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Precambrian in the External Zones of the Variscides: Central and Northern Parts O F the Armprican Massif

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Precambrian in the External Zones of the Variscides: Central and Northern Parts O F the Armprican Massif Chapter 1 Precambrian in the external zones of the Variscides: central and northern parts o f the Armprican Massif Jean Cogné,* Bernard auvray,* Lionel dupretv Jean-Jacques Chauvel,* and Dominique Rabu* * Centre Armoricain d ’Etude Structurale des Socles (CNRS), Institut de Géologie de l’Université de Rennes, 35042 Rennes-Cedex ’Laboratoire de Géologie Armoricaine, Université de Caen, 14032 Caen-Cedex 'B.R.G.M., Service Géologique Régional ‘Pays de Loire’, 10, Rue H. Picherit, 44300 Nantes Contents General framework J. Cogné 414 1.1 Precambrian in the Domnonean domain: the Trégor Massif of north Brittany B. Auvray 415 References 433 1.2 Problem of the Lower Brioverian in the context of a two-phase Cadomian orogenic cycle: the Precambrian of the Penthièvre Massif and its adjoining Mancellian boundary J. Cogné 434 References 443 1.3 The Proterozoic of northeastern Armorican Massif L. Dupret 444 References 460 1.4 Brioverian in central Brittany J.-J. Chauvel and D. Rabu 462 References 470 1.5 Geodynamic evolution of the Cadomian fold belt in the Armorican Massif: present data and new problems J. Cogné 470 References 482 413 Precambrian in the external zones o f the Variscides 415 Cadomian tectonic effects appear relatively minor defined in this massif (Cogné, 1959) and even if the and are seen to diminish in importance as one goes existence of an ancient basement is very likely, there towards the south and east away from the is no evidence so far for rocks older than 1000 Ma. Domnonean ‘Cordillera’ (ridge). The deformation This is in contrast with the situation further north style of these external parts of the Cadomian chain in the Trégor massif and in the Channel Islands is characterized by a lack of cleavage and a (Fig. 1). development of synsedimentary breccias and In fact, the only massif of the Domnonean superficial recumbent folds produced by local domain that has been intensively studied is the sliding over an unknown basement. Trégor massif (Auvray, 1979). However, it is not Both the Brioverian and its Palaeozoic cover are possible here to observe the full sequence of actively affected by Hercynian deformation; in fact, Precambrian series as, for example, further south most of the schistosity developed in the Brioverian in the Baie de St Brieuc where lower Brioverian of Central Brittany relates to Variscan orogenesis. volcano-sedimentary formations are exposed The southern limit of the domain is marked by (Cesson-Lanvollon). Such formations are not the northerp branch of the South Armorican Shear represented in the Trégor, except perhaps towards Zone, which is itself guided by the Canvaux the western limit of the area (l’Armorique) where granitic-gneiss belt. Intrusives along this axis have some metabasic rocks associated with metasedi­ Ordovician and Silurian ages and appear as the first ments could possibly be assigned to the lower signs of crustal distension that, further south, Brioverian. This interpretation is still controversial characterizes the early stages in the evolution of the and agreement has not been reached between Ligerian active margin. different workers (Auvray, 1979; Autran et ai, 1979). Whatever the correct interpretation might be, the present study is concerned with a description 1.1 PRECAMBRIAN IN THE DOMNONEAN of the Precambrian of the Domnonean domain DOMAIN: THE TRÉGOR MASSIF OF within the confines of the Trégor massif. This is NORTH BRITTANY because most of the information necessary to Bernard Auvray establish a stratigraphic sequence in the midst of (Translated by M. S. N. Carpenter) the Precambrian of Armorica comes from the Trégor. In addition, this information will prove Of all the different palaeogeographic and structural useful in constructing a model of geodynamic provinces which make up the Armorican massif, it evolution before the Palaeozoic for the northern is only in the Domnonean domain that Precambrian Armorican massif. terrains are well preserved with their original characteristics (see Fig. 1). The Trégor Massif—General Framework Within this domain several core-areas and massifs have been shown to exist, separated from each other The massif is a geological unit of limited size (ca. by narrow tectonic belts which have taken up most 40 km x 15 km) having a rectangular outcrop and of the deformation during the Hercynian orogeny situated in the most northerly part of Brittany. It is (Cogné, 1974). In fact, these ‘mobile belts’ were bounded on three sides by the Channel, giving an already active at the end of the Cadomian orogeny indented coastline which aids geological observations. since they delimit graben structures which corres­ The southern limit of the massif corresponds to the pond to Palaeozoic sedimentary basins. trace of a major E-W fault (Fig. 2; F2) known as Our geological understanding of the different the Trégorrois fault. This fault is particularly core-areas in the Domnonean domain is very apparent in the east of the Trégor, where its dis­ patchy, with many stratigraphic, petrological, placement during the Hercynian has juxtaposed Bri­ structural, and geochronological problems waiting overian rocks with Palaeozoic formations (Cambro- to be answered. This is particularly the case for the Ordovician Red Beds; Auvray et al., 1980a). Pentevrian massif, where probably the most com­ Geologically speaking, the Trégor can be divided plete Precambrian sequence is exposed but where into two sub-areas separated by another important reconnaissance studies and characterization of the E-W fault running parallel to the Trégorrois fault; rock types are perhaps the most poorly developed. this fault is known as the Tréguier-Lézardrieux For this reason, even if the Pentevrian was first fault (see Fig. 2; Fj). 416 Precambrian in Younger Fold Belts Fig. 1. Sketch map of the Domnonean domain. The different massifs or core-areas of Precambrian age are separated by Hercynian ‘mobile belts’. 1 —The Léon Massif; 2 — The Trégor Massif; 3 — The Penthièvre Massif; 4 — The Lower Normandy Massif; 5 — The Channel Islands Massif (after Cogné, 1974; in Vidal et al., 1981) The northern sub-area is composed essentially of Lower Proterozoic basement are to be found as plutonic crystalline rocks and metamorphic schists xenoliths/enclaves within the batholith (Auvray, and gneisses, the greater part of the outcrop being Chariot, and Vidal, 1980); the only evidence for occupied by the North Tregor Batholith dated at Palaeozoic phenomena in the area is also recorded 600-650 Ma (Adams, 1967; Auvray and Vidal, as various magmatic episodes (dolerite dyke swarms 1973; Vidal, 1976; Auvray, 1979). Fragments of a and plutonic activity in the form of monzonitic and CHANNEL Fig. 2. Geological and structural sketch map showing the major faults affecting the Trégor Massif. 1 —Ancient basement gneisses; 2—Volcano-sedimentary formation of l’Armorique-Trédrez; 3—The North Trégor Batholith; 4 —Volcanics and sediments of the southern sub-area of the Trégor; 5—Palaeozoic Red Bed formations (Bréhec-Plouézec-Plourivo); 6—Hercynian granites (in Auvray, 1979) Precambrian in the external zones o f the Variscides 417 Radiometric age in Ma Late-Stage N-S faulting 300 Emplacement of Ploumanac'h granitic complex Emplacement of Tredrez Granite 320-310 Mouvement along major E-W fault Emplacement of Plouaret-Begard batholith O LÜ < CL 330-440 Doleritic dyke swarms 470 Deposition of Red Bed formations with associated volcanism Alkali-granitic and rhyolitic igneous activity (granites, 530-540 microgranites and ignimbrites) 600-580 Dioritic intrusions (Keralain-Taurel) CADOMIAN II DEFORMATION C CO Deposition of Roche-Derrien sandstone and shale series ' U<1) > o (=Binic series) Eruption of Paimpol spilites Deposition of Treguier keratophyric tuffs (Locquirec tuffs 640-600 o 1^1o as well) oc Basal conglomerates with granodiorite clasts h—LÜ ooc Emplacement for Perros-Guirec-Br^hat batholith(granites and granodiorites) CL CL CADOMIAN I DEFORMATION Deposition of St Efflam greywacke formation < 1,000 L'Armorique-Trédrez volcano-sedimentary series (=Lanvollon volcano-sedimentary formation) Basement gneisses and orthogneisses (Tr^beurden and Port- > 1,800 Beni Gneisses) =Icartian or Pentevrian. (?) plutonic ac­ <D I . 3 -O O -H tivity at 2,000 and 1,800 Mb - I ZZ I Fig. 3. Chronology of events rcorded in the Tregor and adjoining areas (Petit Tregor and Baie de Saint Brieuc) (after Auvray, 1979) sub-alkaline granites; e.g. Trédrez granite, Plou- following stratigraphic succession (from base to top): manac’h ring complex described by Auvray, 1979 —Treguier keratophyric tuffs and Barrière, 1977 respectively). —Paimpol spilites The southern sub-area is made up of Upper —Roche-Derrien sediments (equivalent to the Binic Brioverian volcano-sedimentary formations with the series further south). 418 Precambrian in Younger Fold Belts Rare gabbro-dioritic intrusions of limited size are where they can be correlated with the Morguinen to be found in this southern half of the Trégor. gneisses and the Moulin de la Rive gneisses (Verdier, These intrusions are the local equivalents of much 1968). All these different fragments of gneissic larger granodioritic and dioritic intrusions found basement taken together constitute a NE-SW elsewhere in the Armorican Massif which have been oriented belt which seems to mark out the northern dated from St Quay-Portrieux (Vidal et al., 1971), limit of the Armorican Massif. This belt runs from Plévenon-Fort La Latte (Vidal et al., 1974) and in the Moulin de la Rive gneisses in the southwest, the Mancellian domain of Lower Normandy (Jonin through the Icart gneisses of the Channel Islands and Vidal, 1975). to the Cap de la Hague gneisses in the northern part A magmatic episode common to both sub-areas of the Cotentin peninsula (Adams, 1967, 1976; of the Trégor occurred during latest Proterozoic Roach, 1966, 1977; Leutwein et al., 1973; Cogné, times (<ca. 550 Ma ago) and is observed as ignimbrite 1959, 1962, 1974; Auvray and Vidal, 1973; Calvez flows in the south and as a microgranite/granite and Vidal, 1978; Auvray, Chariot, and Vidal, 1980; intrusive event in the north (Auvray and Vidal, Vidal et ai, 1981).
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