Precadomian relicts in the Armorican Massif: Their age and role in the evolution of the western and central European Cadomian-Hercynian belt

Precambrian Research, 14 (1981) 1--20 Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands

1

PRECADOMIAN RELICTS IN THE ARMORICAN MASSIF: THEIR AGE AND ROLE IN THE EVOLUTION OF THE WESTERN AND CENTRAL EUROPEAN CADOMIAN--HERCYNIAN BELT

P. VIDAL, B. AUVRAY, R. CHARLOT and J. COGN]~ Centre Armoricain d 'Etude Structurale des Socles, CNRS, Universite; de Rennes, Campus de Beaulieu, 35042 Rennes Cedex (France)

(Received August 16, 1979;revision accepted July 3, 1980)

ABSTRACT Vidal, P., Auvray, B., Charlot, R. and Cogn~, J., 1981. Precadomian relicts in the Armorican Massif: their age and role in the evolution of the western and central European Cadomian--Hercynian belt. Precambrian Res., 14: 1--20. Well-dated Precambrian is mostly developed in the north of the Armorican Massif, in which area voluminous Cadomian magmatism is dated at between 650 and 550 Ma. Much older relicts occur at Cap de la Hague, in Guernsey and in the Tregor, and are also found in the northern continental margin of the Iberian Peninsula. In all these occurrences, whole-rock systems have been opened, so that the true ages cannot be determined by the Rb-Sr whole rock isochron method. Four U-Pb zircon ages are between 1.8 and 2 Ga (in Guernsey: Icart orthogneisses; in Tregor: Port Beni, Trebeurden, Morguignen orthogneisses). There is no evidence from strontium isotopes that these isolated and scattered relicts have a wide extension or that such ancient continental crust played an important role in magma genesis from 650 Ma to 270 Ma ago. On the contrary, the evolution of initial 8~Sr/86Sr ratios with time shows that the observed mid- and west European continental crust is probably not older than 700 Ma. The increase of the initial STSr/S~Sr ratios of the magmas with time suggests that, after its formation in Cadomian times, this segment of continental crust evolved virtually as a closed system and Hercynian magmatism arose principally from re-melting of relatively young sialic components.

GEOLOGICAL SETTING The Armorican Massif, located on the western edge of the European continent, constitutes a vast area where several pre-Mesozoic orogenies have been recorded. The superposition of numerous metamorphic events, from Precambrian to Hercynian times often makes elucidation of the timing of each geologic event very difficult. In particular, the lithologic succession and stratigraphy o f t h e P r e c a m b r i a n v o l c a n i c - - s e d i m e n t a r y s e q u e n c e s is d i f f i c u l t t o e s t a b l i s h . Nevertheless, some regional syntheses have been attempted (Graindor, 1957; C o g n 6 , 1 9 5 7 , 1 9 5 9 , 1 9 6 2 , 1 9 7 0 , 1 9 7 4 ; L e C o r r e , 1 9 7 7 ) . O f t h e five d o m a i n s 0301-9268/81/0000--0000/$02.50

© 1981 Elsevier Scientific Publishing Company

defined by Cognd (1974), the Domnonean domain is the least affected by the Hercynian event and is thus the most favourable area for studying Precambrian history (Fig. 1). It is possible to reconstruct the geologic history of this northern domain as follows (Fig.2): (1) The oldest known terranes are represented by metamorphic complexes composed of gneisses, mica-schists, amphibolites and quartzo-feldspathic gneisses. These rocks are well exposed in southern Guernsey, but are also found as inclusions in the Cadomian rocks of Tregor and Cotentin. These Lower--Middle Proterozoic rocks are the oldest known in the Hercynian belt of Europe (Adams, 1967, 1976; Leutwein et al. 1973; Vidal 1976, 1977; Calvez and Vidal, 1978; Auvray, 1979) and were called Pentevrian by Cogng (1959). (2) These remnants are followed in the Armorican Massif by a group of Upper Proterozoic rocks, known as Brioverian. The lower part of the Brioverian begins with a volcanic--sedimentary series, now variably metamorphosed, extending from Lannion Bay in the west ("Armoric series") to the St Malo area in the east (St Malo gneisses and

•

.

.

.

.

OOMAIN

CENTRAL ARMORICAN DOMAIN

Fig. 1. Domnonean domain. Cadomian basement blocks with older relicts separated by Hercynian mobile belts (after J. Cogn~, 1974). (1) Leon Massif; (2) Tr~gor Massif; (3) Penthievre Massif; (4) Lower Normandy; (5) Channel Islands Massif. Fig. 2. Simplified geological map of the North Armorican Massif (Domnonean and Mancellian domains). A stratigraphic column of the Upper Proterozoic (Brioverian) in this part of the Armorican Massif is established. (1) Pentevrian basement (gnei~es, artaphibolites, micaschists, orthogneimes); (2) St. Malo migmatite belt and Cesson-Lanvol~n series (sedimentary and volcanic rocks metamorphosed in the amphibolite facies); (3) Lamballe series (schists and black cherts metamorphosed in the greemmhist facies), (4) Perros-Guirec --Brehat plutonic complex (diorites, granodi0rites, granites); (5) Treguier--Paimpol (calcalkaline volcanic series from basaltic to acidic rocks); (6) Binic series (unmetamorphosed sedimentary rocks: pelites, greywackes, sandstones); (7) Mancellian batholiths (quartzdiorites, granodiorites, granites); (8) Late Precambrian magmatimn (with alkaline affinities: ignimbrites, microgranites, granites).

o

~

o

~

~

~~

ot

I

/f f

migmatite belt), and includes various series in the Saint Brieuc Bay (Cesson-Lanvollon metamorphic series).This volcanic--sedimentary complex is overlain by a thick series of mainly low-grade metamorphosed pelites and black cherts (Lamballe schists and black cherts). These Lower and Middle Brioverian rocks were strongly deformed and folded during the Cadomian orogeny. This folding phase (Cadomian I) is synchronous with several large granodioritic intrusions (Perros--Guirec--Brehat plutonic complex in the north of Tr~gor). After the syn-Cadomian I intrusions (690--650 Ma ago), a thick, Upper Brioverian volcanic series was erupted in the Tr~gor (Tr~guier tufs and Paimpol spilites). Sandstones, quartzites, greywackes and pelites were deposited unconformably on Lower and Middle Brioverian strata in the west of St Brieuc Bay (Binic series). All the Upper Brioverian strata were slightly deformed and metamorphosed by the Cadomian II orogenic phase. This phase was marked by dioritic (St Quay, Fort-la-Latte) and granodioritic intrusions (Mancellian batholiths) at ca. 600 Ma ago. The Precambrian history of the north Armorican Massif ended with ignimbrite activity in the Tr~gor (Ldzardrieux volcanics), the NE of Jersey, and Cotentin (St Germain le-Gaillard). From Cambrian times onwards, this area behaved as a rigid basement. It was affected by Paleozoic transgressions ranging in age from Lower Cambrian in Carteret, to Upper Cambrian--Lower Ordovician in St Brieuc Bay and TrSgor, and up to the Devonian in Normandy. Restricted areas suffered significant Hercynian metamorphism and granitisation. In the southern Armorican region (including the Central Armorican Ligerian, Cornouaille Anticline and West VendEe domains; Cognd, 1974), the succession of different stratigraphic units and of possible Precambrian events is more difficult to decipher, due to (1) a thick Paleozoic succession, and (2) superimposed effects of Hercynian metamorphism, deformation and granite intrusion. Briefly, the Precambrian history of the Armorican Massif can be divided into two major periods: (1) The Pentevrian, which is Lower--Middle Proterozoic in age (1.8--2.0 Ga). (2) The Brioverian which is characterized by an alternation of periods of sedimentation, metamorphism and magmatism during the Upper Proterozoic. This scheme is supported not only by the classic observations of field geology but also by radiometric data, as shown below. GEOCHRONOLOGY Brioverian Introduction Finding pre-Cadomian relicts, and hence evidence for a pre-Brioverian orogeny, pre-supposes a good knowledge of Brioverian chronostratigraphy.

Leutwein (1968) was the first to undertake a geochronological study, using conventional K-Ar data on mafic rocks. However, argon excess in these types of rock and the lack o f atmospheric correction led to serious errors of interpretation. In contrast, whole-rock Rb-Sr isochron ages, as well as U-Pb zircon ages on magmatic sequences enable us to establish a reliable time-scale for Brioverian events.

Lower Brioverian The existence of a pre-Cadomian basement was first proposed on the eastern side of St Brieuc Bay by Cogn~ (1959) who recognised an u n c o n f o r m i t y between the Erquy spilitic series, considered as the stratotype for the Lower Brioverian and the structurally older Coetmieux--Fort-la-Latte dioritic gneisses. These gneisses were therefore regarded as Pentevrian. The dating of the Erquy spilitic series, is more fully discussed elsewhere (Martineau 1976; Martineau et al., in prep.). It suffices to state that a whole-rock Rb-Sr isochron on keratophyres, spilitic sills and the rims and cores of pillow lavas gives an age of 482 + 10 Ma* (Vidal et al., 1971). This was interpreted as an emplacement age, and was subsequently confirmed by micropaleontological investigations (Deunff et al., 1973). In spite of uncertainties relevant to (1) the micropaleontological data, (2) the absolute time scale, and (3) the Rb-Sr analytical errors, a Lower Paleozoic age (Middle Cambrian to Lower Ordovician) is considered much more likely than Lower Brioverian and even Upper Brioverian. We have also attempted to date the Lanvollon quartzofeldspathic gneisses and amphibolites, previously considered metamorphosed equivalents of the Erquy series. Sampling was inadequate for Rb-Sr dating due to the very low and homogeneous Rb/Sr ratios. Furthermore, the age of the Lamballe schists and black cherts remains u n k n o w n due to the lack of associated magmatic rocks which might provide reliable ages. Thus, there is y e t not even an approximate estimate for the age of Lower Brioverian sedimentation. Consequently the minimum age of the pre-Brioverian basement remains uncertain. Upper Brioverian The Binic series rests unconformably on Lanvollon amphibolites. It is intruded by the St Quay gabbro-diorite, which gives a whole rock Rb-Sr age of 583 + 40 Ma and a Rb-Sr biotite age of 564 Ma (Vidal et al., 1972). These ages confirm the Precambrian age of the Binic series, as well as the Cadomian age of its metamorphism and deformation. Further to the northwest, the Paimpol spilites and Tr~guier tuffs, considered as equivalents to the Binic series (Auvray, 1979), have yielded a RbSr WR isochron at 640 + 12 Ma. This age is presently the only accurate age determined within the whole Brioverian cycle. *All

the ages are calculated

h23s U = 0.98485.10

-~ a-1,

with

~STRb = 1.42-10

-1~ a - 1 , ~ 2 3 s U = 0 . 1 5 5 1 2 . 1 0

-~

a -1 ,

Pentevrian in the Saint Brieuc Bay and in the St Malo migmatite belt The gneissic basement (Coetmieux--Fort-la-Latte dioritic gneisses) underlying the Erquy spilitic series has been dated by the Rb-Sr WR m e t h o d at 650 + 60 Ma and by the U-Pb m e t h o d on zircons at 585 + 17 Ma (Vidal et al., 1974). Therefore, this basement belongs to the Cadomian orogeny and not to the Pentevrian. The occurrence of Pentevrian basement further to the east, in the St Malo migmatite belt, has been proposed by Brown et al. (1971). One of the arguments is based on model Rb-Sr ages of potassium-feldspars, of between 900 Ma and 1000 Ma (Leutwein and Sonet, 1965, Leutwein, 1968}. However, when plotted in a ~7 Sr/a6Sr vs. STRb/86 Sr diagram, the potassium--feldspar-muscovite pairs give 530 Ma, whilst potassium feldspar--biotite pairs give 390 Ma (the mean model ages, assuming 87Sr/86Sri -- 0.705, would be 570 Ma for the muscovites and 470 Ma for the biotites, respectively). The discrepancies between muscovite and biotite ages can be explained in terms of Cadomian metamorphism followed ( 1 ) b y slow cooling or (2) by Hercynian partial overprinting. Potassium-feldspars have gained radiogenic strontium lost by micas and consequently give spuriously old model ages. Therefore, geochronological arguments in favour of pre-Cadomian events do n o t exist as far as the St Malo migmatite belt is concerned. On the contrary, Leutwein's data tends to support a Cadomian age, as proposed by Jeannette (1971) and Brun (1975).

Recognized occurrences o f Pentevrian Icart orthogneisses The southern part of Guernsey is mostly occupied by Icart orthogneisses which are intruded in the north b y Cadomian plutonic rocks. Adams (1967, 1976) has established the pre-Cadomian age of the Icart gueisses. The very large scatter in the 87Sr/S6Sr vs. STRb/S6Sr diagram between t w o reference isochrons at 2.6 Ga and 1.9 Ga (Fig. 3) was interpreted as a true emplacement age at 2.6 Ga with a subsequent metamorphism at 1.9 Ga (Adams, 1967, 1976; Roach et al., 1972). Recently, Calvez and Vidal (1978) obtained an age of 2.02 + 0.02 Ga (Fig. 4) on one Icart orthogneiss sample, using the U-Pb m e t h o d on zircons. Due to the conspicuous petrographic and structural homogeneity of the gneisses, they proposed an age of 2 Ga for the whole massif. Alderney and Cap de la Hague gneisses Two preliminary Rb-Sr whole-rock studies have been performed on A1derney gneisses (Adams, 1 9 6 7 ) a n d on gneisses from Greville-Omonvflle, near Cap de la Hague (Leutwein, 1973). In spite of stronger scatter (Fig. 5), a pre-Cadomian age is suggested. Whether or n o t these rocks belong t o a 2 Ga or to a 2.6 Ga old orogeny is still unresolved.

87Sr/ 8GSr

206Pb1238 U

/

0.30 0.800 0.20-

j~s o.o

0.750-

0.I0Y

aZRblS~Sr

0.700

0 [ 0

0

JUL~

207 Pb/ 235U

i

i

i

i

i

|

1

2

3

4

5

6

i,

Fig. 3. Whole-rock Rb-Sr data for the Icart gneisses (after Adams, 1967).

4. U-Pbzircon Concordia plot for an Icart gneiss sample (after Calvez and Vidal, 1978).

Fig.

87Sr/ 86Sr

8z Srl 86$r

o

%.%%~o o/

0.750

/

0.800

• +

Port Beni Trebeurden Morguignen

/o-~

0.725-///~

o/

0.750 +o oo

0.700

0

.

0:5 ~

erRb I S~Sr

1:5

0.700

i

i

F

1

2

3

}"

Fig. 5. Whole-rock Rb-Sr data for Alderney and Cap de la Hague gneisses (after Adams, 1967; Leutwein et al., 1973). Fig. 6. Whole-rock Rb-Sr data for Port-Beni, Trebeurden and Morguignen gneisses (after Vidal, 1976, Auvray et

al.,1980).

T~gor Ortho- and paragneisses at Port-B~ni are found as inclusions in the Tregor granodiorite, which is dated at about 670 Ma (Adams, 1967). Since these inclusions have a much more complex tectonic-metamorphic history than their host rocks (Auvray, 1979), they are possibly pre-Cadomian relicts. The Trebeurden orthogneisses, 20 km to the West, as well as the Morguignen orthogneisses in the Petit-Tr~gor, have also been proposed as remnants of older Precambrian rocks.

On a 87Sr/S6Sr vs S~Rb/S6Sr diagram (Fig. 6), all these gneisses exhibit a strong dispersion (Vidal, 1976). Here also, no precise age information can be drawn, but some older reference isochrons could however be related to a pre-Cadomian original age. This scatter, well beyond analytical errors, could be due to initial Sr isotope and age differences between different petrographic varieties and possible open-system behaviour of the whole rocks during later anatexis or regional metamorphic events (Cadomian and/or Hercynian). Pre-Cadomian orthogneisses from Port-B~nf, TrSbeurden and Morguignen have been dated by U-Pb method on zircons (Auvray et al., 1980). All the zircons are discordant (Figs. 6--9) but define good Discordia lines. The upper intersections with Concordia give reliable ages of 2.02 -+ 0.03 Ga for

206Pb/238~,j

0301206pb/238U 930-

15oo~ ~ 020-

000

~.~"

)20-

~"

0.10-

3.102O7pb/235 U

00

~

207Pb/235U 0.0

~

~

~

~

~

•

Fig. 7. U-Pb zircon Coneo~lia for Ttebeurden gneiss sample (after A u ~ a y et a]., 1980). Fig. 8. U-Pb zircon Coneordia for Morguignen gneiss ~rap]e (after Auvray et a]., 1980). 206 Pb / 238 U

2000

030 1 5 0 0 0,200100.0

ZO~'pb / 235 U

1'.0

210

310

4',0

510

610

"

Fig. 9. U-Pb zircon Concordia for Port,Beni gneiss sample (after Auvray et al., 1980).

Trebeurden, 1.99 -+ 0.03 Ga for Morguigen and 1.79 -+ 0.02 Ga for Port-B6ni gneisses. Owing to the orthogneissic nature of these rocks, we interpret these ages as the timing of emplacement of the precursor rocks of the gneisses.

Conclusions Gneisses from la Hague, Alderney, Icart, Port-B~ni, Tr~beurden and Morguignen all present the following original features: (1) they belong to a pre-Cadomian orogeny; (2) they are located in the D o m n o n e a n Domain as small enclaves in Cadomian or Hercynian rocks; (3) Rb-Sr WR data scatter widely in excess o f analytical errors, b u t reference isochrons generally suggest pre-Cadomian ages; (4) U-Pb zircons ages are slightly discordant and give upper intersections with the Concordia curve of around 2 Ga. All these relicts may belong to a single orogenic episode. CADOMIAN--HERCYNIAN ISOTOPIC EVOLUTION OF THE ARMORICAN MASSIF

Sr isotopic data Radiometric dating has established the existence of isolated occurrences of pre-Cadomian rocks in the northern part of the Armorican Massif. Could this old basement have been involved in the genesis of Upper Precambrian and Phanerozoic magmas? Strontium isotope geochemistry provides some very useful constraints. In the following discussion, the STSr/~6Sr ratio of 0.705 is taken as the upper limit on mantle evolution. We have compiled the available initial ~TSr/~6Sr ratios from whole rocks isochrons, eliminating data with large errors (an arbitrary cut-off of 0.002 was chosen) as well as data suspected n o t to correspond to the true magmatic ratios due to internal Sr redistribution (i.e., some silicic volcanics). The data are taken from Adams (1967, 1976), Barriere et al. (1971), Vidal (1972, 1976), Cogn6 and Peucat (1973). The initial STSr/S6Sr ratios range from 0.702 to 0.716, although most of them do n o t exceed 0.710. Some granitoids actually lie close to the mantle evolution line, as can be seen in Fig. 10, where initial STSr/S6Sr ratios are plotted vs emplacement age. Rocks such as the South Armorican Ordovician granites (Vidal, 1972), the Mancellian Cadomian granodiorites (Jonin and Vidal, 1975), and the Ploumanach D o m n o n e a n Granite (Vidal, 1976) are therefore directly derived from the mantle or more probably are the melting products of short-lived, mantle-derived crustal rocks. It is apparent that the data points are n o t randomly distributed b u t that a rather good correlation exists. The older the rock, the less radiogenic is the initial strontium. If the points located above the trend are withdrawn (mostly leucogranites), a correlation coefficient of 0.7 is obtained (for Rb/Sr of ca. 0.4). The envelope

10

around the data points intersects the mantle evolution line in the range of 700--500 Ma (Fig. 10). Discussion

The following models for the origin of magmas between ca. 650 and ca. 300 Ma ago are n o w discussed in the light of the isotopic data presented in Fig. 10: (1} Reworking of old continental crust (ca. 2 Ga old "North Armorican Precambrian Old Relicts" or " N A P O R " ) (2} C o m m e n c e m e n t of continental crustal growth at ca. 700 Ma-600 Ma ago with subsequent open-system evolution or with subsequent closedsystem evolution. (1) Reworking o f old continental crust ca. 2 Ga old Icart gneisses, with their mean Rb/Sr ratio of ca. 0.8 (Adams, 1967) would have had a mean STSr/S6Sr as high as ca. 0.750 at 600 Ma ago (Fig. 11). Their remelting at that time would have produced magmas with similarly high initial ratios. The other N A P O R rocks have Rb/Sr ratios in the same range (Auvray, 1979). It follows that the involvement of such basement in the genesis of Upper Precambrian and Phanerozoic magmatism was negligible. One could argue that N A P O R rocks are n o t representative of the whole continental crust in this area. A comparison with estimates of continental crust R b / S r ratios given b y several workers is useful: Rb/Sr = 0.33 from Gast (1960); 0.25 from Faure and Hurley (1963), Taylor (1964), Hart and Tilton (1966); 0.35 from Shaw et al (1967); 0.15 from Armstrong (1968), Hurley (1968); 0.18 from Faure and Powell (1972); 0.19 from Shaw

~rSr/86Sr

o ).710

0.705

tMa 600

5oo

500

30o

200

~ oo

o

0.700

Fig. 10. Initial sTSr/S6Sr ratio vs. time for Armorican igneous rocks (data from Adams, 1967, 1976; Barri~re et al., 1971, Cogn6 and Peucst, 1973; Vidal, 1972, 1976).

11

(1975). The last three estimates take into account R b depletion in granulite facies (Lambert and Heier, 1968), a depletion probably typical of lower crustal rocks. According to Hurley (1968), the upper continental crust has a mean Rb/Sr ratio of 0.25, the lower crust, 0.03, the whole crust averaging 0.15. This last figure requires more data, because South Australian Precambrian granulites have Rb/Sr ratios as high as 0.21 (Lambert, 1971), and those of North Norway, ratios of 0.12 (Heier and Thoreson, 1971). If we chose a mean value of 0.20 and an age of 2 Ga, the evolution line is still clearly, above the observed initial ratios (Fig. 11). However, a mean value of 0.707 for the initial 87Sr/86Sr ratios for the 650--300 Ma period would imply a mean Rb/Sr ratio of 0.05 for a hypothetical 2 Ga source. This ratio is 3--4 times lower than the figures for the most depleted granulite terrains of the Hercynian belt (see Discussion on p. 000) though it is in agreement with Hurley's estimate for the lower crust. Perhaps the major argument against this model is that remelting of an old basement cannot explain the rather fast isotopic evolution from 0.703 to 0.710 over some 300 Ma. In this discussion, it is assumed that orogenic processes cannot completely erase the isotopic m e m o r y of an older continental fragment b y equilibrating with a large Sr reservoir like the mantle. Such equilibration processes have been proposed by Armstrong (1968) and Shaw (1975) although they have 87Sr

se Sr 0.760

~ 0

'

2

0720 '

1

i

06

,

0.3

--o

700

Fig. 11. Strontium isotopic evolution line for Icart gneisses and Cadomian-Hercynian pattern.

presumably played a limited role in orogenic processes, due to the unsubductibility of continental sialic crust Moorbath 1977, 1978). (2) C o m m e n c e m e n t o f continental crust growth at ca. 700--600 Ma A more likely model is that the Armorican continental crust came into existence 700--600 Ma ago by the accretion of mostly mantle-derived materials in a subduction zone. This hypothesis implies that during the Upper

12

Precambrian, the Armorican Massif, overlain by a thick supracrustal series (brioverian), was essentially a continental margin domain and was probably located at the northern border of Gondwana (Auvray et al., 1980). During the Cadomian orogeny, the 2 Ga-old basement played a very limited role in the Cadomian magma production which is calc-alkalic and probably related to the closing of an oceanic domain to the north (Auvray, 1979). CADOMIAN-HERCYNIAN ISOTOPIC EVOLUTION OF WESTERN AND CENTRAL EUROPE

Occurrence of pre-Cadomian rocks in the Hercynian belt

The occurrence of Cadomian magrnatic rocks is rather well documented from geochronological studies: in the French Central Massif (Bernard-Griffiths et al., 1977; Hamet and Allegre, 1972), in the Pyrenees (Vitrac and A1legre, 1971) and in the Alps (Hamet and Albarede, 1973). Regarding a hypothetical pre-Cadomian orogeny, some workers suggest that the retrograde granulites found nearly everywhere in the Hercynian belt (Hameurt, 1967; Den Tex and Floor, 1971; Forestier et al. 1973; Leyreloup, 1974; Marchand, 1974; Lasnier, 1977) belong to such an orogeny (Forestier, 1977; Zoubek, 1977). These granulites generally give Middle Paleozoic to Hercynian ages (Jager and Watznauer, 1969; Vitrac and Allegre, 1971; Arnold and Scharbert, 1973; Bonhome and Fluck, 1974; Grauert et al., 1974; Pin and Lancelot, 1978; Van Calsteren et al., 1979). However, Hamet et al. {1975) obtained Rb-Sr whole-rock ages of 800-900 Ma on granulitic inclusions in the Bournac Neogene volcano (Central Massif). They suggest that the underlying basement could be pre-Cadomian in age. Furthermore, the granulites on the northern Iberic continental margin (Banc Le Danois) give Rb-Sr whole-rock data scattered between two reference isochrons at 1.5 and 3.3 Ga and Rb-Sr mineral ages of 1.5 Ga. The origin of these dredged rocks is not firmly established, however. Western and Central European post-Cadomian Sr isotopic evolution

Using the same selection criteria, as for the Armorican Massif, and normalizing data to the value of 0.7080 for the Eimer and Amend Sr CO3, we plot initial S7Sr/S6 Sr ratios versus emplacement age (Figs. 12 and 13). The data are from Arnold {1970), Arnold et al., (1973), Bernard-Griffiths et al. (1977), Borsi e t al. (1974), Brewer and Lippolt (1974), Hamet and A1legre (1972, 1973), Hamet and Mattauer (1977), Hameurt and Vidal (1973), Hanny et al. (1974), Kohler et al. (1974), Roques et al. (1971), Vialette and Sabourdy (1977), Vidal (1976), Vidal et al. (1975), Vitrac et al. (1968, 1969, 1971, 1973) and Wendt et al. (1970). The data have been separated into two geographic groups: (I) Central Massif and Pyrenees (Fig. 12), and (2) Vosges, Black Forest, Harz, Alps and Bohemian Massif (Fig. 13). All the

13

87 S r / 86Sr

o

%("

0/

Zo o,, Ao- o°/ /o

z

/"

/

o

i

o /o o

600

o o

o

-

0.715

,yoo: "Yo

/ @

o

/o

500

J

¢

0.710

oo

o°

o

400

500

200

Fig. 12. Initial SVSr/S6Sr ratio vs. time for Central Massif and Pyrenees igneous rocks (data from Bernard-Griffiths et al., 1977; Hamet and All~gre, 1972, 1973; Hamet and Mattauer, 1977, Roques et al., 1971; Vialette and Sabourdy, 1977; Vitrac et al., 1968, 1969, 1971, 1973).

/

.o

\c~//

/

°°o~y o o

oo

o

o

87Sr/86Sr

o

0 715

o

o o

)710

o ~

...........

)705

)) : 127

? i

r

tMo 600

500

400

300

200

0 700

Fig. 13. Initial S~Sr/S~Sr ratio vs. time for Vosges, Black Forest, Harz, Bohemian Massif and Alps igneous rocks (data from Arnold, 1970; Arnold et al:, 1973; Borsi et al., 1974; Brewer and Lippolt, 1974; Hameurt and Vidal, 1973; Hanny et al., 1974; KShler et al., 1974; Vidal et al., 1975; Wendt et al., 1970).

] 4

Sr/"

) 800

/

. •

,~.

-,

/7

•

-,,

~

::

•

S,

"

.

~/e •

Il.O.Z¢Oo • /o •

,

•

I tGa

,

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/

"h',.,.w"" ~ •

"

° ~/

•

•o

•

,

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•

.'~:

,,/

. . . . .

MANTLE

.

-,--,~0705 1

t Mo 1 600

i 500

i 400

I 500

I 200

I 1O0

0 700

Fig; 14. I n i t i a l S~Sr/SISr r a t i o vs. t i m e f o r t h e C e n t r a l a n d w e s t e r n e u r o p e a n C a d o m i a n Hercynian belt.

data, including those of the Armorican Massif, are plotted in Fig. 14. Similar trends to the Armorican Massif are observed: The same age range and increasingly radiogenic character of the magmas with time is to be noted. The slope of the trend is somewhat higher, and corresponds to Rb/Sr ratios of 0.5--1, typical of granitic rocks. Discussion

As for the Armorican Massif, we can draw the following conclusions: (1) The envelope of initial Sr isotopic ratios has its origin near the mantle evolution line, in the 700 600 Ma time span. This implies that this segment of continental crust was created during the Upper Precambrian and the Lower Paleozoic. The reworking of old Precambrian basement of NAPOR type is not totally rejected but must have been of small importance. Grauert and Arnold (1968), Grauert et al. (1973, 1974), Arnold and Scharbert (1973) and Jager (1977), reached a similar conclusion with the Moldanubian and the Alpine basements, but with a different approach, using data on zircons and the initial 87Sr/86Sr vs. (t) applied to individual geologic units. (2) The strong post 700 000 Ma increase in initial strontium ratios suggests that the continental crust evolved with intense sialic reworking during the Paleozoic. Hercynian m ~ is in part juvenile but most granites have ratios too high to have been directly or indirectly derived from the mantle. In particular, it should be emphasized that the s 7Sr/86St pattern in Hercynian times is very different from that observed in magmatism associated with most

15 of the Mesozoic and Cenozoic subduction zones, which is much more homogeneous and lower, 0.703--0.706, except where a thick and old piece of continental crust, such as in the central Andes, might have induced some high STSr/S~Sr ratios of up to 0.709 (James, 1978) possibly by a process of selective contamination in radiogenic strontium (Briqueu and Lancelot, 1979; Hawkesworth, 1979). As seen before, such hypothetical old continental crust played no major role in the production of the Cadomian to Hercynian magmas and there is presently no field and geochronological evidence for its widespread extension. Therefore, we suggest a model involving a significant intracrustal subduction of Himalayan type for the Hercynian orogeny (Cogn~, 1977). Our model reaches the same conclusions as Vitrac et al. (1975). Working on a restricted segment of the Hercynian belt (Eastern Pyrenees), they propose that Hercynian magmatism largely originated b y Cadomian (and pre-Cadomian?) basement recycling. D u t h o u (1977) showed that, on the scale of the northwestern Central Massif, an original suite of cambrian mantle-derived magmas has been reworked throughout Lower Paleozoic times and that a second suite of early Hercynian age, and also of mantle or lower crustal origin, has been recycled throughout the Hercynian orogeny. We can try to place some constraints on the types of rocks involved in the genesis of Hercynian magmas. As seen above, the trend of Fig. 14 has a slope corresponding to a Rb/Sr ratio of 0.5--1. The granulitic rocks incorporated in the Hercynian belt are characterized by unusually high Rb/Sr ratios: Bohemia (Arnold et al., 1973) Saxony (Jager and Watznauer, 1969) Vosges (Bonhomme and Fluck, 1974} Agly - - P y r e n e e s (Vitrac-Michard et al., 1975) Castillon -- Pyrenees (Vidal, unpubl.) La Picherais, Brittany (Vidal et al., 1980) Central Massif (Dupuy et al., 1976)

3.6 3.4 0.45 0.45 0.22 1 0.15

These granulites could therefore be reasonable candidates as source rocks during Hercynian magma genesis. On the other hand, Upper PrecambrianLower Paleozoic sedimentary rocks could also be considered as good candidates for remelting. More than 50 samples of unmetamorphosed sediments from the upper Precambrian and Palaozoic basins of Central Brittany and of amphibolite-facies metasediments from the South Armorican zone have been analyzed (Vidal, 1976). 87Sr/S6Sr ratios were generally between 0.710 and 0.720 during the Hercynian orogeny. Finally, the Rb-Sr--STSr/8~Sr systematics of Lower Paleozoic granitoids make these rocks also good candidates for Hercynian remobilization. However, strontium isotopes alone are not discriminant enough as tracers and more information is needed in order to determine which of these three (granulites, sediments, granitoids) sources has been involved and to what extent. In this respect, Pb-Pb, Sm-Nd and oxygen isotope data should be helpful.

t6 In s h o r t , we have selected a b o u t o n e h u n d r e d initial STSr/S6Sr ratios in m a g m a t i c r o c k s f r o m t h e l i t e r a t u r e . Whilst s o m e bias in s a m p l i n g p r o c e d u r e s is p r o b a b l e f o r various reasons, we n e v e r t h e l e s s , p r o p o s e t h a t : (1) T h e n o r t h A r m o r i c a n ca. 2 Ga-old gneisses p l a y e d a v e r y limited role in t h e origin of t h e C a d o m i a n - - H e r c y n i a n m a g m a t i s m . (2) T h e c o n t i n e n t a l c r u s t in Western and Central E u r o p e was essentially d e v e l o p e d in U p p e r P r e c a m b r i a n t o Paleozoic t i m e s ( 7 0 0 - - 3 0 0 Ma) (3) D u r i n g t h e H e r c y n i a n o r o g e n y , sialic r e w o r k i n g a p p a r e n t l y p l a y e d a g r e a t e r role t h a n t h e a d d i t i o n o f m a n t l e - d e r i v e d rocks. ACKNOWLEDGEMENTS This p a p e r b e n e f i t t e d f r o m discussions w i t h o u r colleagues at R e n n e s , in p a r t i c u l a r . MM C a p d e v i l a , C a r p e n t e r , J a h n and P e u c a t , T h e final English version has b e e n i m p r o v e d b y M. C a r p e n t e r a n d b y an a n o n y m o u s reviewer. REFERENCES Adams, C.J.D., 1967. A Geochronological and Related Isotropic Study of Rocks from North-Western France and the Channel Islands (United Kingdom); Ph.D. thesis, Oxford. (unpubl.) Adams, C.J.D., 1976. Geochronology of the Channel Islands and adjacent French Mainland. J. Geol. Soc. London, 132: 233--250. All~gre, C.J., Hamet, J. and Leyreloup, A., 1975. Etude STRb/S7 Sr des enclaves catazonales remont~es par les volcans n~og~nes du Velay: presence d'une socle ant~cadomien sous le Massif Central. 3~me R~un. Ann. Sci. Terre, Soc. G~ol. Fr., p. 8. Armstrong, R.L., 1968. A model for the evolution of strontium and lead isotopes in a dynamic earth. Rev. Geophys., 6(2): 175--199. Arnold, A., 1970. O n the history of the Gotthard Massif (Central Alps, Switzerland). Eclog. Geol. Helv., 63(1): 29--30. Arnold, A. and Scharhert, H.G., 1973. Rb--Sr Alterbestimmungen an granuliten des Siidlichen B~hmischen Masse in Osterreich. Schweiz. Mineral. Petrogr. Mitt., 53 (1): 61--78. Auvray, B., 1979. Gen~se et ~volution de la Crofite Continentale Dans le Nord du Massif Amoricain. Th~se d'Etat, Rennes. Auvray, B., Charlot, R. and Vidal, P., 1980. Donn~es nouvelles sur le Prot~rozoique inf~rieur du domaine Nord-Armoricain (France): ~ge et signification. Can. J. Earth. Sci.,

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