Journal of the Geological Society, London, Vol. 149, 1992, pp. 151-155. Printed in Northern Ireland

Discussion on the timing and kinematics of Cadomian deformation (Northern Brittany)

Journal, Vol. 147, 1989, pp. 423426 Special Publication, No. 51, 1990, pp. 133-150 Special Publication, No. 51, 1990, pp. 151-168

Jean-Pierre Brun writes: Strachan et al. (1989), Strachan & comments (e.g. ‘The St Brieuc Thrust where it is exposed at the Roach (1990), Treloar & Strachan (1990) as well as Strachan Plage de Chateau Serein is a late subvertical fault of limited et al. (1990) givean interpretation of Cadomian deformation in displacement which displays none of the deformation features Northern Brittany in terms of a single event of strike-slip tec- to be expected if this were the synmetamorphic crustal scale tonics at 540 Ma andhave proposed a model of ‘terrane amal- structure inferred by Bale & Brun’; Strachan et al. 1990, p. 81). gamation’ which I find in strong contradiction with field and We are obviously aware of the distinction between brittle and geochronological data (Bale & Brun 1983, 1989; Brun & Bale ductilestructures and have never maintained that Chateau 1990). Our analysis takes into account regional-scale evidence Serein (cf. location on fig. 1, Bale & Brun 1989) might be a (geological maps, strain patterns and geophysical data) and type-locality for observing the characteristics of the St Brieuc all available and reliable geochronological data. Our result- Thrust. It is located along what we consider to be a lateral ramp ing interpretation stresses that Cadomian deformation started and not the thrust itself. The ‘deformation features to be ex- around 590-580Ma ago, combining thrusting and sinistral pected’ have been described in detail by Bale & Brun (1989) wrenching shear zones, and ended at 540 Ma with the uplift of (maps of foliation and lineation, strain measurements, shear the St Malo migmatite. I would like to discuss three main criteria, texture goniometry analyses . . .). Our analyses and points that, for clarity, are dealt with under the following interpretation can be summarized as follows. The St Brieuc headings. Thrust system has an arcuate shape(fig. l), joining the NW-SE- oriented front of the NE-SW-trending lateral ramp. The whole Deformation timing. Bale and myself consider that the Coet- thrust pile displays two major thrust slices. The lower one is mieux-Fort la Latte intrusion, dated at 593 Ma (Vidal et al. composed, from base to top, of coarse-grained amphibolites 1974), is synkinematic and provides an age estimate for early derived from gabbros and diorites (Yffmiac Formation), fine- Cadomian deformation in the St Brieuc area. A similar situ- grained amphibolites with strongly deformed pillow lavas ation is observed in Normandy, where the Coutance diorite (Lanvollon Formation) and metasediments (LeguC Forma- gives an age of 584Ma (Guerrot & Peucat 1990). Strachan & tion). Several serpentine pods occur along the basal contact. Roach (1990, p. 144) write: ‘We do notbelieve that parallelism The rocks are strongly deformed and display a SW- or WSW- of an igneous fabric with a regional cleavage can alone demon- trending stretching lineation. The upper thrustslice iscomposed strate synchroneity of intrusion and regional deformation’. of only two of the previous formations which correspond, from This pluton is strongly elongated with an aspect ratio of 1 :4 base to top, to fine-grained amphibolites with moderately de- for the visible part, and presents a well-defined L-S fabric that formed pillow lavas (Roselier formation, equivalent of Lan- developed in a sub-solidus state (‘magmatic’ s.1.) parallel to vollon) and metasediments (Binic Formation, equivalent of strongly deformed basic inclusions. The pluton and its internal Legue). The rocks in this upper thrust unit are moderately to fabric are both parallel to the regional structural grain. How weakly deformed and less metamorphosed than those of the can Strachan and his colleagues explain such a structural pat- lower slice. Stretching lineations trend SW or WSW. In the tern if not in terms of a synkinematic emplacement? Despite the whole pile,shear criteria indicate a top tothe SW or WSW sense obvious deformationof this pluton, the Rb-Srsystem shows no of shear and strain intensity increase towards the base. resetting towards a younger (c. 540Ma) age, and therefore it The basal units of the St Brieuc Thrust system are brought appears that early Cadomiandeformation occurred in the into contact with the Lamballe formation (pelites, pelitic range 590-580 Ma. This accordswith recent 40Ar/39Ardating of sandstones and black cherts) which are strongly migmatized in metamorphic and magmatic amphiboles that indicates ‘a main the GuingampBelle Isle en Terre area. The paragneisses, mig- regional tectonothermal event prior to c. 570 Ma’ (Brown et al. matites and associated leucogranites present unequivocal shear 1990~). criteria indicating a top to the SW or WSW sense of shear. To the SE, the St Malo migmatite belt is separated from the St Kinematics of deformation. Bale & Brun (1989) and Brun & Brieuc units by the St Cast sinistral shear zone which runs Bale (1990) describe the kinematics of Cadomian deformation parallel to the basal lateral ramp of the St Brieuc thrust sys- in terms of thrusting and wrenching (fig. 1). Strachan and col- tem. The migmatites (dated at 549 Ma; Peucat 1986) and para- leagues concentrate their criticism on thrusting and ignore our gneisses are first thrust toward the S-SSW and then strongly analysis of wrenching. Their criticisms range from puredenial sheared along the St Cast and Cancale sinistral shear zones. (e.g. ‘There is no field evidence to support the suggestion that Finally, it is important tonote that the contour geometry of the major crustal units were amalgamated by thrusting’, Strachan frontal and lateral ramps of the St Brieuc thrust system are & Roach 1990, p. 148) to fallacious citations and misleading well marked by strong magnetic and gravimetric anomalies,

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and picked out by weakly dipping reflectors on the SWAT 10 I am very grateful to all my colleagues and students involved in the seismic line (Lefort & Bardy 1987; Brun & Bale 1990). study of the Cadomian of North Brittany: S. Aitomar, B. Auvray, P. My main criticisms of Strachan and colleagues are as Bale, M. Ballevre, P. Bardy, R. Charlot, J. J. Chauvel, J. Cogni, D. follows. Gapais, V. Genestier, P. Graviou, E. Hallot, C. Le Corre, J. P. Lefort, P. Maillet, H. Martin, J. J. Peucat, P. Vidal from the Rennes 1 Univer- (1) Their 540 Ma strike-slip shearzone interpretation sity and J. Chantraine, E. Egal, E. Le Goff, C. Guerrot and D. Rabu ignores regional-scale structures as they are displayed on geo- from the B.R.G.M. As a group and as individuals, all of them have logical and geophysical maps. These are first order structures contributed to some extent to the ideas that I defend here. Several of that must be considered in making a regional scale interpreta- them share my opinions and could have written a rathersimilar discus- tion. I would suggest that Strachan and his colleagues have sion. Thanks are dueto M. S. Carpenter for improvements of English spent little time mapping inland, leading them to write: ‘The style and to W. Gibbons for very constructive suggestions. poorly exposed Yffiniac metagabbros’ (Strachan et al. 1990, p. 72) or ‘Difficulties in accepting this model arise from the 10 December 1990 almost complete lack of inland exposures’ (Strachan & Roach 1990, p. 145). The 1/50.000 cooperative mapping programme undertaken by the B.R.G.M. and Rennes 1 University demon- R. A. Strachan, M. Brown, R. S. D’Lemos, R. A. Roach & P. J. strates that,even in this areaof poor exposure, it is possible to Treloar reply: The Short Paper of Strachan et al. (1989) sum- find enough outcrops to make precise maps. marizes the work of Strachan & Roach (1990), Treloar & (2) On the coastal maps of Strachan & Roach (1990) and Strachan (1990) and Brown et al. (1990a). We interpret the Treloar & Strachan (1990), only ENE-WSW-trending and Cadomian belt of the North Armorican Massif as a collage of shallow-plunging stretching lineations are portrayed. There is variably displaced terranes produced by the amalgamation at no trace of NNE-SSW-trending lineations. One exception c. 540Ma of Cadomian continental arcs and marginal basin exists for a single area on fig. of Treloar & Strachan (1990). 4 complexes by sinistral transpression along a continental mar- But this single recognized thrust is interpreted as due to mig- gin above a subduction zone. This model contrasts with that of matite diapirism during strike-slip shearing. Evidence for SW- Bale & Brun (1983,1989) and Brun & Bale (1990)who propose trending lineationshas been known for a long time (Brun 1977 that thrusting at c. 590-580 Ma resulted in the obduction of a in the St Malo migmatite belt; Bale & Brun 1983 for the St ‘back-arc basin’ (Baie de StBrieuc) southwestwards over a Brieuc thrust system). ‘continental margin’ (St Malo-Mancellia). The resultant (3) Strachan et al., whilst failing to recognize the importance crustal thickening is thought by them to have led to partial of thrust-related structures, emphasize the presence of steep melting of Brioverian rocks at depth and tothe formation of the sinistral strike-slip zones. They fail, however, to give fair St Malo migmatites and the Mancellian granites (cf. also recognition to the fact that these strike-slip structures have Graviou & Auvray 1985; Graviou et al. 1988). We thank Brun been known for some time (e.g. Brun 1979, 1981; Bale& Brun for his discussion of our papers and model, and welcome the 1989; Brun & Bale 1990; Aitomar 1982; Bale 1986). opportunity to debate the various points of contention outlined Terrane amalgamation. Strachan and his colleagues describe by him. the Cadomian of North Brittany as a collage of three terranes, namely St Brieuc, the St Malo and the Mancellian terranes. Timing of deformation. It is agreed that themain regional According to Treloar & Strachan (1990): ‘Each of these has a Cadomian deformationin the St Maloregion (St Malo terrane) distinct tectonothermal and magmatic history and thus con- of NE Brittany is broadly synchronous with the formation of forms to current definitions of a terrane’. This definition, how- the c. 540Ma St Malo migmatite belt (Brun & Bale1990; ever, allows a considerable freedom of interpretation. Stra- Treloar & Strachan 1990). The timing of Cadomian deforma- chan et al. (1990, p. 87)write: ‘In this contribution we stress the tion further west in theBaie de St Brieuc (St Brieuc terrane) has differences in the Cadomian magmatic and tectono-thermal been a matter for discussion. Bale & Brun (1983, 1989) have histories apparent between three terranespresent in North Brit- interpreted the parallelism of igneous fabrics within the Fort tany, each of which represents a displaced segment of the con- de la Latte quartz diorite with the regional structural grain to tinental margin arc complex. There is insufficient evidence at indicate that emplacement at 593 f 15 Ma (U-Pb zircon, present to define the original palaeogeographic and relative Vidal et al. 1974) accompanied regional deformation in this settings of each of these blocks’. They concentrateon ‘differen- part of the belt. Near Erquy and onthe west side ofthe Baie de ces’ instead of trying to elucidate ‘similarities’ between la Fresnaye the intrusion is in faulted contact withweakly domains; one of the main common features is thrusting, which cleaved, sub-greenschist facies Brioverian volcanic rocks which they totally overlook. Their so-called ‘terranes’ are very simi- do not display any signs of contact metamorphism (Shufle- lar to the ‘domains’ identified by Cogne (1959,1962, 1974, botham 1987; Roach et al. 1990; Strachan & Roach 1990). 1979) which have constituted the fundamentalbasis for modern Given the relatively poor exposure which does not allow a studies. The so-called ‘tectonothermal histories’ have been precise three-dimensional picture to be constructed of the summarized in full detail and interpreted in clear and under- foliations within both the intrusion and the wall rocks, and the standable. palaeogeographical terms by Rabu et al. (1983, lack of any cleavage

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this area. Inview of these field relations, Strachan et al. (1989) features consistent with its supposed status, and at least com- argued that the timing of deformation in the StBrieuc area was parable with other shear zones interpreted by Brun & Balk as not unequivocally resolved, and could possibly be as young as lateral ramps (e.g. St Cast). The lack of any such features at c. 540 Ma. Plage de Chateau Serein suggests that this contact is not a A precise lower limit for regional deformation in the St major tectonic boundary, but simply a slightly sheared intru- Brieuc area may be provided by the U-Pb zircon age of sive contact. 587?*, Ma obtained by Guerrot & Peucat (1990) for the A more likely location for a majortectonic boundary is the amphibolite exposed in the quarry at LaCroix Gibat, although nearby Fresnaye-St Castshear zone system (Treloar & therelationship between these rocks andother units is Strachan 1990) which separates the StBrieuc and St Malo ter- unknown due to poor exposure. An upper limit is provided ranes. Leucogranites which have been interpreted as late frac- by recently acquired 40Ar/39Ar mineral cooling ages of tions of the c. 540 migmatite melt (Brown 1978) are emplaced c. 575-565 Ma obtained from (1) metamorphic amphiboles in syn-tectonically into steep, sinistral shear zones at StCast foliated meta-basic intrusions, (2) igneous amphiboles in the (Gapais & Bale 1990;Treloar & Strachan 1990), thus constrain- late to post-tectonic St Quay quartz diorite, and (3) recrystal- ing the timing of final amalgamation across this boundary. lized muscovites within the aureole of the St Quay intrusion (3) There is a lack ofprimafacie outcrop evidence for thrust- (Dallmeyer et al. 1991~).These provide unequivocal evidence ing in the Baie de St Brieuc region. The cross-sections drawn by that regional deformation and metamorphism within the Balk & Brun (1983, 1989) and Brun & Balk (1990) along the southern part of the Baie de St Brieuc occurred prior to west side of the Baie de St Brieuc imply that all Brioverian c. 570Ma,. and thus significantly earlier than the c. 540Ma units are separated by a series of listric thrusts which at surface tectonothermal event recorded in the St Maloregion. Whether dip c. 6WO"in a northerly direction, sub-parallel to beddingl these are two entirely separate and kinematically unrelated foliation within the supposed thrust sheets. Whilst we are pre- events, or whether they represent different stages in a single pared to accept a degree of interpretative licence, in our opinion regionally diachronous accretion event at present is unknown. these cross-sections are highly schematic and may mislead, because contacts between the major Brioverian metavolcanic Kinematics of deformation. The Cadomian structures of the and metasedimentary units are either only poorly exposed or North .Armorican Massif have been the focus of much recent unexposed, and most are inferred by Strachan & Roach (1990) research, and we acknowledge the major contributionsmade in to be subvertical, brittle faults. In places, these faults are co- this area over the last 10 years by Brun and his co-workers. We incident with shear zones (e.g. north of Martin Plage), but in apologize to those who, because of space limitations, were not other places there is no indication of abnormally high ductile cited in our Short Paper. Full referencing of previous work in or brittle strain (e.g. north side of Plage de St Laurent). Foli- the region is found in Strachan & Roach (1990), Treloar & ation and lithological banding within the Brioverian rocks Strachan (1990) and Brown et al. (1990~). between Cesson and Martin Plage (an across-strike distance of The steep strike-slip ductile shear zones exposed along the c. 7-8 km) are dominantly subvertical or steep to the south, North Brittany coastline (e.g. Cancale, StCast, Bonaparte and certainly not moderate to the north as depicted by Balk & Plage) are interpreted by Treloar & Strachan (1990) as first- Brun (1983, 1989).Younging evidence at Martin Plage suggests order structures which reflect (polyphase) transpression during that the repetition of the Lanvollon Volcanics at the Pointe de the Cadomian orogeny. Displacements across dip-slip shear Roselier results from upright folding, and not thrust stacking zones at Martin Plage and along the River Rance are thought (Strachan & Roach 1990; Roach et al. 1990; cf.also Rabu et al. to have occurred synchronously with strike-slip movements as 1983). a result of strain partitioning in an oblique convergent setting. (4) The available geophysical datacannot be used to This model is at variance with that of Bale & Brun (1983, 1989) support exclusively athrusting model at the expense of a and Brun & Balk (1990) who interpret the same structures as transpressional model. Displacement directions cannot be lateral ramps to a series of southwest-verging frontal thrusts derived unambiguously from the dip direction of magnetic and inferred to exist in poorly exposed inland areas. We note that gravimetric anomalies and seismic reflectors, given the com- Brun & Balk (1990 p. 99) acknowledge that '. . . an interpreta- plex array of transport directions to be anticipated within zones tion in terms of a strike-slip duplex . . . would also be possible of oblique convergence (cf. Oldow et al. 1990). The origin of (cf. Treloar & Strachan 1990)'. In response to Brun's comments the north-dipping reflectors observed on the SWAT lines is we make the following points. unclear. The only structures exposed on the coast which have a comparable orientation are moderately-inclined semi-ductile (1) The first order structuresin the region are,in our to brittle fault zones such as those exposed at Martin Plage, opinion, those which are exposed, and any model should be Port Goret and Bonaparte Plage (cf. Strachan & Roach 1990) based on these rather than ongeophysical data andconjectural and which clearly post-date the pre-570Ma Cadomian struc- structures inferred to exist inland where, despite Brun's asser- tures. These faults occur within major litho-tectonic units and tions to the contrary, interpretationof geological relationships we do notinterpret them to represent major displacement is necessarily speculative. zones. Those at Martin Plage and Port Goret have a reverse (2) Irrespective of whether the contact between the Fort de sense of movement. The age of these faults is uncertain. They la Latte quartz diorite and the Brioverian volcanics rocks at may reflect the effects of the c. 540 Ma terrane amalgamation Plage de Chateau Serein is a lateral ramp or a frontal thrust,it event in the StBrieuc terrane, with local thrusting occurring in is part of the same fault labelled as the 'St Brieuc Thrust' the region of strike curvature (= constrictional bend?) within by Brun & Bale(1990, figs 13 & 14) and inferred by them the strike-slip system. Alternatively, they may have formed to represent fundamentala tectonic suture formed at during the Variscan event. c. 59CL580 Ma which separates an obducted back-arc basin We would point out that the main terrane boundary in the from continental margin rocks. It is reasonable to expect that transpressional model, the Fresnaye-St Cast shear zone this structure where it is exposed would display deformation system, which separates the St Brieuc terrane, composed of

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arc-related geological elements, from the St Malo and Mancel- succession and the high-level characteristics of the Mancellian lian terranes,composed of behind-arc geological elements, granites, are consistent with a model of moderate thickening by corresponds to the main break on geophysical maps, such as homogeneous strainduring sinistral transpression, but in- the gravimetric and magnetic maps illustrated by Brun & Bale consistent with thecrustal overthrusting envisaged by, for (1990). Thismajor terrane boundary coincides also with a example, Graviou & Auvray (1985). The structurally deeper change in the isotopic characteristicsof the protolithsources of level exposed in the St Malo terrane in comparison with the Cadomian granitoid magmatism (Brown & D’Lemos 1991 and Mancellian terrane is to be expected from this model given the unpublished data). shallow c. N060”E-plunging lineations throughout the strike- (5) We note with interest Brun’s evidence for SW-directed slip system. Thus, these two terranes may be simply different thrusting inland in the GuingampBelle Isleen Terre area. structural levelsof only one larger terrane. The StMalo Since these structures deform migmatites and gneisses com- terranethen represents the outboard edge of this larger monly correlated with the c. 540Ma StMalo migmatites St Malo-Mancellian terrane which was imbricated as a conse- (Brun & Bale 1990), their relationship to the pre-570 Ma struc- quence of docking of the St Brieuc terrane during the main tures in the Baie de St Brieuc region remains uncertain. regional deformation in the St Malo region (Brown er al. In summary, we see no reason to alter our interpretation 19906). that Cadomian deformation innorth Brittany occurred within a major zone of transpression. We stress that itis not a question 25 February 1991 of whether both thrust and strike-slip displacements have occurred, but a question of whether a transpressional model of terrane amalgamation explains better the observed data than thedominantly thrust-accommodated shortening model of References Brun and his co-workers. We urge geological field parties AITOMAR,S. 1982. Evolution de la diformation duns la zone de cisaillement de St visiting this well-known area to compare critically published Cast. Rapport Diplome d’Etudes Approfondies, Rennes. AUVRAK,B. 1979. Gent& et Pvolution de la eroute continentale duns le Nord du cross-sections and maps (Brun& Balk 1990; Strachan & Roach Massif Armoricain. These Docteur es-Sciences Naturelles, Rennes. 1990; Treloar & Strachan 1990), and toevaluate forthemselves AUVRAY,B. & MAILLET,P. 1977. Volcanisme et subduction au proterozoique the validity or otherwise of these contrasting models. supirieur dansle massif Armoricain (France).Bulletin de la Sociitd Ggologi- que de France, (9,19,953-957. BALE,P. 1986. Tectonique cadomienne en Bretagne Nord. These d‘universite, Terranes in the Cadomian belt. The recognition that the Rennes. Cadomian belt of the North Armorican Massif consists of a - & BRUN,J. P. 1983. Les chevauchements cadomiens de la baie de saint- series of contrasting tectonic units separated by steep, strike- Brieuc. Compte-rendus de I’AcadPmie des Sciences, Paris, 297, 359-362. slip shear zones implies that the terrane concept (Soper et al. -& -1989. Late Precambrian thrust and wrench zones in northern Brit- 1989, and references therein) may be a useful and objective tany (France). Journal ofStructural Geology, 11, 391405. BROWN,M. 1978. The tectonic evolution of the Precambrian rocks of theMalo St basis for future research. This is reinforced by recently region, Armorican Massif, France. Precambrian Research, 6, 1-21. acquired isotopic data which confirms that each terrane has a -& D’LEMOS, R.S. 1991. The Cadomian granites of Mancellia, north-east different tectonothermal history, and which implies the exist- Armorican Massif of France: relationship to the St Malo migmatite belt, ence of an additional outboard terrane in the Guernsey-La petrogenesis and tectonic setting. Precambrian Research, 51, 393427. -, POWER,G. M,, TOPLEY,C. G. & D’LEMOS,R. S. 1990a. Cadomian mag- Hague region (D’Lemos er al. 1990; Brown er al. 1990a; matism in the North ArmoricanMassif. In: D’LEMOS,R. S., STRACHAN,R. A. Dallmeyer et al. 19916). It is regrettable that Brun is unable to &TOPLEY,C. G. (eds) The Cadomian Orogeny. Geological Society,London, acknowledge the application of this concept as a new contri- Special Publication, 51, 181-213. bution to Cadomian geology, particularly so given that there is -, DALLMEYER,R. D., D’LEMOS, R.S. & STRACHAN,R. A. 19906. Transpres- scant consideration of the terrane conceptin the French litera- sionaltectonics in N.W.France: the Cadomian Orogeny. Penrose Con- ference on Transpressional Tectonics of Convergent Plate Margins, Bell- ture. The perspicaceous and influential early work of Cog& ingham, USA, Abstract. has guided muchsubsequent research, including our own, ___, , & - 1990c. Extent, chronology and significance of late although itis important toemphasize that in Brun’s model the Paleozoic tectonothermal activity in the Cadomian Orogen of North-East differences between Cogne’s ‘domains’ (largely equivalent to Armorica, France. Geological Society of America, Abstracts with Programs, A156. our terranes) arise partly from magmatic and metamorphic BRUN,J. P. 1977. La zonation structurale des dames gneissiques.Un exemple: le events which followed tectonic amalgamation at c. 59& Massif de StMalo (Massif Armoricain, France). Canadian Journal of Earth 580Ma.In our model these differences arise from events Sciences, 14, 1697-1707. which occurredeither prior to or duringamalgamation at - 1979. Le Massif de Saint-Malo: in damemigmatique cadomien. In ‘Excur- sions giologiques dans le Massif Armoricain’ 26bme Con& (Xologique c. 540 Ma. International. Bulletin de la Sociiti Giologique et Mindralogique de Bretagne, We have suggested elsewhwere (Brown & D’Lemos 1991) C, XI, 1-2, 81-84. that the geochemical features of, and the similar age of,, the St -198 1.Instabilitds gravitaires et diformation de la crolite continentale. Applica- Malo migmatites and Mancellian granites supports a common tion au diveloppement des d6mes et desplufons.These d’universitt, Rennes. origin by anatexis of the Brioverian succession. Any model -& BALE,P. 1990. Cadomian tectonics in Northern Brittany. In: D’LEMOS, R. S., STRACHAN,R.A. & TOPLEY,C. G. (eds) The Cadomian Orogeny. which purports to explain the timing and kinematics of the Geological Society, London, Special Publication, 51, 95-114. Cadomian deformation must account for the structures and DALLMEYER,R. D., STRACHAN,R. A. & D’LEMOS,R. S. 1991~.Chronology of plutonichistory of the Mancellian region as well asthose Cadomian tectonothermal activity in the Baie de St Brieuc (North Brittany), France: evidence from “Ar/39Ar mineral ages. Canadian Journal of Earth further west. The model for the Cadomian belt proposed by Sciences, in press. Bale & Brun (1989), based on their recognition of the St Brieuc -, D’LEMOS.R. S., STRACHAN,R. A. & MUELLER,P. A. 19916. Tectonother- Thrust system, requires SW-directed thrusting above a NNW- mal chronology of early Cadomianarc development in Guernsey and Sark, dipping subduction zone. This model offers no interpretation Channel Islands. Journal of the Geological Society, London, 691-702. of the Mancellian region. The steep cleavage and steeply- D’LEMOS, R.S., DALLMEYER,R. D., STRACHAN,R. A. & MUELLER,P. A. 1990. Tectonothermal evolution of a late Proterozoic, Cadomian, magmatic arc; inclined folds of the Mancellian region, in combination with U-Pb zircon, and 40Ar/39Ar mineral age constraints. Geological Society of the low grade of regional metamorphism of the Brioverian America Annual Meeting, Dallas, USA, Abstracts with Programss A99.

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GAPAIS, D.&BALE, P. 1990. Shearzonepattern andgraniteemplacement withina la lithostratigraphiedu Brioverien (Proterozorqque supkieur)et pour Cadomian sinistralwrench zone at St Cast,N. Brittany. In: D’LEMOS,R. S., I’evolution geodynamiquecadomienne en baie deSaint Brieuc Massif. STRACHAN,R. A. & TOPLEY,C. G. (eds) The Cadomian Orogeny, Geological Bulletin de la Societe Giologique de France. 25, 615-621. Society Special Publication No. 51, 169-180. -, CHANTRAINE,J., CHAUVEL, J. J.,DENIS, E., BAL~,P. & BARDY,P. 1990. The COGN~,J. 1959. Donnies nouvelles sur I’Antecambrien dans I’ouest de la Brioverian (UpperProterozoic) and the CadomianOrogeny in the France:Penttvrien et Brioverien en Baie deSt Brieuc (CBtes-du-Nord) Armorican Massif. In: D’LEMOS,R. S., STRACHAN,R. A. & TOPLEY, C. G. Bulletin de la Societe Gdologique de France, (7), 1, 112-1 18. (eds) The Cadomian Orogeny. Geological Society, London, Special Publica- - 1962. Le Brioverien. Bulletin de la Societe Geologique de France, Q, IX, tion, 51, 81-94. 413430. ROACH,R. A., LEES, G.J. & SHUFFLEBOTHAM,M. M. 1990. Brioverian volcanism - 1974. Le Massif Armoricain. In: DEBELMAS,J. (ed.) Geologie de la France. and Cadomian tectonics, Baie de St Brieuc, Brittany: stages in the evolution Dovin, Paris, 105-161. of a late Precambrianensialic basin. In: D’LEMOS,R. S., STRACHAN,R. A. & GRAVIOU,P. & AUVRAY,B. 1985. Caracterisation pttrographique et geochimique TOPLEY,C. G. (eds) The Cadomian Orogeny, Geological Society Special des granitoydes cadomiensdu domaine nord-Armoricain implications Publication No. 51, 4147. geodynamiques. Comptes-rendus de 1’Academie des Sciences, Paris, 301, SHUFFLEBOTHAM,M. M. 1986. The Fort de la Lattle Complex: late Pentevrian 315-318. plutonism in Penthievre, Brittany. Proceedings of theUssher Society, 6, -, F’EUCAT,J. J., AUVRAY,B. & VIDAL,P. 1988. The Cadomian Orogenyin the 344-351. northern Armorican Massif: petrological andgeochronological constraints - 1987. The evolution of the Penthievre crystalline massif and the adiacent on a geodynamic model. Hercynica, IV, 1-13. Brioverian supracrustal rocks, North Brittany, France. PhD thesis, Univer- GUERROT,C. & PEUCAT,J. J. 1990. U-Pb geochronology of the Late Proterozoic sity of Keele. Cadomian orogeny in the North Armorican Massif, France. In: D’LEMOS, SOPER,N. J., GIBBONS,W. & MCKERROW,W. S. 1989. Introduction: Displaced R. S., STRACHAN,R.A. & TOPLEY,C. G. (eds) The Cadomian Orogeny. terranes in Britain and Ireland. Journal of the Geological Society, London, Geological Society, London, Special Publication, 51, 13-26. 146, 365-367. LEFORTJ. P. & BARDY,P. 1987. Mise en evidence de chevauchements cadomiens STRACHAN,R. A. & ROACH,R. A. 1990. Tectonic evolution of the Cadomian sur le profil sismique SWAT n”10 leve en Manche. Bulletin de la Societe belt in north Brittany. In: D’LEMOS,R. S., STRACHAN,R. A. & TOPLEY,C. Giologique de France, (8), 111, 6, 1189-1 194. G. (eds) The Cadomian Orogeny. Geological Society, London, Special OLDOW,J. S., BALLY,A. W. & LALLEMANT,H. G. 1990. Transpression, orogenic Publication, 51, 133-150. float and lithospheric balance. Geology, 18,991-994. -_, & TRELOAR,P. J. 1990. Cadomian terranes in the North Armorican PATERSON,S. R. & TOBISCH,0. T. 1988. Using pluton ages to date regional Massif, France. In: STRACHAN,R. A. & TAYLOR,G. K. (eds) Avalonian and deformations:problems with commonly used criteria. Geology, 16, Cadomian Geology of the North Atlantic. Blackie, Glasgow and London, 1108-1111. 65-92. -, VERNON,R. H. & TOBISCH,0. T. 1989. A review ofcriteria for theidentifica- -, TRELOAR,P. J., BROWN,M. & D’LEMOS,R. S. 1989. Cadomian terrane tion of magmatic and tectonic foliationsin granitoids. Journal of Structural tectonics and magmatism in the Armorican Massif. Journal of the Geo- Geology. 11, 349-363. logical Society, london, 146, 423426. PEUCAT,J. J. 1986. Behaviour of Rb-Sr whole-rock and U-Pb zircon systems TRELOAR,P. J. & STRACHAN,R. A. 1990. Cadomian strike-slip tectonics in NE during partial melting as shown in migmatitic gneisses from the St Malo Brittany. In: D’LEMOS,R. S., STRACHAN,R. A. & TOPLEY, C. G.(eds) The Massif, NE Brittany, France.Journal of the Geological Society, London. 143, Cadomian Orogeny, Geological Society, London, Special Publication, 51, 875-886. 151-168. -, HIRFIEC,Y., AUVRAY, B., COGNE,J. & CORNICHET,J. 1981. Late Proterozoic VIDAL,P,, DEUTSCH,S., MARTINEAU,F. & COGN~,J. 1974. Nouvelles donnees zircon age from a basic-ultrabasiccomplex: a possible Cadomian orogenic radiometriques en baie deSt Brieuc. Leprobleme d’un socle ante- complex in the Hercynian belt of Western Europe. Geology, 9, 169-173. Cadomiennord-armoricain. Compte rendusde I’Academie des Sciences, RABU, D., CHAUVEL,J. J. & CHANTRAINE,J. 1983. Nouvelles propositions pour 279, 631434.

J. P. BRUN, Laboratoire de Tectonique, Institut de Gologie,Universite de Rennes, Campus de Beaulieu, Avenue de Ghtral Leclerc, 35042 Rennes Cedex, France R. A. STRACHANdr R. S. D’LEMOS,Department of Geology, Oxford Polytechnic, Oxford OX3 OBP, UK M. BROWN, Department ofGeology, University of Maryland at College Park, MD 20742, USA R. A. ROACH,Department of Geology, University of Keele, Keele, Staffs ST5 5BG, UK P. J. TRELOAR,School of Geological Sciences, Kingston Polytechnic, Kingston-upon-Thames KT1 2EE, UK

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