Use of Zircon Typology for the Study of Some Granites from the Massif Central, France

RENDlCQNTI DEUA SOCIET). ITAllANA DI MlNERALOGIA E PETNOLOGIA, 1988, \bI. 4).2. pp. 46H76

Use of zircon typology for the study of some granites from the Massif Central, France

BERNARD BARBARlN* Univcrsiti Blaise Pascal et U.A. 10 C.N.R.S., Departement de G~IOIie et Mi~ralogie, " rue Kenler, F·6}O}S Clermont·Ferrand Cedex (France)

ABsn.J.cr. - The zircon morpholosy method is • arranged in two main groups: one is of hybrid convenient, reli.ble, and. ec;onomical tool in granite origin, and the other of crusta! origin (DIDIER petrology. 11 gives useful information on the tempenturC', water rontent, .nd chemistry of the magma and LAMEYRE, 1969; DlDIER et al., 1982). In and reasonably precise indications of the nature and many plutons, the largely dominant origin of the material. monzogranite and granocliorite types are Use of the zircon morphology method facilitates commonly K-fe1dspar porphyritic and biodte solution of several kinds of regional problems related to the petrology r:i granites in the Massif Central, rich. When these granites are neither France. This method commonly is determinant or hornblende.bearing nor muscovite· or represents an additional constraint in comparing granites cordierite-bearing, petrographic and eJl:poscd in the same llI'ea or eoclaves of granites in geochemical data are often insufficient to granitic hosts. Study of zircons can also provide a general outline of the magmatism in a large granitic arca such compare the closely-spaced units and to define IS the Forez horst, or a definition cl magmatic zoning their origin. The typologic study of zircon in a fragment of the Hercynian chain such as the entire populations is commonly determinant in the Massif Central. distinction of these two groups of granites The selected aamples of regional gmUte problems (PtwrN, 1980). described in this rep'"- also show that the zircon method is an exteUent OOIntXmmtto fidd.geolosY, petrography, In this paper, the zircon method is and. geochemistry. Together with these methods, it plays performed on some selected plutons from the a fundamental role in either relating or discriminating Massif Central (Fig. 1) to solve various types between granitic plutons. of petrologic problems such as the comparison K~ words: Zircon typology, granites, Massif Central, of closely. spaced granitic units, the Prance. comparison of granitoid enclaves enclosed in granitoid hosts, the origin of the different plut

* Ptesent addre": Universit~ de Paris·Sud, 208). From the variations of prism or Labontoire de Petrognphie-Volcanologie, Bit. '04, pyramidal faces, a zircon type can be defined P·9140' Onay Cedex (Pnnce). for each crystal. A study of a population of 464 8. 8ARBARIN

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Fig. 1. - Distribution of the various types of granitoids in the Massif Central, France (from DIDJE.It and LAMEYRE, 197 1), and local ion of the diflerenl. discussed areas and plutons (CA.: ChouJts-Aigun; Cc.: Chambo1J-k.ChaI~au; H .: Hmnitag~; G.: Ctfln; M.: M~mlC). USE OF ZIRCON TYPOLOGY FOR THE STUDY OF SOME GRANITES ETC 465 more than one hundred crystals from a single granitic piu ton (FERNANDEZ and TEMPlER, sample allows definition of a zircon 1968). However, petrography and population, two indexes, and a list of features geochemistry reveal differences between the which characterize the studied rock. These two units (NEGRONI, 1981). data provide useful information on the sample The zircon method clearly distinguishes the petrology. Within the same fairly Claveix granite from the Gelles granite (Fig. homogeneous geological formation (e.g. a non 3). Although the zircon populations and complex granitic pluton), even the study of indices of these two granites are very similar, one sample gives a reasonably precise the morphological features of the crystals are indication of the nature and origin of the shaply distinct. Zircon crystals from the material. The zircon method can also be used Claveix granite are colorless, clear, and as a tool for the classification and elongated; they frequently contain relic cores. interpretation of granitoids (PUPIN, 1980). Zircon crystals from the Gel1es granite are Differem types of granite and some of their colored and stubby; they never contain relic special features can be defined by the cores but commonly show overgrowths and systematic study of zircon crystal typology (see associalions of several crystals. These features Fig. 7a in PUPIN, 1980, p. 214). suggest crystal growth in a water-rich magma As the formation of the different crystal (PuPIN et ai. , 1978). The indices and faces is constrained by the temperature, water morphological features indicate that the content, and chemistry (major, trace, and Claveix granite belongs to the group of R.E.E .) of the magma (PUP IN , 1985), anatectic (par)sutoch[Onous, aluminous variations of these parameters during granites of crustal origin, and that the Gelles fractionation and crystallization are recorded granite belongs to the group of intrusive, by the zircon populations of the different a1uminous monzogranites and granodiorites fades of the plutons. For instance, in the of essentially crustal origin according to the Hermitage two-mica pluton, Forez (Fig. 1), PuPIN (1980) genetic classification. The zircons of the final aplitic fades are largely presence of scarce mafic magmatic enclaves distinct from those of the early-crystallized in the Gelles pluton is in accord with the slight fades (Fig. 2). Zoning in a single large crystal contribution of mantle-derived material. reveals an inner zone with a crystal type In the same part of the Massif Central, the similar to the dominant type in the early Meymac pluton occurs 40 km south-west of crystallized fades, and an outer zone similar the Gelles plutoo and on the other side of the to the type largely dominant in the late aplitic SWon HouWer fault (Fig. 1). Comparison of fades (Fig. 2). This evolution can be the GelIes and Meymac monzogrm.icic plutons correl ated with the variation of the main underlines the similarity of their petrological, parameters during crystallization of the geochemical, metalogenic, structural, and magma (BARBA Rl N, 1983). gravimetric characteristics (MEzURE snd NEGRONI, 1983). These two porphyritic Comparison of closely-spaced p!utons monzogranites also have similar populations, close indices, and similar crystal morphological features (Fig. 3). The zircon me thod 1. Ge/les, Claveix and Meymac granites complements the other data which suggest In the central part of the Massif Central, that these monzogranites are cogenetic, and the Gelles porphyritic granite is locally in probably belong to the same pluton divided contact with the Claveix granite. This by the SWon Houiller shear. fault (MEZuRE later is a fine-grained, non-porphyritic and NEG RONI , 1983). monzogranite-granodiorite with the same minerals as in the Gelles monzogranite. As there is no clear contact between these two 2. Forez potphyritic monzogranites-granodiorites units, the Claveix granite has been considered In the central part of the Forez horst (Fig. as an aplitic fades of the Gelles porphyritic 1), the two-mica Hermitage pluton is intrusive 466 B. BARBARJN

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Fig. 2. _ Zircon popuialions from the different fades of the Hermitage two-mica granite, and plots of their mean points on the lA-l,T diagram ( $ : aplitic jacks). Also reported on this diagram are the positions of the twO zircon types observed in a zoned zircon crystal from the Hermitage two-mica granite. USE OF ZIRCON TYPOLOGY FOR THE STIJDY OF SOME GRANITES ETC. 467

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Fig. 3. - Zircon populations from the Gdles, Claveix, and Mcymac porphyritic monzogranitcs, and plots of their mean points on the lA·IT diagram. into porphyritic, biotite·rich monzogranites Julien-la.Vetre pluton. 'Ibis facies shows many and granodiorites (BARBARlN, 1 983). To the petrological and geochemical similarities with north, the two-mica pluton is in contact with the dark-colored and biotite-rich, porphyritic the darkest facies of the laterally zoned Saint granodiorite which is in contact with the two- 468 B. BARBARIN mica piu ton to the south. From these data, those in the Margeride granite. The zircon the porphyritic granoc:liorite has been populations, indices, and crystal considered as part of the dark facies of the morphological features of the two porphyritic Saint Julien-Ia-Vetre porphyritic monzogranite monzogranites are very similar (Fig. 5). As the separated from the main plulon by the Margeride granite, the Chambon-Ie-Chateau intrusion of the Hermitage granite. granite belongs to the group of intrusive, Study of zircon populations from many a1uminous monzogranites and granodiorites samples of the Saint Julien-la-Vetre of essentially crustal origin according to the porphyritic monzogranite and from the PuPIN (1980) genetic classification. In this southern, porphyritic granodiorite is case, the zircon method is compatible with determinant in the distinction of these the hypothesis that the Chambon-le·Chateau independent granitic units (Fig. 4) , The [lOO] granite represents a fine-grained margin fades prismatic and [101] pyramidal faces are largely of the Margeride granite (LEMOINE, 1967), dominant in zircon crystals from the northern though it is still possible that it is an monzogranite, whereas the [110] prismatic independent unit (CmrruRIE, 1977). and (211] pyramidal faces are largely dominant At about 1 km from the northern contact in zircon crystals from the southern of the Margeride laccolith with the country granodiorite. Indices of zircon popuiations metamorphic rocks, the Chaudes-Aigues from the two units are totally dislinct. In this porphyritic monzogranites forms a 1 km2 unit central part of the Forez horst, the zircon (RESnnrro, 1971 ). This fine-grained granite method permits discrimination of two granitic contains small K-feldspar megacrysts but plutons with similar petrographic and becomes microgranular and non-porphyritic geochemical features. Data from this method close to the contact with the surrounding also indicate that the Saint Julien·la·Vetre metamorphic rocks. The Chaudes-Aigues porphyritic monzogranite belongs to the group granite has been described as either part of of intrusive, calc-alkaline granites of hybrid the dike swarms or the fine-grained margin origin, whereas the southern Peri·Forez of the Margeride granite (COUN, 1966). The porphyritic granodiorite belongs to the group zircon populations and indices of the Chaudes· of intrusive, a1uminous monzogranites and Aigues porphyritic monzogranite and the granodiorites of essentially crustal origin Margeride porphyritic monzogranite are according to the PUPtN (1980) genetic clearly distinct (Fig. 5). Developments of the classification. [lOO] and [110] prismatic faces and of the [l01] and [211] pyramidal faces are relatively J. Margeride and surrounding porphyritic similar in zircon crystals from the Chaudes monzogranites Aigues granite, whereas the [110] prismatic and [210] pyramidal faces are largely dominant In the southeastern part of the Massif in zircon crystals from the Margeride granite. Central, the Margeride porphyritic The zircon method thus confirms the monzogranite forms a vast laccolith about 100 hypothesis that the Chaudes-Aigues granite km long and 50 km wide. In this body, three forms an independent, small granitic unit, main fades and a local biotite cumulate have intrusive into the metamorphic rocks been distinguished (COUTURlE, 1977). Zircon (RES1m.rro, 1971 ; Co1.JTURIf, 1977; LAOOUE, populations of various fades from localities 1982). scattered all over the laccolith are decidedly similar considering the extent of the unit and Comparison of granitoid endaves and the variety of the fades (PuPlN, 1976). granitoid hosts To the west, the Margeride porphyritic monzogranite presents a fault contact with the Chambon·le-Chateau porphyritic 1. Porphyritic granite enclaves enclosed monzogranite. This latter is finer.grained and in the Margeride granite contains K.feldspar megacrysts smaller than Near its northern contact, the Margeride USE OF ZIRCON TYPOLOGY FO R THE STIJDY OF SOME GRANITES ETC. 469

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Fig. 4. - Selet:ted zircon populations from the Saim Julic:n.la.Vetre (SlY) porphyridc: monzogranitc: and from the Pn-i·Fore! porphyritic granodiorite, IIOd. plots of their mean points on the lA·IT diagram. (p.G.: porphyritic RfI1ni~).

potphyritic monzogranite remains scarce crystal morphological features identical to ovoid enclaves of porphyritic granite (LAooUE, those of the differem fades of the enclosing 1982). These eOClaves range in size from some Margeride granite (Fig. 5). These porphyritk tens of cemimeters to one meter in diameter. enclaves may thus represent a variety of the They have sharp comacts with their host, !ight-colored microgranular eOCl aves of DIDlER from which they can be distinguished by their (1964, 1973) and correspond to fragments of different color. Zircon typology on one of marginal or early·crystaIli.zed parts of the these enclaves gives population, indices, and Margeride piu ton. 470 B. BARBARlN

• MARGEIIIIIl' P.G.· uthtot '\'PO • IlAIIGEIIIIIl' P.G~ MMI~m IypO • M... IIGEJlIDE P.a.· Dofb, ,.,~ • M... IIGEAlDE P.G.· "llout ...•

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(1) Fig. 5.- (1) Zircon populations from the different facies of the Margeride porphyritic monzogranite, from the Chambon·le·Chateau and Chaudes-Aigues porphyritic mon:rogranites, from the Velay analectic granite, and from some enclaves of porphyriric granites enclosed in the Velay anatectic gNnite and in the Piri-Forez porphyritic granodiorite, and (2) plots of their mean points on the lA-IT diagram (P.G.: porp/ryritic granite).

2. Porphyritic granodiorite enclaves enclosed in the Velay anatectic granite where in the complex, where they are either isolated or fonn swarms (DE MONTRA VEL, In the central-eastern part of the Massif 1986) (Fig. 6). These enclaves have been Central, the huge Velay anatectic complex is considered as fragments of the older, mainly composed of oordierite-bearing granite, surrounding Margeride or Tournon biotite-bearing granite near the margins, and porphyritic monzogranite pIu tons, enclosed by kilometer-sized screens of metamorphic rocks the Velay granite during emplacement (DIDIER, 1964; DupRAz, 1983, 1986). Many (COtmJRIE 1969; CHENEVOY et al., 1974). of the abundant enclaves scattered in the Zircon typology of the porphyritic various anatectic granites all over the pluu)n granodiorite enclaves clearly indicates that are made of qark-colored and commonly they are invariably distinct from the highly deformed porphyritic granodiorites Margeride porphyritic monzogranite (PuPIN (DIDIER, 1964). These granodiorite enclaves and TURCO, 1975; DE MONTRAVEL, 1986) range in size from some centimeters to more (Fig. 5). Zircon features of the porphyritic than one kilometer. They are located every- granodiorite enclaves are decidedly USE OF ZIRCON TYPOLOGY FOR TIlE STUDY OF SOME GRA NITES ETC. 471

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12) homogeneous throughout the Velay complex. that generated the Velay anatectic complex. The dominant [100] prismatic and [101 ] These metaluminous granites come from a pyramidal faces in zircons from these enclaves lower structural level, where they were give them high A and T indices, whereas the enclosed in the Velay anatectic complex and dominant [110] prismatic and [211] pyramidal carried up during the doming intrusion in the faces in zircons from either the Velay anatectic aluminous porphyritic monzogranite plutons granites or the Margeride porphyritic to a higher structural level. PIu tons of monzogranite give them low A and T indices metaluminous, porphyritic granites of the (Fig. 5). Therefore, the porphyritic same type as the porphyritic granodiorites granodiorite enclaves are not fragments of the enclosed in the Velay complex only occur essentially crusta! aluminous porphyritic further north in the Montagne Bourbonnaise monzogranites (e.g. Margeride or Tournon area or further south in the Cevennes region. monzogranites) which surround the Velay In the Forez area, besides the common, complex (PuPIN and TURCO, 1975; DE small, porphyritic granodiorite enclaves, there Mol'ITRAVFJ.., 1986). They are best interpreted are two kilometer·sized bod.ies. They are as restitic fragments (DIDIER, pets. comm.) of enclosed, respectively, in the northern end of metaluminous, porphyritic granitic plutons the anatectic complex at Gumieres, and in the involved in the high-grade metamorphic event Peri-Forez aluminous porphyritic 472 B. BARBARlN

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.. ,

!r\"hl leucogranites ~ gneiss and micaschistes

1... ++1 Velay granites and migmatites sedimentary and volcanic rocks Ij.-';::"'I other granites • porphyritic granodiorite enclaves

Fig. 6. - Locations of the enclaves of porpbyrilic granodioriles in the Velay anlleclic complex (DE MONTRAVEL, 1987). monzogranite which forms a belt around the porphyritic monzogranites confirms the northern end of the anatectic complex, at distinction between porphyritic granites Saint JusHn-Bas (BARBARlN, 1983). As with enclosed in the Vday complex and those the smalJ er enclaves, these dark-colored. surrounding this complex (Fig. 5). porphyritic granodiorite units are strongly Furthermore, it suggests that, in the Monts sheared. The occurrence of this type of du-Forez area, there are three main successive enclave, even in the surrounding aluminous. plutonic events: 1. intrusion of metaluminous, USE OF ZIRCON TYPOLOGY FOR TIiE STIJDY OF SOME GRANITES ETC. 473

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-- MONTAG"'E BOURBONNAISE - MO/'IITS DU FOREZ- Mayet-de-Monl.agne plulon Henmage plulon • Mayet!ype .. aplitic IypC <$ Droiturier type 6. Hcnnitage type Bois-Noirs plulon • Croi.x.-de-Ladret type o PaUaduc!ype Pm-Forezgraniles ~ Noirttable type o two-mica granite Saint-Julien-Ia-Velre pluron 13 Bt-bearing granite 11 porphyritic granite • Bt-bearing P.G. Thimplulon • Quam: diorite .t;I. Thiers type • St-lust-en-Bas P.G. (enclave) 1(. Pont-de-Tarentai:.: type Velay-Forez anatedic complex • Bt-bearing anatcctic granite o Cd-bearing anatcctic granite • Gumihes P.G. (enclave)

Fig. 7. - Distribution on the lA-IT diqum of the mean points of zircon populations from the various granites of the Forez horst. porphyritic granite pIu tons probably in the 3. Sheared granite enclaves enclosed metamorphic series; 2. intrusion of the in the Bois-Noirs granite aluminous, porphyritic monzogranite plutons; and 3. doming of the Velay anatectic complex. 10 the northern part of the Forez horst (Fig. 474 8. 8ARIWUN

Definition of p-anitic provinces Use of the zircon method in the study and comparison of the different granitic plutons of the Forez horst (Fig. 1) also permits distinction between two groups of granitoids (BARBAJUN, 1983, 1984). In the group located in the northern part of tht: host called Montagne Bourbonnaise (Fig. 8), the metaluminous monzogranites are characterized by zircons with dominant [lOO] prismatic and (101] pyramidal faces indicating a hybrid origin (Fig. 7). In tht: other group, located in the southern part of the horst called Monts-du-Forez (Fig. 8), the aluminous monzogranites or granodiorites are Fig. 8. - Sketched map of the Feru hom, showing characterized by zircons with dominant (110) the Moncagne Bourbonnaisc and the Monts-du-Forez prismatic and [211] pyramidal faces indicating granitic provinces divided by the Hermitage: shear zone a crustal origin {Fig. 7) . In this case, the zircon (Hsz). method not only complements the other data but is determinant in the distinction between 1), local east-west shear zones cut across the the Montagne Bourbonnaise and Monts-du normally-zoned Bois-Noirs pluton. These Forez granitic provinces. deformed zones frequently contain meter These two granitic provinces are divided sized, lenticular enclaves of strongly sheared by the Hermitage ductile shear zone (Fig. 8) granites. Lack of exposure makes the which is part of a major Variscan crusta! suture structural data ambiguous. Petrographic and in the eastern part of the Massif Central geochemical studies of these highly deformed (BARBARIN and BEUN, 1982). This fracture enclaves have not determined whether they separates the north-eastern part of the Massif represent pieces of the Bois-Noirs granite Central where the granites are of the same locally involved in the shear zones, or older types as in the Montagne Bourbonnaise sheared granites enclosed in me Bois-Noirs granitic province, from the central-eastern granite. However, zircon typology on one of part where the granites are of the same type these enclaves at Lavoine (PuPIN, pets. as in the Monts-du-Forez granitic province, camm.) clearly favors the second hypothesis. the latter being a northern extension of the The sheared granite enclaves contain zircons huge Velay anatectic complex. with [110] prismatic and [211} pyramidal faces On the scale of the whole Massif Central, dominant. They belong to the group of the zircon method leads to the definition of intrusive, aluminous granites of essentially magmatic zonation in the Variscan orogeny crustal origin. In contrast, the enclosing Bois (PuPIN, 1981, 1985). From the study of Noirs monzogranite has zircons with hundreds of zircon populations, it is possible dominant [100) prismatic and [101] pyramidal to quantify the abundana=, in each area of the faces and belongs to the group of intrusive, Massif Central, of various granitic types metaluminous granites of hybrid origin defined in the genetic classification (PuPIN, according to the PuPlN (1980) genetic 1980). The same type of study was carried out classification. The zircon method should be in other Hercynian segments such as Brittany applied to the other enclaves and also to the or Corsica (PuPrN , 1985). Le Donjon sheared granite to the north in order to determine whether the Montagne Conclusions Bourbonnaise metaluminous granitic plutons were emplaced in an area occupied by older The solution of various types of granite aluminous granitic piu tons. problems in the Massif Central using the USE OF ZIRCON TYPOLOGY FOR TilE STUDY OF SOME GRANITES ETC. 475 zircon method emphasizes the quality and Cot.nvR.It}.P. Mm -u mtUJif grm;tiqw tk la Margmk reliability of thls petrological tool. However, (Mani!~ tralfranflliJ)_ These d'Etat, Ann. Fie. Sd. Univ. Clermont-Ferrand (France), 62, 319 p. this method has its limits and should be used DE MOHT1lAVEL C. (1986) - Ln enclavn ck "aniU together with field-geology, petrography. porpbymltk dll g",niu dll Vela,: k>t:4lisatio" tt !tutk geochemistry, and isotopic work, if possible. tks zircons. Bull. Soc. Linneenne Lyon, ", pp. From the various examples, it is clear that this 2}}-247. method is determinant in the distinction DIDIER}. (1964) - Etutk p/trogrllfJhiqw tks mclavn tk qlldq~ granitn dll Mljnif ~tral /ranfais. These between granites of different types in the d'Erlt, Ann. Fie. Sci. Univ. OermOnJ-Ferrlnd Pupin genetic classification. However, it (France), 2}, 2'4 p. cannot specify whether or not two granites DIDlEa J. (l973) • GnmikJ and thdr mclavn. The burin,. of mciavts 0" the origin 0/ graniln. Dev. Petrol. 3, of the same type are cogenetic. Zircon Elsevier, }9} p. typology is an excellent complement to the DmlEa J., DtrnlCRJ j.L_. l..+.MEYRE}. (1982) - Mantk tIIId classical methods. Considering the quantity cruJtal vanikJ: gtn~tic dassi[iation of omgmic gnmitn of information provided compared to its and the fIIlturt of their mclalm. J. Vole. Geotherm. efficiency and cost, the zircon method is Res., 14, pp. 12'-132. OIDIER J., UMEYRE J. (1%9) - Us gnmiln dll Massif profitable and should be systematically used ~tral fnmftlis. Etudt comparit tks kuc~nitn ~t in granite studies. granodiorikJ. Contrib. Mineral. PetroL , 2', pp. 219-2}8. DlDffiR J., UMEYREj. (1971) - Ln mew granitiqlln dll Aclenowldgmmts. - I am very grateful to Or Jean.Pierre MaJfif Central. In symposium j. Jung: "Gwlogie, Pupin, University of Nice (France), who kindly taught gwmorphologie et structure profonde du Massif me his method for Sludying zircon morphology, and Central fran~ais_, Edition Plein Air Service, maintained his inlerest throughout the progress of my Clermont·Fenand, pp. 1}}- I". research. Many tdleagues from the Department of DIDIEJ.j., LAMEYRE}. (1980) - Ln vaniroidtsdll Ma"i! Geology in Clermoru-Ferrand, were al the origin of Ihis ~tral. In «Evolutions gwlogiqucs de la France_, 26e report. I thank}ohn Clemens for his assistance on early CG.I., Paris, coil. C7, Mem. B.R.G.M. 107, pp. version of the mamacrip!. Helpful comments were also 63-70. received from Pro f ~. }elln Didier andJean Lameyre, and DUI'RO\Z l. (l98}) - Evolution du compb:~ anat«tique an anonymous reviewer. du Vtlay et yptist d~ la coniit!ritt (Ma"if Cmtrai jranfaiJJ. Thbe de }e cycle, Univ. Cierman! U (France), 176 p. D UI'Ro\zJ. (1986) - Lt batholit~ du Vtlay tt son mcais$tlnt mltamorphiqut tt plUJoniqllt (Massi! Centra/panfais). REFERENCES C R. Aatd. Sci. Paris., }02, pp. 461-466_ Dl1T1touJ _L., CMfro\GREl-j.M., DIDIF.IJ., VIAlETTE Y. BAJ.IIAJ.IN B. (l98}) - Us graniln carbonjpm du Fora (1984) _ Palatozoic granitoids from tht French Massi! upt~trio nai (Massif Central fra"f4is). TypokJgi~ tt Central: agt and origin studied by " Rb_I1Sr Jystem. rtlatiOflf entrt In di/lbmts ma"ijs. These de }e cycle, Phys. Earth Planet. Inter.,}!i, pp_ 01-144. Univ . ClerIJlQnt U (France). 177 p. FERNO\NOEZ A.N., TID.1PIER P . (1968)-Surla mi~m plau BAJ.llARIN B. (1984) - Mist en tviaence tl l'aitk tk la du granit~ dt Gtlles (Puy-tk-D6mt). Rev. Sci. 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