REND/CONT/ Socio:td Itallana dl Mlneralo"la e Po:trolollla, JI (2): pp. 617-621 COmunlCllzlone presentata all.. Rlunlone della SIMP In Abano (Padoya) 1\ 3 gluino 1982
MONTICELLITE-CLINTONITE BEARING ASSEMBLAGES AT THE SOUTHERN BORDER OF THE ADAMELLO-MASSIF
PETER ULMER lnstitut Hit Ktistallographie und Petrographie, ETH·Zentrum, CH..a092 Zurich
ABSTRACT. - Some lenses of matly dolomites are porta localmente ad una metamorfosi retrograda localized in the contact zone between gabbroic sovrapposta con formazione di dorite e serpemino, cumulates and the intermediate intrusions of the (osl come i1 .. breakdown 1O della monticellite. Dia· Re di Castdlo Pluton at the southern border of the grammi di fase (a!colati mOSlrano che la prima Tertiary, ca!calkaline Adamdlo Massif. The following metamorfosi di contatto si sia svolra ad un valore mineral assemblages formed in these lenses as a di Xco.. alto e ad ahe temperature. result of a high temperature contact metamorphism: La Seconda fase raggiunse invece circa 550· C monticdlite-forsterite-clintonite (near the contact oon un valore di Xoo. basso. with the gabbroic cumul,ues) and forsterite.diopside. dintonite (away from the conlact). Nd caso di coesistenza tra forsterite e monticel· A later intrusion of intermooiate plutonic rocks lite, si pub usare la sostituzionc: ddla forstedte locally produced retrograde overprinting with the nella struttura ddla momicdlite e viceveru, come formation of chlorite and serpentine, as wdl as the misura della temperatura. Profili di temperatura breakdown of monticellite. Calculated phase dia· misurati in grani di momicdlite e fOTSterite nelle grams show that the first contact m.etamorphism paragenesi che non hanno subito una metamorfosi took place under monticdlite facies conditions and retrograda, mostrano una distrihuzione omogenea a high value of Xoo. in the fluid. The second phase, oon temperatute che si aggirano ua gli 850 e i 90lY C. Nelle paragenesi marcate da metamorfosi on the contrary, took place at about 550· C and retrograda le misurazioni diedero una divisione ddla a low value of Xoo2 , temperatura in due parti. Un settore centrale con In the case of coexistence of monticellite and temperature di 800-9OlY C e un bordo di 550-650G C. forsterite grains, the substitution of forsterite in the Questi due differenti metamorfismi negli stessi struclUre of monticdlite and vice versa has been C1Irbonati dimostrano che sia i cumulati gabbrici used as a gcothermometer. Chemical profiles through cosl come le intrusioni intermediarie cristallizzarono monticdlite and forsterite.grains show homogeneous in situ in un Jivdlo crostale moho alto (capertura distribution palterns for a temperature around massima 2 km). 850· C in cases where no retrograde overprinting is obvious. Assemblages affected by retrograde over· printing result in a division of the temperature 1. Introduction distribution pattern into the cores of the grains with temperatures between 800 and 90lY C and In the Lago della Vacca area, at the into a border zone with"" 550·650" C. southern border of the Tertiary, southern These twO metamorphic events in the same car· bonate rocks suggest that the gabbroic cumulates, Alpine, calcalkaline Adamello intrusion (Alpi as wdl as the intermediate intrusions, have crystal Bresciane, Northern Italy) occur some lenses lized in situ at a very shallow cnlSlal levd (max. of high temperature contact metamorphic, overburden 2 km). siliceous, argillitic marbles. The area lies RIASSUNTO. - Nella zona di comano tra i cumu· within the Re di Castello Pluton, the lad gabbrid e le intrusioni intermediarie del plutone southernmost intrusion of the Adamello mas di Re di Castello al bordo meridionale dell'Adamello sif, which intruded the southern Alpine ba appaiono lenti di dolomia argillosa. In queste ul· time, come conseguenza del metamorfismo di con· sement and the Permian and Lower to Middle tallO di alto grado dovuto all'intrusione dei gabbri, Triassic sedimentary pile 47 to 40 m.y. ago. si formarono le seguenti paragenesi: monticellite The igneous rocks in this area have been forsterite·dintonite (xamhofitlite) (vicino al contalto) e forsterite- Lago detla Vacca Area x X XX 30001 x X X X [±J Re d; Coll$tello Pluton X Geological x 00 Adamello Pluton Map B Crystal;ne Basement D Sedimentary Cover Legend GJ Re di Castello Tonalites ~ Re d; Castello Oior;tes t-:;~l Gabbroic Cumulates • Contact Metamorphic Sediments loo! Pso Croce Domini I:·..:::'!l Quarternary ,.' .:..... Cntdi Blumone Fig. I. _ Geological map of the Lago della Vacal area at the southern botder of the Adamello massif (RI:' di Castello Pluton). MONTICELLlTE-CLlNTONITE BEARING ASSEMBLAGES ETC. 619 LEGARI, D. COLBERTALDO, R. MALARODA). a few dm to 300 m long at the Passo Blu In the western and northern part of the mone. studied area the rocks mainly consist of in In this large lens, an old sedimentary termediate plutOnic rocks with the local layering can be observed. Layers consisting name Re di Castello-Tonalite (fig. 1). The of clinopyroxene and clintonite, partially ronalite corresponds to a fine- to medium with boudinage structures, alternate with grained biotite·hornblende tonalite with a forsterite-monticellite layers, each approxima large number of femic, endogenic xenoliths. tely 5 cm thick. This layering corresponds The tonalites are internally heterogeneous and to primary argillitic and dolomitic layers. show transitions to diorites along fluidally A 5 m thick layer of Predazzite (calcite oriented zones (e.g. near the Rifugio Ga hrucite rock), corresponding to a dolomite, briella Rosa). Lenses of gabbroic rocks is found at the Passo Blumone. D. COLDER (cumulates) or clusters of hornblende and TALDO and R. MALARODA have placed these plagioclase crystals with clear affinities to rocks in the Lower Anisian (Formazione di the nearby cumulates of the Cornone di Angolo inferiore). However, in the author's Blumone are locally concentrated along more opinion due to total overprinting of the or less straight zones. primary mineral assemblages and the lack of The eastern part of. the atea is composed fossils or direct contact to the sediments in of gabhroic rocks of the Cornone di Blumone. the Alta Valle del Caffaro (R. MALARODA, This complex mainly consists of coarse 1955; U. ROESLI and P. ULMER, 1982) no grai ned ti tanomagnetite -cl inopyroxene-horn definite age can be determined. Based on blende-plagioclase (An 80-90)-gabbros which lithological aspeCts, both Lower Anisian and display cumulate structures. Olivine-chromi Lower Ladinian (Form. di Livinalongolo) or ferous spinel-cpx-opx-hbl-cumulates occur at Upper Anisian to Lower Carnian (Form. di the base of this complex or isolated in the Esino s.l.) ages can be postulated. tonalites. The whole complex has been af fected by a post-accumulation event of brec 2. Some mineralogical a8pect8 dation and hornblenditisation which has also produced pegmatitic dikes consisting The mineral determinations have been of enormous hornblende and plagioclase made by thin section observations, XRD and crystals. The cumulates are clearly older electron microprobe analyses of the mineral than the tonalites as indicated by stOpping speCies. phenomena at the contact as well as intrusion The following minerals have been found: into the cumulates from the side and below. In addition, the tonalites cross-cut the mafic I. Fassaitic pyroxene (diopside series) complex in large dikes. 2. Olivine (forsterite content 92-95 %) 3. Calcite . The sedimentary carbonate lenses are lo calized in the northeastern part near Passo 4. Hen;ynitic spinel Blumone between the cumulates and the 5. Chlorite (c1inochlore composition) tonalites. To the southwest, the more or less 6. Andraditic garnet (always accompaing straight zone of lenses intersects the tonalites monticellite as an inclusion) and are accompanied by cumulate lenses and 7. Clintonite or xanthophyllite the so-called «diorite a tozze» containing quadratic hornblende. These diorites contain Clintonite was discovered and has al abundant hornblende and plagioclase crystals ready been described from this area and coming from the cumulates. The southern from nearby (Costone di Valbona) by part of the sedimentary lenses seems to fol A. BIANCHI, O. HIEKE and E. SANERO (1940, low the old contact with the cumulates. 45, 46). Clintonite is a green, calcium brittle However, these cumulates are broken up and mica with a phlogopite structure (niocta have been intruded by the tonalites, resulting hedral) with the approximate formula in « diorite a tone» and some lenses of less Ca(Mg,Alh(AI,Si)~OH~OHh. Clintonite is a affected cumulates. The sedimentary carbo relatively rare mineral. This is more the nate lenses have dimensions ranging from consequence of the spedfic bulk composition 620 P. ULMER of the rock which permits the formation of this mica rather than thermodynamic con· 1450r-r--.-/~~~-.,.-,-.\-, ditions, as clintonite has a large stability field (J. RICE, 1979, and this work). Clin 1\"Cl lonite will only be formed if enough ma· gnesium, calcium and alumina is available and only a minimal amount of potassium is present, otherwise the farmalian of phlogo pile would be preferred. These conditions 1200 are: fulfilled by an argillitic dolomite with kaolinitic or montmorillonitic clay minerals instead of ilIilic clay. ta,n;1 1000 CQ2SiO, ",-ot~ in._~c,Q", CaMgSiO{ F. $iO, P,2kb 800 -"1fjfj::- Mo Fe ...,c------Oli~ill. ' Ns2siO, F·2SiO, Fig. 3. - Experimental solvus Ixtwcen forslttite Fo,sttril Fay"lil .md mOl'llioe.llile al 2 kbars in a lemperature venus ;Ktivity (of forslc:rite}-diagnm. (From R. WAIIND: Fig. 2. - Composition diagram of the olivioes Ind and W. LUTH, 1972). c..()livinc:s in the system Mg.SiO,.FcoSiO•.c..,siO•. solubility of the components (forsterite and 8. Monticellite monticellitel of about 18 % between 1300 and 1400° C according to the existing pres As yet this mineral has not been described sure. In the constructed phase diagram (6g. from the Adamello aurrole. Monticellite ap 4). it can be seen that monticellite reacts to pears near the comact with the cumulates or merwinite (Ca.Mg[Si~h) and forsterite at in the middle of the lenses isolated in the 13200 C and 10 kbars. The solid solution tonalites. It forms only under high tempe behaviour of coexisting forsterite and mon ratu~. Contact metamorphic conditions in ticellite can be used as a geothermometer carbonate rocks. Monticellite is an onhosili (see chapter 4l. cate which has the same space group (Pbnm) as olivine and a similar struCture to latter. 3, The paragenelle8 The difference with olivine lies in the dimen sions of the cation sites in the crystal lattice. The mineral assemblages can be divided Olivine has two similar sites for Mg and intO twO groups. One group shows one high Fe(lI'). Momicellite has a larger one for Ca temperature metamorphic event with a few and a smaller one for Mg and Fe(II·). This water bearing minerals and is found near the also explains why monticellite and olivine contact with the gabbroic cumulates in the show a large miscibility gap (6g. 2). P.so Blumone area. A second group shows two differt'nt metamorphic events: an older R. WARNER and W. LUTH (1973) have one which corresponds to group one and is experimentally investigated this miscibility overprinted by H 20 bearing minerals of a gap. Their experiments were carried out from second retrograde evem. These rocks are 1-10000 bar and 800-1400° C. Fig. 3 shows localized in the isolated lenses in the tonalites the experimental salvus for 2 kbars which is and near the contact zone between tonalites practicaHy symmetric and attains a maximum and sediments. MONTICELLlTE-CLlNTONITI:. BEARING ASSEMBLAGES ETC. 621 1"('0 1400 Me + FOJ$ 1300 ,Me • Mo. 1200 1100 I P=10kb Mo 0.2 0.4 0 & 0.8 Fo alo Fig. 4. _ Phasaelationships in the lynem mon Fig. 5. - Microsraph of diopsidc: free: dintonitc: ticcllile-fontc:ritc: at 10 kbal'$ in a temperatufe+ bc:ai-ing monticdlitc: rock ~~ P.50 Blumone: aclivity~iqram. (From R. WUNEl and W. LtlTH, Di == diopsidc:, Mo == monlicdlllC:, Fo = fontc:ntc:, 1972). Cbi = chloritc:. Cl:::: dintonltc:, Sp = spinet GROUP corresponding to Ihc= formarion of monti Mon/ialli/e, lo,s/~,iI~, ca/ciu, din/oni/e, cdlite. + d;opsid~, + spine/ Diops;d~, lo,ste,it~, ca/cit~, Fig. , shows a thin section of this rock. dintonite, Thc= spine! rims have bc=en rc=placed by c1in + spin~l tonite suggesting· that thc= following rc=action, Away from the contact wilh the cumulates, although incomplete, has takc=n place: in the same lens, one finds coexisting diopsi monticellite + spinel = de, forsterite and calcite rather than mond c1intonite + forsteritc= + calcite. cellite. Fig. 6 sho,¥~ another c1intonitc= In order for this interpretation to be: cor bearing reaction: rc=ct the monticellite must have been formed spinel diopside calcite = clinlonite. bc=fore the c1intonile. This is'confirmed by + + Ihc= prc=sc=nce of monlicellite inclusions in In this rock no forstc=rite appears. Howe the forsteritc= grnins. Morc= commonly thc= op ver, in parageneses containing forsterite positc= situation is o~rved: inclusions of c=xactly thc= same reaction tOok platt. This forsterite and diopside and/or calcite are demonstrates, that nc=ither forstc=ritc= nor found in monticellitc= grnins as relicts of the chloritc= is involved in this reaction, as sug reaction: gested by J- RICE (1979). Forsteritc= and forsrerite + diopside + calcite = spinel coaist only if diopside is absent in monticellitc= thc= mineral assemblagc=. 622 P. ULMER Fig. 6. - Micrograph from a momicellilc·forslcritc free dimonirc bearing diopsidc rock from Pso Blumone. Fig. 7. - Micrograph from a momicc:l1ilc free chlorilC bearing rock from Pso Blumonc:. GROUP 2 This assemblage can be found in the iso Chlorite, serpentine (on forsterite) and cal. lated lenses in the tonalites. Diopside is no cite have mainly been formed as retrograde longer present after formation of monticel· assemblages. lite. and chlorite has been formed by the CJintonite, diopside, forstuite, calcite, chlo breakdown of momicellite, forsterite and rite, + spine! clintonite after the reaction: clintonite + forsterite + monticellite = This rock' sample is laken from the same chlorite + calcite. lens as the those previously discussed along Monticel!ite, diopside, forsterite, calcite the contact with the tonalitic rocks. Fig. 7 Along veins in some monticellite rocks, at shows the formation of chlorite and calcite the P.so Blumone, the breakdown of monti· by the breakdown of clintonite, forsterite cellil('~ to forsterite, diopside and calcite is and diopside after the reaction ~ observed. All these are retrograde and fluid· clintonite + forsterite + diopside = consuming reactions have not attained equi· chlorite + calcite. librium, as the fluid must have been con sumed before hand. Chlorite is nOt homo In the lower right, relicts of the clintonite geneous under the microprobe and shows forming reaction are preserved as inclusions strong chemical gradients towards the of consumed spinels and diopsides in din bordering mineral species. tonite grains. 4. Geothermomelry Montiullite, fOTsterite, clintonite, chlorite, When forsterite and monticeIlite coexist, calcite, + spinet the substitution of the forsterite component MONTICELLITE-CLINTONITE BEARING ASSEMBLAGES ETC. 623 2OrT'--,--r-r'--V-~T70 in the montj~llite structure and vice versa f'(Kb.d can be used as a measure of the temperature attained during the contact metamorphism. The experimental results from R. WARNEll and W. LUTH (1972) have shown (6g. 8) that the activity of monticellite in the fame rile solid solution at temperatures lower 2 3 than 1100" C is practically indepen -lnK.2 U l ... 3.0 3.0 '.5 '/n"Cl "':L;---:-;;---;';;--7f-==-;-.-c!.'/n·Cl '·r'".-----.,".,--,,".,,----"Ui-"--"'-.,.,.--,.T.,,-, D.. D.. I.G U U'ID-3 (a) (b) Fig. 9. - (.) I"K, versus lIT plot for the er:perimenlll dau. at 2 kban; I"K, = activity of. mooticd Iile in fonleritC.. (b) InK. versus 1fT pkM: for the er:pcrimcntal dau. at 2 kb.n; InK. = Ktivily of. fonlCritc in moolicdlile. 624 P. ULMER T(Q 800 Forsterite MonticeHite eoo 700 9 TI'IJ TI'c1 750 Forsterite 700 600 core margm margin core ma.rgln ma. co ma. 75;rJ-~·~~~~ Da ta points Data points Data points (,I (bl lel H'CI Monticellite Forsterite TI'Q TI'O 9 850 Forsterite 850 800 ma co ma margin core margin m. Jm.' ,," , , , 800 750 Data points Data points Data points Id J IeI If) Fig. 10. - (a) Temperature profile Ihrough a forSlerite grain. . (b) Temperature profile through a forsterite grain. _ (c) Temperature profile through a momicellite grain. - (d) Temperature profile through a momicellite grain.. (~) Temperature profile through a forsterite grain. - (j) Temperature profile through grain 10 (t) with fine focussed electron beam. 5587.87 Results I I1 n'el = Some profiles have been made through 2.4659 -lna ~~Q forsterite and monticellite grains using an ARL-electron microprobe with wave· and energy-dispersive detection systems ate pre 4429.16 sented in fig. 10 in terms of temperatures. (2) TI'C) = They can be separated into two different groups: MONTICELLITE-CLINTONITE BEARING ASSEMBLAGES ETC. 625 F,,..._=;;, S, Fig. 12. _ Solid .5OIulions in c1intonile .00 c1ino pyroxeoc: ilCOOrding 10 c:lectron microprobe .n-Iysis in lhe: syslan SiOrMgO.AIA. Di 0:= diopsidc:, Mo 0:= monticc:llilC:, Fo 0:= forslerill:, ,6,1 Chi 0:= chlorile. Cl 0:= dinlOnile. Sp 0:= spine!. ",0 50 2°) one from K. BUCHER (1976) on the Bergell Fig. 11 ..- SiOrMgO-AIA diagnffi proi«tc:d from aureole. have been published concerning alciu:: .nd fluid. c1imonite-hearing marbles, however both without any monticellite occurrence. 1here 10 (a), (b) and (c) represent profiles from fore petrological studies on the phase rela rocks which have been affected by retrograde tionships have been made here. overprinting. These profiles show a core with 1he following relevant phases have been temperatures between 800 and 900" C and chosen: a border zone with temperatures ranging Forsterite, diopside, dintonite, monlicel from 5500 to 6500 C. lite. spinel, chlorite and calcite plus a The profiles 10 (d) and (d represent rocks H~-C02 lIuid. without any visible overprinting of the pri The best diagram for presentation IS a mary assemblage and show a homogeneous Si02-AI~,()3-MgO diagram projected from temperalUre distribution pattern around calcite and fluid (Fig. Ill. This is an idea 850" C. lized diagram, showing only the magnesium Geothermometry confirms the observa end members. The minerals are presented as tions from thin sections that two different points neglecting tschermak exchange vectors metamorphic events have occurred, of which in the phases. the second one has not affected all the rocks. A more realistic diagram is presented in Fig. 10 (f) shows the same forsterite as fig. 12 based on electron microprobe analyses, 10 (e) but analysed with a fine focussed but without chlorite. Clinopyroxene and din electron beam (I !-Lm). The homogeneous di tQni{e show a strong dependence on the stribution pattern is lost in the central part coexisting assemblage and vary in ,he di of the grain. X-ray dislCibution patterns of rcedon of the tschermak exchange vector. Ca-Ka rays show that the central part of the Magnesium endmembers have been used forsterite has exolved calcium-rich lamellae. for the calculation of the reactions: However they are too fine to determine exactly (monticellite?). Exolution is limited forsterite Mg2SiO~ to the central part of the grain and is pro diopside CaMgSi06 bably the result of cooling after formation monticellite CaMgSiO~ at high temperatures, as it is known for spinel MgAbO~ dinopyroxenes (Diallag). chlorite M8:lAbSiaOu,(OH)", (dinochlorel 5. Pha8e relationship. and dintonite has been set on the tieline In the literature only two studies one from diopside-spinel. According to electron micro J. RtCE (1979) on the Boulder aureole and probe analyses and thin section observations 626 P. ULMER Fo,Ct) "'ft / \ +c.co3 A \ ::d -'F+-+,*Z"Rk' \ A tchu t---6pfio) l0- (ot.ro) Fig. 13. - ,-Xcoz diagram at 500 bars for the system SiO.MgO-ALO*CaO-HO-CO" with the phases forsterite, diopside, monticellite, clintonite, chlorite, spinel, calcite, HrO and COr. (no forsterite or chlorite involved in the 500 bars constructed after the method of formation of clintonite) the composition is: H.rW. Dev (1,972\ and checked in the tem- perature range from 350-800" C. clintonite Ca(Mg2.ssAl. ea ) ( Alz.ueSit.ss)Oro( OH )z 6. Diecuseion of the resulta in the T-X"ro diagram In this system (6 phases, 3 components) with two degenerations,L1 reactions can be The first high temperature contact meta- calculated,Tab. 1). When available, thermo- morphism can be associated with certainty dynamic data from HrrcnsoN et al. (1978) with the formation of the cumulates (temp. have been used in the calculation of the up to 900" C) which developed at extremely stable T-X-2 sections. A new enthalpy value high X.o, as indicated by the formation of for chlorite was calculated from an expe- monticellite before clintonite. This means that riment of T. \0Trnrvrenr(1980). Since no reaction 1 occurred before reaction 2. During data are available for clintonite, the entropy this primary phase of metamorphism, an an- and heat capacity terms have been estimated nealing texture of monticellite formed to- and the enthalpy is calculated from an expe- gether with spinel gtains. In a second phase, riment of G. Hoscnx (1976). The pres- X"o, diminished. Large cystals of clintonite sure was fixed at 500 bars according to formed by consumption of monticellite and stratigraphic considerations (about 2 km of spinel. Clintonite was mainly formed by at- overburder at the time of intrusion). taining.reaction 11 in a monticellite-bearing Fig. 13 shows the stable T-X*, section at assemblage. Diopside must have been con- MONTICELLITE-CLlNTONITE BEARING ASSEMBLAGES ETe. 627 TABLE 1 stopped before equilibrium could be attained. Reactions and reactions parameters for the The equilibration of monticellite and forste system 5iO, - MgO - Al~O., . CaO . HiO rite at grain boundaries confirms the obser CO ~ with the phllses vations in the T-Xcoz diagram, as far as mon dcellite is only stable at temperarures as low '. <".$o.(~l) , '0' 'I • 2<, • _ • 2CO as 550" C, if the Xco. is 0.1. The low value l < l. (CM."'.'o' , ~.1 • UII • c•• 3HzO • ltt • ClIl of the Xco.. is on one hand the product l. Ho••n , 2C>\ • 3« • 1e<' 1$0' 11 "HzII' 1COz of H~O rich fluids coming from the tonalitic •• (.0,...., , SCM •• '1 • lle<' lS 3« • 17HzII' l1ClI rocks which carry much more volatile com l S. ('0.«' , co, • 01 • &. ( n , SCM • 'e< " 700 ...... le•• 11HzII • >COl other hand the result of the lOtal consumption '. (, , 15cM • U« • M.o • "1 • GC, • 5"'211 • nc. of the carbonates during the first meta morphic event. I. "'.0Il , CM • 2c, • '0 • l'" • So • ''\10 • lClIl •• REFERENCES BIANClll A. e DAL PIAZ G.B. (1937) - If UIIOT~ Carnegie Insl. Wash. Y"k Book, vol. 6J, pp. mNidionll/~ dtl massiuio d~J/'Adamtllo. Rtlll~it> 81.83. ni sul r,lt/}.m~nlo ~ sludi prtliminlrri dtlla ~ona MAUIODA ROBEtTO (19.'54) - Geolot,ill tieU. VII1I~ rompr~s. Ir. t. V.II~ Sftlb,o t 1'111111 VIIUt dtl dl B/umon~ (Alla Vaffe tirf C.8I11O) ntll'Atill' C1I8111O. Boil. UII. GnJI. hal., Roma, vol. LXII, mti/o MNidionll1~. Mem. 1s1. Geol. Mincr. Univ. pp. 1-87. Padova, vol. XVII, pp. 148. BIANCHI A. COAL PIU G_B. (1937 a) • Alltlnu OUSCH M. and SEIFFElT F. (1976) . Slability and t,tOiot.;ct>pttrot,rllftco dtlJ'Adllmt/lo m~rtd;onalt. PMU ftlalions 01 IflCJtlMdrll/ c./dum briul~ rttiont Irll 10 Stllbio td II C118t11O. Mem. 1s1. miclIs (clintonilt I.roup). Jour. of PeltOlO£y, vol. Gcol. Univ. Padova, vol. 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