lIENDICONTI SoclltA lt41lono dl MlneNllogio e PetrolOfll1l _ 13 (I) - 281-334 (1971)
ROBERTO CoMPAGNONI·, GIORGlO V. DAL PIAZ ••, JOHANNES C. HUNZIKER ..., GUlDO GoSSO·, BRUNO LoMBARDO ••••• PAUL F. Wu..LlAMS •••••
THE SESIA-LANZO ZONE, A SLICE OF CONTINENTAL CRUST WITH ALPINE HIGH PRESSURE-LOW TEMPERATURE ASSEMBLAGES IN THE WESTERN ITALIAN ALPS
1. I NTRODUCTION: THE SESIA-LAI"ZO ZoNE AND THE SUilROUNIHNG UNITS 2. OLDER CONCEPTS ON THE SESIA-LANZO ZoNE 2.1 S~s i a-Lanzo Zon~ 22 D~nt Blanch~ Nap~ 23 Post-m~tamorphic alpin~ magmatism 2.4 Geophysical data 3. MonEilN IDEAS ON THE SESIA-LANZO ZONE 3.1 StruClural outline of the composite Austroalpi n~ nap~ (S~sia-Lanzo Zon~ and Dent B1anch~ Nappe) 32 Th~ Upper El~ment 32.1 Pr~.Alp in e lithology 322 Alpin~ evolution 3.23 Geochronology 33 Th ~ Lower Element 33.1 Eclogitic Micaschist Compl~x 332 (Gneiss Minuti , Complex 333 Gcochronology 33.4 Pr~ .A l pi ne lithology 335 Alpine m~tamorp hi c evolution 33.6 poT conditions during the early·Alp i n~ ~vc nt 3.4 Structural data 35 D~nt Blanche Nappe 4. DISCUSSION
• IstirUfo di Pcrrografia dell'Univenid., Torino . •• Istituto di Geologia ddl'Universiti, Padova . .... Mineraiogisch.Pcrrographisches Institut def Universitiir, Bern . .... Centra di Studio per i Probiemi deU'Orogeno delle Alpi Occidentali, rodno...... Gcologisch en Mineflliogisch Irutitut def Universiteit, Lciden. 282 k. COMPAGNON I, C. v. DAL Pli\Z, J. C. HUNZIKER E "LTRI
4.1 Alpine metamorphism in the Austfoalpine system 4.1.1 Early-Alpine event 4.12 Lepontine event of 38 m.y. 42 Alpine orogenic evolution and geodynamic models 42.1 Distribution of the early-Alpi ne event in the Western Alps 4.2.2 Regional significance of the UPIXT Cretaceous event 4.23 Discussion of tectonic models 42.4 Oligocene magm:nism and timing of continental collision 5. COXCLUSIO~ R£F£1I.F.NCIS
The present paper summarizes the literature on the Sesia-Lanzo Zone and Dent Blanche Nappc: produced till 1975; it evolved from Internal Report 1 (1974), a collection of factual data previously dispersed in a literature span ning over one hundred years. We have tried to give a unified presentation of the relevant
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Fig. I. - Ttcfonjc s.o\:ctc'" mop 01 f"'(! Alps. Stippl...!: Pennine Nappes; EW == Engadine Window ; TW = Tauern Window. CA :: CanavcM: Line; TO = Tonale Line; GI = Giudiuria Linc; PU = PU51eria Line; GA = Gail!al Linc:; SV:: Sc,$tri.vol13ggio Linc; EN = Engadinc: Line. literature; tectonic models of the Sesia-Lanzo Zone metamorphism presented in the last five years are also discussed. A preprint version of this paper (Internal Report 2, 1975) has been distributed to the participants to the Genova meeting (Sept. 24-29, 1976) of the Societa ltaliana cli Mineralogia e Petrologi::a. THE S ESI/I.-LANZO ZONE, A SLICE 0 1' CONTINENTAL CRUST ECC. 283
1. Introduction: the Sesia-Lanzo Zone anti the surrounding units
The Sesia-Lanzo Zone is one of the main structural units of the internal Western Alps C). The Sesia-Lanzo Zone, in plan view, is an elongated body (25 X 90 Km), trending NE (rom the river Stura di Lanzo, near Turin, to the Ossola valley, where it narrows, and finally terminates near Locarno (fig. 1 and 2). It comprises poly metamorphic and some monometamorphic rocks and is pan of the AuStfoalpine continental crust. It is characterized mainly by widespread jadeitic pyroxene-garnet-quartz assemblages in paraschists and metagranitoids (DAL PIAZ et al., 1972, 1973; CoMPAGNONI and MAFF".Q, 1973; H UNZIKER, 197'1). The internal side of the Sesia-La nzo Zone is marked by the Canavese tectonic line which di vi des it fro m the narrow Canavese Zone and from the Ivrea Zone. The Canavese Line consists of a set of fau lts which may have been active during pre Alpine ti mes, and certainly during Upper Cretaceous (see the internal plane of the early-Alpine subduction zone, DAL PIAZ et al.. 1972). It is interpreted as the suture of the narrow Mesozoic intraconti nental Canavese basin. After the begi nning of the Oligocene period, a new Canavesc line formed under conditions involving no crustal shortening or folding. This sharply divides the rising Alpine belt and Lcpontine thermal dome from the Southern Alps. The Ivrea Zone or Zona Dioritico.Kinzigitica Ivrea-Verbano ( F RANCHI , 19(5). comprises rocks of dee p continental crustal origin that grade eastward into the Strona-Ceneri Zone. T ogether, they represent the crystalline basement of the Southern Alps (Insubric plate). This basement consists of high.grade metamorphic rocks intruded by Permian granites (e. g. Baveno Granite) and overlain by Permian rhyolites and Mesozoic sediments. The ages of biotites from the Inea basement range from 240 to 140 m.y. and are interpreted as cooling ages of the H ercynian metamorphism, which overprints an earl ier Ordovician-Silurian metamorphism (GRAESER and H UNZIKER, 1968; H UNZIKER, 197'1). The southern part of the Scsia-La nzo Zone is flanked to the southeast by the Lanzo Lherzolite whose contact is marked by partly recrystallized mylonites. The Lanzo massif is considered to be a fragment of subcontinental mantle of the Southern Alps (NICOUS. 1974). The external side of the Sesia·Lanzo Zone overrides the Piemonte Zone (or Piemonte nappe), also known as the Mesozoic complex of 5chisus Irutrh with metaophiolites (c: Caiusciui con Pj~tr~ V ~dj :. of the Ital ian literature). The Pie· monte nappe is composite, being built up of several sheets and is overthrust Onto the Penninic continental crust which fo rms part of Brianyonnais Zone and a nappe comprising the Monte Rosa, Gran Paradiso, Dora Mai ra and Valosio units. The Piemonte Zone consists of two composite teetonic units, which are distinguished on the basis of their lithologies, paleogeographic position, meta morphic
('l The terms inlernal and ex ternal are referred to the concave and convex side of the Alpine arcuate belt . 284 R. COMPAGNONI, G. V. OAr. PIAZ, J. C. HUNZIKER E ALTRI
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association and tectonic evolution. Both units originated during Jurassic time in the oceanic basin between the European and the Insubric plate (fig. 3). The units of external provenance (Western Piemonte Zone or Combin Zone s.l.) consist of rare Permian, Triassic sequences, Liassic schistose marbles and a heterogeneous complex of ophiolite-bearing calcschists. A unique feature of the Combin Zone s.l. is the regularly repeated interbedding of calcschists and prasinites C) . Rare wedges :llldjor olistolithes of metagabbros and serpentinites are also present in the Combin Zone d .. Metacherts (metaradiolarites) arc frequently
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Fig. 3. - Hypat/ul;("1I1 pal~ogrog'lIphy 01 th~ C~ntrlllll"d Wul .... " A/pI d"';"K IlIu ,"'IIU;C. I) Continental crust of the European (HO, SbB, Bf>, VB and COZ) and Insubric Plate (DBN, MA, SLZ, CaB, E, Southern Alps). 2) Oceanic crust. 3) Internal b<.>undary of the early.Alpine metamorphism. SbB = Subbriano;onnais basin. VB = Valais basin (probably non·oceanic). BP = Brian~onnai. platf<>rm. Pll = Piemomc basin; COZ = Combin Zone, non·oceanic basin; ZSZ = Zermatt·Saas Zone, oceanic basin. l.Z = Ligurian Zone; W = Western Ligurian Zone (o«anic); E = Eastern Ligurian Zone (non· oceanic); Call = CanavcSC' basin. SLZ = S"ia-Lan7- (2) In the French and Italian literature the term « prasinite * (see NOVARESE, 189.5, for a discussion) corresponds to chlorite-actinolite~idote.albite metabasites (sometimes quartz.phengite carbonate-bearing) derived from submarine basaltic flows and tuffs. 286 R. COMPACNONI, C. V. DAl PIAZ, J. C. HUNZTKEIl £. ALTRI oceanic crust, as they have been deposited over the thinned continental crust of the Penninic margin (European plate) which was the basement of the western side of the Piemonte basin. The units of internal provenance (Eastern Piemonte Zone or Zermau-Saas Zone 5.1.) show, on the contrary, characteristics of an oceanic crust, although the original basi n was probably narrow (R ed Sea T ype?). They consist mostly of metaophiolites, with large slabs of mantle ultramafics (up to 2-3 Km thick), gabbros (sometimes layered with cumulus textures) and submarine basalts. The emplacement of gabbros and basalts is probably Middle (?) to Upper Jurassic. The oMesozoic sediments consist of pure and impure limestones, gray wackes and cherts ; the latter are sometimes Mn-rich. They overlie the basaltic Rows and may be intercalated with them. eu-Fe sul phide ores may also occu r. The whole Piemonte basin has been completely tectonized during the Alpine collision between the European and the Insubric plates. It has been subducted and then partly thrust towards the external si de of the belt as tectonic slabs (composite Piemonte nappe). Thus the proposed paleogeographic reconstruction (fig . 3) can be regarded only as an approximation. It was proposed by BORTOLAMI and DAL PIAZ (1970) and improved later by ELTER (1971, 1972), DAL PIAZ et aL (1972), STURANI (1973), D ... L p""z (1974 a), H UNZIKER (1974). West of the Sesia-Lanzo Zone the Piemonte na ppe (or Piemonte Zone) is overlain by the Dent Blanche N appe, which is subdivided into several units (or Klippen) : Dent Blanche s.s., Mc Mary and Mt. Emilius on the external side; Pillonet and Glacier-Rafray on the internal side (Fig. 4 and 5). The Sesia-Lanzo Zone and the Dent Blanche Nappe are fragments of a big, composite tectonic unit, ascribed to the Penninic domain by ARGANO (1906, 1911, 1934, nappe VI). H owever it is now considered to be of Austroalpine origin (NW margin of the Insubric plate) because of its position with respect to the T ethyan oceanic basin, the complete lack of ophiolites within the Canavese basin and the Austroalpi ne affinity of its Mesozoic cover (Mt. Doli n and Roisan Zone). According to ARGANO the Scsia-Lanzo is the autochtonous root zone of the Dent Blanche Nappe. However more recent studies (BoRToL... M I and DAL PIAZ, 1970 and D ... L PIAZ et al., 1972) showed that the Sesia-Lanzo Zone must be considered part of the Austroalpine nappe. The Austroalpine and Penninic units of the W estern Alps underwent two cycles of Alpine metamorphism. The first one, called the early-Alpine event (90-70 m.y.), occurred under high pressure-low temperature conditions and developed during the early-Alpine subduction. It reached the highest pressure conditions Od-Qz assemblages) on the internal side. The second metamorphic event (Lepontine event in the Ossola-Tessin area) developed after the thrusting phase and reached its thermal peak at about 38 m.y .. This event developed a regional overprint ranging from greenschist facies (Western Alps) to amphibolite fades (Ossola-T essin region). In the Piemonte composite nappe, the Zermatt-Saas Zone s.1. is characteri zed by regional Dent Slanche NilPPC Sre'thorn L)'s ~ amm VlI1!'ell,nc • .. • ~ 'i z~ Dora Salhll ,iver Z ~ "> • " ~ -i'" •• MI . ~ .~i.li \.ls r "'\ MI . Gl acier Gran Paradiso ~lO ~ B T05)"'_ + + G' + + + + ~ + + + + + + + + + . " "z "'r:;> +++++++++ .:::!\:){::::::\:? b " +T++++ +++++++'+ ,;.:;.:'.;:::::::> ~ ...... 76 ~ C [TI, L.:....:...:...2~ CD J 1+ +1 "~!I.. [::::1···········6 ~7~ ll...U..IG:Jsmm ~ . , a b II b a b B Fig. 4. - Cross uct;o"s lloro"tlo flu Nor/"MI~st"n Alps; for location see Fig. 2. I) Hdvctic b~scment (fI) and cover (b). 2) SUbbrian~nn ~is Zone. J) Bernhard Nappe and Bria~nnai i Zone. 4} MOnle Rosa (MR )-Gran Paradiw (GP)-N~ppe. S) P~monte Zone (fI), Antrona Syrw:line (b). 6) Lower Element of Austroalpine (Sesia·Lanzo Zone (St), Monte Emillul Klippe, GlacicT- Rafra), Klipl'C, Arolla Series). 7) POIt·metamorphic magrmtWn; ." syenite_rnonzooite stocks of BielJa and Travenella; b: trachyandesite yolcanics. S,,) 500tmn Alps (SA) and Upper Element of Austloalpine (11 Zona ~ ~ Diorito-kin::tigitic.a (1I0K), Va lpellinc Sc r ~). 8b) Tectonic wedges of IUbcomincntal upper m~ntlc. ~ 288 R. CO!\{P ... GNONI, G. V. D,l.L PIAZ, J. C. HUNZIKER E ALTRI occurrence of early-Alpine assemblages. These range from edogitic (internal side) to blueschist (external side) and are more or less overprinted by the Lepontine greenschist-facies metamorphism. The Combjn Zone s.L bears a 38 m.y. greenschist facies imprint and does not seem to have suffered the high pressure-Iow temperature conditions of the early.Alpine phase. x xx X XXX X X X XXXX X X X XX xxxxxx X y X X CA"NO"- " DOSSOl-~• + + + + + + + H' + + + + + s ....•••••. •" Fig. 5. - T«I"n;c sknd. mtJp oll"~ inumal Norlhw(StN"" Alps. 1) Lower Pennine Nappes. 2) Bcmhard Napp" (SB) ami Camughcra-Moncucco Unit (CM); a: sWimcntar}' cover; b: ba~mcnt . J) MOnlc Rosa (MR) . Gran Paradiso (GP) Nap!>c; a: ~imentary cover: b: basement. 4) Piemonte Zone (PZ) and Amrona Sync!inc (AS). 5) Dent Bbnchc Nappc and Sesii!.·Lano:o Zone; Q; sedimelllary cover (Roi$.1.n Zone (RZ) and Mont Dolin (MD»); b: Lower Element (MM = Mont Mar)' Klippe; EM = Montc Emitius Kli ppe: GR = Glacier·Rafray Ktippe; PI = Pillonet Klippc); c: Fobcl1o·Rimclla Schisu (FR); d: Upper Element; vp"" ValpcUine Seri,,; 11 OK = 11 Zona Diorito·kino:igitica (I: IJ Zona Oiorilo kino:igitica •.•. ; 2, Val Vogna·Valle cl; Gres,.,nc)' Ktippc; 3: Vasaro Klippe). 6) Can3"csc Zone (CA). 7) Southern Alps (SA). 8) BieHa and Traversdta stocks (a), !r3chyanclcsite VOkan1cS (b). 9) l.3nzo Lherzolitc. SCt.. = Sempione . Ccmova1li Line: CL = Canavcse Line. Additional rtJertnces - An Engiis review of the paieogeography and structure of the Western Alps, with extensive bibliography, can be found in DEBELMAs and LEMOINE (1970), TR!iMPY (1960, 1973) and G. V. Du PIAZ et al. (1975). For a summary on the metamorphism THE SESIA-LANZO ZON E, A SL ICE OF CONTINENTAL CRUST ECC. 289 the non_Alpine reader is referred to ERNST (197) a) and to the special issue of the Schweizerische Mineralogische und Petrographische Mitteilungen: .. Alpine Metamorphose in den Alpen" (65/2, 1974). For the Ivrea Zone, in addition in the special issue «Symposium Zone Ivrea-Verbano" (Schweiz. Min. Petr. Mitt., 40/1, 1968 ), see BoRlANI and SACCH I (1973), and MEHNERT (1975). 2. Older concel)1II on the Sellia·Lanzo Zone The fi rst valuable contributions to the understanding of the Sesia-Lanzo Zone and the Dent Blanche Nappe are by GERLACH (1869, 1871) and GIORDANO (1869). At the end of the last century, FRANC HI , MATTIROLO, NOVARESE, STELLA and ZAC CAGNA began to map the Italian Western Alps on the 1 :25.000 scale, fo r the Italian Geological Survey: 30 years fieldwork produced twenty 1:100.000 sheets C) and a 1 :400.000 geological map of the Western Alps (R. Ufficio Geologico, 1908). 2.1. S~Jia-Lanzo Zon~ FRANCHI, MATTIROLO, NOVARESE and STELI... divided the Sesia-Lanzo Zone into three main lithological complexes (, Gneiss Minuti ), , Micascisti eclogitici ) , ' n Zo na Diorito-k inzigitica ) and a marginal shear belt e). The latter comprises the Fobello-Rimella schists and occurs on the internal side at the north end of the zone. The northeasternmost part of the Sesia-Lanzo Zone, the Ossola region, was mapped using a different subdivision (STEU.A) and will be dealt with separately because of its different lithology and metamorphism. a) , Gn~iSJ Minuti ) Compl~x The term , Gneiss Minuti ) (= fi ne-grained gneisses) introduced by GASTALOI (187 1, 1874), represents a sequence of albite-white mica gneisses and schists. They are generall y fine-grained, but locally have a porphyroclastic texture (K-feldspar relics). The , Gneiss Minuti ) outcrop mostly along the entire external side of the Sesia-Lanzo Zone; the complex contains in its central and non hern part several wide areas of metagranitoids (S~Jia Gn~iJJ~J). FRANCHI mapped the metagranitoids in several places and also described massive types (1905, 1911 ), very slightly transformed, with relics of K-feldspar and magmatic amphibole. b) , M/caJcis!; Eclogitici ) Compl~x The , -Micascisti Eclogitici ) (eclogitic micaschists) were defined by STEl..ld (1894) and outcrop in the internal Sesia-Lanzo Zone, from Lanzo to the Sesia Valley. This complex contains micaschists with jadeitic pyroxene-garnet ± glaucophane (of the colourless variety known as gastaldite) ± chloritoid, with intercalated (l) The Sesia-Lanw Zone is represented on the sheets Domodosso!a (15), Monte Rosa (29), Varallo (30), Gran Paradiso (41), Ivrea (42), SuSItaly. The Dent Blanche Nappe is represented on the sheets Aosta (28), Monte Rosa (29), Gran Paradiso (41) and Ivrea (42). (4) On the 1: 100,000 sheets, however, a more simple division was used, and the first two complexes were combined; however local details were mapped such as the granitoids, mafic rocks, marbles and other units. 290 R. COMP~GNONl, G. V. DAL PIAZ, J. C. HUNZIKER E ALTIII edogites, glaucophanites and marbles e). F RAI\'CHI (1901, 1902c) described the metamorphic evolution of the eelogitic micaschists (Na-pyroxenes - g/aucophane ...... blue green- and green-amphiboles) of the Sesia-Lanzo Zone and indicated the rare occurrence of kya nite. c) ( ll Zona Dioritico-Kinzigitica» T he name was given because of the close li thological similarity with the Ivrea Zone or (first) .. Zona D iorito-kinzigitica :t . ARTINI and MI'.LZ I (1900) and NOVARESE (1929, 1931), stressed the intimate association of *' kinzigites» (6) with ( diorites:t and marbles in the area between the Strona and Gressoney Valleys, and in the Oreo Valley. STELLA (Varallo sheet) showed the occurrence of simila r rocks in the Ossola Valley. The Italian geologists thought that the three complexes were intedingering within a single unit, the Sesia·Lanzo Zone. Their data and this hypothesis were largely used by ARGAND in interpreting the Sesia-Lanzo Zone as root-zone of the entire Dent Blanche Nappe. Ultimately, NOVARES E (1929) expressed the opinion that the e ll Zona Diorito-Kinzigitica ~ and the Mt. E milius Klippe were equivalent lithologically, except for the eclogitic metamorphism. After a gap of about twenty years the study of the Sesia-Lanzo Zone was resumed by MICHEL (1953) who extensively applied the metasomatic theories of JUNG and R OQU ES to the Sesia-Lanzo Zonc and Gran Paradiso massif; the presence of glaucophane in the eclogitic micaschim and of albite in the c Gneiss Minuti :. was asc ribed to a large scale latc-Alpine metasomatism . This interpretation was refuted by BIANCHI and Gs. DAL PIAZ (1959, 1963), who gave the first modern petrographical, chemical and mineralogical description of some sectors of the Eclogitic Micaschist Complex . Particular attention was paid to the metamorphic evolution of the high prC'Ss ure-low temperaturc assemblages. BlANCH! et al. (1965) described in the glaueophanite masses of Corio and Monastero (near Lanzo) primary magmatic assemblages (hornblende, clinopyroxene and sauss uritic plagioclase) altered to high-pressure mewmorphic minerals and later retrograded to grcenschist-facies assemblages. (5) FUNCH!, NOVARESE and STELLA delineated in the internal part of the Western Alps a wide zone of .. eclogite» facies rocks extending from Liguria to the Sesia Valley. It consists not only of edogitic micaschists in the $esia·Lanzo Zone and in the Dent Blanche Nappe (Mt. Emilius), but also of metaophiolites of the Piemonte Zone and of edogitic boudins in the Penninic con tinental crust (Monte Rosa and Dora Maira massifs). It must be pointed ou t that FR ANCH! (1897, 1902) recognized also the first occurrence of lawsonite-glaucophane assemblages in the Western Alps, suggesting that thl.! ophiolitic edogites and the prasinites originated from rocks of basaltic composition. Analogous transformations were described in Ihe edogitic ophiolites by G B. D u PIAZ (1928) and by CoRNELl US (1935 ). These early findings seem to have been forgotten by subsequent workers. (6) The term .. kinzigite» is extensively used in the Alpine litera ture for sillimanite garnec·biotite gneisses of the Tvr('a Zone and the 11 Zona Diori t ()-Kin~igitka. It will be here used as a unit name for the high.grade paragneisses with interbedded marbles and amphibolites. These amphiboli{es correspond to {he « diori{es » of the older literature. THE SESIA-LANZO ZONE, A SLICE OF CO~'T I NEI'''Tl\L CRUST ECC. 291 New chemical and mineralogical data for the Sesia-Lanzo rocks were also provided by CALLEcARr and VITERBO (1966); CX;"'IBEN (1968); V!TfJl.BO BASSANI and BLACK BURN (1968); FIORENTINI Pcm.:NZlI and MORELLI (1968); FIORENTINI POTENZlI (1969 b); Enc.", et ,I. (1969). d) Fobello-Rimdla Schists The Fobello-Rimella Schists (GEIlLAC H, 1869) are retrograded and strongly sheared rocks occurring all along the Canavese L ine, on t he internal margin of the Sesia-Lanzo Zone. between the Sesia and Strona Valleys (fig. 5). They merge on one side into either . Gneiss Minuti , or , 11 Zona Diorito-Kinzigitica,. and on the other side into rocks of the Ivrea Zone (PORlIDA, 1967). e) Ossola Region This part of the Sesia-Lanzo Zone was mapped by STELLA (Varallo sheet). It shows peculiar lithological and metamorphic features. Eclogitic assemblages are completely lacking in the banded gneisses, kinzigites and mctagr:lIlitoids. The Scsia Lanzo Zone on the eastern side of the Ossola Valley has been mapped in detail by REI NHARnT (1966) who described high.temperature pre-Alpine metamorphic rocks with A lpine overprinting ranging from greenschist. to amphibolite facies. Additional rt/trtnus - BAGGIO and FRlZ (1968); BERTOLlINI (1964 a and b); F.... NCHI (I89~, 18%, 1900, 190), 1 90~, 1907, 1908, 1911); HE!I..M.lINN (19}8); NOVARESE (1894, 189', 1901, 19O), 190' a, 190,b); STELLA (1894, 190} ); VITERBO {l%I J. 22. Dmt Blanche Nappe The first detailed description of the lithologies of the D ent Blanche Nappe was given by ARCAND (1908. 1909, 1911 , 1934). H e distinguished tWO main formations: the Aralia Series and the Valpellint Strin The former corresponds to the Aralia Gneisses of Gerlach (1869) and consisls of metagranitoids and less common paraschists. The latter (.: Valpdline Gesteine, of GERLACII) consists of paragneisses, marbles and metabasic rocks, with granulite- to amphibolite facies assemblages which were interpreted by ARCAN I> as a thermal product of the Aralia granite. The geological correspondence between the Valpelline Series, the, II Zona D iorito.kinzigitica, and the Ivrea Zone, was repeatedly messed by NOVARESE (1929. 193 1). Eclogitic micaschists were found in the Mt. Emilius Klippe (NovAREs£, 1904. 1931) and in tbe G lacier·Rafray Klippe (ARcAND, 1911 ; D AL PIAZ and NERVO, 1971). DIEHL, MA SSON and STun (1952) mapped in detail the southern part of the D ent Blanche and Mc Mary Klippen. They defined the chemistry and the mineralogy of the Arolla metagranitoids. Well preserved high-grade assemblages of pre-Alpinc age were fo und in the Valpelline Series. The large bodies of oli vi ne.bearing gabbras occurring as tectonic intercalations in the Arolla Series of Matterhorn and M t. Collon were described by BRUNN (1892, 1894, 1899), BARTHOl.~IEs (1920) and AR C.-I.N D (1908, 1934). R. COt.fPACNONI, G. V. DAL PIAZ, J. C. nUNZIKER £ ALTRI On top oE the Arolla Series arc some small scattered outcrops of calcareous and detrital Mesozoic sequences (Mt. Dolin Series and Roisan Zone), considered to be relics of the original Auslroalpine sedimentary cover. In ARGAI.m's synthesis the Dent Blanche Nappe corrcsponds to thc overturned limb of a big recumbent fold, the whole Sesia-Lanzo Zone being its root-zone. On the contrary, SC HMIDT (1906) was convinced that the Dent Blanche Nappe, as a whole, originated from thc Ivrea Zone. Accordi ng to AR GANO the calcschists of the Piemonte Zone are in normal contact with the Arolla Series, which, in turn is in normal contact with the Valpelline Series. DII:'HL, MASSON and STun, however, considered the Arolla and the Valpdline Series as two independent sheets and denied the existence of ARGAND'S recu mbent fold. They postulated a sliding tectonic style (c Gleitbretf" T~kl onik ~), overprinted by several folding phases. Additional rt!trtm:tJ _ AMSTun (19'4, 1962); ARGAND (1906, 19(8); ELTEI. (1960); GERLACH ( 1871); GIORDANO (1869); HAGEN (1948); LUGEON .nd ARGAND (l9 23. Post-mttamorphic Alpin~ Magma/ism Stocks and dykes of igneous rocks intrude the Sesia-Lanzo metamorphic rocks. The metamorphic rocks are in turn overlain by volcanics, now only locally preserved . ' .. • c • ' ''.oh ." ., •• • ••: 0 ..... " • • l-. • 0 . ; ," '" '" " Fig. 6. - Clonn;ctJ vtu1arion in 11ot' Olitoant' i,nrollr ,...... t, of 11o~ IVrsl(T'n Alps. or) FMA plot; taUI iron plotted as Fo:O; dashed lint: Hawaii alblim: trend. h) Alkali·silica plot; the boundariQ betwun the tho!ciite series, the high-alumina series and the albli olilline·basalt series arc after Ku ... o (1966). Solid dOls: Biena stO(k; optn dOh: Tra,·cudla stO(k; diamonds: trachpndesite and banlt dikes; open oquares: laonprophyre dike.; wlid oquare. : truh)'andcsitc flowl. Data from: Nov"un (1943, with tarlicr references); DE MA "CO (1958); PEnosli.. PAG"'"'''' ( 1961): B'ASGt" and Cl. D".. P,.u: (1963), D.... PI"~ et al. ( 19H); D.... P' ''~, unpub. (dikes f,om Valle Arlogna and Bcco. Torch':), BUINO, unpub. (Traversclla lIock and dikes from Valchiusella). (Traversella ltock and di kes from Vakhiuselb). THE SESIA'LANZO ZONE, A SLICE OF CONTINENTAL CRUST ECC. 293 These rocks were first described by GAS,'ALDI (1871, 1874) and partially mapped on the 1 :100.000 scale Geological Map of Italy. Fig. 6 presents bulk chemical variation of these rocks. a) Bid/a and Trav"ul/a stocks The BieUa stock consists of a core of porphyritic granite surrounded by syenites aod monzonites (FIORENTINI POTEN7..A, 1959). The Traversdla stock consists of diorite and monzonite (NOVARESE, 1943). Both produced contact aureoles which in the inner parts reached temperatures corresponding to the K-feldspat Additional rtlUtnCtl - AHREN DT (1969); AMATUCCI (l934); BURR! and NIGGU (194'); CoLOMBA {I912, 19U, 1929); CoSSA (1876); DE MAleo (19'8); DEUTSCH and LoNCINELU (19'8); FENOGLlO (1924); FIORENTINI POTENZA (1961,1969); MATTIROLO (1899); MUL LER (1912); Nux:u (1922); NOV ....£SE (1901, 193'); PEnoNEL PAGLlANI (19'9, 1961); PREISWERK (1906); TItAVEISO (1894); WALTER (1950); ZAMBONINI (190'). 2.4. G~ophysical Data Some of the more interesting geophysical data available for the internal Western Alps are summarized below. On the internal margin of the Alpine arc, gravimetric surveys reveal a large positive Bouguer anomaly (fig. 7), called Cuneo-Ivrea-Locarno anomaly (VECCHIA, 1968). A regional magnetic anomaly with the same pattern is found north of Torino (L.\I"7.A, 1975, with references). Seismic surveys (see GIF.5E and MOREL1.I, 1975, for a review) reveal the existence of an eastward..dipping zone of high velocities (more than 7 Km/sec.) within the upper crust, with a low-velocity layer underlying it. These anomalies are asc ribed to the so called Ivrea body and their interpretation led to a well known struClurll model (BERCKHEMER'S beak, 1968). What is less known, but has been pointed out since 1968 by CoRON and GUlL LAUME, is that the Ivrea anomaly is within the external side of the lvrea Zone only between Locarno and the u rvo Valley, near Biella. Southwest of the Ccrvo Valley the anomalous gravity zone shifts into the Sesia·Lanzo Zone. It extends along the axis of the Zone, approximately parallel to the c II Zona Diorito-kinzigitica ~ and to the bou ndary between the high pressure.low temperature domain and the greenschist fades domain (fig. 7). Towards the south, the Ivrea anomaly runs parallel with the Lanzo Lherzolite and then enters the area of the Pennin;c continental crust THE SESIA-LA.VZO ZONE, A St.ICE OF CONTINENTAL CRUST ECC_ 295 ",• > > D- D, ~ , ~. ~, /' , ----- ' Fig_ "7. - Zoni"6 of I"~ Alpi,,~ m~l(lmorp""lm ;" I"~ Su;a-/..J:nz;o Zon., and D.,,,, Blan.,,,., Nap~. 1) Areas of Upper E I ~men t wilh Alpine ov~ r pr i nl on pre-Alpine high-Iemperature uoemblagel. 2) Early Alpine «Iogilit area l. 3) Early-Alpine non-cdogitic areas. 4) Areal with grcen$Chin-faciel alSC'mbbges of Lepontinc age (gbucophane and aeS}·.ine rdia of indctffminale Alpine age OCCUr in the Pi llonct klippe). 5) Arras with l.cpontine overprint on carly-Alpine alSC'mblages. 6) PO$itivc Bouguer anomaly of the Ivrea body, in mga!' 7) u.riy alpine i~rade,. A, B" B, and Ho refer to the corresponding subdivisionl of the Lower Element of the Selia-unto Zone. Sce text for discuslion. (Dora ..Maira nappc)_ An ot h~ r ~xplanat i o n is that the high-velocity zon~ and the large positive gravity anoma li~s mark the position of a ma ntl~ fragment dipping internally and underthrusting the Sesia-Lanzo Zone. 196 R. COMPACNONI, G. V. IML PIAZ, J. C. H CNZ1KER E ALTRI 3. Modern ideas on the Sesia-Lanzo Zone The: lite: rature: discusst':d above mostly cove:rs the ce: ntury 1869.1969. The structural and lithological setting, the geochronology and the metamorphic evolution of the Sesia.L'l.nzo Zone have been more recently modified. These new data, integrated with some preliminary informations from the Sesia-Lanzo work group (1974 field. work), are outlined below. 3. 1. Structural ol/tlin~ of t/l~ composiu AUHroaipinc nappc (Suia.Lanzo Zone and Dent Blanche Nappc) Wu.uo et al. (1970) and DAL PI AZ ct 31. (1971) recogniud a sharp mylonitic contact betwe:e:n the e ll Zona Diorito-kinzigitica .. and the two other complexes of the Se:sia-La nzo Zone. Therefore the: e ll Zona Diorito-kinzigitica .. is not only a lithological complex but also a tectonic unit. Its provenance is outside the Sesia·Lanzo Zone, in the sutured border of the Ivrea Zone. The Scsia Lanzo - Dent Blanche system can thus be subdivided into two 'tectonic elements, without any trace of interposed Mesozoic sequences (fig. 4). The upper element of the Sesia-Lanzo Zone consists of several Klippen of the c II Zona Diorito.kinzigitiea .. ; the lower element consists of the Eclogitic Mieaschist Complex and of the c Gneiss Minuti .. Complex. In the Dent Blanche Nappe s.l. the upper element corresponds to the Valpelline Series. the lower element to the Arolla Series (in the Dent Blanche, Mt. Mary and Pillonet Klippen) and to the Eclogitic Mieaschist Complex (in the Mt. Emilius Klippe). 32. Th~ Upp~ El~m~nt Klippcn of the Upper Element outcrop in three different areas of the Sesia Lanzo Zone (fig. 5) ; in the northeast, between the Ossola and Sermenza Valleys (11 Zona Dioritfr"kinzigitica s.s.; C."RItARO et al ., 1970; BERTOLAN I, 197 1; ISLER and ZI NGG, 1974); in the north, between the Sesia and the Ayas Valleys (c Val Vogna Valle di Gressoney .. Klippe; D AL PIAZ et al., 197 1) and in 1he south (small Klippen of Vasaro and M. Cialmera; C"RRARO et al., 1970). Other kinzigites, commonly strongly retrograded occur in the Aosta Valley near Verres, in the Ayas Valley near Colic Pallasina and at Becca T orche, and in the Oreo Valley. It is not yet clear if these kinzigitic sequences actually belong to the Upper Element and are tectonically enveloped by the Lower Element, or if they are pre.Alpine re lics of the latter (,B ORU.NI et al., 1975). 32.1. Pr~-Alpin~ Iithologi~! In spite of structural reworking and metamorphic retrogres~ i o n of Alpine age, pre-Alpine high-grade assemblages are perfectly preserved in places. The high-temperature associations mainly belong to the amphibolite fades of H ercynian age (DAL PIAZ et al., 1972; H UNZIKER, 1974) but also contain some relics of older granulite assemblages. The lithologies and mineral assemblages closely resemble those of the I vrea Zone. mE SESIA-LAN""LO ZONE, A SLICE OF CONTINENTAL CRUST ECC_ 297 The main lithologies of pf(~-Alpine age arc: a) kinzigites, b) amphibolites, c) marbles d) ultramafics. a) Kinz;g;us arc rocks with a gneissose to schistose texture, which are locally massive and granulitic, and generally coarse-grained. A smaJl-scale mineralogical la ye ring is present, due to the alternation of biotitc-garnet-sillimatite and quartz...two feldspar layers. The most common assemblages are: biotite-garnet-quartz-K-feldspar ± sillima nite ± plagioclase; quartz-K-feldspar ± plagioclase ± muscovite; plagioclase-garnet. The high-grade paragneisses are closdy associated with p~gmato;ds that fo rm generally concordant intercalations and discordant veins with migmatitic appearance. T he pegmatoids are coarse-grained and are composed of quartz...K-fddspar-plagioclase -biotite ± muscovite ± garnet. Generally itey appear more retrograded than the associated paragneisses. These pegmatoids seem to represent the anatectic product of the Hercynian meta morphic cycle. Massive rocks of granulitic appearance (plagiodase-garnet; plagioclase-garnet -quartz-biotite) are locally preserved in the kinzigites. b) Amphiboliu! are commonly subordinate to the associated gneisses in all the Klippen of the Upper Element and may occur as lens-like bodies or layers from cm to a few m thick. They are mostly massive, fine- to medium-grained, and may show a fine layering of plagioclase-rich and amphibole-rich layers. The most com mon assemblages are: brown hornblende-plagioclase ± hiotite; brown hornblende-plagioclase-garnet ± hiotite ± quartz; brown hornblende-plagio clase-pyroxene-garnet ± quartz. c) Marb/~! of various thicknesses are interlayered with the kinzigitic gneisscs and are an important structural marker. The marbles contain, as pre-Alpine minerals, garnet, diopside, phlogopite, amphiboles, quartz and large white-mica flakes. Alpine metamorphism produced widespread tremotite-acti nolite and epidote-group minerals. d) A small harzbttrg;u body outcrops in the upper Artogna Valley, between the Gressoney and Sesia Valleys (ART/NI and MF.LZI, 1900; DAL PIAZ et al., 197 1). The harzhurgite consiSlS of olivine (50 to 70 % in volu me), orthopyroxene, opaque spine! and locally strongly deformed phlogopite Aakes. Along its boundaries the harzburgite is serpentinized (antigorite). Two-pyroxene ± amphibole dikes were also found cutting the body. 311. Alpin~ evolulion The rocks of the Upper Element com monly show a strong Alpine retrograde metamorphism, the final product being greenschis,t-facies miLaschists and albite- 298 R. COMPACNON1, C. V. I)AI. PIfoZ, J. C. nUNZIKI!R E. ALT"I .amphibolites; the ~gmatoids are transformed into white·mica quartzites. T he:: retrograded rocks exhibit two different Alpine metamorphic c=pisodes. Relics of the first ~pjsod~ are only locally preserved. T hey 3rc: kyanite. blue amphibole, phengite and fine-grained garnet. The blue amphibole. although in small amounts, occurs both in micaschists and in metabasics. In the former it replaces the Hercynian biotite, while in the latter it devdopes mainly at the expense of brown hornblende (DAL P1AZ el aI., 1971). Extensivdy developed blue amphibole is found only in parts of the Strona Valley (BE.RTOLANI, 1971). Kyanite develops as very fine-grained aggregates pseudomorphing si llimanite. Similar kyanite pseudomorphs are also found in the Valpelline Series of the Oent Blanche Nappe. The se<:ond metamorphic episode is of gr~nschist fades and is more uniformly developed than the liut. It is polyphasic and is accompanied by multiple deformation that results in complete tl!xtural reworking in soml! arl!a$. A lint pha$!! is represl!ntl!d by: phl!ngitl!, grl!l!n biotite, epidote, chlorite, acti nolitl!, albitl! ± garntt (?) ± scarcl! stilpnomelane. Locally there is a seco"d generation of sti lpnomelane and albitl!, soml!times with ol igoclase rims. The Alpine metamorphic evolution of the marbles is not fully understood, but the following minerals can bt: re<:ognized as Alpine: actinolitl!, epidole, quartz, albite, while mica and chlorite. 3.23. G~ochronology T he various metamorphic evelllS dl!scribt:d above in the Upper Ell!ment can be dated by radiometric ages or by comparison with other units (Valpelline Series and Ivrea Zone). T hl! oldl!st metamorphic eVl!nt, characterized by high.grade assemblages, may be Hercynian as suggested by a fl!w K/Ar and Rb/Sr ages of micas (OAL PIAZ tt al., 1972; H UNz IKE R, 1974). T he$!! ages confirm the close relationship of the Sesia Lanzo Upper Elemem and the Ivrea Zone. The two latest episodes are Alpine as is apparem from overprinting relationships and from comparison with the metamorphism in other units of the Western Alps, particularly in the Lower Element of the Sesia·Lanzo Zonl!. The first episode corresponds to the early-Alpine event, the se<:ond one to the 38 m.y. event (s~ OAL P IAZ et al., 197 1). The latter, and the relatl!d multipll! ddormations terminatl!d prior to thl! I! mplacl!ml!nt of the andl!sitic and lamprophyric dik l!s of Oligocenl! age (OAL PI AZ el al., 1972, 1973). 33. T h~ LoWa'" E/~m~nl T he Lower Element is distinguished from the Upper mainly by the occurrence of abundant pre-Alpine (late H ercynian?) granitoids and by diffl!rent grade of Alpi ne metamorphism. The granitoids intrude a sequence of high-temperature metamorphics. The old distinction of the Lower Element into two complexes, THE SESIA-UN7.Q ZONE, A SLICE 01' CONTINENTAL CRUST Eec. 299 c Eclogitic Micaschist ~ Complex and c Gneiss Minuti ~ Complex, based on litho logical criteria, n~ds to be modified in view of new petrographical and geoch ro nological data. A distinction bt:twun the two complexes prior to tht: high pusstlrt:-!ow urn pt:Talllre mt:tamorphism ;s mt:aningless brxause both bdong to a nngfe, though ht:tt:rogt:nous, pre-Alpine basement; thcy became distinguishable only during the Alpine history, as areas with prevailing eelogitic or grcenschist-facies assemblages respectively. The Eelogitic Micaschist Complex can be considered a portion of the Sesia-Lanzo Zone characteri zed by early-Alpine eelogitic assemblages (sector A). The c Gneiss Minuti ~ Complex can be viewed as a sector with greenschist-facies assemblages (sector B) (fig. 7). Within the c Gneiss Minuti ~ Complex three new subdivisions need to be introduced (fig. 7): 1) Northeastern sector (sector BI), with no traces of eelogitic metamorphism, characteri zed by low-grade parageneses (probably of grecnschist faci es) which are at least partly of earl y-Alpine age (OAL PI AZ et al., 1972; H UNZI KER, 1974). 2) External sector (sector B2), extend ing all along the external boundary of the Lower Element and characterized by typical greenschist-facies assemblages of u pontine age (HUNZ IKER, 1970, 1974). 3) Intermediate sector (sector B3) between A and B2 and between BI and B2, chamcterized by overprinting of ea rl y-Alpine by upontine. 33.1. Edogitic Micaschist Complex St:ctor A is characterized by early-Alpine eelogitic assemblages. It consists of coarse-grai ned garnet-bearing micaschists (ortho- and paraschisls) characterized by widespread occurrence of Na-pyroxenes and blue amphiboles and by numerous interbedded edogius and glaucophanius. The complex also contains frequent marble layers and lenses, and rare quarlzjt~s. The high pressure-low temperature minerals of this sector yield radiometric ages of 90 to 70 m.y. and predate the thrusting of the Austroalpine nappes over the Piemonte Zone, which is considered to have occurred at the Cretaccous-Paleocene boundary (DAL PIAZ et aI., 1972; H U"''ZIKER, 1974). This sector extends h orn the Sesia Valley to the Lanzo Valley, in the internal portion of the Lower Element (fig. 7). MicQ!chi!ts - The typical assemblages arc: 'luartz-phengite-garnet ±glauco phane± pyroxeoe Gadeite to omphacite)± epidote ± paragonite. The white micas are mai nly phcngite (fig. 8) and mi nor paragonite, occurring separately or together in the same rock (LIEBEAUX, 1975). They may occur as different polytypes in the sa me specimen; the most commonly observed phengite being the 3T polytypc (FIORENTINI POTENZA and MORELLI, 1968; F10RENT1Nl Po TEN'ZA, 1969 b). Garnet is represented by several generations; locally the oldest generation is 300 R. COMPACNONI, C. V. DAL PIAZ, 1. C. HUN'ZIKI!R £ ALTRI prc-Alpine. Alpine garnets (fig. 9) 3re almandines (5(}.()() % Aim), rich in grossular component (25-40 %); pyroPl= component may amount up to 25 %, but is usually around 10 0/0; spcssarli ne is very subordinate (0-5 %). If plotted in the triangle Gr-Py-Alm + Sp, most compositions fall close to garnets from type-C eclogites of CoLEMAN et 31. (1965). A few garnets from (elogit;c schisu arc zoned (LIE8EAUX, 1975), with cores showing Aim contents significantly higher (70-80 70 ), which possibly represent rdics of prc-Alpine garnets; the outer portions of these garnets have:: composi tions close: to those of un zoned garnets. Chemical analyses of Na-pyroxenes from the eelogitic micaschists fall in three gmup' (fig. 10). t " '" .' 00 to AI F•• ""'I 0 0 -... 0 0 o 0 • • 0 • • • 5(l ~ "'~K • F'~"UK { ., '0 '" Fig. 8. - Si.Al,.,.-(Fc .... + Mg) diagram for while micas from micasc hists (squares) and metabasics (dots) of the EclOgilic M;nschii\ Complex. Data afler FJ OIlESTINJ POTtiNZ .. "nd MOl< ILJ.LI ( 1968), F'DaILl The pyroxene=s of the first group are jadeites in which the Jd component ranges from 80 % to over 90 %; the Ac component does not exc~d 1010. In this group fall the Na-pyroxenes from metagranitoids and edogitic micaschists probably deriving from granitoids. The pyroxencs of the second group arc omphacites with Jd component ranging from 45 to 60 %, and Ac usually less than 5 70 . These pyroxenes are found in edogitic micasc hists which probably derive from high-grade paragneisses. The third group is r epr~nte::d by second stage::-pyroxe::nc=s (see p. ) showing a marked e:: nrichme::nt in the:: Ac compone:: nt and plotting in the chlore melanite field. Several generations of blue amphiboles were observed in the micaschists; they appear to have:: grown later than the jadeite-rich pyroxene and may. be in stable association with albite. Optically they are glaucophanes of the very light AI .. Sp A Py Fig. 9. - MoI«u11lT prQportiQIt, 0/ IllTnrll /rQm Ihr ErlQfi1ir MiC4Khilt CQmpltr. Dou: gunc:ts from edogile., omphacililCS and glaucophanitcs; square.: garnet$ from mKaKhim. Tic-lines show :tOOing from core. (light squares, .solid dou) IQ rim. (.solid squares, light dQu). The compositiQn Qf one pre·Alpine glIrnC"t frQm 11 ZQna DiQritQ- Kintigitica is a150 shQwn (uiangle). CompositiQnal boundaries of garlic", from different types Qf «Iogitcs aflcr CoU.MAN el a1. (1965). Data after CALLEG Allt and VtTIt .... o ( 1966), VELDE and KIEOIMST ( 1973), LATTAIID ( 1974), LIUIlAUK ( 1975). Fig. 10. - ItI-(Di+HtI}Ac proportiQns 1.1/ Na-WOrt"tl from ,11 .. &IQlil;c M;cast:llitt CQmpl.-r. - PyroKenes from miClKhisu. "luarcs; frQm metagranitoids, diamOfwb; from «\ogiles and giaucophanites, solid dots; from Na-pyrQ:u:niIQ. upngla. Composition of ble·.tage pyroxcne, from edogilic mic2Khistl (solid "luarcs) and Na·pyroxenites hlarl) il aho shown. Tie·!ines show zoning from c:ores (solid Iymbob) IQ rim •. Dala after OoNIUN (1968). Eoc ..... Cl al. (1968). Vnol. and KIENAST (1973). LtUli:AUJ: (1975). CompositiQ nal lidd. after EIUIENE and Fvn ( 1967). 302 R. COMPAGNONI, C. V. 0,0\1.. P","Z, J. C. HUN'I.IKEJ. E At.T1U variety known as gastalditc. Only few chemical analyses of these amphiboles are presently available. As shown in fig. 11 where they 3rc plotted in the triangle Fetot.-Mg·Alv" Na-amphiboles fro m eclogitic micaschists mostly fall in the glauco phane field (as defined by MIl'ASHIRO, 1957), close to the Gl-Fe-GI boundary; glaucophanes from metabasics, on the other hand, are more magnesian and plot closer to the Cl-Mg-Rich tie-line. These relations suggest that the higher content in Fe in glaucophanes from the eclogitic micaschists is possibly caused by differences in bulk chemistry of the host rocks. The epidote group is generally represented by zoisite with minor c1 inozoisite, and fa rc Fe~pidote forming in this order; they represent different stages of metamorphism. Zoisite and probably also dinowisite can be referred to the early Alpine event, while the Fe-epidote could be of Lepontine age. AI'" AI '" ~ " fe- Cl e, D o 0 0 0 0 ~ !J o °0 0 .. C'01' .. T f R,.b M lI- R, . b ·'01 ...... M. Fig. 11. - Ar'·Fe .... ·Mg piot for NiNlmph,hoJ.., from the Edogitie Mlc.sd/ilt Complez. Amphiboles ftom mic:nchitlS: ~uares; amphiboles from eclogiles, glaucophanitn and omphadles: dolS. OUa from 0 ..... PlAt Cl: al. ( 1973) and LIUUlJX ( 1975). Composilio~1 ficlds after M1YASmt.o (1957). Chloritoid of two generations is a major phase in high-alumina paraschists but is uncommon in other rocks. It appears la be coeval with the first glaucophane generation, but later than kyanite. Kyanite is ve ry rare and generally pseudomorphs pre-Alpine sillimanite; in the high-alumina paraschists it seems to be older than the first generation of chloritoid (DAL PIAZ ct al., 1972). Rutile is a typical accessory mineral which transforms to sphene, as omphacite becomes unstable. Locally the micaschists show a weak transformation into greenschist-facies as semblages with development of blue-green to green amphibole, albile, chlorite, green biotite and Fe-epidote. THE SESIA-LANZO ZONE, A SLICE OF CONT INEl'o'TAL CRUST ECC. 303 Metabasia (edogites and related rocks) are widely distributed within the micaschists as layers or as lenses of variable size (cm to hundred m). In addition to typical eclogites, glaucophane edogites and glaucophanites with minor ompha citites also occur; these different types may occur together in thinly layered bodies. The edogites are generally massive but may exhibit fol iations in the glaucophane and/or white mica·rich types. Bulk chemistry of the edogites (VITERIIO BASSANI and BLACKBUJ..N, 1968; LIEBE!r.UX, 1975) grossly corresponds to alkaline basalt; almost all the analyses show normative 01 (2-20 0/0 ) and about half of them also normative Ne (2-10 %). The most common assemblages are: omphacite-garnet-rutile±glaucophane± ± white mica ± zoisite. Fe-epidotes and blue green4.o green amphiboles appear in the partially retrograded types. More than one generation of omphacite and blue-amphibole is recognized. The available analyses of pyroxenes from the metabasics show omphacitic compositions (fig. to), with Jd ranging from 45 to 60 %; Ac component is mi nor, and usually less than 5 %. Minor zoni ng may occur, Na decreasing and Ca and Mg increasing in the external part of the omphacites (LIEBEAUX, 1975). Secondary pyroxenes, forming in the Na-pyroxenites are strongly enriched in the Ac component and plot in the chloromelanite field (LIEBEAUX, 1975). Garnet occurs in various shapes and sizes (idioblasts, irregular or atoll-like grains). Chemical data for garnet point to compositions dose to Alm!>5.ee-Gru-4o -PY5-IO (fig. 9); as in garnets from eclogitic micaschisLS spessartine component does not exceed a few percents. A few garnets showing Py component significantly higher (up to 40 0/0) possibly represent pre-Alpine relics. Lawloniu-bean'ng melabanles have been found in the intcrnal part of the Eclogitic Micaschist Complex near Alpeue (WRON and SALIOT, 1969), and Corio (OAL PIAZ et al., 1972; LIEBEAUX, 1975). At Ivozio, in the lower Val d'Aosta (CoMPAGNON I. this volume) and Pont Canavese (LIEBE!r.UX. 1975) lawsonite also occurs, but as pseudomorphs consisting of zoisite and white micas. These rocks are glaucophanites (OM. PIAZ et al., 1972; LIEBE!r.UX, 1975) with chlorite, zoisite, white mica and garnet±quartz; pumpellyite may also occur, rimming the lawsonite (CARON and SALIOT, 1969). The lawsonite.bearing metabasites analized by LIEBEAUX show t holeiitic compositions, with 5 _ to % normative quartz. Pseudomorphs on lawsonite consist largely of zoisite and white micas (phengite and pamgonite), with minor albite±chlorite, blue-green amphibole and sphene. MarbleJ occur as intercalations in the paraschists (a few cm to more than a hundred m thick) often severely stretched and with internal Row structures. The most common assemblages are: carbonates (calcite, ankerite, dolomite)±phengite± ± garnet ± zoisite ± quartz. Particularly interesting is the occurrence of omphacite in these rocks, which often fo rms at the contact between marbles and micaschists. Other minerals, like 304 a. COMPAGSONI, G. V. OAL PIAl:, J. C. HUNZIKER E ALTIU S9 y _-""-,, v,t.RALLO AOSTA GP WHITE MIC"'S • 90-60 m.y l u.rl)' - AIP,ne ) A 90-60m.), ov~rl>nntcd " by J8m,), o 038 m 'J ILcPo~ \, n.) ~ 2 <1 0-1<10 m.y * 33·29 m.y o 240·140 my- o 3Sm)' * JJ-28m.y Fig. 12. - Ag.. de,"mi"AlioM lrom fir .. St'li"·u.,,zl> ZOM, D.. ", 81t", .." .. Nap~, I".... " Zo" .. and Oli,o«"" m",matic- 'W~s. Sa: rderences in H U"~ lIt EIl (1974). HI = Biella stock ; MI = Miallliano Slack ; Tit = Traversdla stock. FA = Fav:lJ"o trachyand<'Si tc; FL = Fal1ctti trachy;:mdesitc; SS = Boa:hclto Sa.scn. trach),a ndnitc. FA = Palluina la mptophyre. Slipplcd areu: 11 ZoN Dioril... Kinzigitica aRCi Valpdline s"rin. epidote, albite, tremolite and chloriu: are considered to be related to the late green schist-facie", retrogression. In addition to these marbles, rocks consisling of q uanz-carbonate ± white micas± THE S ESIA-LANZO ZONE, A SLICE OF CONTINENTAL CRUST ECC. 305 ±chlorite (similar to the ~ calcesc i sti ~ of the Piemonte Zone) and rare carbonate microbreccias are found locally (e.g. near Quincinetto in the Aosta Valley and near Pont Canavese in the Orco Valley). Quarzite; - In the Eclogitic Micaschist Complex very rare thin layers of quarzites (possibly metacherts) were found near Sparone in the O rco Valley. They contain spessartine ± phengite± Na-pyroxene ± blue amphibole. The spessartine component in garnet is about 75 '1'0 (MINNIGH, pers. comm., 1975). 33.2. ~ Gn~iJ! Minuti ~ Complex S~ctor 81: North~astern uctor with ~arly-Alpine non~clogitic asgmblages - It consists of metagranitoids, augengneisses and minor paraschists containing mineral assemblages referred to the greenschist bcies (DAL PIAZ et al., 1972; HUNZIKER, 1974). This sector is still poorly known and seems to extend from the middle Gressoney Valley (Gaby) to the Ossola Valley (fig. 7). Since the mineral assemblages are essentially the same as those of B2, this sector could not have been distinguished without radiometric dating. In the Bl sector a metamorphic grading towards sector A occurred during the early-Alpine event. This grading ranged from higher-pressure conditions (southern and central portions of the internal sector of the Lower Element) to lower-pressure conditions in the northeastern sector. T emperature probably re mained constant throughout the whole element. T he «Gneiss Minuti ~ of the sector BI consist of metagranitoids, augen-gneisses and minor paraschists containing albite, phengite, Fe-epidote, chlorite, actinolite, green biotite ± blue amphibole± garnet± zoisite ± rutile. Blue amphibole occurs in a narrow area, rimming the eelogitic zone to the north. The researches in progress in sector Bl reveal a few mineralogical differences between the early-Alpine greenschist-facies assemblages and those cf Lepontine age occurring in sector B2. Zoisite, rutile, garnet and glaucophane are widespread in the fo rmer, while they seem to be generally lacking in the latter. Sector 82: Greenschist-facies sector of Lepontim! age (38 m.y.) - This sector approximately corresponds to the «Gneiss Minuti ~ Complex of the literature and extends all along the external margin of the Lower Element (fig. 7). Mineral assemblages contain albite-phengite-quartz-Fe-epidote ± chlorite± green biotite ± acti nolite ±stilpnomelane; albite is very abundant and commonly poikiloblastic. T hese assemblages are identical and coeval with those of the Aralia Series in the Dent Blanche Nappc, and similar to the assemblages in the underlying Combin Zone of the Piemonte Zone. In the overprinting sector (sector B3) they overprint the earlier high pressure.low temperature assemblages. K/Ar and Rb/Sr determinations on phengite and green biotite give Eocene ages (HUNZIKER, 1969, 1970, 1974). These assemblages belong to the second episode of the Alpine metamorphism which corresponds to the " Lepontine ~ thermal dome in the Ossola-Ticino region. 306 ... COMP,,"CNONI, G. V. DAl. P IAZ, J. C. H UNZIKU, I!. ALTa, In the B2 sector the most common rock types are fine·grained albite-bearing gneisses with minor prasinites. Chemical data for these rocks and thei r minerals (phengites, biotilCS, amphiboles, garnets and slilpnomelanc) arc reported in LAlTARD (1974). These rocks are commonly banded and consist of leucocratic layers interbed ded with layers rich 10 white micas, chlorite. epidotes and actinolite. The banding is largely a transposition of primary aplilic dikes. W hithin more homogeneous albitc.bearing gneisses the original magmatic texture of the porphyri tic granitoi ds can be recognized. The relict minerals belonging to the magmatic assern :b13ge are K-feldspar, brown hornblende, faded brown biotite and largely saussuritic plagioclas.:::. These granitoids, which ar.::: mainly transformed into augen-gneisses, occur widely between the upper Sesia Valley and the Champorcher and ChiuseUa Valley. In places, metagabbros with relics of the primary minerals are also found (Mt. Pinter, upper Gressoney Valley). The transi tion from metagranitoids to fine-grained gneisses indicates an igneous origin fo r at least some of the .. G neiss Minuti). This field evidence in confirmed microscopically by the occurrence of K-feldspar (more or less transformed into chessboard albite) and of allanite relics. Possibly the rest of • G neiss Minuti ) may have been derived from paraschisls (some of wich may even be of post-H ercynian age) but in the absence of relics it is impossible to draw any conclusion. Th.::: possible occum:nce of acid mctavolcanics of Permian age (said by AMST UTZ, 1971, to be of regional extent) is as yet unproved in the Sesia-Lanzo Zone. In some parts of this sector very intensively retrograded kinzigitic schist5 with some marbles and amphibolites are intercalated in the c Gneiss Minuti). There are relics of pre-Alpine garnet and biotite in the kinzigitic schists and of diopside in the marbles. So far it has not been possible to show whether the association of kinzigitic schists and c Gneiss Minuti ) in this sector is primary, or whether the ki nzigitic schist5 are tectonic intercalations of the . 11 Zona Diorito-kinzigitica ) in the c Gneiss Minuti ). Locally the c Gneiss Minuti ) Complex contains imercalations of calcschist like marbles, sometimes closely associated with distinctive micaceous q uartugarnet rocks which contain as accessory minerals piemontite±braunite±ardennite. The latter outcrop in the L1nzo Valley, near Ccres (ZAM80NIN I, 1922; GENNARO, 1925). Similar rocks occur near Ciusal road H one-Issognc) in the lower Aosta valley. S~ctor Bl: Ofl"printing uclor _ This sector comprises a narrow band of a few hundred meters to one kilometer widlh and is defined as the area in which high pressure-Iow temperature minerals, characteristic of the Eclogitic Micaschists, are overprinted by greenschist-facies minerals, characteristic of the c Gneiss Mi nuti ) (fig. 7). Radiometric data confirm the overprinting and allow extension of this sector towa rds the northeast. In this area early-Alpine greenschist-facies assemblages are THE SESIA-I.ANZO ZONE, A SLICE 01' CONTINENTAL CaUST ECC • 307 ca 8-• • 1+ + + 11 Lall' [2], !&B' - ". Fig. B. _ Dirlrih .. ,;O" 0/ fW"·;flpi",, 1;I1101oliu i" ,II~ LAW" EJ"mml 01 ,11" SU;iI·Ul1llO Zrm~. I) Palaschins and marble, (black); 2) Silicic 10 inlcrm~ialC mctagnnilic rocks; 3) Mct~gabbros; 1) Gneiss Minuti Coml.iu; 5) Upper Elcm~nl: 6) Fobdl<>-Rimdla SchiSlS; 7) TCfli~ry plutonic rocks ~nd contact au,,,,,lcl (dou). - Data from the I: 100.000 maps listo:d in footnote (3); DAL PIA~ Cl aI., 1971 , 1973; MINNIGH, pen. comm.; and personal observations. ovc=rprintc=d by Lc=pontine grc=enschist facies. These two c=vents wc=rc= originally distinguished by dating, but more recc=ntly minor pctrographical diffc=rencc=s havc= bc=e n recognized. The upontine assemblages include porphyroblastic albite and grttn biotite, not found in the c=arlier assemblagc=s. 333. G~ochronology Fig. 12 shows the location of samplc=s datc=d by Rb/Sr and K/Ar methods ID 308 ... OOr.n>AGNONI, C. V. DAL PIAZ, J. C. H UNZIKER E ALTlI the Se:sia-I.anzo Zone, D ent Blanche Nappe and Oligoccne magmatic rocks (HUN ZIKU, 1969, 1970, 1974; DAL PIAZ et al., 1972. 1973; BOCQUET et al.. 1974). These data are reported and discusst:d in more detail by H UNZI KER (1974). Eleven phengites of the Lower Element of the Sesia-Lanzo Zone yield early Alpine ages (90 to 67 m.y.); eight of them come from the • edogitic. sector (A in fig. 7) and three come from the greenschist sector (Bl) of the northeastern Sesia-Lanzo Zone. Two micas from the 83 overprinting sector give ages ranging from 90 to 40 m.y.; these mixed ages are interpreted as early-Al pine ages overprinted by Lepontine ages. Preliminary whole rock data for the Mt. Mucrone metagranitoids give Permian ages, while the relict igneous biotites yield an ea rl y-Alpine age. 33.4 . P,.~ -Alpin~ lithology Ikfore 1969 only a few pre-Alpine relics were known in the Sesia-Lanzo Zone: granitoids in the c Gneiss Minuti , Complex and rare gabbros in the Edogitic Micaschists. Recent investigations have revealed the occurrence of more pre-Alpine relics (Hercynian granitoids and high-temperature paragneisses) both in the Eclogitic Micaschist Complex and in the c Gneiss Minuti, Complex (OAl. PIAZ et aL, 1972, 1973; CoMPAGNON I and MAFPEO, 1973). The Lower Element consisted of two main lithological complexes before the Alpine orogeny: high-temperature metamorphics and granitoids. Both include meta basics which are now mainly edogites or prasinites. Small intrusions of gabbroic rocks were associated to the high-temperature metamorphics. Fig. 13 shows the distribution of the pre-Alpine lithologies in the Lower Element. a) Th~ high-t~mp"atllr~ pa,.agn~jss~s Mineralogical and structural relics of paragneisses with high-temperature assem blages were found in very small areas ne.1 r the Mucrone Lake t0Al. Pu.z et aL, 1972, 1973). These paraschists contain numerous pegmatite veins, mctabasic nodules and layers and rare marble lenses. In the field the paraschists appear very similar to the kinzigites of the c II Zona Oiorito-kinzigitica, and of the Valpelline Series. Under the microscope the following pre-Alpine assemblages can still be recognized: biotite-garnet-sillimanitNwo feldspars-quanz (in the paraschists); two feldspars-quartz±biotite± muscovite (in the pegmatites). They indicate amphibolite facies conditions of indeterminate pressure. W ithi n the high-temperature paragneisses the more widespread pre-Alpine relic is biotite: it occurs as red-brown Rakes mainly concentrated in the sillimanite garnet layers. SiIlimanite can occur c.-ither as millimeter-long nematoblasts or as fibrolitic aggregates; it is very rarely preserved and generally is completely transfor med into finc.--grained kyanite pseudomorphs (DAl. PIAZ et aL, 1972). Garnet is coarse-grained and texturally similar to the kinzigite garnc.-ts. K-feldspar is generally preserved but the plagioclase is always pseudomorphically replaced by a jadeite- THE SESIA-LAN7.0 ZONE, A SLICE OF CONTINENTAL CRUST ECC . 309 -zoisite-quanz intergrowth. As elsewhere in the Valpelline Series and the Ivrea Zone it is noteworthy that graphite is quite common in these rocks. In many areas the appearance in outcrop of the eelogitic micaschists is remi nescent of high-temperature pa ragneisses though prc-Alpine minerals are lacking: under the microscope only the pseudomorphs of phengite+garnet after pre-Alpine biotitc are recognizable. High-temperature paragneisses with illlerbedded marbles and amphibolites, with a strong grecnschist-facies overprint, were also found in the external. Gneiss Minuti :. Complex (e.g.: Colle Pallasina and Becca Mortens in the Ayas Valley; between Bard and Venes in the Aosta Valley). However, as seen before (Sector B2, pag. 310), their significance is still in debate. b) T he granitoid$ The granitoids arc present throughout the Lower Element but arc most com mon in the north of the Aosta Valley, between Mt. Mucrone, Lago della Vecchia, Piedicavallo (Cervo Valley), Gaby (Gressoney Valley) and the Sesia Valley. The granitoids are clearly intrusive, as shown by their sharp and discordam contacts with the surrounding pa rasc histsj swarms of aplite and minor pegmatite dikes crosscut both the paraschists and the granitoids. The granitoids are represented by biotite ± hornblende granodiorites and tonalites with minor granites (WLLEGAR I et al., 1976). Commonly the igneous mineralogy is completely transformed into Alpine assemblages: the only relict igneous minerals are K-feldspa r (particularl y in the porphyri tic granitoids transformed into augen-gneisses) and the characteristic accessory allanite. In a very few areas these rocks escaped pervasive Alpine deformation and the igneous texture is perfectly recognizable. c) T he majic rocks T he metabasics of the Lower Element can be subdivided into two groups, according to their pre-A lpine lithology: 1) cognate dark inclusions in granitoids; 2) metabasics interbedded in high-temperature paraschisu. 1) Cognat~ dark incJlfJions in m~tagranito i ds. The $<:sia-Lanzo metagranitoids are rich, and locally very rich, in cogna te dark incl usions n the size of which ranges from a few centimeters to one meter. In the Eclogitic Micaschist Complex they can be recognized on ly as Slructural relics for they are completely transformed into fine-grained quartz-bearing garnet-omphacite-phengite rocks. In the • Gneiss Minuti:. Complex, however, thei r mineralogy (plagioclase, biotite and minor amphiboles) is still locally well preserved and similar to that of the surrounding (7) We caIl cognate dark inclusions the inclusions in the granitoids referred to as «dark microgranular enclaves :. by DIDIU (1973). 310 11. COMPACNONI, c. V. 0"', P ' "Z, J. C. IIUNZ1KER £ ALTRI mctagranitoids; they differ from the latter only in having a very low quartz content and more abundant hiotite: and amphibole. 2) M~labasics inurhedded with the high-temperature paragnnstes. The eclogites interlayered with the original high-temperature paragneisses are interpreted as deriving from primary amphibolitcs and basic granulitcs because such rocks arc commonly associated with the p:lraschists (*' Kinzigites , ) in the lvrea Zone, *' 11 Zona Diorito-kinzigitica :. and Valpelline Series. Recently significant remnants of pre-Alpine amphibolites have been fo und (see Co"IPACNONI, this issue). d) T he gabbroids The primary gabbroic nature of the metabasics of Corio and Monastero (SoUlhern Edogitic Micaschist Complex) is shown both from microstructural and mineralogical relics (brown hornblende. clinopyroxene and saussuritized plagioclase) and by their bulk chemimy ( BIAN"CI-II et al., 1%5). A gabbroic body was found in the (Gneiss Minuti .. Complex at Ml. Pinter (Upper Gressoney Valley), close to the tectonic contact with the Picmonte Zone calcschists. The gabbros, cut by leucocratic differentiated dikes, are transformed into fine-grained greenschist-facies schiSlS; the contact is marked by a thick mylonite band_ Also at Mt_ Pinter the igneous mineralogy consists of brown hornblende, clinopyroxene and s3ussuritized plagioclase_ • 33.5. Alp;n~ m~/amorphic ~voltll;on The Alpine metamorphic evolution of the Lowcr Element gi ves a very complex polyphase picture and is characterized by the two main metamorphic events known as the early-Alpine and Lepontine respectively_ a) EarJy-Alp;n~ ~volution a.l) Eclogitic Mica!chists (5«tor A )_ The fi rst metamorphic phase is characte rized by the highest pressure-lowest temperature conditions, the most significant reactions in mewgranitoids and paragneisscs being: plagioclase -- jadeite+quartz+ +zoisite, biotite -- phengite+garnet+rutile, and sillimanite -- kyanite_ Such reactions occur panly under non pervasive deformation as shown by the extensive preservation of pre-Alpine magmatic and metamorphic textures. A second early-Alpine stage is characterized by development of foliations on a regional scale, by recrystallization of jadeite and by growth of omphacite, glaucophane, zoisite and chloritoid. During this stage the typical (eclogitic .. ortho and paramicaschists were formed. Lawsonite too grew during this stage (CoMPA GN"ONI , this volume, p_ 359). In the eclogiles these two stages ca nnot yet be distin guished: probably more than one generation of omphacite grew together with garnet, glaucophane, phengite, zoisile and rutile. The third early-Alpine stage is characterized by the breakdown of the pair jadeitic pyroxene+quartz, with development of albite, white-mica, dinozoisite+ THE SESIA'LANZO ZONE, A SLICE OF CONT INENTAL CRUST Eec. 3lI +acmite·rich pyroxene and by the growth of a second generation of glaucophane, then of blue-green amphibole and st ill late r of actinolite. Up to this stage (glauco phanitic-gretnschist stage) the eelogit ic assemblages survived in the metabasics. A final greenschist.facies stage locally transforms eelogites into albite-amphibo· lites with widespread development of albite-actinolite intergrowths. The first stages of this complex evolution are surely ea rly·Alpine; the age of the last onc, characterized by the breakdown of glaucophane, is doubtful. This stage shows mineralogical features similar to those of the Lepontine event; it differs from the latter in that it lacks green biotite and porphyroblastic albite; moreover it has only a local gccurrence . In any case it seems evident that the evolution in the earl y-Alpine event is characterized by a marked pressure decrease. a2} North~asUrn uctor, with ~arly-Alpin ~ non~clogitic asumblagu (S~clor BI). The data concerning the early-Alpine event in this sector are very scarce. The main transformations of tbe igneous minerals of the granitoids are as follows: K-feldspar ..... albite; biotite ..... white mica (phengite)± rutile ± garnet; plagioclase- saussurite (albite + fin e-grained zoisi te and/or e1inozoisi te); recrystallization of magmatic quartz to polygonal aggregates. As previously discussed the phengites of this sector give ea rly-Alpine ages. In places plagiodase is preserved, generally as relics in K-feldspar, indicati ng that the poT conditions for its breakdown were not attained. In the intermediate area between the early-Alpine greenschist-facies sector and the higher.pressure (c eelogitic , ) sector, glauco phane (largely transformed) is quite common. In this area too the phengites give early-A lpine ages. a3) Surar B2. The present knowledge of the c: Gneiss Minuti , Complex al lows two different interpretations of the early-Alpine metamorphic zonation in this area: 1) The c: eelogitic , assemblages develo~d in the whole Lower Element south of the boundary Gaby-Lower Ayas Valley, but were completely obliterated in Sector B2 by the Lepontine event. 2) South of the boundary Gaby-Lower Ayas Valley the early-Alpine event developed mineral assemblages of lower pressure, as is observed in the North eastern sector (Sector B1). b) Lepontin~ m~tamarph;c evolution This event extends over the external portion of the Lower Element (c:Gneiss Minuti , Complex). It is characteri zed by assemblages typical of the greenschist fades with albite. Fe-tpidote, phcngite, chlorite, green biotite, ± Mn-rich garnet, ± actinolite, ± sti lpnomclane as main constituents. These minerals belong to stable assemblages, as shown by microscopic obser vations and by the partition coefficients of Fe and Mg between coexisting bi()(ite, chlorite and amphibole (LAlTARO, 1974). Albite, which is a main constituent of the c: Gneiss Minuti :., occurs as at least 312 It. COMPACNONI, C. V. DAL PIAZ, ,. C. IIUNZIKER E ALTIU two generations; the second one is later in the textural evolUlion and develops mainly at the expense of white mica layers. Stilpnome1ane also seems to occur as two generations: the first one has a restricted occurrence and coexists with gretn biotite and chloritc; stilpnome1ane II appears later as unoriented sheaf.like aggre gates and needles in the groundmass. Lepontine assemblages are fou nd overprinting early-Alpine assemblages (sector B3) or H crcy nian magm:J.lic assemblages (sector B2), but can also occur in rocks (.Cneiss M inuti~) devoid of ea rlier relics. In the sector where overprinting relations occur, the most significant ·transformations 3fC: albite + actinolite: b" " Na-amphiboles and pyroxenes a IbIte:" + c hi·onte: ± g re:e:n lome: garne:t chlorite: ± gre:e:n biotite: ± e:pidote: rutile: $phe:n e: zoisite: Fe:-epidote: C haracte: ristic albite: + chlorite: ± actinolite: ± gr(:e:n biotite: pS(:udomorphs, mainly af((:r glaucoph:me:, are: common in the:se: rocks. In the: e:xt(:rnal s(:Ctor (B2) the: min(:rals of the: granitoids show transformations which ar(: comple:tdy or largdy rdated to th(: upontine: phas(: (Du PIAZ (:t al ., 1971). Th(:se transformations ar(: : ch(:ssboard albite K.fddspar albit(: + ph(:ngite plagioclase epidol(: + ph(:ngil(: + albite white mica + rutil(: -- albite + rutile: biotite green biotit(: ± chlorite: actinolite hornbl(:nd(: chlorite: ± albit(: gre(:n biot ite Eve:n wh(: r(: lhe:se transformations are: almost compl(:t(: (giving riS(: to the: typical c Gndss Minuti:. appearanc(:) rdics of K.fddspar and aJlanil(: can still be found, which show that most c Gn(:iss Minuti :. must com(: from granitoids. 33.6. P·T conditions d"ring th ~ ~arly.Alpjn~ ~ tI~n l During the: (:ariy.Alpin(: mctamorphic event the S(:sia·Lanzo Zone reached its highest pressures in the Eclogitic Micaschist Complex, which is characterized by: a) bre:akdown of albite to produce jad(:itc+quartz; b) wid(:s pre:ad occurrence: of almandin(:.rich garnet, zoisil(:, glaucophane and phengitic micas. THE SESIA-LANZO ZO~E, A SLICE OF CONTINENTAL CRUST ECC. 313 Temperatures of 300-400· C and pressures in excess of 8-14 kb were inferred by DILL PIAZ et al. (1972) and CoMI'AGNONI and MAFFEO (1973) from comparison of the phase assemblages occurring inl' the Edogitic Micaschist Complex with experimentally determined poT equilibria. The inferred pressures roughly correspond to a minimal lithostatic load of 30-40 Km. The early-Alpine metamorphic gradient in the Lower Element of the Sesia Lanzo Zone was considered to be characterized by a transition from glaucophanitic greenschist facies assemblages (sector El) to jadeite-quartz-garnet assemblages (sector A). The early-Alpine high pressure-Iow tcmperature metamorphism was believed to evolve through a progressive pressure decrease accompanied by a constant or slightly increasing temperature. Other estimates of the recrystallization temperature in the Edogitic Micaschist Complex (VELDE and KIENAST, 1973; LIEBEA UX, 1975) are based on questionable assumptions (e.g. the stable coexistence of jadeite and omphacite with albite); moreover, the temperatures inferred for the northeastern part of the Eclogitic Micaschist Complex by VE!..DE and KIENAST (T = 700 0 C with PH.o = 11-12 Kb) appear much too high in view of the occu rrence of chloritoid (not of staurolite) throughout this part of the Sesia-Lanzo Zone. The poT conditions inferred for the Sesia-Lanzo Zone mineral assemblages from experimental petrology may be wrong in detail, but not in order of magnitude. The experimental data show that for the development of the early-Alpine high pressure-low temperature metamorphism a kynematic model is required which allows the achievement of high pressures and their subsequent decrease in a relatively short time. It is necessary that this process be rapid in order to avoid the conversion of the high pressure-Iow temperature assemblages into amphibolite-facies assemblages due to the otherwise unavoidable rise in temperature. For this reason we also believe that the initial temperature (during the highest pressure) must have been below 500" C. This petrological interpretation agrees with that proposed by P. BEARTH (1967, 1974) for the high pressure-Iow temperature metamorphism in the meta ophiolites of the Piemonte Zone. 3.4. Structural Data Detailed structural work is in progress in the Gressoney Valley and in the Bard and Mt. Mucrone areas. Thrusting of the Upper over the Lower Element is found to post-date the high-pressure metamorphism and to be, in turn, overprinted by later foldi ng. At present a minimum of four generations of folds is recognized in all areas in rocks belonging to both the Upper and Lower Element. At least three of these generations (F2-F4) post-date the thrusting. Of the th ree the earlier two (F2 and F3) are cut by andesite dikes and must therefore be older than 30 m.y .. No overprinting relationships are known between the youngest foJds (F4) and the dikes. In all the 314 R. COMPACNONI, C. V. IM.l. PIAZ, ,. C. HUNZIKER F. ALTRI three areas the last three fold generations are represented by large scale structures 50 that the overall structure is very complex. This is almost certainly true also of the whole Sesia-Lanzo Zone and means that structural interpretations should not be made without detailed analysis of small scale structures. 3.5. D~nt Blanche Nappe The Dent Blanche Nappe shows the same tectonic duality as the Sesia-Lanzo Zone and a very strong lithological and metamorphic analogy. The Upper Element (Valpelline Series) corresponds to the . 11 Zona Diorito kinzigitica :t. The Lower Element comprises the Arolla Series (equivalent to the c Gneiss Minuti ) Complex of the Sesia-Lanzo Zone) and Edogitic Micaschists (equivalent to the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone). The former occurs mainly in the Northern Klippen (Dent Blanche Nappe s.s., Mt. Mary, Pillonet) and the Jancr only in the Southern Klippcn (Mt. Emilius and Glacier Rafray). 1) Pr.-:-Alpin.-: lithology and evolution. The Upper Element consists of granulites, high-temperature paragneisses, amphibolites and marbles. The pre-Alpine evolution can be outlined as follows (BoR'ANI et al., 1974, 1975; H UNZIKER, 1974): a) deposition of a pelitic sequence, with minor carbonate rocks, accompanied by igneous activity (pre.Ordovician); b) granulite-facies event (Ordovician-Silurian); c) low.pressure (cordierite) amphibolite-facies metamorphism of H ercynian age (Carboniferous?), with subsequent cooling ages of micas of 200-180 m.y. (DAL PIAZ et al., 1972; H UNZI KER, 1974). In the Low.-:r Elem..:nt, the Arolla Series mainly consists of late-Paleozoic granitoids (mostly granites and granodiorites, STun, 1940); their original assem blages are rarely preserved, and they are mostly transformed into greenschist-facies orthogneisses and albite-bearing • Gneiss Minuti :. (Arolla Gneisses of the older authors). The stratified gabbro complexes occurring as tectonic intercalations in the Arolla Series of M atterhorn and Mt. Collon comprise two pyroxene ± olivine gabbros with minor plagioclase peridotites and anorthosites (DAL PIAZ, 1974 a). T he Eclogitic Micaschists of the Mt. Emilius Klippe comprise metagranitoids and more abundant pa raschists derived fro m high-grade paragneisses, with marbles and metabasics, very similar to the kinzigitic sequence (BEARTH et aL, in prep.). 2) Alpin..: M..:tamorpllism. In the Upp.-:r El..:m.-:nt the Alpine metamorphism is very similar to that of the . 11 Zona Diorito-kinzigitica ,. : i.e. it is polyphase, of low temperature, hetemgeneously distrihuted and occurs mainly in areas of strong Alpi ne deformation. The earliest Alpine assemblages in the kinzigitic sequence are characteri zed by kyanite+phengite+ .'!:arnct ± chloritoid. An important difference with respect to the . n Zona Diorito-ki nzigitica:. is the app:lrent lack of blue amphibole. THE SES'A-LANZO ZONE, A SLICE OF CONTINENT"L CRUST ECC. 315 In the Lower Element early-Alpine eclogitic assemblages are found in the Mt. Emilius Klippe (well preserved) and in the Glacier-Rafray Klippe (as relics). Glauco phane- and aegirine-relics (of indeterminate Alpine age) occur in the Pillonet Klippe. No traces of high pressure-low temperature minerals have been found in the Dent Blanche· and Mt. Mary Klippen which seem to have escaped the celogitic metamorphism. The second metamorphic event (Lepontine, 38 m.y.) produces greenschist-facies mineral assemblages in both tectonic elements. In the Upper Element, the Lepontine retrogression is very irregularly distri 'buted and occurs mainly in areas of more intense deformation. The Lepontine event produces a partial to complete rcerystallization of the Hercynian metagrani toids and metamorphics in the Northern Klippen of the Lower Element (Dent Blanche, Mt. Mary, Pillonet); in the Mt. Emilius and Glacier-Rafray Klippen these assemblages strongly overprint the earlier high pressure-Iow temperature assem· blages. 3) Alpine folding. The entire Dent Blanche Nappe shows polyphase Alpine folding with longitudinal and transversal trends. These deformations are also visible on a regional scale and give rise to tectonic enveloping of the Upper Eleruent by the Lower Element and of the Dent Blanche Nappe by the Piemonte Zone. 4. Discu81ion The Sesia-Lanzo Zone and the Dent Blanche Nappe provide evidence of a high pressure-low temperature metamorphism taking place in the continental basement and overprinting pre.Alpine metamorphics and granitoids. The occur rence of jadeite-quartz.garnet-zoisite assemblages in the continental crust is most uncommon throughout the rest of the world. Furthermore the high pressure-Iow temperature assemblages of the Austroalpine system are now embodied in the topmost crystalline elements of the W estern Alps nappe pile. Thus we believe that the Sesia Lam·.o-Dent Blanche system represents an unusual tcetono-metamorphic situation very significant from a geodynamic point of view. 4.1. Alpine metamorphism in the Austroalpine system 4.U. Early-Alpine event The occurrence of edogitic and greenschist-facies assemblages of the same age in adjacent sectors of the Lower Element enabled DAL P,,,z. et al. (1972) to ap proximately locate an earl y-Alpine isograde (line a-b in fig. 7). This line divides an internal Northeastern sector characterized by greenschist facies assemblages, from an internal Southern secto r, characterized by celogitic assemblages. Throughout the external secto r of the Sesia-Lanzo Zone, the Lepontine 316 11.. COMPACNONI, C. V. DAL P1AZ, J. C. HUNZIKER E. ALTlU metamorphism masks the western extent of the ea rly-Alpine metamorphic zones and related isogrades; the early-Alpi ne metamorphi~m of the Sesia-Lanzo Zone is sharply bounded to the south-east by the suture of the non-ophiolitic Canavese basin, which was interposed during Triassic to Cretaceous times between the Austroalpine domain and the Insubric domain (Southern Alps). In the Dent Blanche Nappe. the picture is even morc complicated by the tectonic dismemberment of the Lower Element. H owever, possible: extension of the early-Alpine isogrades bey~md the Sesia-Lanzo Zone can be argued. The Lower Element of th..: Dent Blanche Nappe did not reach the pressure conditions of the Eclogitic Micaschist Complex. This is proved by the widespread occurrence of relict igneous pJagioc\ases in metagranitoids. In the Eclogitic Micaschist Complex (e.g. Mt. Mucrone) on the contrary the plagioclases are always transformed into jadeite+ + zoisite+quartz. It is therefore concluded that in the Northern Klippen of the Dent Blanche Nappe (Dent Blanche Nappe s.s., Mt. Mary and Pillonet) the early-Alpine meta morphism, where developed, should have comprised greenschist-facies assemblages. The Southern Klippen of the Dent Blanche Nappc (only consisting of Lower Element) show on the contrary widespread high pressure-Iow temperature assem blages. These para geneses are locally well preserved at Mt. Emilius, whereas they are intensively transformed by the later gr~nschist-facies Lepontine episode in the Rafray-Glacier Klippe. It is believed that the early-Alpine isograde in the Lower Element of Dent Blanche Nappc should run approximately along the Aosta Valley (line b-c in fig. 7). Taking into account these data and the early-Alpine isograde preserved in the &sia-Lanzo Zone (line a-b), it was proposed that all the Sesia-Lanzo Zone between the Aosta and Lanzo Valleys underwent high pressure-low temperature conditions during the early-Alpine metamorphic event. This means that the .. Gneiss Minuti :t of this sector should represent the original c eclogitic micaschists) completely transformed during the Lepontine phase. H owever, significant high pressure-low temperature relics seem to be lacking in this sector (see also LAnARD, 1974). Another hypothesis can be proposed for the present position of the early-Alpine isograds, which tahs into' account the Lepontine deformations. According to this hypothesis the early-Alpine isograds have been folded and transposed; it is therefore ' possible that the isograds could be transposed into approxim:lte parallelism with the major axis of the Sesia-Lanzo Zone and could at present run - masked by the Lepontine overprint - across the c Gneiss Minuti :) Complex, between the Aosta and Lanzo Valleys (line b-d in fig. 7). Such a discussion is rclev3nt to the paJeogeographicaJ reconstruction. The first interpretation enables us to directly connect the Southern Dent Blanche Klippen to the external Sesia-Lanzo Zone margin, while the second hypothesis obliges us to refer these Klippen to some internal part of the Eclogitic Micaschist Sector. TilE SES)A-LANZO ZONE, A SLICE OF CONTINENTAL CRUST ECC. 317 Howt:vt:r. any nt:w imt:rpretation of tht: t:volution of tht: Sesia-Lanzo Zont: - Dent Blancht: Systt:m must takt: into account the following points: 1) Tht: t:arly-Alpint: mt:lamorphism has difft: rt:nt charactt:ristics in tht: two structural dt:mt:nts of the Austroalpint: system. In tht: Upper Elt:mt:nt the high prt:ss urt:.low temperaturt: assemblages of tht: • II Zona Diorito-kinzigitica " and Valpellint: Series are very similar. On tht: other hand a metamorphic fades transition can bt: obst:rvt:d in tht: Lower Elt:ment. 2) T ht: high pressurt:.low temperaturt: aSSl!mblages occu r not only doSl! to tht: thrust plant:s but are homogt:ncously distributed throughout the Lowt:r Elt:mt:nt. 3) The t:arly-Alpint: isogradt:s do not intt:rSl!ct tht: tt:Ctonic boundaries of Iht: Lowt:r Elt:mt:nt, i.t:. tht:y do not t:ntt:r tht: Upper Elt:mt:nt or tht: underlying Pit:monte Zont:. but art: cut by tht: thrust boundarit:s. Thus the t:arly-Alpint: high prt:ssurt:-low tt:mperaturt: metamorphism prt:datcs thrusting and is not dt:pendent on depth in tht: nappe pilt:. 4) In the Eclogitic Micasc hist Complex tht: early-AI pin t: isograds art: still unconformablt: to the It:ngth of the Sesia-Lanzo Zone and the Canavt:st: Lint:. Tht: t:arly-Alpinc metamorphic gradit:nt rises to tht: South and lies almost paralld to tht: thrust plant:s. Tht: foregoing data It:ad to tht: following conclusions: The t:a rly-Alpint: mt:tamorphism prt:d:ltt:S tht: final thrusting of tht: Austroalpint: system over tht: Pit:montt: Zont:. and of the Upper Elemt:nt over the Lowt:r Elt:mt:nt. T ht: early-Alpint: mt:tamorphism is therdort: preservt:d in c tt:ctonically transportt:d rock bodit:s". It originatt:d bdore the prt:st: nt devdopment of the nappe pilt: and its subSl!q uent folding. 4.12. Lepontine event 0/ 18 m.y. Tht: Sl!cond Alpine metamorphic t:pisodt: (Lcpontint:) net:ds to be discuSSt:d further. It is well known that its isogrades cut tht: nappes discordant..ly (FREY t:t al.. 1974. with rdert:nees). In tht: OS50la-Ticino rt:gion, wht:rt: tht: Lepontine rt:acht:d amphibolitt:-facies conditions, tht: metamorphism is typically post-tt:ctonic with rt:spec:t to tht: main ddormations (. Lt:pontinc crystal li zation ,). Sevt:ral authors (EUENBERCER, 1958; NIGGLl, 1970) bdit:vt: that the Lt:pontint: metamorphism is rdated to tht: gcothermal rt:adj ustt:mt:nt produced by tht: nappt: pilt:. Others (t:.g. WENK, 1%2, 1970; THO~ ( PSON, 1976; STRECKEISEN and E. WENK. 1974). on th t: contrary, bdit:ve that it is tht: const:qut:nct: of a lhermal domt:, of dt:t:p origin, unrdatt:d to the nappt: ovt:rburden. With rt:gard to the St:sia-Lanzo - Dent Blancht: systt:m, tht: occurrt:nct: of tht: 38 m.y. metamorphic t:vt:nt in tht: Lowt:r Elt:mt:nt appears to be unrdatt:d to tht: tt:Ctonic load rt:sulting from the Upper Elt:mt:nt. The Upper Elt:mt:nt. in places. overlies • Gneiss Minuti " of Lt:pontine age but dsewhere ovt:rlies t:clogitic micasc hisu of early-Alpint: agt:. Furtht:r mort:. tht: maximum magnitudt: of tht: 318 R. COMPAGNONI, G. V. DAL PIAZ, J. C. HUN'lIKER E ALTRI 11 M [ TIM E G . OCV~ AM'C W. A l ' $ , .• , • s m •• , m,Y· , ('; 0 "0"' "'> I< S '-----0 • " ,o ~, 38 1 ,~ , •, • I ., I 'I :I'j I I, -~- -4- - Fig. 14. _ Ag~ lhurm;nal;o", 0/ ophioliu Itmplaanliml, In the Ossola-Ticino region, the internal boundary of the thermal dome would correspond to the Insubric Line that divides a Northern domain (Pennine Zone) with an Alpi ne metamorphic imprint, from a Southern domai n (Southern Alps) lacking Alpine metamorphism. In the Nonhwestern Alps the internal boundary of the Lepontine metamorphism is no longer coi ncident with the Ca n ave~ Line, but it traverses the Sesia-Lanzo Zone. 41. Alpine orogenic evolution and geodynamic models 41. 1. Distribution of the early-Alpine cvent in thc Wcst"n Alps A high pressure-Iow temperatu re event occurs in other units of the Western Alps. In some areas it is proved to be of earl y-Alpine age (fig. 14) and elsewhere, though not yet demonstrable, it is believed to be of the sa me age: ~. ....,,,..,... C•• ~m ..- m ,~ "--/" " " " 00' • " • • +, . .. ~m"'~. r 'j'" ", ...... ~...... -~o __• ~+ GP + + + - '::> ~ . + + •• • ••• • + + --V + ' ++++ . ++ ••• S8 . / . 'I- + • + •• • •• + • + + ••• b ~/ +++ - - + . + + ~ r:-J !+:"Cl Fff.T'l • - •••... Cl 0 7 ~ 1 L.:-J 2 ~ l l.!Li:l:l.4 =LW 5 ~..... Fig. 15. - Diltrib..ri()1J o/tlu tiVly·Afpi"t iIIumbf/litl i" tht i"UT",J Northwtlltrll Alpl. I) Eelogitic UKmblagcs in the continenui cruS!. 2) Eelogitic as...mblagcs in meuophiolites of the Ztrmans·Sus Zorw:. 3) Eelog;lie .uso:mbbges in metab:uK.:s of Ihe Monle Rou-Gran Par~d iw N ~ ppe. 4) Kyanitc: a) T he sheets of oceanic crust of the Piemonte napp/= contain high pressure low temp/= rature assemblages ranging from ed ogitic (internal side) to lawsonite -glaucophane ± Na-py roxenes (external side). Their age has been determined radio metrically (BocQUET et aI., 1974) and by overprinting relations in the Western Alps (DAL PI AZ, 1974 b). These assemblages do not incl ude the later generation of Na-a mphiboles and locally occurring lawsoni te; some of the$(: can be attributed to more recent metamorphic phases (H UNZl.K ER, 1974 ; 80cQUET et al., 1974; CutoN. 1974). It also seems possible to trace in some areas an isograde for the high pressure -Iow temp/= rature metamorphism in the oceanic pa rt of the Piemonte nappe. As in the Sesia-Lanzo Zone the pressure increases towards the internal side. 320 R. COMPACNONI, C. V. DAL PIAZ, J. C. HUNZIKER E ALTRI b) Rdics of eelogitic associations, very likely of early-Alpine age (certainly post-Hercynian and pre-upomine), occur in the metabasic lenses of the internal Pennine crust (Monte Rosa, Gran Paradiso and Data-Maita); kyanite-phengite 42.2. Regional significance of the Upper CretaceolU event Evidence of an early-Alpine tectonic event in the Western Alps is recorded also in some sedimentary sequences as indicated by the occurrence of the well known Cretaceous Flysches and by stratigraphic and structural observations in the French Western f\lps and in the Southern Alps (LEMOINE, 1972; HI\CCARD et al., 1972. with references). The regional significance of the Upper Cretaceous event is confirmed in the central and Eastern Alps, where KOB£R (1955), TOLL~1I\NN (1963), OBF.RHWSER (1964), CLAR (1965), WOLETL (1967), MULLER (1973) and TRUMPY (1973) postulated a Cretaceous tectonic event, mainly on the basis of sedimentological and strati graphic criteria. An early-Alpine metamorphic event (90-65 m.y.), followed by a Tertiary tectono-metamorphic event has been radiometrically dated in the Eastern Alps (fig. 12) by Ox BURGH et al. (1966). CLIFF et al. (1971), ScHMIDT et al. (1%7), MILLER et al. (1967), H"RRE et al. (1968). CLIFF et al. (1971), SI\TiR (1974). 423. Discussion of tectonic models Experimental work indicates pressures equivalent to 3!H0 Km depth of burial for the high-pressure rocks of the Western Alps. We do not believe that such high pressures can be explained by tectonic overpressure models (see e.g. RUTLAND, 1964; BRACE et al., 1970) nor they can be explained by burial by the nappe pile, si nce this can be shown to have been too thi n (DI\L PIAZ et al., 1972; D"L PIAZ, 1974 b; HUNZIKER, 1974). T he only reasonable possibility therefore is a subduction model (HUNZIKER, 1970, 1971 , 1974; ERNST, 197 1, 1973 a, 1975; D"L PIAZ, 1971. 1971 a and h; D.u PIAZ et al., 1971, 1972, 1973; BOCQUET, 1974; WUBSCHER, 1975). OXBURGH & T URCmTE (1968). OXSURGH (1972), DAL PIAZ (1974 a and b). DIETRICH (1974), H AWKESWORTH et al. (1975) proposed an analogous interpretation for the Central and Eastern Alps and for the Apennines. Others have ignored or underestimated the geody namic significance of the high pressure-low temperature metamorphism in considering only the paleogeographic and kinematic evolution of the Alpine chain (LAUIISCHER, 1970, 1971; GIESE et aI., 1970; DEWEY and BIRD, 1970; BOCCALETII et al., 1971; SCHOLLE, 1970; RO£DER, 1973). All proposed models appear too simple to explain the complexity of the Alpine tectonic and metamorphic evolution. This situation undoubtedly arises from the still incomplete knowledge of the characters and distribution of the Alpine meta morphism in the Western Alps. Furthermore, structures coeval wi-th the high pres sure-low temperature early-Alpine event have not yet been deciphered at a scale large enough to show the sense of regional coupling. H owever, consensus is apparent on a few points. Evidently. the western Alpine chain is produced by a continental 322 It. CO:lotPAGNONI, G. V. DAL PIAZ, J. C. HONZIKEI\ £. ALTJlI collision with closing of the intervening oceanic basin, followed by a subsequent thermal and tectonic episode (Eoccne-Lower Oligocene) involving further shortening of the crust and finally buoyant uplift (Oligocene to Recent). The latter phase is accompanied by trachyandesitic m:lgmatism and surfieial sliding possibly due to gravity ('). ERNST'S first model (197 1) implies the subduction of a single large plate which comprises the EUfopean continental margin, the ophio li te basin and the Sesia Lanzo Zone. This plate is SUbdUC1Cd under the Southern Alps without disrupting the:: main paJeogeographic setting. A regular monophase metamorphic zonation results from this dynamic event. Pure buoyant uplifting allows the preservation of the high pressure-Iow temperature associations. In later models (EaNST, 1973 a, 1973 b, 1975) the complexity of the metamorphic pattern in the Nonhwestern Alps is considered and the original si ngle plate is dismembered in an imbricated sub duction zone, with external you nging of progressively lower-grade metamorphic recrysta llization; westward stepping of the subd uction zone accou nts for the uplifti ng of the more deeply subduCled imernal pam. DAL PIAi~. et al. (1972) and HUNZIKER (1974) have proposed a different model wh ich takes into account twu tectono-metamorphic events in the Western Alps. The early-Alpine event (90-70 m.y.) produces subduction tectonics with related eclogitic to greensc hist-facies and lower-grade assemblages. The d osing: of the Piemonte basin involves thrusti ng under a low thermal regime. This model also suggests a subduction process involving a plate that is subdivided into several sheas interplaying in a complex way. In this view relative displacements between the sh~ts and different subduction velocities can be justified and a progressive nonh-westwards migration in time and space of the subduction zone is more feasib le. T he internal limit of the subduction zone is represented by the early Canavese line. A rapid rise of fragments of oceanic and continental crust from a depth of JO.35 Km in the subduction zone is required in order to preserve the newly fo rmed early-Alpine metamorphic associations of the Sesia-Lanzo Zone. According to DAL PIAZ. et al. (1972) a 25-35 Km rise appears to be accomplished not simply by vertical uplift, but mostly by movements with a significant oblique component, consistent with the subduction geometry but with opposite sense of translation. This process leads to the fi rst major th rusting phase (early-Alpine). T he earl y-Alpine event is followed by a period of tectonic inactivity relatively to the surface (TRUMPYs, Paleocene restoration). From Eocene to Lower Ol igocene a thermal dome develops, produci ng the second metamorphic event (38 m.y .. Lepontine) and activating polyphase ductile deformation, giving rise to large scale folding of the nappe pile leading to pronounced crustal thickening. C') References to the classic works of LUGEON, SotAIlDT, GAGNI!.BIN .nd SaiNa:GANs on gnvitltion.l tectonics in the Western Alps can be found in HILLS (1963, p. 337-338) and GocUI!.L (196', p. 239-240). THE SESlA-WN"lO ZONE, A SLICE OF CONT INENT,\L CRUST ECC. 323 The general upli£t of the axial part of the nappe belt n, the deposition of the marginal Molasse (Middle Oligocenc to Recent) and the trachyandesitic magmatism (33-25 m.y.) mark a sharp change in the tectonic regime and the beginning of the cXlensional conditions. In the Mio-Pliocene new nappcs involving only the sedimentary cover develuped at the margins of the chain; it is said that they arc related to gravity tectonics and do not necessarily imply a significant amount of shortening in the crystalline basement. 42.4. O{igocene magmatism and timing of continental col/ision Two important topics deserve further discussion: a) the geodynamic significance of the Oligocene magmatism, and b) the timing of the continental collision in the Western Alps. The Oligocene magmatism of the internal Northwestern Alps is represented by volcanic and plutonic rocks. The volcanics comprises andesites, trachyandcsites and lamprophyres, with minor basalts including shoshonitic types CO). The plutonic rocks range from granites to monzonitcs and quartzdiorites. The Oligocene magmatism has a peculiar di stribution both in space and in time: it occurs not only in the overriding Insubric plate (Southern Alps), but also in the Austroalpine and - locally - in the Piemonte nappe. That is to say it occurs also in the sutured subduction zone, with a maximum distribution in a band along the lnsubric line. Volcanic graywackes of Upper Eocene-Lower Oligo cene age, containing up to 90 percent andesite fragments, occur in the external part of the Western Alps (Gres de T aveya nne: MARTI NI, 1968; Thanes syncline, Champ saur, Clumanc, St. Antonin syncline: BIJU-DuVAL & GUBLER, 1961), but the source area of this volcanic material is still unknown. The Oligoccne magmatism develops after the continental coll ision and thus does not represent a volcanic belt of circum-pacific type: ("). DAL PI,\Z et al. (1972, 1973) suggested relating the Oligocene magmatism to the early-Alpine subduction of the Piemonte oceanic crust C1. The ti me gap (over 30 m.y.) between subduction and magmatism according to them may be due to lack of crustal extension until Middle Oligocene. At this time extension begins and melting becomes possible. This interpretation requires a post (9) CLAJ.K and J.... EGEa (1969) quantitativc:ly evaluated the: uplift f'tte IS 0,4-4,0 mm/year. (IQ) New work on the gc:ochc:mistry and gc:ochronology of these rocks is in progress. (11) An Upper Cretsceous andc:sitic magrnatism was rc:cc:ntly ascertained in the ovc:rriding Insuhric plate in the Central (GANSSEI, 1968; c;...S ....TI et aI., 1976) and Eastc:rn Alps (GATro et al., 1976). (12) In the present authon' opinion the Qligocc:ne magmatism cannOt be related to Oligocene subduction of the: more c:xternal Valau basin, bc:cause thil buin does not 5c:c::m to have: had an oceanic cruu. 324 R. COMPACNONI, C . V. OAL PIAZ, J. C. H UNZ1KEIl E ALTJU A simpler mean of explaining the Oligocene magmatism has been propo~d by TR UMPY (1973, 1975). According to this model, closing of the oceanic basin, subduction, continental collision and thrusting occurred during the c Meso-Alpim:_ phase (Upper Eocene-Lowcr Oligocene); crustal shortening would amount to at least 300 Km. This model, however, is not tenable because it ig nores both the high pressu re-low temperature assemblages and the Upper Cretaceous magmatism, and their petrological significance. Furthermore it ca nnot be applied to the Western Alps. An intermediate solution has been devised by LAUBSCHER (1975). A very long subduction episode, occurring from Upper Cretaceous to Eocene, explains the early-Alpine metamorphism ,lIld the Oligocene magm:l.tism. This model, actually, implies a very wide oceanic basement. which contradicts the sedimentological evidence of the Piemonte Zone. Moreover, the si nking into the mantle of such a huge volume of ocean ic crust should have produced an extended Upper Cretaceous to Oligocene volcanic belt of circumpacific size. No trace of such a belt has yet been found in the Alps. In addition a very long subduction phase such as that postulated by L AUBSCHER poses additional difficulties to the rapid exuming required for the subducted materials to escape thermal reequilibration of the early-Alpine high pressure·low temperature assemblages. 5. Conclusion8 The preservation of superposed Alpine metamorphisms in the Sesia-Lanzo Zone - Dent Blanche Nappe and Piemonte metaophiolite nappe makes it possible to discuss the feasibi lity of applying theoretical plate-tectonics models to the Western Alps. The most peculiar feature of the Alpine evolution appears to be the marked duality of its: I) pre- and 2) post-collisional, tectonic, metamorphic and magmatic history. 1) The pre.-<:ollisional history of the Western Alps, even if it is broadly consistent with an Andean-type evolution, differs from this model in some essential aspects ; 0) subduction and high pressure.low temperature metamorphism involve both continental and oceanic crust; b) andesite activity within the overriding plate is ran':; c) there is an apparent lack of high-temperature metamorphism (paired belt) in the overriding plate. Continental collision in the Western Alps does not result in exhumation of the subducted materials in the form of tectonic melanges but in the form of nappcs in which the continuity of large units and their deformation history are preserved. 2) T he posl-collisional evolution of the Western Alps seems to be characte rized by; THE SESIA-L.o\NZO ZONE, A SLICE OF CONTINENTAL CRUST ECC. a) the occurrence of a thermal dome of regional extent (Lepontine meta morphism) overprinting the earlier high pressure-Iow temperature belt. and b) by crustal shortening and thickening accompanying folding of the early Alpine nappes. T he latter process is followed by extensional conditions leading to the andesite magmatism and buoyant uplift. Acknowledgements _ The authors ate indebted to R. G. CoLEMAN and E. V.LLEGAJ.l for pelpful suggestions which much im prove:d an earlie:r draft, 10 W. G. ERNST and H. J. ZWAJ.T for reviewing the: te:xt. Financial support from Cooperative: Project Italy - U.SA. 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