bosse_early-orogenic exhumation eclogites champtoceaux complex (variscan belt, france)_geological...

GEOLOGICAL JOURNAL

Geol. J. 35: 297±325 (2000)

Late Devonian subduction and early-orogenic exhumationof eclogite-facies rocks from the Champtoceaux Complex

(Variscan belt, France)

VALERIEBOSSE,1* GILBERTFERAUD,1GILLESRUFFET,1MICHELBALLE© VRE,2

JEAN-JACQUESPEUCAT 2 andKOENDEJONG3

1GeÂosciences Azur (UMR-CNRS 6526), UniversiteÂ de Nice-Sophia Antipolis, Nice, France2GeÂosciences Rennes (UPR-CNRS 4661), UniversiteÂ de Rennes, France

3Geological Survey of Japan, Geology Department, Tsukuba, Ibaraki, Japan

In order to de®ne the mechanisms involved during exhumation of the eclogite-facies rocks from the Champtoceaux Complex(Armorican Massif, France), Sm-Nd, Rb-Sr and 40Ar/39Ar methods are combined with a petrological study to construct a pres-sure±temperature±time (P±T±t) path for the Cellier Unit.

The Champtoceaux Complex is a crustal-scale thrust sheet located in the South Armorican Domain. The lower unit, the Cel-lier Unit, consists of leucocratic gneisses, mica-schists and well-preserved eclogites. Petrological data on selected samples fromdifferent lithologies show (1) preservation of growth zoning in garnet, (2) no amphibolite- or greenschist-facies overprint in theeclogite and (3) variations in the Si content of phengite as a function of bulk-rock chemistry, P±T conditions and partial re-equilibration during decompression.

Sm-Nd analyses on the eclogite sample give a Grt±Cpx±whole-rock age of 362�25 Ma, consistent with the U-Pb age of356�8 Ma (recalculated) obtained from the same sample by J. L. Paquette in 1987. Preservation of growth zoning in the garnetand the absence of late overprint show that resetting of both Sm-Nd and U-Pb systems is unlikely. The age of c. 360 Ma is thusinterpreted as the age of the high-pressure event.

Eight 40Ar/39Ar plateau ages, ranging from 352.0�1.6 to 340.5�1.4 Ma, are obtained from phengite single grains from sixsamples. The existence of Ar inheritance is unlikely, because (1) 40Ar/39Ar ages are younger than the age of the high-pressureevent as deduced from U-Pb and Sm-Nd ages, (2) duplicates display a high reproducibility of plateau ages in all cases, and (3) aconcordant Rb-Sr age is obtained on one common sample. These plateau ages probably represent closure temperatures (pos-sibly on the order of 450±500�C) for the best preserved and oldest samples, whereas the younger plateau ages may represent alater closure of the K/Ar system due to continuous deformation and chemical re-equilibration during retrogression. Copyright# 2000 John Wiley & Sons, Ltd.

Received 6 January 2000; revised version received 6 August 2000; accepted 18 September 2000

KEY WORDS eclogite-facies metamorphism; exhumation; geochronology; Variscan belt; Armorican Massif; Champtoceaux Complex; France

1. INTRODUCTION

Eclogite-facies rocks are widely distributed in most orogenic belts of Late Proterozoic to Recent age (Maruyama

et al. 1996). They are key indicators of the burial to great depths (in excess of about 30 km) of large, coherent slices

of oceanic and continental crust during the early stages of the orogenic evolution. Open questions concern mainly

the exhumation mechanisms of the high-pressure (HP) rocks and the rheological behaviour of the crust at great

depth. In order to de®ne the possible mechanisms involved during exhumation of HP rocks, accurate constraints

are needed on their pressure±temperature (P±T) evolution, the timing of the (HP) event and the cooling rate. This

Copyright # 2000 John Wiley & Sons, Ltd.

* Correspondence to: Dr ValeÂrie Bosse, Laboratoire de GeÂochronologie-GeÂochimie, GeÂosciences Azur (UMR-CNRS 6526), UniversiteÂ de Nice-Sophia-Antipolis, Parc Valrose, 06108 Nice Cedex 2, France. E-mail: [email protected]

koendejong

Sticky Note

Presently Assocate professor of TectonicsSeoul National UniversitySeoul 151-742, [email protected]

paper is an attempt to solve some of these questions in a speci®c case, the Champtoceaux Complex (Armorican

Massif, France), which belongs to the Variscan belt of Late Palaeozoic age. High-pressure relics are found through-

out the Variscan belt from the Iberian Massif to the Bohemian Massif (O'Brien et al. 1990). Although different

types of high-pressure rock have been distinguished in the Ibero-Armorican arc (Godard 1988; BalleÁvre et al.

1993), previous geochronological data suggest that they belong to an early episode of Silurian age (about 400±

440 Ma, U/Pb; Pin and Peucat 1986; Paquette et al. 1995). Younger ages have also been obtained from eclogites

in the Armorican Massif, as in the Champtoceaux Complex (about 360 Ma, U-Pb; Paquette 1987), as well as in

other areas of the Variscan belt, such as the Black Forest (about 330±340 Ma, Sm±Nd; Kalt et al. 1994). Never-

theless, these latter ages have generally been regarded as unreliable because of methodological problems, or were

considered to be the result of late resetting of the chronometers (Faure et al. 1997). Such discrepancies are crucial

for the choice between different possible models of the tectonic evolution of the Variscan belt. The Champtoceaux

Complex offers an opportunity to clarify these issues by means of a combination of petrological data and isotopic

data obtained by different methods (Sm-Nd, Rb-Sr and 40Ar/39Ar) from a few carefully selected samples. We shall

tentatively propose a model of isotopic closure of phengites, dependent either on temperature or on deformation-

induced chemical re-equilibration. If this model is valid, it implies closure temperatures for the K-Ar system in

phengites higher than those generally assumed.

2. GEOLOGICAL SETTING

2.1. Main units

The Champtoceaux Complex (CogneÂ 1966; Marchand 1981; Bayer and Hirn 1987; BalleÁvre et al. 1993) is a crus-

tal-scale thrust sheet located in the South Armorican domain (Brittany, France), bounded to the north by the Nort-

sur-Erdre Fault and to the south by the southern branch of the South Armorican Shear Zone (SASZ) (Figure 1). In

this area four major superposed units are identi®ed, brie¯y described as follows from bottom to top. The Para-

autochthon, the Mauves Unit, consists of a monotonous sequence of metagreywackes of unknown age. The

Champtoceaux Complex is thrust over the Mauves Unit. It consists of several stacked units distinguished mainly

by lithology and metamorphism. Because the thrusts bounding the different units are ductile, and because most

rocks have a mylonitic fabric, identi®cation of the thrusts relies essentially on discontinuities in lithology and/or

metamorphic history. Two major subunits can be distinguished. The Lower Allochthon includes the Cellier Unit,

consisting essentially of leucocratic gneisses enclosing numerous eclogite lenses and overlain by mica-schists, and

the St Mars Unit, a strongly deformed leucocratic gneiss. In both units, the leucocratic gneisses have been derived

from granitic bodies of Early Ordovician age (Vidal et al. 1980; Paquette et al. 1984). The Middle Allochthon dis-

plays migmatitic orthogneisses with poorly-preserved relics of eclogite lenses (Champtoceaux Unit) and a

sequence of deformed gabbros with some peridotites (Drain Unit), as well as metavolcanics and metasediments

(HaÃvre Unit). The Upper Allochthon, the Mauges Unit, consists of weakly- to strongly-deformed Proterozoic sedi-

ments and volcanics, unconformably overlain by Cambrian to Silurian sediments (Cavet et al. 1966). The relation-

ships between the Champtoceaux Complex and the Mauges Unit are not well exposed.

The Ancenis Basin, northeast of the Champtoceaux Complex, is in fault contact with the stacked units described

above. The sedimentation within the Basin consists essentially of purple shales with interbedded sandstones, with

conglomerates dominating the top of the sequence (RivieÁre 1977). The beds are undeformed or tilted, and have

been intruded by minor stocks of microgranite and a two-mica granite. Poorly known fauna and ¯ora suggest a

continental (¯uviatile or limnic) environment and an Early Carboniferous age (BeaupeÁre 1973; Cavet et al. 1978).

2.2. Deformation history

Three main deformational events can be distinguished in the Champtoceaux Complex (Lagarde 1978; Marchand

1981; BalleÁvre and Marchand 1996). Relics of the eclogite-facies deformation are scarcely recognized in some

298 v. bosse et al.

Copyright # 2000 John Wiley & Sons, Ltd. Geol. J. 35: 297±325 (2000)

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exhumation of eclogite-facies rocks, champtoceaux complex, france 299

boudins, but cannot be used for determining a strain pattern at regional scale. For this reason, the ®rst event was the

main ductile deformation, easily recognized in the ®eld by a pervasive foliation and a prominent stretching linea-

tion. This deformation is associated with the thrusting of the Champtoceaux Complex (from east to west) over the

Parautochthon (Mauves Unit). The second event is the ductile deformation associated with the exhumation of the

Champtoceaux Complex with respect to the Mauges Unit, the contact being interpreted as a major ductile detach-

ment. The third event corresponds to the folding of the earlier structures (i.e. foliations, stretching lineations, thrust

surfaces between units within the Complex, and folding of the detachment fault at the contact between the Champ-

toceaux Complex and the Mauges Unit) around an east±west axis which plunges towards the east. This late fold-

ing, which is responsible for the present-day outcrop pattern of the Champtoceaux Complex, was probably

synchronous with the dextral strike-slip motion along the SASZ, the southern part of the Champtoceaux Complex

being strongly sheared along the SASZ. Intrusion of two-mica leucogranites (Vigneux Granite (Hussein 1960);

Mortagne Granite (Guineberteau et al. 1987; Roman-Berdiel et al. 1997)) during dextral movement along the

SASZ is associated with the development of an andalusite-bearing contact aureole.

3. ECLOGITE-FACIES ROCKS FROM THE CELLIER UNIT

Eclogite-facies rocks of the Champtoceaux Complex are found mainly in the Cellier Unit, where their excellent

preservation has attracted attention since the last century (Lacroix 1891). The Cellier Unit consists essentially of

®ne-grained leucocratic gneisses (i.e. leptynites) derived either from granites and granodiorites (Lasnier et al.

1973; Lagarde 1978) and/or from volcanic or volcaniclastic rocks of dacitic to rhyolitic composition. The granites

have been dated as Lower Palaeozoic (Vidal et al. 1980; Paquette 1987). The leptynitic gneisses display abundant

small (1±5 m) eclogite lenses. The basic rocks have a rather restricted range of compositions and rare earth element

patterns ranging from N-MORB to T-MORB types (Bernard-Grif®ths and Cornichet 1985; Paquette 1987) and

contain relict doleritic textures (Godard 1988); they can thus be interpreted as representing a former dyke swarm

intruded into a thinned continental crust. The leptynites and their eclogite lenses are structurally overlain by gar-

net-bearing mica-schists, which also contain a few eclogite lenses.

The ma®c rocks display well-preserved eclogitic parageneses (Lacroix 1891; BrieÁre 1920; Velde 1966, 1970;

Godard et al. 1981; Godard 1988) consisting of combinations of garnet, omphacite, sodic-calcic amphibole (bar-

roisite), quartz, zoisite, phengite and rutile. In some samples from the eastern part of the Cellier Unit, glaucophane

crystals developed as late porphyroblasts (Godard et al. 1981). Coexisting kyanite and omphacite are observed in a

few eclogites from the western part of the Cellier Unit (Lacroix 1891; BrieÁre 1920; Velde 1970). In a few quartz-

rich rocks from Fay-de-Bretagne, parageneses such as garnet � omphacite (Jd55) � kyanite or garnet � jadeite

(Jd95) � phengite have been reported (BalleÁvre et al. 1987).

Mineral parageneses in the mica-schists also suggest an increasing grade of the eclogite-facies event from east to

west (BalleÁvre and Marchand 1991). In the eastern part of the unit (along the Loire River), peak assemblages

include garnet � chloritoid � chlorite, whereas samples from the western part of the unit display garnet � kyanite

assemblages, with relict chloritoid and staurolite inclusions in garnet (BalleÁvre et al. 1989). Estimated P±T con-

ditions for the eclogite-facies event are about 15±20 kbar, 550�C and 20±25 kbar, 650�C for the eastern and wes-

tern part, respectively, of the Cellier Unit (BalleÁvre and Marchand 1991). In both areas, near-isothermal

decompression allowed the growth of chlorite or staurolite � chlorite.

Well-preserved relics of eclogite-facies parageneses in granitic or rhyolitic rocks are unknown in the Cellier

Unit. Garnet coronas around biotite in undeformed pods were originally ascribed to a granulite-facies event (Las-

nier et al. 1973; Vidal et al. 1980), but later studies have emphasized their similarity with coronas developed in

jadeite-bearing metagranites (e.g. Sesia Zone, western Alps (Compagnoni and Maffeo 1973; Koons et al. 1986);

Dora-Maira, western Alps (Biino and Compagnoni 1992); Malpica-Tui Unit, Iberian Massif (Gil Ibarguchi 1995)).

Although jadeite has not been found in the plagioclase pseudomorphs, the coronas are interpreted as recording

partial re-equilibration during the high-pressure event (BalleÁvre et al. 1989). In strongly deformed granitic to rhyo-

litic rocks, the high grossular content of garnet (Le Goff and BalleÁvre 1990) and the high Si content of phengite

300 v. bosse et al.


porphyroclasts (Le Goff 1989) are possible indicators of the high-pressure metamorphism. The main ductile defor-

mation (D1 of Lagarde 1978) took place in the stability ®eld of albite.

4. PETROLOGY OF THE STUDIED SAMPLES

The sampling strategy was to combine petrological and geochronological studies on carefully-selected samples.

Criteria for selecting samples were as follows. The only studied eclogite (sample CX5c) was chosen because (1) a

U-Pb zircon age was previously obtained from it (Paquette 1987) and (2) it appeared to be a good candidate for Sm-

Nd and 40Ar/39Ar dating because of the abundance of garnet and the presence of phengite. This eclogite was col-

lected in a quarry (la BreÂhardieÁre) located on the left bank of the Loire River, 2 km southwest of la Varenne (Figure

1). The other selected rocks, which represent a large spectrum of whole-rock compositions, were collected in two

closely-spaced localities, namely Fay-de-Bretagne and Campbon (Figure 1). They consist of three mica-schists

(FAY 24, FAY 29 and CH 20), a leptynite (FAY 13) and a quartz vein (CAM 5).

4.1. Mineral parageneses

The eclogite CX5c (Table 1) is especially well-preserved, undeformed (except for a few narrow albite-bearing

fractures) and shows a paragenesis consisting of garnet, omphacite, glaucophane, phengite and rutile. Most garnet

grains present an atoll shape, the atoll being ®lled with phengite, and minor omphacite or quartz. Some garnet

grains are full idioblastic crystals, which sometimes contain inclusions of dark blue-green sodic-calcic amphibole

in the core and a pale green omphacite in the rim. Two types of omphacite are present: larger grains with abundant

inclusions of rutile needles, quartz, minute paragonite and rare plagioclase, and ®ner, inclusion-free grains. The

former are thought to represent omphacite topotactically developed on the site of a magmatic phase (augite?) while

the latter are newly-grown sodic pyroxenes. Both types have the same chemistry (about 50±55 mol % of jadeite).

Glaucophane crystals are idioblastic porphyroblasts which contain a few garnet inclusions. Phengite is mainly

found in the atoll garnets. This sample presents many similarities with the one described by Godard et al.

(1981), except that phengite and clinopyroxene are present in the atolls instead of quartz.

Two types of metapelite have been collected. Samples FAY 24 and FAY 29 are garnet±kyanite mica-schists simi-

lar to those previously described (BalleÁvre et al. 1989). The high-pressure paragenesis consists of quartz � phen-

gite I � garnet � kyanite � rutile. Garnet from sample FAY 24 shows an inclusion-rich core, with chloritoid and

staurolite � graphite, and a rim with abundant kyanite and rare staurolite inclusions. Chloritoid and staurolite

grains are also found in the matrix, close to kyanite. Sample FAY 29 is similar, with abundant inclusions of kyanite

and rare staurolite in garnet. In both samples, phengite I is distinguished by its large size (about 0.2±0.8 mm), and

kyanite is replaced by minute grains (about 10±20 mm) of phengite II and paragonite. The second type of meta-

pelite, represented by sample CH 20, shows an intense ductile deformation, where high-pressure minerals are now

porphyroclasts in a ®ne-grained matrix. Large relict phengite I grains are bent or folded, and rimmed by ®ne-

grained biotite. Undeformed or foliated aggregates of phengite II and albite have replaced paragonite (or jadeite?).

Garnet contains inclusions of quartz, rutile and rare paragonite and is partially resorbed. Minerals crystallizing in

its pressure shadows include biotite and oligoclase. Rutile, common as inclusions in garnet and phengite I, is

rimmed by ilmenite in the matrix.

The felsic rocks, volumetrically the most abundant but the poorest in terms of preservation of the HP paragen-

eses, are represented by sample FAY 13, a very-®ne grained leucocratic gneiss. Relics of magmatic phases include

allanite and rare recrystallized K-feldspars. The foliation is de®ned by the shape fabric of quartz, albite, white mica

and biotite. Clinozoisite forms either single grains or surrounds allanitic cores. Garnet is common, as small atoll-

shaped grains around undeformed phengite grains. In the matrix, phengite grains are mostly large deformed clasts,

whereas biotite grains are minute crystals de®ning the foliation.

The quartz vein (sample CAM 5) consists of quartz and white mica, and displays a well-developed foliation.



Table 1. Mineral parageneses of the studied samples. Garnet rim compositions are plotted in Figure 3b. Phengite compositions are plotted in Figure 4b. mg � MgO/(FeO�MgO). Mineral abbreviations are from Kretz (1983)

Sample

Prograde history and peak PT conditionsÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐ

Paragenesis Reaction Deformation

Retrograde historyÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐ

Paragenesis Reaction Deformation

Eclogite CX5c Grt±Omp (Jd46±54)±Gln(mg 0.73-0.84)±Phe±Rt±Sulphide

Grt � Omp� H2O�Qtz�Gln

orOmp� (K2O�H2O)� Grt atoll�Gln�Phe

Unknown Rare blue-green Amp None NoneRare fractures

Grt±Kymica-schists

FAY 24 Qtz±Grt±Ky±Phe I±Rt±Tr±(St I±Cld I±Gr)

St�Cld �Grt�Ky�H2O

Inclusions de®neinternal schistosity

Pg±Phe II±Chl(mg 0.58)±Ilm±St II±Cld II

Grt�Phe I�ChlGrt�Phe I�St IIor Cld IIKy! Phe II�PgRt! Ilm

Weak

FAY 29 Qtz±Grt±Ky±Phe I±Rt±Tr±(StI)

St I�Qtz�Grt�Ky�H2O

Unknown Pg±Phe II±Chl(mg 0.50)±Ilm±St II

Grt�Phe I�ChlGrt�Phe I�St IIKy! Phe II�PgRt! Ilm

Weak

Grt mica-schist CH 20 Qtz±Grt±Phe 1±Rt±(Pg) Unknown Phe II±abundant Bt (mg 0.53)±rare Chl (mg 0.51)±Oligoclase (An 13±17)±Ab±Ilm

Grt�Phe I�BtRt! IlmJd �PheI�Ab�PheIIor Pg�Phe I�Ab� Phe II

IntenseBt foliation andshear bands

Leptynite FAY 13 Qtz±Grt±Phe±Czo±(Kfs) Unknown Ab-altered Bt (mg 0.09) Grt�Phe I�BtPhe I�Kfs�Bt

IntenseStrong foliation

Quartz vein CAM 5 Qtz±Phe Unknown UnknownDepends on ¯uidcomposition

IntenseStrong foliation

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4.2. Garnet chemistry

Because of its importance for P±T estimations and Sm-Nd geochronology, garnet chemistry has been checked care-

fully in order to ascertain whether growth zoning has been preserved or erased by later diffusion. In all samples

except FAY 13, garnet grains display a chemical zonation. Garnets from metapelites are characterized by very

smooth changes in chemistry, with decreasing spessartine and increasing pyrope contents from core to rim. This

pattern is a typical growth zoning, recording increasing temperatures during garnet-producing continuous reac-

tions. In the eclogite (sample CX5c, Figure 2a), the zoning pattern is less regular, characterized by increasing

Figure 2. (a) Compositional rim-to-rim zoning of a garnet grain from a well-preserved eclogite (sample CX5c). Mineral abbreviations afterKretz (1983). Note regular increase in pyrope content from core to rim and anticorrelated almandine and grossular variations. A few inclusionsof sodic-calcic amphibole are present in the core. The pattern is interpreted as growth zoning. Simpler patterns of growth zoning are alsorecorded in the other samples. (b) Rim composition of garnet from the investigated eclogite-facies rocks. Note the correlation between garnet

chemistry and bulk-rock composition.



pyrope and almandine contents and decreasing grossular content from core to rim. Anticorrelated changes in

almandine and grossular contents, which may be related to the amphibole-consuming and omphacite-producing

reactions, gives to the growth zoning a more complex shape.

The composition of garnet rims (Figure 2b) is related to the bulk-rock chemistry, i.e. to buffering assemblage.

Garnet from the leptynite is an almandine±grossular solid solution, with negligible pyrope and spessartine con-

tents. It contains a high amount of grossular (about 55 mol%), as already observed in other samples of leptynites

from the Cellier Unit (Le Goff and BalleÁvre 1990). Garnets from the metapelites are essentially almandine±pyrope

solid solutions with negligible spessartine contents. Garnet in sample CH 20 has a slightly higher grossular content

than samples FAY 24 and FAY 29 (about 10, 2±3 and 2 mol%, respectively). Finally, garnet from the eclogite is a

ternary solid solution (about Alm61 Prp20 Grs18 Sps01).

4.3. Phengite chemistry

Because phengite is the only mineral analysed by the 40Ar/39Ar method in this study, detailed analyses have been

undertaken in order to understand its chemical behaviour. As usual for metamorphic minerals, phengite composi-

tion is a function of both whole-rock chemistry and P±T conditions (Massonne and Schreyer 1987) (Figure 3). At a

given P and T, the Si content is higher in low-alumina than in high-alumina bulk-rock compositions, i.e. in K-feld-

spar±garnet compared to garnet-kyanite assemblages (Figure 3a). With decreasing pressures during exhumation,

phengites tend to achieve lower Si contents (Figure 3b). An additional complexity, not shown in the AK(FM) pro-

jection, results from the FeMg-1 partitioning between phengite and coexisting phases. Experimental data (Mas-

sonne and Szpurka 1997) show that, at similar P±T conditions, phengites coexisting with pyrope and kyanite

are more Si-rich than those coexisting with almandine and kyanite. The phengite analyses from the studied sam-

ples can now be understood with this theoretical and experimental background.

In all studied samples, phengites show a large spread in composition (Figure 4a). This spread can result either

from the existence of several generations of phengitic micas (samples FAY 24 and CH 20) or from core-to-rim

zoning in individual grains (CAM 5, FAY 13 and CH 20). For the eclogite CX5c, the variable composition is

not related to the grain geometry, but corresponds to different grain compositions. In the ®rst case, the minute

grains (i.e. phengite II) developing at the expense of kyanite (FAY 24) or de®ning the late foliation (CH 20) present

low to very low (up to 6.2 (pfu) per formula unit) Si contents. In the second case, lower Si values are found at grain

margins in rocks which have been highly strained after the eclogite-facies event (CH 20, FAY 13 and CAM 5),

Figure 3. Phengite chemistry as a function of buffering assemblage (i.e. bulk-rock composition) and P±T conditions (see text for furtherexplanation).

304 v. bosse et al.


where phengites behave as clasts in a deforming, quartz-rich, matrix. Late biotite growth in samples CH 20 and

FAY 13 is probably associated with partial dissolution of phengite, and accompanying modi®cation of rim com-

position by intracrystalline diffusion. The same process is possible in the quartz vein but, due to the absence of

another K-bearing phase, the chemical re-equilibration probably took place through elemental exchange with the

¯uid phase. Remarkably, the smallest spread in composition is observed in the sample FAY 29, and both the smal-

lest spread in composition and the highest Si-content are observed in the eclogite sample (CX5c) which experi-

enced no deformation or reaction during decompression.

Once the mechanism of phengite re-equilibration during decompression is understood, one can compare the core

compositions, which should retain information about peak P±T conditions. As expected, phengites from garnet±

kyanite mica-schists are less substituted than in garnet mica-schists (Figure 3). However, the Si-contents of phen-

gites from the K-feldspar±garnet sample (FAY 13) are similar to those of the Grt±Ky mica-schists (Figure 4a), not

higher as expected. Two hypotheses can be proposed to explain this observation. Firstly, the phengites in sample

FAY 13 could have grown at lower pressures than phengites in the mica-schists. Secondly, the phengites could have

crystallized at P±T conditions similar to those of the other sample, but the lower Si contents result from their much

Figure 4. Phengite chemistry. (a) (Fe�Mg) versus Si diagrams for investigated phengites from the Cellier Unit. (b) XMg versus Si diagram forinvestigated phengites from the Cellier Unit, showing the variations of Si content in phengites related to the whole-rock chemistry. XMg� Mg/

Fe�Mg. Open symbols, phengite I; solid symbols, phengite II.



more Fe-rich compositions (Figure 4b). The use of phengite core composition for estimating P±T conditions, or for

assessing their relative age in different samples (if based on their chemistry), is thus dependent on the whole-rock

composition, hence on the buffering mineral assemblage.

4.4. Timing of main ductile deformation relative to P±T path

The main purpose of this paper is not to discuss in detail the P±T path of the Cellier Unit but, rather, to stress the

timing of the main ductile deformation with respect to the P±T path. Observed parageneses and mineral chemistry

of the studied samples are consistent with previous estimates of the P±T conditions during the eclogite-facies event

(Godard et al. 1981; BalleÁvre et al. 1987, 1989; BalleÁvre and Marchand 1991) (Figure 5).

The studied eclogite lacks a fabric and was therefore not deformed during its burial and exhumation. The inclu-

sions in garnet preserve no evidence of an included fabric which could have developed during the prograde history.

The matrix phases are not equigranular with triple junctions. Consequently, the lack of fabric does not result from a

complete annealing during the eclogite facies, which should have preserved the internal schistosity in the garnet

and recrystallized the matrix phases. A few late fractures record a brittle behaviour of the eclogite during exhuma-

tion, in the albite stability ®eld. The leptynites, as usual, were so intensely deformed in the albite stability ®eld that

no record of a previous deformation has been preserved. Consequences for geochronological studies of the contrast

Figure 5. Schematic P±T path of the studied eclogite-facies rocks from the Cellier Unit. Some equilibrium curves are plotted for reference. TheKFMASH grid is taken from Powell et al. (1998). Oligoclase-in after Maruyama et al. (1983). The Fay-de-Bretagne and la Varenne areas record

different peak P±T conditions (BalleÁvre and Marchand 1991).

306 v. bosse et al.


in mechanical behaviour between ma®c rocks (i.e. eclogites) and their surrounding environment (i.e. leptynites)

will be discussed below.

The studied Grt±Ky metapelites record part of the prograde history, either as garnet growth zoning or as inclu-

sions within garnet (chloritoid inclusions in la Varenne, staurolite and chloritoid, then kyanite inclusions in Fay-de-

Bretagne). Peak assemblages (garnet±chloritoid±phengite±ilmenite at la Varenne and garnet±kyanite±phengite±

rutile at Fay-de-Bretagne) suggest lower P±T conditions in la Varenne compared to Fay-de-Bretagne (BalleÁvre

and Marchand 1991). Peak P±T conditions of the order of 12±18 kbar, 550�C and 20-25 kbar, 650�C are estimated

for la Varenne and Fay-de-Bretagne, respectively (Figure 5). Late growth of chlorite at the expense of garnet and

chloritoid in la Varenne, and of chlorite and staurolite at the expense of garnet and kyanite in Fay-de-Bretagne

(BalleÁvre et al. 1989), record partial re-equilibration during decompression, and further constrain the P±T path

(Figure 5). No signi®cant deformation took place after the peak pressure in some metapelites (FAY 24 and FAY

29), but others were foliated during the decompression (CH 20).

5. GEOCHRONOLOGY

5.1. Analytical procedures

5.1.1. Sm-Nd and Rb-Sr methods

For Sm-Nd and Rb-Sr analyses, minerals were separated into different size fractions after crushing. Garnet and

omphacite were then concentrated from 120±150 mm size fractions using magnetic separation and heavy liquids.

Two white-mica size fractions from sample FAY 13 (500±300 and 300±200 mm) and three from CAM 5 (800±500,

500±300 and 300±200 mm) were concentrated. All fractions were ®nally carefully handpicked under a binocular

microscope.

Garnet (250 mg) and omphacite (100 mg) were separated for Sm-Nd analysis and phengite (50±70 mg ) for Rb-

Sr analysis. Two dissolution steps were carried out with acid digestion (2/3 HF and 1/3 HNO3) in a sealed Savilex

beaker on a hot plate (80�C) for 7 to 15 days at ®rst, and for two or three days for the second step. Whole rocks (100

mg) were dissolved using the same method as the minerals for four or ®ve days for the ®rst step. Total dissolution

was checked for and con®rmed in all cases. Sm, Nd, Rb and Sr concentrations were determined by isotope dilution

using a mixed 149Sm/150Nd spike and separate 87Rb and 86Sr spikes. All samples (whole rocks and minerals) were

spiked before dissolution.

Isotopic ratios and concentrations were measured using a seven-collector Finnigan MAT - 262 mass spectro-

meter. All ratios were normalized against 88Sr/86Sr � 8.3752. The long-term analyses on NBS-987 Sr standard

yielded 87Sr / 86Sr � 0.710249�8 (12 analyses). 143Nd/144Nd ratios were normalized against the value146Nd/144Nd � 0.721900. During the period of acquisition the AMES Nd standard gave 143Nd/144Nd �0.511961�5 (49 analyses) which is equivalent to the La Jolla value 143Nd/144Nd � 0.511860. Sr blanks were

lower than 200 pg and Nd blanks were 360 pg. Isochron calculations follow Ludwig (1999). The decay constant

is 0.0142 Ga-1 for 87Rb and 0.00654 Ga-1 for 147Sm. Input errors used in age calculations are 87Sr/86Sr � 0.005%,87 Rb/ 86 Sr � 2%, 143Nd/144Nd � 0.005% and 147Sm/144Nd � 0.2%. All errors on ages are quoted at 2�.

5.1.2. 40Ar/39Ar method

The 40Ar/39Ar analyses were performed on single grains of phengite I by the step-heating procedure. Grains ran-

ging from 0.5 to 1.2 mm were carefully selected under binocular microscope. The samples were irradiated in the

nuclear reactor of the McMaster University in Hamilton, Canada, in position 5c. The total neutron ¯ux density

during irradiation was 1.6�1017 n cmÿ2 with a maximum ¯ux gradient estimated at �0.2% in the volume where

the samples were included. We used the Hb3gr hornblende as a ¯ux monitor with an age of 1072 Ma (Turner et al.

1971).

The step-heating procedure is described in detail by Ruffet et al. (1991). The heating was carried out by a Coher-

ent Innova 70-4 continuous argon-ion laser. Each laser step heating lasted 3 min, 1 min for heating, and 2 min of



clean-up of the released gas, before introducing the gas into the mass spectrometer. The laser-beam size, adjusted

onto a cleavage-faced mica, was at least twice the sample size in order to obtain a homogeneous temperature over

the whole grain. The temperature is not known, but its homogeneity is controlled by observing the heated mineral

with a binocular microscope coupled with a video-colour camera. The fusion of the mineral was achieved by

focusing the laser spot.

The mass spectrometer is a VG 3600 working with a Daly detector system. The blanks of the extraction and

puri®cation laser system were measured every third step and subtracted from each argon isotope from the subse-

quent gas fraction. Typical blank values were in the range of 8.4±21.0, 0.7±2.1, 2.1-2.8 and 1.3±1.4�10ÿ13ccSTP

for masses 40, 39, 37 and 36, respectively.

The criteria for de®ning plateau ages were as follows: (i) a plateau age should contain at least 70% of released39Ar; (ii) there should be at least three successive steps in the plateau; and (iii) the integrated age of the plateau

should agree with each apparent age of the plateau within a 2� error con®dence interval. All errors are quoted at the

1� level (except the plateau ages that are given to the 2� level for comparison with data from other methods) and do

not include the errors on the age of the monitor. The error on the 40Ar*/39Ark ratio of the monitor is included in the

plateau age error bar calculation.

5.2. Results

5.2.1. Sm-Nd data

Sm-Nd analyses were performed on garnet, clinopyroxene and whole rock from sample CX5c. Results are reported

in Table 2. The garnetÿwhole-rock and garnetÿclinopyroxene pairs give concordant ages of 359�7 Ma and

371�7 Ma, respectively. Including garnet and whole-rock data previously obtained by Paquette (1987) in the same

sample gives an age of 362�25 Ma, with a high mean square of weighted deviates (MSWD) of 6.8 (Figure 6). This

may suggest that isotopic equilibrium between the whole rock and the minerals was not completely achieved dur-

ing the eclogite-facies metamorphism. The two garnet fractions from Paquette (1987) and this study show different

Sm/Nd ratios, probably because of a higher purity of the garnet population from the present work.

5.2.2. Rb-Sr data

For the leptynite (sample FAY 13), two phengite fractions (WM 1: 300±200 mm and WM 2: 500±300 mm) and the

whole rock yield an isochron age of 336�6 Ma (Figure 7a, Table 3). Three phengite fractions (WM 1: 800±500

mm, WM 2: 300±200 mm and WM 3: 500±300 mm) and whole-rock replicates from the quartz vein (sample CAM

5) yield an isochron age of 320�6 Ma (Figure 7b, Table 3).

5.2.3. 40Ar/39Ar data

In the la Varenne area (eastern part of the Cellier Unit, Figure 1), phengites from eclogite CX5c display concordant

plateau ages of 352.0�1.6 and 351.2�1.4 Ma (Table 4, Figure 8). Phengites from three mica-schists, a leptynite

Table 2. Sm-Nd results from the whole-rock and mineral separates from eclogite (sample CX5c). Data from Paquette (1987) arealso included

Sample No. Analysis No. Type Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd

CX5c Eclogite 7935 Whole-rock 2 5.6 21.3 0.1592 0.512778�7

Grt 1.6 1.1 0.8944 0.514507�12

Cpx 4.6 18.2 0.1521 0.512705�5

7935 Whole-rock 1 0.1633 0.512772�22

Paquette (1987) Grt 0.5024 0.513525�16

308 v. bosse et al.


Figure 6. Sm-Nd Grt±Cpx and whole-rock (WR) isochron for the eclogite CX5c. Open symbols are from Paquette (1987). Solid symbols arefrom this study.

Figure 7. Rb-Sr isochrons. (a) Isochron for leptynite FAY 13 obtained from two phengite fractions (Wm1: 300±200 mm and Wm 2: 500±300 mm)and the whole rock (WR). (b) Isochron for quartz vein CAM 5 obtained from three phengite fractions (Wm 1: 800±500 mm, Wm 2: 300±200 mm

and Wm 3: 500±300 mm) and two whole-rock replicates (WR1 and WR2).

Table 3. Rb-Sr results from the whole-rock and phengites from leptynite (sample FAY 13) and quartz vein sample (CAM 5)

Sample No. Analysis No. Type Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr

FAY 13 Leptynite 14043 Whole rock 113.6 50.5 6.5 0.740050�7

Wm 2 414.5 19.9 62.1 1.005241�9

Wm 1 420.5 21.4 58.2 0.987472�9

CAM 5 Quartz vein 14046 Whole rock 1 28.3 7.0 11.7 0.763676�10

Whole rock 2 28.2 6.8 11.9 0.763299�7

Wm 1 350.2 31.4 32.7 0.858035�7

Wm 2 190.4 15.6 35.9 0.871973�7

Wm 3 328.3 27.8 34.6 0.868513�7



Table 4. 40Ar/39Ar analytical results obtained from phengite single grains from the Cellier Unit. All isotopic measurementswere corrected for K and Ca isotopic interferences, mass discrimination and atmospheric argon contamination. Correctionfactors for interfering isotopes were (39Ar-37Ar)Ca � 7.06�10ÿ4, (36Ar-37Ar)Ca� 2.79�10ÿ4, (40Ar-39Ar)K � 2.95�10ÿ2.

Step no. 40Aratm(%) 39Ar (%) 37ArCa/39ArK

40Ar*/39ArK Age (Ma)

CX5c Phengite 1

1 12.49 1.93 ± 5.89 357.0�16.0

2 14.03 2.44 ± 5.91 358.0�12.3

3 3.07 12.28 ± 5.71 347.3�3.2

4 0.00 12.13 ± 5.82 353.0�1.8

5 0.00 21.27 ± 5.82 353.0�1.4

6 0.00 24.19 ± 5.82 353.4�1.1

7 2.26 2.96 ± 5.75 349.5�5.3

8 3.69 3.33 ± 5.64 343.5�6.1

9 0.97 3.32 ± 5.80 351.9�5.2

Fuse 0.34 16.15 ± 5.83 353.8�1.4

Integrated age 352.3�0.8

CX5c Phengite 2

1 39.09 0.29 ± 7.10 422.2�40.7

2 14.83 0.81 ± 6.52 391.5�8.9

3 7.11 0.90 ± 6.25 376.6�11.2

4 7.59 0.93 ± 5.85 355.1�7.0

5 9.38 1.01 ± 5.71 347.1�6.9

6 3.65 1.09 ± 5.94 359.5�6.4

7 6.40 1.48 ± 5.91 358.067�5.2

8 0.00 1.19 ± 6.05 365.942�7.1

9 0.00 1.29 ± 6.07 366.768�4.6

10 10.70 2.49 ± 5.83 353.611�3.1

11 6.60 19.22 ± 5.79 351.685�0.7

12 1.03 6.57 ± 5.82 353.219�1.2

13 1.30 11.43 ± 5.79 351.711�0.8

14 1.67 11.35 ± 5.81 352.377�0.9

15 2.67 8.45 ± 5.77 350.705�1.1

16 3.94 7.12 ± 5.75 349.307�1.4

17 4.15 3.71 ± 5.74 348.917�3.2

Fuse 2.79 20.68 ± 5.77 350.293�0.8


FAY 29 Phengite 1

1 20.37 4.22 ± 10.44 305.7�3.5

2 3.49 5.80 ± 12.62 363.4�2.4

3 0.12 4.79 ± 12.50 360.3�2.3

4 0.71 6.46 ± 12.39 357.5�2.4

5 0.11 16.50 ± 12.17 351.5�1.0

6 0.33 18.95 ± 12.09 349.5�1.1

7 0.59 5.89 ± 12.12 350.3�3.6

8 0.54 3.73 ± 12.20 352.3�5.1

9 3.09 3.26 ± 11.97 346.4�4.4

10 1.14 3.57 ± 12.24 353.4�4.0

11 0.77 13.89 ± 12.11 350.1�1.2

Fuse 0.14 12.92 ± 12.18 351.9�1.2


310 v. bosse et al.


Table 4. (Continued)



FAY 29 Phengite 2

1 74.80 0.28 ± 6.89 207.5�9.5

2 45.05 0.45 ± 11.50 334.0�5.9

3 39.59 0.55 ± 12.12 350.2�5.9

4 11.42 0.83 ± 12.41 357.9�4.4

5 5.90 1.20 ± 12.42 358.1�2.7

6 2.38 1.59 ± 12.44 358.6�1.4

7 5.41 6.41 ± 12.48 359.7�0.7

8 0.88 32.90 ± 12.14 350.8�0.5

9 0.00 17.08 ± 12.16 351.4�0.7

10 0.02 9.26 ± 12.04 348.3�0.5

11 0.00 3.47 ± 12.06 348.6�0.7

12 0.00 2.31 ± 12.08 349.2�1.2

13 0.08 11.16 ± 12.08 349.2�0.6

Fuse 0.00 12.50 ± 12.06 348.8�0.8


FAY 24 Phengite 1

1 65.69 0.23 ± 10.83 316.4�23.7

2 33.83 0.72 ± 11.43 332.5�12.5

3 24.08 1.32 ± 12.58 362.7�3.9

4 0.00 0.80 ± 13.35 382.6�5.4

5 11.96 1.09 ± 12.45 359.2�3.6

6 4.33 3.58 ± 12.48 360.1�1.9

7 0.00 45.87 ± 11.90 344.9�0.5

8 0.16 29.94 ± 11.82 342.7�0.5

9 0.29 1.95 ± 11.94 345.9�2.7

10 0.67 1.85 ± 11.87 344.0�2.4

11 0.00 0.19 ± 12.18 352.1�23.3

Fuse 0.00 12.44 ± 11.87 344.0�0.6


FAY 24 Phengite 2

1 59.20 0.21 ± 10.75 314.3�50.5

2 0.00 0.18 ± 14.54 413.2�56.5

3 4.21 0.95 ± 12.87 370.3�8.8

4 6.15 2.26 ± 12.30 355.3�4.0

5 3.69 3.81 ± 12.17 351.9�2.5

6 1.00 20.26 ± 11.93 345.5�0.9

7 0.06 57.39 ± 11.93 345.6�0.5

8 0.00 2.64 ± 12.03 348.3�3.9

9 0.00 1.75 ± 12.56 362.1�5.2

10 0.00 1.38 ± 12.32 355.8�6.8

11 0.00 3.69 ± 12.13 351.0�2.9

Fuse 0.90 5.47 ± 11.84 343.2�2.5


CH 20 Phengite 1

1 45.49 0.31 ± 9.05 267.3�9.7

2 9.56 1.06 ± 12.59 361.8�3.3

3 2.29 2.44 ± 12.25 353.0�1.3






4 0.57 12.35 ± 12.12 349.5�0.5

5 0.00 22.95 ± 12.08 348.6�0.5

6 0.00 31.89 ± 12.09 348.7�0.5

7 0.09 8.90 ± 12.02 347.1�0.6

8 0.18 4.16 ± 12.00 346.5�0.9

9 0.16 3.33 ± 12.02 346.9�1.1

Fuse 0.10 12.63 ± 12.04 347.5�0.8


CH 20 Phengite 2

1 77.53 0.13 ± 6.87 206.4�33.2

2 56.36 0.07 ± 9.47 278.6�42.3

3 33.88 0.24 ± 12.46 358.6�12.6

4 17.04 1.00 ± 12.95 371.3�3.4

5 12.24 1.08 ± 12.64 363.3�2.5

6 10.64 1.43 ± 12.35 355.6�2.1

7 6.44 1.83 ± 12.70 364.8�1.6

8 1.53 0.79 ± 12.21 351.9�3.4

9 2.95 1.96 ± 12.50 359.5�1.8

10 2.93 3.21 ± 12.48 358.9�1.1

11 1.62 4.07 ± 12.28 353.9�1.5

12 0.59 2.79 ± 12.21 351.9�1.2

13 0.79 4.10 ± 12.25 352.9�1.0

14 1.19 4.46 ± 12.18 351.2�0.8

15 0.76 8.92 ± 12.18 351.3�0.6

16 0.72 11.75 ± 12.15 350.4�0.6

17 0.07 4.54 ± 12.16 350.6�0.8

18 0.46 11.10 ± 12.14 350.2�0.6

19 0.68 7.99 ± 12.13 349.8�0.6

20 0.95 3.58 ± 12.14 350.1�0.9

21 0.93 1.97 ± 12.12 349.5�1.7

22 0.00 1.61 ± 12.29 354.1�1.9

23 0.66 8.12 ± 12.18 351.2�0.6

24 1.24 4.54 ± 12.16 350.6�0.8

25 1.85 2.28 ± 12.13 349.7�1.1

26 3.01 0.56 ± 11.93 344.6�4.5

Fuse 0.70 5.86 ± 12.20 351.7�0.8


FAY 13 Phengite 1

1 39.05 1.37 ± 10.59 310.3�5.8

2 6.25 3.62 ± 11.82 343.0�2.6

3 2.72 5.08 ± 11.67 339.1�1.8

4 0.02 6.17 ± 11.88 344.6�1.4

5 0.16 18.94 ± 11.69 339.7�0.6

6 0.08 24.27 ± 11.71 340.1�0.5

7 0.00 13.30 ± 11.76 341.4�0.9

8 0.00 7.09 ± 11.73 340.6�1.3

9 0.00 7.09 ± 11.74 340.9�1.3

Fuse 0.00 13.05 ± 11.76 341.4�0.8


312 v. bosse et al.





FAY 13 Phengite 2

1 85.51 0.45 ± 7.26 218.4�51.3

2 21.96 0.93 ± 11.04 322.3�25.4

3 16.27 1.77 ± 11.20 326.6�16.7

4 0.00 1.80 ± 12.94 372.4�14.5

5 0.00 0.89 ± 13.30 381.7�29.4

6 0.93 24.54 ± 11.83 343.2�1.2

7 0.34 54.99 ± 11.69 339.7�0.7

8 3.37 4.68 ± 11.56 336.2�5.3

9 1.36 3.77 ± 11.72 340.4�6.3

Fuse 10.00 6.16 ± 11.72 340.5�5.6


CAM 5 Phengite 1

1 33.26 0.81 ± 12.61 362.7�13.2

2 8.51 1.48 ± 12.27 353.9�7.4

3 4.36 1.67 ± 12.06 348.4�6.2

4 9.17 1.45 ± 11.36 330.0�6.8

5 3.76 2.69 ± 11.72 339.4�2.4

6 2.27 3.60 ± 11.82 342.1�1.7

7 1.68 10.36 ± 11.77 340.8�1.0

8 0.92 10.82 ± 11.92 344.6�1.3

9 1.71 4.21 ± 11.80 341.5�1.7

10 0.87 7.39 ± 11.87 343.2�1.2

11 0.56 36.83 ± 11.83 342.4�0.9

12 1.39 7.85 ± 11.76 340.5�1.1

13 2.53 3.65 ± 11.62 336.7�1.9

14 0.00 2.46 ± 11.94 345.3�2.6

15 0.68 2.97 ± 11.88 343.6�2.3

Fuse 0.00 1.75 ± 11.99 346.4�3.5


CAM 5 Phengite 2

1 71.77 0.24 ± 9.39 276.8�19.7

2 61.27 0.06 ± 15.26 430.5�60.0

3 89.77 0.12 ± 9.91 290.9�38.1

4 53.20 0.14 ± 11.58 335.8�30.2

5 44.36 0.25 ± 11.82 342.2�20.6

6 35.66 3.12 ± 12.27 353.8�2.6

7 8.42 1.47 ± 12.41 357.6�3.4

8 8.91 1.35 ± 12.37 356.5�3.9

9 5.97 1.61 ± 12.30 354.7�3.2

10 5.22 2.99 ± 12.50 359.8�1.5

11 3.21 3.46 ± 12.43 358.2�1.4

12 2.40 1.19 ± 12.66 364.1�3.7

13 3.69 4.99 ± 12.24 353.0�1.0

14 4.39 8.07 ± 12.23 352.7�0.8

15 2.04 5.05 ± 12.11 349.7�1.0

16 1.82 3.57 ± 12.11 349.7�1.3

17 1.67 5.93 ± 12.10 349.5�0.9

18 1.10 8.37 ± 11.98 346.3�0.7



and a quartz vein (Table 4, Figure 8) from Fay-de-Bretagne and Campbon, in the western part of the Cellier Unit

(Figure 1), were also analysed. Phengites from two Grt±Ky mica-schists (samples FAY 24 and FAY 29) and a Grt

mica-schist (sample CH 20) display plateau ages of 350.7�1.4 Ma (FAY 29), 345.7�1.4 and 344.1�1.4 Ma (con-

cordant plateau ages on FAY24), and 348.3�1.4 Ma and 350.9�1.4 Ma (concordant plateau ages on CH20). A

second grain from sample FAY 29 does not provide a plateau age, but the weighted mean age of 350.0�1.4 Ma

(calculated on steps 8 to 14, representing 88.7% of 39Ar released) is concordant with the plateau age obtained on

the other grain. All these age spectra are characterized by more or less regular higher ages at low temperature. Two

phengite grains from the leptynite (FAY 13) give similar plateau ages of 340.5�1.6 and 340.5�1.4 Ma. Finally,

two phengite grains from sample CAM 5 (a quartz vein) were analysed. The ®rst displays a plateau age of

342.2�1.4 Ma which is concordant with the plateau ages obtained in the other samples. The second gives a highly

disturbed age spectrum characterized by increasing apparent ages at low temperature (from 342 to 364 Ma, steps 5

to 12), followed by a regular decrease down to 340 Ma (over 77% of the total 39Ar degassing), then increasing

apparent ages for the last ®ve steps. The 36Ar/40Ar versus 39Ar/40Ar correlation diagrams are not given because




19 0.82 5.23 ± 11.92 344.6�1.0

20 0.49 8.53 ± 11.91 344.4�0.8

21 1.36 3.66 ± 11.78 341.0�1.0

22 1.22 6.26 ± 11.76 340.4�0.9

23 0.98 4.64 ± 11.74 340.0�0.9

24 0.65 3.79 ± 11.74 339.8�1.1

25 0.43 4.91 ± 11.76 340.6�0.8

26 0.54 4.30 ± 11.73 339.7�1.3

27 0.35 3.31 ± 11.80 341.5�1.2

28 0.79 1.46 ± 11.86 343.2�2.5

Fuse 2.07 1.93 ± 12.07 348.6�2.0


Figure 8. 40Ar/39Ar age spectra obtained from single grains of phengite. The numbers indicate plateau ages with error bars at the 2� level.Apparent ages are given at the 1� level.

314 v. bosse et al.


they do not provide useful information due to the clustering of ratios. The 37ArCa/39ArK ratio is always very low

and not always visible because of the long delay between the irradiation and the experiment. The 37ArCa/39ArK

spectra are therefore not shown.

6. THE AGE OF THE ECLOGITE-FACIES METAMORPHISM

Previous U-Pb analyses on zircons (Paquette et al. 1985; Paquette 1987) were performed on two eclogite samples

(CX14 and CX5c), which differ in their location (Fay-de-Bretagne and la Varenne, respectively) and chemistry

(andesitic and basaltic compositions, respectively) and hence mineral paragenesis (garnet±omphacite±kyanite±

quartz and garnet±omphacite±glaucophane±phengite, respectively). Zircons from sample CX14 provided a lower

intercept age of 413�44 Ma (recalculated using Ludwig 1999), which was interpreted as the age of the HP event

(Paquette et al. 1985). This result was consistent with the then-available ages of the HP metamorphism in the

Armorican Massif (Peucat et al. 1982) and Massif Central (Pin and Lancelot 1982; Ducrot et al. 1983). Later,

Paquette (1987) reconsidered the U-Pb data on sample CX14. Because the zircons are highly discordant, the

lower-intercept age of 413 Ma may have no geological meaning (Paquette 1987; Mezger and Krogstad 1997). Zir-

cons from the second eclogite sample (CX5c) display an upper intercept age of 356�8 Ma (recalculated using

Ludwig 1999). Because the zircons are nearly concordant, this age is interpreted as dating the growth of the zircons

(Paquette 1987). Three interpretations are nevertheless possible.

Firstly, the 356 Ma age can be considered as the minimum age for the eclogite-facies metamorphism (Paquette

1987). This interpretation is based on a combination of an observation (i.e. zircon grains are only found as inclu-

sions in glaucophane) and an interpretation on the timing of glaucophane growth, according to which glaucophane

idioblasts in the Champtoceaux eclogites mark a temperature decrease during the earliest stage of the cooling his-

tory (Godard et al. 1981). Nevertheless, new observations show that zircon grains are found not only in the glau-

cophane crystals (where they develop pleochroic haloes, hence explaining their easy identi®cation) but also in the

garnet and in the clinopyroxene, i.e. in all high-pressure minerals. Zircon growth is thus not linked with glauco-

phane growth, and therefore there is no clear-cut observation indicating when zircon growth took place. Based on

petrographic criteria, zircon could either pre-date the high-pressure paragenesis or be synchronous with it. The Sm-

Nd data obtained are thus critical for testing the latter two hypotheses.

Secondly, the 356 Ma age can be interpreted as the age of crystallization of the high-pressure assemblage. This

hypothesis is supported by the new Sm-Nd data (362�25 Ma) obtained from the same sample, despite its large

error bar. The slight isotopic disequilibrium could be a consequence of either mineral inclusions in garnet and

clinopyroxene, or the presence of two different types of clinopyroxene in this sample (see above). Nevertheless,

the isochron ages obtained separately on Grt±Cpx and Grt±whole-rock pairs (371�7 Ma and 359�7 Ma, respec-

tively) are concordant and in agreement with the U-Pb age. Preservation of growth zoning (i.e. zoning for major

elements) indicates that the minor elements, known to diffuse more slowly than Fe, Mn and Mg and at rates lower

than or similar to Ca (Hiroi and Ellis 1994; Lanzirotti 1995; Spear and Kohn 1996), have not been modi®ed after

garnet growth. Temperature estimates based on the FeMg-1 exchange between garnet and clinopyroxene (Krogh

1988) give values on the order of 550±600�C for the studied sample. This temperature is slightly lower than the

minimum estimated closure temperature for the garnet Sm-Nd system (600�30�C: Mezger et al. 1992). It must be

emphasized that the analysed garnets are very small (about 300±400 mm for the larger grains), once again laying

emphasis on the kinetic constraints on the Sm-Nd age. The situation would have been radically different if the

analysed garnets, rather than preserving a growth zoning, had shown a diffusion zoning, in which case the signi®-

cance of the Sm-Nd age would be less straightforward.

A third interpretation would be to assume that the 356 Ma age is younger than the high-pressure metamorphism

(which should have taken place at 400±440 Ma, as suggested by previous U-Pb data), due to a resetting of the U-Pb

clock (Faure et al. 1997). In fact, resetting of both the Sm-Nd and the U-Pb system in sample CX5c is highly unli-

kely because of: (i) preservation of the growth zoning in the garnet; (ii) preservation of prograde inclusions in

garnet (sodic-calcic amphibole) as well as matrix omphacite (quartz and plagioclase); (iii) preservation of the



idioblastic shape of the garnet grains, indicating that no garnet resorption occurred during decompression; and (iv)

lack of amphibolite- or greenschist-facies overprint except in a few fractures.

7. TIMING CONSTRAINTS ON THE AGE OF THE EXHUMATION

7.1. Multimethod age consistency

Although they vary from 352.0�1.6 to 340.5�1.4 Ma, the 40Ar/39Ar plateau ages appear in general agreement

(i.e. are slightly younger than) the U-Pb and Sm-Nd ages obtained on at least one common sample. They could

therefore represent cooling ages. Note also that the Rb-Sr isochron and the 40Ar/39Ar plateau ages obtained on

sample FAY 13 are concordant. Taking into account the estimated closure temperatures of white micas, i.e.

350�50�C for the K-Ar system and 500�50�C for the Rb-Sr system (JaÈger 1979), an obvious interpretation

of the above data is that the cooling rate was very fast so that: (i) 40Ar/39Ar plateau ages are only slightly younger

(or within error) than the U-Pb and Sm-Nd ages, thought to re¯ect crystallization of the eclogite-facies paragenesis

at temperatures lower than the estimated closure temperature for the U-Pb and Sm-Nd systems; (ii) the lack of

difference between Rb-Sr and K-Ar ages. Nevertheless, this model of fast cooling during decompression does

not explain the range of 40Ar/39Ar plateau ages, which is higher than the analytical uncertainties. Potential expla-

nations of this spread are now discussed.

7.2. Spread of 40Ar/39Ar ages

7.2.1. Absence of correlation between 40Ar/39Ar ages and spatial distribution of studied samples

The spread of 40Ar/39Ar ages could result from differences in cooling rate. Indeed, the studied samples have been

collected over a large area (Figure 1), and differences in P±T histories have been found between the two main

sampling localities (Figure 5). Differences in cooling ages, and possibly cooling rates, can thus be expected.

Nevertheless, the two oldest plateau ages are obtained from samples from both the eastern (le Cellier: CX5c)

and the western (Fay-de-Bretagne: FAY 29) parts of the Cellier Unit. Consequently, differences in P±T±time paths

cannot be responsible for the spread of 40Ar/39Ar ages.

7.2.2. Absence of evidence for excess argon

The spread of 40Ar/39Ar ages could also result from the incorporation of variable amounts of undetectable excess

argon, as previously observed in HP phengites (Tonarini et al. 1993; Li et al. 1994; Arnaud and Kelley 1995; Ruffet

et al. 1995, 1997; Scaillet 1996). Although this cannot be ruled out in the Champtoceaux Complex, it is unlikely

that the data are affected by excess argon for the following reasons.

(1) The possibility that excess argon was present when plateau ages were obtained arises because the 40Ar/39Ar

ages are older than the U-Pb and Sm-Nd ages obtained from the same locality, or even the same sample, as

observed, for example, in ultra-high-pressure eclogites from China (Li et al. 1993) and in high-pressure eclo-

gites from the Himalaya (Tonarini et al. 1993). This multimethod approach is followed in the present study of

the Champtoceaux Complex. Because the 40Ar/39Ar plateau ages are always younger than (or within error of)

the ages obtained by the U-Pb and Sm-Nd methods (e.g. sample CX5c), this precludes the presence of large

amounts of inherited argon. Moreover, although we would expect an older Rb-Sr age, the concordance of Rb-

Sr and 40Ar/39Ar data in sample FAY 13 also argues in favour of a lack of excess argon.

(2) The investigated samples are monometamorphic rocks derived from sedimentary or magmatic rocks, with very

different K-contents (and therefore radiogenic Ar content, before the HP event), with ages on the order of 400±

500 Ma (Paquette et al. 1984), which should introduce correspondingly different amounts of excess argon. We

do not observe any relationship between the plateau age and the type of rock: the two pelites FAY 29 and FAY

24 give different plateau ages, whereas the eclogite CX5c and the pelite FAY 29 display concordant plateau

ages.

316 v. bosse et al.


(3) In the case of phengite affected by excess argon, different grains of the same sample or even different splits of a

same grain, generally give different plateau ages (Ruffet et al. 1995), which is clearly not the case for the

Champtoceaux duplicates which show high reproducibility of plateau ages.

The ®rst parts of some age spectra are affected by more or less regular higher and decreasing apparent ages at

low temperature. They may originate from 39Ar loss from ®ne-grained phases by recoil during the irradiation, or

may correspond to some inherited 40Ar trapped by the less retentive ®ne-grained phases during the ®nal closure of

the whole grain. Although this last possibility cannot be ruled out, both the previous discussion on eventual excess

argon and the fact that there is no relationship between these higher low-temperature apparent ages and the cor-

responding plateau ages, lead us to favour the recoil effect.

7.2.3. Correlation between 40Ar/39Ar plateau ages and deformation

Plateau ages are correlated with the degree of strain and deformation-induced chemical re-equilibration of micas

during the decompression history. Indeed, the oldest plateau ages are obtained from well-preserved samples (CX5c

and FAY 29) that show neither deformation nor important core-to-rim chemical variation in phengite. Conversely,

the youngest plateau ages are obtained on phengites from rocks that have been highly strained during their exhu-

mation (i.e. FAY 13 and CAM 5) and show strong core-to-rim variations in Si-content.

A relationship between plateau age and deformation and/or phengite re-equilibration is less clear for samples

FAY 24 (344.1�1.4 Ma) and CH 20 (348.3�1.4 Ma and 350.9�1.4 Ma). Nevertheless, whereas no foliation can

be observed in sample FAY 24, some phengite I grains show undulose extinction. This feature may explain the

younger plateau age in this sample, which can be considered as intermediate between the older (CX5c) and the

younger plateau ages (FAY 13). Sample CH 20 has been highly strained in the albite stability ®eld, but phengite I

porphyroclasts are preserved in a ®ne-grained matrix. Therefore, the old plateau age (350.9�1.4 Ma) may be given

by the phengite I porphyroclasts.

Finally, in sample CAM 5, the strongly disturbed 40Ar/39Ar age spectrum obtained from one grain and the young

Rb-Sr age (320�6 Ma) suggest a high disturbance of both isotopic systems, possibly due to deformation and ¯uid

¯ow during the exhumation history.

7.3. Closure mechanism of the K-Ar system

7.3.1. Closure temperature of the K-Ar system in phengite

Because phengite grains (and other high-pressure minerals) tend to re-equilibrate with decreasing pressure, most

grains show chemical zoning to some extent. The possible in¯uence of this re-equilibration on the 40Ar/39Ar

chronometer has been the focus of much attention (e.g. Hammerschmidt and Franz 1992). In the studied samples,

microprobe analyses show the existence of two generations of phengite in two samples, and core-to-rim chemical

variations of variable extent in four samples (Figure 4a). In the case of the analysis of mixed phases of phengite

(old HP phengite surrounded by ®ne-grained recrystallized phengites) during dating experiments, we would expect

younger ages at low temperature, which is not the case. On the other hand, more than 80% of the age spectra show

unvarying ages corresponding to the original and homogeneous HP phengite. It follows that the oldest plateau ages

obtained on the best-preserved samples (CX5c, FAY 29) may represent closure temperature ages, whereas younger

plateau ages could represent a later closure of the K-Ar system, resulting from continuous deformation of the rock.

Textural (well-developed foliation in the granitic gneisses, suggesting high ductility) and petrological arguments

(oligoclase and biotite growth in sample CH 20; biotite development at the expense of phengite in the leptynites)

suggest that the temperature range during the late ductile deformation was around 400±500�C. Assuming a closure

temperature of 300±400�C for the K-Ar system in phengite (JaÈger 1979), ductile deformation would have ended

before closure of the K-Ar clock in phengite. The 40Ar/39Ar ages should be similar in all types of rock, irrespective

of their ductile deformation during retrogression. Because the contrary is observed, we must admit that the closure

temperature of the K-Ar system in phengite is higher than the temperature corresponding to deformation of



phengite, c. 450±500�C (Figure 9). This temperature interval is evaluated from the following observations. The

lower limit (450�C) must be higher than the temperature at which deformation ends (c. 400�C). The upper limit

(500�C) is given by the maximum temperature for the eclogite facies in the la Varenne area (500±550�C) where the40Ar/39Ar plateau age obtained on the undeformed CX5c sample (352.0�1.6 Ma) is only slightly younger than the

U-Pb and Sm-Nd ages (360 Ma).

Figure 9. P±T±time path for the eclogite-facies rocks from the Cellier Unit in la Varenne and Fay-de-Bretagne. U-Pb data are from Paquette(1987), Sm-Nd, Rb-Sr and 40Ar/39Ar ages from this study. Closure temperatures are shown for the K-Ar and Rb-Sr systems in phengite (JaÈger1979). The data cannot be reconciled with a simple model assuming (i) rapid cooling, and (ii) a closure temperature for the K-Ar system inphengite of 350�50�C. Because the oldest 40Ar/39Ar ages were obtained from undeformed (CX5c) or poorly-deformed samples, and becausethe youngest ages were obtained from the most strongly deformed rocks (FAY 13), closure of the K-Ar system in phengite must have occurred at

a higher temperature than the ductile deformation (i.e. about 450±500�C).

318 v. bosse et al.


In conclusion, if (i) all plateau ages correspond to the closure of the K-Ar system, and (ii) the plateau ages are

related to the deformation of the phengites, the closure temperature of the K-Ar system in the phengite would be

around 450±500�C, i.e. signi®cantly higher than generally assumed. The oldest plateau ages in undeformed rocks

(350±352 Ma in CX5c and FAY 29) would correspond to a closure temperature of about 450±500�C (the closure is

temperature-dependent only) (Figure 9). The younger plateau ages (around 340 Ma) displayed by deformed rocks

(e.g. FAY 13) correspond to a later closure of phengites at a lower temperature of about 400�C, where continuous

deformation during retrogression would represent the main parameter responsible for the closure of the K-Ar sys-

tem (Figure 9). This interpretation is in closer agreement with the concordant 40Ar/39Ar and Rb-Sr ages.

7.3.2. Contrasting rheologies and consequences for 40Ar/39Ar ages

As a ®nal point, let us discuss the distribution and role of the late ductile deformation. As pointed out above, some

rocks (e.g. the eclogites) were left undeformed during the albite±epidote±amphibolite-facies overprint while others

were highly strained (e.g. the granitic gneisses). The primary control on this rheological behaviour is the bulk-rock

composition, which either allows or precludes the stability of easily deformed minerals at speci®c P±T conditions.

Consider the case of the ma®c eclogite lenses enclosed in the granitic gneisses. During retrogression (at about

400±500�C), garnet and sodic pyroxene cannot be deformed in a ductile manner. Reaction-enhanced ductility

would be possible provided that H2O enters the eclogite, allowing the growth of ®ne-grained amphibole±plagio-

clase aggregates at the expense of the eclogite-facies paragenesis. This is indeed observed along the margins of the

eclogite lenses, which are strongly foliated. In contrast, granitic gneisses contain a high modal proportion of quartz

(and possibly jadeite), with minor phengite and garnet as the high-pressure paragenesis. During retrogression,

deformation-induced recrystallization of quartz is easy, and jadeite � quartz reacts to form albite. With decreasing

pressure, the modal proportion of phengite decreases, and its chemistry evolves towards lower Si contents (Figure

3). The associated continuous reaction liberates a small amount of H2O, which could potentially in®ltrate the eclo-

gite lenses along their margins or within fractures. Textural observations of the granitic gneisses show that phengite

is less ductile than the quartzo-feldspathic matrix (e.g. samples FAY 13 and CAM 5). Ductile deformation of phen-

gite porphyroclasts is limited to bending and development of ®ne-grained, recrystallized tails (Choukroune and

Lagarde 1977; Lagarde 1978). Whether or not H2O enters the granitic gneisses thus has no major in¯uence on

their rheological behaviour, which is controlled by the mechanical properties of quartz and feldspar. In short, duc-

tile deformation during retrogression is con®ned to (i) rheologically-weaker (i.e. quartz-rich) rocks, or (ii) domains

where an externally-derived in¯ux of H2O allows reaction softening.

Similar interactions between deformation, metamorphim and ¯uid behaviour are common in most high-grade

terranes, e.g. the Alpine belt in Switzerland and Italy (Heinrich 1982; Rubie 1990) and the Caledonian belt in Nor-

way (Austrheim et al. 1997; Krabbendam and Wain 1997; Austrheim 1998; Krabbendam et al. 2000). Conse-

quences for the behaviour of the Rb-Sr and K-Ar systems are now better understood. In rocks that preserve

relics of an older orogenic cycle, the Rb-Sr system can preserve ages of the older cycle under dry and static con-

ditions (KuÈhn et al. 2000). Excess Ar in phengite is commonly observed in basement rocks (i.e. in Variscan rocks

overprinted by the Alpine deformation) from the Alps, whether or not they preserve mineralogical relics of the

Variscan cycle (Hammerschmidt and Frank 1991; Arnaud and Kelley 1995; Ruffet et al. 1995, 1997; Scaillet

1996, 1998). In the latter case, restricted ¯uid ¯ow at high pressure probably facilitated the maintenance of a high

Ar partial pressure with high 40Ar/36Ar ratio (originating from older pre-metamorphic rocks most often rich in

potassium, and therefore in radiogenic argon), which was trapped into the phengites at their closure time.

In the Champtoceaux Complex, no petrological evidence for a tectono-thermal event prior to the eclogite-facies

metamorphism has been found (BalleÁvre et al. 1989). The differences in 40Ar/39Ar ages observed in the Champ-

toceaux Complex result from the interplay of bulk-rock chemistry (de®ning which minerals can be stable at a par-

ticular stage of the P±T history) and ductile deformation during retrogression. Despite these intricacies, the

observed 40Ar/39Ar ages cannot be reconciled with a low closure temperature for the K-Ar system in phengite

(300±400�C: JaÈger, 1979; Hurford 1986; Hunziker et al. 1992), but suggest a higher temperature, as previously

proposed (e.g. von Blanckenburg et al. 1989; Villa 1998). The proposed value (450±500�C) is a rough estimate,



which should be re®ned when better estimations of the P±T conditions during retrogression become available. In

low-strain volumes (e.g. the eclogite CX5c), undeformed phengites record older ages, because closure of the K-Ar

system took place earlier than in high-strain volumes (e.g. the leptynite FAY 13) where phengite porphyroclasts

remained open with respect to the K-Ar system up to the end of the deformation. Externally-derived H2O played a

minor role, with the possible exception of the quartz vein (CAM 5).

8. GEOLOGICAL IMPLICATIONS

The combined petrological±geochronological approach of this study gives strong constraints on the exhumation

history of the Champtoceaux Complex (Figure 10). The main stages are as follows.

(1) Subduction of the continental crust is recorded by relics of the eclogite-facies metamorphism, dated at about

360 Ma (i.e. latest Devonian).

(2) The Cellier Unit is an allochthonous thrust sheet, in which the high-pressure metamorphism pre-dates the

thrusting and its associated deformation. The latter took place in the albite±epidote±amphibolite facies, i.e.

Figure 10. A summary chart of the geochronological constraints of the tectonic evolution of the Champtoceaux Complex. Isotopic data aregrouped in the upper part of the diagram, palaeontological data in the lower part. Age boundaries are taken from Okulitch (1999). Exhumation ofthe Champtoceaux Complex occurred during sedimentation in the Ancenis Basin and before strike-slip movement along the South ArmoricanShear Zone and associated faults. Thus, exhumation of the high-pressure rocks took place during an early-orogenic, syn-convergence stage of

the Variscan Orogeny.

320 v. bosse et al.


at about 400±500�C, 5±10 kbar. In eclogite-facies rocks where the late deformational or chemical overprint is

lacking or poorly developed, cooling is recorded by 40Ar/39Ar plateau ages at about 350±340 Ma. Taking into

account the 40Ar/39Ar ages and Rb-Sr ages obtained in the most strongly deformed rocks from the Cellier

Unit, the thrusting can be dated at c. 330±340 Ma, i.e. from the Early Carboniferous.

(3) The Ancenis Basin contains a thick sequence of continental detrital material, dated as Early Carboniferous

(BeaupeÁre 1973; Cavet et al. 1978). This shows that sedimentation in the Ancenis Basin is broadly coeval with

exhumation of the eclogite-facies rocks from the Champtoceaux Complex (Figure 10).

(4) The Nort-sur-Erdre Fault, which bounds the Champtoceaux Complex to the north, is marked by the alignment

of coal-bearing basins of Namurian to early Westphalian age (Cavet et al. 1978), i.e. early Late Carboniferous.

The coal-bearing sequences may have been deposited in narrow pull-apart basins which opened during dextral

movement along the Nort-sur-Erdre Fault.

(5) Finally, dextral movement along the South Armorican Shear Zone is coeval with the intrusion of two-mica

leucogranites, namely the Vigneux Granite (Hussein 1960) and the Mortagne Granite (Guineberteau et al.

1987; Roman-Berdiel et al. 1997). The latter is poorly dated by a Rb-Sr whole-rock isochron at

313�19 Ma (Guineberteau 1986; recalculated using Ludwig 1999). Late, brittle deformation along the SASZ,

recorded by a few coal-bearing basins in western Brittany, took place during the Stephanian (Picquenard

1920), i.e. at the end of the Late Carboniferous. The large-scale antiformal folding of the Champtoceaux Com-

plex may have taken place during this period.

Geochronological data show that exhumation of the Champtoceaux Complex occurred during the Early Carbo-

niferous, and that it pre-dates strike-slip movement along the Nort-sur-Erdre Fault as well as along the SASZ.

Exhumation of the high-pressure rocks is thus an early-orogenic event, taking place while convergence was still

active. Although extensional structures, possibly related to the gravitational collapse of the Variscan belt, are

known in the South-Armorican Domain (Gapais et al. 1993; Brown and Dallmeyer 1996), they occurred at a much

later stage of the evolution of the Variscan belt (300±310 Ma). Exhumation of the Champtoceaux Complex was

synchronous with sedimentation in the Ancenis Basin, for which two main models are possible. The Ancenis Basin

could have developed as a pull-apart basin (Diot and Blaise 1978), or as an extensional basin located in the hang-

ing-wall of a crustal-scale detachment fault. Although both interpretations need to be substantiated by additional

structural data, their consequences are brie¯y examined here. In the ®rst hypothesis, exhumation of the Champto-

ceaux Complex resulted from the down-dip component of the strike-slip movement. In the second hypothesis,

exhumation took place by contemporaneous thrusting at low structural levels and detachment at higher structural

levels. A potential candidate for the location of the detachment fault is the contact between the Champtoceaux

Complex and the Upper Allochthon, for two main reasons. Firstly, the contact is associated with a sharp meta-

morphic break, with lower-grade rocks (the Upper Allochthon) structurally overlying higher-grade rocks (the

Champtoceaux Complex). Secondly, a low-grade, mylonitic fabric overprints the Precambrian structures of the

Mauges Unit close to the Champtoceaux Complex. The mylonitic zone has a thickness of the order of 1±2 km

and dips at about 45� to the east, with top-to the-east shear criteria (Wyns et al. 1998). In other words, the slice

of rocks between the two faults (i.e. the Champtoceaux Complex) was `extruded' in a manner similar to the models

advocated, for example, in the Himalayan Range (Burch®eld et al. 1992; Gapais et al. 1992). Some similarities

with analogue models (Chemenda et al. 1995) are also clear, except that the extruded material in the analogue

models does not deform whereas the Champtoceaux Complex has been highly strained during exhumation.

9. CONCLUSIONS

1. Sm-Nd and U-Pb data on the eclogite-facies rocks from the Lower Allochthon (the Cellier Unit) of the Champ-

toceaux Complex yield an age of c. 360 Ma (U-Pb: 356�8 Ma, Sm-Nd: 362�25 Ma) for the high-pressure

event, much younger than those proposed in other units of the Armorican Massif. This leaves open the

possibility of either a diachroneity of the high-pressure event at the scale of the Armorican Massif during a



continuous convergence, or several episodes of subduction±collision related to the accretion and shortening of

back-arc basins and related arcs.

2. Concordant 40Ar/39Ar plateau ages and Rb-Sr data constrain the cooling history. Although small amounts of

inherited argon cannot de®nitely be ruled out, the oldest 40Ar/39Ar plateau ages (around 350 Ma) obtained in

the best-preserved samples probably represent closure temperatures. The relationship between plateau ages and

the ductile deformation of phengite suggests that closure temperature of undeformed phengite is on the order of

450±500�C. The youngest plateau ages (down to 330±340 Ma) may be the result of a later closure of the K-Ar

system, due to continuous deformation of the rheologically weakest rocks.

3. Exhumation of the high-pressure rocks occurred shortly after subduction, and at the same time as deposition of

¯uviatile to limnic sediments in the nearby Ancenis Basin. This favours a model of `extrusion' of the Champ-

toceaux Complex by coeval displacements along thrusts and detachments during the same period (Early Car-

boniferous). Exhumation of the Champtoceaux Complex thus occurred in an early-orogenic, syn-convergence

geodynamic setting. Strike-slip displacement along the SASZ and associated faults (Nort-sur-Erdre Fault) dur-

ing the Late Carboniferous reworked all the above structures, but did not contribute signi®cantly to the exhu-

mation of the Champtoceaux Complex.

ACKNOWLEDGEMENTS

This work has been undertaken as part of the project GeÂofrance3D-Armor2 (BRGM-University). The facilities

provided by M. Bohn (Microsonde Ouest) made our time spent behind the microprobe a pleasure. The technical

staff in both Rennes (M.A. Chassonneau, O. HeÂnin, J. MaceÂ and N. Morin) and Nice universities (Y. Ageon, M.

Manetti) are thanked for their help at various stages of the geochronological work. Participants in the Metamorphic

Studies Group Exhumation of Metamorphic Terranes meeting and ®eld excursion are thanked for their insightful

questions. The comments of two anonymous referees are greatly appreciated. Publication GeÂofrance-3D nb 104.

Contribution Geoscience Azur nb 331.

REFERENCES

Arnaud NO, Kelley SP. 1995. Evidence for excess argon during high pressure metamorphism in the Dora Maira Massif (Western Alps, Italy),using the ultra-violet laser ablation microprobe 40Ar/39Ar technique. Contributions to Mineralogy and Petrology 12: 1±11.

Austrheim H. 1998. In¯uence of ¯uid and deformation on metamorphism of the deep crust and consequences for geodynamics of collisionzones. In When Continents Collide: Geodynamics and Geochemistry of Ultrahigh-Pressure Rocks, Hacker BR, Liou JG (eds). KluwerAcademic Publishers: Dordrecht; 161±201.

Austrheim H, Erambert M, Engvik AK. 1997. Processing of crust in the root of the Caledonian continental collision zone: the role ofeclogitization. Tectonophysics 273: 129±153.

BalleÁvre M, KieÂnast JR, Paquette JL. 1987. Le meÂtamorphisme eÂclogitique dans la nappe hercynienne de Champtoceaux (Massifarmoricain). Comptes-Rendus de l'AcadeÂmie des Sciences de Paris (Series II) 305: 127±131 (in French with English abstract).

BalleÁvre M, Marchand J. 1991. Zonation du meÂtamorphisme eÂclogitique dans la nappe de Champtoceaux (Massif armoricain, France).Comptes-Rendus de l'AcadeÂmie des Sciences de Paris (Series II) 312: 705±711(in French with English abstract).

BalleÁvre M, Marchand J. 1996. ItineÂraire 14: De Nantes aÁ Ancenis: une coupe de la nappe de Champtoceaux. In Guides GeÂologiquesReÂgionaux: Bretagne, Lardeux H (ed). Masson: Paris; 173±178 (in French).

BalleÁvre M, Pinardon JL, KieÂnast JR, Vuichard JP. 1989. Reversal of Fe-Mg partitioning between garnet and staurolite in eclogite-faciesmetapelites from the Champtoceaux nappe (Brittany, France). Journal of Petrology 30: 1321±1349.

BalleÁvre M, Marchand J, Godard G, Goujou JC, Wyns R. 1993. Eo-Hercynian events in the Armorican Massif. In Pre-Mesozoic Geology inFrance and Related Areas, Keppie JD (ed). Springer Verlag: Berlin; 183±194.

Bayer R, Hirn A. 1987. DonneÂes geÂophysiques sur la structure profonde de la crouÃte hercynienne dans l'arc ibeÂro-armoricain et le Massifcentral francËais. Bulletin de la SocieÂteÂ GeÂologique de France Series 8, 3: 561±574 (in French with English abstract).

BeaupeÁre C. 1973. Contribution aÁ l'Etude de la Flore Fossile du Culm du Synclinal d'Ancenis. PhD Thesis, UniversiteÂ de Paris VI (in French).Bernard-Grif®ths J, Cornichet J. 1985. Origin of eclogites from South Brittany, France: a Sm-Nd isotopic and REE study. Chemical Geology

52: 185±201.Biino G, Compagnoni R. 1992. Very-high pressure metamorphism of the Brossasco coronite metagranite, southern Dora-Maira massif,

Western Alps. Schweizerische Mineralogische und Petrographische Mitteilungen 72: 347±363.BrieÁre Y. 1920. Les eÂclogites francËaises. Leur composition mineÂralogique et chimique; leur origine. Bulletin de la SocieÂteÂ Francaise de

MineÂralogie 43: 71±222 (in French).

322 v. bosse et al.


Brown M, Dallmeyer RD. 1996. Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphicbelt, France. Journal of Metamorphic Geology 14: 361±379.

Burch®eld BC, Chen ZL, Hodges KV, Liu YP, Royden LH, Deng CR, Xu J. 1992. The South Tibetan Detachment System, Himalayanorogen: extension contemporaneous with and parallel to shortening in a collisional mountain belt. Geological Society of America SpecialPaper 269: 1±41.

Cavet P, Gruet M, Pillet J. 1966. Sur la preÂsence de Cambrien aÁ Paradoxides aÁ CleÂreÂ-sur-Layon dans le NE du Bocage vendeÂen (Massifarmoricain). Comptes-Rendus de l'AcadeÂmie des Sciences de Paris (Series D) 263: 1685±1688 (in French).

Cavet P, Arnaud A, Blaise J, Gruet M, Lardeux H, Marchand J, Nicolas A, RivieÁre LM, Rossignol JC. 1978. Notice de la CarteGeÂologique de la France aÁ 1/50000, Feuille Ancenis (452). BRGM: OrleÂans (in French).

Chemenda AI, Mattauer M, Malavieille J, Bokun AN. 1995. A mechanism for syn-collisional rock exhumation and associated normalfaulting: Results from physical modelling. Earth and Planetary Science Letters 132: 225±232.

Choukroune P, Lagarde JL. (1977). Plans de schistositeÂ et deÂformation rotationnelle: l'exemple du gneiss de Champtoceaux (MassifArmoricain). Comptes-rendus de l'AcadeÂmie des Sciences de Paris (Series D) 284: 2331±2334 (in French with English abstract).

CogneÂ J. 1966. Une nappe cadomienne de style pennique: la seÂrie cristallophyllienne de Champtoceaux en bordure meÂridionale du synclinald'Ancenis (Bretagne-Anjou). Bulletin du Service de la Carte GeÂologique d'Alsace-Lorraine 19: 107±136 (in French with English abstract).

Compagnoni R, Maffeo B. 1973. Jadeite-bearing metagranites l.s. and related rocks in the Mount Mucrone area (Sesia-Lanzo Zone, WesternItalian Alps). Schweizerische Mineralogische und Petrographische Mitteilungen 53: 355±378.

Diot H, Blaise J. 1978. Etude structurale dans le PreÂcambrien et la PaleÂozoõÈque de la partie meÂridionale du domaine ligeÂrien (Sud-Est du Massifarmoricain): Mauges, synclinal d'Ancenis et Sillon Houiller de la Basse-Loire. Bulletin de la SocieÂteÂ GeÂologique et MineÂralogique deBretagne (Series C) 10: 31±50 (in French with English abstract).

Ducrot J, Lancelot JR, Marchand J. 1983. Datation U-Pb sur zircons de l'eÂclogite de la Borie (Haut-Allier, France) et conseÂquences surl'eÂvolution anteÂ-hercynienne de l'Europe occidentale. Earth and Planetary Science Letters 62: 385±394 (in French with English abstract).

Faure M, Leloix C, Roig JY. 1997. L'eÂvolution polycyclique de la chaõÃne hercynienne. Bulletin de la SocieÂteÂ GeÂologique de France 168: 695±705 (in French with English abstract).

Gapais D, PeÃcher A, Gilbert E, BalleÁvre M. 1992. Synconvergence spreading of the Higher Himalaya Crystalline in Ladakh. Tectonics 11:1045±1056.

Gapais D, Lagarde JL, Le Corre C, Audren C, Jegouzo P, Casas Sainz A, Van Den Driessche J. 1993. La zone de cisaillement de Quiberon:teÂmoin d'extension de la chaõÃne varisque en Bretagne meÂridionale au CarbonifeÁre. Comptes-rendus de l'AcadeÂmie des Sciences de Paris(Series II) 316: 1123±1129 (in French with English abstract).

Gil Ibarguchi JI. 1995. Petrology of jadeite metagranite and associated orthogneiss from the Malpica-Tui allochthon (Northwest Spain).European Journal of Mineralogy 7: 403±415.

Godard G. 1988. Petrology of some eclogites in the Hercynides: the eclogites from the southern Armorican massif, France. In Eclogites andEclogite-Facies Rocks, Smith DC (ed.). Elsevier: Amsterdam; 451±519.

Godard G, KieÂnast JR, Lasnier B. 1981. Retrogressive development of glaucophane in some eclogites from "Massif Armoricain" (east ofNantes, France). Contributions to Mineralogy and Petrology 78: 126±135.

Guineberteau B. 1986. Le massif granitique de Mortagne-sur-SeÁvre (VendeÂe). Structure, gravimeÂtrie, mise en place, distribution de U-Th-K.MeÂmoires du Centre de Recherches sur la GeÂologie de l'Uranium (NancËy, France) 11: (in French).

Guineberteau B, Bouchez JL,Vigneresse JL. 1987. The Mortagne granite pluton (France) emplaced by pull-apart along a shear zone:structural and gravimetric arguments and regional implication. Geological Society of America Bulletin 99: 763±770.

Hammerschmidt K, Frank E. 1991. Relics of high pressure metamorphism in the Lepontine Alps (Switzerland)-40Ar/39Ar and microprobeanalyses on white K-micas. Schweizerische Mineralogische und Petrographische Mitteilungen 71: 261±274.

Hammerschmidt K, Franz G. 1992. Retrograde evolution of eclogites: evidences from microstrutures and 40Ar/39Ar white mica dates,MuÈnchberg Massif, northern Bavaria. Contributions to Mineralogy and Petrology 111: 113±125.

Heinrich C. 1982. Kyanite-eclogite to amphibolite facies evolution of hydrous ma®c and pelitic rocks, Adula Nappe, Central Alps.Contributions to Mineralogy and Petrology 81: 30±38.

Hiroi Y, Ellis DJ. 1994. Preservation of Ca, P, REE growth zonation patterns in garnets which have undergone partial Fe, Mg and Mn volumediffusion re-equilibration: examples from Japan and Sri Lanka. Eos, Transactions, American Geophysical Union, 75: 185±186.

Hunziker JC, Desmons J, Hurford AJ. 1992. Thirty-two years of geochronological work in the Central and Western Alps: a review on sevenmaps. MeÂmoires de GeÂologie, Lausanne 13: 1±59.

Hurford AJ. 1986. Cooling and uplift patterns in the Lepontine Alps, South Central Switzerland and an age of vertical movement on theInsubric fault line. Contributions to Mineralogy and Petrology 92: 413±427.

Hussein HAM. 1960. Etude radiogeÂologique de la reÂgion situeÂe au Nord de Nantes (Loire-Atlantique). Sciences de la Terre 7: 1±114 (inFrench).

JaÈger E. 1979. Introduction to geochronology. In: Lectures in Isotope Geology, JaÈger E, Hunziker JC (eds). Springer-Verlag: Berlin; 1±12.Kalt A, Hanel M, Schleicher H, Kramm U. 1994. Petrology and geochronology of eclogites from the Varsican Schwarzwald (FRG).

Contributions to Mineralogy and Petrology 115: 287±302.Koons PO, Rubie DC, FruÈh-Green G. 1986. The effects of disequilibrium and deformation on the mineralogical evolution of quartz-diorite

during metamorphism in the eclogite facies. Journal of Petrology 28: 679±700.Krabbendam M, Wain A. 1997. Late-Caledonian structures, differential retrogression and structural position of (ultra)high-pressure rocks in

the Nordfjord-Stadlandet area, Western Gneiss Region. Norges Geologiske Undersokelse Bulletin 432: 127±139.Krabbendam M, Wain A, Andersen TB. 2000. Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway:

indications of incomplete transition at high pressures. Geological Magazine 137: 235±255.Kretz R. 1983. Symbols for rock-forming minerals. American Mineralogist 68: 277±279.Krogh EJ. 1988. The garnet-clinopyroxene Fe-Mg geothermometer- a reinterpretation of existing experimental data. Contributions to

Mineralogy and Petrology 99: 44±48.



KuÈhn A, Glodny J, Iden K, Austrheim H. 2000. Retention of Precambrian Rb/Sr phlogopite ages through Caledonian eclogite faciesmetamorphism, Bergen Arc Complex, W-Norway. Lithos 51: 305±330.

Lacroix A. 1891. Etude peÂtrologique des eÂclogites de la Loire-InfeÂrieure. Bulletin de la SocieÂteÂ des Sciences Naturelles de l'Ouest de la France1: 81±114 (in French).

Lagarde JL. 1978. La DeÂformation des Roches dans les Domaines aÁ SchistositeÂ Subhorizontale (Champtoceaux ± Canigou-Roc de France).PhD Thesis, UniversiteÂ de Rennes (in French).

Lanzirotti A. 1995. Yttrium zoning in metamorphic garnets. Geochimica Cosmochimica Acta 59: 4105±4110.Lasnier B, Leyreloup A, Marchand J. 1973. DeÂcouverte d'un granite "charnockitique" au sein des "gneiss oeilleÂs". Perspectives nouvelles sur

l'origine de certaines leptynites du Massif armoricain meÂridional (France). Contributions to Mineralogy and Petrology 41: 131±144 (inFrench with English abstract).

Le Goff E. 1989. Conditions P±T de la deÂformation dans les orthogneiss: modeÁle thermodynamique et exemples naturels. MeÂmoires du CentreArmoricain d'Etude Structurale des Socles 29: 1±321 (in French).

Le Goff E, BalleÁvre M. 1990. MeÂthodes d'estimation des conditions P±T dans les orthogneiss: analyse des relations de phases. Comptes-rendusde l'AcadeÂmie des Sciences de Paris (Series II) 311: 119±125 (in French with English abstract).

Li S, Wang S, Chen Y, Liu D, Qiu J, Zhou H, Zhang Z. 1994. Excess argon in phengite from eclogite: evidence from dating of eclogiteminerals by Sm-Nd, Rb-Sr and 40Ar/39Ar methods. Chemical Geology 112: 343±350.

Li S, Xiao Y, Liou D, Chen Y, Ge N, Zhang Z, Sun S-S, Cong B, Zhang, R, Hart, SR, Wang S, 1993. Collision of the North China andYangtse Blocks and formation of coesite-bearing eclogites: timing and processes. Chemical Geology 109: 89±111.

Ludwig KR. 1999. Isoplot/Ex Version 2.06. A geochronological toolkit for Microsoft Excel. Berkeley Geochronology Center, SpecialPublication 1a.

Marchand J. 1981. Ecaillage d'un "meÂlange tectonique" profond: le complexe cristallophyllien de Champtoceaux (Bretagne meÂridionale).Comptes-Rendus de l'AcadeÂmie des Sciences de Paris (Series II) 293: 223±228 (in French with English abstract).

Maruyama S, Liou JG, Terabayashi M. 1996. Blueschists and eclogites of the world and their exhumation. International Geology Review 38:485±594.

Maruyama S, Suzuki K, Liou JG. 1983. Greenschist-amphibolite equilibria at low pressures. Journal of Petrology 24: 583±604.Massonne HJ, Schreyer W. 1987. Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite and quartz.

Contributions to Mineralogy and Petrology 96: 212±224.Massonne HJ, Szpurka Z. 1997. Thermodynamic properties of white micas on the basis of high-pressure experiments in the systems K2O-

MgO-Al2O3-SiO2-H2O and K2O-FeO-Al2O3-SiO2-H2O. Lithos 41: 229±250.Mezger K, Essene EJ, Halliday AN. 1992. Closure temperature of the Sm-Nd system in metamorphic garnets. Earth and Planetary Science

Letters 113: 397±409.Mezger K, Krogstad EJ. 1997. Interpretation of discordant U-Pb zircon ages: an evaluation. Journal of Metamorphic Geology 15: 127±

140.O'Brien PJ, Carswell DA, Gebauer D. 1990. Eclogite formation and distribution in the European Variscides. In: Eclogite Facies Rocks,

Carswell DA (ed). Blackie: Glasgow; 204±224.Okulitch AV. 1999. Geological time chart, 1999. Open ®le 3040. Geological Survey of Canada: Ottawa.Paquette JL. 1987. Comportement des systeÁmes isotopiques U-Pb et Sm-Nd dans le meÂtamorphisme eÂclogitique. ChaõÃne hercynienne et chaõÃne

alpine. Documents du Centre Armoricain d'Etude Structurale des Socles 14 (in French).Paquette JL, Peucat JJ, Bernard-Grif®ths J, Marchand J. 1985. Evidence for old Precambrian relics shown by U-Pb zircon dating of

eclogites and associated rocks in the Hercynian belt of south Brittany, France. Chemical Geology 52: 203±216.Paquette JL, Marchand J, Peucat JJ. 1984. Absence de tectonique cadomienne dans le complexe de Champtoceaux (Bretagne meÂridionale)?

Comparaison des systeÁmes Rb-Sr et U-Pb d'un meÂtagranite. Bulletin de la SocieÂteÂ GeÂologique de France Series 7, 26: 907±912 (in Frenchwith English abstract).

Paquette JL, Monchoux P, Couturier M. 1995. Geochemical and isotopic study of a norite-eclogite transition in the European Variscan belt:implications for U-Pb systematics in metabasic rocks. Geochimica Cosmochimica Acta 59: 1611±1622.

Peucat JJ, Vidal P, Godard G, Postaire B. 1982. Precambrian U-Pb zircon ages in eclogites and garnet pyroxenites from south Brittany(France): an old oceanic crust in the west European Hercynian belt? Earth and Planetary Science Letters 60: 70±78.

Picquenard C. 1920. Sur la ¯ore fossile des bassins houillers de Quimper et de Kergogne. Comptes-rendus de l'AcadeÂmie des Sciences de Paris170: 55±57 (in French).

Pin C, Lancelot JR. 1982. U-Pb dating of an early Paleozoic bimodal magmatism in the French Massif Central and its further metamorphicevolution. Contributions to Mineralogy and Petrology 79: 1±12.

Pin C, Peucat JJ. 1986. Ages des eÂpisodes de meÂtamorphisme paleÂozoõÈque dans le Massif central et le Massif armoricain. Bulletin de la SocieÂteÂGeÂologique de France Series 8 2: 461±469 (in French).

Powell R, Holland TJB, Worley B. 1998. Calculating phase diagrams involving solid solutions via non-linear equations, with examples usingTHERMOCALC. Journal of Metamorphic Geology 16: 577±588.

RivieÁre LM. 1977. Le Culm frasno-dinantien du synclinal d'Ancenis (SE du Massif armoricain) au nord de la Loire. Bulletin de la SocieÂteÂGeÂologique et MineÂralogique de Bretagne (Series C) 9: 19±57 (in French with English abstract).

Roman-Berdiel T, Gapais D, Brun JP. 1997. Granite intrusion along strike-slip zones in experiment and nature. American Journal of Science297: 651±678.

Rubie DC. 1990. Role of kinetics in the formation and preservation of eclogites. In Eclogite Facies Rocks, Carswell DA (ed.). Blackie:Glasgow; 111±140.

Ruffet G, FeÂraud G, Amouric M. 1991. Comparison of 40Ar-39Ar conventional and laser dating of biotites from the north TreÂgor Batholith.Geochimica Cosmochimica Acta 55: 1675±1688.

Ruffet G, FeÂraud G, BalleÁvre M, KieÂnast JR. 1995. Plateau ages and excess argon on phengites: a 40Ar-39Ar laser probe study of alpine micas(Sesia Zone). Chemical Geology 121: 327±343.

324 v. bosse et al.


Ruffet G, Gruau G, BalleÁvre M, FeÂraud G, Philippot P. 1997. Rb-Sr and 40Ar-39Ar laser probe dating of HP phengites (Sesia zone, WesternAlps): underscoring of excess argon and new crystallization age constraints. Chemical Geology (Isotope Geoscience) 141: 1±18.

Scaillet S. 1996. Excess 40Ar transport scale and mechanism in high-pressure phengites: a case study from a eclogitized metabasite of the DoraMaira nappe, Western Alps. Geochimica Cosmochimica Acta 60: 1075±1090.

Scaillet S. 1998. K-Ar (40Ar-39Ar) geochronology of ultrahigh pressure rocks. In: When Continents Collide: Geodynamics and Geochemistry ofUltrahigh-Pressure Rocks, Hacker BR, Liou JG (eds). Kluwer Academic Publishers: Dordrecht; 161±201.

Spear FS, Kohn MJ. 1996. Trace element zoning in garnet as a monitor of crustal melting. Geology 24: 1099±1102.Tonarini S, Villa IM, Oberli F, Meier M, Spencer DA, Pognante U, Ramsay JG. 1993. Eocene age of eclogite metamorphism in Pakistan

Himalaya: implications for India-Eurasia collision. Terra Nova 5: 13±20.Turner G, Hunecke JC, Podosek FA, Wasserburg GJ. 1971. 40Ar/39Ar ages and cosmic ray exposure ages of Apollo 14 samples. Earth and

Planetary Science Letters 12: 19±35.Velde B. 1966. Etude mineÂralogique d'une eÂclogite de Fay-de-Bretagne (Loire-Atlantique). Bulletin de la SocieÂteÂ Francaise de MineÂralogie et

Cristallographie 89: 385±393 (in French).Velde B. 1970. Les eÂclogites de la reÂgion nantaise (de Campbon au Cellier, Loire-Atlantique). Bulletin de la SocieÂteÂ Francaise de MineÂralogie et

Cristallographie 93: 370±385 (in French with English abstract).Vidal JJ, Peucat JJ, Lasnier B. 1980. Dating of granulites involved in the Hercynian fold-belt of Europe: an example taken from the granulite-

facies orthogneisses at la Picherais, southern Armorican massif, France. Contributions to Mineralogy and Petrology 72: 283±289.Villa IM. 1998. Isotopic closure. Terra Nova 10: 42±47.von Blanckenburg F, Villa I, Baur H, Morteani G, Steiger RH. 1989. Time calibration of a PT-path from the Western Tauern Window,

Eastern Alps: the problem of closure temperatures. Contributions to Mineralogy and Petrology 101: 1±11.Wyns R, Lardeux H, Moguedet G, Duermael G, Gruet M, Biagi R. 1998. Notice Explicative de la Carte GeÂologique de la France aÁ 1/50000,

Feuille ChemilleÂ (483). BRGM: OrleÂans (in French with English abstract).



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