Metadiorites in the Dora·Maira Polymetamorphic Basement (Cottian Alps)
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RENDICONTJ DEllA SQCIETA ITAUANA DI MINERAlOGIA E PETROLOG IA, 1988, VoL 4.H, pp, 593·608 Metadiorites in the Dora·Maira polymetamorphic basement (Cottian Alps) R.iCCAROO SANDRONE Dipartimento di Georisorse e Territorio del Po!.itecnico, corso Duea degli Abruzzi 24, 10129 Torino, Italia ROSAlINO SACCHI, MARCO CORDOlA, DARIO FONTAN Dipartimento di Scienze della Terra (Sezione Geologica) deIl'Universiti\; Centro di Studio sui Problemi ddl'Orogeno delle Alpi Ocddentali, C.N.R., via Accademia delle Scienze 5, 10123 Torino, Italia IGOR M. VILLA Istituto di Geocronologia e Gwchimica lsotopica, C.N.R., via Cardinale Maffi 36, 56100 Piu, ltalia ABSTRACT. - Previously unreported bodies of pre·Alpine diorite bodies into a small area, both in a metamorphic diorite occur in the polymetamorphic supposedly allochthonous basement unit and in the basement of the north-central part of the Dora·Maira supposedly autochtonous terrains of the Pinerolo inlier, Massif in VaI Pellice (Cottian Alps), their size being no calls for critical assessment of the nappe tectonics model more than 100 m. for the Dora-Maira Massif, especially the existence of The relation of these intrusives to the country rock large·scale displacements. is obscured by low.angle tectonics; the rock may be either massive or SChist05e, and displays abundant melanocratic Key words: diorites, Hercynian plutonism, Alpine inclusions, as well as mafic and aplitic dikes; when metamorphism, Pu/Ar dating, Dora·Main Massif. unfoliated, it preserves igneous textural features, whereas the mineralogy was re-equilibrated during Alpine-age metamorphism. The paragenesis includes quartz, a1bite, Introduction zoisite and/or dinozoisite, amphibole, chlorite, white mica ± biotite and garnet; accessory minerals are rutile The data in the literature on intrusives and and/or sphene, ap;ltite, zircon and ore. Major element their metamorphic products referable to the chemistry confirms the classification of the intrusives Hercynian magmatic cycle in the Western as mainly diorite. The REE panerns show uniform downward·convex trend for HREE with LREE Alps (rewiev in D ' AMlCO c., 1974; BONIN B. enrichment, no or slight negative Eu anomaly and no et aI., in press) reveal a distinct prevalence of substantial fractionation of HREE. On the whole, acid terms over intermediate and basic chemical and geochemical data suggest that these rocks lithologies. Moreover, in the Penninic formed by variable degrees of partial melting of 1) amphiholitic sources. Domain, the latter lithologies (Fig. appear Two amphiboles were analysed by the <OArfJ9Ar to characterise only Brian~onnais Units method. The spectra are internal1y discordant and (Internal Zone and Ambin Massif) or probably have no immediate chronological value. The Brian~onnais-like Units (<< Serie Grafitica» of K/Ar ages differ (63 -84 Ma). The Ca{K ratios are also the Dora-Maira Massif), and their intrusion erratic. We therefore supPOse that the step·ages reflect irregularities in the individual micfO$tructural domains, level is very high. rather than a gwchronological memory. However the In the Internal Zone of the Great St. presence of step· ages around 16 Ma in both samples Bernard- Brian~onnais Zone, which is the suggests a final disturbance in the Lower-Middle Aosta Valley equivalent of the Vanoise·Mt. Miocene. Similar rocks in nearby Val Chisone intrude the Pourri Zone (BOCQUET J., 1974), the Cogne Carboniferous sequence and, on general geological Val Savaranche pluton is in fact composed of grounds, are thought to be Permian. The crowding of prevalent tonalites with minor granodiorites, 594 R. SANDRONE, R. SACCHI, M. COROOLA, D. FONTAN, I.M . VILLA diorites and gabbros, which were probably upper member formed of pre-Carboniferous intruded in Permian times (NOVARESE V., metasediments and metabasites, and a lower 1894, 1909; FENOGUO M., RICAULTG ., 1957, member composed of probably (Permo) 1959, 1962; AMSTUTZ A, 1962; GRASSO E., Carboniferous metasediments. Both members 1974). contain metagranitaids with granite to tonalite In the Ambin Series (Ambin Massif), chemistry (VIALON P., 1966; MrCI·IARD A., metadiorites associated with aplhic gneisses 1967; ZANETTIN LoRENZONI E., 1967; have been reported by CALLEGARJ E. et al. BORToLAMI GC., OAt. PIAz G.V., 1970; (1980), and small metagabbroic masses have BORGHI A. et al., 1984). also been described by POGNANTE U. et al. A «Lepontine » metamorphic re (1984). equilibration in the greenschist facies may be If we leave aside the small Guglietta mass observed in all these lithologies. Edogite relics (Valiane Giulian Cruello) in upper Val PeIlice, of probably early-Alpine age are locally identified by the authors of the Pinerolo Sheet preserved in the merabasires (STELLA A., of the geological Map of Italy (1913), dioritic 1895,1896; FRANCHI S., 1900; COMI'AGNONI gneisses in the broad sense have so far only R., SANDRONE R. , 1981; SACCHI R. et al., been described for the Dora-Maira Massif 1983; POGNANTE U., 1984; BORGH! A. et al., wi thin the (Permo)-Carboniferous cover 1984, 1985; POGNANTE U., SANDRONE R., exposed in the «Pinerolo Tectonic Half submitted). Moreover, assemblages including Window» (FRANCHI S., NOVARESE V., 1895, chloritoid, zoisite, phengite, paragonite, ZANETI1N E., 1964; VIALON P:, 1966; kyanite, garnet and rutile, are reported in the MICHARD A., 1967; ZANE'ITlN LoRENZONI E., metasediments (GRILL E., 1925; VIALON P., 1967). The Brian~onnais affinity of this cover 1966; BORGIO A. et al., 1984, 1985). An has been accepted by MICHARD A. (1967), unusual paragenesis containing coesite and OGNIBEN L. et al. (1975) and BORGHI A. et pyrope has recently been described in the 01. (1984). central portion of the Massif (CHOPIN C., New detailed surveys have led to the 1984). discovery in the Pellice-Angrogna ridge of By contrast with the well·documented several diorire bodies within the micaschisrs picture for the other internal crystalline of the polymetamorphic basement of the massifs of Monte Rosa (BEARTI-I P., 1952; OAt. Massif (Fig. 2). PIAZ G.V., 1971) and Gran Paradiso This paper describes the petrographic and (COMPAGNONl R. et al., 1974; OAL PIAZ G.V., geochemical features of these rocks and LoMBARDO B., 1986 and bibliography), the discusses their geological implications as far Oora-Maira Massif displays blurred evidence as the reconstruction of the structure of the of a pre-Alpine metamorphic event Oora-Maira massif is concerned. (COMPAGNONI R., SANDRONE R., 1981; BeRGH! A. et al., 1984, 1985; SANDRONE R. et al. , Regional geological setting 1986; SANDRONE R. , submitted). T he sedimentary protoliths in the pre The Oora-Maira Massif is a Penninic unit Carboniferous basement are mainl y made of continental crust. I t consists of an represented by pelites with subordinate, more Fig. l. - Slruclural $ket,h.map of Weslern Alps with kx:ation of I!lel.d.ioricic bodies in the Penninic Domain (grey reclangle: Dora·Maira massif in Fig. 2). 1: Oauphinois.Hdvetk Domain (dashed line: external crySlallinc: mllSSifs); 2: Prealps; }: Subbri~nnais Zone:; 4: Bria~nnais Zone (grey: pl:uton of Cogne-VaI SlVararKhe; Slat: meudiorites in Ambin MIlS$if); ,: Scmpione: Tidno Nappc:5 and Camughc:n.Monc:ucco Zone; 6: Inlernal cryslalline massifs (MR .. Mome Ron, GP .. Gnn Paradiso, OM .. Don·Maira); 7: Piedmontesc: Zone, Versoyen, Monlenotle and Scslri.Voltaggio Unils; 8: Alpine: Hdmimhoid F1ysch of Ubayc:-Embnmais arxl Liguria; 9: Scsia·Lanzo Zone: (St) and Oml Blan,he Nappc: (OBL); 10: Soulhern Alps; 11 : Terliary, posl.kinematic intrusives of Traversella and Valle dd Cervo; 12: Apennines and Monferrato; U: Quaternary and Terliary sedimenlS of Ligurian-Piedmontese basin and Po Plain; 14: Canave$!: Line (C Lland boundary belween Alps and Apennines (Sestri· Voltaggio Line Au,t.). Heavy dashed line: SC:Clion$ of Fig. 8. METADIORITES IN THE DORA·MAIRA POLYMETAMORPHIC BASEME NT (COTTIAN ALPIS) )95 BI E LLA Po Pl li n 1I~... 12 8 Liguri l n S .. "0 ~, " " 14-- 'm , . 596 R. SANORONE, R. SACCHI, M. COROOLA, D. FONTAN, I.M. VILLA .. ••. '.4. '. ,.-.•.•• ••.. 'D 'D..... 'D. • • · 0 • ' . I •I Fig. 2. - Sketch map of north-ccntral Dora-Maira Massif (from VIALON P. (1966) and authors' unpublished data). 1: Quaternary cover; 2: Permian/Carbonifcrous terrains (.sent Gtafitiepl; ) : mctadiorites; 4: granitoids; , : pre Carboniferous, polymelamorphic basemenl (al and Mesozoic metaophiolites and . $china Lustris_ of PiedmontcK Zone (h); 6: new occurrences of mcur.dloritcs (1 : Giasset; 2: Cone dei Fons; ) : Subiaschi). or less dolomitic limestones, whereas coarser COMPAGNONl R. et al. , 1982·1983; VIALON'S clastic facies predominate in the (Permo)· «porphyro"ides arkosiques& and «Jouglard. Carboniferous cover. Selleries Complex &gneisses of BORGHI A. et In the light of field evidence and ,I.); petrographical and chemical data, the - gneisses with pluricentimetric, metagranitoids can be divided into: polycrystalline feldspar eyes, abundant red· - gneisses with granite chemistry and brown biotite, and granodiorite chemistry mainly augen structure, consisting o f quartz (VIALON'S «gneiss amygdalaires&); + microcline + albite + dinozoisite/epidote - medium· to fine-grained dioritic, + biotite + white mica (VIALON'S «gneiss granodioriric and tonalitic g ne isses, oeilles homogenes& and the «Freidour·type & consisting of quartz + plagioclase (albite gneisses of B ORGHl A. et aL) ; oligoclase) + biotite + chlorite + blue-green - gneisses with granite chemistry and amphibole + c1inozoisite/epidote and/or an augen st ructure, a very much