Structure and Tectonic Evolution of the Armorican Massif

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J. geol. Soc. London, Vol. 137, 1980, pp. 211-216. Printed in Northern Ireland. Conference Report

Structure and tectonic evolution of the Armorican Massif

R. A. Roach

Report of a meeting of the Tectonic Studies Group held onthe foliated granodiorites and quartz-diorites, to at Burlington House, 14 March 1979. The meeting was helpdistinguishto Pentevrian andCadomian organized by Dr A. J. Barber and Dr P. R. Cobbold. magmatic-metamorphic episodes. Whiledeformation, metamorphic and magmatic The papers presented provided a timely review of the episodes occurring between 670 and 490 Ma are well major advances during the last decade in understand- establishedfor the northern part of theMassif, the ing the structure and tectonic evolutionof the Armori- significance of Cadomian structures in central Brittany canMassif. The establishment of specialresearch has been recently questioned. According to Hanmer, groups at the Universities of Rennes, Brest, Caen and Le Corre & Bertht,who presented a considerable Nantes, supported by the CNRS, has been the prime amount of supporting data, the main syn-metamorphic factor in this development. As a result, the geological deformation recognized within the Upper Brioverian evolution of this region is probably better understood of central Brittany is Hercynian in age. Less certain, thanthat of otherNercynian Massifs in central and however, is their conclusion that the Upper Brioverian western Europe. is thepost-tectonic molasse to the main phases of Thegeological evolution of theArmorican Massif Cadomianorogeny. CognC,in his model,envisaged canbe traced back more than 2000 Ma, beginning deposition of thecentral Brittany Brioverian on the withthe deposition of aLower Proterozoic or late southerncontinental slope of thePentevrian micro- Archaean supracrustal sequence which was polyphase continent, whereas Auvray & Lefort suggested deposi- deformed,metamorphosed, and intruded by granitic tion within a marginal sea. andquartz-dioritic magmas between 2500 Ma and Anothermajor advance in understandingthe tec- 1900Ma to produce the Pentevrian basement of the tonic evolution of the central and northern parts of the northernpart of theMassif. Professor CognC por- Massif has been the recent identification of important trayedthe Pentevrian as a fragment of continental deformation episodes in late Devonian to early Car- crust originally closely related to the W African Cra- boniferous times, corresponding to the Bretonic phase ton,which similarly exhibits magmatism around of the Hercynian orogeny. Details of these episodes, 2200 Ma to 1900 Ma (Eburnian event). whichcomprise folding, thrusting and tear-faulting, Itwas upon this basement and within adjacent were given for W Brittany and Finistkre by Darboux et oceanicdomains thatUpperthe Proterozoic al., Rolet,and Cabanis. These authors were able to Brioverian Supergroup was deposited. The closure of demonstrate polyphase folding of Devonian and very a portion of this ocean, the segment lying NW of the earlyCarboniferous successions and a comparatively Capde laHague-Tregor Pentevrian axis and S of simpler structural chronology for the post-Tournaisian continentalcrust represented by thePrecambrian of succession. The nature and importance of Hercynian southernBritain, was, according to the model prop- and pre-Hercynian shear belts within N Brittany was osed by Auvray & Lefort, the result of south-easterly demonstrated by Williams & Watts in a detailed and subductionunder the northern part of the Massif illuminating study of the Bourbriac-Quintin sector of between 750 and 550 Ma. This closure resulted in the the Molkne-Moncontour lineament. Cadomian orogenic event. Such features as the nature S of theWNW-ESE trending S ArmoricanShear of theCadomian igneous activity and the proposed Zone (Zone BroyCe Sud-Armoricaine), the Massif ap- existence of alate Precambrian ophiolite body at pears to have been partof a zone of continuous crustal depth under the western part of the English Channel mobilityfrom Ordovician to Upper Carboniferous are essential ingredients of this model. Details of the times. Within this zone the two most critical units in nature of thePentevrian basement, the Brioverian interpretations of the evolution of this region are the successionand the Cadomian orogenic event as en- blueschist facies rocks of the Ile de Groix and the S countered in the Channel Islands, Cap de La Hague Brittany I,P-HT migmatites.According to CognC’s and Tregor were given by Bishop, Power, and Auvray model, further separation of the central and northern & Lefort respectively. The former two speakers stres- parts of the Massif frommore southerly continental sed the need for more isotopic age determinations on massescommenced in latePrecambrian to early rocks from the Pentevrian basement areas, particularly Phanerozoic times with the formation of a Palaeozoic

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212 R. A. Roach

Proto-Tethysto the S of Brittany.Closure of this could be traced some 40 km inland, perpendicular to ocean during Silurian-Devonian times (420-375 Ma) the regional structural trend. A model was then pre- led to the development of the blueschist facies rocks sented whereby the porphyroids lay structurally below withinanortherly dipping subduction zone above higher grade rocks within a pile of nappes formed and which the highly elongated S Brittany migmatite belt subsequently folded in mid-late Carboniferous times. was developed within an active margin. Closureof this The gneiss-migmatite terrains in the S of the Massif ocean terminated with continental collision during the were, in this case, not elongated domes, as generally Carboniferous. There is uncertainty as to whether the thought, but formed synforms within the folded nap- Groixblueschist facies rocks are products of apre- pes. This model, which Sougy suggested could also be Hercynian (Ligerian) or an Hercynian metamorphism. appliedto S Brittany,is amajor departure from The main period of migmatization has been dated at previous models on the evolution of the region, and approximately 370 Ma. It has not yet been established, willcertainly be the focus of considerable debate in therefore, if therewas atruly synchronous paired the near future. metamorphic belt developed in S Brittany. Quinquisdescribed the deformation accompanying ROBERTAINSLEY ROACH,Department of Geology, Univer- the HP-LT metamorphism on Groix as one of intense sity of Keele, Keele, Staffordshire, ST5 5BG. progressive shear along shallow-dipping surfaces. The recognition of inverted metamorphic zones on Groix Principal stages in the creation and evolution of Annorican was explained in terms of northerly obduction of the continental crust in the context of Western Europe J. Cognt blueschist facies rocks subsequent to continental collision. Richards, from a comparative structural study of Two principal stagesmark the crustal evolution of Ar- the Groix rocks with those of the adjacent mainland morica: (1) The first stage is thecreation of new crust of area(the Pouldu Series), concluded that both sequ- Upper Proterozoic (Brioverian)age (Cog1161970), a consequ- ences were part of the same crustal unit. Audren & ence of the development of an oceanic domain and its closure Peucat gave details of the metamorphic and structural dunng the Cadomianorogeny, between the Precambrian N Atlantic continent and the more southerly continentalmasses histories for the rocks on Groix and for the migmatites of ‘Gondwana’ affinity. Theopening in S Armorica (and of theGolfe du Morbihan. In the latter area the southernEurope) of the Palaeozoic Proto-Tethys,begun syntectonicGneiss de Roguedas has been used to during the latest Brioverian (Infra-Cambrian), terminatesthis date the D1 episode isotopically at 463 Ma. The main evolution. The result is the attachment to the southern mar- migmatization is broadly synchronous with DZ/D3, the gin of the N Atlantic continent of a ‘piece of Africa’ rep- closely associated anatectic granites crystallizing about resented by the Cadomian belt (cf. Panafrican) and nuclei of 370 Ma. A later episode observed in mica-schists is Pentevrian rocks (cf. Eburnian). (2) The Cambro-Ordovician associated with the emplacement of numerous leuco- opening of Proto-Tethys is followed duringthe Siluro- granites during Carboniferous times and is related by Devonian by a period of closure (the Ligerian orogeny) and subsequentcontinental collision (theHercynian belt), evi- CognC to continental collision. Brown, Friend & Top- dence of whichis located along thesouthern margin of ley, in their account of the mineral chemistry of the Armorica and the length of theMoldanubian Zone of the Morbihan migmatites, were able to estimate a temper- Variscan belt S.[. (Cognt 1976). This collision of northward ature of 700°C at5 kb for theLP-HT metamorphism. moving Gondwanancontinental masses, against the then Itis now established that the S ArmoricanShear sub-equatorial Europe, occurred at the end of the Devonian, Zone isa major structural lineament separating reg- resulting in thedevelopment of theHercynian chain s.s., ions of contrasting Palaeozoic development. Jegouzo characterizedduring the Carboniferous by majorcrustal traced the development of this shear belt from mid to thrusting and shearing and the ensialic reactivation (anatexis, late Palaeozoic times. It was suggested that, within the production of leucogranites) of the Cadomian basement. Shear Zone, the early overthrusting movements to the SW and the main dextral shearing were the result of Review of the geology of the Channel Islands A. C. Bishop eventual continent plate collision in the later stages of theHercynian orogeny. The presence of deformed TheChannel Islands are largely made of rocks formed Stephaniandeposits within the S ArmoricanShear before the onset of Variscan events and maybe compared Zone at Baie des TrCpassCs indicatesthat the shear with similar old rocks exposed in Brittany and the Cotentin. belt was active at least until early Permian times. This review touched briefly on the formation of the Archaean The SE part of theArmorican Massif (W and early Proterozoic Pentevrian metamorphic complexes of VendCe), which has not been as intensely studied as Guernesey, Alderney and Sark, the deposition of Brioverian S Brittany,was, according to Sougy, a criticalre- turbidites and associated volcanic rocks in late Precambrian and early Palaeozoic times, and their deformation during the gionin our understanding thetectonic evolution of Cadomian orogeny. The intrusive igneous activity associated of theMassif. He suggestedthat the problematical with the Cadomianmovements and thepost-orogenic porphyroids in the SE of the Massif were cataclastic molasse deposits of Jersey and Alderney was described, and rockswhich had been thrust over the Silurian sedi- the significance of this region in the evolution of Armorica mentsin the Bretignolles area. This thrust contact discussed. Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

Structure and tectonic evolution of the Armorican Massif 213

The development of the Armorican Massif: evidence from La (St-Renan) B 280 Ma (Aber-Ildut); (2) desorthogneiss de Hape G. M.Power deux types: les orthogneiss de Brest-Landivisiau situds sur la At La Hague, NW of Cherbourg, Pentevrian rocks show bordure sud et est, sont des granodiorites d’ige caltdonien striking similarities to those of theChannel Islands and (datationsur zircons B 450Ma) comparables B la enable the geological history of these old constituents of the trondjhdmite de Douarnenez. Les orthogneiss de Trtglonou Massif to be reconstructed. In addition, La Hague contains deet Plouvenez-Lochristsitu6 aucoeur des stries examples of syn- and post-Cadomian igneous activity and of mttamorphiques du Pays de Lton, sont d’anciens granitoides Brioverian and Lower Palaeozoic sedimentation. d’ige ddvonien (385 Ma); (3) des stries paramttamorphiques d’Lge infra-ordovicien. Le Bassin de Morlaix, situ6 B la limite Evolutiongeodynamiqne du Massif Armoricain au du Lton et du Trtgor, comprend des stries d’lge dtvonien Proterozoique Superieur B. Auvray & J. P. Lefort inftrieur et en ltgtre discordance cartographique unesdrie L‘interprttationdes donntes gtologiques,gtochimiques, volcano-stdimentaired’ige strunien. Cet ensemble a ttd gtophysiqueset gtochronologiques(Auvray 1979; Lefort vigoureusementplisst etmttamorphist B I’Hercynien. On 1975; Vidal 1976) recueillies dans la partie septentrionale du distingue deux phases principales de plissements: Massif Armoricain (domaines tmergt et immergt) permet de (a) une phase majeure B plis couches isoclinaux synschis- conclure qu’entre 800 (?) et 550 Ma cette region constituait teux d’orientation submtridienne, la bordure sud d’une zone de convergence. Malgrt les com- (b) et une phase de plis droits ou dtversts d’lxes N 60”, B pressions et lescisaillements resultantde la tectogenese schistositt de crtnulation, des phases tardives. hercynienne, on reconnaitencore la zonationstructurale Les m&mes dtformations ont pu &tre observtes dans les propreau fonctionnement d’une subduction B plongement stries paramttamorphiquesdu Pays deLton avecun vers le SE: mttamorphisme gtntral du typeintermtdiaire de basse- (1) Au Nord, le corpsbasique du centre de la Manche, pression,progressivement croissant depuis le Strunien assimilt B un cortegeophiolitique, reprtsente lacicatrice tpizonalde Morlaixjusqu’aux paragneiss de Lesneven (suture cryptique) tdmoignant de I’existence d’un octan au- anatectiques. Dans le m&me temps, le socle prtcambrian du jourd’hui disparu. Trtgor situ6 au contact du Bassin de Morlaix, a encaisst les (2) AuCentre (Domnonte-Mancellia), une croiitecon- deuxphases dedtformations hercyniennesmais seule la tinentalerenfermant, outre les formationsbriovtriennes premitre y estbien exprimie avecun mttamorphisme (stdimentsbordure,de magmatismes calco-alcalins, tpizonal. mttamorphismesintermtdiaires ou de basse pression),des En conclusion, nouspouvons dire qu’en Bretagnenord- fragments de croOte sialique ancienne (Calvez & Vidal 1978; occidentale,mttamorphismes et structuresactuellement Auvray et al. 1979)d’ige Protkrozoique inftrieur (2000- dtcelablesdatent des temps hercyniens,la premierephase 1800 Ma). majeure ayant dibutt posttrieurement au Strunien. (3) Au Sud du bloc Domnonte-Mancellien qui constituait la marge active, une distension provoque I’amincissement de la croiite, voire m&me sa rupture avec tmission volcanique de Cadomian or Hercynianmain deformation in centralBrit- typeophiolitique calco-alcaline;la mer marginaleainsi tany? The Hercynian granitesreply! S. Hanmer, Cl. Le dessinte prtfigure le domain centre-armoricain. L‘arrgt de la Corre & D. Berth6 subduction (550 Ma) induit une tectonique verticale au cows de laquelle apparaissent des grabens dans lesquels vont s’ac- The late Proterozoic rocks of central Brittany, represented cumuler les stdiments B partir du Paltozoique inftrieur. essentially by Upper Brioverian sediments, are considered to bea late to post-tectonic molasse to theCadomian chain. Faolts rocks as indicators of progressive shear deformationin The poly-orogenic character (Cadomian andHercynian) of the Guingamp area G. D. Williams & M. J. Watts the deformation structures affecting these rocks is noted from Deformation fabrics along a 20 km belt from Bourbriac to the Baie de Douarnenez to the Rennes region. However, the Quintin, including Chauris’s (1969) classic locality of a major main syn-metamorphic deformation event isclearly Hercy- shearzone in theTrieux valley, areexamined in terms of nian. Furthermore, this event and theassociated metamorph- their tectonic significance. A zone of fault rocks possessing ism are directly related to the emplacement of the Hercynian highly parallelizedplanar and linear structural elements leucogranites. The syntectonicnature of the leucogranites, trending 290-110” attains a maximum width of 4 km in the with respect to the deformation of their envelope, is there- central part of the belt. N of Quintin, representatives of the fore affirmed and the concept of a ‘Brioverian basement’, in fault rock series are absent and deformation is restricted to the mechanical senseand contrasted with a ‘Palaeozoic narrowtracts of foliatedHercynian granite with a similar cover’, must be abandoned. structural trend. It is suggested that the various fault rocks representboth ductile (aseismic) andbrittle (seismic)displacement products. This, together with structural and petro- TheBretonian phase in westernBrittany J. R. Darboux, fabric data indicatesa deformation historyin which the J. Rolet, A. Pelhlte, M. Barritre & M. Gravelle main movementsare pre-Quentin granite intrusion, and probably pre-Hercynian. Recentstudies upon the western part of the ‘Domaine centre-armoricain’ provide several argumentsof (1) paleogeog- Les evenements hercyniens dans le nord-est de la Bretape raphic, (2) structuraland (3) geochronologicalkind, that de IRon et Bassin de Morlaix) Cabanis (Pays R. lead us to emphasize the ‘Bretonian phase’ in Brittany. Le Pays de Lton comprendtrois ensembles gtologiques (1) Visean-Namurian detritalmetasediments of theCar- distincts: (1)des granites hercyniens s’tchelonnantde 350 Ma boniferous ‘Bassin de Chiteaulin’ display the characteristics Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

214 R. A. Roach of a post-orogenic deposit and occupy an eccentric outcrop continuous compositionalchanges have beenoverstepped area with respect to that of the Devonian formations. as a result of decrease in Plead during high-grade metamor- (2) The Devonian (including Fammenian 11) gives evidence phism and anatexis. The reactionsare: garnet orbiotite+ of a complex structural evolution with at least 2 phases of AI-silicate +quartz+cordieritezt hercynite* ilmenite*excess folding and cleavage (F,-S,, F,-S,), followed by tangential components. P-T conditions during early metatexis are esti- thrusting, while, exceptnear its tectonicboundaries, the mated to have been P = 7 kb and T = 650-660 “C, based on Carboniferousnever presents the marks of superimposed the plagioclase-garnet-AI-silicate-quartz geobarometer as- deformations. suming T to correspondto the condition of muscovite+ (3) The radiometricage of theHuelgoat granite, whose albite +quartz + vapour- kyanite + liquid for in the thermalmetamorphism post-dates strain-slipcleavage (F,- range’P,,,, to 0.6 P,,,,. P-T conditions during diatexis are S,) in theDevonian, confirms thatthese formations have estimated to have been P = 5 kb and T = 700 “C with PHZO= undergonean early complex tectonicand metamorphic 0.4 Ploadr basedon the Mg content of cordierite occurring in evolution. the assemblage quartz + plagioclase + AI-silicate + biotite + We conclude that, in western Brittany, the major Hercy- corditrite +garnet co-existing with melt. The negative slope nian tectonics took place after the deposition of the Upper of the P-T path defined by these estimates during the prog- Devonian and before the Visean, during a period of tectonic ression from metatexis to diatexis is consistent with anatexis activity at least 15 Ma long that we can call the ‘Bretonian being aresponse to thickening of thecontinental crust by phase’. overthrusting and subsequent erosion. This, in turn, is consistent with a suggestion that the Ligerian Orogeny may repres- Le rBle de coulissages dans la structuration de la couverture enta collision-type ratherthan simply a cordilleran-type paleozoiqueouest-armoricaine J.Rolet orogeny. Les mttastdiments paltozoyques du ‘Domainecentre- Is the ne de Groix geologically as well as geographically armoricain’ occidental affleurent & la faveur d’une aire sync- isolated? L. E. Richards linoriale encadrte par des accidents de nature, d’importance et d’Lge varies: ‘faille de I’Elorn’, satellites de la ‘zone broyte The blueschists of the Ile de Groix have attracted detailed nord-armoricaine’,accident lintamentaire de Briec- geochemical, petrographic,structural and geochronological Montagne Noire et failles du Menez Be1 Air. attention and are among thebest known rocks in the Armori- A l’inttrieur de cette aire, les terrainssont dtcoupts en can Massif. Their mineral assemblages suggest that they may lentilles tectoniques par un rtseau dense de failles tresstes be relicts of a Variscan subduction zone ‘complex’. However, (fractures de Riedel, R et R’) ntesen rtgime gdntral de their significance in the understanding of Variscan tectonics dtcrochement dextre. depends to a great extent upon their relationship to the rocks Les coulissages, responsableslocalement d’une veritable exposed on the adjacent mainland. The nearest comparable dilactrationdes terrains ante-carboniftres, ont tgalement outcrops consists of metasedimentary schists of theLe induit I’aire subsidente dedtpBt du Visten (‘Bassin de Pouldu series, intruded by various granitic masses. The struc- Chlteaulin’) ainsi que le volcanisme fissural du Carboniftre turaland metamorphic evolution of the schists of theLe inftrieur. Poulduseries and the Groixschists have beenstudied in L’essentiel deces cisaillements sesitue versla fin de la detail with the aim of achieving asuitable framework for ‘phasebretonne’, posttrieurement auxplis P2 (Darboux & comparison.Both havecomplex histories of varying Garreau 1976) et aux grands dtcollements tangentieb (Rolet metamorphic conditions punctuated by episodes of deforma- & Thonon 1978), et anttrieurement au Visten. tion. The points of similarity between the two histories are 11s vont se poursuivre, comme des tchos atttnuts, aprts le numerousand indeed greater contrasts canbe observed Namurien et le Sttphanien. within the confines of the Ile de Groix than between the TheSouth Armorican Shear Zone P. Jegouzo island and the mainland. The details of the comparisons and contrasts provide valuable evidence concerning the tectonic The late Hercynian S ArmoricanShear Zone, in S Brit- evolution of the region. tany, is morethan 300 kmlong andseveral kmwide. It affectsprincipally mica-schists, gneisses andgranitoids of Geodynamicimplications of shearingin the ne deGroix pre-Hercynianand Hercynian ages. Overthrustingtowards blueschists H. Quinquis the SW precededthe main episode of dextralshearing. The high pressure/lowtemperature metamorphism at- The main features and the succession of events are described. tained by the blueschists of the Ile de Groix (S Armorican The occurrence of this major shear zone is discussed in terms Massif)is eitherpre-Hercynian (Peucat & Cognt 1977) or of current geodynamic models. Hercynian (Carpenter & Civetta 1976; Maluski 1976). Con- The petrochemistry of some migmatites from hesqu’ile de temporaneous progressive shearing along a shallowly dipping surface resulted in the formation of sheathlike foldswhich Rhuys,Morbihan, and the tectonic implications M. Brown, C. R. L. Friend & C. G. Topley point along the N-S shear direction (Quinquis er al. 1978). An inversion of themetamorphic zonation suggests deep Metatexitesand diatexites from Rhuys carry abundant seated overthrusting (Graham & England 1976). The sense garnet, frequently with quartz and plagioclase inclusions, and of displacement (S to N for the overlying block) is indicated occasional kyanite relicts in plagioclase (although sillimanite by syntectonic garnet rotation and sheath-fold geometry. If is the more common Al-silicate), but carry no primary mus- oneinterprets the Hercynian orogenyin terms of plate coviteand no K-feldspar. Textural relations between, and tectonic theory, the sense of shear deformation observed is mineral chemistries of, ferro-magnesian phases in the diate- compatiblemore withan obduction (or the flake-tectonic xites,suggest that2 discontinuous reactions and additional model of Oxburgh (1972), or even an antithetic Himalayan Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

Structure and tectonic evolution of the Armorican Massif 215 type overthrust) than with the northward subduction hitherto deformation a schistositt subhorizontale gtntratrice de plis envisaged. non-cylindriques dont I’initiation, puis la rtorientation suiv- ant une direction majeure N-S caracttrise un mtcanisme de Structurede Is doubleceinture mdtamorphique silnro- cisaillementprogressif (Quinquis et al. 1978). Les lges ob- devoniennede Bretsgne meridionale C.Audren & J. J. tenus ne sont pas aistment corrdlables avec la deformation Peucat observte dans lesschistes B glaucophane. MaisI’existence d‘une p&ricde de mttamorphisme HP-BT B 420-375 Ma Les risultatsLes structuraux, pttrologiques gto- et (Peucat & Cognt 1977) permet de montrer un synchronisme chronologiques rtcentsmontrent qu’ilexiste en Bretagne de fonctionnement avec la ceinture de haute temptrature du mtridionale,au sud de la zone broyte sud-armoricaine, continent. Les tvtnements hercyniens provoquentpar la unsystbme dedouble ceinture mttamorphique dont I’lge, suitedes recristallisationsjusqu’a 280 Ma(Maluski 1976: initialement considtrt comme hercynien (Carbonifltre) Carpenter & Civetta 1976). (Carpenter & Civetta 1976),est plusprobablement prt- hercynien (Siluro-Dtvonien) (Cognt 1977; Peucat & Cognt Reflections and hypotheses on the structure of the Caledono- 1977; Hanmer 1977; Audren & Lefort 1977; Peucat et al. HercynianArmoricnn Massif J. Sougy 1978). La ceinture de haute tempe‘ratureest composte de gneiss, Syntheses of the structure of the Armorican Massif have migmatites, granites et micaschistes (Cognt 1960). paid scant attention to the SE part of the region, although Les gneiss et migmatitessont caract6ridspar un mtta- this area maywell provide an important key to the under- morphisme plurifacial (Le MBtour 1976, 1978): d’abord de standing of thebelt. Analysis of the publishedgeological type moyenne temperature B relativement haute pression M, maps suggests that the thrust of the ‘porphyroids’ over the synchrone d’un premier ipisode de dtformation D, (Audren Silurian sequence, on the coast at Bretignolles, extends in- & Le Metour 1976), il tvolue vers un type haute temperature land for some 40 km with an ENE-WSW trend, normal to moyenne a basse pression M, conduisant B l’anatexie est post the S Armoricandirection (WNW-ESE). This thrust may D, et montre deux stades: la mttatexie ant6 a syn-D, (plis P, well havea wider regionalimportance. From this stems 9 planaxial subverticalet axe subhorizontal) et la dlatexie several hypotheses, e.g. the highest grade metamorphic ter- syn i post D, (plis P, a plan axial subvertical et axe subverti- rains (i.e. the gneiss and migmatites of the Saint-Nazaire-Les cal). La mise en place terminaledes granites d’anatexie Essarts anticline and by association theCornouailles antic- marque lafin de I’tpisode M,/D,-D,, et apour risultat line)could be at the top of a pile of nappes. The ‘por- essentiella structurationdes migmatites parune foliation phyroids’would bethe lowest nappe of theallochthonous subverticale centrtesur cesgranites. Unerttromorphose sequence, restingwith strongtectonic discordance on the gintraliste M, clos I’tvolution mttamorphique. folded Palaeozoic sequence. One consequence of this Les micaschistes montrent une tvolution mttamorphique hypothesisis thatthe above-mentioned anticlines are in parallde(Audren 1974) mais caracttrisentdes zones ?I realitysynforms of thenappes, and the Palaeozoicrocks schistosittsubhorizontale interprtttes commele toit de occur in tectonic windows, e.g. La Roche-Sur-Yon and La diapirs anatectiques non affleurants. Chataigneraie. The distribution of the Namuro-Westphalian Troisgtnkrations de granites sont connues: les granites rocks in narrow strips between these metamorphic units and ordoviciens (460 a 500 Ma) de type Moelan I1 (Vidal 1973, the Palaeozoic developments suggests more than chance. One 1976; Calvez 1976)et Rogutdas (Peucat et al. 1978)-1es explanationamong others is thedisplacement of molasse granitesd’anatexie divoniens moyen (370Ma) de type depositsduring the final movements of theallochthonous Hennebont-Vannes (Vidal 1973)-1es granitescarboniftres units. (340-300 Ma) (Vidal 1973). On thescale of the whole massif, the palaeogeographicbio- L’analyse desrelations granites/mttamorphisme/dtfor- stratigraphicand structural contrast on either side of the mation montre que la mise en place des granites ordoviciens Lanvaux-Angers line is remarkable. This line appears on the est ant6 B syn-tpisode MID, (Le Metour & Audren 1979) mapas a fropt. To the S, thePalaeozoic units areoften etque lamise en place desgranites dtvoniens moyenest narrow and contrasted, suggesting distinct palaeogeographic syn a postBpisode M,D2/D,. L‘essentiel desBvtnements units considerably shortened in a NNE-SSW direction. This tectonomttamorphiques s’est donc dtroult entre I’Ordovicien contrasts with the broad expanses of cover developed N of et le Dtvonien moyen. the line, where gently folded sequences rest with progressive I1 existe unephase de dtformation plus tardivedans les faciesvariation ona Cadomian basement and present the mica-schistes, et qui est lite aux granites carboniftres: cette characteristics of a foreland region., phase est le rtsultat possible d’une collision de plaques. Finally, there is the problem of the straight faulted con- La ceinture de hnute pression montre des associations de tacts: are they strike-slip faults or thrust faults subsequently haute pression/basse ?I moyenne temptrature 21 1’Ile de Groix rotated to a vertical position? These remarks and hypotheses (Cognt1960; Felix 1972; Makanjuola & Howie1972; arepresented to draw attentionto data whichhave been Triboulet1974; Carpenter 1976). Ces associations sont more or lessneglected in previous syntheses.Before new rttromorphostes dans le facilts schistes verts. models in terms of plate tectonics are presented, it would be Laptriode de haute pression estassocite a unlitage useful to consider further the interpretation of the structure originel (Boudier & Nicolas 1976), puis aune phase de of the massif.

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