RENDlCONTJ OELLA SOCIETA ITALIANA DI MINEAAlOGlA E PETROlOGIA, 1988, \bI. 0·2, pp. .\1"428

Rh-Sr Geochronology of Lower Penman plutonismin Massiccio dei Laghi, Southern Alps (NW Italy)

LAURA PINAREL.I..I, Awo Da MOROhtituto di Gca:ronoJogi. e Geochimica Isotopica de:!. C.N.R., Vi. M.£fi J6, ,:6100 Pin

ATI1UO BoRIANtDipntimento di Sdem:e dell. Terra, Univenid di Milano, Via Bonicdli 2J, 20133 Milano

ABSTRACT. - Based on Rh-Sr whole· rock isochrons.ndRh-Sr biotiu~ determinatiOl1$, the complacemcont agco ofthe Momorfano, B.veno-Mollarone and Pell. imrosivesis 2H·280 Ma.

The: Quama, Roocapicotra granites and Sacromomco diBriuago, Pcdemorlle, M. Zuccaro, Quan. and Valsesi.calcalkaline imermcdille·mafic rocb, which lie mill ornear the Coss.tc>MC'rBozm-Briuago (CMB) lineament,give significamly )OWlgC'l" Rh-Sr bioQtco ages. 1besebiotite dates are~ • result cl the: post • Permianreactivation of this f.ult.

11lC' initial Sr isotopic; ratios of the granitic bodies,around 0.7100, lUCO gconera1ly higher than those of theappinites, which an: between 0.7051 and 0.7l0J, Asregards the protholith of the parent:al magmas, the twoseries of Sr.isotopic values do not permit .n univocalintcorpretation. The suggested hypotheses ate: a) aheterogcneow anomafuecl mantlco; bl. «normal» m.ntle,with later comamination by crustal m.terial: or c). lowercruu.

K~ words: Rh-Sr dating, Sr Isotopic lleochemistry,magmatogenesis, Southern Alps, Hercynian plutonism.

Introduction

Jp the westttn sector of the Southern Alps,the Sludied Hercynian plutonism occurs insome composite intrusive bodies, alignedparallel along or inside the NE·SW trendingCossato-Mergozzo-Brissago (CMB) lineament(Fig. I).

This plutonism comprises twogeographically associated bot separately

intruded magmatic series: a syn-orogenicprevalently gabbrodioritic to granodioritic(termed appinitic by BoRlANI & SACCHI, 1973)suite and a late-orogenic, quartz-dioritic togranidc one. The former outcrops in smallmasses near the CMB sySlem fauh inducingpartial melting in the country rocks betwttnValdossola and Valsesia. The latter iscomposed of the major plutonic complexes ofMomor£ano, Baveno-Mouarone, Quarna,Alzo·Roccapietra (and Biella, not consideredin this paper).

Concerning the granitic complexes, manygeological, geochemical and petrographicalstudies have het:n performed in the last 20years (BURANI, 1965; MORTEN & ROSSI, 1971;D'AMICO & Mm'uNA, 1974; GANDOLFI &

PAGANELLI, 1974; ORIGONI GIOBBI et al.,1975; FONTANA, 1976; ZEZZA, 1977; ZEZZAet al., 1984; SASSI, 1985; SESANA, 1985;TOLOMIERl, 1985), whereas t~ appinitic suiteis less well.known and an essentiallygeopetrographic study is that of BORlANI et.J. (1974).

As regards geochronological knowledge,during the last thirty years many isolatedradiometric data have been carried oOl onboth series, which made it possible todetermine a Lower Permian emplacement age.Among the more reliable results, we mentionthe papers of JAEGER and FAUL (1959, 1960)

412 L. PlNARELU, A. BORIANI, A. DEL MORO

I:\~~:I Permo - Mesozoic\lol(:ano- sedimentary coyer

[B late Hercynian granites..~ Caledonian granites..I2l Mt'lasediments, metabasitE's

0 20km m Basic and ultra basic rocks(Ivfu-Verbano)

!ill Kinzigites (Ivru - Verbano)..0 Appinitic series

FiI· 1. - Simplified geologio.l sketch map of Graniu dei Laghi (after 80wxI er al., 1988). a) Montonlno; b)Blveno-Motlllrol1e; cl Quam.; d} Alzo.ROCl;tpietn; co) M. Zucaro; 0 Vl1sesia; g) Pedemome; h) Sacromonte diBdssago.

.740..--------,--------------,

"Sr/·Sr

.730

,no

.710 -..o

Fij:. 2. - Rl:¥Sr whoINock isochron for Montorfano ~ples. C" granite; JP .. granite- porphyry; U" g.r=n graniteof MerSOUO; bd .. basic dyke.

Rb.St GEOCHRONOLOGY OF LOWER PERMIAN PWTONISM IN MASS1CClO DEI lAGHI. SOUTHERN ETCA13

R'

'"

'"

'"

lOO

"

........~".00-'. • ...

.~..

@ ...

@II'

• Montothno~ Bauno - MOllatOnt10 Quatna• Alzo _Roccap;.lta

,2 , • • 10 12 Rb/St

Fig. }. _ Rh vs. Rl:¥$r plol for granitic rocks of Mootorfano, B,lveoo-Moltarone, Quama and Alz.o-Roccapietra.

which give Permian K·Ar and Rh-Sr mineralages on the Montorfano and Baveno granites.

Later, KoEPPEL (1974) analyzed a monaziteconcentrate determining an V-Ph age of 280± 5 Ma for the Montorfano granite, whileKOEPPEL and GRUNENFELDER (quoted inCUf..1MlNG et aL, 1987) ohtained an age of 285± 5 Ma on an appinitic diorite using the samedating methoo.

Lastly, hy the Rh-Sr whole-rock isochronmethod, HUNZlKER and ZINGG (1980) dateda heterogeneous set of rocks coming fromdifferent plutonic bodies whose radiometricage (276 ± 5 Ma) was strongly affected bythe high Rh-Sr ratio specimens of the Baveno­Mottatone mass.

In this paper, we perform a detailedradiometric Stlx:ly on the Hercynian granitoidsof the Massiccio dei Laghi, in order toascertain possible temporal differences insideand between the two magmatic suites,differences that are suggested by thegeological evidence. Moreovtt. we would liketo provide funher isotopic data dealing withthe magmatogenesis of the Hercyniangranitoids present in the circum­Mediterranean regions.

With this aim, eighteen granites (fromMomorfano, Baveno-Mottarone, Abo­Roccapietra arx:I Quarna plutons), fifteenappinites (from Sacromonte di Brissago,Pedemonte, M. Zuccaro, Quama, Valsesia),one granitic neosome (Va1~ia) and onemetapelitic·kinzigitic country rock (M.Zuccaro) were studied from a radiometricpoint of view (Tabs. 1-2). Moreover, majorand some trace element data (Rb, Sr, Ba, Y,La, Ce, Nb, Ni, Cr, V and Zr) are presentedfor the appinitic samples of the M. Zuccaroarea (Table 3). The numerous and goodstructural, petrographical and geochemicaldata collected from this area provide solidgrounds for setting up such a discussionwhich, moreover, may throw light upon thedebated problem regarding the origin of theHercynian magmatism.

The South Alpine basement rocks, crossedby the Hercynian plutonites of the Massicciodei Laghi, have been fairly ~ll investigatedand characterized from the Sr isotopic pointof view (refen:nces qUOted in DEL MORO,1987) and so the: genetic rdatimships betweenthe parental magma(s) and the country rocks

414 l. PIN.... RELU, A. BORIANI, A. DEL MORO

TABLE 1Rh/S, whole rock analytical data. Rb and S, contents were ana'.1..zed by I.D. method using 87Rb98% and 845, 99.89% mixed spik~. Uncertanity 0/ the 87RbroS, ratio was evaluated ± J%.The mor on 875,/65, ratio is 2a",. lsotopit: ratios were ~o""ed by a Varum MAT TH5 mallrp«lrome~equipped with an automatic 4Cquisition and el4boration tUlla syslmI. AtullyslJ: GwnnottiU. and Pardin; c.c. Reproducibility was periodkol/y checked by tktemti1llltions ofNBS 987 87S,j86S,

ratio giving a value of 0.71028 % :; (2a,w n" 23)

Sample Rb ppm se ppm 87Rb/86s r (87Sr/86srlm (87sr/86Sr) 1

GRNHTIC SUITE

""""..."'"GL , 195 " 5.76 0.73319+ • 0.7103GL 2 181 112 4.67 0.72822'+ • 0.7097GL 3 199 96 6.02 0.73405+ • 0.7101GL • 140 216 2,93 0.72162+7 0.7101GL 5 163 19.5 24.43 0.80545+31 0.7083GL 6 70 370 0.55 0.70877+ 4 0.7066GL 7 44.1 57 2.26 O.72196! 4 0.7130

BIWEHO-HO'M'llRONE

BB 1 212 68.9 8.95 0.74599+11 0.7104BB 2 255 25.4 29.34 0.82468+21 0.7080BB 3 234 19.3 35.53 0.84945+29 0.7082BB • 224 80.5 8.09 0.74214+29 0.7100BB 6 1B7 107 5.09 0.n003! 9 0.7098

"'~.BB 5 120 19. 1. 79 0.71671= , 0.7096

.......GL 24 157 155 2.94 0.72117=10 0.7095

Il:OCCAPt£1'RJl.

GL 22 ,B5 175 3.07 0.7192H 7 0.7071GL 23 110 160 2.02 0.71903+ 5 0.7'10BB 8 139 '16 3.47 0.72462+12 0.7108BB , 120 201 1. 73 0.71681! 5 0.7099

wiJI be less problematic than In othergeological areas.

Analytical results

Montot/ano uanite

This is the northernmost portion of the«Granid dei Laghi~. The four granitic samplesanalyzed are fairly uniform in Rb/Sr ratio(0.7·2.0) and provide an isochron age of 283± 14 Ma (2 a) with an initial Sr isotopic ratioof .709g ± 9 (Fig. 2).

If we also consider the high Rb/Sr ratioporphyritic sample (GL 5), the Rb-Sr whole­rock a~e i~y-ractically unchanged (277 ± 8Ma; (8 Sr/ooSr)j'" .7101 ± 5). This sample isrepresentandve of an acidic dyke and, on thebasis of its anomalous position on the Rb vsRb/Sr diagram (Fig. 3), a derivation of thedyke from the granitic magma of Montorfanoby a simple differentiation process can beexcluded. The Rh-Sr ages for the biodtes oftwo granitic samples are concordant (274 :I::8 and 278 :l:: 8 Ma) with the whole-rockisochron age, within analytical uncertainty.

A gabbroid dyke from the northern sector

Rb-S. GEOCHRONOLOGY OF LOWER PERMIAN PWTONlSM IN MASSICCIO DEIl..AGHI, Smm-IERN ETC.415

TABLE 1 (Continued)

Salllple Rb ppm " pp. 81Rb/86s r (81sr/86Srlm /81sr/86sr)1

APPINITIC SUITE

SACJICM:lNTE 01 BRlSSAGO

ee 13 133 273 1.41 i)'112641: 1 0.1010

...........ee 14 30.6 388 0.23 0.10914.:!:. 5 0.1082

M. Zucx:ARO

GL , 2J6 18.1 38.26 0.85041+ 7GL 10 126 31. 1.16 0.11095+16 0.1063GL 11 180 29J 1. 78 0.71739+12 0.7103GL 12 47.6 J27 0.42 0.10676+17 0.7051GL 13 25.6 21' 0.27 0.70694+11 0.7059GL 14 1.5 J22 0.01 0.70661! 3 0.1066

QlJARNA

BB 11 116 25' 1. 30 0.71477.:!:. 5 0.7096

VALSESIA

GL 16 19.6 478 0.12 0.71034+ 7 0.7099GL 17 17.0 342 0.14 0.11012+ 7 0.1102GL 18 58.4 350 0.48 0.71249+ S 0.7106GL " 43.5 ." 0.26 0.70976+ 7 0.7087GL 20 50.0 390 0.37 0.71149+ S 0.7100GL 21 75.4 JSS 0.61 0.71231+ 4 0.7099

HETAPELITIC ROCK

GL 8 B5.9 35. 0.70 0.72126+ , 0.7245

GRANITIC MOBILIZED

GL 1S 72.6 256 0.82 0.71420+ 4 0.7109

of the graniric body is not similar in inirialSr isoropic composirion to the rest of thepIuton, having a calculated ratio as low as.7066.

Lastly, the hydrothermally chIoritized andalbitized «Green granite of Mergozzo» issignificantly depleted in both Rb and Srrelative to the llllaltered~ranites, but it seemsrelatively enriched in 8 Sr, having an initialcalculated 81SrfS6Sr rario (280 Ma ago) of0.7UO.

This value, intermediate between that ofthe unaltered graniric fades (.7098) and thatof the country rocks (Fig. 4), may be the resultof an isotopic exchange between granite andcrustal component enriched fluids.

&fJeno·Mottarone

Southwards, beyond the alluvium of the

Toce river, the composite, granitic Baveno­Mottarone pluton occurs. We were notconcerned with the xenolithic granodiorite,but analyzed five samples belonging to themore diffused granitic fades: the porphyritic(BBl), white (BB4, BB6), pink (BBJ) andtransitional between the last two (BB2). Thepink fades shows pneumatolitic.hydrothermaleffects (PEYRONEL PAGUANI, 1948 andreferences quoted therein). BBl, BB2 andBB4 come from the northern end, BB3 andBB6 from the southern part of the pluton.

All the samples define a good Rb-Sr whole·rock isochron of 276 ± 7 Ma (2 0), with aninilial 87SrfS6Sr ratio of 0.7102 ± 8 (Fig. 5).

The rocks of the transitional and pink fadesare characterized by the highest Rb/Sr ralioof the plutonic complex. These Rb-rich andSr-poor values are typical of highlydifferentiated magmatic produas. 1be perfect

416 L. PINAREW, A. BORIANl, A. DEL MORO

TABLE 2Rh/5rmin~l ages. Rb and 5T contents were determined by I.D. Analysis: Giannotti U. and PardiniG. C. Rh analyses were performed on an a/iquot 0/mineral solution. Mineral ages were calculatedwith the correrponding whole-rode (except for ~atite CL 9, whose age was calculated usingbiotite + feldspar), using the tkray cons14nl 0/ Rh: 1,42 x 1f),JJ/a. Quoter1 errors are 2a",

Sample MlIterl",l Ob pp. Sr p~ 87Rb/86sr 181sr/86sr l. AGE 1"..Il

GRANITIC SUITE

~~

LL , .. ." .., H).Ct 2.46302+125 278+ 8LLS .. ,0< '.3 854.66 4.04310!tS6 214+ 8

M_~

" , .. '" '.3 177.53 '.40776+ " 276+ 9

'" .. 1029 11.6 285.22 1.83981. " 279+ 9

'" .. 1065 8.' 446.13 2.48285+ " 260+ ," . .. 8" '.3 282.20 1,83769+ 34 281'+ 9

" . .. '" 1 5.0 144.49 1.285701 42 2801 ,.,-

,ss .. S83 ,. , 256.47 1.5100)1:. 26 219:!:. •~~

CL 22 .. '" 8.' 366.47 1.92200+ 43 233+ 7CL 2J .. S" 3.' 430.61 2.05143137 2191 6

~

CL 24 Bi ". .., 482.90 2.60288.!.12S 276:. 8

APPINITIC SUITE

SACJli:MCfI'Tl! Dl BlUSSAGO

BB 1] Bi '" '-' 803.23 2.65094! 77 110+ 5

M. """""

LL ,M" '" '.8 1525.84 6.32992+145

259+ 8H <OS 30.3 39. J4 0.84152'. "GL 10 B' '" 10.7 89.21 1.02608+ 21 25H ,

GL 11 Bi '" '-' 363.25 1.12239!: 29 196+ 6

.,~.

'B " Bi U8 ,., 514.51 2. U054! 99 236! 7

V.o.LSEsIA

LL " Bi+Chl US 30.6 14.01 0.15344+128 218+ 9GL 18 " '" 1 O. 2 18.16 0.94928+ 82 214+ 6GL 20 " m 11.6 91.06 1.04154!: 15· HO!: 7

M£'TAP£LITIC '00'LL8 Bi '" '.8 216.55 1.3805H 44 213+ 6

"" >to it> S.43 0.74604+ , 219!:10

" ,.. m 0.03 0.72489!: •GRANITIC MOBILIZED

CL 15 ~ ", n.1 19.60 0.78528!. " 266! 8

linear trend of the samples along the isochron interacting and partially modifying the earliersuggests an exclusive magmatic origin for the crystalline phases (biotire. fddspars).hydrothermal and/or pneumatolitk fluids The Rb·Sr biotite ages confirm that of the

Rh·S. GEQCHRONOLOGY OF lOWER PERMI....N Pll110NISM IN MASSICCIO DEI u.GHI. SOUTHERN ETC.417

Iv rea- Verbano (n:21)' ...and ...... ,

"

Strona. Ceneri Zone (n :7)

metapelites "Caled.gr.ogn.(n.16)

x

("SrjI6S r)2H1Ma ;;-'~,, ,,,,,,,,,,,,,,,,,,,,,,,\ p - --

,~--------qg

.71S

.720

.HS

X Kinzigit.

~."0 0

a migmilltit.

10O ~ 000 o grillnit.

0 • ViIlls.siill appinit•• .. toll Zuttaro appinit.

0 + amphibolit.0 '"'".70S '"

o 1 2 3

Fig. 4. - (l7Sr{l6Sr)lIO Ill. vs. Rb/Sr plot showing relationship betwtcn Htrcynian guniloids and mainmelasedimentuy basement rOtks of Iv~a·Vcrbano and Slrona·Ccnc:ri zones (GRAESEI\ '" HUNZlK£l\, 1968; HAMET'" ALBAREDE, 1973; HUNZIKER 05< ZINGG, 1980). Cross: amphibolitc from Alpe Mordlo, Val d'Ossola and Anzolaquarry (DEI. MORO, unpubl. data). Bla:k area: «Caledonian- acidic orthogneisscs (BoRIANI et al., 1982). All dataare corrected for an age of 280 Ma.

'" "Sr/"ISr

." Baveno -Mottarone granitesBB 3...

BB2~-

.eoo

'"

1",276~7Ma

Sri '" .7102~8MSWQ",.66

'00

"Rbj"Sr

o 5 10 15 20 25 30

Fig_ ,. - Rh/Sr whole·rock isochron for Baveno·MOllarone granites."

418 L. PINAREW. A. BORlANI, A. DEL MORO

TABLE 3Ma;or and /raa: element contcJlJ of Va!ses;a samples. MgO and Na2.0 were determined by A.A.

spectrophotometry; ot«r major and trace elements were determined by XRF_.. ..~~-

~" ~ .. ~" ~N ~" ~" ~ .. ~ .. ~ ... IUl '0' • UT' • JS T'-....., ".. N.O ~" ".M Q ... ".. M O1.n ".N ".N n.n so.,.

"'" '.~I '.B ." .. .• .B .u .. '.B ." .... ...'¥> N.• n.8 It." 11.<) ".M 'S. " .s.m I1.U '~.'" ".15 '..~ ...SoI- n." '-" ,.• .." ,." '.B '.• '.N n.70 .." '.• ,.n~ ." ." ." ... .0 .B ." .U ." .0 ." ."~ '.l' '.M '.n '.M .." .." .n '.5' I.u ••• ••• I."~ I1 ••s ,." l.41 S••• .." ••• ••• '0.11 10.91 ••• '.M '.N-, ••• l.O .." l.~O .." l.l' ••• ••• .." .." l." l ...

'i' 1. IS ." '.. .... .. '.N .." ." ... ,... '." '.H..... ." ... ... .• ." ... ." ." ... ." .M ."L.O-I. ... ... ." ." .• ... ." ... 1.S' ." l.n l.ll

--.. " .. • " .. .. .. ", ... " .... ~ ... •• ... m •• ., m •• - •• m

• •• - ... '" "' - 'n • m - - -.. .. .. .. .. " .. • • .. ".. ... ... = ... •• •• .. • ,. m • .." .. .. ... u .. .. • " .. • " .... n • ... • " • " " .. 'M • •• M " N .. " .. • " M " M ..M " • • • • 0 on " • • .".. " .. " " N .. " M' M' 0 ." m• M' M. " • • • .. 'N - M m .n__ ...0-. ...._ ....... -..-....., __._,... _ ..._.__n ...._ ... _ •

........ __ ... -...-._ .................-..._.

.......__" ... L. , .0]'__Ul.

whole·rock isochron as being 276 ± 9 Ma(BBl). 279 ± 9 M. (BB2) ",cl 280± 9 M.(BB}).

Aho·Roccapietra

Our sample collection comes from theRoccapietra quarry and deals with threebiotitic ± amphibolitic granites and onemicrogranitic sample (GL 22).

They do not define any isochron and, whenplotted in the graph (87S r/86S r) vs.(87Rb,t86sr) of FIg. 6, scatter widely along thereference isochron, allowing no datingsuggestive of either open-system behaviour ofthe total rocks or initial Sr isotopicheterogeneity. As the two biotites analyzedexhibit Rb-Sr «ages. significantly youngerthan those of Montorfano and Baveno­Mottarone, open-system behaviour for thetotal rocks is supported.

However. the specimen of the easterngranitic «tongue. of PelIa behaved dif£~ndy:

its biotite did show an unaltered age of 276± 8 Ma, and the whole rock fits fairly weUinto the 277 Ma reference-line of the Srevolution diagram (Fig. 6).

Quama

Only one quartz-dioritic sample (BB5) wasanalyzed for the Quarna pluton, near theeME lineament. This mass exhibits behavioursimilar to that of the AIzo-Roc:capietra pluton,and turns out to be isotopically disturbed (Fig.6).

The biotite also shows a Rb-Sr«rejuvenated. age of 219 ± 6 Ma.

Appinitic suite

The chemical data presented here refer to

Rh-S, GEOCHRQNOLOGY OF LOWER PERMlAN PWTONISM IN MASSICCIO DEI LAGHI, SOUTIIERN ETC.419

olivingabbro to monzogranite specimens (Fig.7) of the Valsesia , M. Zuccaro and La Colmaareas (Table 3). Major elements show a regular

decrease of CaO, MgO and FeOtot. with anincrease in silica content, whereas otherelements are scattered when plotted versus

.73

.71

.-..,..~.71 --!•

••

• *

~_ -oGlU

OGl22refer. isochron 277 Ma

o granite• Valsesia appinite.. Mt.Zuccaro appinite• Quarna appinite* Sacromonte appinite• Pedemont{> appinite

87Rb j86Sr

o 2 3 4 5

Fig. 6. - Rh/Sr ~volUlion diagram for Quarna, Alzo.Roccapi~tra, P~lIa granites and M. Z...:caro, Valsesia andQuarna appinites.

o__ -0

o

GABBRO

400

BOO

1200

1600

2000 r-~---r-~--'--~"-~-T-::=""~~--'--~--'---'" / 0- - 0" 1;A88RO_

"ALKALI / OLlVIN( /' NORIT(, /GA6BRO / GA66RO '\ ,-

.... ,,, / I, ...., ¥,," .............. :- _'.... -....

'/ ,..... )<~ENOG ... 6BROI

" .... .... 1 I,,/ --I-. --..f----

" SVENODIORITE \MONZO-\ OIORITE 0 ,-, , ,OIORoTE \ ........0, , ,

/ _ _ MONZONIIE,). ..-/ '/-__ \ ,_- TON ...LlIE

/ // ---1\-0---'..../ I ,ClU"'RTZ '..... ,-

/ I ,MONZONIIE >.:'"_.. • GRANOOIORITE/ ,<A/ ••-- __

SVENITE I ......" .....~4GRAN'If. - - - __I I .... '1'.:i~!.NOGIiAN;;'- - - ~ _ !:'..... _.-_-

/ /ClUARTZSvENITE/ ---- ..:~~:.~_I I "':c. .....

/ AL~ALIGR... NITE----- '- 1. _

N

'"

o 01L--'----,4J,0"0---'---,B"0"0---,~"12"0"0--'----,,"60=cO=-'---,2"0"0"0--'----24"0"0:-'---,;2"BO"o=-'o Appinit{> R1• Granit{>

Fig. 7. - Pro~tion of analyzed samples on DE LA RocHE'S RI-R2 diagram {after DE LA RocHE et aI., 1980).Open cird~s • appinites; full circles. granites.

420 L. PINARELLI. A. BORlANI, A. DEL MORO

SiOZ.(Fig. 8). In the Al20 J vs. SiOz diagram,the Valsesia samples occupy a distinct field,being A120 3-richer than the M. Zuccaro andLa Colma specimens.

A significant difference between theValsesia appinites and the others may also benoted in the trace element (Rh, Se and Ni)vs. Si02 plots (Fig. 9). Moreover, the GL 21granodioritic appinite from Valsesia isanomalously rich in K20, V, REE, Ze andBa (Fig. 9).

The Rh-Se data were determined onspedmens from the M. Zuccaro (G 10-14) andValsesia (GL 16·21) areas, from Pedemonte(BB14), Sacromonte di Brissago (BB 13), andfrom an enclave in the Quarna granitic mass(BBII). The whole-rock data points of the M.Zuccaro, Sacromonte di Brissago andPedemonte appinites lie markedly below thereference isochron of 277 Ma (Fig. 6), whilethose of Valsesia and Quarna scatter to a lesserextent from that line.

The Rb-Sr biotite dates of these samplesrange from 251 Ma (M. Zuccaro) to 170 Ma(Sacromonte di Brissago) (Table 2), while themuscovite of a pegmatite associated with theM. Zuccaro appinitic series yielded a Rb-Sr«age» of 259 Ma.

The kinzigitic sample collected in the samearea displays a Rb-Sr muscovite age of 275::I:: 8 Ma and also a calc~ated Sr isotopic ratioas high as .7240 for the total rock, whereasthe granitic neosome from Valsesia has anapparent Rb-Sr muscovite age of 266 Ma andan initial Sr isotopic composition of .7110.

Discussion

The granitic masses of Montorfano andBaveno-Mottarone have the sameemplacement age (275-280 Ma) and the sameinitial Sr isotopic composition: this agerepresents the most reliable age of the plutonicactivity of this region. The Sr isotopic ratio,around .7100, recurs in the calc-alkalineHercynian magmatism of the Southern Alpsand is an intermediate values between thoseof a suboceanic mantle and a typicalmetasedimentary crust (Fig. 4). The nearlyuniform values around .7100 exclude arandom contamination process of the magma

during its ascent through the crust or duringits emplacement. Moreover, a directderivation of the granitic series from theappinitic one can be ruled out on the basisof the sample distribution in the Rb/Sr vs.l/Sr plot of Fig. 10 defining two distincttrends. It is more reasonable to hypothesizethat the value of .7100 was acquired duringevolution of the magma in a magmaticchamber or directly from the source region.In fact, the Sr isotopic composition value of.7100 is compatible with a medium-low Rb/Srratio material present in the lower crust orwith a subcontinental mantle, enriched in87Sr before the magmatic event.

According to lead isotopic data ofCUMMING et al. (1986), it is possible toobserve an isotopic evolution of the mantleregion in this area towards an apparently«crustal» characteristic, probably determinedby a subduction related dehydration of thedowngoing sediment, starting from a mantlesimilar to the source of the lower layeredcomplex of the Valsesia (PIN & SILLS, 1986).

The remaining intrusive bodies of thegranitic suite exhibit Rb-Sr brotite agesyounger than 277 Ma, indicating an openingof the mineral systems. Moreover, from thescatter of the whole-rock data points in theisochron diagram (Fig. 6), open systembehaviour may also be inferred for the rocks.It should be pointed out that the latter massesoutcrop nearer the CMB line than theMontorfano and Baveno-Mottarone bodies.This fact suggests a strict relationship betweenthe activity of the fault and the radiometricdata. The open system behaviour of thegranitic rocks in this area may in fact bedetermined by the deformative event relatedto the activity of the CMB lineament.

The «rejuvenated» mineral ages of theappinites, outcropping near the CMB line,confirm that the activity of this fault may havedisturbed their chemical-isotopic system. Asimilar effect may be inferred for those wholerocks which do not fit the isochron of Fig. 6.

On the other hand, the diagram of Fig. 11shows the corrdation between the calculatedinitial Sr isotope ratio and the Sr contentinside each appinitic suite. Such a relationshipseems to exclude a major role of the disturbing

Rb-Sr GEOCHRONOLOGY OF I..(JWER PERMlAN I'LUlONlSN: IN MASSlCCIO DEI LAGHI, SOtTTHERN ETC.A21

23.0 14.0

• ·/'• •AI,OJ • • • CaO•• , • ••• • • • •14.2 2.8 •12.0 0.0

44.0 50.4 SiO, 76.0 44.0 50.4 Si01 76.0

12.0 5.0

• • •• •MgO • • • ••Na20 •• • • ••2.4 • •• 1.8 •• •0.0 1.0

44.0 50.4 Si01 '6D 44.0 50.4 Si01 76.0

14.0 • 6.0 •• •• • •

F~lOJ • •• K,O•• • •

2.0 • 1.2 • • •"I. • •

0.0 0.044.0 50.4 SiO, 76.0 44.0 50.4 SiO, 76.0

2.2 .5• • <5

Ti02 • • •• P,O, ••• <5 •• • ••••.4 • .1 • <5 •.0 .0

44.0 50.' SiO, 76.0 ".0 50.' SiO, 76.0

FJ.g. 8. _ Major elements vs Si01 variation diagrams for appinitk rocks. Full square .. Va!sesia (thU world; fulltriangle .. M. Zuccaro (BUKllNl L., unPlbl. thesis); full triangle enclosed in cirde .. La Colma (B1JItW,1 L., unpobl.thesis).

422 L. PINAREW, A. BORlANI. A. DEL MORO

2<0,,---------,•

Rb 120

'i>•

jo.'i> • •

• •° •" 57

5i02

I

72

120 •

Fig. 9. - Trace c:lcmcms vs. Si01 varialion diagramsfor Ippinitie rocks; symbols as in Fig. 8

•• .......

O+---r-~_.~F__TI.;_~

40 SO 60 70Si02

Ni 60

Rb-S, GEOCHRONOLOGY OF LOWER PERMlAN PurI'ONlSM IN MASSICCIO DEI LACHI, SOlTI'HERN ETC.423

tectonics on the whole rocks, as it ought tohave caused random variations of the Srelemental and isotopic composition. A lineartelation betwe(:n these two parameters isusuaUy linked to mixing processes betwe(:ntwo end-members with different Sr isotopicand geochemical characteristics.

The data on the M. Zuccaro and Valsesiaseries were separately tested for a processof combined assimilation - fractionalcrystallization (DE PAOLO, 1981), startingfrom the more primitive rocks, and assumingas contaminant an average crusta! component

The good agreement of the observed andcalculated trends (Fig. 11) confirms that thegeochemicaI and isotopic variations inside thetwo series may be explained by theeffectiveness of an APC process involving theappinitic magma and a crustal componentderiving, for example, from melting of thecountry-rocks: the neosomatic portion of amigmatized augengneiss outcropping inValsesia (GL 15, Caneto quarry) does havea corrected-age Sr isotopic composition anda Sr content similar to that required for thecontaminant, as can be seen in Fig. 11. At

o V.Sesia } Appinite/1 Zuccaro

I l/Sr Xl03 • Granite

I •

I .'

100"""00

10 f-0

0"

00

"I f-N

0

X

"~

.1 0:

1

• ••

10

•

•

.,,•

•

••

•"•

100

-

-

1000

Fig. 10. - Rb/Sr vs. l/Sr plOI diKfiminaling bct\\"CCn gnlnilic and appinitic 5C'ries.

with Sr content of 150 ppm and (87Sr,t86Sr)ratio of .720. The calculated mixing linesreported in Fig. 11 correspond to a theoreticalAFC process, assuming a Sr bulk partitioncoefficient (D) of 1.5 for the Valsesia sampleseries and 1 for the M. Zuccaro series, andan assimilation to crystallization rate of .5.

The results of the least-squares mixingcalculation based on major dements (STORMERand NICHOu.s, 1978) agree with the assumedD values. The Valsesia major elementvariations may in fact correpond to acrystallization process dominated byplagioclase, whereas those of M. Zuccaro areascribed to a minor amount of plagioclase inthe fractionated solid (Table 4).

the moment, no potential contaminant isknown for the M. Zuccaro appinitic series,as there is no evidence of interaction betwe(:nthese rocks and the kinzigitic ones, e.g. GL8, which, however, would not satisfy theisotopic characteristics of the contaminantend-member.

The lower values of the Sr isotopiccomposition (0.705-0.706), obtained on themore mafic samples of the M. Zuccaro suite,are compatible with a subc:rustaI origin, inagreement with their extremely basiccomposition.

The differences between the more primitivemagma of M. Zuccaro and that of Valsesiastill remains to be explained. In fact, it may

424 L. PINAREL1J, A. BORIANI, A. DEL MORO

TABLE 4Results 0/ least-sqU4re1 mixing calculationbased on major elements (STORMER &

NICHOLLS, 1978) for Valsesia and M.Zuccaro appinitic suites. HO .. Horneblende;PL = P/agioclase; MC = Magnetite;Bl = Biotite; FS - Fractionated So/id;

OX '" Residue Oxides

- m ~ ~ .. '.'. ~

• •~

" " -" " ••• '.' .. ' ... n .• ..u

" " -" .. ... ".2 '.' ••• J'. , •. n

" , '"COUO

~, - ~ " 2] •• '.. ' .... •••• •. n

be seen that the parental magma of theValsesia series is richer in Sr and haS a higherSr isotopic composition than the M. Zuccaroone. Whether these diffe~nces must beascribed to primary or secondarycharacteristics is, at the moment, undur.

Concluding remarks

The results obtained by the investigationof the intrusion age of the late-orogenicgranitic suite of the Momor£ano, Baveno­Mottarone and Pella masses are concentratedaround 275 Ma. This value is similar to thoseof other Hercynian plutonites of the SouthernAlps: Bressanone massif (DEL MORO &

VISONA, 1982), Cima d'futa (BORSI et al.,1974) and Navazze (DEL MoRO quoted inCASSINlS, 1982). For the remaining graniticsamples studied (Alzo-Roccapietra andQuarna), the post-intrusive opening of theRb/Sr system impeded the attainment of areliable emplacement age but, on geologicalgrounds, the same age of the other masses canbe assumed. Evaluation of the intrusion ageof the appinites was hindered by a probablefractional crystallization plus assimilationprocess, involving the parental magma and acrustal material and affecting both the M.Zuccaro and Valsesia magmas. Micas ofappinites and granites outcropping near theCMB line tend to become younger towards

0110

0.10$

••

•B

• •

• Valnsis appn,le

.. ~U.Z..c..'o apPnole• g,an,'ic neOsome

, , , .,S.,0

Fia· 11. - (1ISr/'OSr); V$. IlSr plol fOI" appinites. Lines A and B: calculated trrnd for tWO AFC processes,respectively of M. Zuccvo and Valscsia series. ParaInelcn:A: Init. magma: ~SrI"Sr). 0.70'; Sr. 330 ppm

COnt. magma: (lISrI"Sr). 0.720; Sr. 150 pprnDSr. 1.01; R (Ass. rale!Cryst. rate). 0.'

B: lnil. magma: (17Sr/'6Sr). O. 7087; Sr. 600 ppmCom. magma. (lISrf'6Sr). 0.713; Sf. 260 ppmOSr. 1.2 R; (Ass. rate/Cryst. rate). 0.'

DSr values are obtained according to result of least-squares mixing calculations based on major elements reportedin Table 4.

Rb-Sr GEOCHRONOLOGY OF l.OWER PERMlAN PUJTONISM IN MASSlCClO DEI LAGHI, SOlmiERN ETC.42'

this lineament, up to about 180 Ma,suggesting a post-magmatic tectonic historyof the basement along this fault system.

Based on Sr isotopic data, an anomalousmantle or a lower crust may be invoked as thesource region of the Hercynian granitoids ofthe «Graniti dei Laghi».

APPENDIX, SAMPLE DESCRIPTION AND LOCATION

Montorfano

GL 1 White medium grained granite (qtz, pi, Kf, bi, zir, ap). Cava Donna quarry.GL 2 White medium grained granite. Ponte Toce quarry.GL 3 White medium grained granite. Cirla quarry.GL 4 White medium grained granite (qtz, pI, Kf, bi, ho, zir, ap). Cava near the aqueduct.GL 5 Porphyritic granitic dyke. Along the trail towards Ansolo quarry.GL 6 Basic dyke. Locality as GL 5.GL 7 Hydrothermally altered «Green granite of Mergozzolt (ab, chI, qtz, ser, tit,

earb). Ansolo quarry.

High-grade metamorphite: kinzigite (hi, qtz, sil, KC, gt ± hyp). Near Quaggione.Musoovite-tourmaline-bearing pegmatite. Loc. as GL 8.Appinitic dyke: chilled margin. Loc. as GL 8.Appinitic dyke: leucocratic portion. Loc. as GL 8.Biotite-horneblende-bearing appinitic dyke. Loc. as GL 8.Cumulitic appinitic dyke. Loc. as GL 8.Cumuli tic appinitic dyke. Loc. as GL 8.

M. Zuccaro

GL 8GL 9GL 10GL 11GL 12GL 13GL 14

Va!ses;a

GL 15GL 16GL 17GL 18GL 19GL 20GL 21

Granitic neosome_ Caneto quarry.Quartz-bearing appinitic diorite. Along the Sesia river.Quartz-bearing appinitic diorite. Loc. as GL 16.Granodioritic appinite. 200 m south lsolella.Dioritic appinite. Along the road Isolel1a-Foresto.Granodioritic appinite. 700 m north Isolella, towards Varallo.Granodioritic appinite. Cross·road vat Verde·Riomaggiore.

Ako-RoccapieJra

GL 22 Tounnaline·bearing microgranitic dyke. Roccapietra quarry.GL 23 Horneblende-bearing granodiorite (pI, Kf, bi, ho, qtz). Loc. as GL 22.GL 24 Pink heterogranular biotitic granite. Pella, along the Ona Lake shore.BB 8 Horneblende-bearing granodiorite. Loc. as GL 22.BB 9 White medium-grained biotitic granite with scarce horneblende. Loc. as GL 22.

Baveno-Mottarone

BB 1 - HeteJ'ogranular white granite (Kf, qtz, pI, hi, ms, all, zir, n, fal. Madonna del1aScarpia Sanctuary.

426

BB 2BB JBB 4BB 6

Quarna

BB 5BB 11

L PINAREW, A. BORlANI, A. DEL MORO

Transitional granite. Loc. as BB 1.Pink medium-grained granite. M. Mottarone q. 1200 m. (Giacomini quarry).White granite. Cascine Pironi inactive quarry, near GravelJona.White coarse-granite. Road wnnd near Omegna.

Granite (pI, qtz, hi, scarce ha). Quarna di Souo.Appinitic quanz-diorite. Pathway between Quarna di Sopra and Fontegnooratory.

Brissago

BB 13 - Appinitic granodiorite. Sacromonte cli Brissago.

Pedemonte

BB 14 - Gabbrodiorite. Cascina Lucchini.

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