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Pentevrian and brioverian in the North armorican
massif : characteristics and geodynamic evolution
Bernard Auvray
To cite this version:
Bernard Auvray. Pentevrian and brioverian in the North armorican massif : characteristics and geo-dynamic evolution . 1989, pp.9 + figures. �insu-01576103�
"P E N T E V R IA N A N D B R IQ V E R IA N IN TH E N O R T H A R M O R IC A N M A S S IF : C H A R A C T E R IS T IC S A N D G E O D Y N A M IC EV O LU T IO N ".
Bernard A U V R A Y
Abstract :
The N orth A rm o ric a n M a s s if is one of the rare areas w ithin the H e rcyn ian belt of W estern Europe w here the Precam b rian basement outcrops. W ithin this basem ent, two m ain cycles m ay be distinguish (1) The P entevrian or Icartian (2.2 - 1.8 Ga) com posed of m etam orphic rocks m ainly derived from m agm atic protoliths. (2) The B riove ria n which includes several sedim entary and volcanic units am ong w hich stratigraphic corre lations are not p e rfe ctly established. The num erous plutons em placed during this period allow a
chronology of the m ain tectonom etam orphic events to be established.
Now adays, the La te Precam b rian evolution (660 M a - 540 M a ) of the N orth A rm o ric a n dom ain is interpreted as resulting from a subduction geodynam ic process, but the vergence of the subduction rem ains a m atter for discussion. Nevertheless, m ost of the major structures now visible in this dom ain are related to this P re cam b rian history^
A U VR A Y Bernard Université de Rennes I Institut de Géologie Campus de Beaulieu 35042 - RENNES Cédex Poste de Professeur Université de Rennes I Section CNU : 35ème J.O. n*i6 du 19/01/89
The first com prehensive synthesis concerning the P re ca m b ria n terrains in the N orth A rm o ric a n M a s s if has been proposed by Cogné, in 1962, m ainly based on the ca rtographic data of Barrois, Pruvost, G raindor and C ogné him self. A s it is possible to observe on fig. 1, the Precam brian in this area was subdivided into two m ain cycle s (Cogné 1962).
(1) A highly m etam orphosed basement, granodioritic in com p osition in evidence at Jospinet on the eastern flank of the Baie de St Brie uc and called by C ogné the P entevrian (C ogné 1959).
(2) This gn eissic basem ent was unconform ably overlained by the B rio v e ria n com posed of two stages separated by an orogenic phase : at the base, a volcano-sed im entary sequence; at the upper part, and e xclu sive ly sedim entary series. The m ain phase of m etam orphism , deform ation and granitisatio n occured at the end of the firs t stage and was achieved at the end of tfie second stage w ith the intrusion of the M ancellian granites.
The first questions concerning the validity of this schem e appeared in 1971 when a new datation of the Erquy volcanic form ation, considered as a reference level for the base of the Brioverian, yielded a Low er P a le o zo ic age of about 480 M a (Vidal et al. 1971). One of the main consequences of this new radiom etric data w as to introduce some doubt concerning the existence of Pentevrian at least in the area where it was defined and also to introduce confusion in the B riove ria n stratigraphy.
A n in ve stiga tio n am ong the volum inous bibliography con ce rn ing the N orthern part of the A rm o ric a n M a ssif dem onstrates :
(1) That a large am ount of various available data have been obtained during the last tw enty years.
(2) That the precam brian stratigraphy, especially the B rio v e ria n stratigraphy, has been stron gly m odified.
(3) That all the authors try to explain the geodynam ic evolution during the P re cam b rian by using a plate tectonic model and more precisely a subduction model. But there is no com plete agreem ent concerning the direction o f this subduction.
Consequenlty, it seems now necessary to separate the fa c ts which seem well established from interpretations and hypothesis; this is the m ain purpose of this paper.
II - M A IN C H A R A C T E R IS T IC S OF T H E P R E C A M B R IA N F O R M A T IO N IN TH E N O RT H A R M O R IC A N D O M A IN :
L e t us define the main c h a ra cte ristics of the Precam brian in the N orth A rm o ric a n M a s s if and first of all the Pentevrian.
(A ) The Pentevrian :
The only area w here the P entevrian form ations are w ell-defined is located at the northern m argin of the N orth A rm o ric a n M assif, along a line starting from the T re go r area to C ap de la Hague in the C otentin Peninsula (figure 2).
mm>. The ra d io m e tric data obtained by A dam s (1967, 1976), C a lv e z and Vidal (1978), A u v ra y et al. (1980), P iton (1985), yield a Low er P ro te rozoic age ranging from 2,2 by to 1,8 by. The Pentevrian, particularly well exposed in Guernsey and in the Trebeurden area near to Lannion is m ainly com posed of m etavolcanics, m etasedim ents and m e tagranito’ids. M etavolcanics, varyin g from basic to a cid ic types (am phibolites to acidic gneisses) are largely dom inant; the only well developed sedim entary sequences being the Jerbourg and Pleinm ount m etasedim ents in G uernsey (R oach 1966). G ra n itoid s, actually transform ed into augen-gneisses, are the m ost typical facies with com positions varying from granodiorite to quartz-m onzonite. A ll the m agm atic rocks (including volca n ics and plutonites) show a calc-alka line evolution in the A F M diagram (fig. 3a) and in the Tarney dia gram (fig. 3b). A ll the form ations have been m etam orphosed in the am phibolite fa c ie s and in places, it is possible to observe the first stages of a m igm atisation. Because of the scattering and sm all extent of the outcrops, it is im possible to define a large scale m etam orphic zoneography and structure in this Low er P ro te ro zo ic basement. The ages have been determ ined on the augen-gneisses and consequently are a m inim um age for the volcanics and the sedim ents intruded by these gra n ito id s.
Now here else in the A rm o ric a n M a ssif has this basem ent been recognized w ith certainty, especially in the Jospinet area where it was defined for the first time. A t that place, there is no radiom etric data and no geological evidence, the m ain argum ent (that is the Brioverian age of the volcanic series of Erquy) being highly doubtful.
Fina lly, concerning the name of this period, it is proposed to use the term P e nte vria n as equivalent o f Icartian, considering that in its origina l definition P e n te vria n included ail the terrains older than the Brioverian.
(B) The B rlo v e ria n ; M any attem pts have been made to establish a com prehensive stratigraphy am ong the various lith ologic units belonging to this stage. A ctu ally, these corre lations are not well defined.
A t present, w ith the exception of the Perelle Gneisses in G uernsey w hich have yielded an age around 700 M a (W ilson 1986, unpublished data), the oldest age in the B rio ve ria n has been obtained on a granitic pebble o f the C e sson C onglom erate and is around ¿6 0 M a (G uerrot et al. 1986). Consequently, this conglom erate, w hich is considered as the low er part of the Brioverian, has a m axim um age of 660 Ma.
A ll over the Baie de St B rie uc area, from the Tregor to St M alo, are exposed num erous lith ologica l units com posed of volcanics and sedim entary rocks. The correlations are d iffic u lt to establish between these units since they actually belong to te ctonic blocks joined side by side during the C adom ian tectogenesis. The m ost recent syntheses cle a rly show that there is a tendancy to consider that the va ria tio n s observed in the different units result from lateral facies variatio ns m ore than ve rtic a l ones. This concept is cle arly expressed for instance in the stra tig ra p h ic table proposed by C hantraine et al. (1988) and D issle r et al. (1988) (fig.4) and also by Balé (1986) to explain the correlations between the va rio u s units outcropping in the South part of the Baie de St Brieuc (fig.5).
These units have suffered various degrees of m etam orphism from greenshist facies (in the T re go r or Lam b alle series) to am phibolite facies in the Lan vollon or Y ffin ia c form ations.
The fa c ts which seem w ell established are as follow s (fig.4) :
(1) The volcanics are very abundant in the northern part of the dom ain and decrease towards the South-South E a st in the St Cast, St M a lo and La m b a lle series.
(2) A m on g the sedim entary rocks, the black che rts or "phtanites" are considered as a good stra tigra p hic m arker. Consequently, L e C orre (1977) has proposed to use the term "P h ta n itic B rio v e ria n " to designate all these units which belong to the Low e r Brioverian.
(3) D uring this first stage of Brioverian, and w ith the exception of the 660 M a age of a grano dio ritic pebble in the C e sson conglom erate, radiom etric data are the follow ing :
(a) The 615 M a age o f m agm atism in the Tregor area (Graviou 1974). (b) The 600 M a age o f the basic-ultrabasic sequence o f B e lle -Isle -e n -T e rre - Y f f in ia c (Peucat et al. 1981 ; G uerrot & Peucat this volume).
(4) Except the St M a lo - St C a st and Lam balle units, the tectonic and
m etam orphic evolution ended around 590 Ma, the age of the syntectonic em placem ent of the d io ritic m assifs like that of C oëtm ieux - F o rt-la -L a tte (Vidal et al. 1974 ; G enestier 1984 ; Balé 1986).
(5) Eastw ards, in N orm andy, appears the same phtanitic Brioverian including large am ounts of volcan ics (Le Vast, M ontsurvent, L a Terrette and les M o rte s Fem rres séries, D upret 1986 ; D issle r 1987) m etam orphosed in the greenschist facies; in this area, the m inim um age for the "P h ta n itic Brioverian", is given by the em placem ent of the C outances diorite at 585 M a (G uerrot et al., 1986).
After the Phtanitic cycle, occurs à flysch-type sedim entation with rare
volcanic intercalations like the V a ssy spilites in N orm andy (Dupret et al. 1985). This post-phtanitic cycle is w ell developed in N orm andy (G ranville and L a L a iz e series) but is only known in B ritta ny in the sm all area of Le M inard near to Paim pol (Denis 1987). The only appreciable m etam orphism in this form ation is the m etam orphism developed at the contact of the M ance llian granites which were em placed at 540 M a (G raviou et al. 1988). This age indicates the end of the Cadom ian evolution. D u rin g this post-phtanitic stage, the S t-M a lo - Lam balle and Ranee series suffered a high tem perature - low pressure m etam orphism (Brun <3. M a rtin 1978) characterized by the appearance of m igm atites in the St M a lo area, the age of the m etam orphism being 540 M a (Peucat 1982, 1986 ; Peucat & M a rtin 1985).
The post-p hta nitic Brioverian occurs, then, between 590 M a and 540 Ma. This tim e range includes the m agm atism dated around 550 M a in the N orthern part of the A rm o ric a n M a ssif (A u vray 1975). The ignim britic
volcanics are cha ra c te ristic of this m agm atism and in Norm andy, at St G erm ain Le Gaillard, are directly overlained by the Low er C a m b ria n form ations.
In short, according to the available radiom etric data, the Brioverian cycle cove rs a period of about 120 Ma, the major tectonic events occuring since 590 Ma.
Consequently, the geodynam ic m odels are constrained by the m ain facts sum m arized in the table I.
Ill - G E O D Y N A M IC M O D E L S - D ISC U SSIO N :
It was at the end of the seventies that the first plate tectonic models were proposed to explain the Precam brian evolution in the N orth A rm orican M a ssif. Fig.é show s an exam ple o f th is m odélisation proposed by A u v ra y (1979) w ith an oce a nic plate plunging under a continental m argin constituted by the N orth A rm o ric a n Dom ain.
The m odels w ere based on the follow ing m ain argum ents :
(1) The existence of an oce a nic dom ain outlined by a w ell-m arked m agnetic anomaly, trending S W -N E in the m iddle part of the C hannel (L e fo rt 1975, 1977). (2) The presence of an old sia lic basem ent at the south m argin o f this oceanic domain.
(3) Em placem ent in this continental m argin of vólcan lcs and plutonios of
calc-alkaline a ffin ity largely developed in the T re go r area (fig. 7).
This c a lc -a lka lin e a ffin ity was the stron ge st argum ent favouring the subduction model, w ith a subduction plane dipping tow ards S-SE, the m elting of the plate itse lf or o f the above m antle wedge giv in g rise to the calc-alkaline m agm atism .
A t the same tim e, the age ¿0 0 M a for the Be lle -Isle -e n -T e rre Com plex of oceanic a ffin ity (Peucat et al. 1981), appeared to confirm the opening of a m arginal basin south of the active m argin, as was previously proposed by H irbec (1979) and A u v ra y (1979).
Since this firs t proposal, num erous data concerning the characteristics of the sedim entation, the ge o ch e m istry of the volcanics, the geochem ical and c h ron ologica l zonations in the plutonism have allow ed im provem ents to the model (see C hantraine et al. 1988, for references). Fig. 8, fo r instance, shows the re gio nal zonation of the plutonites as given by G raviou and A uvray (1985 ; this volum e).
— Mo r e recently, Balé and Brun (1983) and Balé (1986), using structural argum ents have dem onstrated in the Baie de St Brieuc area :
0 ) That it is possible to correlate the B e lle -Isle -e n -T e rre and Y ffin ia c form ations, corre latio n con firm e d by datation, (G uerrot and Peucat, this volume) or the m ig m a titic belt o f St M a lo w ith the m igm a titic series of Guingam p as shown on fig. 9.
(2) That the m ajor C a d om ia n structures m ay be interpreted as interactions between im bricate thrust zon e s tow ards the S-SW and ductile sinestral wrench zones trending N 50°. This type of deform ation has been related to an oblique
subduction followed by collision and obduction w hich developed continuously from 600 M a until 540 Ma. In fig. 10, it is possible to observe the resulting structures in the Brioverian form ations in the St B rie uc area.
Fijj-dO > The resuif 0f this new data, is that in m ore recent geodynam ic models two domains are distinguished (fig. 11) :
(a) in the North, a volcanogenic dom ain c h a ra cte rize d by its calc-alkaline
m agm atism and its M -type plutonism s with a m ean age of em placem ent around ¿0 0 Ma but in fact scattered between ¿2 0 M a and 580 M a.
(b) to the South a terrigeneous domain ch a ra cte rize d by the lack (except the
V a ssy spilites in Normandy, fig.4) of vo lcan ism and the nature and age of its plutonism (C -typ$ and 540 M a in age).
(c) between these two domains, a suture zone represents the closure o f the Belle-Isle-en-Terre - Y ffin ia c back-arc basin.
> A ll this concepts are more or less taken in account in the various models proposed by D issler et al. (1988) to explain the P recam brian evolution in Norm andy (fig. 12) and by Chantraine et al. (1988) (fig. 13) or G raviou et al. (1988) (fig. 14) to explain the same evolution all over the North A rm o ric a n Dom ain. In the three models, the subduction plane is dipping towards the South-East. One of the main difference is the m ore com plicated a rc-syste m in the model of D issle r et al. (1988 ; fig. 12). The existence of a fore-arc, an intra-arc and a back-arc basin results from the various geochem ical characteristics of numerous volcanic series (Le Vast, M ontsurvent, la Terrette, L e s M ortes Fem m es series). In fact, some of the che m ica l data are co n flictin g and need further determ inations; consequently, this arc-m odel has perhaps to be sim plified in the future. A nother im portant diffe re n c e is that in the m odel for Norm andy (D issler et al., 1988), the vergence of the deform ations is tow ards the North; it means that all the structures are synthe tic, with regard to the direction of the subduction. In opposition, in the m odels of Chantraine et a l. (1988) and Graviou et__al. (1988), the ve rge n ce o f the deform ations is system atically towards the South, all the stru ctu re s being antithetic com pared to the subduction dipping.
.
However, all these models with a subduction plane dipping tow ards the South become questionable if we consider the stru ctu ra l argum ents of Balé and Brun (1983) and Balé (198¿) :
(1) In the hypothesis of a subduction dipping tow ards the South, the syste m atic antithetic character of the structures, alm ost in the St M alo and St Brie uc areas, is surprising and has never been observed in modern subduction and
collisio n system s.
(2) If we agree w ith the translation hypothesis o f the Leon block proposed by B alé and Brun (1986) during P a le o zo ic tim es (fig. 15), the Channel ocean is not in its origin a l position and m ust be translated towards the SW, in the prolongation of the B e lle -Isle - Y f f in ia c suture, (see Brun & Balé, this volume, for m ore details).
C onsid e rin g these two strong structural argum ents and taking into account all the other constraints, it is possible to propose an alte rna tive geodynam ic model w ith a subduction dipping tow ards the N orth (fig. 16).
- To the N orth, a continental dom ain southern m argin of w hich is represented by the D bm enean dom ain but w hich also includes the P recam brian form a tio ns o f Wales, South England and So u th -E a st Ireland; this P recam b rian block (N orthern continent in fig. 16) m ay be a part o f the A rm o ric a plate defined by the ge o ph ysicists (Van der V o o 1979).
- Then an oceanic dom ain o f lim ite d extension, as is proved by the la ck of true op hiolitic assem blage, the non typ ica l oceanic characters o f the v o lca n ic series, the m arginal type sedim entation and the short tim e for the closure of th is ocean, about 10 M a.
- Finally, a Southern continent, the M a nce llia n dom ain the southern extension o f which is ignored in the paper.
A fte r the opening o f the ocean before 620 M a the closure phase begins by a subduction of the oceanic plate under the N orthern continent. Then follo w s the collision around 590 M a and the underplating o f the southern continent under the northern one until 540 M a. The various stages o f this geodynam ic evolution are sc h e m a tic a lly shown in fig. 16.
But this new m odel is not, again, entirely satisfying, m ainly because there is a problem o f tim ing between the opening of the ocean and the subduction process. The subduction has a m inim um age of 620 Ma, w hich corresponds to the age o f c a lc -a lka lin e m agm atism in the Tregor; this m eans that, at that time, the ocean m ust has been open, but the only ra d io m e tric of the supposed oceanic dom ain (B e lle -Isle - Y ffin ia c ) is at 600 Ma.
A nother question concerns the vergence of the de form ations (see above); it seem s cle a rly established that, in the Baie de St-B rie u c dom ain, the vergence is towards the South (Balé and Brun 1983). In contrast, in N orm andy, the vergence of the C a d o m ia n de form ations has been described as being tow ards the North. (D issle r et al. 1988; fig. 12). M oreover, L e fo rt and B ard y (1987), on the base o f ge o ph ysical argum e nts yielded by the S W A T - E C O R S
se ism ic profile s conclude that the m ain re fle ctors dipping towards the South observed in the Channel, are the trace s of synthetic de form ations related to C adom ian subduction o f the Channel Ocean. This is an im portant problem which needs to be clarified.
The non-existance of the Channel ocean w hich is a consequence of the Balé and Brun hypothesis, does not have a dram atic effect on south dipping subduction m odels because it is alw ays possible to find another ocean, for instance, the C e ltic or Longm yndian ocean as has been previously proposed by C o gn é and W right in 1980 (fig. 17). In that hypothesis, Wales, South England, Southeastern Ireland and N o rth A rm o ric a n M a ssif com pose a single continental m argin (N orthern continent, fig. 17); this would explain the sim ila r geological and geochronological characte rs of these regions (Shackleton 1975 ; D unning 1975 ; M a x 1975 ; C ogn é and W right 1980 ; Thorpe et al. 1984) . The Be lle -Isle - Y ffin ia c -C o u ta n c e s suture zone becom es again the trace of a m arginal basin developed during the subduction. But rem ains the structural problem of synthe tic deform ations; to m y knowledge, no cha ra c te ristic €adom ian structure w ith a northw ard vergence has. been described in the northern m argin o f this N orthern continent, perhaps m ainly because of the strong influence of C aledonian and H e rcyn ia n tectonics in that area.
In conclusion, the problem of the vergence o f the subduction is not solved for the m oment. The m ain fact enhanced in this revue is that there is a great paradox between the large am ount of various geological inform ation acquired concerning the Precam brian dom ain in the N orth A rm o ric a n M a s s if and the d iffic u ltie s encountered in integrating all these data in a global evolution. T h is is pa rtly because, today, we observe only some fragm ents of a puzzle; and a lso perhaps, because the scale of observation is ve ry lim ited and does not correspond to the continental scale required to apply global tectonic m odels.
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D U P R E T , L., L E G A L L , J., D O R E , F., G A TIN O T, F., D IS SL E R , E. 1985 - les
sp ilite s de V a ssy (Calvados), tém oin d'u n volcanism e tholéiitique et distensif, dans la sédim entation du B rio v é rie n supérieur de Norm andie. C om p te s Rendus de l'A c a d é m ie des Sciences, P aris, II, 300, 687-692.
G E N E ST IE R , V. 1984 - P é trologie d'une intrusion calco-alcalin e et de ses
enclaves : le rqassif cadom ien de "F o rt L a L a tte " (Bretagne Nord). D E A , Rennes, 52 p.
G R A V IO U , P. 1984 - Pétrogenèse des m agm as calco-alcalins : exem ples des
g ra n ito ïd e s cadom iens de la région trégorroise (M a ssif A rm oricain). 3 rd cycle thesis, Rennes, 236 p.
G R A V IO U , P., A U V R A Y , B. 1985 - C a ra c té risa tio n pétrographique et
géochim ique des g ra n ito ïd e s cadom iens du dom aine no rd -a rm orica in : im plication s géodynam iques. C o m p te s Rendus de l'A c a d é m ie des Sciences, Paris, 301, (II), 315-318.
G R A V IO U , P., A U V R A Y , B. - La te Precam brian M -type g ra n ito id genesis in the
C adom ian belt of N.W. France. Journal of G eological Society, London, this volum e.
G R A V IO U , P., P E U C A T , J.J., A U V R A Y , B., V ID A L , Ph. 1988 - L 'o ro ge n è se
cadom ienne dans le N ord du M a s s if A rm o ric a in : P étrologie et géochronologie. H ercyn ica, (in press).
G U E R R O T , C., P E U C A T , J.J., D U P R ET , L. 1986 - A ge du Précam brien
sédim entaire (Briovérien) dans le M a ssif A rm orica in , llè m e R éunion des Sciences de la Terre, C le rm o n t-F e rra n d , abstract, p.86.
G U E R R O T , C., P E U C A T , J.J. - U -P b geochronology in the Late P recam b rian of
N o rth Brittany, F ra n ce : the C a d o m ia n orogeny. Journal o f G eological Society, London, th is volum e.
H IR B E C , Y . 1979 - Le com plexe basique de B e lle -Isle -e n -T e rre (C ôte s du Nord). Sa place dans l'é v o lu tio n géodyam ique du Nord du M a s s if A rm orica in . 3 rd cycle thesis, Rennes, 146 p.
L E C O R R E , C l. 1977 - Le B riové rie n de Bretagne C e n tra le : essai de synthèse lithologique et structurale. Bulletin du B R G M , section I, 3, 219-254.
L E F O R T , J.P. 1975 - L e socle p é ri-a rm o rica in : étude géologique et géophysique du socle subm ergé à l'O u e st de la France. Thesis, Rennes, 217 p.
L E F O R T , J.P. 1977 - Possible "C a le d o n ia n " subduction under the Dom nonean domain N orth A rm o ric a n area. Geology, 5, 523-526.
L E F O R T , J.P., B A R D Y , Ph. 1987 - M ise en évidence de chevauchem ents cadom iens sur le p rofil sism ique S W A T n°10 levé en Manche. Bulletin de la Société Géologique de France, (8), III, 6, 1189-1194.
M A X , M .D . 1975 - P re cam b rian rocks of So u th -E a st Ireland in "P re ca m b ria n - A correlation of Precam b rian rocks in the B ritish Isle s". G e o logical Society (London), Special R e p orts nr 6, 96-101.
P E U C A T , J.J. 1982 - G éochronologie des roches m étam orphiques (R b -S r et U-Pb). Exem ples ch o isis au Groenland, en Laponie, dans le M a s s if arm oricain et en Grande Kabylie. Thesis, Rennes, 172 p.
P E U C A T , J.J. 1986 - Behaviour of R b -S r whole rock and U -P b zircon system s during partial m elting as shown in m igm a titic gneisses from the St M alo M a ssif, N E Brittany, France. Journal of G eological Society, London, 143, 875-885.
P E U C A T , J.J., H IR B E C , Y., A U V R A Y , B., C O G N E , J., C O R N IC H E T , J. 1981 -Late P ro te rozoic zircon ages from a b a sic -u ltra b a sic com plex : a possible cadom ian orogenic com plex in the H e rcyn ian belt of W estern Europe. Geology, 9, 169-173.
P E U C A T , J.J., M A R T IN , H. 1985 - A re R b -S r thin slabs m igm a tite ages alw ays m eaningfull? Neues Jahrbuch für M inéralogie, Abh., 152, 1, 1-21.
continentale p roté rozoïq ue (1,8-2,2) au N ord du M a s s if A rm oricain. D E A , Rennes, 53 p.
R O A C H , R.A . 1966 - O utline and guide to the geology of Guernesey. R e p ort
and Transactions, L a Société G uernesiaise. X V III, 751-776.
SH A C K L E T O N , R.M . 1975 - Pre cam b rian rocks of Wales in "Precam b rian - A
correlation of P recam b rian rocks in the B ritish Isles". G eological So cie ty (London), Special Reports. nr 6, 76-82.
TH O RPE, R.S., Ô E C K IN SA L E , R.D., P A T C H E T T , P.J., PIPER , J.D.A., D A V IE S, G .R., EVAN S, J.A. 1984 - C ru sta l grow th and late precam brian - early
paleozoic plate te ctonic evolution o f England and Wales. Journal of G e o logical Society, London, 141, 521-536.
V A N D E R VOO, R . 1979 - P a le o zo ic assem bly o f Pangea : a new plate tectonic
model for the Taconic, C aledonian and hercynian orogenesis. Eos Transactions. A G U , 60, p.241.
V ID A L , Ph., A U V R A Y , B., C O G N E , J., H A M E U R T , J., JEAN N ETTE, D. 1971
-Données géochronologiques sur la série spilitique d 'E rq u y : problèm es nouveaux à propos du B riové rie n de Bretagne septentrionale. Com ptes Rendus de l'A c a d é m ie des Sciences, P aris, (D), 273, 132-135.
V ID A L , Ph., D EU T SC H , S., M A R T IN E A U , F., C O G N E , J. 1974 - N ouvelles
données radiom étriques en Baie de St Brieuc. L e problème d'u n socle antécadom ien nord-arm oricain. C om p te s R e n dus de l'A c a d é m ie des Sciences, Paris, (D), 279, 631-634.
B. A U V R A Y
Institut de G éologie L P C N R S 4661
U n iv e rsité de Rennes I A venue du G énéral Le cle rc 35042 - R E N N E S Cédex (France).
F igu re C a p tio n s
F ig. 1 - Precam brian stra tigra p hy in the N o rth A rm orican dom ain according to Cogné (1962).
F ig . 2 - Lo ca tion (stars) of the outcrops of the Low er P ro te ro zo ic form ations at the northern m argin of the N orth A rm o rica n M a ssif (after A u v ra y 1979) (1) M oulin de la Rive, (2) Trébeurden, (3) Port-Béni, (4) Guernsey, (5) Sark, (6) Alderney, (7) La Hague.
F ig. 3 - C a lc-a lk a lin e a ffin ity o f igneous rocks (volcanics and plutonites) constituing the Pentevrian or Icartian basem ent in the Tregor area (N orth Brittany), (a) A F M diagram , (b) T a rn e y 's dia gram (1976) (after A u v ra y 1979).
F ig. 4 - Stratigraph ic corre latio ns in the Brioverian fo r m a tio n s , o f the N orth A rm orica n M a s s if (after C hantrain e et al. 1988 and D issle r et al. 1987). (1) Port-Béni, (2) Tréguier, (3) Trédrez, (4) Paim pol, (5) La R oche D errien, (6) Le Minard, (7) L e M oulin de la R iv e - Trébeurden, (8) Locquirec, (9) L 'A rm o riq u e , (10) St-M ic h e l-e n -G rê ve s, (11) Y ffin ia c, (12) Cesson, (13) Lanvollon, (14) Binic, (15) St M alo, (16) Lam b alle, (17) L a Rance, (18) L a Hague, (19) Le Vast, M onsurvent, L a Terrette, L e s M orte s Fem m es, (20) P h ta n itic Brioverian o f Norm andy, (21) G ra nville and L a laize, (22) Vassy.
F ig. 5 - C orre la tions between the va rio u s Brioverian lith ological units outcropping in the Baie de St Brieuc (from Balé 1986).
F ig. 6 - Subduction geodynam ic m odel proposed by A u vra y (1979) fo r the Brioverian evolution in the N o rth A rm o ric a n domain.
F ig. 7 - C a lc-a lka lin e characte r o f the brioverian m agm atism in the T re go r area, North B ritta ny (after A u v ra y 1979).
F ig . 8 - G eochem ical and chron ologica l zonation within the briove rian m agm atism (from 620 M a to 540 M a ) in the North A rm o rica n M a s s if (after G raviou & A u v ra y 1985 ; G ra v io u & A u v ra y , this volume). M = m antle type
Fig. ? - G e o lo gica l sketch-m ap o f the Baie de St B rie u c area according to Balé & Brun 1983 and Balé 1986 : (1) M etam orphic and m ig m a titic series of Guingam p and St M alo, (2) Be lle -Isle -e n -T e rre and Y f fin ia c ba sic -ultra b a sic form ations.
Fig. 10 - T e c to n ic interpretation o f the w estern part o f the Baie de St
Brieuc area proposed by Balé <5. Brun 1983. There is a general ove rth ru sting of all the lith ological units tow ards the S-SW .
Fig. 11 - D u rin g the B riove ria n evolution, two dom ains separated by a
suture zone are distinguished in the N orth A rm o ric a n M a s s if : a volcanologie domain to the N orth, a terrigeneous domain to the South (after C hantraine et al. 1988).
Fig. 12 - G eodynam ic m odel proposed by D issle r et al. 1988. (1) Pentevrian
basement, (2) A c tiv e volcanism , (3) Rem nant volcanism , (4) E a rly C adom ian Granitoids, (5) D io r itic intrusions (C outances type), (6) L a te C adom ian G abbroic intrusions, (7) La te C adom ian G ranitoids, (8) P htanites (b lack-sh e rts) form ations, (9) Upper B rio v e ria n sedim ents.
Fig. 13 - G e odynam ic model proposed by C hantrain e et al. 1988.
Fig. 14 - G e odynam ic m odel proposed by G raviou et al. 1988. T = Tregor
m assif ; L P = La n n io n -P a im p o l volcanics, M F = L e s M o rte s F em m es volcanics; M = M ontsurvent volcanics; T e = L a Terrette volcanics; B I = B e lle -Isle -e n -T e rre
series; BS = Baie de St B rieuc series; SQ = St Q uay diorite m assif; C -F l =
C oëtm ieux - F o r t -L a -L a t t e diorite m assif; SM = St M a lo m igm a tite belt.
Fig. 15 - Translation o f the Le o n block from South to N o rth during the
Devonian (from B alé <5. Brun 1986).
Fig. 16 - G eodynam ic evolution o f the N orth A rm o ric a n M a s s if explained by
a subduction process with a subduction plane dipping tow ards N -N W .
Fig. 17 - Subduction model proposed by C ogn é & W right 1980, to explain the
Brioverian evolution. The closure o f the C e ltic O cean induced a subduction towards S - S E under the Pente vrian m icro-plate (Wales, South England, Southern Ireland and N orth A rm o ric a n M assif).
NORMANDIE ! NORMANDIE OCCIDENTALE I MERIDIONALE BRETAGNE SEPTENTRIONALE T F e .S ‘Â .ÿ A re n î ç E 1 T r é m o d o c 1 o Pots de m i e n e l *- g A cedîe n o £ (G é o rç ie n) Gres A r m o r i c e i n s 6r«s feldspothiques S c h i s t e s et colt Grés A r m o n c c m s Plissements _ Groniîes intrusifs neetocenèse ) I ô * E n o v e r ien supé rieu r Schistes , çrés, microbrèches l Etcçe Ce' !o Lcize'J
Séries çtccîoires et périçlccieîres ( P c u C . ce Grenville. St Germern d'Ec tot, IccQuirec/PlusQueîiec )
Sou.tve menîs mttc morph, rfçionot ^ ie ro $ e n è s e ) .£ c Z & E r ic v é rie nC O JT o moyen E n o v e r i e n in f i r i e u r É-S « im> • V ï £.£. I s:» C O RO IL L CR C N O R K A N I E K K ' E ' '“S 11 11 £ •ff “ î
Schiste* et phyiioces (Ce Soint l ô ) c petites intercolloîions çréseuses ou çrése-feiCsp
(Etcçe de Viilîer s-Fossore )
Schistes et phfoniîes ( Ce lombclie ) (Etoçe ce to tende Ces VorCes)
Série cph îolit iqu e d ' E r q u y et cmphibolites ce Lonvoîlon S é r ie c l t r it î ç u e Ce t e s e (c r k c s e s et peucinçues ce Cetsonî 7W s / ) ^ y ... Gneiss çr e n o - C «critique s S o c le
P e n t é v r i e n »»tu» (reno* citniicuri
<v
fgj Gronodioril« (= 620 M.Y) Brioverion VoJccno-stdiroeniory lormcîions oí lh* Domnoneon * Wonctllien domcins
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^ I n r.n
Coni incnl ol domain
v- 6 4 0 M.A.
Ic o r lio n m argii D o c k -o re Ixisin
td n v o llo n - O lnlc P h lo n ilic se ries L a m b a No Chateau ScroWi - 6 2 0 - 6 0 0 M A T re go r m a g m o lis m 'X -O O O - 5 0 0 M.A. C lo su r e o f I lie ^ b o c k - o r e b asin P c n lh ie v r e m a g m o lis m ( F o r f - la - L a M c d io rific type) M in a rd b a sin '-v-SOO - 5 6 0 M A SSBU* S l - M o lo m ig m o lilc s Ho c a g e norm nnd IV a say M o n c c llia n g ra n ite s ^ 5 4 0 M A
MAIN CI IARA C TER ISTIC S O F PRECAM nR IAN IN Tl tE N O R TH AR M O RICAN MASSIF
1° - Occurence of a gncissic bosement - Lower Proterozoic oge (2.0 - 1.0 Go) - in the northern port of the oreo.
2° - Development, between 660 Mo. (perhaps 700 Mo) one) 540 Ma. of a Iwo-stoge Driovcrion cycle : (o) - The phtonitic slogc (650 Ma - 590 Ma) with large amounts of volconics of the base ont) sedimentary
rocks (block cherts = phtanites) towards the upper part.
Emplacement of plutonic rocks of calc-olkalinc affinities all during this stage.
(b) - The post-phtonitic sfoge (590 Mo - 540 Mo) characterized by flysch type sedimentation with rare basic volcanics (Normandy).
3® - Mctomorphism one) deformation occur during « 60 Ma (600 Ma to 540 Ma)
They mainly, but irregularly, affect the formations which belong to the phtanific stage.
4® - Finally, the Precombrian evolution ends with the emplacement at about 550 Ma - 540 Ma. of magmatic rocks : mainly volcanics in the North (ignimbrltes)
and plutonites in the South-Eost (Mancellian granites). Table 1
