The nappes of La Chataigneraie, southwest Massif Central, France

Tectonophysics,

69

157 (1989) 69-79

Elsevier Science Publishers

B.V.. Amsterdam

- Printed

in The Netherlands

The nappes of La Chataigneraie, southwest Massif Central, France SERGE BOGDANOFF, Laboratoire de Gologie

JEAN-LUC

CIRODDE

Historique et Structurale,

and MAURICE

UniuersitP Paris-Sud,

(Revised

version accepted

November

Donnot,

M., 1989. The nappes

DONNOT

91405 Orsay Cedex, (France)

14,1987)

Abstract Bogdanoff,

S., Cirodde,

J.-L.

France.

In: R. Meissner

Drilling,

Geophysics,

The well-known limb

Carboniferous

Geology

inverted

of a recumbent

and and

fold

D. Gebauer

and Geochemistry.

metamorphism with

(Editors),

Tectonophysics,

in the La Chataigneraie

SW-vergence.

Newly

of the European

southwest

Massif

Continental

Central,

Crust:

Deep

157: 69-79. zone has previously

interpreted

data

indicates

that

been interpreted it is the

as a reverse

result

of Early

thrusting.

Intraduction The La Chataigneraie schists are located to the south of Aurillac in the Massif Central (Fig. 1). Intruded by several late Variscan granites, this zone is uncomformably covered by Carboniferous to Neogene sediments and volcanic rocks. To the east, La Chataigneraie is bounded by the La Margeride granite. To the west, the sinistral Carboniferous Sillon Houiller strike-slip fault separates La Chataigneraie from the southern end of the Millevaches and Limousin formations. The studied zone is located between the Marcolb and La Margeride granites (Fig. 1). Previous studies, including those of Boule (1899-1900) Roques (1941), Demay (1946, 1948) briefly described the lithology and an inverted

0 1989 Elsevier Science Publishers

quently involved in a large recumbent fold, overfolded to the south. The present authors offer another interpretation: The eastern part of La Chataigneraie is made up of thrust nappes which shear an already metamorphosed continental crust. The stacking of the nappes may explain the inversion of the metamorphism. The lithology, ships between crystallizations

the structures,

and the relationmetamorphic

deformations and will be discussed.

Lithology

The works BotssC (1980)

metamorphism. The sericitic phyllites and slates located in the southwest of La Chataigneraie gently dip towards the northeast and to the north. These rocks are overlain by staurolite-garnet micaschists which gently dip in the same directions. These authors have interpreted these relationships as being due to a regional metamorphism subse0040-1951/89/$03.50

of La Chataigneraie,

The Evolution

B.V.

of Cheze (1975), Joubert (1978) and Bogdanoff et al. (1980, 1985)

have described five lithological units. From the bottom (southwest) to the top (northeast), they are (Figs. 1 and 2): grey phyllites, bluish phyllites, the La Salvetat schists, quartz greywacke and quartzose schists. (I) Grey phyllites. This formation dips to the northeast. At least 1000 m thick. it is characterized

+La+ +

Fig. 1. Simplified

map of the La Chataigneraie

area, S-Soulaque

tfiangles-_thrust planes; dashed lines with ticks-schistosity

by an alternation metres-decimetres) sericitic phyllites

granite:

of various thicknesses (centiof sericitic and quartzose with a few quartzite beds

(metres-decametres thick) in which it was impossible to demonstrate a sequential order. The mineralogical composition is quartz, albite, sericite, chlorite and brown contact biotite, with ilmenite, zircon and tourmaline as accessories. The sericitic phyllites are chlorite-rich.

La

SW

Salvetat

Cydierite

continuous

and/or

bedding

heavy line-main dip. Line of sections

fault;

lines ornamented with

in Fig. 2 also shown.

sories. (3) Lu Sulvetat schists. This 500-m thick formation rests on the bluish phyllites and dips to the northeast. It is made up of two alternating litholo-

schists Garnet Thlorlte

NE

Muscowte

chlorite

Iknl

Fig. 2. Two sections A-B and A’-B’

+

(2) Bluish phyllites. These overlie the grey phyllites, are about 500 m thick and are devoid of quartzites. The mineral composition is quartz, albite muscovite, sericite, chlorite and brown contact biotite, with ilmenite and zircon as acces-

Muscovtte 0

+ +

(see Fig. 1 for location).

.S,-bedding;

S,-regional

schistosity.

chlorite

71

gies in varying proportions, the order of which is not clear: fine-grained grey or grey-green phyllites composed of chlorite or sericite, and banded medium grey or deep grey medium-grained schists. The La Salvetat schists contain quartz, biotite, garnet, cordierite, albite, muscovite, chlorite and pinite. (4) Quartz greywackes. This formation is generally flat-lying or gently inclined towards the north or the northeast, and exceeds 1000 m in thickness. It is easily separated from the other formations, comprising massive sandstone beds (decimetresmetres thick) sometimes interstratified with thin more-or-less graphitic schists. The quartz greywackes are associated with a thickness of a few metres of leucocratic gneisses (leptynites) which are concordant with the bedding. This is the only formation to contain graded bedding and cross-laminated beds, structures which seem to indicate local inversion of the formation (Nicolas, 1985), and younging to the southwest. However, because of the amphibolite facies metamorphism these criteria are often difficult to interpet. Quartz, biotite, plagioclase, almandine garnet (Cheze, 1975), rare staurolite, muscovite, chlorite and albite, with tourmaline, ilmenite, zircon and apatite as accessories are the minerals found in the grey-wackes. The leucocratic gneisses contain mainly quartz and albite; biotite, chlorite and muscovite are rare. Accessories are zircon and sphene. (5) Quartzose schists. This monotonous formation, exceeding 1000 m in thickness, overlies the La Salvetat schists and the quartz greywackes. The quartzose schists contain thick micaceous quartzites (up to 20 m), a few metres of leucocratic gneisses, thin graphitic schists similar to those of the quartz greywackes and lenses or boudins (decametres in thickness) of quartz greywackes which closely resemble the underlying rocks. The mineralogy is identical to that of the quartz greywackes. Summary

These five lithological units, in which mafic rocks are almost absent (Cheze, 1975; Joubert,

1978), may be grouped into two types which are also defined by geochemical data (Bogdanoff et al., 1987). They are: (1) a phyllitic group, the grey and bluish phyllites and the La Salvetat schists, and (2) a quartzose group of quartz greywackes and quartzose schists. The grey and bluish phyllites were initially shales (the La Salvetat schists were not analysed). The quartz greywackes and the quartzose schists are chemically indistinguishable. Both formations are interbedded with leucocratic gneisses (leptynites) which are very probably variously modified dacitic tuffs. In these two groups the only distinctly bedded formation is the unfossiliferous quartz grey-wacke formation. These rocks resemble the azoic Marcory sandstone attributed to the Lower Cambrian in the Monts de Lacaune (Fig. 6B) by Thoral (1935) and Donnot and GuCrangC (1978). The quartz greywacke could be Cambrian in age although it could be of Cambro-Ordovician age like the Lot schists (Fig. 6B) (Briand and Gay, 1978). Thus, an Early Palaeozoic age is suggested for the La Chataigneraie rocks. In brief, we believe that neither the disposition of the lithological units (Figs. 1 and 2) nor some of the rare and cryptic criteria described above, indicate a large southerly or southwesterly recumbent fold.

structures The structures were studied in the zone situated between the La Margeride and Veinazes granites (Fig. l), avoiding the vicinity of these batholiths as they induce, at least locally some deformation in the enveloping rocks. The zone studied shows that the different structures can be divided into an early and a late group. Ear& Structures (Fig. 3).

Two schistosities and numerous quartz ribbons are present in the formations described above. (1) A regional schistosity S,, gently dips towards the north or northeast except in the localized, late deformed, zones. This surface is present as a penetrative schistosity in the bluish phyllites

l‘!

EARLY

glTRUCTUR=s

Quartz

ribbons

not to lcala Fig. 3. Disposition

of early structures

and in the La Salvetat and quartzose schists. This schistosity is parallel to the quartz greywackes bedding S, and often located in the interbedded schists. Further, S,, is parallel to or intersects at a low angle the ill-defined bedding in the grey phyllites.

in the tectonic

pile.

but frequent in the other formations. They are isoclinally folded on a scale of centimetres-deci-

(2) An occasional S,_, schistosity predates the S, and occurs in microlithons envelopped by S,,,

metres and the folds are overturned towards the south or the southwest, S, being in the axial plane. This latter geometry is developed particularly in the quartzose schists. The ribbons are also parallel to the S, schistosity and may exhibit boudinage, the stretching being approximately in

the angle between S,, and S,_ , varying. This schistosity is observed in the La Salvetat and quartzose schists. These two surfaces are compara-

a N-S direction. With respect to the regional schistosity, S,,, these ribbons seem older (isoclinally folded or boudinaged), although they appear

ble with those observed schists (Fig. 6B).

younger than the S,_ 1 schistosity because they wrap around it. Stretching and mineral lineations (Figs. 3 and 5) are also early-formed structures. Sporadically distributed, they lie on regional schistosity in the quartzose schists and the quartz greywackes. One observes this lineation occasionally in the bluish phyllites coinciding with S,,. This lineation, which plunges in a N20”-N60” direction is an u-type lineation (Bryant and Reed, 1969; Mattauer, 1975). Sheath folds were not observed but some isoclinal folds (centimetres-decimetres) occur, the axial planes of which are parallel with the regional schistosity (Matfauer, 1975). The axis of the latter is parallel to the u-type lineation. Quartz veins (Fig. 3) (centimetres-decimetres

by Pin (1980) in the Lot

(3) In thin section the quartz ribbons (a few millimetres in thickness and many metres in length) (Ramsay and Huber, 1987) are different from quartz laminae in undeformed shales: (a) the quartz grains are often flattened in the ribbon plane, (b) the boundaries of the ribbons are sharp and not blurred, and (c) micas and quartz are well segregated. We suggest that these ribbons have a mechanical origin and do not directly represent the stratification. Boullier and Bouchez (1978) believed these ribbons to be typical of ductile mylonites in a rotational regime. The ribbons are rare in the quartz

greywackes

13

LATE

STRUCTURE

not Fig. 4. Disposition

to

rcmla

of late structures in the tectonic pile.

in

thickness and l-2 metres long), are well developed in the grey phyllites. Flat-lying, parallel to the flat or gently dipping S, schistosity, they are seen to slightly intersect it; they are also isoclinally folded, .S,, being in the axial plane. These veins appear to be located either at the base of thrusts or at the summit of their sole (Andrieux et al., 1981; Brunel, 1983). They could represent hydraulic fractures caused by a high hydraulic pressure which favoured the transport of thrust nappes and slices (Hubbert and Rubey, 1959). Late structures (Fig. 4).

These structures deform the early structures. (1) S-shaped folds. These are non-penetrative (already described by Cheze (1975)) and they deform the S,, schistosity of the quartz greywackes, the quartz ribbons and occasionally the S,, schistosity of the La Salvetat schists. These folds, generally around 1 m in size, are overturned towards the south or the southwest, the axial plane schistosity S, + r dipping towards the north or the northwest. A coaxial microfold-intersection b-type lineation is also present in non-folded zones where,

carried by the S, and S, (bedding) surfaces, it affects the first (a-) type lineation. Nevertheless, the interference between this latter lineation and the S-shaped folds has not been observed directly. The S-shaped folds are similar in geometry and dimensions to those described by Pin (1980) in the Lot schists (Fig. 6B). The S-shaped folds, at least locally, exhibit a rotational south or southwestwards vergence, shown by the N90-N150 axial strike (Fig. 5). (2) Upright folds. These are located at the bottom of the bluish and grey phyllites, and occasionally present in the quartz greywackes and the La Salvetat schists; their relationship with the S-shaped folds have not been observed. The upright folds deform the bedding, the S,, and the quartz ribbons of the grey and bluish phyllites. Metres to decametres in size, and sometimes slightly overturned to the north or the south, the axes of the folds plunge gently in a NllO-N130 direction (Fig. 5). They bear a coaxial microfoldintersection lineation, and often a crenulation-type schistosity in the axial plane. (3) The crenulation cleavage is located not far from the upright fold zones and is generally slightly

Fig. 5. Lower hemisphere, Wulff net representation of the a-type lineation and S-shaped and upright folds. A. a-type lineation in the quartz greywackes and the quartzose schists. Ticks-lineation S-shaped folds

plunge, dots--S,

(regional schistosity) pole and SO (bedding) pole. B.

in the quartzgreywackes. Arrows-fold axis; dots-axial planepole C. Uprightfoldsin the grey and bluish phyllites. Arrows-fold

axis; dots-axial

overturned south or southwestwards. It deforms the same features as the folds. Its axial strike is parallel to that of the upright folds. (4) Cleavage (C) planes (Berth6 et al. (1979) and observed by Roques (1941)) are located in the quartz greywackes, the quartzose schists and sometimes in the La Salvetat schists and the grey

plane pole.

phyllites. Generally N-dipping, more so than the regional schistosity S,, they exhibit small S-directed shears. (5) The occasional flexure, 3-4 hectometres in size (Demange and Nicolas, 1984), is seen to deform the S, of the quartzose schists. They seem to be synchronous with the S-shaped folds as they

75

deform the same tectonic features, their vergence The only structures

which postdate

The following locations

being approximately identical. the late

structures are small-scale structures such as chevron folds and kinks (Cheze, 1975; BoessC, 1980;

question

concerns

the precise

of the thrust surfaces within the litho-

logical pile. The relationships

between deforma-

tions and rnet~o~~c castrations helpful in resolving this problem.

should be

Nicolas, 1985). The S-shaped and upright folds, the crenulation cleavage and the C-planes which deform the S, and the quartz ribbons are likely to be of the same age. The early structures, such as S,._, and S,, the u-type lineation, the quartz ribbons and the quartz veins, indicate a rotational very nearly southwestern the S-shaped indicate the

strain regime which is

a simple shear with a south or a vergence. The late structures, such as folds, the C-planes and the flexures, last increments of this late type of

deformation (with the same vergence) in the quartz greywackes and in the quartzose and La Salvetat schists. Upright folds and crenulation cleavage show that the process ceases in the grey and bluish phyllites. These late non-penetrative structures moderately disturb the regional schistosity resulting from the early deformation, and as a consequence, this generally remains ho~zontal or gently dipping to the north or the northeast. Further, the authors suggest that the early and late structures appear to be more like thrust structures than those seen on the inverse or normal limb of a large recumbent fold. In this context, large (many hectometres) early S- or, for that matter, Z-shaped folds are not found in La Chataigneraie.

S-shaped folds can be

observed in the area but they are only a few metres in size and are late-formed. Also, they classically characterize a normal limb, while according to earlier authors, La Chataigneraie consists only of the inverted limb of a large fold. Finally, in the section on lithology we have seen that: (a) quartz greywackes and quartzose schists contain the same interbedded rocks (leucocratic gneisses and quart&es), (b) the qua*ose schist S, encloses lenses of quartz greywackes, and that (c) the chemical composition of these two formations (major elements) is the same. Therefore, the authors consider that the quartzose schists are mylonitized quartz greywackes. This conclusion favours a thrust nappe interpretation.

Relationships between deformations and crystallizations Boule (1975)

(1899-1900)

and Joubert

Roques (1978)

(1941),

have described

Cheze two

metamo~~c events in La Chataigneraie: (1) an inverted general metamorphism, without linking it precisely to the deformation, and (2) a contact metamorphism relating to the granite intrusions. The

contact

met~o~~srn

su~ounding

each

granite can be readily separated from the general event as the larger minerals (sillimanite, cordierite, etc.) cut the S, at various angles.

Deformation and crystalkation general metamorphism The st~cturally

relationships in the

lower formations, i.e., the grey

and bluish phyllites (Fig. 2), contain muscovite and chlorite located in the S,, and S,_, schistosities. In the superposed formations the following mineral assemblages appear: (1) Microscopic biotite in the S, and in the a-lineation, and staurolite and almandine (Roques, 1941; Cheze, 197.5) enclosed by the S,, occur in the quartz greywackes. These minerals are moreor-less transformed to chlorite and white mica. (2) In the La Salvetat schists the situation is practically the same, with biotite in the 5, while the garnet and cordierite are wrapped by this surface. Again, these three minerals are partly retrogressed to white mica and chlorite, this retrogression being more pronounced in this formation than in the quartz greywackes. (3) In the quartzose schists, biotite is again situated in the S,,, garnet being wrapped or fragmented and aligned within this surface. Retrogression to chlorite again occurs. As a consequence of this, the authors believe that the metamorphism changes abruptly when passing (towards the top) the boundary between

76

grey and bluish phyllites (low grade) on the one hand and quartz greywackes and La Salvetat schists (medium grade) on the other. Furthermore, these changes are not heralded by prograde reactions in the first two formations (the grey and bluish phyllites). We suggest then, that a first thrust plane may be placed at the quartz greywackes - La Salvetat schists base, where, regarding the latter, it is more-or-less hidden by a late vertical normal fault. The grey and bluish phyllites are considered here as a parautochthonous sole. If this thrust plane was not parallel to this base, one would expect to observe syn- or pre-tectonic biotite or garnet or staurolite in the underlying formations; this is not the case. Above this thrust, variations in metamorphic intensity are not very different from one formation to the other. However, as already noted, the quartzose schists are mylonitized quartz greywackes. A second thrust plane may therefore be placed at the base of these rocks which probably has the same dip as the first (to the north). Concerning the boundary between the quartz greywackes and the La Salvetat schists, the authors believe that this is also a thrust plane. Furthermore, although variation in metamorphic intensity is not very intense on either side, the thinning, both in plane and in section across the bedded quartz greywackes, framed by the La Salvetat schists where the S, and the quartz ribbons are penetrative surfaces, seems to support this interpretation. Outside the zone studied, in a northeasterly direction, and above the quartzose schists, one finds staurolite-garnet-sillimanite-tkyanite?)bearing quartz greywackes (Restituito, 1971; Brousse et al., 1973). Conclusions

Abrupt changes in the intensity of regional metamorphism coinciding with the lithological boundaries defined above, the increase in metamorphism towards the top of the lithological pile (towards the northeast), the more-or-less regressive character within the formations, and the change in deformation intensity between quartz greywackes and quartzose schists all suggest

thrusting. The thrust planes, which cut a pre-existing metamorphic zone, involve thrust translation towards the southwest, resulting in the stacking of the higher grade metamorphic zones on the lower grade zones. Thus, the authors believe that the earlier interpretation cannot be retained: namely that the metamorphic inversion is the consequence of the folding of a pre-existing general metamo~~sm into a large recumbent fold, only the inverted limb of which is still present. Comparison with adjoining regions (Figs. 6A and B) Other Variscan thrusts having a south or southwestwards vergence are known in this part of the Massif Central. Above the La Chataigneraie thrusts, towards the north-east, lie the La Truyere thrust ~rn~nly sillimanite gneiss; Burg, 1977; Burg et al., 1984). This thrust is overlain in turn by the Haut-Allier thrust which includes amphibolites with HP-HT relics, migmatites and sillimanitic gneisses (Forestier, 1963; Forestier et al., 1973; Carme, 1974; Marchand, 1974; Mattauer and Etchecopar, 1977; Lasnier, 1977). Towards the east, the La Chataigneraie thrust probably extends as the Lot schists inverted metamorphic sequence (Peyretti, 1971; Komprobst and Poulain, 1972; Briand and Gay, 1978; Pin, 1980) which is presumed to be connected with the Haut-Allier thrust (Burg et al., 1984). Age of thrusting The granites indicated in Fig. 1 are intrusive in the La Chataigneraie thrust units. Undeformed and associated with a rnet~o~~c contact aureole, they are cut by frequently vertical microdioritic to microgranitic veins (Vivier, 1970; Cheze, 1975; Joubert, 1978; Nicolas, 1985). The age of the La Margeride granite is 323 f 13 Ma (Rb/Sr method; CouturiC et al., 1979) or 314 $- 3 Ma (U/Pb method on monazite; Pin, 1980). The age of the Soulaque granite is 278 ILt8 Ma (K/Ar method; Bellon and Gibert, 1981), while the age of the Marco& granite is 305-301 + 13 Ma (Rb/Sr method; Vivier and Lassene,

described above; (c) the Hercynian

age of the

Haut-Allier thrust (Burg and Matte, 1977), and (d) the Visean age of the Marvejols thrust (Pin, 1980), it is possible to propose an Early Carboniferous age for the La Chataigneraie thrusts. These thrusts could be Devonian in age as was suggested by Autran and CognC (1980) for other thrusts in the Massif Central. Nevertheless,

the

authors prefer a Visean age. Although it could not actually be better documented, this age seems to be in better agreement with the age of the nearest thrusts. Acknowledgement This research was financially supported by the Bureau de Recherches Geologiques et Mini&es project “Gites de Depart Acide”. The authors are deeply indebted to J. FabriCs, L. Latouche, G. Vivier and J.-C. Guezou for valuable improvements to the text. Figures were by Mme. Roche and text corrections by B. Purser. References Andrieux,

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