Pre-alpine and alpine evolution of the South-alpine basement of the Orobic Alps

353

Geologische Rundschau 74/2 I 353-366 [ Stuttgart 1985

Pre-alpine and alpine evolution of the South-alpine basement of the Orobic Alps

By ANNIBALE MOTTANA, MASSIMO NICOLETTI, CLAUDIO PETRUCCIANI, Roma, GIUSEPPE LIBORIO, LUISA DE CAPITANI and ROSANGELA BOCCHIO, Milano*)

With 3 figures and 3 tables

Paper presented at the Second E.U.G. Meeting at Strasbourg, April 23-26, 1983

Zusammenfassung

Neue Kali-Argon-Datierungen yon Gneisen und Schie- fern und ihrer abgetrennten Minerale des siidalpinen Grundgebirges wurden in Erg~inzung der Studie yon BOC- CHIO et al., 1981 durchgefiihrt. Die Abkiihtungsalter der Biotite und Muskovite reichen yon 218 bis 331 ma und yon 170 bis 330 ma entsprechend einer bimodalen Entwicktung mit Modalwerten bei 222 und 298 ma Rir Biotit und 182 und 322 ma fiir Muskovit. Die Gesamtgesteinsalter reichen von 43402 ma ohne signifikante H~iufung. Nur wenige Proben geben eine statistisch signifikante Isochrone, woraus sich zwei Gleichgewichtsperioden zwischen 226-245 ma und 312-368 ma ablesen lassen.

Diese Altersbestimmungen unterstiitzen die Schlu~fol- gerungen, die aus regional-petrotogischen Smdien (BOG- CHIO et al., 1980; CRESPI et al., 1980) gezogen wurden und die Aussagen, dag das siidalpine Grundgebirge zwei Hauptphasen der Metamorphose mitgemacht hat, die heute in verschiedenen Gebieten erhalten sind: eine ~iltere herzy- nische Phase unter mittleren Druckbedingungen (Staur- Ky) und eine jiingere postherzynische Phase unter niedri- gem Druck (Staur-Sil), die unter der Uberdeckung einer m~ichtigen Sedimentserie tiber dem Grundgebirge reakti- viert wurde. Dariiber hinaus deuten die radiometrischen Alter auf einen welt verteilten, aber unregelm~il~ig auftre- tenden alpinen Einfluf~ hin, der unter den Bedingungen el- her sehr niedrigen Metamorphose aufgepr~igt wurde. Dies wird dokumentiert dutch das Wachstum postkinemati- scher Stilpnomelane in Gesteinen mit einer entsprechenden Zusammensetzung (CRESPI et al., 1981, 1982) sowie yon :Feink6rnigen Phenigiten.

* Addresses of the authors: A. MOTFANA, Istituto di Mine- ralogia e Petrografia, Citt5 Universitaria, Piazzale A. Moro 5, 1-00185 Roma, M. NICOLETTI and C. PETRUC- CIANI, C.N.R., C.S. Geocronologia e Geochimica delle Formazioni Recenti c/o Istituto di Geochimica, Citt5 Universitaria, Piazzale A. Moro 5, 1-00185 Roma and G. LIBORIO, L. DE CAPITANI and R. BOCCHIO, Diparti- mento di Scienze della Terra, Sezione di Mineralogia, Via Botticelli 23, 1-20133 Milano, Italy

Abstract

New K-Ar age determinations on the gneisses and schists of the south-Alpine basement of the Orobic Alps and their separated minerals have been carried out to supplement and support the previous study of BOCCHIO et al. (1981).

The cooling ages of biotites and muscovites range from 218 to 331 my and from 170 to 330 my respectively, according to a bimodal distribution with modes at 222 and 298 my for biotite, 182 and 322 my for muscovite. The whole rock ages spread from 43 to 402 my with no significant cluster. Only a few samples give statistically significant internal isochrones, and these suggest two periods of equilibration, at 226-245 and 312-368 my.

These age determinations support the conclusion drawn from regional petrologic studies (BOCCHIO et al., 1980; CI~ESPI et al., 1980), that the south-Alpine basement underwent two main phases of metamorphism, now prefer- entially preserved in different areas: an old, Hercynian phase under a regime of intermediate pressure (staur-ky type), and a young, post-Hercynian phase under a regime of low pressure (staur-sill type) reactivated by the cover of the thick sedimentary pile on top of the basement. Furt- hermore, radiometric ages also point out the widespread but irregularly distributed occurence of an alpine overprint under the conditions of a very low metamorphic regime, that is documented by the growth of post-kinematic stilpnomelane in rocks of suitable bulk composition (CI~ESI'I et al., 1981, 1982), as well as by fine-grained phengite.

R~sum~

De nouvelles d&erminations d'ige par la m&hode K-Ar ont ~t~ effectu~es sur les gneiss et les schistes du socle sud- alpin des Alpes Orobiques, ainsi que sur certains de leurs minfiraux, en compl~ment aux travaux ant~rieurs de Boc- CHIO et al. (1981).

L'fige du refroidissement des biotites s'&end de 210 5 331 Ma et celui des muscovites de 170 5 330 Ma, selon une distribution bimodale: modes ~ 222 ~ 298 Ma pour la biotite et ~t 183 et 322 Ma pour la muscovite. Les ~ges mesur~s sur ro- che totale vont de 43 ~ 402 Ma sans concentration significa- tire autour d'une valeur. Un petit nombre d'~chantillons

354 A. MOTTANA et al.

seulement fournissent des isochrones statistiquement signi- ficatives; elies sugg~rent deux p~riodes de raise en 6quilibre, ~t 226-245 Ma et ~t 312-368 Ma.

Ces d&erminations d'~tge appuient les conclusions d'&u- des p&rologiques r~gionales ant6rieures (BoccrtIO et al. 1980; CRESPI et al. 1980), fi savoir que le socle sud-alpin a subi deux phases principales de m&amorphisme, conser- v~es aujourd'hui dans de r~gions diff6rentes: une phase an- cienne, hercynienne, de pression interm~diaire (type staurotide-disth~ne) et une phase jeune, post-hercynienne, de basse pression (type staurotide-sillimanite) r~activ~e sous la couverture s6dimentaire 6paisse reposant sur le socle. De plus, les ~ges radiom&riques mettent en ~vidence une reprise alpine, d'extension r6gionale mais irr6guli~rement di- stribute, dans les conditions d'un m&amorphisme de tr~s faible degr& Cette reprise s'exprime par la croissance de stilpnom~lane post-cin~matique dans les roches de composition appropri~e (CR~sPI et al. 1981, 1982) ainsi que par le d~veloppement de phengite finement grenue.

KpaTKOe col lep:a~aHne

KaK ~IOIIO;lHeHHe K HCC;le2IOBaHHgM E o q q H o H ~p.

(1981), npoBe;lr~ onpefte;lenrIe Bo3pacTa B r14e~Icax I~ c;laHIIaX H BK;ltOHeHHbIX B I-IHX MH14epa;lax !43 10)t(140-

a;lbHHITICKHX KOpeHHblX Fop C HOMOLL~blo K/Ar-MeTO~Ia.

I Io ; lyqHJIH BpeMfl OX;la}K~/IeHHfl 6HOTHTOB H MyCKOBH-

TOB B 218 dO 331 M14:I;lHOHOB net 14 170 llO 330 MHJI;1140- HOB ;leT, COOTBeTCTBeHHO 6IIMO~Ia;lbl-IOMy pa3B14TH1O C

MO~e;lbHbIM14 3rtaqeHi4~iM14 B 222 rI 298 Mrln;114onoB ;let ~sxa 6rtOTHTOB rI 182 a 322 M14;lnI~OnOB neT ~I;la Mycro- BHTOB. O6LIIHI~I Bo3pacT nopoI IbI OXBaTblBaeT n e p n o R B

4 3 - 4 0 2 MHTI;IHOHOB BeT 6e3 KaKI//X-;IH60 3Hattl/ITe;lbHblX

Ko;le6aHrlfi. To;lbKO OqeHb HeMHor14e n p o 6 b I pa3-

peu la tOT COCTaB14Tb CTaTHCTHqeCKH Bax<Hylo H 3 o x p o H y ,

no rOTOpOfI OTMeqarovca RBa nep~Io~Ia paB14oBecn~ Mexjxy 226-245 MrI;l;lrlOHaMH ;leT H 312-368 MrI;l;lHoIta- MrI ~IeT TOMy na3a~I. 9 T H ~aHHl~Ie O Bo3pacTe HOJITBep~jlaloT BblBO~bI

Bo~t,tuo H lip. (1980) n Kpecnn ri ~p. (1980) o TOM, qTO 1OTKHO-a;lbHHI~ICKHe KOpeHI-IbIe nopo~IbI npeTeplie;114 ABe

OCHOBHbIe dpa3bI MeTaMopqbH3Ma: 6o;lee paHHfl~l, r e p -

n14ncra~, qba3a n p o T e K a ; l a np~I c p e 2 I n g x 3HaqeI-IHStX ~aB-

;leHrlSt (Staur-Ky) 14 60;lee no3AH~a, nocxreprlHncra~t, npoTeramnaa np~ HH3KHX Be;lrlqi41-Iax AaB;leHHa (Staur- Sil) ~i pearx14BHpOBaHHaa MOn~HblMH oca~IoqHblMl,I ce- pHflMH, OT;IO~KHBLnHMHCS[ Ha KopeHH/:,IX ropax. ][[aTbl

9TOFO pa~HOMeTpl/iqecKoFO Bo3pacTa yKa3blBatOT Ha

06Ul r lpHoe , HO HepaBHOMepI-IO npoflB;lflto14ieec~t a ; lb-

nH!5tcxoe B.JIH~H14e, BO3HHKalOLRee n p H yC;IOBHgX o q e H b

HH3KOffI CTelIeHH MeTaMOp~H3Ma. OHO Bblpa>KaeTc~ pO-

CTOM HOCTKHHeMaTHqeCKHX CTHYIBUHOMe;laHOB B n o p o -

~ a x COOTBeTCTBylOUlero COCTa~a ( K p e c n r t i,I ~p . , 1981, 1982), a TaKxe Me;lKo3epHHCmblX qbeH~Ir14TOB.

Introduction

Within the basement of the Alps south of the Insu- bric Line, the )~Orobic Sector,, (BORIANI et al., 1974)

plays a paramount role, since the transition from the low-grade metamorphic conditions, dominant over the entire basement to the East, to the high-grade metamorphic conditions, dominant -by contrast-to the West and reaching the acme in the Ivrea-Verbano granulite zone, is accomplished here. In fact, there is no better area where evidence useful for the elucida- tion of the geological history and evolution of these units prior of the alpine collision can be studied.

However, for reasons which are difficult to under- stand, the Orobic Sector has never attracted much attention either from petrologists or from geochro- nologists. After the first, large scale mapping of PoRt~o (1903), it took almost 70 years to have it completely remapped by acceptable modern lithostrati- graphical methods (Carta Geologica d'Italia sheets n.7-18 ,~Sondrio-Pizzo Bernina<~ 1970; and n. 19 ,,Ti- rano<, 1969). Even now only a few small areas can be considered to be described in a satisfactory manner, from a petrological point of view. It is not surprising, therefore, that the ,>Metamorphic facies map of the Alps,, published in 1973, is now considered obsolete for this area, and in fact contained inaccuracies from the very beginning.

Together with a series of field and laboratory inve- stigations on the mineralogy and petrology of the transitional area between high-grade and low-grade regimes of metamorphism, resulting in new and un- expected findings (BoccHIO et al., 1980; CRzsPI et al., 1980, 1981, 1982), a program of radiometric age in- vestigations was started. It was deemed useful to supplement and clarify the mineralogical results. The first contr ibut ion was published some time ago (BoccHIO et al., 1981). However, new results have prompted us to review the present status and remo- del the geological evolution, in order to clarify some mistunderstanding of the geological record due to in- complete data.

The present work may also turn out to be obsolete in a short time: but we believed that it will help to draw attention to an area still poorly known, but full of basic information relevant for any understanding of the basement of the Alps and its correlation to the Variscan basement all over Europe.

Overv iew of the south-a lp ine basement in the Orobic Alps

The ~,Orobic Sector~, of the south-Alpine basement stretches east to west from the Adarnello intru- sion to the Italian-Swiss border, over a lenght of 103 kin. The basement is sharply bound to the Nor th by the almost straight I n s u b r i c L i n e; to the South it is covered by molasse-type conglomerates of

Pre-alpine and alpine evolutior~ 355

post-Westphalian age. In other areas, it is in tectonic contact with even younger sedimentary rocks of the south-Alpine sequence through an irregular swarm of faults known, as a whole, as the O r o b i c L i n e. The average width of the basement is 8 km, but ~windows~ of metamorphic rocks crop out as far as 21 km South of the Insubric Line (Fig. 1).

The age of the basement is controversial. Accor- ding to some Authors it is entirely Archean (the so- called ~Altkristallin~,); according to others it is lower-Paleozoic and was metamorphosed either during the Caledonian or the Hercynian orogeneses.

The scanty radiometric age determinations available till 1980 hinted at the following:

a) Metasediments derive from precursors whose detrital portion crystallized 2050 my ago. This age is the upper intersection of a U-Pb concordia determined for the zircons of a quartzite interbedded with garnet-staurolite schists at Sueglio (GRAuEt~T et al., 1973).

b) A significant metamorphic episode occurred later than 600 my, possibly at430 my. This age is given by the lower intersection of the above-mentioned concordia (Gt~AUEt~T et al., 1973); moreover such an

Fig. 1. Geological map of the upper Lake Como and western Orobic Alps region (compiled and modified after DE SITFER & DE SITTER KOOMANS (1949) and the sheets ~,Como,~, ,~Chiavenna~,, ,,Bergamo,~, ,~Sondrio~ of the Geological Map of Italy 1:100000).

Symbols: l. Quaternary sediments; 2. Austro-alpine Do- main (overthrusted Insubrian plate); 3. Mantello Gneiss formation (leucocratic p.p. granitic gneisses); 4. M. Le- gnone Gneiss formation (leucocratic gneisses); 5. Grave- dona Zone (Lake Schist formation AA., metamorphosed under staur-ky conditions); 6. Dervio-Olgiasca Zone (Lake Schists formation AA., metamorphosed under ,~low pressure~, staur-sill conditions); 7. M. Muggio and Morbegno

Gneiss formation (gneisses and micaschists metamorphosed under ~,intermediate pressure~, staur-ky-sili conditions); 8. M. Pedena phyllites AA. (pelitic schists, chlorite and garnet-beating); 9. ~Gneiss Chiari,~ AA. (leucocratic gneisses); 10. M. Fioraro igneous complex; 11. Val Bian- dino granodiorite; 12. Dazio granite; 13. Upper Paleozoic sediments (basal conglomerates, Collio and Verrucano formations); 14. Triassic sediments. Letters: M. L. Musso line; V.G.L. Val Grande line; P.L. Porcile line; T.S.L. Tre Signori line; V.T.L. Vai Torta line. Numbers refer to the radiometrically dated samples of Table 1, see Table II and BOCCHIO et ai., (1981) for descriptions (for space reasons n.7 is not shown; its position corresponds about to the location of symbol 4).


Assyntian episode is apparently confirmed by a K-Ar age of 385 my, determined for a hornblende in an amphibolite interbedded within kyanite-garnet- staurolite schists at Dosso del Liro (McDowELL, 1970).

c) A second, important metamorphic episode occurred during the main phase of the Hercynian orogenesis, and is indicated by the cooling age 239 + 12 my measured by the K-Ar method on the biotite of a garnet-staurolite schist at Corenno Plinio, only 1.5 km away from Sueglio (PuRDY & JXcER, 1976). Al- ternatively, it was suggested that this age was related to a thermal event, either the Permian volcanic acti- vity (HANSON et al., 1966), or the remobilitization of crustal material shortly before the breakup of the Eu- rasian continental block into plates, with the formation of new oceanic basins (F~Rv, ARA & INNOCENt, 1974).

Our systematic mineralogical and petrological studies carried out over the last two decades on the entire Orobic basement, and especially those con- centrated in the upper Lake Como region for the last five years, indicate that the geological history of the south-Alpine basement is far more complex than initially expected or believed.

Two major rock-types form the basement: apelitic group, trending to moderately psammitic, undoubtably derived from shales and sandy shales, and a quartz-feldspatic group of controversial derivation, either from granitoids or from arkoses (in turn derived from granitoids). These rock-types are unevenly distributed over the entire Orobic Sector (Fig. 1). In general, the latter occur as minor bodies scattered throughout the former, mainly near the Orobic Line, and are frequently disrupted by faults of alpine age. Minor bodies of quartzites, amphibolites and marbles occur at special localities. The basement is intruded by the Val Biandino and Monte Fioraro composite Hercynian plutons (granite to gabbro) and injected by innumerable dioritic dykes, mostly of late-alpine age.

In the upper Lake Como region the metamorphic rocks of pelitic composition can be divided in two units with different metamorphic regimes (Fig. 1). To the East and South there is an >>intermediate-pressure unit<< (Boccmo et al., 1981) characterized by a prograde metamorphic sequence from chlorite to sillimanite via a staurolite-kyanite zone (see also Ct~ESPI et al., 1980). This metamorphic regime dominates over the bulk of the Orobic Sector in spite of major tectonic and lithologic changes, and reaches the Adamello Massif. From there it is tied to the other basement sectors of Eastern Italy such as the lower Val Camonica-Val Trompia area, the Bressanone

quartz-phyllites, the Recoaro and Comelico areas. The western portion of the basement is made up by

a >>low-pressure unit,, (BoccHIO et al., 1981) that ex- tends along most of the western shore of Lake Como and farther West over the Italian-Swiss border. This unit is bound to the North by the alpine Musso tectonic line, across which other rocks, again belonging to a northern-most ramification of the intermediate pressure unit, outcrop as far as the Insubric line. Such a ,,low-pressure unit,~ displays another prograde zonation from biotite to sillimanite via a staurolite zone where kyanite has never been detected (BoccHIO et al., 1980). The orientation of the isograds is essentially E-W, dipping N or N N W , thus at a sharp angle with the orientation of the isograds in the ,,intermediate-pressure unit,,, that trends essentially NW to SE and dips NE.

The different pressure regimes of the two units are made conspicuous by variations of the white mica compositions with increasing temperature over a temperature span that is practically the same: 350o-400 ~ (chlorite/biotite isograde) to 650~ ~ (sillimanite-in isograde). Boccmo et al., (1980) and CR~sPI et al., (1980) found a variation from celadonite-poor (RM = 0.05), paragonite-poor (Na/Na + K = 0.03) muscovites to celadonite-absent but paragonite-rich (Na/Na + K = 0.35) muscovites in the low-pressure unit, as opposed to a more widely scattered variation from celadonite-rich (RM = 0.10), paragonite-absent phengites to celadonite-poor (RM = 0.02), paragonite-rich (Na/Na + K = 0.30) muscovites in the intermediate-pressure unit. According to GUlDOT~ & SASSI (1976)'s interpretation, these different trends would imply a difference of pressure of the order of that existing between the Bosost and Barrovian areas, i.e. a difference of some 3 kbar in the confining pressure. This metamorphic crystallization is responsible for the main schistosity, that is of- ten convolute in the ,,low-pressure unit,,, while being fairly monotonous in the >,intermediate-pressure unit,~.

Field and microscopical evidence point out that another metamorphic episode occurred after the equilibration of the main assemblages and the related formation of the main schistosity.

Such a late metamorphic episode was considerably weaker than the main one. Moreover it affected only small areas along the major faults, the alpine age of which is beyond doubt because they contain sedimentary wedges as young as Norian. It seems therefore a logical conclusion to consider the late metamorphic event also as alpine in age.

The mineralogical evidence of it is given by chloritization of biotites, overall retrograde transforma-

Pre-alpine and alpine evolution 357

tion of staurolites and kyanites (FuMASOLI, 1974) and, above all, recrystallization of the white micas along shear planes or at the margin of large muscovite fla- kes (RM = 0.02; Na/Na + K = 0.16) with produc- tion of small scales of ,,sericite<, of phengitic composition (RM = 0.10; Na/Na + K = 0.07). Such a phengite possesses 3T symmetry, whereas the muscovite lying on the main schistosity is a normal 2Mi type (BoccHIO 1977; BOCCHIO et al., 1980; CRESPI et al., 1980).

Where suitable bulk compositions were available, i.e. in the K- and Si-rich rocks of the quartz-feldspatic group, chloritization of biotites and sericitization of K-feldspars was followed by the growth of stilpnomelane (CREsPI et al., 1981). This mineral occurs as fan-shaped bundles of very fine laths in the blastomi- lonitic matrix surrounding the K-feldspars, in the open cracks of the rocks, and in the quartz-albite veins that recover these cracks. Therefore it was interpreted as post-Insubric in age (CREsPI et al., 1982).

Geochronological results

Following PUV.DY & JXGER (1976), only samples coming from the staurolite, kyanite and sillimanite zones were used for this investigation. In a polyme- tamorphic terrane this is a prerequisite to obtaining meaningful K-Ar mica ages, i.e. ages that are to be interpreted as cooling ages for minerals thoroughly re-equilibrated by the metamorphism and retaining no inherited argon.

However, this prerequisite holds only as far as the main metamorphic zonation of the basement is concerned. As described before, in the above mentioned hlgh-grade zones there is evidence of ,,retrometamorphism<< i.e. of a low-grade overprint probably of alpine age. Thus, all the investigated samples deviate somewhat from the ideal behaviour. Their apparent ages only approach by defect the cooling ages refer- red to as the main metamorphic episode.

This line of interpretation - essentially PURDY & J~GEr.'s line (1976) on the Lepontine dome - will be followed during the discussion to come. We are aware of the current debate on the actual meaning of K-Ar mica ages (CHOPIN & MALUSKI, 1980, 1982; DESMONS et al., 1982), but we will not consider this problem further at the present stage of our work.

Methods

For bulk rock samples (WR) the homogeneous sieved fraction 18 to 35 mesh has been used. For the separated minerals, the fraction 35 to 60 mesh was used. Before the Ar extraction, the WR samples were treated with 5 % diluted HF for 3 minutes under ul-

trasonic, so as to remove all possible clay alteration products and therefore reduce the scatter of the measurements. The analyses were performed by the method of NICOLETTI & PETRUCCIANI (1977) using an A.E.I. mod. MS 10 mass-spectometer for 4~ and I.E. rood. 243 flame photometer for K, with Li as the internal standard. Computation of the age were carried out using the following decay constant (STErnal< & JAGER, 1977): ~.e = 0.581 �9 10 -l~ A- i ; ~.fl = 4.962 �9 10 -l~ A- l ; %40 K = 0.011167 and for the formula: t(m.y.) = 4.183 - 109 �9 loglo

4~ RAD �9 cc STP ] 143.51 �9 + 1 .

g . %K

The errors on the ages were calculated by the Cox-Dalrymple's formula (DALRYMPLZ & LAMPHERE, 1969). Isochrones were calculated both by the Har- per's method (HAY.PER, 1970) and the ,,isotope isochrone method,< (HAYATSU & CARMICHAEL 1970; HAYATSU, 1972). Standard controls performed at the time of the investigation gave the following values: Muscovite P 207 : 80.5 + 1.5 my (recommended in- terlaboratory value 81 + 1 ); Muscovite Bern 4M : 18.3 _+ 0.6 (r.v. 18.7 + 0.5); Biotite LP 6 : 122 + 4 (r.v. 125 +_ 2); Phonolite M2 : 7.5 + 0.7 (r.v. 7.4 + 0.2).

Age results for 7 samples have been published and discussed already (BoccHIO et al., 1981). A further 6 samples is reported here. The discussion will include all the samples, since all the results are completely compatible from any experimental viewpoint. All the data are given in Table 1, where the scanty previous determinations by HANSON et al. (1966) are also given for sake of comparison, but recomputed with the new age constants. Individual ages are reported in the histogramme of Fig. 2.

Mineral Ages

a. Biotites

The eleven measured biotite ages spread over the range 218 to 292 my, plus a single age at 331 my. Within the range there are two maxima at 222 + 4 my and at 263 + 12; the former is rather well defined, the latter not so well, as the standard deviation clearly shows. The older age is essentially in agreement with the sole previous determination (HANSON et al., 1966, sample P 31:239 + 12 my or, after recalculation with the new constants, 245 + 12 my).


Table 1. Analytical data for muscovites, biotites arid whole rocks of the upper Lake Como region.

Sample Mineral 4~ 40ArRAO% K% N a % t • e Locality

DAT-1 BI (Bellano) MS

WR

DAT-2 BI (Corenno) MS

WR

DAT-3 BI (Laghetto) MS

WR

DAT-4 BI (Laghetto) MS

WR

DAT-5 MS (Piona)

DAT-6 BI (Morbegno) MS

WR

BI MS WR

BI MS WR

BI MS WR

BI MS WR

MS WR

BI WR

BI MS WR

MS

DAT-7 (Campo)

DAT-8 (Tremenico)

DAT-9 (Mad. Bondo)

DAT-10 (Fumasi)

DAT- 11 (Fiesso)

DAT-12 (Musso)

DAT-13 (Cos•

P-4 (Olgiasca)

P-10 (Sparese)

P-15 (Roccoli L.)

P-27 (Villatico)

P-30 (M. Viacava)

P-31 (Corenno)

MS

MS

MS

MS

BI

1 . 0 2 7 0 - 1 0 .4 1.1611 �9 10 -4 3.4781 - 10 -s

6 .5700 ' 10 -s 5 .7060. 10 -s 2.2680 - 10 -s

6.4281 10 -s 6.0180 10 -s 3.4640 10 -s

6.5240 10 -s 7.0581 10 -s 3.8804 10 -s

7.9040 10 -s

9.3640 10 -s 1.1576 10 .4 4.3092 10 -s

9.8800 10 -s 1.1120 10 .4 5.9537 10 -s

7.1120 10 -s 5.9980 10 -s 2.7140 10 -s

7.1332 10 -s 5.9333 10 -s 3.9222 10 -s

8.9760 10 -s 6.9590 10 -s 2.2287 10 -s

6.2190 10 -s 2.5499 10 -s

7.5920 10 -s 6.5314 10 -6

7.1678 10 -s 6.8785 10 -s 7.2268 10 -6

7.36 10 -s

7.64 10 -s

7.41 10 -s

6.53 10 -s

7.85 10 .5

6.72 I0 -s

87.73 7.26 n.d. 331 • 10 96.50 8.30 n.d. 330 • 10 91.91 2.85 n.d. 313 • 9

80.30 7.40 n.d. 218 • 5 84.53 7.84 n.d. 180 • 3 91.03 2.65 n.d. 247 • 5

61.85 7.24 n.d. 218 • 6 83.57 7.84 n.d. 190 • 6 91.08 3.18 n.d. 264 • 8

91.92 7.17 n.d. 223 • 5 86.61 7.81 n.d. 221 • 5 91.39 4.24 n.d. 224 • 5

78.13 12.58 n.d. 228 • 6

86.52 8.18 n.d. 276 • 8 89.83 8.30 n.d. 330 • 8 93.06 5.03 n.d. 210 • 5

90.53 8.17 n.d. 29I • 6 91.36 8.68 n.d. 306 • 7 80.24 4.51 n.d. 315 • 8

84.11 6.38 0.40 269 • 9 93.89 6.14 1.32 238 • 7 89.25 2.54 0.80 259 • 7

98.58 6.88 0.27 252 • 6 99.90 6.28 1.46 231 • 5 99.00 2.27 0.76 402 • 9

95.43 7.37 0.67 292 • 9 94.73 7.02 1.99 242 • 7 67.00 2.39 1.86 228 • 7

92,08 9.I2 1.06 170 • 6 93.10 4.79 2.80 134 • 4

92.13 7.25 0.69 254 • 8 82.90 0.80 1.88 201 • 5

92.80 7.63 0.60 229 • 7 95.65 9.15 0.80 186 • 6 83.66 4.35 2.73 43 • 2

74.25 8.50 n.d. 213 • 10

76.67 8.47 n.d. 211 • 11

79.00 8.11 n.d. 223 • 11

91.50 7.92 n.d. 203 • 10

73.00 8.52 n.d. 225 • 11

94.00 6.66 n.d. 245 + 12


no. �84

2-

..VA 5'O ,t 2(~0

I JURASSIC [ TRIASSIC I PERM[AN

Fig. 2. Compilation of all the radiometric ages measured on biotite (BI), muscovite (MS) and whole rocks (WR) of the

On the average, biotite ages of samples undoubtably belonging to the ~>intermediate pressure unit<, are older (298 + 24 my) than those of the ,low-pressure unit~, (240 + 20 my).

The only quartzo-feldspatic rock containing biotite (DAT-13) gives a young age of 229 my, although it comes from the >,intermediate pressure unit~,. This sample shows a late low-temperature overprint (stilpnomelane present, CRESPI et al., 1982 point d), so that such a young age can be interpreted as due to loss of argon.

Following the means of interpretation described above, and therefore assuming that the true cooling age for biotites of each series is that of the sample ha- ving the measured maximal age for each series, the ,>low-pressure unit,~ may be believed to have cooled down to the closure (blocking) temperature (300 ~ + 50 ~ C : PURDY & J~io~R, 1976; D~SMONS et al., 1982) before 269 + 9 my ago; the >,intermediate pressure unit,, before 331 + 10 my ago.

b. Muscovites

The available ages on muscovites are 17, including five of the pegmatites of the Piona swarm (HANSON et al., 1966). Their distribution is essentially bimodal, as the biotite ages were, but shifted toward lower values. In fact, the maxima lie at 188 + 8 my and 224 + 8 my. Furthermore two ages as old as 330 my have also been measured. The age difference between muscovites and biotites in the two units is essentially the same (34-38 my), a feature that suggests a cohe- rent behaviour of the two mica systems.

Since the closure (blocking) temperature of muscovite is higher than that of biotite (500 ~ + 50 ~ C: PURDV & J;~G~t~, 1976), a normal process of cooling during the uplift of the basement cannot explain the observed feature. It is clear that the youngest muscovite ages are measured in samples showing the largest impact of the low-temperature metamorphic event,

iiii!i!iil BL IrJIlllr Ms

WR

300 400' m.y. CARBONIFEROUS DEVONIAN[

basement of the upper Lake Como and western Orobic Alps. The time scale at the bottom is Odin's (1982) scale.

either in the appearance of stilpnomelane or of fine- grained phengite. It is also clear that the youngest ages occur close to the Musso Line, of alpine age. We interprete therefore the muscovite ages as ,~mixed ages,,. In the ,,intermediate pressure unit~ the likely true age of muscovite should be 330 my, thus in good agreement with the biotite age; in the ~,low-pressure unit<, the true age should be at least as old as 238 my (DAT-8).

The formation of the alpine phengite lowered those ages down to 170-186 my in the former unit and 180-190 my in the latter one, thus indicating a rather homogeneous regional distribution of the low-temperature imprint. According to petrogra- phical observations the amount of newly formed phengite is little, so that the drastical reduction of the ages is a further indication that the newly formed mica is very young, alpine or even postalpine (as suggested by CRESPI et al., 1981).

c. Correlation among apparent mineral ages

The standard model of interpretation of K-At ages assumes that the apparent ages of different minerals are not the true ages of their metamorphic equilibration but that they are rejuvenated by selective loss of 4~ due to the different diffusion properties of each mineral. The apparent ages of otherwise coexisting minerals should decrease according to the following experimentally determined pattern tMS>tm>(tKF)>tWR (JAGER & HUNZIKER, 1979).

This is not the case in our samples. Not only whole rock apparent ages are scattered (see below), thus suggesting intervening effects of minerals other than the main phases that equilibrated at the peak of metamorphism, but the ages of the two micas, i.e. of the two most abundant constituents of the examined metamorphic rocks, show diverging behaviours. A group of three samples have tMs>tBu thus behaving as expected; another group of 7 samples have tMs<t m


(Fig. 3). The first group is homogeneous in that it contains only samples coming from the ~,intermediate-pressure unit,,; the second contains all samples from the ~,low-pressure unit~,, plus one sample of the ,~intermediate-pressure unit,,. Tha latter group can be further split into two: one homogeneously com- posed of samples from the ,,low-pressure unit,, and a second, containing samples of both units, indeed very strongly rejuvenated particularly in their muscovite ages. These samples are very close to tectonic lines, and indeed contain either new phengite, or stilpnomelane, or both.

We may therefore generalize that the apparent contrasting behaviour of the two units is due to their different responses to the process of argon diffusion: the ~intermediate-pressure unit,, follows the ~normal,, thermal diffusion process resulting in the ~normal,, sequence of cooling ages; the ,>low-pressure unit~, is ,~abnormal% possibly because of argon occlusion due to a yet unknown metamorphic pro- CCSS.

For some samples we may now assume this process to be the late-alpine overprint observed in thin

section, but for others a second stage of metamorphism older than the alpine overprint can also be conceived of.

S a m p l e a g e s

a. Whole rock ages

The 12 WR ages show the widest scatter of all the present results: one order of magnitude from 43 to 402 my! Furthermore, there is no significant cluster of the ages, either with record to their regional distribution or to their values. It appears to be significant that the oldest ages have been determined for samples of the ,Antermediate-pressure unit<, (313-315 my) and the youngest ones of the >,low-pressure unit,, (210-228 my).

Nevertheless, these clusters are contradicted by the two extreme measured values, the youngest (43 my) of which occurs in rocks belonging to the ~,intermediate-pressure unit,, : the oldest (402 my) in the ,,low-pressure unit,,.

We believe, therefore, that WR ages are better in- dicators than mineral ages of the complex interplay

m.y.

O) > 0 o

E

~ , o 6 l o )

\ \ / . / /

' 7 \ \ O / /

/ /

/ . . . . .

/ ,..'" �9

�9 �9 8

�9

43�9 3[.�9149

!

1so 2oo 2so m. y.

Fig. 3. Correlation between the mineral ages measured on coexisting micas. Symbols: dots: samples from the ,~intermediate pressure unit,,; triangles: samples from the ,,low pressure unit,,; star: sample from the MantelIo Gneiss for-

. - . . . . .

"JOe"-.

b i o t i t e

i

3 0 0 3 5 0

mation. Samples falling above the line follow the ~,normal<~ thermal diffusion process; samples below are ,,abnormal,, and either retain occluded argon or underwent a argon loss, see text for discussion.


of recrystallization and mineral growth experienced by the south-Alpine basement. It is significant, for this line of argument, that the youngest ages have al- ways been determined for samples containing a significant amount of newly formed stilpnomelane, and in areas close to alpine faults.

b. Isochrone ages

Statistically meaningful K-Ar isochrones have been obtained for only 4 samples: two of the ,finter- mediate-pressure unit~ and two of the ,,low-pressure unit,, (Table 2).

A K-Ar isochrone age is believed to reflect the moment when all the phases were in isotopic equili- brium independently of the diffusions or occlusions of the 4~ D. In a metamorphic rock, such a isochrone age reflects a significant phase during the metamorphic cycle. Calculating isochrone ages by the Harper's method (HARPER, 1970), i.e. by plotting %K vs 4~ as it is intrinsic in K-Ar geochronology, is believed to date the final stage of the metamorphic event. In contrast, calculating the isochrone ages by the so-called ,fisotope isochrone method,, (HAYATSU & CARMICHAEL, 1970; HAYATSU, 1972), i.e. by plotting 4~ vs 4~ is believed to date the initial stage of the same event. Both methods, however, are unable to date precisely the start and end points of the metamorphic event.

Isochrone ages for the ,,low-pressure unit,, samples range from 219 to 245 my, whereas those for the ,,intermediate-pressure unit,, range from 283 to 368 my. The isotope isochrone method gives values in average about 30 my older than Harper's method, thus giving support to the above mentioned interpretation, but clearly dividing the rocks examined into two suites that underwent different episodes of metamorphism.

Table 2. Statistically significant isochrone ages

Discussion and Conclusion

The radiometric age determinations, if interpreted following the standard model, lead to the conclusion that two distinct metamorphic events affected the south-Alpine basement in the upper Lake Como region. The metamorphic rocks containing widespread kyanite (~,intermediate-pressure unit,,) would have been metamorphosed very early during the Variscan cycle (possibly even during a ,,Caledoniam~ cycle, BoccHIo et al., 1981), but undoubtably their evolution was finished before the emplacement of the late Hercynian Val Biandino pluton (DE CAPITANI, 1982) and the deposition of the Westphalian molasse. Rocks that are kyanite-free (,,low-pressure unit~,) might in fact have undergone the same metamorphism as the previous one, but certainly underwent a later crystallization that obliterated the previous assemblages and structures as well as resetting the K-At clock. Such a metamorphic event would have started, according to the measured radiometric ages, during the Permian and have lasted at least until mid Triassic (or even Jurassic).

There are no geological contraints precluding the former part of the inferred evolution: on the contra- ry, the possibility of a Mesozoic phase of regional metamorphism, as might be suggested for the ~dow- pressure unit,,, is absolutely to be ruled out from purely geological considerations.

The south-Alpine sedimentary pile on top of the basement is classical and well studied, initially from the stratigraphical and sedimentological viewpoints (e.g. DE SITTER & D~ SITTER KOOMANS, 1949; ASSE- ~ETO & CASAT% 1965; CASATI & GNACCOLINI, 1967) but, lately, also from the structural standpoint (e.g. Ross% 1975; DE JONG, 1979; GA~TANI & JADOUL, 1979). The sequence appears to have evolved conti- nuously, without major breaks although never enti-

HARPER'S ISOCHRONE ISOTOPE ISOCHRONE

4~ + 8 t _+ e r (4~ t + e r

a) ,,low-pressure unit,,

DAT-4 9.29 �9 10 -7 + 3.25 10 -7 219 + 1 0.999 156 + 20 245 + 1 0.999 DAT-8 n.s. 617 + 157 226 + 5 1.000

b) ,,intermediate-pressure unit,~

DAT-1 2.94 �9 10 -6 4- 4.7 10 - 6 338 + 2 0.999 -5 + 38 368 + 2 0.999 DAT-7 5.99 - 10 -6 + 5.95 10 -6 283 _+ 21 0.990 398 + 127 312 + 15 0.999


rely quiet, from the deposition of the Westphalian molasse in a subaerial environment up to the alpine phase of the folding and thrusting, that began probably during Santonian but reached its climax during Eocene and Oligocene (Pyrenean phase: CASATI, 1969).

Evidences of the involvement of the basement in any one of the sedimentary stages is lacking from Westphalian (blocks of metamorphic rocks in the molasse deposits: MELZI, 1891; VZNZO & MAGLIA, 1947; CASATI & GNACCOLINI, 1967) to at least lower Santonian (pebbles of crystalline rocks in the ,,Con- glomerates of Sirone~, a member of the Sarnico Sandstone formation: RossI, 1975; GELA~ & CASCO- NE, 1980).

During such a long time-span (215 my: ODIN, 1982), the basement remained buried under a pile of sediments the maximum thickness of which is difficult to estimate, in view of the presence of at least four N-S oriented transversal troughs developed at different times across the Orobic Prealps (see e.g. DE SITTER & DE SITTER KOOMANS, 1949). However there are indications suggesting a total thickness of more than ten thousand meters of sedimentary rocks (plus overlying water) before the end of the subsidence (Ross~, pets. com.).

Such a thickness is enough to justify the widespread occurrence of burial metamorphism near the base of the sedimentary pile, particularly in rocks of suitable bulk composition and grain-size (,,poffiroidi di Branzi,, : see MOTTANA in NIGGLI, 1978, p. 226, and also MOTTANA & SCHIAVINATO, 1973). However, if we consider the sedimentary pile from Westphalian to middle Triassic (the time of the metamorphic acme, see Fig. 2), the thickness reduces to 2.5-3 km (DE S~TTER & DE SITTER KOOMANS, 1949).

Such a thickness would still be enough to justify the pressure acting on the basement as for the ,,low pressure,, unit metamorphism is concerned. In fact, if we bring back this unit to its original flat lying position as marked by the isograds, and add on top the above-mentioned sedimentary pile, we obtain a total of 8-9 km, i.e. a pressure of 3-3.5 kbar. This is the pressure that we can deduce from the observed assemblages, in particular from the occurrence of sillimanite. The corresponding minimum temperatures would be 510-550~ (HoLDAWAY, 1971), and the calculated geothermal gradient well within the accepted values for a low pressure facies series.

This model, however, suffers a serious drawback: since sedimentation continued undisturbed until lower Santonian (see above), the basement also should have continued in the way down. Therefore, the K-Ar clock should mark a much younger time, even

younger than the age of the subsidence itself: in fact, a cooling age marks the time when the basement crosses the 300~ isotherm in its w a y u p, not in its way down. Thus a pure isostatic model does not fit the geological evidence, nor it explains the observed middle Triassic ages.

The only alternative model left seems to be that the 220 my age marks a crystallisation (or recrystallisa- tion) event due to a localized temperature increase from below, i.e. a high heat flow related to factors such as crustal thinning or Mantle upwelling. During such a heating from below, not only were the metamorphic rocks (re)crystallized, but the mobilisation of crustal material produced injection of the pegmatites into the basement, as well as the widespread, although limited as mass, explosive volcanism of rhyo- litic composition that can be found in the form of re- worked tuffites interbedded within the marine Trias- sic sediments (PAsQUARI2 Ni; ROSSI, 1969; CRIscI et al., 1984).

In a way, this model revives the hypothesis of FER- RARA & INNOCENTI (1974) on the formation of the pegmatite swarm. However, rather than arguing for a direct connection with the break-up of the Eurasian Plate and the opening of the Atlantic Ocean, we sim- ply note the localized phenomenon as possibly related to Mantle upwelling, either as a result of crustal erosion from underneath, or of crustal stretching.

The present model, although it requires a deep- seated mechanism, appears to be simpler and easier to support on geological evidence than our previous model (BoccHIo et al., 1981), based only on geochronological data. At that time, we suggested that a generalized argon loss (due to some unexplained rea- son) could satisfy the observed unrealistic young ages of the ,,low pressure unit~. We supported our suggestion in two ways:

a) the K-At age reported for the muscovite of the Olgiasca pegmatite (P-4: HANSON et al., 1966) is si- gnificantly younger (207 my) than the Rb-Sr age of the same mineral (286 my). Thus the true ages of a 11 the muscovites should be pulled back by some 80 my to fit the -300 my accepted age of the main Hercy- nian metamorphic event.

b) By accepting this, the ages measured in the ,,intermediate pressure unit~ should also be shifted back by some 80 my. This was considered to be acceptable, because they would then fit the -430 my age re- ferred to above and deduced from the equilibration of zircons in the same area (Gt~AUZRT et al., 1973).

Consequently we postulated a model of two sepa- rate metamorphic events for the Orobic Sector: one, 430 my old, ~,Caledoniam~, and a second, -300 my old, typically Hercynian. We found it acceptable not


only on a purely geochronological basis, but also because such ages would make the metamorphic evolution of the Orobic Sector consistent with that observed by several authors in other areas of the prealpine basement: an old event followed by a younger overprint under static low-pressure conditions.

A two-phase model of metamorphism has been as- sumed for the South-Alpine basement of the Eastern Alps by D L MoRo et al., (1980). However, they conceive this two-phase metamorphic evolution as entirely confined to the Hercynian metamorphic cycle, the first phase being Visean (350 + 3 my), and the second Westphalian (317 + 16 my). The type of metamorphism is also different than that encoun- tered in the Orobic Alps, since the Hercynian metamorphism in the Easter Alps developed only within ~:he greenschist facies conditions.

Our new model, although still open to criticism, fits better both the geological evidence as well as some preliminary data obtained by the Rb-Sr me- n,hod: they imply that the main metamorphism of the kyanite-bearing unit is indeed Hercynian (303 my: W. L. GRIFFIN, pers. communication) while the (re)crystallization of micas in the kyanite-free unit is indeed Triassic (217 my: ibidem).

Moreover, some of the still-remaining conjectural aspects of this new model can find their explanation if one takes into account the overprint due to the newly discovered alpine metamorphism (CREsPI et al., 1981).

Although weak, such an overprint is undoubtabIy responsible for most of the anomalous WR ages, as well as for certain low mineral ages (,,mixed ages,,) turned out in spite of our careful selection of samples coming only from localities within the staurolite isograde and apparently free from retrometamorphism. .Argon loss (or argon occlusion) is an obvious expla- :nation of several of the observed features, particularly for samples near the Insubric line and other alpine faults.

Concluding, the radiometric data available so far suggest the following metamorphic evolution for the upper Lake Como area:

1. Formation of the sedimentary precursors during low Paleozoic, but with incorporation of material crystallized as early as during Precambrian times. This is conclusively established by the 2050 my intersection of the concordia determined by GRAUERT et al., (1973) on the Sueglio zircons.

2. Main phase of regional metamorphism under an intermediate-pressure regime (confining pressure at least 5 kbar) prograde up to maximum temperatures of 650~176 (sillimanite-in isograde). The age of such a main phase of metamorphism ranges any-

where from -430 my (lower intersection of the above-mentioned concordia, GRAU~RT et al., 1973) down to -350 my (K-Ar isotope isochrone age). The latter however is more likely.

The metamorphism lasted at least 30-50 my (K-Ar Harper 's isochrone age). The uplift that followed brought the investigated area rapidly at temperatures below the 500 ~ isotherm (Rb/Sr and K-Ar blocking temperature for white mica) as well as the 300 ~ isotherm (Rb/Sr and K-Ar blocking temperature for biotite) since they overlap at 303 my (W. L. Gt~IFFIN, pers. comm.). However this implies argon occlusion in muscovite and biotite since the corresponding K-Ar ages are 330 my.

3. Metamorphic overprint in selected areas, in particular in the Dervio-Olgiasca zone (,,low-pressure unit,,) with complete recrystallization of micas while high-grade metamorphic minerals such as siltimanite remained stable. The K-Ar and Rb-Sr radiometric measurements are concordant in dating this event at

220 my, an age that should probably be considered the age of the maximal thinning of this portion of the Orobic basement due to lateral stretching of the crust or to mantle upwelling. The event occurred after the basement had been uplifted and eroded at the end of the Hercynian orogenesis, and while other blocks remained under a more shallow cover and their rock-forming micas could not re-equilibrate.

We may ascribe to this event, which involves re- mobilizations of chemicals under the leaching effects of H20 freed by partial dehydration reactions, the features typical of the sillimanite zone rocks of the area between Olgiasca and Dervio, such as: the widespread pegmatite veins, the growth of a second gene- ration of sillimanite (poikiloblastic around fibrolite: BoccHIo et al., 1980, figs. 3-4), the formation of cross-cutting quartz veins containing andalusite (Rz- POSSI, 1910; EL TAHLAWI, 1965) and cordierite (MOT- TANA et al., 1983).

The following stages of the evolution of the basement are still conjectural in view of the limited wealth of data available. We may tentatively envisage the following:

4. Lateral movements of slabs of the basement along alpine faults, bringing into sharp contact por- tions affected and unaffected by the low pressure (re)crystallization. These faults are alpine (they contain Norian dolomite) and are probably related to the formation of the Insubric line. Their movements are accompained by a widespread low-temperature overprint on all the rocks, particularly along the Val- tellina foothills, where ,,mixed ages~, varying from 134 to 43 my have been determined. It is noteworthy


that the youngest age occurs the closest to the Insu- bric line.

5. Uplif t and final depressuration, rapidly, along the Insubric line, with opening of cracks permit t ing the free growth of st i lpnomelane, fol lowed at places by deposi t ion of quartz and albite (CRzs~I et al., 1981, 1982). This final stage of depressuration marks the emplacement of the basement at, or near, the present level, possibly around 15 my ago, if the results of WAGNER et al., 1977, and PURDV & J•GER, 1976 in the nearby Ticino area can also be extended to the present area.

Dating of these stages shall be the main aim of our future research, while accumulating other data on all the area may produce a better understanding of the overall history.

Acknowledgements

Work supported by CNR >,Centro di Studio per la Stra- tigrafia e Petrografia delle Alpi Centrali<, (Milano) and >>Centro di Studio per la Geocronologia e Geochimica delle Formazioni recenti,< (Roma). Thanks are due to A. Boriani, P. M. Rossi and R. Crespi (Milano) for detailed discus- sions. Our special thank to Prof. W. L. Griffin (Oslo) for carrying out Rb/Sr determinations, discussing repeatedly the entire problem and critically reading the manuscript. Critical reading by A. Zingg (Basel) and M. A. Carpenter (Cambridge) is also acknowledged.

Appendix

DAT 8 (along the road Dervio-Premana near Tremenico, 32TNS27940270)

Fine-grained gneiss showing rare porphyroblasts of staurolite and quartz. Modal composition: quartz (51.8), biotite (23.0), muscovite (16.0), staurolite (4.9), chlorite (2.2), plagioclase (1.2), opaques (0.7), garnet (0.2). Trace amount of tourmaline has been also detected.

DAT 9 (foot-path to Madonna di Bondo, 32TNS25500535)

Fine-grained gneiss with veins of quartz alternating with micaceous bands. Euhedral garnets and porphyroblasts of staurolite are elongated along the schistosity planes. Modal composition: quartz (45.3), biotite (28.7), muscovite (19.6), staurolite (3.2), garnet (1.5), plagioclase (0.8), opaques (0.8).

DAT 10 (south of Bema, Stalle dei Fumasi, 32TNS43540488)

Fine-grained gneiss (~Morbegno Gneiss~<) with porphyroblasts of staurolite and poikiloblastic plagioclase. Modal composition: biotite (30.2), quartz (29.3), plagioclase (21.6), staurolite (7.7), muscovite (6.6), garnet (4.0), opaques (0.6).

DAT 11 (Fiesso, 32TNS38971148) Fine-grained gneiss with porphyroblasts of perthitic K-

feldspar (49.6). Other components: quartz (27.9), plagio-

Table 3. X-ray determinative data for the radiometrically dated micas

Biotites Sample b �9 doo2(A ) Io04/Ioos (I004 + Ioo6)/Ioos

DAT-8 9.248 10.078 1.27 1.31 DAT-9 9.245 10.083 1.14 1.51 DAT-10 9.241 10.081 0.92 1.33 DAT-12 9.259 10.077 1.25 1.82 DAT-13 9.281 10.078 1.85 2.77

Muscovites Sample b�9 doo2(lk) Polytype

DAT-8 8.985 9.918 2M DAT-9 8.976 9.915 2M DAT-10 8.995 9.928 2M DAT-11 9.019 9.970 2M

9.972 DAT-13 9.028 2M

9.949

Preoalpine and alpine evolution 365

clase (15.2), muscovite (5.6), biotite (1.4), opaques (0.2), apatit e (0.1).

DAT 12 (quarry of Musso, 32TNS21000750) Amphibolite with nematoblastic amphibole and with

scales of biotite parallel to the lineation. Modal composition: amphibole (68.6), biotite (8.3), quartz (12.9), sphene (4.8), plagioclase (2.9), opaques (1.2), apatite (1.0), calcite (0.3).

DAT 13 (Cosio, 32TNS40530902) Fine-grained gneiss containing quartz (48.7), plagioclase

(32.0), K-feldspar (6.5), muscovite (9.0), biotite (3.7). Rare needles of stilpnomelane are also present.

N.B. Location and mineral data of samples DAT 1-7 are given as appendix in BOCCHIO et ah (1981).

References

ASSERETO, R. & CASATI, P. (1965): Revisione della stratigrafia permotriassica della Val Camonica meridionale (Lombardia). - Riv. It. Pal. Strat., 71, 990-1097.

BOCCHIO, R. (1977): 3T muscovite from a staurolite-zone south-Alpine gneiss, Cermeledo, Italy.-Min. Mag., 41, 400-402.

- , CRESH, R., LIBORIO, G. & MOTTANA, A. (1980): Va- riazioni composizionali delle miche chiare nel metamorfismo progrado degli scisti sudalpini dell'alto Lago di Como. - Mem. Sci. Geol., Padova, 34, 153-176.

- , DE CAPITANI, L., LIBORIO, G., MOTTANA, A., NICO- LETTI, M. & PETRUCCIANI, C. (1981): K-Ar radiometric age determinations of the south-Alpine metamorphic complex, western Orobic Alps (Italy). - N. Jb. Miner. Mh., 7, 289-307.

BORIANI, A., DAL PIAZ, G. V., HUNZIKER, J. C., VON RAUMER, J. & SASSI, F. P. (1974): Caratteri, distribu- zione ed eta del metamorfismo prealpino nelle Alpi. - Mem. Soc. Geol. It., 13, 165-225.

CASATI, P. (1969): Le fasi orogenetiche nelle Alpi Meridio- hall. - Arti grafiche Artigianelli Pavoniani, Monza, 30

PP. - & GNACCOLINI, M. (1967): Geologia delle Alpi Orobie

occidentali. - Riv. It. Paleont., 73 (1), 25-144. CHOPIN, C. & MALUSKI, H. (1980): 4~ dating of

high pressure metamorphic micas from the Gran Para- diso area (Western Alps): evidence against the blocking temperature concept. - Contrib. Mineral. Petrol., 74, 109-122.

-- & - (1982): Unconvincing evidence against the blocking temperature concept? A reply. Contrib. Mineral. Pe- trol., 80, 391-394.

CRESPI, R., LIBORIO, G. & MOTTANA, A. (1980): L'iso- grada della sillimanite negli gneiss di Morbegno della bassa Valtellina (Complesso Sudalpino, Alpi Centrali, Italia). Mem. Sci. Geol., Padova, 34, 247-272.

- , - & - (1981): Metamorfismo tardo-alpino di grado bas- sissimo nel basamento a sud della Linea Insubrica. - Rend. Soc. It. Min. Petr., 37 (2), 813-824.

--, - & - (1982): On a widespread occurrence of stilpnomelane to the South of the Insubric line, Central Alps, Italy. - N. Jb. Miner. Mh., 6, 265-271.

CRISCI, C. M., FERRARA, G., MAZZUOLI, R. & ROSSI, P. M. (1984): Geochemical and geochronological data on Triassic volcanism of the southern Alps of Lombardy (Italy): genetic implications. - Geol. Rundschau, 73, 279-292, Stuttgart.

DALRYMPLE, G. B. & LAMPHERE, M. A. (1969): Potas- sium-Argon dating: principles, techniques and applica- tion to geochronology. W. H. Freeman, 258 pp.

DE CAPITANI, L. (1982): Contributo alla conoscenza dei plutoni sudalpini: Le masse intrusive della Val Biandino (Como). - Rend. Soc. It. Min. Petr., 38 (1), 109-118.

DE JONG, K. A. (1979): Overthrusts in the Central Berga- masc Alps, Italy. Geol. Mijnbouw, 58,277-288.

DE SITTER, L. U. & DE SITTER KOOMANS, C. M. (1949): Geology of the Bergamasc Atps, Lombardia, Italy. - Leid. Geol. Meded., 14, 1-250.

DEL MORO, A., SASSi, F. P. & ZIRPOLI, G. (1980): Prelimi- nary results on the radiometric age of the Hercynian metamorphism in the South-Alpine basement of the Eastern Alps. - N. Jb. Geol. Palaont. Mh., 1980, H. 12, 707-718.

DESMONS, J., HUNZIKER, J. C. & DELALOYE, M. (1982): Unconvincing evidence against the blocking temperature concept. - Contrib. Mineral. Petrol. 80, 386-390.

EL TAHLAWI M. R. (1965): Geologic und Petrographie der nord6stlichen Comerseegebietes (Prov. Como, Ita- lien). - Mitt. Geol. Inst. E.T.H. & Univ. Zfirich, N.F., 27, 199 pp.

FERRARA, G. & INNOCENTI, F. (1974): Radiometric age evidence of a triassic thermal event in the Southern Alps . - Geol. Rundschau, 63, 572-581, Stuttgart.

FUMASOLI, M. W. (1974): Geologie des Gebietes n6rdlich und siidlich der Jorio-Tonale-Linie in western von Gravedona (Como, Italia). - Mitt. Geol. Inst. E.T.H. & Univ. Ziirich, V.N.F., 194, 230 pp.

GAETANI, M. & JADOUL, F. (1979): The structure of the Bergamasc Alps. - Rend. Acc. Naz. Lincei, ser. 8, 66 (5), 411-416.

GELATI, R. & CASCONE, A. (1980): Le successioni terri- gene cretaciche della Bergamasca. Rend. Soc. Geol. It., 3, 39-40.

GRAUERT, B., HKNNY, R. & SOPTRAJANOVA, G. (1973): Age and origin of detrital zircon from the Pre-Permian basements of the Bohemian Massif and the Alps. - Con- trib. Mineral. Petrol., 40, 105-130.

GUIDOTTI, C. V. & SASSI, F. P. (1976): Muscovite as a pe- trogenetic indicator mineral in pelitic schists. N. Jb. Miner. Abh., 127, 97-141.

HANSON, G. N., EL TAHLAWI, M. R. & WEBER, W. (1966): K-Ar and Rb-Sr ages of pegmatites in the South Central Alps. - Earth Plan. Sci. Lett., 1,407-413.


HARPER, C. T. (1970): Graphical solutions to the problem of radiogenic 4~ loss from metamorphic minerals. -Eclogae Geol. Helv., 63 (1), 119-140.

HAYATSU, A. (1972): On the basic assumptions in K-At dating method. Comments on Earth Sciences. - Geophy- sics, 3, 69 pp.

- & CARMICHAEL, C. M. (1970): K-Ar isochron method and initial argon ratios. - Earth Plan. Sci. Lett., 8, 71-76.

HOLDAWAY, M. J. (1971): Stability of andalusite and the aluminum silicate phase diagram. - Amer. Journ. Sci., 271, 97-131.

J;~GER, E. & HUNZIKER, J. C. (Eds.) (1979): Lectures in isotope geology. - Springer-Verlag, Berlin, 312 pp.

Mc DOWELL, F. W. (1970): Potassium-Argon ages from the Ceneri Zone of the southern Swiss Alps. - Contrib. Miner. Petrol., 28, 165-182.

MELZI, G. (1891): Ricerche microscopiche sulle rocce del versante Valtellinese della Catena Orobica occidentale. - Giorn. Min. Crist. Petr. 2, 1-34.

MOTTANA, A. & SCHIAVINATO, O. (1973): Metamorfismo regionaie e di contatto nel settore nord-occidentale del Massiccio dell 'Adamello.- Mem. Ist. Geol. Min. Univ. Padova, 29, 71 pp.

- , FusI, A., BIANCHI POTENZA, B., CRESPI, R. & LIBO- RIO, G. (1983): Hydrocarbon-bearing cordierite from the Dervio-Colico road tunnel (Como, Italy). - N. Jb. Miner. Abh., 148, 181-199.

NICOLETTI, M. & PETRUCCIANI, C. (1977): I1 metodo K-Ar: modifiche metodologiche al processo di estra- zione dell'argon. - Rend. Soc. It. Miner. Petr., 34, 549-557.

NIGGLI, E. (1978): Metamorphic maps of the Alps. 1 : 1.000.000-Explanatory text. Subcomm. for Cartogr. of the Metam. Belts of the World, Leiden, Unesco, Pa- ris, 181-242.

ODIN, G. S. (1982): The phanerozoic time scale revisited. - Episodes, 3, 3-9.

PASQUARE', G. & ROSSI, P. M. (1969): Stratigrafia degli orizzonti piroclastici medio-triassici del Gruppo delle Grigne (Prealpi Lombarde). - Riv. Ital. Paleontol. Stra- tigr. 75, 1-87.

PORRO, C. (1903): Carta geologica delle Alpi Bergamasche, scala 1:100.000 con note illustrative. Ditta Artaria di F. Sacchi e F.Edit. Milano.

PURDY, J. W. & J~.GER, E. (1976): K-Ar ages on rock-forming minerals from the Central Alps. - Mem. Ist. Geol. Miner. Univ. Padova, 30, 1-30.

REPOSSI, E. (1910): L'andalusite di Musso (Lago di Como). - Rend. R. Acc. Lincei, 19 (5), 291-295.

RossI, P. M. (1975): Structural and stratigraphical pattern of the Lombardy Southern Alps. - Quaderni de La Ri- cerca Scientifica, Roma, 90, 67-119.

STEIGER, R. H. & J)~GER, E. (1977): Convention on the use of decay constans in geo-and cosmochronology. - Earth Plan. Sci. Letters, 36, 359-362.

VENZO, S. & MAGLIA L. (1947): Lembi carboniferi tras- gressivi sui micascisti alia ,,Fronte sedimentaria sudal- pina~, del Comasco. - Atti Soc. It. Sc. Nat. Museo Civ. St. Nat., Milano, 86, 33-70.

WAGNER, O. A., REIMER, G. M. & JKGER, E. (1977): Cooling ages derived by apatite fission-track mica Rb-Sr and K-Ar dating: the uplift and cooling history of the Central Alps. - Mere. Ist. Geol. Min. Univ. Pa- dova, 30, 1-27.

Documents

Pre-alpine and alpine evolution of the South-alpine basement of the Orobic Alps