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The Cadomian in the North Armorican : a brief review

R. S. D'LEMOS, R. A. STRACHAN & C. G. TOPLEY Department of , Oxford Polytechnic, Oxford, OX3 0BP, UK

The North Armorican Massif (NAM) of ages of 2600-2000 Ma were initially obtained NW is the type locality of the late from the Icart granitic of 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 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 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 , 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 . However, complexes and syn-orogenic 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). 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 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, and structures and the geotectonic setting of the quartz 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 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 ~ Cadomian diorites ~ '~Sark Metamorphic belts of uncertain age Cadomian belts Jer ~es. NASZ [ ] Brioveriansuccession Foliated granites of Guernsey, & 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 ; 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 ; 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 , siltstones Leutwein (1968) and Leutwein et al. (1973) and mudstones, with minor conglomerates, provided the basis for early tectonic models impure 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. More recent geochronological the NAM. The stratigraphic subdivisions of studies, utilizing U-Pb zircon and high precision Barrois (1895, 1932, 1934), Graindor (1954, Rb-Sr whole-rock analyses, support a more ex- 1957) and Cogne (1962, 1970) have in common tensive time span of approximately 275 Ma, a subdivision into three major groups, namely from c. 700-650 to c. 425 Ma for Cadomian Lower (Xa), Middle (Xb) and Upper (Xc) magmatism (Pasteels & Dore 1982; A. M. Bland Brioverian. The Lower Brioverian was thought 1984, 1985; Peucat 1986; D'Lemos 1987a, b; to be represented by sandstones and conglom- Guerrot & Peucat 1990; Brown et al. 1990; erates at its base, overlain by basic volcanic Power et al. 1990a). rocks and fine-grained clastic rocks. The Middle Early Cadomian foliated quartz diorite com- Brioverian was characterized by / plexes occur in the Channel Islands (Guernsey, sequences with interbedded phtanites. Sark, Alderney), at La Hague, in Lower Roblot (1962), Graindor (1964), Cogne (1962) Normandy (Coutances) and around the Baie de and Jeanette & Cogne (1968) considered that St Brieuc (Fig. 1). Isotopic evidence suggests a the sandstones and conglomerates of the Upper probable age range of c. 700-580 Ma (Guerrot Brioverian rested unconformably on the earlier & Peucat 1990; Power et al. 1990b) for these Brioverian. quartz diorite complexes which occur in three More recent work suggests that these attempts distinct settings: emplaced into Icartian base- at subdivision are premature. The tectonically ment (Guernsey, Sark (?)), locally overlain un- segmented nature of the belt, the likelihood of conformably by the Brioverian (Penthievre rapid facies changes and the lack of chrono- complex) and intruded into the Brioverian stratigraphic markers all indicate that while local (Coutances). A U-Pb zircon age of 614 +- 13 Ma subdivision on formation level is possible, obtained from a unit of the Perros-Guirec regional correlations are problematic (Rabu et complex of the Tregor region suggests that at al. 1982, 1983; Cabanis et al. 1987; Strachan & least part of this multicomponent batholith was Roach 1990). Although there is no unequivocal also emplaced during this period (Graviou & field evidence for an intra-Brioverian uncon- Auvray 1985). Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

6 R.S. D'LEMOS ET AL

In the Channel Islands and at La Hague, late in composition from gabbro to granite (s.s.). Cadomian plutonic complexes were emplaced They have medium to high K contents and show during and - similarities to high K orogenic . On times. On Guernsey and at La Hague an im- Pearce type discrimination diagrams (Pearce et portant magmatic episode is recorded at c. al. 1984) the granitic rocks (>56% SiO2) plot 500 Ma (D'Lemos 1987a, b; Power et al. 1990a), mainly within the volcanic arc granite field with whilst on Jersey magmatism continued until some overlap of the more evolved members much later (A. M. Bland 1984, 1985), its final into the within plate granite field (Brown et al. manifestations being seen in the Jersey Dyke 1990). The geochemistry supports an origin for Swarm which may be as young as 425 Ma (Lees many of the rocks by derivation from mantle 1990). Late Cadomian magmatism in this area wedge melts which have undergone significant is typified by mixed-magma complexes which assimilation and fractional crystallization (AFC) commonly contain a wide range of compositions processes whilst pooled at the base of the from gabbro to granite (Topley et al. 1990; and during subsequent emplacement. Marked Power et al. 1990a). exceptions to this rule are the St Malo and Magmatism apparently coeval with Cadomian associated migmatite belts, and possibly also deformation is recorded by the St Malo, Dinan the Mancellian granites, where anatexis of and St Cast migmatite belts of NE Brittany juvenile Brioverian sediments took place to (Fig. 1). Contacts between these migmatite belts produce and diatexites. and adjacent Brioverian rocks are frequently Evidence of Cadomian extrusive igneous tectonic (Brown 1978; Brun & Bale 1990; activity is found within the Brioverian suc- Treloar & Strachan 1990). In the Rance Valley, cession. Thick sequences of basic submarine however, Brioverian phyllites pass transitionally pillow lavas are exposed in the Baie de St into upper facies metatexites and Brieuc, in the Tregor region and near Coutances diatexites which in turn grade into homogeneous in Lower Normandy (Roach et al. 1986, 1988, anatectic granites (Brown 1974; Brun 1975, 1990; Lees et al. 1987; Cabanis et al. 1987), and 1977; Treloar & Strachan 1990), demonstrating generally pre-date the deposition of Brioverian that the migmatites have been derived by the turbidites. Basic dykes which cut the Penthievre partial meltipg of Brioverian sediments. Field complex and the Icartian gneisses of Guernsey evidence indicates that leacogranites and are inferred by Lees et al. (1987) to have been anatectic granites inferred to represent late contemporaneous with this phase of basic fractions of the migmatite belt were emplaced magmatism. The tectonic setting of these lavas syn-tectonically into sinistral strike-slip shear has proved difficult to discriminate using geo- zones (Brown 1978; Treloar & Strachan 1990). chemical criteria, since they have been inter- Rb-Sr whole rock isochrons and U-Pb zircon preted on this basis as either continental flood ages indicate that anatexis occurred at c. 540 (Lees et al. 1987) or tholeiitic within-arc/ Ma (Peucat 1986). back-arc basalts (Cabanis et al. 1987). Andesites The Mancellian batholith of east Brittany and and rhyolites overlie Brioverian sediments on Lower Normandy (Fig. 1) comprises a number Jersey and at St Germain-le-Gaillard in Lower of largely undeformed bodies of dominantly Normandy, and an Rb-Sr isochron age of 533 _+ granodioritic to granitic composition, with only i6 Ma has been reported by Duff (1978) for the minor basic components (Jonin 1981; Georget Jersey volcanics. Auvray (1979) has argued that 1986; Brown & D'Lemos 1990). Conclusive the 'Tufts de Treguier' and the 'Ignimbrites de isotopic age data are not available, although Lezardrieux' of the Tregor region also comprise U-Pb monazite ages of c. 540 Ma from the a Brioverian volcanic succession correlatable Vire-Carolles granite (Pasteels & Dore 1982), with the volcanic rocks of Jersey and Normandy. a composite Rb-Sr whole-rock isochron for volu- Strachan & Roach (1990) have suggested how- metrically minor of 525 _+ 6 Ma ever, that these units mainly comprise high and an Rb-Sr whole-rock isochron age of 502 -+ level intrusives which are a marginal facies of 37 Ma for part of the Bonnemain granite (Jonin the Perros-Guirec complex and are not pri- & Vidal 1975) all indicate a late Cadomian age. marily extrusive in nature. The evidence for a Brown & D'Lemos (1990) have demonstrated widespread phase of 'late' Brioverian andesitic- that the Mancellian granites are geochemically rhyolitic is therefore questionable. similar to the anatectic granites of the St Malo There is general agreement that Cadomian migmatite belt, and may therefore have also magmatism occurred within a zone formed by anatexis of Brioverian sediments. setting, and available isotopic data imply that Overall, the Cadomian plutonic rocks exhibit this activity spanned the period c. 700-425 Ma. a clearly defined calc-alkaline trend and range Although an oversimplification of what was Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

THE CADOMIAN OROGENY: A REV1EW 7 probably a continuum of igneous activity, two with the regional foliation in the Brioverian. main periods of magmatism can be broadly Strachan & Roach (1990) have argued that the identified. The first is recorded by early parallelism of fabrics cannot alone be used Cadomian foliated plutonic complexes and as evidence for simultaneous deformation and Brioverian basic volcanics. The second is as- intrusion, and conclude that the timing of de- sociated with generally post-tectonic, often formation in this area has still to be clarified. mixed magma complexes, and andesitic- All workers are agreed however, that the c. rhyolitic volcanism. Further detailed isotopic 540 Ma St Malo migmatites were affected by investigations are needed to date more precisely an important deformational event. Whether these periods of magmatism and to constrain there are two quite distinct deformation events, the characteristics of the source regions for the at 590 and 540 Ma, or only one major event at . 540 Ma, or whether there was simply a con- tinuum of deformation over a long period Structural evolution of the Cadomian belt remains to be determined. The extent of Cadomian deformation on It is in the area between the Tregor region and Guernsey, Sark, Alderney and in the La Hague Cancale in Brittany (Fig. 1) that Cadomian area is difficult to evaluate as a result of the structures are best exposed and this area has general absence of Brioverian rocks. Planar and been the focus of several recent studies (Brown linear fabrics within the early Cadomian quartz 1978; Bale & Brun 1983, 1989; Bale 1986; Brun diorites are at least partly magmatic in origin & Bale 1990; Treloar & Strachan 1990; Strachan (Gibbons & Power 1975), implying that these & Roach 1990). Brioverian sedimentary and intrusions were emplaced syn-tectonically. The volcanic rocks are deformed by E-W to NE- apparent sub-parallelism of these fabrics with SW trending upright folds which carry a variably some structural elements in the adjacent Icartian developed axial-planar cleavage or schistosity. gneisses might imply that the basement in this Metamorphic grade varies from prehnite- area was at least locally reworked during pumpellyite facies to mid-amphibolite facies. Cadomian events. Detailed field studies are All workers are agreed that the Cadomian strain therefore needed to clarify the extent and nature was heterogeneous and concentrated in steep of Cadomian structures in this northern part of ductile shear belts such as those which are spec- the Armorican Massif. tacularly exposed at St Cast. Furthermore, it is clear that displacement across many of these shear belts was strike-slip and sinistral in nature Post-orogenic as demonstrated by a wealth of minor structures (e.g. Bale & Brun 1989; Treloar & Strachan Continental conglomerates and sandstones on 1990). Quite different interpretations have been Jersey, Alderney and in northern Brittany attached to these structures however. Bale & rest unconformably on various components of Brun (1983, 1989) have interpreted these shear the Cadomian belt (Fig. 1). Although these belts as steep lateral ramps to a series of SW- sediments have in the past been assigned a directed thrusts which they infer to be located variety of ages ranging from the Cambrian in poorly exposed areas inland. The thrusts to the (Barrois 1908; Cogne 1963; form the basis of a crustal thickening model for Bonhomme et al. 1966), there is now general the Cadomian belt of Brittany proposed by agreement that they are of Cambro-Ordovician Bale & Brun (1989). Treloar & Strachan (1990) age. Auvray et al. (1980b) obtained an Rb-Sr and Strachan & Roach (1990) prefer to interpret isochron age of 472 + 5 Ma from trachy- the evolution of the belt as being fundamentally andesites interbedded with the Plourivo- strike-slip in nature and only see limited evi- Plouezec red-beds in north Brittany, and trace dence of thrusting, which they suggest is con- fossils within the Cap Frehel red-beds are also sistent with local transpression within the consistent with an Upper Cambrian-Lower evolving strike-slip system. Ordovician age (B. H. Bland 1984). The Rozel The elucidation of the timing of deformation Conglomerate of Jersey rests unconformably on in different parts of the belt in Brittany requires the Jersey Volcanic Formation dated at 533 + further geochronological work. Bale & Brun 16 Ma (Duff 1978) and is itself cut by inter- (1983) have suggested that main phase de- mediate dykes dated at 427 -+ 13 Ma by Adams formation in the Baie de St Brieuc area ac- (1967) using the K-Ar method. companied emplacement of the Fort de la Latte Sedimentological analysis indicates that these quartz diorite at c. 590 Ma, based on the paral- red beds were mainly deposited in alluvial fan- lelism of igneous foliations within this body braided channel systems which drained a series Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

8 R.S. D'LEMOS ET AL

of E-W trending basins. dispersal have resulted in the partial melting of Brioverian was generally towards the Cambrian palaeo- sediments and the formation of the St Malo shoreline in lower Normandy (Went & Andrews migmatites and the Mancellian batholith at c. 1990). The parallelism of the post-orogenic 540 Ma. A similar model is proposed for the basins with the structural grain of the Cadomian evolution of the Cadomian belt in lower belt implies a possible origin by the reactivation Normandy (Dupret et al. 1990), although here of Cadomian shear zones and faults. Basal the back-arc basin is closed by subducting the red bed sequences in Lower Normandy are basin lithosphere south beneath Mancellia. succeeded by shallow-marine sandstones and Detailed discussion on various aspects of this carbonates of Cambrian age (Dore 1972). model is presented in Lees et al. (1987), Shufflebotham (1990), Treloar & Strachan Geotectonic setting of the Cadomian belt (1990), Strachan & Roach (1990) and Brown & D'Lemos (1990). There is general agreement that the Cadomian An alternative model is proposed by Strachan belt of the North Armorican Massif comprises a et al. (1989) and Treloar & Strachan (1990) series of calc-alkaline volcanic arc complexes who argue that the North Armorican Massif and marginal basin sediments which formed comprises a series of displaced which in an Andean-type continental margin above a were amalgamated along sinistral strike-slip major southerly-dipping subduction zone shear zones at c. 540 Ma. They interpret the (Graviou & Auvray 1985; Cabanis et al. 1987; Cadomian belt in terms of three major tectonic Graviou et al. 1988; Strachan et al. 1989; Brown units, the St Brieuc, St Malo and Mancellian et al. 1990; Dupre et al. 1990; Rabu et al. 1990). terranes, each representing displaced segments The precise location of this subduction zone is of the subduction zone complex. Regional de- unclear; whilst Lefort (1977) suggested that formation is thought to reflect oblique sub- it was represented by the duction and collision of volcanic arcs and magnetic anomaly, other workers have con- marginal basins along a continental margin. No sidered that there is simply insufficient evidence major crustal over-thickening is envisaged to substantiate this claim (Lees et al. 1987; during the Cadomian orogeny. Formation of Strachan et al. 1989). The geotectonic setting of the migmatite belts of NE Brittany is thought to the component parts of the Cadomian belt and have been initiated in an extending marginal the mechanism by which they were juxtaposed basin with high heat flow, and to have continued is controversial, and two major tectonic models during sinistral transpression with the syn- have been proposed for the evolution of the tectonic emplacement of anatectic granites. If belt. these workers are correct in their supposition A crustal thickening model for the Cadomian that Cadomian deformation is fundamentally belt of north Brittany has been proposed by strike-slip in nature then this would invalidate Bale & Brun (1983), Graviou & Auvray (1985), orthogonal sections across the belt (e.g. Cabanis Cabanis et al. (1987); Graviou et al. (1988) and et al. 1987). A strong argument against any Rabu et al. (1990). They propose that the major crustal thickening during the Cadomian Perros-Guirec granite complex of the Tregor orogeny is provided by the present-day normal region, and its possible along strike correlatives crustal thickness of the North Armorican in the Channel Islands and Normandy, represent Massif, combined with only low metamorphic an active volcanic arc, possibly located on grade at the existing erosion level over much of Icartian basement, situated to the northwest of the Cadomian belt (Brown et al. 1990). a back-arc basin floored by oceanic-type crust. This back-arc basin, they suggest, is represented by the Brioverian basic volcanic rocks and Conclusions turbidites in the Baie de St Brieuc, with the The evolution of the Cadomian belt of the Yffiniac and Belle-lsle-en-Terre metagabbros North Armorican Massif is most convincingly interpreted as the obducted remnants of the interpreted in terms of an Andean-type orog- back-arc basin crust (cf. Bale & Brun 1983 and eny. The events recorded within this region Peucat et al. 1981). Following the structural reflect a continuum of tectonic and magmatic work of Bale & Brun (1983) it has been proposed processes within an active plate margin possibly that this arc-basin complex was thrust to the located along the southern margin of the SW at 590 Ma over a 'continental' domain evolving lapetus Ocean. There is no evidence represented by the Brioverian rocks of the for -continent collision. Although St Malo and Mancellian areas. The crustal thick- understanding of the evolution of the Cadomian ening consequent upon thrusting is thought to belt has progressed considerably in recent Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

THE CADOMIAN OROGENY: A REVIEW 9 years, a number of topics still require further ships between magmatism and deformation and study. In particular, precise isotopic studies are the timing of will allow currently needed to date early Cadomian magmatic proposed tectonic models to be assessed criti- activity and to elucidate the nature of the cally and refined, and facilitate comparison with source regions for Cadomian magmatism as a related parts of the Cadomian-Avalonian belt whole. Detailed investigation of the relation- elsewhere in the North Atlantic region.

References ADAMS, C. J. D. 1967. A geochronological and related Jersey, with particular reference to the South-West isotopic study of rocks from North-western France Granite Complex. PhD thesis, CNAA (Oxford and the Channel Islands (United Kingdom). DPhil Polytechnic). thesis, University of Oxford. BLAND, B. H. 1984. Arumberia Glaessner & Walter, 1976. Geochronology of the Channel Islands a review of its potential for correlation in the and adjacent French Mainland. Journal of the region of the Precambrian-Cambrian boundary. Geological Society, London, 132, 233-250. Geological Magazine, 121, 625-633. AUVRAY, B. 1979. Gendse et dvolution de la cr6ute BONHOMME, M., COGNE, J., LEUTWEIN, F. & SONET, continentale dans le Nord du Massif Armoricain. J. 1966. Donn6es nouvelles sur l'fige des series Th6se Docteur es-Sciences Naturclles, Rennes. rouges du golfe normano-breton. Compte rendus , CHARLOT, R. ~ VIDAL, P. 1980a. Donn6es de l'Academie des Sciences, Paris, 262, 606-609. nouvelles sur le Prot6rozoique inf6rieur du BROWN, M. 1974. The Petrogenesis of the St Malo domaine nord-Armoricain (France): fige et sig- Migmatite Belt, North-Eastern Brittany, France. nification. Canadian Journal of Earth Sciences, PhD Thesis, University of Keele.

17, 532-538. -- 1978. The tectonic evolution of the Precambrian --, MACE, J., VIDAL, P. & VAN DER Voo 1980b. rocks of the St Malo region, Armorican Massif, Rb-Sr dating of the Plouezec volcanics, N France. Precambrian Research, 6, 1-21.

Brittany; implications for the age of red beds -- & ROACH, R. A. 1972. Precambrian rocks South ('Series Rouge') in the northern Armorican of Erquy and around St Cast, C6tes-du-Nord. Massif. Journal of the Geological Society, Nature (Physical Sciences), 236, 77-79.

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volcanism in the Northern Massif Armoricain, graphique, g6ochemique et structurale le long France. In: PHARAOH, T., BECKINSALE,R. D. & d'une traversale en baie de Saint-Brieuc. Rapport PACKARD, D. T. (eds) Geochemistry and Mineral- BRGM et Document BRGM, No 66, (1983), isation of Proterozoic Volcanic Suites'. Geological Orleans. Society, London, Special Publication, 33, , & 1983. Nouvelles propositions 503-523. pour la lithostratigraphie du Brioveri6n (Pro- 1990. The geochemical character of late trrozoique sup6rieur) et pour l'6volution gro- Cadomian extensional magmatism in Jersey, dynamique cadomienne en baie de Saint Brieuc Channel Islands. In: D'LEMOS, R. S., STRACHAN, Massif. Bulletin de la Socidtg Gdologique de R. A. & TOPLEY, C. G. (eds) The Cadomian France, 25, 615-621. Orogeny. Geological Society, London, Special RABU, D., CHANTRAINE, J., CHAUVEL, J. J., Publication, 51,273-291. DENIS, E., BALE, P~ & BARDY, P. H. 1990. LEEORT, J. P. 1977. 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