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
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
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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 finer grain, zoisite. and richer in white mica (<< Luserna& gneisses W ith the exception of the certainly older of P AG LlANI G. 1954, BARlSONE G. et al., «gneiss amygdalaires» (SANDRONE R. et al., 1979, SANDRONE R. et al. , 1982, and 1986), these metaintrusives are regarded as METADlORITES IN THE DOM·MAIM POLYMETAMORPHIC BASEMENT (C01TIAN ALPIS) 597
Hercynian by analogy with lithologies dated with «gneiss amygdalaires». All the lithologies in other sectors of the Penninic Domain di~lay a transposed foliation as well as (review and discussion in DAL PIAZ G.V., isoclinal folds with a subhorizomal axial plane, LoMBARDO S., 1985). and referable to Alpine tectonics by A different view is expressed by ZANETTlN comparison with the deformation of the E. (1?64) and VIALON P. (1966). Mesozoic «Schistes Lustres». Preserved magmatic relations and thermometamorphic effects cannot be Field occurrences discerned in any of these outcrops. The examined metadiorites are located in Minor occurrences of semi-in-situ diorite the ridge between the Pellice and Angrogna (not indicated in Fig. 2) were found at Pra Valleys, where they form a series of lenticular del Torno and Rocca Sparviera in Angrogna bodies of varying, though always relatively Valley. modest, size. They outcrop as more or less dark grey greenish gneisses wi th a regular medium to / fine grain. The original magmatic structure can still be at least partly discerned in the best / preserved facies. In this case, the rocks have / a granular appearance. They are usually more deformed on the borders of the bodies with a structure that is first flaser, with small, pale, very flat eyes, and then more or less markedly banded. ?--7'7"~t=~c-> , .. o. A body of approximately 100 m dimension _ .. ~ 0 outcrops along the bed of the Angrogna .- stream near the Giasset district between 1520 o • and 1700 m above sea level. The country rocks are garnet and chloritoid micaschists , belonging to the polymetamorphic basement Fig. 3. - R1-Rl diagram (de LA ROCIIE H. et al., 1980) of the Dora-Maira Massif. The contact is for dioritic-granodioritic-tonalitic metaintrusives from masked by a detritic-eluvial cover. Apli te the Dora·Maira Massif (full ci rcles; analyses 1 to 10 of dikes parallel to the regional foliation (dipping Table 1; open ciclcs; literature data from VIALON P. 10° towards 280°) and displaced by shear (1966) and ZANETIlN LoIlENZONI E. (1967) and our two unpublished analyses for Val Chisone rocks; gb-di: zones cut the metadioritic body locally. gabbro-diorite; di; diorite; mo; monzonite; to; tonalite, A second outcrop of comparable size lies grd; granodiorite; gr: granite). on the crest separating VaI Pellice from Valle Angrogna near the CoIle dei Fons (2200 . Petrography 2296 m a.s.l.). The rock is well exposed on a vertical wall and contains abundant basic Microscopic examination shows that these dikes of decimetric thickness, always parallel metadiorites are composed of quartz, albite, to the regional foli ation (dipping 30° towards zoisite and/or clinozoisite, amphibole, white 10°). The contact with the surrounding mica, biotite, chlorite and garnet, with micaschists is marked by mylonitic levels. accessory rutile, sphene, apatite, zircon and A third outcrop is located around 1300 - ores. 1500 m a.s.1. in the Subiaschi Valley, a left The mineralogy is in keeping with the well hand tributary of the Pellice stream. The documented metamorphism of the northern metadiorites contain basic dikes and part of the Dora·Maira Massif (BORGIIl A. et centimetric to decimetric melanocratic al. , 1985, with references; POCNANTE V., inclusions. They SAOW a strongly tectonised SANDRONE R., submitted), namely, an earlier contact with metabasites and a gradual contact event under high·pressure conditions followed R. SANDRO NE, R. SACCHI, M. CORDOLA, D. FONTAN, I.M. VII.LA
510, • .~ · "., • 19,.0 0 • • , • • , , • ·. , , • . • ,. .- • . · " • • .. ..· .. • • 0 , , , •• • "'':0'1:11 , , • , .~ ,• , • . • '. • • " · •• • "". , • · • , • , · · , • • • · , ... ~ · , •• • • • , · • • ". , • .· • , .. ~ ,". , " • • · , , • , , .• • .:, , 110, .' · • •• • 'r, , · . , • , , , · , • , ,~ , · • , • • • • .' , " • • ! '.· · • • , , • • • -. , , ,- ' • ' " • · , · , , , , , " • • , , ~.,o. . ~. ''''. 'r , , , , ',0, • • • , , '.· • • . , •, , -, · • • .. , " , , · , · • • , , • ... , . • St,"8o , , - • • • · ...... , , , ., , .. ro .. .. ro -. • L-I. " u . " Fig. 4. - Major dements vs. Larsen Index diagrams for meladiorile5 (cirde5), mdanocralie inclusion (square) and mafie (diamonds) andaplilic (triangle) dik«. Full symbols refer 10 samples of Table 1; open symbols represent literalUn: data (VIALON P., 1966; ZANETl1N loR£N7..ON1 E., 1967) and our two unpublhhed analyses for Val Chisone rocks. by re-equilibrations in the greenschist fades. poikilitic pycnochlorite filled with colourless The primary paragenesis was probably actinolite, locally containing relics of represented by quartz, plagioclase, hornblende glaucophane, and/or biotite-bearing and biotite. This, however, has been symplectites pseudomorphous on a previous completely transformed by metamorphic amphibole. In the more deformed rocks, recrystallisation, though the sites of the however, it is replaced only by biotite original magmatic minerals can still be symplectite, at whose expense blue-green discerned in the less deformed facies. actinolitic hornblende and ripidolite grew. Primary plagioclase is transformed into Microprobe checks confirmed the a1bite + zoisite, itself often indpiently or even compositions of amphiboles and chlorites. competely ~placed by c1inozoisite ::t: garnet, The original biQtite sites can usually be the latter usually marking the contacts with readily identified owing to their local the original amphibole and biotite. enrichement in titaniferous minerals (rutile :I: The magmatic amphibole has undergone sphene) and the growth of coronitic garnet. two types of transformation. In the less O ther biotite trasformation products are: deformed rocks it is replaced by millimetric, white mica, chlorite, blue· green amphibole METADlORITES IN THE DORA·MAlRA POt.YMETAMORPHIC BASEMENT (CarTIAN At.PIS) ~99 and newly formed biotite. and, apart from the greater abundance of Melanocratic inclusions and mafic dikes mafic minerals and ores and the lack of quartz, show very similar mineralogic compositions their minerals are the same of the host metadiorites . .. • ~ · · Chemistry Table 1 sets out the analytical data for the major elements, some trace elements and rare • · • · · earth elements (REE) determined in the • • · diorites, melanocratic inclusions and dikes • • • cutting the dioritic bodies. Except for an aplitic dike, the chemistry ~ is remarkably uniform: intermediate Si0,,2 • values (52·59%), distinct peraluminous trend, •• • • and prevalence of Na20 over K~O. The • · analyses fall within the diorite and gabbro. • • · diorite fields (Fig. 3) on the classification · · diagram of de LA ROC HE H . et al. (1980), • in keeping with their mineralogical characters and their normative composition (MIELKE P., W INKLER H.G.F., 1979). This covers " a relatively broad range, though it is • · • • substantially formed of quartz (0·26%), • plagioclase (18-54%, composition An ·An'2)' • • • • · S • hornblende (5·49%), usually predominant • · over biotite (7-22%), and corundum (1- 6%), in keeping with the peraluminous nature of , • these rocks. There is considerable scattering of the • • • • • · ·· representative points on the major elements • · vs . Larsen Index (1.1.) diagrams (Fig. 4), which also include data from the literature and
[ ~El our two unpublished analyses of diorites s.l. • from the (Permo)·Carboniferous terrains of · Val Chisone, for the purpose of comparison. The data of the literature, though 20 years , old, compare we ll with our unpublished '00 • • • analyses plou ed in figures 3 and 4. • • • · Si02 and (albeit with even greater scattering for the Val Chisone rocks) K20 are L.IV. positively correlated with the L. I. The values • of AGO) are so scattered that a significant · trend cannot be detected, even though the samples from ValIe Angrogna and Val Pellice seem to display a positive correlation; taken 0 · · · all together, the data are arranged in a bell· • • • • · · shaped curve. The values for CaO, Na~O, -, 0 , L.t and P20, are rather scattered, though a Fig. 5. - Ba, Rb, Sr, Y, I: REE and U/Yb~. Lanen negative correlation with the 1.1. can be Index diagrams (symbols 15 in Fig. 4). observed for CaO, MgO, total Fe20) and 600 R. SANDRONE. R. SACCHI, M. CORDOLA, n FONTAN, I.M. VILLA
MnO (not shown in Fig. 4). TiO~ tends to nil Eu anomaly (Eu/Sm = 0.22-0.33) and a display two different trends, one with a slope total REE range of 52 to 252 ppm. (distinct negative correlation) given by the The patterns for the melanocratic inclusion three samples from the Giasset body plus the and the basic dike are also very similar: a slight data from Val Chisone bodies, the other with negative Eu anomaly CEu/Sm = 0.20-0.25) and a nearly-flat trend. The G iasset analyses also less REE fractionation than in the diorites d isplay higher MgO and Fe20) contents for (L.N/Yb N " 3.73·2.92; the same L.1. DYN/ybN = 1. 06· 1. 0)). The trace elements vs. 1.1. diagrams are set If the twO analyses of the Giasset out in Fig. 5. Apart from the rather flat metadiorites (MDM 143 and MOM 147 of pattern for Y, there is a variously marked Table 1) are left out, and only the patterns positive correlation. In the diagrams for Ba, falling in the grey fields of Fig. 6 are I; REE and LafYb the highest values for the considered, both LREE enrichment and total same 1.1. belong [0 the points representing REE range decrease (LaNfYbN = 8.19-3.58; the Giasset diorites. 52-114 ppm). We are then dealing with rather The REE patterns in Fig. 6 are normalized flat patterns, downward-convex for HREE. to chondrite Cl (EVENSEN N.M. et al. , 1978). We are aware that the chemical and The trends are very similar: LREE enrichment geochemical features of metamorphic rocks (LaN/YbN = 15.87-3.85), absence of should be viewed with caution (see e.g. substantial HREE fractio nation H UMPHRlS S.E., 1984; VOCKE R.D. et aI., ( Dy~ b N - 1.3 4-1.07), slightly negative or 1987, with references); however. the above
500
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u "• 50 j ..... _- ," ----., ,• " "• ,. - --
5
La Ce Nd Srn Eu Gd D, ,. Vb lu Fig. 6. - Rare earth element patterns for 11 metadiorites, one melanc:x;ratic inclusion (heavy line) and one mane dike (heavy line with circles). Thin dashed lines and thin full lines: patterns with respectively slightly negative and nil Eu anomaly. Except for MDM 14) and MDM 147, all metadiorite analyses (Table 1 and two un(:lublished analyses for Val Chisone rocks) plot in grey field. METADlORlT£S IN THE DORA·MAlRA POLYMETAMORPHIC 8t\SEMENT (COTIlAN ALPIS) 60 1
TABLE 1 Chemical analyses for metodiorites (anal. 1·10), me/anocratic inclusion (anal. 11), mafic (anal. 12·1) and aplitic (anal. 14) dikes. MDM 139, MDM 143 and MDM 147: Giasset metodiorites (normal, mica-rich and /oliated/ades; U. T.M. coordinates: 32TLQ53J69.5); MDM 1.58 and MDM 168: Pra del Torno metmiiorites (massive /acies; coord. 32TLQ5.57703); MDM 169 and MDM 170: Pra del Torno metodiorites (foliated and dark /acies; coord. 32 TLQ.5.57703); MDM 164: metodiorite from Vallone Subiaschi (coord. 32TLQ517664); MDM 16.5: metodiorite from Colle dei Fons (coord. 32TLQ.537679); MDM 167: metodiorite from Rocca Sparoiera (coord. 32TLQ52770I); MDM 161: melanocratic inclusion in MDM 1.58; MDM 163: mafic dike in MDM 164; MDM 166: mafic dike in MDM 16.5; MDM 134; ap/itic dike in Giasset metmiiorite. *: Fel~.J as total Fe 1 2 3 4 5 6 7 8 9 10 11 12 13 14 s.Jnple -,. -~ -,n -'M -'M -'H -~ -_ -_ -,n -u, -_ -,. -'M (wt. \)
Sl02 5 • . 23 56.96 57 .48 54.63 56.83 58 . 37 52 .58 55. 41 52 . 0 55.87 52.95 52.07 52 . 40 72 . 46 TI02 1.65 1.37 0.98 0 .87 0 . 10 0 . 80 O. H o.ao 0 . 72 0 . 85 0.13 0.85 0 .81 0 . 21 -'120) 17.08 16.78 18.92 19.06 U . H 18.14 15.10 16.11 19.14 11 . 25 14.41 16.74 15.54 14.91 FeW3 • 9.79 8.59 7. SO 8.06 7.30 6.95 7.71 7.73 •. 59 7. 53 9.01 • . 77 9 . 87 1. 89 0 . 17 0.11 0.09 0.14 0.14 O.il O.il O.H 0 . 17 0.13 0.15 0.12 0.19 0.05 ... 6.H 4.27 2 .48 3.54 3 . 43 2.10 7 .42 6.39 4. 79 3.51 '.64 7 .'2 7.10 0.69 -<~ 7.)5 5.21 4.87 7.4) 6 . 71 6.24 5.56 7.14 7.72 7.U 6.09 5 . 71 7.93 3.1i1 .. ,. 2.00 3.0. 2.1l 2.66 3 . 41 3.41 4.46 2.73 3.U 2. 16 4.32 4.44 3.56 1.26 .,. o.n 1.87 2.ll 1.71 1 . 11 1.95 1.20 1.41 0.11 1.63 1.01 0 . 52 0.71 2.27 0.46 0.1I 0 .4\ 0.27 0.21 0.19 0 . 22 0.21 0.21 0.25 0.22 o.n 0 . 21 0.16 .""J.t. 1.97 1.60 2.36 I.U 1.27 1.32 J.lt 1.59 I . U 1.13 1. 54 3 . 0 4 1 . 09 1.56 Total 100. 04 100. 13 99.35 99." 100 . Jl 100.)0 98.63 99.19 99.41 " . 81 99.07 100. 10 99." n.41
(ppool 296 667 684 •• 185 .57 OH 186 46 121 Hl 21 63 69 '".. '" 0' 38 48 '" )5 19 13 67 4U 385 571 ... " 160 240 " 101 235 162 791 21 II 2a '" '" 19 n.d. '" 45 n.d. 19 n.d. B.05 57.92 62.S4 18.57" 1I.n" 20.30" 1],01 '.n 1t. 07 17. 60 10. 22 n.lI 106. 08 111.47 37.65 44 .10 4] . 61 25 .67 20 . 52 41.43 45.46 24 . 6' 24.50 46.66 48.99 16.60 19.76 20.06 13.00 9." n.17 27.11 10.90 S.25 I.U 8.n 3.92 4.57 4.61 1.B 2.74 4.H 7 . 15 2.89 1.74 1.88 2.55 I.U 1.11 1.16 0.90 0." 1.25 1. 44 0 . 71 4 .56 7.11 1.14 l.n 4 .51 4 .62 ] .54 2.94 4.46 7.09 3.10 3.73 5.12 4.61 1.H 4. 01 4.17 1 .10 2 . 10 1.98 6.65 2." ~.Ol 2 . 74 2.59 1.19 2 . 3 4 2.ll 1.74 1. 70 ~.21 1.U 1.71 1.90 l . Sl 2.66 1.91 2 . 21 2.47 1.73 1.66 2.19 4.07 LIS 0 . 29 0 •• 1 0.41 0.39 0.31 0.33 0.29 0.21 0.41 0 . 62 O. )0 data suggest some inferences, namely: amphibolitic protolith is indicated by the 1. The various diorite bodies of Valle patterns in Fig. 6; Angrogna are co-magmatic; 4. The patterns of MDM 143 and MDM 2. The good linear correlation of La/Srn 147 display rather high LREE values for these and La/Yb ratios with La (codfident of lithologies, although the La/Srn to La and determination r2 "" 0.966 and 0.980 LafYb to La ratios are not unlike those of the respectively) suggeslS that these rocks formed other samples. The Giasset rocks might thus by variable degrees of partial melting (see be taken to represent the product of either TREUIL M., JORON ].L., 1975); early·phase partial melting or metamorphic 3. Among the various protoliths deemed modification. The two samples belong, in fact, suitable to yield melts chemically similar to to an abnormally mica·rich and a markedly those of the Angrogna rocks (see e.g. CUllERS foliated rock respectively; R.L. , GRAF J.L., 1984; STERN R.). , 5. On chemical grounds, the metadiorites GOTTFRIED D., 1986, with references), an from VaI Chisone and Valle Angrogna would 602 R. SANDRONE, R. SACCH I, M. COROOLA. D. FONTAN, I.M. VIUA
TABLE 2 40Arp 9Ar step-heating data. Ar concentrations all in pi/g. Entry «rad» in total is radiogenic 40Ar, in nI/g. K concentrations are determined from known production ratios, total J9 Ar and ]. Total ages represent conventional KjAr age. • (101.) A' .n, 9 .J. 1.01 1-' ... , ' 0"., )0"., HA. "M , ,~,
.H • . U ~ G.'" 0.,,, ,"".' .' , • , • , • !'.....' 'N u •.• ...... '.It, "., , .. , <0 ,. • , , , ., ...... un, t.'" 1. '1] O.il] 11.' , , " , • , • , ., ,.,. ,.. , n ... ' .. ... , 0.". "'.l , • , " , .. , • , ., 11 •• ..., l .a, n. , .. O.US 7 1. 1 , , " , .... , • !' .• ,... 116.' G.'" , .. ,'.'0 •' ,',' " , • !'.....' UH .,,, •. uS •. nl" , , O.l", ". , .. , , • , !l. ' - To,.. L C..... '. SI) '.n) ".7 ". Fig. 7. - Age and CalK spectrum for actinolilic hornbleooes A3 and A4. All spectra are heavily . ~ , ' ••11- ' .. ", · disturbed; see lext for di$CUssion. , )lA'. HA. , ,~, <0... . ' lA, hornblendes was only partly satisfactory. The crushed rock was sieved and the mafic <0. ,m ,.. " . 2." ,'. ". ,11 •• • " ! . " • .. minerals magnetically enriched. However, ... u n 1 . ••• ..,. I.U' n. ' , • , .. , • , , , .. visual inspection of the finest fraction (100 'H' ,u .., ,1. '.f ,n." ,•• ,96, ,t •.• Ilm) revealed the ubiquitous overgrowths of .. • .. other minerals besides amphibole. Treatment ,n. "'! ..f" I.U 0.6" n.' , • , " , • , • , .. with heavy liquids produced a series of density 'nO ..., ..... " , •. ,u "0.' , , U , ..• , • , ••• fractions, none of wich was completely pure. u. ,... , ,G. '", ,'.n ,.. n-o ,.. ... The least contaminated fractions were chosen . • " ... Irradiation and analysis of samples follow Tot •• 1." • '. '71 ... Ion) ' 0.' ODDONE M., VlLLA I.M. (1986) . The data are shown in Table 2, after mass spectrometer seem to belong to one and the same magmatic background and discrimination and 37 Ar suite, the Val Chisone rocks being the most decay corrections. Ages calculated after fractionated products (Fig . 4). correcting for Ca interference are shown in Fig. 7. Both age and Ca/K spectra are strongly Ar/ Ar data discordant, indicating a very disturbed system. Two new samples (A 3 and A4) of massive Ages vary betwec=n 14 and 170 Ma, and metadiorite from Pra dd Tomo were collected interpretatio ns are necessaril y very for Ar/Ar dating in order to pinpoint the speculative. The low ages of steps A3· 1200 recrystallization phase and to look for possible and 1400 suggest that most of the gas released chronological links with known tectonic by A3 is fraught with excess Ar. Its two 14 phases. Ma step ages are also compatible with that of The separation of pure ac tinolitic A4· 700. The spectrum of A4 could , in METADlORITES IN THE DORA-MAIRA POLYMETAMORPHIC BASEMENT (COITlAN AU'lS) 603 principle. reflect a very strong Oligo-Miocene discuss these authors' results because no thermal disturbance of a sample older than measured isotopic composition are given, and 200 Ma. Even if this is consistent with the calculated ages may be wrong because of an presumed geological history of the Dora-Maira incorrect initial. Our limit, on the contrary, Massif, the present data cannot be used to conflicts with the conclusions of CARPENA support it. Firstly, samples A3 and A4 seem j. et a!. (986) for the Piedmontese unit. to record two antithetical histories, but the Three points are controversial, in their petrography fails to show a substantial reconstruction: difference such as to justify their contrasting 1. The temperature is postulated to have behaviour. Secondly, the pervasive remained below the fading temperature for recrystallization visible both in thin section apatite, 100°C, during the last 44 Ma. This and from the CalK spectrum should have implies that the uplift rate was at most 70 prevented the newly formed amphibole from Jlm/a (or less, according to the assumed preserving chronologically meaningful 40Ar gradient in the Dora-Maira nappe). This is not gradients. implausible in itself, but should be proved by The CalK spectra reveal a number of independent, non-radiometric arguments; features. The ratios change by two orders of 2. The evidence for a Cretaceous magnitude. which is fairly surprising in a greenschist phase rests on the erroneous separated mineral. The very irregular determination of Te in zircons by HURPORD behaviour points to two explanations: A.]. (1983, 1986), who calibrated it against L At temperatures below 1 100°C, the biotite Te, using the discredited value of presence of micron-sized biotite intergrowths 300°C (see discussion in DEL MORO A. et al .• (confirmed by microprobe analyses on the 1982). C.... RPENA j. et al. (1986) themselves separates) has the effect of lowering the CalK realize, but quickly dismiss, that 350°C did ratio, at the same time contributing some of not reset zircons in the Schistes Lustres the uncomfortably high K concentration blueschists. This means that the observed (T,bl.2). zircon ages do not conflict with a Tertiary 2. At temperatures where biotite is greenschist phase; degassed, the CalK ratio is still not constant, 3. The existence of an orogen-wide very probably owing to complexities in the Lepontine Phase at 38 ± 2 Ma is also being amphibole microstructure. These complexities challenged in more recent works (Purr ].P .• are expected to occur in a low-temperature 1986.1987) and extensive radiometric data reaction such as fluid-induced compilation (DESMoNs J.. HUNZIKER ].c., recrystallization, where successive generations 1987) shows that the situation is more shaded of unmixing episodes follow from varying than a simple single peak. It is thus not longer regimes of subsolidus reactions. necessary to twist all the data to fit an The presence of biotite intergrowths may interpretation in the light of a synchronous be used to draw one final chronological uplift of the whole Alpine chain. and the inference. The step with the lowest CalK northward migration pattern of the data of ratio, A3-900, is presumably the richest in CARPENAj. et aI., (1986) simply means that newly formed biotite. It is well known that rocks which were originally 30 km deep were excess Ar makes metamorphic biotite one of brought up at different times in the different the worst chronometers there is (see e.g. portions of the orogen. Clearly 2 amphiboles. BREWER M.S .• 1969). Its age (59 Ma) thus 3 biotites, 3 apatites and 4 zircons are not represents a strict upper limit for the quite sufficient to unravel the complete recrystallization event. history of the Dora-Maira nappe. This limit is consistent with the limit of «29 In conclusion, the Ar/Ar data show that the Ma or younger» (30 Ma with the new decay amphiboles are very disturbed, which we constants) given by VIALETTE Y., VIALON P. already knew, and that they recrystallized for (1964) for biotite in a metagranodiorite from the last time in the Tertiary, possibly as Val Chisone. However it is impossible to recently as the Miocene. 604 R. SANDRONE, R. SACCH I, M. COROOLA. D. FONTAN, I.M. VIllA ,. .. "' CC[ C; ~IO N. I""'eco !, " \. " ~, ______"lfc-______~~ O k~ Fig. 8 - Contrasting interpretations of Dora-Mair. Massif by VlALON (A) and ARG.-.NO (B) (from VIALON P., 1966). A - I: Don-Main pre-Carboniferous; 2: Carboniferous; J: Permi,n and «siliceous. Trias; 4: Schistes Lustres and «carbonaticlO Trias. B - 1: .. Serie Grafiticu (Great St. Bematd Nappe); 2: Don-Maira ~-C arboniferous basement (Montc Rosa Nappe); }: Trias and Sc;hiues Lustt~ . Discussion and conclusions The relation between this series and the In the last 60 years, the: Middle Penninic poly metamorphic terrains in the Dara-Maira crystalline massifs (Dora-Maira, Gran Massif has been the subject of controversy Paradiso and Monte Rosa) have been of ever since the publication of the Argentera central importance in the controversy Dronero (78-79), Pinerolo (67) and 5usa (55) concerning the nappe structure of the Alps. sheets of the 1:100,000 Geological Map of Although the debate has now virtually Italy. ARGANO's «nappe» interpretation (1911) subsided, owing to general acceptance of the was contested by the autochthonistic view of view that the chain has inde«l. a nappe FRANCHl S. (1929) and his colleagues from the structure, this consensus become less general Regia Ufficio Geologico. These contrasting when the discussion turns to the details of the views were taken up again several decades structure and to the relative importance of later by VIALON P. (1966) and MICHAR D A. fragile and ductile tectonics. (1 967). In ARGANO'S opinion, shared by The Gran Paradiso and Dora-Maim Massifs OG NIBEN L. et al. (1975) in the «Structural are of particular in terest in this connection. Model of Italy», in the Dora-Maira Massif a In the former, COMPAGNONI R. et al. (1 974) palaeogeographically more internal unit have detected a sequence of metasediments (polymetamorphic crystalline schists with the affected by Alpine age metamorphism only _ remains of Permian-Mesozoic « Piedmontese » the Money Complex<- outcropping in a covers) was tectonically superimposed on the tectonic window below the polymetamorphic more external, Briam;onnais unit, i.e. the terrains. They regard it as Carboniferous in «5erie Grafitica del Pinerolese», which view of its affinity with terrains of this age outcrops through a tectonic window in the (<<5erie Grafitica del Pinerolese») in the Dora Pinerolo district. According to the Maira Massif. «autochtonists», on the other hand, the METADIOR ITES IN mE OORA·MAlRA POU'METAMORPHlC BASEMENT (COTIIAN ALPIS) 60.5 Carboniferous terrains constitute the DM PIAz G.V. et al., 1977; BIGIOGGERO B. stratigraphic cover of the overlying et al. , 1981, 1983). In particular, BIGIOGGERO poly metamorphic complex. These two B. et al. (1981, 1983) described several bodies viewpoints are illustrated in sim ple graphic of anorogenic Ti-rich olivine d.iabase spatially form in Fig. 8. The allochthonist associated with the late Hercynian granite in interpretation has recently been adopted by the Biella district. It may also be noted that SACCI'I! R. et al. (1983) and BORGHI A. et al. a similar petrogenetic mechanism (melting in (1984). the deep crust triggered by the rise of A few words may now be said with regard asthenospheric material) appears in the early to the contribution offered by our data to the orogenic phase in evolution moods widely problems just described. accepted for the Proterozoic, such as that of We feel it reasonably certain that the KRONER A. (1983). In NW Scotland, a similar eruptive bodies we have identified in the evolution accompanitd the early phases of the Pdlice basin are coaeval and consanguineous opening of the Atlantic Ocean, as shown by with the more extensive pluton of the lower Sr and Pb isotope compositions (MOORBATH Val Chisone (Malanaggio diorite and satellite S., 1978, p. 410). bodies). The lithology and fabric are analogous 2. The age and position of the diorite and the chemistries are compatible, as al ready bodies are of substantial relevance to a argued. problem of Alpine tectonics. In the lower Val As far as the age of this magmatism is Chisone the diorites intrude rocks belonging concerned, no geological or radiometric to a Brian~onnais formation (the evidence has come forward against its Carboniferous in the Pinerolo district). Some traditional attribution to the Permian. It 7 to 8 km away - a very small distance on should not be forgotten, however, that this the scale of the orogen - the rocks we have is based on somewhat slender grounds, namely described are includtd in a basement of (1) the intrusive relations with the terrains Piedmontese affinity. This crowding of pre assumed as Carboniferous in age and (2) the Alpine diorite bodies into a small area, both general metamorphic transformation into in a supposedly allochthonous basement unit greenschist facies, this being manifestly of and in the supposedly autochthonous terrains Alpine age. A pre-Cretaceous age would be of the Pinerolo inlier, imposes critical re confirmed if high-pressure metamorphic assessment of the nappe tectonic model for transformations were to be detected in the Dora-Maira, especially the existence of large metadiorites. Such evidence, in fact , is given scale displacements. As an alternative, the age by the occurrence of glaucophane, Mg of intrusion could be doubted, a problem that chlorite, zoisite and rutile. has not yet been settled by the available If the Pennian age is accepted, our findings radiometric data. involve certain implications that invite reflection. Acknow!edgml:lltJ. - The wrilers wish 10 acknowledge 1. As has been stated, the chemistry of stimulating discussions on the subje<:t of this paper with the diorites points to an origin by melting of R. Compagnoni (University of Turin) and G. Poli (University of Perugia). Mineral analyses were carried amphibolites. This does not conflict with the out by means of the ARL SEMQ instrument installed data of regional geology, if we consider the and maintained by C.N.R. (National Research Council) widespread metabasites interbedded within at the Department of Eanh Sciences of the Universit y pre-Carboniferous metasediments. The most of Milan: V. Oidia, A. Borghi and P. Cadoppigave kind help. Chemical analyses were perfomed at the Centre likely scenario is mande activity under poSt· de Recherches Pl!trographiques et Geochimique of orogenic stretching conditions that served as C.N.R.S. (Vandoeuvre.les.Nancy, FraTICel. This study the vehicle for the surplus heat responsible was carried out with the financial support of C.N.R. fo r partial melting in the deep crust. (Centro di Studio sui Problemi dell'Orogeno delle Alpi Occidentali, Turin) and Ministry of Education (M.P.I. In the more internal portion of the Western 60%·1985 granl to R. Sacchi; M.P.I. 40%·1985 gram Alps, there is no lack of products indicative 10 R. Sandronc). Constructive criticism by J. Desmons of mantle activity in Permian times (see e.g. and by an anonymous referee is gratefully acknowledged. 606 R. SANDRONE, R. SAceHI, M. COROOLA, D. FONTAN, I.M. VILLA REFERENCES petrografici delle Alpi Code Italiane tra la Val di 511$a e la Va!!e Po. Ass. Pi em. Miner. Paleem ...F. Meda», AMSTUT.£ A. 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