , •. on 11" ~~. Ill' oo lU' flU 11 ,. ,. 1011 '" =.=" ,. ., .,,. ~. - llO-l~ • Ul m''" '"m •• ""- _to.... Ul .....11 lIS' ~ ". Ul .,..1...... w w ". .~------If , If_" .... -",..... ~~. ",."" m" ...... , -­.. ~. "' ...... a...... -.. Ul ....." •• ...... _ _ ...... ~all_r ,•. 0.''" ,,,. --...... La 11·' lU" m w·

Fig. 6. - Winkkr's diagrams for 13 representltive For sample IOClllion see legcnd of rabIes 1·2·.l. samples of the ,. Scrie dei Laghi ». For imcrpre­ ration see tCl't. This is probably the explanation for the presence of the two apparently different In the K~O/Na~O diagram (fig. 4) the trends in the above illustrated diagrams. three groups (hornblende-bearing, horn­ The Rb/Sr and the Rb/Ba diagrams (fig. blende-free and augen-gneissesl are well 5) give the same indications. For comparison defined by their different alkali ratio; the purposes we have traced the lines, calculated hornblende-bearing gneisses can be split in by MCCARTHY & ROBB (1978), representing twO groups at lower and higher Na~O the effect of fractional crystallization of a content. One can see a major rectilinear granitic melt on Ba, Sr and Rb concentrations trend from high Na low K to high K low in solid and liquid. One can see that most Na members that corresponds to a magmatic of the points lie near the line AB, Le, in differentiation with different degr«s of the -fractionation interval corresponding to mixing of cumull\te and melt (MCCARTHY & the crystalliza"tion on the plagioclase-quanz ROBB, 1978); the minor trend, resulting cotectic surface. One of the homblende·£r~ from the alignement of the points repre­ and all the augen gneisses lie in the line senting the low Na group of the hornblende­ representing the liquid during the cotectic bearing gne.isses, can be interpreted as due crystallization of plagioclase, quartz, K-feld­ to concentrations of cumulate homblerKk. 'p"'. 200 A. BORIANl, E. ORIGONI GIOBBI, A. DEL MORO

TABLE .5 Geochronology: methods and results Whole rtXk isotopic ralios for the «Se,;e dei 1.ogh;. The radiometric studv \vas carried out on «total rocks.. as well'as on separated mi· nerals by means of the Rb/Sr method. 17lb/'&.s. I1S.'''5,.=.I • ..... Rb and Sr con~ntrations were measured ~~, 0." O.Ton:.J by isotope dilution with 81Rb (98 %) and ~~, D.n a.TOU:.l ~Sr (99,9 %) «spikes ... Sr isotope compo­ UI fO-' 1.5. o.nro:.z sitions we~ determinated on fractions of the ~~. J.19 o.nu:,l Ul 10-. l.IZ a.un:.? solutions containing the Sr «spike... All ~~. I.J' o.n.o.=.. the analyses were performed with a Varian Ul to-IO 1.19 0.7177.:.2 Mat TH5 mass spectrometer. The least UlIO-U I.n 0.1110.=.: Ul IO-U I.JZ o.nn:.: square method was used to calculate ages Ul '0-10 1.1\ o.nu.:.: and relative errors according to YORK (1966) Ul 10-IS 0." 0.7011:.: with a 87Rb decay constant of 1.42· III 10-17 I.n 0.7611.:.2 1O-1l /y. UI 10-19 n.'~ \.OH1.:.2 L.Il 10-10 t.t. o.nu:.z UI 10-/1 6.01 0.1490.:.: LO U-l I.U O. ll15.:.3 Age 0/ intrusion 1.0 11-' 9,11 0.110t:.: The analytical results on total rocks are For sample location see legend of tables 1·2·3. listed in Tab. 5. We must notice that the hornblende­ TABLE 6 bearing and the hornblende-free orthogneis­ Data for mineral separates from the ses generally show a Rb/Sr ratio lower than «Se,;e dei Lagh;,. orthogneiJseJ that of the augengneisses, on account of their minor concentration of Rb and a higher content of Sr. However this difference is not reRected in the distinction of two rock .., .., _.• ... 1._,":'" ..,. groups with different isotopic characlers. olod... ~ m- ...... ~ "':' All of them de6ne an isochron (6g. 7) of .-.. 11>" _-~ ..• m l.'loOl:.lo4 466 ± 5 m.y. and a value of initial isotopic ...... _. .., ,.~ "'"..1:.1 Sr composition of 0.7087 ± 0.0002. These .~ .- .... olot'" - •• ....:.,.. ..,. data were obtained on 13 samples (full 1.0 .1·' ....,.. ~ I,UlI:.J' ..,. - -•• circles) of hornblende·bearing, hornblende­ free and augen gneisses with the exclusion For sample 10000fion see legend of lables 1- 2·3. of the samples (open circles) with clearly different chemical and isotopic characters. Samples LM80-2, LM80-J and LM80-15 fall • Age value of the tWl)-points isochron (mica + well below the isochron (hornblende-bearing whole·rock). gneisses) and show low Rb/Sr ratio. If they behaved like closed system since their origin, In fig. 6 a few representative analyses their isotopic Sr ratios were 0.7040, 0.7044 (mesonorms according to MIELKE & WIN­ and 0.7054 respectively at the moment of KLER, 1979) are ploued in Winkler's dia­ their solidification. These values, definitely grams; it can be seen that only the ploned not typical of crustal magmas, indicate that hornblende-free orthogneisses fall near the the origin of the «granites It of « Serie dei plagioclase-quartz cotectic surfaces, whilst Laghi.. cannot be confined to the highest the hornblende-bearing gneisses fall well levels of the lithosphere. Also the augen within the plagioclase volume and the gneiss LM80·19 was excluded from the augengneisses in the quartz volume. calculation of the isochron, since its Sf But, for the preceding considerations, it isotopic composition is quite different from is very difficult to attribute a real liquidus that of the other samples; its use would composition to our rocks. yield an age value of 469 m.y. COMPOSITION, LEVEL OF INTRUSION AND AGE ETC. 201

However it is possible that even other (1969) on zircons from a «Strona·Ceneri» samples used for the isochron calculation orthogneisses (400'.450 m.y.). KOPPEL & were originally characterized by a value of GRUNENFELDER (1971) determined an age Sr isotopic composition slightly different of 430-500 m.y. on zircons from the «Strona­ from 0.7087. This possibility is suggested Ceneri» rocks and placed at this time the by the small dispersion of some points «anatectic granitization» of this zone. around the isochron line. Slight modifica- It is important to emphasize the difference. _

0775 , "

0.750 1.10 '" ,

MO

'" 0.725 T=466!5m.a. (16'-) lM13"" (81Sfr~Sf)i=O.7087!O.0002 (111"") "' ,~,"" 0.700+----__,-----~----~----__,-::__;::,._-.,...J o 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 in the legend of table 1- 2 - 3.

tions to our calculated age could result from between the age obtained for the « Strona. the analyses of other surely comagmatic Ceneri» orthogneisses and that recently rocks. This would presumably lead us to determined by HUNZIKER & ZINGG (1980) the individuation of a system of parallel for the peak of the regional metamorphism isochrons with slightly different initial iso­ (478 ± 20 m.y.) in the adjoining «Ivrea­ topic Sr ratios, but for the moment our data Verbano» Zone. If metamorphism was indicate unquestionably that the orthogneis­ coeval to the magma formation and if we ses of «Serie dei Laghi» belong to the admit that «Serie dei Laghi» and «Ivrea­ late-Ordovician magmatic cycle. Verbano» belonged to the same part of the Our results are similar, within the limits basement (as also suggested by the incomplete of analytical uncertainity, to those obtained data of HAMET & ALBAREDE (1973) recalcu­ with the same method on other plutonic lated by HUNZIKER & ZINGG (op. cit.» bodies of the alpine region connected to the the difference of 10-15 m.y. could be «Caledonian» magmatism, i.e. on the Anter­ real and correspond to the time employed selva and Casies orthogneisses (BoRSI et aI., by the magmas to rise from the source zone 1973) and on those of the Oetztal region up to the level of their emplacement as (SATIR, 1975; BORSI et al., 1980). plutonic bodies. Similar ages were obtained also by means It is interesting to compare the analytical of the V-Pb method by PIPGEON et al. data 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 (open cirdet) and of the pangneissn of the .. lvre•.verbano .. zone (full squa~). COMPOSITION, LEVEL OF INTRUSION AND AGE ETC. 203

onhogneisses with those obtained by HUN­ ZIKER &. ZINGC (1981) on the metapelites (<< kinzigites» and «stronalites») of the «Ivrea-Verbano» zone; the twO rock suites, though genetically different, show essential­ ly the same initial Sr isotopic composition, but different corresponding amounts of Rb and Sr. The onhogneisses of « Serie dei La­ ghi » (open circles) show, at the same Rb/Sr ratio, a higher concentration of both Rh and Sr than the paragneisses of the ~ Ivrea­ Verbano» zone (full squares), fig. 8. These differences could be due to an originally dif­ ferent chemical composition between different crustal levels or could represent the conse­ quence of a «degranitization» process (SCHMtD, 1978-79; HUNZIKER &. ZINGC, 1980) that caused a depletion of Rb and Sr.

Metamorphic evolution The « Caledonian granites» of « Serie dei Laghi» were metamorphosed during the hercynian cycle; the stratigraphical relations indicate that in the area under investigation, it occurred before Westfalian B, because this is the age of the base of the post­ metamorphic sedimentary sequence. Many radiometric data on the hercynian meta­ morphism are available for this sector of the Southern Alps. GRAESER &. HUNZIKER (1968) determined an age of 329 ± 19 m.y. (recalculated with)" = 1.42. 1O-1l y-t), for the peak of the hercynian metamorphism with the Rb/Sr method applied to separated slabs of a' single «stronalite» sample. Mc DoWELL (1970), on the basis of K-Ar determinations on micas and hornblende concentrates, concluded that in die « Suona­ Ceneri » the metamorphism occurred around Fig. 9. - Evolutionary scheme of the ... Serie dei 325 m.y. ago. KOPPEL (1974), determined Laghi "". the minimum age of the metamorphism of 1) Lowl!f Ordovicion. From the bottom upwards « Ivrea-Verbano» and «Strona-Ceneri» the sequence is; S.L. = metapelites of the «Scisti zones around 295 m.y. with the V.Pb me­ dei Laghi,," (probably low grade}; 0 = basic extnJ­ sives; S,c. = psammitic rocks of the .Strona-Ce. thod on monazites and zircons. Furthermore, neri '" (probably very low metamorphic grade). on the basis of other K-Ar and Rh-Sr data 2) Late Ordovician. Diapiric intnJsion of semi· on mineral concentrates from the «Massic­ molten magma of granodioridc composition entrai· cio dei Laghi» region (JAEGER .et al., 1967; ning migmatite blocks (m). Fiher·pre5S mechanisms during emplacement generate differentiation. The MAC DoWELL &. SCHMIO, 1968; GRAESER intrusive bodies reach also the S.C., but this unil is mostly affened by extremely ftactionated liquids 3) Westphalian. Hercynian folding of both supra· that generate pegmatites (p}, porphyritic granites and infrastructure; sraurolite grade metamorphism and, to some extent, feldspathization of the country and generation of the present schistosity in the rocks. Comact aureoles are developped around plu­ orthogneisses. Comae! aureoles are strongly deformed tonic bodies and pegmatite dykes. and overprinted by regional metamorphism. 204 A. BORIAN1, E. ORIGONI GIOBBI, A. DEL MORO

& HUNZIKER, 1968; Mc DoWELL, 1970; ConclU8ion8 HUNZIKER, 1974), it was ascertained a syste­ Orthogneisses with a Rb/Sr age of in­ matic variation of the cooling age values that trusion of 466 m.y. are one of the characte­ decrease gradually in NW direction iowards ristic rock types of both « Scisti dei Laghi » the «Ivrea-Verbano» zone. From values and « Strona-Ceneri ». 8'$r/8oJ$r initial ratio around 325 m.y. E of Lago Maggiore, the (0.7087) favours a crustal origin of the in­ K-Ar and Rh-Sr mineral ages concentrate trusives that can be considered cogenetic towards 170 m.y. in the NW part of «Serie though not strictly comagmatic; individual dei Laghi» and in the adjoining «Ivrca­ lower initial ratios reflect a deeper origin of Verbano ». part of the magma. This cooling age pattern was attributed The chemical characteristics are compa­ to a posr-hercynian thermal evolution that tible with magmatic fraetionation mostly involved this part of South-Alpine basement without a real separation of cumulate and in a different way from place to place: the melt, such that each rock type results from NW part of the « Serie dei Laghi » towards mixing of different amounts of both ma­ the «Ivrea-Verbano », underwent a more terials, giving a hulk composition ranging prolonged cooling period in consequence of from tonalite to granite. Only the augen­ a slower combined process of post+(Jtogenic gneisses show a marked character of re­ uplift and erosion. Alternatively all the data sidual melt. could be interpreted as due strictly to mag­ In spite of the presence of superimposed matic cooling of the basic-ultrabasic Ivrea regional metamorphism and deformation, complex. one can recognize the igneous nature of the Our results fit well in the already described original contacts with the country rocks. geochronological frame. The age of the The already proposed structural setting of muscovite concentrate of sample LM80-14 the « Serie dei Laghi j) at the end of Ordo­ is very close to the inferred age of the her­ vician (a weakly metamorphic « Strona-Ce­ cynian metamorphic peak. The biotites neri j) overlying a more deeply metamor­ become younger, going from the eastern phosed « Scisti dei Laghi») is confirmed: shore of Lago Maggiore towards the « Ivrea­ the level of intrusion seems to be more Verbano» in an even more marked way shallow in the « Strona-Ceneri j) than 10 since .they range from 315 m.y. of sample «Scisti dei Laghi j). LM80-2 to 234·244 m.y. of two samples The proposed evolutionary scheme is collected near the « Strona·Ceneri j).« Ivrea­ sketched in 6g. 9. Verbano j) boundary (Table 6). Acknowledgments. _ The Authors are grateful to If we assume the values suggested by G. PEYRONEL PAGlIANI, A. GREGNANIN, R. SACCHI PURDY & JAEGER (1976) for the temperatute for the fruitful discussion and to J.C. HUNZIKER, of the Rb-Sr system closure, we obtain from A. ZINGG, A. MOTTANA and F. AUMENTO for the age differences of the micas coexisting critically reading the manuscript. Lavora escguito nell'amblto delle ricerche del in sample LM80-19, a local cooling rate of Centra di Studi sulla Straligrafia e Petrografia delle only 4° e/m.y. Alpi Centrali del CN.R.

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