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Structure and Tectonic Evolution of the Armorican Massif

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Structure and Tectonic Evolution of the Armorican Massif Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021 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 the Armorican 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 partsof 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 southern Britain, 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 0016-7649/80/0300-0211$02.00 @ 1980 The Geological Society Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021 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, will certainly be the focus of considerable debate in therefore, if therewas atruly synchronous paired the near future. metamorphic belt developed in S Brittany. Quinquis described 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 colli- sion. 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 the Moldanubian 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,
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