Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021 The Cadomian orogeny in the North Armorican Massif: a brief review R. S. D'LEMOS, R. A. STRACHAN & C. G. TOPLEY Department of Geology, Oxford Polytechnic, Oxford, OX3 0BP, UK The North Armorican Massif (NAM) of ages of 2600-2000 Ma were initially obtained NW France is the type locality of the late from the Icart granitic gneiss of Guernsey Precambrian Cadomian orogenic belt. The (Adams 1967, 1976) and similar lithologies NAM largely escaped reworking during the from La Hague (Leutwein et al. 1973). U-Pb Upper Palaeozoic Variscan cycle, with the result zircon dating has confirmed the antiquity that earlier geological relationships are com- of the Icart gneiss (2018 -+- 15 Ma, Calvez & monly preserved intact. A large part of the Vidal 1978) and established the existence Cadomian belt of the NAM comprises variably of similar aged gneisses as enclaves within deformed and metamorphosed supracrustal the Cadomian Perros-Guirec complex of the rocks of the Brioverian succession (Barrois Tregor region (1790 -+ 10 Ma, Auvray et al. 1895, 1896, 1908; Graindor 1957; Cogne 1962; 1980a). Gibbons & Power (1975) have argued Le Corre 1977; Rabu et al. 1982, 1983; that the gneisses of Sark are also a part of Chantraine et al. 1982). The Cadomian orogeny, this pre-Cadomian basement, although these which takes its name from Cadomus (Caen) in rocks remain undated by isotopic methods. The Normandy, was defined by Bertrand (1921) as extent of these basement gneisses beneath the the late Precambrian orogeny which in the NAM Cadomian belt is uncertain. Vidal et al. (1981) resulted in the folding and uplift of the considered that the low initial S7Sr/86Sr ratios Brioverian succession, prior to the deposition (0.702-0.716) displayed by Cadomian in- of Lower Palaeozoic red-bed sequences. Events trusions in the NAM precluded the presence which have been ascribed to the Cadomian of extensive areas of continental basement at orogeny span the period c. 700-425 Ma, during depth. The NE-SW trending belt of basement which time the Brioverian was heterogeneously gneisses exposed between La Hague and the deformed and metamorphosed, and intruded Tregor region has accordingly been interpreted by calc-alkaline igneous complexes. The evol- by some workers (e.g. Rabu et al. 1990) as a ution of the belt has been interpreted in terms fragment of c. 2000 Ma Gondwanan basement of the gradual cratonization of calc-alkaline arc detached from the African craton. However, complexes and syn-orogenic sediments in an initial 87Sr/S6Sr ratios may not be used to fully Andean-type continental margin (Brown et al. constrain the extent of crustal involvement in 1990; Dupret et al. 1990). magma genesis (Brown et al. 1990) and large This contribution reviews briefly the geology areas of continental basement may therefore lie of the Cadomian belt of the NAM (Fig. 1) and beneath the Cadomian belt of Brittany and outlines some of the more controversial debates Normandy. Sm/Nd studies currently in progress concerning the distribution and status of pre- (R. S. D'Lemos pers. comm.) may help to Cadomian basement, the stratigraphy of the resolve this problem. Brioverian succession, the nature of Cadomian A complex sequence of variably deformed magmatism, the kinematic significance of major calc-alkaline granitoids, amphibolites and structures and the geotectonic setting of the quartz diorites exposed on the east side of the component parts of the belt. Baie de St Brieuc (Fig. 1) were formerly thought to have a minimum age of c. 1200-900 Ma (Leutwein et al. 1968; Roach et al. 1972), and to Distribution and status of pre-Cadomian represent the type area of a pre-Cadomian 'Pentevrian' basement (Cogne 1959, 1962, basement 1964). The westernmost components of this The existence of small areas of pre-Cadomian igneous complex are probably overlain uncon- basement in the Channel Islands, at La Hague formably by the Brioverian succession in this and in the Tregor region (Fig. 1) has been area (Brown & Roach 1972; Roach et al. 1986, confirmed on the basis of isotopic evidence. 1988; Strachan & Roach 1990; see however Poorly constrained Rb-Sr whole rock isochron Bale & Brun 1983). Isotopic work has, however, From D'Lemos, R. S., Strachan, R. A. & Topley, C. G. (eds), 1990, The Cadomian Orogeny, Geological Society Special Publication No. 51, pp. 3-12. Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021 4 R.S. D'LEMOS ET AL Alderney<~ La Hague Mesozoic sediments Variscan intrusions • hs.~nexEngishC i ~riu c~urg Palaeozoic successions Guernsey Cadomian granites ~ Cadomian diorites ~ '~Sark Metamorphic belts of uncertain age Cadomian migmatite belts Jer ~es. NASZ [ ] Brioveriansuccession Foliated granites of Guernsey, Alderney & La Hague Penthi~vre complex I Icartian gneisses ~ ..,~Tregor region ~ ,::'~:s '~ ~ :-:.:.::: ::::::::: :~ _ . : .... CF - SM Cn - " ~" " "'-~" N 7. I I ~iiii!i~' ,'~":~,~?::-:i::: Fig. 1. Geological map of the North Armorican Massif. NASZ, North Armorican Shear Zone; SASZ, South Armorican Shear Zone; Ct, Coutances; VC, Vire-Carolles Granite; Bn, Bonemain Granite; Dn, Dinan; RV, Rance Valley; Cn, Cancale; SM, St Malo; SC, St Cast; CF, Cap Frehel; FdlL, Fort de la Latte quartz diorite; Yf, Yffiniac; BdSB, Baie de St Brieuc; BI, Belle-Isle-en-Terre. demonstrated that much of Cogne's Pentevrian prior to deposition of the local Brioverian. Bale belt is significantly younger than originally & Brun (1983), however, consider that the thought. The Fort de la Latte quartz diorite, Brioverian conglomerates are not proximal in which forms a large part of the Pentevrian origin and were derived from a now unexposed belt (Fig. 1), has yielded a U-Pb zircon age of arc; they interpret the Penthievre complex as 593 + 17 Ma (Vidal et al. 1974). The affinities the basal segment of a Cadomian ophiolite of the remainder of the complex, which has which was obducted from a back-arc basin been collectively referred to as either the during regional deformation. Irrespective of the Penthievre complex (Strachan et al. 1989; precise affinities of these rocks most workers Shufflebotham 1990; Strachan & Roach 1990) now agree that Cogne's Pentevrian belt was or the Langeux-Yffiniac complex (Guerrot emplaced at various stages during the Cadomian & Peucat 1990), are controversial. Basal cycle and does not therefore constitute pre- Brioverian conglomerates in the Baie de Cadomian basement sensu stricto. Other rocks St Brieuc contain granitoid boulders which formerly assigned to the 'Pentevrian' basement, Shufflebotham (1987) has correlated with parts such as the migmatite belts of NE Brittany of the Penthievre complex. U-Pb zircon ages of (Brown et al. 1971; Brown 1974, 1978) and the 667 +- 4 Ma and 656 -- 5 Ma obtained from high grade rocks on the west side of the Baie de these boulders (Guerrot & Peucat 1990) have St Brieuc (Ryan & Roach 1975), are now also led several workers (e.g. Strachan et al. thought to have formed during the Cadomian 1989; Strachan & Roach 1990; Shufflebotham orogeny (Brun 1975; Peucat 1986; Treloar & 1990) to infer an early Cadomian age for the Strachan 1990; Strachan & Roach 1990; Brown Penthievre complex which they argue represents & D'Lemos 1990). The evidence for a wide- a volcanic arc complex uplifted and eroded spread pre-Cadomian 'Pentevrian' basement in Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021 THE CADOMIAN OROGENY: A REVIEW 5 North Brittany is therefore lacking. formity separating lower and upper successions The term 'Pentevrian' has been used pre- (Bale & Brun 1983), the concept of a two-stage viously by many workers to refer collectively Brioverian stratigraphy with an 'upper' se- to not only the supposed basement of North quence derived from the erosion of a 'lower' Brittany but also the c. 2000-1800 Ma gneisses sequence is still a central theme of some recent of the Channel Islands, La Hague and the papers (e.g. Dupret et al. 1990; Rabu et al. Tregor region. Given that the type area of 1990; and references therein). This hypothesis Cogne's 'Pentevrian' basement has now been rests on the presence within rocks assigned to demonstrated to be of Cadomian age, continued the 'upper' sequence of phtanite clasts thought usage of this term is inappropriate. Therefore to have been derived from the erosion of a we propose that the term 'Pentevrian' should be lower phtanite-bearing succession which was abandoned entirely, and that the basement subject to earlier deformation and metamor- gneisses of c. 2000-1800 Ma age should be phism. Further petrographic and field studies referred to as 'Icartian'. are needed to clarify this. Brioverian stratigraphy Nature of Cadomian magmatism Brioverian supracrustal rocks are widespread Extrusive and intrusive calc-alkaline magmatism throughout the North Armorican Massif. Micro- of Cadomian age is widely recorded within the palaeontological evidence is consistent with a North Armorican Massif. Field, petrographic, Vendian age (Downie 1975; Chauvel & Mansuy geochemical and isotopic studies of these rocks 1981), although there is considerable uncer- have provided valuable insights into the nature tainty as to the more precise period of sedimen- and evolution of the Cadomian belt. The Rb-Sr tation within the period c. 750-540 Ma. The and K-At isotopic data of Adams (1967), dominant lithologies are sandstones, siltstones Leutwein (1968) and Leutwein et al. (1973) and mudstones, with minor conglomerates, provided the basis for early tectonic models impure limestones and carbonaceous quartzites which identified Cadomian magmatism as oc- (phtanites). Sedimentological studies suggest curring between c. 690-500 Ma. The geological deposition in a variety of submarine fan environ- meaningfulness of some of the 'dates' provided ments (Ryan 1973; Squire 1974; Denis & by these early studies is, however, questionable Dabard 1988). Basic and acid volcanic se- due to shortcomings in sampling and analyti- quences occur sporadically over the whole of cal techniques.