Paleozoic Orogenies in the French Massif Central a Cross Section from Bèziers to Lyon

B22_ copertina_R_OK C August 20-28,2004 Florence -Italy

Field Trip Guide Book - B22 Pre-Congress Associate Leaders:J.M.Lardeaux,P. Matte Leader: M.Faure BÉZIERS TOLYON A CROSSSECTIONFROM FRENCH MASSIFCENTRAL PALEOZOIC OROGENIESINTHE 32 Volume n°2-fromB16toB33 GEOLOGICAL CONGRESS nd INTERNATIONAL B22 28-05-2004, 17:28:58 The scientific content of this guide is under the total responsibility of the Authors

Published by: APAT – Italian Agency for the Environmental Protection and Technical Services - Via Vitaliano Brancati, 48 - 00144 Roma - Italy

Series Editors: Luca Guerrieri, Irene Rischia and Leonello Serva (APAT, Roma)

English Desk-copy Editors: Paul Mazza (Università di Firenze), Jessica Ann Thonn (Università di Firenze), Nathalie Marléne Adams (Università di Firenze), Miriam Friedman (Università di Firenze), Kate Eadie (Freelance indipendent professional)

Field Trip Committee: Leonello Serva (APAT, Roma), Alessandro Michetti (Università dell’Insubria, Como), Giulio Pavia (Università di Torino), Raffaele Pignone (Servizio Geologico Regione Emilia-Romagna, Bologna) and Riccardo Polino (CNR, Torino)

Acknowledgments: The 32nd IGC Organizing Committee is grateful to Roberto Pompili and Elisa Brustia (APAT, Roma) for their collaboration in editing.

Graphic project: Full snc - Firenze

Layout and press: Lito Terrazzi srl - Firenze

B22_ copertina_R_OK D 21-05-2004, 10:42:13 Volume n° 2 - from B16 to B33

32nd INTERNATIONAL GEOLOGICAL CONGRESS

PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON

AUTHORS: M. Faure (University of Orléans - France) P. Ledru (BRGM, Orléans - France) J.M. Lardeaux (University of Nice - France) P. Matte (CNRS, University of Montpellier - France)

Florence - Italy August 20-28, 2004

Pre-Congress B22

B22_R_OK A 21-05-2004, 10:50:54 Front Cover: View of gneiss-migmatite of the Montagne Noire axial zone (visited during D2) looking to the north. The village of Olargues is located in the micaschist envelope of the dome. The picture is taken from the northernmost part of the recumbent folds of the Montagne Noire southern side (visited during D1).

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Leader: M. Faure Associate Leaders: J.M. Lardeaux, P. Ledru, P. Matte 1. Introduction The formation of the continental substratum of The French Massif Central is one of the largest Medio-Europa occurred in Paleozoic times. The pieces of the Variscan Belt. The whole Massif Central names of “Hercynian” or “Variscan” are used to deal provides a reference cross section throughout the north with the geodynamic processes that took place from Gondwana margin deformed and metamorphozed Cambrian to Carboniferous. It is now widely accepted during the Paleozoic. During the last two decades, that this Paleozoic Belt that crops out from Iberia to and recently through the GéoFrance 3D program, Bohemia (Fig. 1) results from a complex interplay of developments made in the areas of geochronology, rifting, convergence and collision between three large structural geology, metamorphic and magmatic continents, namely Laurentia, Baltica and Gondwana petrology, allow us to draw a comprehensive and several microcontinental stripes such as Avalonia structural map of the Massif Central and to discuss or Armorica (Matte, 2001). Continental drifting and a possible scenario accounting for the Paleozoic welding resulted in the opening and closure of several tectono-thermal evolution. oceans such as Iapetus, Rheic and Medio-European. This ﬁ eld trip presents representative lithological, There is however a wide range of opinions concerning structural, magmatic, metamorphic and the location and width of these oceanic domains and geochronological data of the French Massif the number, kinematics and timing of collisional Central from unmetamorphozed kilometer-scale processes (e. g. Autran and Cogné, 1980; Franke, recumbent folds to UHP metamorphic rocks. Most 1989, 2000; Ledru et al., 1989; Matte, 1991; 2001; of the controversial aspects of collisional orogens Faure et al., 1997). such as continental subduction and exhumation

Figure 1 - Location of the French Massif Central in the frame of the Paleozoic belt of Medio-Europa (Matte, 1991). Volume n° 2 - from B16 to B33 n° 2 - from Volume

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure Neoproterozoic toOrdovician ageis stratigraphic agesarelacking,a beds andvolcanic rocks. Although micaschists”) withsomequartzite series (alsocalled“Cévennes of athickmetapelite-metagrauwacke overthrusts theprevious unitconsists ii) The Para-autochthonous Unitthat area (Arthaud,1970). well observed intheMontagneNoire within kilometer-scale recumbentfolds Carboniferous. The seriesisdeformed spanning fromEarlyCambrianto more orlesscontinuoussedimentation margin/platform seriesrecordinga Thrust Beltinvolves asetofcontinental i) The Southern Palaeozoic Fold and tectonic unitsaredistinguished. also fromsouthtonorth,sixmain 1994 Fig.3).Fromtoptobottomand is astackofnappes(Ledruetal.,1989, structure oftheFrenchMassifCentral It isnow widelyacceptedthatthe French MassifCentral. 2.1. A structural mapofthe geological setting 2. Regional Tarare. St-Symphorien-sur-Coise; n°697 Yssingeaux; n°745St-Etienne;n°721 n°839 Langogne;n°840Burzet;n°792 n°862 Mende;n°863LeBleymard; n°1014 St-Chinian;n°988Bédarieux; Geologic mapsBRGM1/50000: Etienne-Le Puy;n°51Lyon-Grenoble. Mende; n°59Privas-Alès; n°50St- n°65 Béziers-Montpellier;n°58Rodez- Topographic mapsIGN1/100000: Field References addressed. setting ofplutonemplacementwillbe tectonics, crustalmeltingandtectonic syn- topost-orogenicextensional kinematics, inverted metamorphism, of ultrametamorphicrocks,nappe Lèvezou klippe(Matte,1991). an alternative sectionthroughthe and throughMassifCentral(C).C’ is Massif Armoricain to Ardenne (B) Figure 2-Cross-sectionsfromthe

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metamorphism is reached locally near Lyon, coesite-eclogite facies rocks crop-out (Lardeaux et al., 2001). The protoliths of the UGU also include metasediments and granitoids. The upper part of the UGU consists of migmatites formed by the partial melting of pelitic and quartzo-feldspathic rocks within which amphibolite block are preserved as restites. Radiometric dates show that the magmatism occurred in Early Ordovician times (ca. 480 Ma) and the high-pressure metamorphism in Late Silurian (ca. 420-410 Ma, Pin and Lancelot, 1982; Ducrot et al., 1983). Due to the occurrence of rare metagabbros and serpentinized ultramafi cs, the UGU is considered by some authors as a remnant of an oceanic domain, the Medio- European Ocean, that opened in Early Paleozoic times during the rifting that led to the separation of Armorica from North Gondwana (e. g. Dubuisson et al., 1989; Matte, 1991). However, it is worth noting that the Upper Gneiss Unit is not a true ophiolitic sequence since oceanic sedimentary rocks Figure 3 - Structural map of the Massif Central such as radiolarites or siliceous (adapted from Ledru et al., 1989). shales are lacking and ultramafi cs or serpentinites are rare. A likely interpretation would be to consider generally accepted. that the UGU is a transitional crust between true iii) The Lower Gneiss Unit (LGU) is lithologically continental and oceanic ones. quite similar to the Para-autochthonous Unit. Early v) The Thiviers-Payzac Unit that crops out in the Cambrian and Early Ordovician alkaline granitoids, south Limousin, is the highest tectonic unit of the now transformed in augen orthogneiss, are also allochthonous stack in the French Massif Central. It is widespread. Both the Para-autochthonous Unit and formed by Cambrian metagraywackes, rhyolites and Lower Gneiss Unit are interpreted as Proterozoic- quartzites intruded by Ordovician granite. Conversely Early Paleozoic remnants of the northern Gondwana to the underlying UGU, the Thiviers-Payzac Unit Volume n° 2 - from B16 to B33 n° 2 - from Volume margin that experienced crustal thinning and rifting in never experienced the high-pressure metamorphism. Ordovician times. As revealed by seismic refl ection line (Bitri et al., iv) The Upper Gneiss Unit (UGU) is made up of a 1999), these relatively low grade rocks tectonically bi-modal association called “leptynite-amphibolite” overly the UGU. sequence which is a peculiar assemblage of mafi c vi) In the NE Massif Central, near Lyon, the and felsic rocks. This unit experienced a higher Brevenne Unit consists of mafi c magmatic rocks metamorphic pressure under eclogite and HP (pyroclastites, pillow basalts, diabases, gabbros), granulite facies (ca. 20Kb). Ultra high-pressure serpentinized ultramafi cs, acidic volcanic rocks,

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure acidic rocksaredatedof366 and siliceoussediments(radiolarites,siltites). The Structural informationrelatedtothehigh-pressure 2.2. The tectono-metamorphicevolution. Carboniferous ageislikely (seebelow). unconformity ofLeGoujet(eastLyon) anEarly the Early Visean calcareoussandstoneofthefamous but sincethemetamorphic rocksareconcealedbelow 1999). The preciseageofthethrustingisunknown dextral strike-slip (Feybesse etal.,1988;Leloix to theNWover theUGUfollowed byaNE-SW 1998). The Brevenne Unitrecordsanearlythrusting Ohnenstetter, 1986;Pin,1990;PinandPaquette, back-arc basinopenedwithintheUGU(Siderand oceanic sequenceformedwithinanora extension intheBeaujolaisareaisaMiddleDevonian geochemistry show thattheBrevenne Unitandits on zircon,PinandPaquette, 1998).Petrologyand Figure 4-Structuralandgeochronologic features relatedtotheLateSilurian-MiddleDevonian D1event. ± 5Ma(U/Pbmethod

rocks (e.g. Villedé d’Ardin,Génis,Somme,Belfort Devonian stratigraphicageoftheunmetamorphozed rocks. The radiometricdatescomplywiththe of theexhumation ofthehigh-pressuremetamorphic upper partoftheUGU,itoccurredduringoratend the D1event isfoundinthemigmatitesthatform al., 1994;RoigandFaure, 2000;Figs.4,5).Since Costa, 1992;BoutinandMontigny, 1993;Duthouet Ma (e.g.Floc’h,1983;QuenardelandRolin,1984; metamorphism andanatexis datedaround385-380 an intermediatepressure/intermediatetemperature a top-to-the-SWshearingdeveloped coevally with characterized byaNE-SWtrendinglineationwith The earliestdeformationfoundintheUGU,D1,is main synmetamorphicductileevents arerecognized. draw ageneralview ofthisevent. Moreover, three known onlyasrelics.Itisthereforequitedifﬁ evolution ispoorlydocumentedsincetheserocksare metamorphism andtheprogrademetamorphic 21-05-2004, 10:51:56 cult to cult PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

Figure 5 - P-T paths of the Silurian-Devonian and Carboniferous events for the different units.

areas, Fig. 4). Although a direct unconformity is never this Early Carboniferous deformation is coeval with observed, ﬁ eld relationships suggest that D1 is older the closure of the Rheic Ocean and collision between than Middle Devonian. Gondwana and Laurussia. The second event, D2, is characterized by a NW- The third event, D3, is restricted to the southern part SE trending lineation coeval with a barrovian type of the Massif Central. In the Para-autochthonous metamorphism (Figs. 5, 6). 40Ar/39Ar dates on biotite, Unit of Cévennes-Albigeois, upper greenschist to muscovite and amphibole range around 360-350 Ma. amphibolite facies rocks are deformed by top-to-the- Most of the shear criteria developed along the NW-SE south ductile shearing along a submeridian lineation lineation indicate a top-to-the-NW shearing. In the (Fig. 7). Available 40Ar/39Ar dates on the metamorphic Rouergue area, the Naucelle thrust is related to this minerals yield Visean ages around 340 Ma (Monié event (Duguet and Faure, in press). The last increment et al., 2000; Faure et al., 2001). This thrusting of the ductile deformation in the metamorphic series propagates southward in the Fold and Thrust Belt is associated with the emplacement of peraluminous where kilometer-scale recumbent folds develop from cordierite bearing granitoids such as the Guéret Visean to Namurian. Although in the South Massif pluton that is the largest massif of this type. These Central south-directed compressional regime lasts granitoids exhibit magmatic to sub-solidus fabrics from Visean to Namurian (345 to 325Ma), conversely, that comply with the synkinematic character of these in the northern part of the massif, the Late Visean (ca. plutons (e. g. Roig et al., 1998). A similar tectonic- 340 Ma) is a turning point in the tectonic evolution. metamorphic-magmatic pattern is also recognized in From Morvan to Limousin, the Late Visean time the south part of the Massif Armoricain. The closure corresponds to the onset of syn-orogenic extension Volume n° 2 - from B16 to B33 n° 2 - from Volume of the Brevenne oceanic basins is chronologically caracterized by a huge crustal melting. Structural and kinematically in agreement with the D2 event studies indicate that the syn-orogenic extension is (Leloix et al., 1999). The geodynamic signiﬁ cance controled by a NW-SE maximum stretching direction of the NW-SE lineation parallel to the belt is not (Fig. 7). The NW-SE spreading of the inner part of clearly understood yet. Several hypotheses have the Massif Central is also partly accommodated by been proposed (e. g. Burg et al., 1987; Bouchez and ductile wrench faults well developed in Limousin Jover, 1986; Mattauer et al., 1988) but none of them (e. g. La Courtine or S. Limousin faults, Fig. 7) and appears fully convincing. As discussed in section 2.4, in the Massif Armoricain. In the scale of the whole

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure metamorphozed andincludedinthestackofnappes. or Ordovician magmatic rocksareductilelydeformed, amphibolite complex in theUGU,andCambrian responsible fortheformationofleptynite- detail here. The EarlyOrdovician bimodalmagmatism, 2.3.1. The pre-orogenicmagmatismisnotpresentedin et al.,1984). migmatites andgranitoidsarerecognized(e.g.Duthou derived fromcrustalmelting.Several generationsof also characterizedbyavoluminous magmatismmainly Like allthe Variscan massifs,theMassifCentralis 2.3. A magmatic outline. they willbeconsideredinthenext section. belt. Sincethey arecloselyassociatedtomagmatism, younger ones)tookplaceduringthecollapseof The lastductiledeformationevents (ca.320Maand thrusts in Ardenne andSEEngland. responsible forthedevelopment ofnorth-directed belt, theLate Visean toNamuriancompressionisalso

2002). The migmatitesandcordieritegranitesofthe Chemical U/Th/Pbmethodonmonazite(BeMezème, anatexis isdatedbetween 333and324Mabythe or pre-Velay migmatites(Faure etal.,2001). The ortho- andparagneisscalled“ the MasméjeanUnit ” Para-autochthonous Unit isunderlainbymigmatitic orogenic collapse.InthenorthernCévennes, the a NW-SE stretchingrelatedtotheearlystageof structural controlofdyke intrusioncomplieswith magma isalsolikely (PinandDuthou,1990). The Moreover, amantlecontribution asthesourceof melting was triggered byheatimputfromthemantle. microgranites. Geochemistryindicatesthatcrustal series”, rhyolitictodaciticdykes andhypovolcanic pyroclastic deposits, called“Tufs anthacifères consists inaerialproductswithlava fl north andwestpartofthemassifCentral(Fig.7).It 2.3.4. The Visean magmatismiswelldeveloped inthe same strainfi eld thantheD2deformation(Fig.6). emplacement was controlledbythe fabric suggests thatthoseplutons aluminous plutons. Their magmatic by theGuéret-typegranitesper- collisional magmatismisrepresented 2.3.3. TheTournaisianlate- be discussedinsection2.4. arc. Itsgeodynamicsignifi the deeppartofsameDevonian Lameyre, 1971),areinterpretedas time intheLimousin(Didierand alcaline rockswellknown foralong magmatic arc.Moreover, mafi interpreted astheaerialpartofa Meilleraie series. These rocksare basaltic pillow lavas formthe Armoricain, Eifelian-Givetian 1989). InthesouthpartofMassif (Fig. 8;Pinetal.,1982;Delfour, series) belongtoamagmaticarc Morvan area(calledtheSomme clastic rocksthatcropoutinthe calc-alkaline volcanic andvolcani- 2.3.2. The MiddletoLateDevonian Paris Basin). event (AMBP:Magnetic Anomaly of Late Devonian-Early Carboniferous D2 geochronologic features relatedtothe Figure 6-Structural,magmaticand ows, ignimbrites, cancewill 11-06-2004, 8:50:00 c calk- PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

Figure 7 - Structural and geochronologic features related to the Visean-Namurian D3 tectonics and Late Visean magmatism (AMBP: Magnetic Anomaly of Paris Basin).

Montagne Noire Axial Zone (cf D2 ﬁ eld itinerary in section 3) yield similar ages. In the present state of knowledge, this Late Visean event is still poorly studied. It is likely that other pre-Velay migmatites are not yet recognized also within the Velay dome.

2.3.5. The Namurian-Westphalian plutonism corresponds to the main period of magma production in the French Massif Central. It is well acknowledged (Didier and Lameyre, 1971) that this magmatism is represented by two types of granitoids, namely porphyritic monzogranites, such as the Margeride or Pont-de- Montvert-Borne plutons, and biotite-muscovite leucogranites such as the Brame or Millevache

Figure 8 - Map showing B16 to B33 n° 2 - from Volume the distribution of the Devonian plutons and volcanic rocks related to the magmatic arc and ophiolites (Brè venne, Ligne Klippes) interpreted as back arc basins (adapted from Faure et al., 1997).

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure and correlatively driftingandrewelding of Armorica opening andclosureoftheMedio-EuropeanOcean cycle, (Cambrian toEarlyDevonian), isrelatedtothe 1997). According tothismodel,anEarlyPaleozoic out apolycyclic evolution (Pin,1990;Faure etal., 1991, Lardeauxetal.,2001). The secondonepoints from SiluriantoEarlyCarboniferous(e.g.Matte, convergence betweenGondwana andLaurussia ﬁ proposed. The for theevolution oftheFrenchMassifCentralare geodynamic evolution model.Presently, two scenari The above-presented dataallow ustodiscussa 2.4. A possible geodynamic scenario. Carboniferous St-Etiennebasin. north ofthePilatfault formthesubstratumofLate and micaschistsbelongingtotheLGUthatcropout ductile normalfault (Malavieille etal.,1990).Gneiss bounded tothenorthbyadetachmentfault, thePilat formations inthecoalbasins. The Velay domeis rarely alkalinebasaltsinterlayerswithterrigeneous dome, andalsobyacidictuf,ashlayersmore by cordieritegraniteandmigmatitesofthe Velay 2.3.6. The Stephanian magmatismisrepresented (Faure, 1995). orogenic extensional tectonicsoftheMassifCentral pattern isinterpretedastheconsequenceofsyn- the Argentat ductilenormal fault. This structural slickenline. A similar kinematicsisalsofoundalong normal fault thatalsoexhibits aNW-SE trendinghot pluton isboundedtothewestbyNantiatductile rocks (Fig.9).InthenorthLimousin,Brame ans andalusitecontactmineralsintheplutonhost lineation. The sametrendisalsoobserved inbiotite NW-SE trendingmineral,stretchingandmagnetic that thesebodiesarecharacterizedbyaconspicuous studies oftheNamuri-Westphalian plutonsshow types emplacedcoevally. Petro-structuraland AMS geochronology show thatthesetwo magmatic from different magmas,but ﬁ the northandwestparts. The two typeswerederived of themassifandlatertypeismoreabundant in is bestrepresentedinthecentralandsouthernparts out throughouttheMassifCentral,formertype massifs (Fig.9). Although bothgranitetypescrop Figure 9-Distribution ofthemaingraniticplutons tectonics. Duringthisevent, thepre-Visean GuÈret coeval withthestretching lineationandkinematics related totheNamurian-Westphalian extensional rst oneemphasizesacontinuous pluton behaves asarigidbody. eld relationshipsand

the leptynite-amphibolitecomplexes. As amatterof continental riftingiscoeval withtheemplacementof Duthou, 1990).IntheUGU,crustalthinningdueto to crustalcontamination(Duthouetal.,1984;Pinand calc-alkaline” geochemistryofthesegranitoidsisdue continental rifting.Itisworth notingthat“pseudo- alkaline Ordovician granitoids alsocomplywith pelite series(PinandMarini,1993).InLGU,the lava), diabasedykes, gabbrointrudethegrauwacke- Unit, alkalinemaﬁ inferred frommagmatism.InthePara-autochthonous al., 1994).EvidenceofanOrdovician riftingisalso as theresultoferosionontiltedblocks(Robardetet Late Ordovician andSiluriandepositsisinterpreted in Devonian byacarbonateplatform. The lackof corresponds toaterrigeneousenvironment, followed study ofMontagneNoire,theCambrian-Ordovician which extends fromSouth America toChina.Fromthe belongs tothenorthernpassive margin ofGondwana From CambriantoEarlySilurian,theMassifCentral 2.4.1. The breaking ofthenorthGondwana margin. model, thefollowing stagesareacknowledged. of Gondwana andLaurussia. Whatever thepreferred which theclosureofRheicOceanandcollision Middle Devonian toCarboniferousaccountsfor to Gondwana. A secondorogeniccycle rangingfrom c volcanics (sometimeswithpillow 21-05-2004, 10:52:09 PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

Ocean started in Silurian. All authors accept a northward subduction of the Gondwana margin, however, structural constraints (i. e. kinematics coeval with the development of high-pressure assemblages) or geodynamic evidence (i. e. relics of a magmatic arc) are lacking. By Middle Devonian time, the Armorica microcontinent is rewelded to Gondwana. In NE Massif Central, (North of Lyon), undeformed and unmetamorphozed Givetian sedimentary rocks unconformably cover the migmatites and high pressure rocks (Delfour, 1989; Godard, 1990). Subduction of oceanic and continental rocks is followed by their exhumation in Early to Middle Devonian, around 390-385 Ma. The lack of large volumes of Devonian clastic rocks suggests that exhumation was tectonically assisted. Exhumation results in the extensive retrogression of the high-pressure rocks of the UGU and migmatisation of the pelitic parts.

2.4.3. Mid-Devonian magmatic arc-back arc system. Frasnian-Fammenian calc-alkaline volcanism in the NE Massif Central and Vosges argue for subduction. In addition, the 380-370 Ma calc- alkaline diorite, tonalite, granodiorite plutons that crop out in NW Massif Central are interpreted as the deep part of this magmatic arc. However, in their present position, these plutons are rootless and tectonically included into the Hercynian nappes. Southward subduction of the Rheic Ocean is viewed as the cause of the calc-alcaline magmatism. At the same time, distension also Figure 10 - Devonian geodynamic reconstruction (map occurred in the upper plate, giving rise to limited and section) showing the closure of the Rheic Ocean oceanic zones such as the Brévenne in the Massif by southward subduction below Gondwana and related Central or other areas in the Massif Armoricain and microcontinents (from Faure et al., 1997). Vosges. Therefore, an arc-back arc pattern appears as the most likely geodynamic setting for Devonian fact, the Cambrian-Ordovician period is characterized times (Fig. 10). However a discussion of the Léon by the formation of continental stripes, such as the and microcontinents is beyond the scope of this Armorica microcontinent drifted from the north presentation. Gondwana margin. The question of the maximum width of the intervening Medio-European Ocean has 2.4.4. The closure of the Rheic Ocean and the B16 to B33 n° 2 - from Volume not been not settled yet (see discussion in Robardet, Tournaisian collision. 2003). A rough estimate suggests that this oceanic Since the Late Fammenian, a complete closure area was of limited extension (i. e. between 500 and of the Rheic Ocean led to a collision between the 1000 km). North European continent made by the assembly of Laurentia, Baltica and Avalonia during the Caledonian 2.4.2. The closure of the Medio-European Ocean. orogeny and Gondwana, including Armorica On the basis of available dates on the high-pressure microcontinent rewelded to it. Intracontinental metamorphism, the closure of the Medio-European shortening follows the Lizard ophiolite obduction

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure In theCentralandsouthernMassifCentral,thermal of kilometer-scale recumbentfoldsandthrusts. still undercompressionasshown bythe development such asMontagneNoireand Ardenne respectively, are and northernexternal zones oftheHercynian Belt, inner partofthebelt(Fig.7).However, thesouthern for anincipientstageofsyn-orogeniccollapseinthe by aNW-SE maximumstretchingdirectionandargue Visean plutonsanddykes emplacementiscontrolled related plutonism. The structuralanalysisoftheLate for the“tufsanthracifères”acidicvolcanism and Massif Centralexperienced crustalmeltingresponsible As soonasLate Visean (ca335Ma),thenorthern syn-convergence extension. 2.4.5. Late Visean-Namurian Fold and Thrust Belt. in thePara-autochthonous Unitto330-325Mainthe progressively youngingsouthward: ca.345-340Ma shearing andrecumbentfoldingdevelops witha In thesouthernpartofMassifCentral,southward widespread inwesternandnorthernMassifCentral. metamorphism, anddatedaround360Maisalso coeval withmiddletemperature/pressure in themaﬁ under uppergreenschist-lower amphibolitefacies area ischaracterizedbytop-to-the-NWshearing Massif Central,theclosureofBrévenne oceanic the SouthofEnglandto Ardennes. Inthenorthern of Paris Basin).North-directedthrustsdevelop from (which probablyextends alongtheMagnetic Anomaly lithosphere scalecross (modified fromFaure magmatism andheat regimes (extensional and compressional), Belt inLate Visean. section throughthe Mantle lithosphere contrasted tectonic French Hercynian flow inthecentral delamination may account for the part ofthebelt Interpretative et al.,2002). c rocks. Top-to-the-NW ductileshearing Figure 11- bounded bypurenormalfaults structural typesofbasinsarerecognized:1)half-graben setting ofintra-mountainStephaniancoalbasins. Two belt. Extensionalregime iswellrecordedbythetectonic Massif Centralcorrespondstothecollapseofwhole The laststageoftheHercynian orogeny intheFrench 2.4.6. Stephanianpost-orogenicNNE-SSWextension mesothermal golddeposits. corresponds alsotothemainmetallogeneticepochfor maximum stretchingdirection. This tectonicstage synkinematic plutonsexhibit thesameNW-SE and MassifCentralthewrenchorextension controlled it isworth notingthatbothintheMassif Armorican dextral wrenching(e.g.Berthéetal.,1978).However, There, they aresyn-kinematicplutonscoeval with Massif Armoricain, leucograniticarealsowidespread. granodiorites intheMassifCentral(Fig.9).In by emplacementfabrics ofleucogranitesand Ma), orogenparallelextension iswellrecorded From Namurianto Westphalian (ca325-310 account forthemagmatism(Fig.11). delamination, islikely toplayasignificant roleto lithospheric mantlefromcrust,i.e. tectonic regime) isnotsettledyet.Decouplingof tectonic setting(namelyextensional orcompressional event. However, inthepresentstateofknowledge, the of thelateCarboniferous Velay massifbelongtothis crop outintheMontagneNoire Axial Zoneandsouth granite formation. The ca330Mamigmatitesthat overprint isreponsibleformigmatite andcordierite or normal faultswitha 21-05-2004, 10:52:14 PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

Figure 12 - Massif Central map showing the Carboniferous extensional structures: coal basins, Velay granite- migmatite dome. Volume n° 2 - from B16 to B33 n° 2 - from Volume

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure and Vielzeuf,1983). temperature granulitizationofthelower crust(Pin Heat imputfromthemantleisresponsibleforhigh migmatitic-granitic Velay dome(Ledruetal.,2002). most spectacularstructureisthe100kmdiameter than duringsyn-convergence extension. The to LateCarboniferousextension arelessdeveloped Magmatism andmid-crustaldeformationassociated that accommodatedifferent amountsofextension. and Argentat fault areinterpretedastransferfaults trending wrenchfaults suchastheSillonHouiller the Massif Armoricain. Several N-StoNNE-SSW rare inwesternMassifCentralandalmostlacking are widespreadineasternMassifCentralbut are east. NE-SWextension andcorrelatively coalbasins The amountofextension increasesfromwestto NE-SW stretching,NW-SE andvertical shortening. pattern ofStephanianextension ischaracterizedby In thescaleofMassifCentral,deformation graben isnotsettledyet(MattauerandMatte,1998). control, eitherasaleft-lateralpullapartorhalf- of theStephanianstage.Nevertheless thestructural famous sinceitcorrespondstothepara-stratotype basins, theSt-Etiennecoalbasinisoneofmost wrench faults (Fig.12). Among theseintra-mountain strike-slip componentor2)pull-apartcontrolledby (1970). and Arthaud Figure 13-StructuralmapoftheMontagneNoiremodifiedfromGèze(1949) column ofthePaleozoic seriesfound intheMontagne Noire southernsiderecumbentfolds Figure 14-Schematic stratigraphic (adapted from Arthaud, 1970). stratigraphy (Lower Cambrian, worldwide famous forPaleozoic during D2. The SouthernSideis the Axial Zonewillbepresented the previous two. The geologyof This lastareaislessstudiedthan and aNorthernSide(Fig.13). Southern Side,an Axial Zone from SouthtoNorth,intoa Noire areaisclassicallydivided Arthaud (1970),theMontagne Following Gèze(1949)and A. Geologicsetting de Ladarez Cessenon D136toSt-Nazaire Montpellier--> Béziers --> Noire Southern Side Montagne Recumbent folding inthe 3. Fielditinerary DAY 1 21-05-2004, 10:52:24 PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

Figure 15 - Sketch of the main units observed in the eastern part of the Montagne Noire.

Five tectonic units are recognized in the southern side of the Montagne Noire, namely from top to bottom (Fig. 15): i) The Pardailhan Nappe (or recumbent fold) ii) The Mont Peyroux Nappe iii) The Monts de Faugères Unit iv) The Cabrières Unit v) The Para- autochthonous domain The Pardailhan Nappe consists of Lower Ordovician, Devonian, Carboniferous) and the folded and overturned Cambrian to Devonian rocks. development of kilometer-scale recumbent folds (or The Mont Peyroux Nappe includes Ordovician to nappes). The stratigraphic column is schematically Visean rocks. The Monts de Faugères Unit consists summarized in Fig. 14. of several overturned folds of Devonian to Visean rocks. The Cabrières Unit is an olistostrome, with large-scale olistoliths of Carboniferous and Devonian limestones, Silurian volcanites and Ordovician turbidites are resedimented within a wild-ﬂ ysch matrix corresponding to the foreland basin of the belt (Engel et al., 1980). The Pardailhan Nappe exhibits a conspicuous axial planar cleavage, whereas in the Mont

Peyroux Nappe, the B16 to B33 n° 2 - from Volume transition between ductile deformation with axial planar cleavage folds and synsedimentary structures can be observed. On the basis

Figure 16 - D1 route map.

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure 17). Para-autochthonous domainunderneath(Figs.16, structural aspectsoftheMontPeyroux nappeand The stopswillshow mostofthelithological and B. Stopdescription Zone dome(cf.D2). deformation linked withtheformationof Axial develop afterrecumbentfoldingduringanupright to North,however mostoftheobserved structures deformation andmetamorphismincreasefromSouth to MiddleCarboniferous(Visean). Along thisroute, ranges fromEarlyOrdovician (Tremadoc-Arenig) river (Fig.16). There, thestratigraphicsuccession through aSouthtoNorthcrosssectionalongtheOrb deformation oftheMontPeyroux recumbentfold The aimofthisﬁ Visean-Namurian times(around330-325Ma). of stratigraphy, those nappeswereemplacedin The landscapeshows Eocene limestone Visean flyschwithlimestoneblocks. Stop D1.1: Figure 17-SyntheticcrosssectionoftheMontagneNoirewithlocationD1andD2stops. rst dayistopresentthepolyphase and nearly200mtothenorthofthisstop. Visean turbiditeisseenontheothersideofvalley limestone blocks.Devonian limestoneoverlying the exposes disruptedbedswithsandstonelensesand (20E 60). The northern partthatisfoldedandsheared ﬂ rocks. The S.partofthesectionexposes Visean unconformably overlying atlow anglePaleozoic extinction inthe Earth history(Kapperetal.,1993). Crisis” responsibleforoneofthemostsevere mass series correspondstothe“Famenian Biological Section fortheFrasnian/Famennian boundary. This section hasbeenchosenastheGlobalStratotype “Griotte marble”(griotteisatypeofcherry). This Devonian (365Ma)Goniatitelimestonecalled de Janeiro). The vertical beds(N30.90)areLate in Washington, ortheMaisondelaFranceinRio exported allover theworld (e.g.the White House This oldquarrywas minedforrednodularlimestone -Frasnian/Famennian boundary. Coumiac quarry(protected area) Stop D1.2: ysch withcontinuoussandstonebedsdippingeast 21-05-2004, 10:52:32 PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

Stop D1.3: phases. Recumbent isoclinal folds (F1) are deformed Early Ordovician turbidite. NE of Lugné. by upright open folds (F2) with axes plunging 50° In the landscape, looking to the SE, the vertical cliffs NW. F1 are related to the Mont Peyroux recumbent are Early Devonian limestones continuous with those fold and F2 belong to the kilometer-scale upright seen in the previous stop (D1-2) in the Coumiac folding responsible for the Roquebrun synform, quarry. Vieussan synform, and Axial Zone antiform (Fig. Sandstone-mudstone alternations lie subhorizontally, 17). however graded bedding and load cast show that the sequence is upside down. Folds are apparently Stop D1.7: overturned to the north but correspond in reality to Ordovician-Devonian contact. North the inverted limb of the Mont Peyroux recumbent of Roquebrun. fold. This Early Ordovician turbidite is interpreted This stop shows the inverted stratigraphic contact as deposited along the northern passive margin of between Ordovician detritals and Devonian carbonates Gondwana. in the northern limb of the Roquebrun synform. From south to north: Ordovician turbidite with top-to-the-S Stop D1.4: base (with load casts) dipping southward is underlain Early Ordovician turbidite. by Devonian calcareous sandstone, followed by A few hundred meters from the previous stop. limestone and dolomite. At the northern end of the The subvertical to north dipping upside down of outcrop, undeformed crinoid stems can be observed the beds exhibits numerous load casts, ripple marks in the Devonian carbonates. and bioturbation evidence (worm burrows). Locally Lingula shells can be abundant. Sandstones contain Stop D1.8: abundant fl oatted muscovite and heavy minerals. Visean fl ysch. Chapelle St-Poncian, S. of Ceps. In this outcrop, like the previous one, cleavage is Looking to the NW, the white rocks above the village lacking. of Ceps are inverted Devonian limestone, and to the W and SW, the vineyards are located in the Ordovician Stop D1.5: turbidite. The highest white cliff in the background Panorama on Roquebrun synform and the Orb (La Tour du Pin summit) is the northern extension of River. Col de la Vernède. Devonian formations. Below the cliff and up to Ceps, Below the road, the vineyards and Orb river are the lowest parts of the mountains are made of Visean located in the Ordovician turbidites which form the fl ysch, belonging to several tectonic units. core of the Roquebrun synform. Looking northward, The outcrop exposes Visean mudstone-sandstone with above the Roquebrun village, the white cliffs are limestone intercalations dipping south-westwards Devonian limestones. In the background, the hills (S0-1: 130 SW 50). Contrasting with the Visean with bushes are Visean fl ysch and in the distance, the rocks observed at the fi rst stop (point D1-1), here the last hills are made of Devonian limestone belonging Visean pelites are slightly metamorphosed (sericite) to the Monts de Faugères Unit. To the West (left), and exhibit a N70E trending crenulation lineation. the highest mountain is the Pic de Naudech made Chevron folds and south-directed brittle shear zones of inverted Cambrian rocks overlying inverted with quartz veins deform S0-1. Along the road, Ordovician turbidites belonging to the Pardailhan Devonian rocks are not observed, a late fault separates Nappe. Lastly, the farthest mountain to the NW is the the Visean and Devonian rocks. Mt Caroux composed of orthogneiss belonging to the Axial Zone. Stop D1.9: Volume n° 2 - from B16 to B33 n° 2 - from Volume Along the other side of the road, the Ordovician Monts de Faugères Unit. Large curve of Orb River turbidite is complexly folded. Superimposed folds are below Chapelle St-Geminian. observed in the next stop. Tournaisian (?)-Visean limestone and sericite metapelite present a westward (170W40) dipping Stop D1.6: foliation and a well marked mineral, stretching and Superimposed folds in Ordovician turbidite. 200 m crenulation lineation trending N 70E. Pressure- down to Roquebrun. solution is the dominant deformation mechanism. The Ordovician turbidite experienced two folding S0-1 is also cut at high angle by west dipping tension

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure marked bynumerousquartzveins (nostop). autochthonous seriesandtheMontPeyroux nappeis Back tothemainroad,contactbetweenPara- Pin andleLaufolds)overturned to theSouth. autochthonous seriesisfoldedbytwo anticlines(Le and intersectiondevelops. Regionally, thispara- due toelongatednodulesandgoniatites,crenulation the anticlinegeometry. A N70Ecompositelineation cleavage refractioninsandstonebedscomplywith Pin “anticline.Bedding-cleavage relationshipswith respectively. This S-SEverging foldiscalled“Le Stop D1.10: gashes fi Stop D1.11’: top-to-the-NE senseofshear. direction. InXZsection,pressureshadows indicate to thefoldaxisandthusathighangletransport is worth notingthatstretchinglineationtrendsclose folds belongingtothesamelarge-scale structure.It Plate 1,B)andlimestoneareinvolved inisoclinal normal limb. To thesouth,radiolarianchert(Color overturning. The horizontalpartoftheoutcropis Bedding-cleavage relationshipsshow asouthward folded LateDevonian limestone(partlydolomitized). This famous outcrop(ColorPlate1, A) exposes a de Graïs. Recumbent fold inDevonian limestone.Moulin Stop D1.11: limestone to Visean fl the successionbecomesnormalfromDevonian Faugères) and Visean fl (lydiennes) andnodularlimestones(calcairesde cleavage; Tournaisian radiolarianblackcherts (S0: 60NW60)withaninverted limbsubhorizontal nodular limestone(griottemarble)withgoniatites down sequenceconsistsoftheUpperDevonian red the Orbsection.FromNorthtoSouth,upside of LePin,thisoutcropexposes thedeepestpartof the underlyingPara-autochthonous Domain.North Turning righttotheroadofLePin,wecanobserve anticlines. Para-autochthonous Domain.LePinandLau Devonian limestones. limb ofthe Vieussan antiform,wellmarked inthe the rightprovides aclearpanoramaofthenorthern Turning leftonD177,toBerlou,thelarge curve to Optional. Landscapeonthe Vieussan antiform. lled byfi brous calcite. ysch. Northofthisoutcrop, ysch frombottomtotop, event occurringinthehinterland. tourmaline supportapre-Devonian metamorphic volcanic quartzgrains,mica,garnet, zircon,rutile, northern sourceoftheterrigeneoussediments.Detrital that sedimentologyofeo-Devonian rocksindicatesa in theinternalzoneofBelt.Itisworth noting metamorphic event thattookplacemoretothenorth as asedimentaryconsequenceofremotetectonic- the MontagneNoiresouthernsidecanbeinterpreted the lackofLateOrdovician-Silurian rocksinmostof between Ordovician andDevonian formations,and the Ordovician sandstone. The angularunconformity Isoclinal foldswithcurved hingescanbeobserved in the leftandEarlyDevonian sandstonetotheright. stratigraphic contactbetweenOrdovician turbiditeto recumbent fold(Fig.18). The outropexposes inverted along thebasalthrustcontactofPardailhan masses correspondingtoDevonian limestoneboudins Looking West, thehillslopeshows several white Ordovician/Devonian contact.N.of Vieussan. D1.12: Back to Vieussan bythesameroad. shearing, thelimestoneisweaklyorundeformed. the tightcurve northofEscagnès.Inspiteintense of thePardailhan recumbentfoldcanbeobserved in limestone boudinsmarkingthebasalthrustcontact the MontPeyroux recumbentfoldandtheDevonian contact betweenOrdovician turbiditebelongingto deformed bothbyisoclinalanduprightfolds. The turbidite oftheMontPeyroux recumbentfold Southward, theroadgoesthroughOrdovician fold “Queuedecochon(pig’s tail)” Basal thrustcontactofthePardailhan recumbent Stop D1.11»: foliation axialplanartoF2. few biotitegrainscanbeobserved inthevertical S2 trending folds(F2)andN50Eisoclinal(F1). A pelite andsandstonedeformedbyuprightN80E At theoutcropscale,Ordovician rocksareblack Vieussan antiform. is Ordovician turbidite atthewesternpericlineof built onDevonian marbles(D1-14),thefrontview d’Heric visitedonD2. The villageof Tarassac is Axial ZonegneissandtheentranceofGorges Looking tothenorth,landscapepresents Peyroux recumbent fold. Nof Vieussan. Ordovician turbiditeinthenorthpartofMont Stop D1.13: .

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Stop D1.14: 17). The foliation of micaschists and gneiss defines a para-autochthonous Devonian marble. Tarassac, NE-SW long axis elliptical dome whose western part parking of VVP. is disturbed by the Eocene Mazamet thrust. Some Muscovite bearing Devonian marble with pink calcite authors argued that the Axial Zone metamorphic crystals corresponding to deformed crinoid stems rocks correspond to the Precambrian basement of the exhibit a southward dip (70S60) and well marked Paleozoic series observed in the recumbent folds. In subhorizontal mineral and stretching lineation. This the present state of knowledge, there is no argument marble is separated from the overlying Ordovician to support the existence of a Neo-Proterozoic (i. e. turbidite by a major thrust contact corresponding Cadomian) orogen in the Massif Central. Therefore, to the basal thrust surface of the Mont Peyroux the reality of a Precambrian basement in the Montagne recumbent fold. The Devonian marble and the Noire Axial Zone is not supported by the data. The underlying metapelites attributed to Ordovician augen orthogneiss seen in the gorges d’Heric, are (not seen here) are a normal sequence belonging porphyritic granites intruding a Neo-Proterozoic to to the Para-autochthounous Unit. 40Ar/39Ar date on Paleozoic metasedimentary series of micaschists muscovite gives 297 ± 3 Ma which is interpreted as and gneiss and transformed into augen gneiss during the age of a Late Carboniferous gravity sliding event Hercynian tectonics. Recent U/Pb dating supports an related to the formation of the Axial Zone (Maluski Early Paleozoic age for the magmatism. The presence et al., 1991). of penninic style recumbent folds overturned to the north has also been assumed (Demange, 1975). End of the 1st day. Overnight stay in Olargues Although possible, this interpretation cannot be demonstrated, mainly due to poor outcrop conditions.

Figure 18 - Panoramic view of the contact between the Pardaillhan (top) and Mont-Peyroux nappe (bottom) marked by Devonian limestone boudins called “ pig’s tail ”.

DAY 2 The Axial Zone gneiss experienced a HT/LP type Migmatite dome of the Montagne metamorphism up to partial melting giving rise to Noire Axial Zone migmatites and anatectic cordierite granites (e. g. Laouzas granite). U/Pb dating on single grain zircon A. Geological setting and monazite give a ca 330 age. Isograds of this HT/ B16 to B33 n° 2 - from Volume The Montagne Noire Axial Zone remains one of the LP metamorphism define the same domal geometry most controversial area in the geology of Massif as the foliation. Within the micaschist enveloppe, Central (cf extensive references in Soula et al., 2001). kyanite relics are locally found (K in Fig. 13). P-T The Late Visean-Early Namurian recumbent folds paths for the gneiss core and metamorphic envelope examined during D1 are overprinted by metamorphic have been proposed (e. g. Soula et al;, 2001; Fig. 19). and structural features related to a granite-migmatite It is worth noting that some amphibolites included in gneiss dome developed in the Axial Zone (Figs. 13, the gneiss are retrogressed eclogites (E in Fig. 13) with estimated pressure and temperature around 9 ± 2

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure kbar and750 (from Soulaetal.,2001). and newHTmetamorphicmineralscrystallize of themigmatiticcore,earlyisogradsaredeformed the MontagneNoire Axial Zone.Duringupward doming Figure 20-Interpretationofthepresent-daygeometry southern andnorthernsidesoverprints therecumbent NE-SW trendingstretchinglineationwhichinthe The Axial Zoneischaracterizedbyaconspicuous folding ofthePaleozoic sedimentarysequence. to compressionaltectonicscoeval withrecumbent of theductiledeformationgneissisrelated is thatthehigh-pressuremetamorphismandapart is available fortheeclogites,apossibleinterpretation at ca25-30kmdepth. Although noradiometricdate units situatedundertherecumbentfoldwereburied These high-pressurerockssuggestthatthetectonic ± 50°Crespectively (Demange,1985). from Soulaetal.,2001andDemange,1985). Axial Zonemicaschist andmigmatiticgneiss(modified Figure 19-P-Tpathsinferred for theMontagneNoire the development ofaNE-SWcrenulation,strengthens lineation keeps thesameN60-70Etrend.Furthernorth, After thesecondbridge,foliationfl dykes arefl southwards whereasnearthetopofmountain, entrance ofthegorges d’Héric,mostofthedykes dip pegmatitic dykes obliquely cutthefoliation. At the are themostcommonrock-types. Tourmaline-garnet The Hericaugenorthogneissandparagneisssepta SE partofthe Axial Zone,called“CarouxMassif”. The morningisdedicatedtotheobservation ofthe gorges d’Hérictrack,1.5km,aneasywalk. Cross section oftheCaroux Massifalongthe Stop D2.1: B. Stopdescription 1998). on zirconandmonaziteat327 that crosscutsthemigmatitefoliationisdatedbyU/Pb the Axial Zone.Indeed,the Vialais granite(Fig.21) hypothesis cannotaccountforthebulk structureof Graissessac coalbasin);theextensional gneissdome (Stephanian) tectonics(e.g.syntectonicinfi role toaccountfortheLateCarboniferous Although extensional tectonicsplaysanimportant in Soulaetal.(2001). 92). A recentdiscussionofthisproblemcanbefound core complex” (Van denDriesscheetBrun,1991- 1960, Faure etCotterau,1988);iv) “metamorphic diapir andregional NE-SWshortening(Schuilling, et al.,1998;iii)interferencebetweenmigmatitic NE-SW antiformalstack(Mattaueretal.1996,Matte (Nicolas etal.,1977;EchtlerMalavieille, 1990);ii) proposed, namely:i)NE-SWductilewrenchzone doming remainsdisputed.Several interpretationsare the tectonicsignificance oftheMontagneNoire to theupliftofmigmatiticcore(Fig.20).However, combination ofthetectonicandthermalstructuresdue The presentshapeoftheisogradsresultsfroma morning. famous sectionof“gorges d’Héric”visitedinD2 shear criteriaareambiguous,asseenalongthe However, alongthesubvertical domelonglimbs, is down dip,i.e.top-to-the-NEandSWrespectively. northeastern andsouthwesternterminations,shearing provides contrastedshearcriteria. Around thedome folds (cf.stopD1-14). The kinematicanalysis at lying. (Fig. 21) ± 5Ma(Matteetal., attens but the ll ofthe 21-05-2004, 10:52:41 PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

Figure 21 - D2 route map

the gneissic linear fabric. In the paragneiss, meter folds in augen gneiss and meter to millimeter-scale scale isoclinal folds are refolded by NE-SW trending crenulation. upright folds. Pegmatites dykes are also deformed by upright ptygmatitic folds and veins parallel to the fold axes are Stop D2.2: boudinated. Along the stream, amphibolite boulders Augen gneiss and sheare pegmatite veins at the (containing garnet locally) including retrogressed eastern part of the Caroux dome. Le Vernet . eclogites, boulders of migmatites and migmatized From gorges d’Héric Æ east to Le Poujol Æ north augen gneiss, sometimes containing sillimanite (left) to Combes. nodules are widespread. Outcrop of migmatite will Fine grain augen gneiss (Sx 40SE 30) with a N70E not be visited along this route. trending stretching lineation at the eastern termination In spite of a clear stretching and mineral lineation, of the Caroux antiform. Sigmoidal K-feldspath the sigma-type porphyroclast systems exhibit both indicates a down-dip, top-to-the East sense of shear.

dextral and sinistral asymmetry at the outcrop Pegmatite dykes cross cutting the foliation are also B16 to B33 n° 2 - from Volume scale. The ambiguous sense of shear might be due sheared to the East. Along the road next curves, to superimposed deformation (namely doming many asymmetric pegmatite boudins can be observed overprinted upon low angle shearing) or to strain within weathered gneiss (no stop). partitioning with a signiﬁ cant component of pure shear (Color Plate 1, C) during doming. Stop D2.3: The bulk structure of the Heric section is a gneiss Biotite-Garnet-staurolite micaschists. Crossing of antiform overturned to the north (Figs. 13, 17). the road to Forêt des Ecrivains Combattants. The antiform hinge zone is parallel to upright The gneiss-micaschist series experienced a high-

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure on topofthemountains. quality, EarlyCambriancarbonates canbeobserved Northern Side.Inspiteofgloballypoorexposure North ofGinestet,begins theMontagneNoire et al.,1991). muscovite andbiotiteindicate297 antiform andtheEspinousedome. folded HT/LPmicaschistsbetweentheCaroux Rosis synformwhichconsistsofcrenulatedand After theColdeMadale,roadrunswithin Plate 1,D). shear bandsindicateatop-to-theEastshearing(Color pressure shadows, heliciticgarnetandstaurolite, to-the-E shearcriteriadevelops. Inthinsection,quartz stretching lineationtrendingN70Ealongwhichtop- (160E15) bearsacompositecrenulation,mineraland tight isograds. The eastward dippingfoliation characterized bybiotite,garnet,andalousite,staurolite temperature andlow-pressure metamorphism unconformably cover thePaleozoic (Cambrian- can beobserved too.Stephanian bedsdipping110S10 Cambrian limestoneblockthatisprobablyanolistolith and limestone. Along theroad,adecameter-scale cm tomsizeblocksofEarlyCambriansandstone The LateCarboniferousconglomeratecontains exposed. landscape, theEarlyCambrianlimestonecliffs are the StephanianGraissessaccoalbasin.Innorthern At thepassofCroixdeMounis,roadentersinto Stephanian conglomerate.Falaise d’Orques. Stop D2.5: to theEspinousenormalfault. and bearsaN60Etrendingslickenline corresponding shearing, theNW-SE trendingfoliation dips50NE The migmatiticorthogneissexperiences aductile S. ofGinestet. Sheared augen gneissandmigmatite. Stop D2.4’: Optional there. and parkingdifﬁ Vialais granite,however duetopoorexposure quality the pass,roadcrossesnorthernborderof preserved. Migmatisationisdatedat330Ma. After orthogneiss but MFK andgneissicfabric arestill Partial meltingdevelops attheexpense oftheHeric Migmatitic orthogneiss.Coldel’Ourtigas. Stop D2.4:

culties, theexcursion willnotstop 40 Ar/ ± 3Ma(Maluski 39 Ar dateson basin. Espinouse domefromtheStephanianGraissessac ductile dextral-normal Espinousefault separatesthe Espinouse gneissdomedippingnorth-eastward. The the landscapetoeastshows thefoliationof At thecrossingwithD22EroadtoCastanet-le-Haut, the depositionofStephanianrocks. Graissessac basin(Castanetfault) isreactivated after Thus strictlyspeaking,thesouthernborderfault ofthe Ordovician) sedimentaryrocksoftheNorthernSide. Axial Zonemicaschists.Nostop. forms anextensional allochthonemplaceduponthe de St-Gervais Unit)belongstotheNorthernSide,and Cambrian (grèsdeMarcory). This Unit(calledMonts sandstone andpelitecorrespondingtotheEarly unmetamorphosed andundeformedgreenish East of Verenoux, theroadgoesthrough overturned totheSEcanbeobserved atthisoutcrop. the Axial Zone.Centimeter- tometer-scale folds outermost partofthemetamorphicrockssurrounding greenish sandstone-peliteseriescorrespondtothe Weakly deformedandweaklymetamorphosed Castanet-le-Bas. Weakly metamorphosed schists. West of Stop D2.7: the Espinousefault. Quartzveins arewelldeveloped. Paleozoic rocksoftheNorthernSide)isshearedby The Graissessacbasinsubstratum(i.e.early N. ofNougayrols. Sheared rocks alongtheEspinouse fault. Stop D2.6: Permian (Autunian) conglomerate. Stop D2.9: there. but unfortunatelythecontactcannotbeobserved conglomerates, sandstonesandpelitesdipping0E20, rocks areunconformablycovered byEarlyPermian the Southcanbeobserved. These Late Carboniferous mine (notvisited)synsedimentaryfoldsoverturned to mesures. IntheancientopenpitofGraissessac Decollement surfaces maydevelop alongcoal dipping 120NE30withcoalintercalations. Graissessac coalbasin. They areﬂ The LateCarboniferousrocksbelongtothe mesures. East ofLaMouline. Stephanian massive sandstoneandcoal Stop D2.8: uitl deposits uviatile 21-05-2004, 10:53:29 PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

West of La Tour-sur-Orb. Leaving Millau to the North, the road crosses Continental conglomerates with quartz, sandstone the Jurassic limestone plateau called Causse de pebbles. Sauveterre. Near Séverac-le-Château, Toarcian black shales were mined for oil. The Jurassic sedimentary Stop D2.10: series observed along the road, is deformed by Permian extensional tectonics. decameter to kilometer-scale wave-length upright Mas d’Alary quarry. folds and reverse faults. These structures are related In the ancient open pit formerly mined for uranium, to the Eocene compression due to the Pyrenean continental red beds are cut by north dipping normal orogeny. faults. Alike other early Permian basins in S. France (e. g. St-Affrique, Rodez), the Lodève basin is a half- A. Geological setting graben bounded to its southern margin by normal The Marvejols area (Fig. 22) is a famous place in the faults. geology of the French Massif Central since it is one of the ﬁ rst places where Variscan syn-metamorphic End of the 2nd day. Drive to Millau (ca 60 km), nappe thrusting has been documented on the basis overnight. of geochronology, metamorphism and tectonics (Pin, 1979). The metamorphic inversion with HP rocks of DAY 3 the Upper Gneiss Unit upon the Lower Gneiss Unit The stacking of Upper and Lower Gneiss Units was used as an argument for tectonic superposition and post-nappe crustal melting (Fig. 23). The contact between the two units is a MillauÆ Highway to La Canourgue Exit n° 39 high temperature mylonitic zone (Faure et al., 1979). Moreover, radiometric dates document the tectonic

Figure 22 - D3 route map Volume n° 2 - from B16 to B33 n° 2 - from Volume

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure 342 from theLotSeriesmicaschistyields351 by various magmaticbodies: succession ofmetapeliteandmetagrauwacke intruded locally theLotSeries,consistsofamonotonous evolution ofthearea. The Lower GneissUnit,called (Fig. 24). in thefootwall oftheUpperGneissUnitoverthrust example oftheinverted metamorphismdeveloped metapelites. The Marvejols areaisoftentaken asan andalousite andmuscovite arewidespreadinthe pressure metamorphism.Biotite,garnet,staurolite, coeval withanintermediatetemperature-intermediate and stretchinglineation. This ductiledeformationis foliation andaconspicuousNW-SE trending mineral Lower GneissUnitischaracterizedbyasubhorizontal As seenintheﬁ lineation whichiswidespreadthroughouttheMassif signiﬁ . Anacidicaugenorthogneisswithmyloniticzones 3. 2. Pink K-feldsparorthogneissofalkaline 1. Quartz-diorite orthogneissisdatedat540 4. Other smallgneissmassesarerecognizedinthe 4. Figure 23-GeneralcrosssectionfromMarvejols tothe populations gives alower interceptageof346 Lower GneissUnitcontact. An U/Pbageonzircon crops outimmediatelybelow theUpperGneiss- an EarlyCambrianageisgenerallyaccepted. orthogneiss intheMontagneNoirenorthernside, zircons. BycomparisonwiththeMendic composition isnotwelldatedduetoinherited 352 linear structure.40Ar/39Ardatingonbiotitegives exhibits awellmarked post-solidusplanarand populations (PinandLancelot,1978). This rock Ma byU/Pbisotopicdilutionmethodonzircon metasediments (Pin,1981). as hypovolcanic granitesorvolcaniclastic Lower GneissUnit,someofthemareconsidered data). Ma isobtainedonmonazite(A.Jolyunpublished A preliminarychemicalU/Th/Pbageof370 developed intheLower GneissUnit(cfbelow). particularly withtheNW-SE stretching lineation does notcomplywithmicrostructuraldataand the UpperGneissUnit.However, thisconclusion synkinematic plutoncoeval withthethrustingof radiometric agethisorthogneissisinterpretedasa 8 Ma(Pin,1981).Duetoitstectonicsetting,and Truyère areashowing thetectonicsuperpositionof ± cance oftheNW-SE trendingstretching 4Marespectively (Costa,1989). The tectonic ± 1Ma(Costa,1989). Upper GneissUnitupontheLower GneissUnit 40 Ar/ rst stopsofD3day, theLotSeriesof 39 Ar datesonbiotiteandmuscovite and theshapeofMargeridepluton ± 4Maand ± ± 12 12 ±

21-05-2004, 10:53:34 PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

Figure 24 - Metamorphic map of the Lower Gneiss Unit around the west part of the Margeride pluton (Marvejols, Truyère, Châtaigneraie) showing the inverted metamorphism (adapted from Burg et al., 1984).

Central is not clearly settled yet. metagabbro at 484 ± 7 Ma. An orthogneiss intrusive The mylonitic zone at the base of the Upper in the paragneiss gives 478 ± 6 Ma. These dates Gneiss Unit is characterized by a N-S trending are interpreted as the evidence for an Ordovician mineral and stretching lineation. In spite of intense magmatism related to the rifting of Armorica from recrystallization and post-kinematic annealing, Gondwana. Moreover, the 415 ± 6 Ma age of zircon top-to-the-south shearing can be observed. 40Ar/ populations from a high-pressure trondhjemite is 39Ar dating of synfolial biotite gives 358 ± 4 Ma considered as the age of melting coeval with eclogite interpreted as the age of thrusting. Moreover, a late facies metamorphism. Pressure and temperature muscovite developed upon the early foliation is dated constraints on this rock are 16 ± 4 kb and 800 ± 50°C at 340 ± 4 Ma (Costa, 1989). respectively. In the Marvejols area, the Upper Gneiss Unit consists Upper and Lower Gneiss Units stacking was followed of a lower part called “leptynite-amphibolite” by huge crustal melting produced under distinct P-T sequence and an upper part with migmatitic gneiss conditions (Fig. 25). The second part of the D3 and the and micaschist. The leptynite-amphibolite sequence whole D4 days are devoted to the observation of some B16 to B33 n° 2 - from Volume contains metamorphosed maﬁ c rocks of magmatic or manifestations of the Middle to Late Carboniferous sedimentary origin such as metagabbro, metabasalt, crustal melting. From older to younger, three stages amphibolite rare serpentinites and acidic rocks (i. e. are distinguished. leptynites) Meter to centimeter scale acidic-maﬁ c alternations are probably of volcani-clastic origin. 1. Pre-Velay Cévennes migmatites, dated between Several U/Pb ages on zircon populations are available 333 to 324 Ma by chemical U/Th/Pb method on (Pin and Lancelot, 1982). Namely, an amphibolite monazite (Be Mezème, 2002). boudin is dated at 487 ± 6 Ma, and a coronitic 2. Namurian-Westphalian plutonism dated around

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure 3. Velay migmatitesandcordieritegranitedated NW-SE deformation. This wellfoliatedandlineatedrockexperienced a corresponding tothetectonicevent (Costa,1989). 40 Stop D3.1: B. Stopdescription Velay area(simplifi Figure 25-P-Tpathsofpre-Velay and Velay events constructedfrommetamorphicrocks sampledinthesouthpart ofthe Lancelot, 1978)fortheemplacementofthispluton. populations gives a540 to theLower GneissUnit.U/Pbdatingonzircon originally intrudesmicaschists(stopD3-2)belonging A fi Ajustons, S.ofMarvejols. Cambrian quartz-diorite.N88,EastofPont des Ar/ during D4. around 300Ma. This large massifwill beexamined are coeval. relationships show thatthesetwo magmatictypes massifs derived fromdifferent magmas,fi (Signaux andRoclesplutons). Although distinct Montvert-Borne plutons)andalsobyleucogranite porphyritic monzogranite(Margeride orPont-de- 320-315 Ma. This magmatismischaracterizedby 39 ne-grained biotite-hornblendequartzdiorite Ar datesonbiotiteprovide a352

ed fromMonteletal.,1992). ± 12Maage(Pinand ± 1Maage eld Ma (Joly, unpublisheddata). An U/Th/Pbchemicalageonmonazitegives 370 gives alower intercept age of346 SE senseofshear. U/Pbdateonzirconpopulations asymmetric K-feldsparaugenindicateatop-to-the- the foliationandparalleltoNW-SE lineation, with myloniticfabric. Insectionsperpendicularto The Lower GneissUnitincludesaugenorthogneiss Pessil. Augen orthogneisswithintheLotseries.Pont de Stop D3.3: 39 shear (ColorPlate1,E).Biotiteandmuscovite stretching lineation,indicatingatoptoNWsenseof foliation bearsaconspicuousNW-SE mineraland porphyric graniteandmetadiorite. The ﬂ ± The Lotseriesarecomposedofbiotite-garnet Lower GneissUnitoftheLotseries. Stop D3.2: unconformably overly themetamorphicrocks. 1989). Inthebackground,EarlyJurassiclimestones The UpperGneissUnitoutcropsNorthof Gneiss Unit.LeCroisier. Coronitic metagabbro belonging totheUpper Stop D3.4: staurolitemicaschistsoriginallyintrudedby Ar agesarerespectively 351and342

± 8Ma(Pin,1981). ± 4Ma(Costa, a lying at 21-05-2004, 10:53:38 40 ± Ar/ 12 PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

Marvejols city. Metabasites, locally with mylonitic amphibolite complex which becomes migmatized fabric, underwent the Eovariscan high-pressure northward. The migmatite is not dated here. By metamorphism (Color Plate 1, F). However, due to comparison with other places in the Massif Central, a the heterogeneity of deformation, magmatic textures ca 380 Ma age can be inferred. are still preserved. This rock is dated by U/Pb method on zircon populations (upper intercept) at 484 ± 7 Ma Stop D3.7: (Pin and Lancelot, 1982). Middle Carboniferous Margeride pluton. Truc de Fortunio. Stop D3.5: Drive to St-Amans Æ Estables Leptynite-amphibolite complex The Margeride massif is one of the largest granitic (Upper Gneiss Unit). Along the main road (N9). plutons in the French Massif Central (3200 km2). It This outcrop exposes a typical section of the consists mainly of a porphyritic monzogranite with “leptynite-amphibolite complex” made of alternations large (up to 10cm) K-feldspar megacrysts. On the of maﬁ c and acidic rocks considered as a volcani- basis of biotite content, three facies, namely dark, clastic formation. The foliation exhibits meter to intermediate and light facies, are distinguished

Figure 26 - Example of ponctual dating on monazite decameter-size amphibolite boudins (Color Plate 2, single grain by U/Th/Pb chemical method A). Zircon populations from an amphibolite give a U/ (Be Mezème et al., 2003). Pb age of 487 ± 6 Ma age (upper intercept) interpreted

as the rock formation age (Pin and Lancelot, 1982). B16 to B33 n° 2 - from Volume The lower intercept at 340 ± 4 Ma is close to the (Couturier, 1977). Nevertheless, more than 80% thermal event (ca. 345-330 Ma) that overprints the of the massif is made of the intermediate facies. tangential tectonics. Moreover, muscovite-K-feldspar leucogranite intrude the monzogranite. Most of the leucogranites Stop D3.6: are meter-scale dykes, but east of the massif the Early Variscan migmatisation. Grandrieu leucogranite represents a kilometer-sized Gorges du Val d’Enfer. pluton. The monzogranite yields a Rb-Sr whole rock The road runs from South to North in the leptynite- age of 323 ± 12 Ma (Couturier, 1977) and an isotopic

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Figure 27 - A: Map of the planar and linear structures in the Margeride pluton inferred from AMS study (from J-Y. Talbot, unpublished). B: Stereograms represent the magnetic lineation (K1) and the pole of foliation (K3). PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

dilution U/Pb monazite age of 314 ± 3Ma (Pin, 1979); the Grandrieu leucrogranite is dated at 305 ± 4 Ma (U/Pb method, Lafont and Respaut, 1988). The Margeride pluton is a subhorizontal of 4 to 8 km thick (Fig. 23). Petrostructural and AMS studies (J-Y. Talbot, work in progress, Fig. 27) show a complex pattern of the foliation trajectories. Although the foliation trajectories do not show a consistent shape, at the scale of the whole pluton, the foliation presents a ﬂ at lying attitude. The lineation pattern exhibits a well-deﬁ ned NW-SE trend. It is worth noting that such a trend is widespread throughout the whole Massif Central and corresponds to the maximum stretching direction of the Namurian-Westphalian syn- orogenic extension (Faure, 1985). Stop D3.8: Augen orthogneiss below the Para-autochthonous Unit. Drive to Châteauneuf-de-Randon Æ Montbel Æ NE of Belvezet SE of the Margeride pluton, the Cévennes area exposes the para- autochthonous Unit of the Massif Central. Below the unconformity of the Mesozoic sedimentary rocks, an augen orthogneiss and its host rocks crop out in a tectonic window below the Para-autochthonous Unit (Cévennes micaschists). The augen orthogneiss exhibits a subhorizontal foliation and NE-SW trending stretching lineation. The age of the granitic magmatism is not determined here but assumed to be Early Cambrian (ca 550 Ma) by comparison with other orthogneiss B16 to B33 n° 2 - from Volume in the Massif Central. The chemical U/Th/Pb age of 560 ± 18 Ma in the core of monazites grains from migmatitic orthogneiss supports this interpretation (cf. Stop D3-9)

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure Vanderhaeghe etal.,1999). The Velay migmatite- (Brown andDallmeyer, 1996;Gardienetal.,1997; the roleofpartialmeltingduringorogenicevolution migmatitic complexes isofgreatinteresttostudy exposes numerousgranitic intrusionsandlarge and Teyssier, 2001). Thus, the Variscan belt which its behaviour duringorogeniccollapse(Vanderhaeghe rheology ofthethickened crustandlargely control collision tectonicsimpliesamajorchangeinthe development ofapartiallymoltenmiddlecrustduring (Clemens, 1990;Brown, 1994).Moreover, the inaccessible locationoftheirproductionsites still discussedinparticularbecauseofthedeep, by crustalmeltingduringorogeny isaprocess The generationoflarge granite-migmatite complexes during orogenic evolution melt generationandgraniteemplacement The Velay dome(French MassifCentral): chemical U/Th/Pbmethodonmonaziteat316 the metamorphicrocks.Muscovite dyke isdatedby Pink granitedykes andleucogranitedykes intrude Boudinaged dykes ofleucogranites. interpreted astheageofmigmatization. Mezème, 2002,Cocherieetal.,inpress). The latteris ± U/Th/Pb datesonmonazitegive 560 orthogneiss fabric remainswellpreserved. A chemical melting giving risetodiatexites. Locallythe To theeast,orthogneissexperiences apartial Migmatitic orthogneiss.BarragedePuylaurent. Stop D3.10: A few kilometerseastward, biotite,garnet( Stop D3.9: End ofthe3 again alongthe Allier river. to Langogne,themigmatiticorthogneisscropsout (Schistes desCévennes). FromLaBastidePuylaurent, the roadislocatedinPara-autochthonous Unit of Permianage.SouthLaBastide-Puylaurent, The roadfollows thebrittleleft-lateral Villefort fault In Prévenchères, turntothenorth(left)Langogne. (Be Mezèmeetal.,2003;ﬁ correspond totheorthogneisshostrock. cordierite) paragneissandquartz-micaschist 6Maforthegrainscoreandrimrespectively (Be rd day. Overnight inLangogne

DAY 4 DAY 4 g. 26). ± 18Maand324 ± 5Ma ±

• • • Variscan belt,byshowing: dome inconnectionwiththetectonicevolution ofthe generation andgraniteemplacementofthe Velay The aimofthefourthdayistoillustratemelt on theevolution ofthe Variscan orogeniccrust. generation ofalarge volume ofpartiallymoltenrocks anatexis andtheconsequencesofpresence thermal conditionsrequiredforwidespreadcrustal 28, 29,30)offers auniqueopportunitytoexamine the granite domelocatedintheSEMassifCentral(Fig. • • • • models work inthisarea provided thefollowing resultsand (Didier, 1973;DuprazandDidier, 1988).Previous (25%) andgranites(5%)ofvarious natureandsize and prismaticcordieritebyenclaves ofgneisses (about 70%)characterizedbyabundance ofnodular diameter, iscomposedofperaluminousgranites The Velay dome(Fig.28,29),about100kmin A. Geologicalsetting the Velaydome. Etienne Stephanianbasinonthenorthernfl extensional tectonicsandformationoftheSt the relationbetweengraniteemplacement, the cordierite-bearinggranitesandmigmatites; southern margin; pre-tectonic graniteandmetasedimentsonthe incipient stagesofmeltingintheLateNeoproterozoic (Malavielle etal.,1990) developed duringthecollapseof Variscan belt zone ofthePilat,onemajornormalfaults expansion ofthegranitesbelow thedetachment pattern ofthe Velay domerecordssouthward lateral Lagarde etal.(1994)suggestedthatthedeformation during late-orogenicextension. instabilities withinapartiallymoltenmiddlecrust migmatites andgranitesrefl the amplifi Burg and Vanderhaeghe (1993)proposed that dehydration conditions. with biotitestablefollowed bymeltingunderbiotite of anatexis, fi Montel etal.(1992)describetwo successive stages underplating ofmafi evolution overprinted byathermalpeakduetothe Velay domehasfollowed aclockwiseP-T-time and Barbey etal. (1999), indicatethatthe Williamson etal.(1992),Montel(1992) Geochemical andpetrologicaldatapublishedby cation ofthe Velay domecoredby rst underwater-saturated conditions c magmas(Fig.31). et gravitational ects 11-06-2004, 8:43:52 ank of ank PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22 Volume n° 2 - from B16 to B33 n° 2 - from Volume

Figure 28 - Simpliﬁ ed geologic map of the eastern margin of the Massif Central, Day 4 and Day 5 localities

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure • • • phenomena. dome resultsfromtheconjunctionofseveral that theformationof Velay migmatite-granite petrologic andgeochronologicaldataindicate According toLedruetal.,2001,structural, indicated asspecificgeochronological markers. Day 4and5localities. Ages ofsomegranitesare main tectonicunitsandtheirstructuralrelationships, eastern marginoftheMassifCentralshowing the Figure 29-Simplifiedcrosssectionthroughthe • al., 1994a,b): the collisionhistoryof Variscan belt(Ledruet dome ishostedbygneissicunitsstacked during 1. The hostrocks. The Velay granite-migmatite 28, 30): illustrated duringthisfourthday, aredeﬁ Four mainstructuralzones,thatwillbepartially The formationofthe Velay dome,coeval with presently inanuppergeometricposition,thisunit of EarlyPaleozoic oceanicormarginal basins is Partial meltingofthethickened cruststartedat Partial meltingtookplacewithinadominantly the UpperGneissUnit,thatcontainsremnants the migmatite-granitedome. decompression associatedwithexhumation of prograde metamorphicpathandendedduring and indicatethatmeltingstartedattheendof solidus todestabilizationofhydrousminerals evolved fromthewater-saturated granitic fertile peliticcompositions.Meltingreactions metasedimentary crustallayerdominatedby hence lastedabout40My. generation oftherocksforming Velay dome Ma. Crustalanatexis responsibleforthe during collapseoftheorogeniccrustat~300 of the Variscan beltwas stillactive, andended about 340Ma,whilethrustinginthehinterland collapse. molten crustallayerinresponsetogravitational potentially correspondstoﬂ the activation ofcrustal-scaledetachements, the migmatitesandgranitesof Velay dome. large volume ofthethickened crustandgenerate provided theextra heatsourcerequired tomelta upwelling coeval withorogeniccollapsehave mantle-derived magmasandalsoasthenospheric However, itisproposedthatheatadvection from the evolution ofthe Variscan orogeniccrust. likely causedariseintemperatureduring heat productionfollowing crustalthickening Thermal relaxationandincreasedradioactive ow ofapartially ned (Fig. ned 21-05-2004, 10:53:53 PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22 Volume n° 2 - from B16 to B33 n° 2 - from Volume

Figure 30 - Extension of Meso and Neovariscan metamorphism and foliation trends within the host rocks of the Velay migmatite-granite dome

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure the southernhostrocks ofthe Velay granite(oblique-ruledboxes, afterMonteletal.,1992) ruled boxes, afterGardienetal.,1997,andMontel1992),(3)inthemigmatiticpartof migmatitic orthogneissandgraniteinthesouthernpartofdome(oblique- andhorizontal- stage isindicatedbyarrows (1)inthePilatmicaschist (afterGardienetal.,1990),(2)inthe biotite; As =aluminiumsilicate;Lliquid. ThetransitionfromtheM3toM4metamorphic Grt =garnet;Msmuscovite; Qtz=quartz;Kfspotassiumfeldspar; Pl=plagioclase;Bt Mineral abbreviations: Ky =kyanite;Silsillimanite; And =andalusite;Cdcordierite; Figure 31-Pressure-temperatureevolution fromthegneissesto Velay granite • Devonian, priorto350Maintheinternalzone the UpperGneissUnitwhichoccurredduring metamorphism attributed tothethrustingof margin underwentageneralmedium-pressure sedimentary parautochthonoussequence. This R’Kha Chahametal.,1990),and(b)amainly granite datedat528±9 Ma (Rb-Srwholerock, Fix”) originatingfromperaluminousporphyric and argilites, andaugenorthogneiss(the“Arc de composed ofmetasedimentsderived frompelites is representedby(a)aLower GneissUnit The northGondwana continentalmargin al., 2001). while subductionwas stillactive (Lardeauxet 420-400 Matolessthan30kmat360-380 show theserockswereexhumed from90kmat (450-400 Ma).Structuralandradiometricdata Eovariscan stageoflithosphericsubduction are preserved withinbasiclayersmarkingan tuffs andgrauwackes. Numerouseclogiticrelics granites, microgranites,acidandbasicvolcanics, at itsbaseoverlain bygneissesderived from contains dismemberedbasic-ultrabasiccomplexes • identifi the gneissichosts,following meltingreactionsare the roofof Velay dome.Inthemigmatites andin 2. The gneiss-migmatitezone,attheperipheryand quartz-feldspathic rocks,withbiotiteremaining conditions exceeding thoseforwater-saturated The fi ( to Ar diffusion hasbeendatedat335-340Ma Burg, 1993; Arnaud, 1997). The closureofmicas thickening estimatedatabout15km(Arnaudand synchronous withsouthward thrustinganda muscovite-chlorite-garnet paragenesesbeing are thereestimatedat500 °C, 5 kbar, withthe conditions duringthemetamorphicevolution the parautochthonousdomain.MaximumP-T the Cévennes micaschistsareinterpretedas so-called “vaugnerite”. Inthesouth(Fig.2), Ma, includingmagnesio-potassicplutons,the of the Velay domeemplaced between335-315 is intrudedbysyntectonicgranitesprecursor (Mesovariscan period). The Lower Gneiss Unit 40 ed (Fig.31): Ar/ rst meltingstagedeveloped underP-T 39 Ar, Caronetal.,1991). peraluminous contain 1997). They Malek, zircon, Aït U-Pb respectively on (207Pb-206Pb and ± 3and314±Ma are alsodatedat313 mantellic affi monzodiorite, with High-K magnesian (Mougeot etal.,1997). age of314±5Ma indicates aminimum U-Pb monazitedate Malek etal.,1995). A this meltingevent (Aït prograde characterof of muscovite andthe the initialpresence enclaves confi corundum paragneiss 1992). The presence of stage ofMonteletal., the graniticcore(M3 envelope, 5 kbar in the metamorphic within 4kbar 700 °C, stable: around 21-05-2004, 10:55:49 nity rms PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

xenoliths that record a fi rst stage of isothermal (Vitel, 1985). Cordierite may be prismatic, decompression at 700-800 °C, 8-10 kb, consistent cockade-type or mimetic overprinting previous with a source located more than 30 km deep, biotite - sillimanite assemblages. Most of the followed by a stage at 5-6 kb (Montel, 1985). In heterogeneous granites indicate mixing between view of the water-saturated conditions, it is unlikely melts of lower-crustal origin and melts from the that large quantities of granite (i.e. < 10-20%) were para- and ortho-derived host rock (Williamson et produced and extracted at this stage (Patiño Douce al., 1992). and Johnston, 1990). - a homogeneous leucocratic biotite-cordierite • The second stage of melting is characterized by granite with mainly cockade-type cordierite. Its high-temperature metamorphism in the cordierite emplacement has been dated at 301 ± 5 Ma using stability fi eld, with biotite destabilized: 760-850 °C, the U-Pb method on monazite (Mougeot et al., 4.4-6.0 kbar (stage M4 of Montel et al., 1992). 1997). Leucosomes were dated at 298 ± 8 Ma based on - a homogeneous granite with biotite and prismatic Rb-Sr whole rock isochron (Caen-Vachette et al., cordierite as a primary ferromagnesian phase, 1982), and Rb-Sr whole rock-biotite isochrons with few enclaves. The heterogeneous and yield ages between 305 and 276 Ma (Williamson homogeneous granites with prismatic cordierite, et al., 1992). An age of 301 ± 5 Ma was obtained with a high Sr content, have a mixed isotopic for the homogeneous parts of the granite using the signature between the host rocks and a lower- U-Pb monazite method (Mougeot et al., 1997). crustal origin. The deep source is considered to Therefore, this second melting stage is considered be the melting of the lower mafi c/felsic plutonic to be generally synchronous with emplacement of crust (Williamson et al., 1992). the main cordierite-bearing granites. The volume of - a leucocratic granite with cockade-type cordierite, cordierite-bearing granites generated makes a case without enclaves. The cordierite-quartz aggregates for massive partial melting at this stage, associated posdate primary biotite bearing assemblages and to destabilization of hydrous minerals. probably prismatic cordierite. • The late magmatic activity that includes: 3. The migmatite-granite domain. The various - homogeneous granite with K-feldspar granites that appear in the Velay dome defi ne a suite, porphyrocrysts and common prismatic cordierite, with 3 main granite types distinguished according basic and micaceous inclusions (the Quatre Vios to age, structure, homogeneity, mineralogy and massif) (Fig. 10d). These granites are defi ned as geochemistry: late-migmatitic and are considered to originate • A heterogeneous banded biotite granite, found from the melting of aluminous sediments at mainly on the western margin of the dome and 4.5-5.5 kbar and 750-850 C, under water- locally on the southern and eastern margin. It undersaturated conditions and have a signifi cant corresponds to the fi rst generated granite of the mafi c component (Montel et al., 1986; Montel Velay suite. Foliation trajectories are in continuity and Abdelghaffar, 1993). Ages at 274 ± 7 Ma with porphyric granites in the external rim of the (Rb/Sr whole rock, Caen Vachette et al., 1984) dome suggesting continuity between these precursor are considered to be partially reset during granites and the development of the heterogeneous Permian or Mesozoic hydrothermal event. banded biotite granite. - Stephanian leucogranites, microgranite and • A main biotite-cordierite granite, in which several aplite-pegmatite dykes, Permian rhyolites. sub-types may be distinguished, in particular Microgranite dykes have been dated at 306 ± according to the cordierite habitus (Barbey et al., 12 and 291 ± 9 Ma and a Permian hydrothermal 1999). event at 252 ± 11 and 257 ± 8 Ma (microprobe B16 to B33 n° 2 - from Volume - a heterogeneous banded granite with abundant dating of monazite, Montel et al., 2002). enclaves. Most of these enclaves represent incorporated and partly assimilated pieces of 4. The Stephanian intracontinental basin of St- the Lower Gneiss unit and precursor plutons Etienne. This basin is formed along the hanging wall originating from the host rocks, although some of the Mont Pilat extensional shear zone (Malavieille enclaves with refractory composition or granulite et al, 1990). The Mont Pilat unit, attributed to the lower facies metamorphism have a lower crustal origin gneissic unit at the scale of the French Massif Central

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure (3- 5Kbarand700-780°C,Gardienetal.,1997). pressure -hightemperaturemetamorphicconditions was coeval withtheprogressive development oflow Ma (39Ar/40Ar, Malavieille etal.,1990). This event with atop-northextension datedbetween322and290 criteria, observed atdifferent scales,arecompatible whole rock,Caen-Vachette etal.,1984).Shear plane andhave beendatedat322±9Ma(Rb/Sr outcrop moreorlessparalleltothismainfoliation stretching lineation.Numerousleucograniticpods north-dipping foliationplanebearinganorth-south orthogneisses andamphibolites. This unithasagently consists ofaluminousmicaschists,metapelites, the inheritedstructure.Structuralorientationsthen second meltingstagetendtobediscordantwith with cockade-typecordieriteproducedduringthis Critical MeltPercentage(Arzi,1978).Leucosomes of meltbeyond 30-50%, thevalue oftheRheological breakdown ofbiotiteisresponsible fortheproduction although K-feldsparaggregates arepreserved. The that areinequilibriumwiththegraniteeutecticpoint, plagioclase aggregates arereplacedbyassemblages accompanied bythegrowth ofsillimanite.Quartz- existing foliation. The breakdown ofmuscovite isthen ﬁ detail inthiszonebyDallainetal.(1999). Anatexis annealing tomeltingprocesshasbeenstudiedin textural evolution inthetransitionfromsubsolidus The progressive development oftheanatexis and orthogneiss. “vaugnerite”) areintrusive andboudinatedwithinthe melanosome. Magnesio-potassicdykes (theso-called the quartzofeldspathicaggregates andbiotite-rich marked bytheelongationandcrystallizationof type ofthegraniteprotolithwhilefoliationis phenocrists ofK-feldsparattesttheporphyric bearing graniticpatches(ColorPlate2,B).Large inherited foliationandlocallydiscordantcordierite- the segregation ofcordierite-freemeltsalongthemain granite-migmatite dome. The meltingismarked by constitutes analmostcontinuousrimaroundthe Velay Ma (Rb-Srwholerock,R’KhaChahametal.,1990), 9 from peraluminousporphyricgranitedatedat528± An augenorthogneiss(the“Arc deFix”),originating within theorthogneissofLower GneissicUnit. Stop D4.1shows theincipientstageofmelting N102, Roadcut. Meyras, Roadfrom LePuy-en-Velay to Aubenas, Stop D4.1: B. Stopdescription rst develops withtheresorptionofquartzalong (Fig. 28,29) randomly oriented. increases, withfoldsbecomingabundant and become morevaried astheleucosomeproportion attributed toS3(Barraudetal.,2003). saddle reefandaxialplanesofthefoldingthatis melts: cordierite-freeleucosomesaccumulatein that playanactive roleinthesegregation ofanatectic Plate 2,C). The outcropischaracterizedbyopenfolds (named regionally S2)andpolyphasedfolding(Color tectonic evolution thatresultedinacompositefoliation microstructure developed duringthepre-migmatitic Numerous resistersfromrefractorylayerspreserve 2003). (Macaudière etal.,1992)andfolding(Barraud of earlymeltingiscontrolledbyfoliationanisotropy refractory quatz-richandcalciclayers) The location intrusive insediments(pelitesandargillites, including metagranite observed atthestop4.1.isoriginally within paragneissoftheLower GneissicUnit. The Stop D4.1shows theincipientstageofmelting Aubenas, N102,River banksofthe Ardèche river. Pont deBayzan, Roadfrom LePuy-en-Velay to Stop D4.2: the earlyphaseofmeltingtoendmagmatic presence ofameltphase.Cordieriteisformedfrom cordierite isformedattheexpense ofbiotite,inthe (Color Plate2,D).Detailedobservation indicates that petrographic typeofthe Velay migmatite-granitedome cockade-type cordierite,thatrepresentsoneofthe Stop D4.4shows anhololeucocratic granite,with D578, Road-cut. Volane river, Roadfrom Aubenas toMezilhac, Stop D4.4: granite andmigmatitesyieldaU-Pbageat289 the NorthandPradesbasininSouth. Apatites in found intheconglomeratesofStephanianbasin Stephanian asbouldersofgranitesandgneissesare Velay granite-migmatitedomeoccurredduringthe migmatitic paragneiss. The exhumation ofthe sandstone over alteredgraniteand biotite-sillimanite Stop D4.3shows theunconformityofMesozoic Ucel, Roadfrom Aubenas toMezilhac,Road-cut. Stop D4.3: this unconformity. fresh roadcutprovides aspectacularillustrationof the Trias sandstone andconglomerate. A recentand regional unconformityischaracterizedatthebaseof of the Vealy region (Mougeotetal.,1997).Finally, a that isinterpretedasacoolingageduringtheuplift 21-05-2004, 10:55:56 ± 5Ma PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

evolution of the Velay granite. However some garnets Stop D4.7: are present in cordierite nodules, indicating that, in Moulin de Sezigneux, Road from St Chamond to that area, melting started in the garnet stability fi eld. Le Bessat, D2, Road cut. The cordierite-andalusite-bearing micaschist of the Stop D4.5: Pilat Unit, seen at stop D4.7, displays extensional Volane river, Road from Aubenas to Mezilhac, tectonics and HT-LP metamorphism within the Pilat D578, Road-cut. series (Lower Gneissic Unit). This outcrop offers a Stop D4.5 shows a late-migmatitic homogeneous good example of a low-pressure metamorphic unit, granite and relations with migmatitic gneisses and with micaschists and paragneisses intruded by syn- heterogeneous cordierite-bearing granite. In the upper tectonic pegmatite dykes and pods. The main foliation part of the outcrop, a late-migmatitic granite, the plane is gently dipping to the North and shear criteria Quatre-Vios granite, is intruding the heterogeneous are well marked mainly around the pegmatitic pods banded granite: it is a coarse grained peraluminous indicating top to the North extensional tectonics. In granite with prismatic cordierite and frequently the metamorphic rocks, the common mineralogy oriented feldspar phenocrysts. It contains abundant is: quartz + feldspars + biotite + andalusite and /or mafi c microgranular enclaves and surmicaceous cordierite ± muscovite. enclaves including biotite, garnet, cordierite, sillimanite, hercynite, ilmenite and rare plagioclase. D4.8: Moulin de Sezigneux, Road from St Chamond This unusual mineralogy (absence of quartz and to Le Bessat, D2, River banks. potash feldspar) and the corresponding chemical Structure and metamorphism of the orthogneiss composition indicate that these enclaves are restites. of the Lower Gneissic Unit are illustrated at stop P-T conditions calculated from this mineralogy D4.8. Ultramylonite and pseudotachylite textures yielded water-undersaturated conditions estimated at are developed within the orthogneiss and S-C-C’ 4.5-5.5 kbar and 750-850 C. The mafi c microgranular structures indicate top to the North shear criteria (biotite+plagioclase) enclaves correspond to frozen compatible with extensional tectonics. Shear bands blobs of mafi c magma. Locally, another type of late- are underlined by syntectonic recrystallized biotites migmatitic fi ne-grained granite, with typical acicular dated around 320-300 Ma (40Ar/39Ar). In the highly biotite and devoid of enclaves, crosscuts the Quatre- sheared parts of the outcrop ultramylonite bands and Vios granite. pseudotachylites are observable. The main foliation In the lower part of the outcrop, another dyke of plane bears a north-south oriented stretching lineation. late-migmatitic granite is intrusive within migmatitic The metamorphic mineralogy is characterized by the orthogneiss in which the second stage of melting is association of quartz + feldspars + biotite + muscovite. well marked by the biotite breakdown that produces Rare small-sized cordierites can also be observed. Fe-rich garnet and cordierite. This zone is itself enclosed within the heterogeneous banded granite that End of the 4th day. Overnight stay in St Etienne contains a lot of enclaves of migmatitic paragneiss. DAY 5 Stop D4.6: High to ultra-high pressure metamorphism and Mont Gerbier-de-Jonc, Road from Mezilhac to Le arc magmatism: records of subduction processes Puy-en-Velay, D378, Sight as seen from the road. in the French Massif Central The Velay volcanism is made up of an eastern chain of Mio-Pliocene basaltic to phonolitic volcanoes and The main goal of Day 5 is to present the geological a western Plio-Quaternary basaltic plateau (Mergoil and petrological records of subduction processes Volume n° 2 - from B16 to B33 n° 2 - from Volume et al., 1993). The road to Le Puy-en-Velay shows in the eastern Massif Central that preceeded the nice sightseeing of this phonolitic chain that extends development of the Velay migmatite-granite dome over more than 55 km with more than 180 points of illustrated during Day 4. These records are: remnants extrusion, emplaced between 14 and 6 Ma. of high to ultra-high pressure metamorphism in both The Mont Gerbier-de-Jonc is known as the spring crustal and mantle-derived lithologies in the Mont du of the Loire river. It is a phonolitic protrusion that Lyonnais unit, a partially preserved back-arc derived displays a rough prismatic jointing. ophiolitic sequence, the Brévenne unit.

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Figure 32 - Geological maps of the eastern margin of the Massif Central through time showing the progressive edification of the belt. The geodynamic cartoons show the possible position of the Monts du Lyonnais eclogites during the four critical periods illustrated (a to d). PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

A. Geological setting lavas that, fi nally, are overlain by siltstones with 1. Eclogites and garnet peridotite from The Monts pyroclastic intercalations (Milési et Lescuyer, 1989; du Lyonnais unit (Fig. 32a) Feybesse et al., 1996). A prograde greenschist to The Monts du Lyonnais unit belongs to the upper lower amphibolite facies metamorphism is recorded gneissic unit (Lardeaux, 1989; Ledru et al., 1994a). (Peterlongo, 1960; Fonteilles, 1968; Piboule et al., It comprises metasediments, orthogneisses (with 1982; Feybesse et al., 1988). The Brévenne ophiolite protoliths of Ordovician age), leptynites (i.e. meta underwent a polyphase deformation. An early event, rhyolites), amphibolites and minor marbles. This unit well developed in the northern part of the unit, is also contains lenticular relics of either crustal (mafi c characterized by a NW-SE stretching lineation with and acid granulites, eclogites, Lasnier, 1968; Coffrant top to the NW shearing (Leloix et al., 1999). During and Piboule,1971; Dufour,1985; Dufour et al.,1985; the second event, the ophiolitic unit is overthrusting Lardeaux et al.,1989) or mantle origin (garnet and / the Monts du Lyonnais along a dextral transpressional or spinel bearing peridotites, Gardien et al., 1988, zone in which syntectonic granites emplaced between 1990). Eclogites outcrop, in close association with 340 and 350 Ma (Fig. 32c, Gay et al, 1981; Feybesse garnet-bearing peridotites, in the southernmost part et al., 1988; Costa et al., 1993). Subsequently, of the Monts du Lyonnais unit. Eclogites and related monzonitic granites of Namurian- Westphalian age garnet amphibolites also occur in a similar structural and a contact metamorphism aureole postdate this situation farther north (in the Morvan unit, Godard, tectonics (Delfour, 1989). 1990) and also southeast of the Monts du Lyonnais unit (in the Maclas-Tournon area, Gardien and 3. The development of the Velay migmatite granite Lardeaux, 1991). In the Monts du Lyonnais unit, three dome and the collapse of the orogen (Fig. 32b) ductile strain patterns were distinguished (Lardeaux The tectonic evolution of the eastern Massif Central and Dufour, 1987; Feybesse et al., 1996) and related is achieved during Westphalian and Stephanian. to high pressure and medium pressure metamorphic The formation of the Velay dome, coeval with the conditions: activation of crustal-scale detachements, potentially • The relictual high-pressure structures corresponds to fl ow of a partially molten crustal layer • A main deformation imprint, contemporaneous with in response to gravitational collapse. amphibolite facies conditions, corresponds to a NW -SE crustal shortening with a fi nite NNE-SSW B. Stop description (Fig. 28, 29) stretching direction (Fig. 32b) • A deformation event developed under a Stop D5.1: transpressional regime dated between 335 and 350 The Bois des Feuilles, Road from St Symphorien- Ma (Rb/Sr whole rock, Gay et al., 1981; 40Ar/39Ar, sur-Coise to Rive de Gier, D2. Costa et al., 1993) which is correlated to the main This outcrop consists of garnet bearing peridotites deformation within the Brévenne ophiolite in and eclogites occuring as boudins within garnet relation with its overthrusting. (Fig. 32c) sillimanite paragneisses. The coesite-bearing With respect to this transpressive strain pattern, in eclogite occurs in the southern part of the Monts the southern part of the Monts du Lyonnais unit, du Lyonnais unit, near St Joseph in the Bozançon the eclogites outcrop exclusively in the strongly valley (1/50.000 geological map “ St Symphorien folded domains where they behave as rigid bodies Sur Coise “, Feybesse et al., 1996) in association with in a deformed ductile matrix. We never found any “common” eclogites and serpentinites. In the whole eclogitic body within the shear zones. area, eclogites are preserved in low-strain lenses 2. The uppermost part of the magmatic arc: the (meter scale boudins) wrapped by amphibolites or Volume n° 2 - from B16 to B33 n° 2 - from Volume Brévenne ophiolite (Fig. 32b) amphibolite facies paragneisses. For practical reasons The Devonian Brévenne ophiolitic unit consists of an (diffi cult access), we shall observe only garnet- association of metabasalts and metarhyolites together peridotites and “common eclogites”. with intrusive intruded by trondhjemitic bodies Well-preserved peridotites occur as metric to (Peterlongo, 1970; Piboule et al., 1982,1983). The decametric scale bodies within the paragneisses. In ophiolitic unit was initially emplaced in a submarine the less retrogressed samples, garnets in equilibrium environment (Pin et al., 1982; Delfour et al., 1989). with olivine, clinopyroxenes and orthopyroxenes can These intercalations are cut and overlain by intrusive be observed. Frequently, garnets contain inclusions gabbros and dolerites and by submarine basaltic of spinels and pyroxenes, while in some samples

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure Piboule, 1971;Coffrant, 1974; Blanc, 1981;Piboule As pointedoutbyvarious authors (Coffrant and • • • can bedistinguished: Petrographically, threetypesof eclogitefacies rocks bearing amphiboliteswithrelicsofeclogiticminerals. of thecases,mafi conditions, andin80% amphibolite facies under granuliteand strongly retrogressed Lyonnais unitare the Montsdu the eclogitesfrom As ageneralrule, temperature decrease. by bothpressureand decrease followed isothermal pressure fi evolution characterized document aretrograde transformations These mineralogical chlorite andserpentine. talc, amphibole, are transformedinto olivine andpyroxenes the porphyroclastsof orthopyroxene while replaced byspineland garnets arepartly In many samples, (Gardien etal.,1990). temperature increase to amoderate increase associated a strongpressure T pathinvolving metamorphic P- during aprograde lherzolite facies from spineltogarnet indicate anevolution These microstructures by coronasofgarnet. spinels arereplaced rst byastrong facies conditions. textures) onlypartly re-equilibratedundereclogite coarse-grained meta-gabbros(withcoronitic eclogites fi fi ne-grained light-colored,oftenkyanite-bearing ne-grained dark-colored kyanite-free eclogites c boudinsarecomposedofgarnet and titaniumrich(Fe0+Fe and Briand,1985),dark-colouredeclogitesareiron contents (Al while light-colouredeclogiteshave higheraluminium (TiO magnesium values but lower titaniumcontents that theseeclogitescanberegarded asthevariably (Blanc, 1981;PibouleandBriand,1985)have shown > 2%), Al-poor metabasalts(Al Figure 33-P-TpathoftheMontsduLyonnais coesite- 2 <1,3%).Detailedgeochemicalinvestigations bearing eclogite(seedetailedlegendofthereactions 2 O 3 near17-20%),andhigheraverage 2 O in Lardeauxetal.,2001). 3 near13%and TiO 2 O 3 near13-15%), 21-05-2004, 10:56:28 2

PALEOZOIC OROGENIES IN THE FRENCH MASSIF CENTRAL A CROSS SECTION FROM BÈZIERS TO LYON B22

fractionated members of a volcanic tholeiitic suite. • Clinopyroxene + orthopyroxene + plagioclase ± amphibole + ilmenite, In the less retrogressed samples, the following mineral • Garnet + clinopyroxene + othopyroxene + assemblages, representing the relicts of eclogite facies plagioclase ± amphibole + ilmenite. metamorphism, are recognized in the dark (1-2) and In acid granulites (Dufour, 1982; Lardeaux et al., light (3-4) eclogites from the Monts du Lyonnais (Fig. 1989), the common mineralogy consists of quartz 33): + plagioclase + K-feldspar + garnet + sillimanite ± • Garnet - omphacite - quartz - zoisite - rutile - apatite spinel ± biotite. Kornerupine - bearing granulites have - sulfi des, been also locally recognized in this outcrop (Lardeaux • Garnet - omphacite - quartz - zoisite - colourless et al., 1989). amphibole - rutile - sulfi des, In this northern part of the Mont du Lyonnais unit, • Garnet - omphacite - quartz (or coesite) - zoisite the metamorphic imprint is typical for Intermediate - kyanite - colorless amphibole - rutile, Pressure granulite facies and there is no trace of • Garnet - omphacite - quartz - zoisite - kyanite - eclogitic high-pressure facies metamorphism. phengite - rutile. Coesite and quartz pseudomorphs after coesite Stop D5.3: were exclusively detected as inclusions in two The Yzeron quarry, Road from Ste-Foy- garnet grains within one sample of kyanite-bearing l’Argentière to Craponne, D489. eclogite (Color Plate 2, E, F). SiO2 polymorphs were Stop D5.3 shows the migmatitic orthogneisses distinguished optically, i.e. coesite was fi rst positively from the northern Monts du Lyonnais. These identifi ed relative to quartz by its higher refractive rocks are metamorphosed and strongly foliated index, and then confi rmed by Raman spectroscopy, under amphibolite facies conditions. Their typical by observation of the characteristics Raman lines mineralogy is an association of quartz + plagioclase 177, 271, 521 cm-1. Only two coesite grains are + K-feldspar + biotite ± sillimanite. Muscovite preserved as relics and, generally, coesite is otherwise and chlorite are developed during retrogression. completely transformed into polycrystalline radial In the Yzeron quarry, superposed fold systems quartz (palisade texture) or into polygonal quartz have been described in relation with progressive surrounded by radiating cracks. The extremely rare deformation under amphibolite and greenschist preservation of coesite in the Monts du Lyonnais facies metamorphic conditions. The main foliation eclogites is clearly the result of the high temperature observed in the orthogneisses is folded and / or conditions (near 750°C, see details in Dufour et reworked in the ductile strike-slip shear zones related al., 1985 and Mercier et al., 1991) reached during to the development of a regional scale transpressive decompression as well as the consequence of fl uid regime. infl ux (hydration) during retrogression. Indeed, the The northern Monts du Lyonnais area, with kinetics of the coesite ---> quartz transformation are intermediate–pressure granulite and amphibolite strongly temperature and fl uid dependent (Gillet et facies metamorphic rocks can be interpreted as al., 1984; Van der Molen and Van Roermund, 1986; remnants of the upper overriding continental crust on Hacker and Peacock, 1995; Liou and Zhang, 1996) which the magmatic arc and the Brévenne back-arc and consequently in the studied area, coesite has been were emplaced. In this model, the southern part of the almost entirely transformed into quartz. Monts du Lyonnais unit corresponds to a subduction complex (remnant of subduction channel) located on Stop D5.2: the top of the lower plate (i.e. Pilat – Velay units). St-André-la-Côte, Road from St-André-en-Haut to Mornant. Stop D5.4: B16 to B33 n° 2 - from Volume Migmatites and granulitic rocks outcrop in the Brévenne valley, road cut: bimodal magmatic northern part of the Monts du Lyonnais. Near St- sequence. André-la-Côte village, mafi c and acid granulite Stop D5.4 shows different lithologies, like facies rocks are well exposed. In mafi c granulites the metabasalts, metarhyolites, metapyroclastites and following metamorphic assemblages are described metasediments, typical for the Brévenne ophiolitic (Dufour, 1985; Dufour et al., 1985): unit. All the lithologies are metamorphosed under • Garnet + plagioclase + orthopyroxene + ilmenite, greenschist facies conditions. In mafi c lithologies, the • Garnet + plagioclase + amphibole + ilmenite, common mineralogy corresponds to an association of

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure End ofthe5 upper plateofasubductionsystem. Devonian, inaback-arcbasindeveloped uponthe origin oftheBrévenne ophioliticsequence,during Recent geochemicalinvestigations supportthe Monts duLyonnais unitataround350–340Ma. regional transpressive regime whichaffects alsothe vertical axialplanes. These foldsarerelatedtothe involved intoaregional scalefoldsystemwithsub- The different lithologiesarealsodeformedand ± et l’anticlinoriumdel’Iglesiente(Sardaigne). Thèse : lesnappes delaMontagneNoire(France) hercyniens microtectonique comparéededeuxdomaines Arthaud, F. (1970).Etudetectonique et Paris plagioclase +actinolitechloritesphene détermination deleurcinématique. chevauchements varisques danslesCévennes et des mylonitesschisteuses:cartographie Arnaud, F. andBurg, J.P. (1993).Microstructures 286, 351pp. Polytechnique deLorraine,DocumentsduBRGM, schisteuses. Thèse 3èmecycle, InstitutNational déformation dansleszonesdecisaillement français). Microstructuresetmécanismesde varisque :lesCévennes centrales(MassifCentral barométrique d’unsystèmedechevauchements Arnaud, F. (1997). Analyse structuraleetthermo- Acad. Sci.Paris implications pourlemétamorphismeardéchois. : une vaugnérite del’Ardèche(Massifcentralfrançais) Bertrand, J.M.(1995).Desxénolites àcorindondans Ait Malek,H.,Gasquet,D.,Marignac,C.and l’INPL, Univ. Nancy, 297pp. de l’anti-Atlasoccidental(Maroc). Thèse doctoratde exemples duSE Velay (Massifcentralfrançais)et géochronologie U/Pbd’associationsacide-basiques: Ait Malek,H.(1997).Pétrologie,Géochimieet 184. of partiallymeltedrocks. Arzi, A.A. (1978).Criticalphenomenaintherheology References cited O. Monodisthanked forhisreview ofthefi Acknowledgements preparing thisguidebook. provided agreathelpduringthefi of thisguidebook.MrsS.MatratandD.Quiniou quartz. 317,1441-1447. th dayinLyon (airportandrailway station) 321,959-966. Tectonophysics C. R. Acad. Sci. nal stagesfor 44,173- rst draft rst ± calcite C. R. Appalachians andtheUrals. Africa: resultofrightlateralshearzonebetween strike slipfaulting insouthernEuropeandnorthern Arthaud F. andMatteP. (1977)-LatePaleozoic d’Etat, Univ. Montpellier, France,175pp. sédimentaires post-hercyniens, 26 Géologie del’EuropeduPrécambrienauxbassins hercynienne. (J.CognéJ.andM.Slansky M.Eds.), et saplacedansledéveloppement delachaîne de l’orogenèsevarisque dansl’OuestdelaFrance Autran, A. andCogné,J.(1980).Lazoneinterne p. 1305-1320. minéralogique etgéochimique. Thèse Doctorat3 des montsduLyonnais. Etudepétrographique, Blanc, D.(1981).Lesrochesbasiquesetultrabasiques 777. central français). sismique réfl et despièges associésdanslachaînevarisque : paléochamps hydrothermaux As-Sb d’échellecrustale P., Milési,J.P. andRoigJ.Y. (1999)-Imageriedes Bitri, A., Truffert, C.,Bellot,J.P., Bouchot, V., Ledru, français. tectono-metamorphic event inwesternMassifCentral J.L. (1977)-Radiometricevidence foran Acadian Bernard-Griffi 40 pp. Massif Centralfrançais.Masterthesis,Univ. Orléans, de migmatitesetgranitoïdestardi-hercyniens du de datationàlamicrosondeélectroniquemonazite Be Mezème,E.(2002). Application delaméthode migmatites. P. (2003). Analog modelsofmelt-fl Barraud, J.,Gardien, V., Allemand, P. andGrandjean, zone. of granite:theexemple oftheSoutharmorican shear Orthogneiss myloniteandnoncoaxialdeformation Berthé, D.,Choukroune,PandJegouzo, P. (1978). Petrology Velay granitecomplex (MassifCentral,France emplacement ofcrustalgraniticmagmas:the growth texture andtheconditionsofgenesis J., Gasquet,D.andJabbori,J.(1999).Cordierite Barbey, P., Marignac,C.,Montel,J.M.,Macaudière, XCIX, 90-111. Int., Coll.C6,Paris 1980. nord-ouest. : unchevauchement detypehimalayenvers l’ouest- Bouchez, J.L.,andJover, O.(1986).LeMassifCentral cycle, Univ. Lyon 1,152p. Boutin, R.andMontigny, R.(1993).Datation J. Struc.Geol. Contrib 40,1425-1441. C. R. Acad. Sci.Paris exion verticale (GéoFrance3D:Massif J. Struct.Geol ths, J.,CantagrelJ.M.andDuthou . Mineral. Petrol C. R. Acad. Sci. Paris 1,31-42. Ann. Soc.géol. Nord . inpress. Geol. Soc. Am. Bull . 61,199-212. 302,675-680. ème ow infolding Cong.Géol. 329,771- 21-05-2004, 10:56:34 , Lille . 88, ). J. ème

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure région deMarvejols (Lozère). leptyno-amphibolique surlesschistesduLotdansla mylonitiques associéesaucharriagedugroupe Faure, M.,Pin,C. andMailhé,D.(1979).Lesroches Tectonics extensions inthe Variscan FrenchMassifCentral. Faure, M.(1995). Lateorogeniccarboniferous sédimentation. Noire : rapportsentremiseenplacedesnappeset Carbonifère anté-stéphaniendelaMontagne Engel, W., Feist,R.andFranke, W. (1980).Le Tectonophysics complex (MontagneNoire,SouthernMassifCentral). collapse basininaLate Variscan metamorphiccore tectonics, basementupliftandStephano-Permian Echtler, H.andMalavieille, J.(1990).Extensional Sci. Paris. du Lyonnais (MassifCentralfrançais). Rb/Sr Dévonien moyen desmigmatitesàcordiérite Duthou, J-L.,Chenevoy, M.andGay(1994). Age system. Massif Central:ageandoriginstudiedby87Rb/87Sr Y. (1984).Paleozoïc granitoïdsfromtheFrench Duthou, J.L,Cantagrel,J.M.,Didier, J.and Vialette, Géologie delaFrance structure d’ensembleetévolution géologique. anatectique du Velay (MassifCentralfrançais) : Dupraz, J.andDidier, J.(1988).Lecomplexe de 3 du Lyonnais (MassifCentralfrançais). Thèse Doctorat formations orthométamorphiquesacidesdesMonts Dufour, E.(1982).Pétrologieetgéochimiedes Planet. Sci.Lett. anté-hercynienne del’EuropeOccidentale. (Haut-Allier, France)etconséquencessurl’évolution Datation U-Pbsurzirconsdel’éclogitelaBorie Ducrot, J.,Lancelot,J.R.andMarchand,J.(1983). France. Variscan terranesofthewesternMassifCentral, Frison, J.Y. (1989).Evidenceforalostoceanin Dubuisson, G.,Mercier, J.C.C.,Girardeau,J.and Clermont-Ferrand, Plein Air Service. 288, 167-170. evolution. area: aneo-Hercynian plurifacial metamorphic Eclogites andgranulitesintheMontsduLyonnais Dufour, E.,Lardeaux,J.M.andCoffrant, D.(1985). 113 metabasites (MassifCentralFrance) and retrogressive evolution oftheMontsduLyonnais Dufour, E.(1985).Granulitefacies metamorphisme ème cycle, Univ. Lyon 1,241p. Nature Phys. EarthPlanet.Int. 14,132-153. C.R. Acad. Sci,Paris 319,791-796. 337,23,729-732. 177,125-138. Bull. BRGM 18,97-113. 4,73-87. 2,341-389. C. R. Acad. Sci.Paris 35,131-144. 300,141-144 Lithos C. R. Acad. 18,97- Earth

part oftheFrenchMassifCentral.New C. (2001).Late Visean thermalevent inthenorthern Faure, M.,Monié,P., Maluski,H.,Pin,C.andLeloix, Bull. Soc.Geol.France L’évolution polycyclique delachaînehercynienne. Faure, M.,Leloix,C.andRoig,J.Y. (1997). Acad. Sci.Paris de laMontagneNoire(MassifCentral,France). migmatitique carbonifèremoyen delazoneaxiale cinématiques surlamiseenplacedudôme Faure, M.andCotterau,N.(1988).Données orogenic extension. Rb-Sr isotopicconstraintsontheHercynian syn- Noire (FranceMéridionale), la sédimentationsynorogéniquedansMontagne de Cabrières(Hérault).Implicationssurladurée d’âge namurienprobabledansleflyschàolistolithes Feist, R.andGaltier, J.(1985).Découverte deflores Geological SocietyofLondon. Tanner, Eds.)pp.35-61.SpecialPublications,179, Variscan Belt ”(W. Franke, V. Haak,O.Oncken, D. Processes. boundaries andplatetectonicevolution, inOrogenic the Variscides: tectonostratigraphicunits,terrane Franke,W. (2000). The mid-Europeansegment of America, 230. Ed.), pp.67-90.Specialpaper, GeologicalSocietyof ” (R.D.Dallmeyercircum-Atlantic Paleozoic orogens Variscan beltofcentralEurope. Franke, W. (1989). Tectonostratigraphic unitsinthe (3), 1-54 la Brévenne (MassifCentralfrançais). volcaniques paléozoïquesdelaBruche(Vosges) etde processus despilitisation.Etudecomparéedesséries Fonteilles, M.(1968).Contribution àl’nanalysedu France, 445pp. Limousin central. Thèse d’Etat,Univ. Limoges, Floc’h, J-P. (1983).Lasériemétamorphiquedu BRGM Orléans. (1/50000), feuilleStSymphorien-sur-Coise (721). Thieblemont, D.(1996).CartegéologiquedeFrance Y., Lemiere,B.,Mercier, F., Peterlongo, J.M.and Feybesse, J.L.,Lardeaux,J.M., Tegyey, M.,Kerrien, 996. collision varisque. métamorphique desMontsduLyonnais àlaﬁ une unitédévonienne charriéesurlecomplexe La sériedelaBrévenne (MassifCentralFrançais): M., Dufour, E.,Lemiere,B.andDelfour, J.(1988). Feybesse, J.L.,Lardeaux,J.M.,Johan, V., Tegyey, 300, 207-212. In “ Quantiﬁ 307,Il,1787-1794. C. R. Acad. Sci. Paris Int. J. Geol cation andModellinginthe 168, 695-705. C. R. Acad. Sci.Paris 91,53-75. In “ Terranes inthe Bull. BRGM 40 Ar/ 307,991- 39 21-05-2004, 10:56:37 n dela Ar and C. R. 2,

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Gardien, V., Lardeaux, J.M. and Misseri, M. (1988). Lardeaux, J.M. and Dufour, E. (1987). Champs de Les péridotites des Monts du Lyonnais (Massif Central déformation superposés dans la chaîne varisque. français) : témoins privilégiés d’une subduction de Exemple de la zone nord des Monts du Lyonnais lithosphère paléozoique. C. R. Acad. Sci. Paris 307, (Massif Central français). C. R. Acad. Sci. Paris 305, 1967-1972. 61-64. Gardien, V. (1990). Reliques de grenat et de staurotide Lardeaux, J.M., Reynard, B. and Dufour, E. (1989). dans la série métamorphique de basse pression du Granulites à kornérupine et décompression post- Mont Pilat (Massif Central français): témoins d’une orogénique des Monts du Lyonnais. C. R. Acad. Sci. évolution tectonométamorphique polyphasée. C. R. Paris II 308, 1443-1449 Acad. Sci. Paris 310, 233-240. Lardeaux, J.M., Ledru, P., Daniel, I. and Duchène, Gardien, V., Tegyey, M., Lardeaux, J.M., Misseri, M. S. (2001). The variscan French Massif Central - a and Dufour, E. (1990). Crustal-mantle relationships new addition to the ultra-high pressure metamorphic in the french Variscan chain: the example of the «club»: exhumation processes and geodynamic Southern Monts du Lyonnais unit (eastern French consequences. Tectonophysics 323, 143-167. Massif Central). Journ. Metam. Geol. 8, 477-492. Lasnier, B. (1968a). Découverte de roches éclogitiques Gardien, V. and Lardeaux, J.M. (1991). Découvertes dans le groupe leptyno-amphibolique des Monts du d’éclogites dans la synforme de Maclas: extension Lyonnais. Bull. Soc. Géol. Fr. 7, 179-185 de l’Unité Supérieure des Gneiss à l’Est du massif Ledru, P., Lardeaux, J.M., Santallier, D., Autran, A., central. C. R. Acad. Sci. Paris 312, 61-68. Quenardel, J-M., Floc’h, J-P., Lerouge, G., Maillet, Gardien, V., Lardeaux, J.M., Ledru, P., Allemand, P. N., Marchand, J. and Ploquin, A. (1989). Où sont les and Guillot, S. (1997). Metamorphism during late nappes dans le Massif Central français ? Bull. Soc. orogenic extension: insights from the French Variscan Géol. France 8, 605-618. belt. Bull. Soc. Géol. Fr. 168, 271-286. Ledru, P., Autran, A. and Santallier, D. (1994a). Gèze, B. (1949). Etude Géologique de la Montagne Lithostratigraphy of Variscan terranes in the French Noire et des Cévennes Méridionales. Mem. Soc. Géol. Massif Central. A basic for paleogeographical France 24, 215. reconstruction. In: “Pre-Mesozoic geology in France Gay, M., Peterlongo, J.M. and Caen-Vachette, M. and related areas”, (J. D. Keppie, Ed.). pp. 276-288. (1981). Age radiométrique des granites en massifs Springer Verlag. allongés et en feuillets minces syn-tectoniques dans Ledru, P., Costa, S. and Echtler, H. (1994b). Structure. les Monts du Lyonnais (Massif Central français). C. In: “Pre-Mesozoic geology in France and related R. Acad. Sci. Paris 293, 993-996. areas”, (J. D. Keppie, Ed.). pp. 305-323, Springer Gillet, P., Ingrin, J. and Chopin, C. (1984). Coesite Verlag. in subducted continental crust: P-T history deduced Ledru, P., Courrioux, G., Dallain, C., Lardeaux, J.M., from an elastic model. Earth Planet. Sci. Lett. 70, Montel, J.M., Vanderhaeghe, O., and Vitel, G. (2001). 426-436 The Velay dome (French Massif Central): melt Godard, G. (1990). Découverte d’éclogites, de generation and granite emplacement during orogenic péridotites à spinelle et d’amphibolite à anorthite, evolution. Tectonophysics 332, 207-237. spinelle et corindon dans le Morvan. C. R. Acad. Sci. Leloix, C., Faure, M. and Feybesse, J.L. (1999). Paris 310, 227-232. Hercynian polyphase tectonics in north-east French Hacker, B.R. and Peacock, S.M. (1995). Creation, Massif Central : the closure of the Brévenne preservation, and exhumation of UHPM rocks. In: Devonian-Dinantian rift. Int. J. Earth. Sci. 88, 409- “Ultra-high-Pressure Metamorphism”. (Coleman, 421. Wang, Eds.), Cambridge University Press, Cambridge, Liou, J.G. and Zaang, R.Y. (1996). Occurrences of pp. 159-181 intergranular coesite in ultrahigh-P rocks from the B16 to B33 n° 2 - from Volume Lagarde, J.L., Dallain, C., Ledru, P. and Courrioux, Sulu region, eastern China: implications of lack of G. (1994). Deformation localization with laterally ﬂ uid during exhumation. Am. Mineralogist 81, 1217- expanding anatectic granites: Hercynian granites of 1221 the Velay, French Massif Central. J. Struct. Geol. 16, Macaudière, J., Barbey, P., Jabbori, J. and Marignac, 839-852. C. (1992). Le stade initial de fusion dans le Lardeaux, J.M. (1989). Les formations développement des dômes anatectiques : le dôme du métamorphiques des Monts du Lyonnais Bull. Soc. Velay (Massif Central français). C. R. Acad. Sci. Paris Géol. Fr. 4, 688-690 315, 1761-1767.

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure thickened crust. An Variscan tectonicevolution bythinningofanearlier Maluski, H.,Costa,S.andEchlerH.(1991).Late Geodinamica Acta mais unanticlinalpost-nappeàcœuranatectique. n’est pasun“ metamorphic corecomplex ”extensif La zoneaxialehercynienne delaMontagneNoire Matte, P., Lancelot,J-R.andMattauer, M.(1998). 1128. Armorica microplate : areview. (480-290 Ma)andthetectonicdefi Matte, P. (2001). The Variscan collageandorogeny 329-374. for thevariscan beltofEurope. Matte, P. (1991). Tectonics andplatetectonicsmodel dextre. bassin pull-apartenrelationavec undécrochement extension tardi-hercynienne généralisée:c’estun stéphanien deSt-Etiennenerésultepasd’une Mattauer, M.andMatte,P. (1998).Lebassin 315. Turland M.(1993).Le Velay. Sonvolcanisme etles Mergoil, J.,Boivin, P., Blès,J.L.,Cantagrel,J.M.and 010 DEX(fi contrat MAIM-0030-F(D).Rapp.BRGM89DAM their enrichedzonesinFranceandPortugal.CEE markers ofhigh-grademassive sulphidedepositsof Central, France).Project:identifi Brévenne volcano-sedimentary belt(easternMassif Zn-Cu-Ba massive sulphidedepositandtheDevonian Milési, J.P. andLescuyerJ.L.(1989). The Chessy 10, 131-140. paths ofthefrenchMassifCentraleclogites. On thetectonicsignifi Mercier, L.,Lardeaux,J.M.andDavy, P. (1991). 287-304. Noire, southernMassifcentral,France. Montagne Noire. étirement subhorizontaldansleversant Suddela Plissements hercyniens synschisteuxpost-nappeet Mattauer, M.,LaurentP. andMatteP. (1996). 306, 671-676. hercynienne duMassiffrançais. une nouvelle analogieentrel’Himalayaetlachaîne Failles normalesductilesetgrandschevauchements : Mattauer, M.,Brunel,M.andMatte,P. (1988). Carboniferous basin. Pilat extensional shearzoneandStEtienneupper Variscan crustintheFrenchMassifCentral:Mont and Gardien, V. (1990).Collapseofthethickened Malavieille, J.,Guihot, P., Costa,S.,Lardeaux,J.M., Geodinamica Acta nal report) C. R. Acad. Sci.Paris. 11,13-22. 40 Tectonophysics cance oftheretromorphicP-T Ar/ 39 11,23-31. Ar studyoftheMontagne C. R. Acad. Sci.Paris cation ofdiagnostic Terra Nova Tectonophysics 177,139-149. nition ofthe 322,309- Lithos Tectonics 13,122- 126, 26,

Faure, M.andRoig,J.-Y. (2000). Monié, P., Respaut,J.-P., Brichaud,S.,Bouchot, V., Géologie delaFrance formations associées,noticedelacarteà1/100000. and aluminousgranulites. implications fortheoriginofperaluminousgranitoids equilibria andmeltproductivity inthepeliticsystem: Patiño Douce, A.E. andJohnston, A.D. (1990).Phase continental shearzones. J-P. (1977).Geologicalaspectsofdeformationin Nicolas, A., Bouchez,J-L.Blaise,J.andPoirier, Eur. J. Mineral of the Velay anatecticdome(FrenchMassifCentral). C. (1997).U-Pbchronologyonaccessoryminerals Mougeot, R.,Respaut,J.P., Ledru,P. andMarignac, (French MassifCentral). the Hercynian low-P, high-T Velay anantecticdome M. (1992). Thermobarometry andgranitegenesis : Montel, J.M.,Marignac,C.,Barbey, P. andPichavant, France Velay (MassifCentralFrançais). C. andCeret,K.(2002). Age desmicrogranitesdu Montel, J.M.,Bouloton,J., Veschambre, M.,Pellier, et géochimiques. Central): principalescaractéristiquespétrographiques granites tardi-migmatitiquesdu Velay (Massif Montel, J.M.and Abdelghaffar, R.(1993).Les Sci. Paris genèse desgranitestardi-migmatitiques. du Velay (MassifCentralfrançais) et conditionsde (1986). Thermobarométrie dudomaineanatectique Montel, J.M., Weber, C.,Barbey, P. andPichavant, M. 301, 615-620. profonde tardihercynienne. français). Indicationssurl’évolution delacroute les doléritesduPeyron, en Velay (MassifCentral Montel, J.M.,1985.Xénolithesperalumineuxdans Minières, Orléans. BRGM 297,BureaudeRecherchesGéologiqueset in Europe”,(V. Bouchot,Ed.),pp.77-79.Document Massif Central,France). the Cévennes andChâtaigneraiedistricts(Southern geochronology appliedto Au-W-Sb metallogenesisin Central Français). métamorphique delaBrévenne (Rhône,Massif variolitique etmatricebasique danslasérie Peterlongo, J.M.(1970).Pillows-lavas àbordure Fac. Sci. monts duLyonnais (MassifCentralfrançais). Peterlongo, J.M.(1960).Lesterrainscristallinsdes 107, 202-218. 1,15-20. Univ. Clermont-Ferrand4,(1),187 302,647-652. . 9,141-156. Géologie delaFrance C.R. Acad. Sci.Paris 3,3-96. Tectonophysics J. Metam.Geol. In : “Orogenicgolddeposits Contrib C. R. Acad. Sci.Paris 40 . Ar/ Géologie dela Mineral. Petrol 39 42,55-73. 1,15-28. 2,190-194 Ar andU-Pb 10,1-15. C. R. Acad. 21-05-2004, 10:56:41 Ann. .

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Piboule, M., Briand, B. and Beurrier, M. (1982). bimodal suite in the Hercynian Belt: Nd isotope and Géochimie de quelques granites albitiques dévoniens trace element evidence for a subduction-related rift de l’Est du Massif Central (France). Neues Jb. Miner. origin of the Late Devonian Brévenne metavolcanics, Abh., 143, 279-308. Massif Central (France). Contrib Mineral Petrol 129, Piboule, M., Beurrier, M., Briand, B. and Lacroix, P. 222-238. (1983). Les trondhjemites de Chindo et de St-Veran Quénardel, J.M. and Rolin, P. (1984). Paleozoic et le magmatisme kératophyrique associé. Pétrologie evolution of the Plateau d’Aigurande (N-W. Massif et cadre géostructural de ce magmatisme Dévono- Central, France). In “Variscan tectonics of the North Dinantien. Géologie de la France I, 2, (1-2), 55-72 Atlantic region” (D. Hutton and D. Sanderson, Eds.), Piboule, M. and Briand, B. (1985). Geochemistry of pp. 63-77, Geol. Soc. London Spec. pub, 14. eclogites and associated rocks of the southeastern area R’Kha Chaham, K., Couturié, J.P., Duthou, J.L., of the French Massif Central: origin of the protoliths. Fernandez, A. and Vitel, G. (1990). L’orthogneiss Chem. Geol. 50, 189-199 oeillé de l’Arc de Fix : un nouveau témoin d’âge Pin, C. (1979). Géochronologie U-Pb et cambrien d’un magmatisme hyper alumineux dans le microtectonique des séries métamorphiques anté- Massif Central français. C. R. Acad. Sci. Paris 311, stéphaniennes de l’Aubrac et de la région de 845-850. Marvejols (Massif Central). Thèse 3° cycle, Univ. Robardet, M., Verniers, J., Feist R. and Paris, F. Montpellier, France, 220pp. (1994). Le Paléozoïque anté-varisque de France, Pin, C. (1981). Old inherited zircons in two contexte paléogéographique et géodynamique. Géol. synkinematic variscan granitoids: the “granite du de la France 3, 3-31. Pinet” and the “orthogneiss de Marvejols” (southern Robardet, M. (2003). The Armorica ‘microplate’: French Massif Central). N. Jb. Miner. Abh. 142, 27- fact or ﬁ ction? Critical review of the concept 48. and contradictory palaeobiogeographical data. Pin, C. (1990). Variscan oceans: ages, origins and Palaeogeography, Palaeoclimatology, Palaeoecology geodynamic implications inferred from geochemical 195, 125-148. and radiometric data. Tectonophysics 177, 215-227. Roig, J-Y. and Faure M. (2000). La tectonique Pin, C. and Lancelot, J. (1978). Un exemple de cisaillante polyphasée du Sud-Limousin (Massif magmatisme cambrien dans le Massif Central : les Central français) et son interprétation dans un modèle métadiorites quartzites intrusives dans la série du Lot. d’évolution polycyclique de la chaîne hercynienne. Bull. Soc. Géol. France 7, 203-208. Bull. Soc. Géol. Fr. 171, 295-307. Pin, C. and Lancelot, J. (1982). U-Pb dating of an Roig, J-Y., Faure, M. and Truffert, C. (1998). Folding early paleozoïc bimodal magmatism in the French and granite emplacement inferred from structural, Massif Central and of its further metamorphic strain, TEM, and gravimetric analyses: the case study evolution. Contrib. Mineral Petrol, 79, 1-12. of the Tulle antiform, SW French Massif Central. J. Pin, C., Dupuy, C. and Peterlongo, JM. (1982). Struct. Geol. 20, 1169-1189. Répartition des terres rares dans les roches volcaniques Sider, J-M. and Ohnenstetter, M. (1986). Field and basiques dévono-dinantiennes du nord-est du Massif petrological evidence forfor the development for central. Bull. Soc. Géol. Fr. 7, 669-676. an ensialic marginal basin related to the Hercynian Pin, C. and Vielzeuf, D. (1983). Granulites and orogeny in the Massif Central, France. Geol. related rocks in Variscan median Europe: a dualistic Rundschau 75, 421-443. interpretation. Tectonophysics 93, 47-74. Soula, J.C., Debat, P., Brusset, S., Bessière, G., Pin, C. and Duthou, J.L. (1990). Sources of Hercynian Christophoul, F. and Déramond, J. (2001). Thrust granitoids from the French Massif Central: inferences related, diapiric and extensional doming in a frontal from Nd isotopes and consequences for crustal orogenic wedge: example of the Montagne Noire, B16 to B33 n° 2 - from Volume evolution. Chemical Geology 83, 281-296. southern French Hercynian Belt. J. Struct. Geol. 23, Pin, C. and Marini, F. (1993). Early Ordovician 1677-1699. continental break-up in Variscan Europe: Nd-SR Van den Driessche J. and Brun, J-P. (1991-92). isotope and trace element evidence for bimodal Tectonic evolution of the Montagne Noire (French igneous associations of the southern Massif Central, Massif Central): a model of extensional gneiss dome. France. Lithos 29, 177-196. Geodinamica Acta 5, 85-99. Pin, C and Paquette, JL (1998). A mantle-derived Vanderhaeghe, O., Burg, J.P. and Teyssier, C. (1999).

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Volume n° 2 - from B16 to B33 B22 - B22 Leader: M.Faure The pressurepathofsoildinclusionsinminerals:the Van derMolen,I.and Van Roermund,H.L.M.(1986) 451-472. melting andﬂ Vanderhaeghe, O.and Teyssier, C.(2001).Partial London and S.D. Willett (Eds.)181-204, ﬂ “Exhumation processses:normalfaulting, ductile Canadian CordilleraandFrench Variscides. Exhumation ofmigmatitesintwo collapsedorogens: ow anderosion”U.Ring,M.T. Brandon,G.S.Lister , SpecialPublications,154, ow oforogens. Tectonophysics Geological Society, 342, In:

Royal Soc.Edinburgh Velay granitecomplex, MassifCentral,France. underplating andgranitegenesis:anexample fromthe (1992). The relationshipbetweencrustalmagmatic Williamson, B.J.,Downes, H.and Thirlwall, M.F. ParisSci. R. Acad. amphibolite danslesenclaves basiquesdu Velay Vitel, G.(1985).Latransitionfaciès granulitefaciès 19, 317-324 retention ofcoesiteinclusionsduringuplift. 300,407-412. , Earth Sciences 83,235-245. 21-05-2004, 10:56:44 Trans. Lithos . C.

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