Thiviers-Payzac unit

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The Thiviers-Payzac Unit is a Neoproterozoic to Ordovician metasedimentary rock sequence in southern Limousin France . Geologically it belongs to the Variscan basement of the north-western massif Central . Tectonically, it occupies the highest position in its ceiling structure . Their allochthonousness towards the underlying gneiss blankets is based on observations of a metamorphic and structural nature as well as on seismic profile lines.

designation

The Thiviers-Payzac unit, sometimes also called the Thiviers-Payzac ceiling or the Bas-Limousin group, was named after two places in its area of ​​distribution: Thiviers and Payzac , both located in the northeast of the Dordogne department .

Occasionally, the term Thiviers-Payzac unit is also used in a broader sense and refers to the Leyme unit at Figeac (Upper Quercy) and to the Saint-Sernin-sur-Rance ceiling or to the Saint-Salvi-de-Carcavès- Cover extended in the Rouergue . The latter units as well as the Gartempe unit near Bellac , the Saint-Salvadour unit near Uzerche and also the rocks in the Chantonnay synclinory in the Vendée have a comparable lithological structure and a similar tectonic position as the actual Thiviers-Payzac unit.

Geographical distribution

Geological overview map for the location of the Thiviers-Payzac unit (in green)

The Thiviers-Payzac unit, part of the Bas-Limousin (Südlimousin) landscape, follows a roughly 70-kilometer-long, bent circular arc segment starting slightly west of Thiviers in the north of the Dordogne via Lanouaille , Payzac, Orgnac-sur-Vézère , Donzenac to a little east of Brive in the Corrèze department . Its strike direction is initially WNW-ESE (N110), but then turns north of Orgnac in the NW-SE direction (N135). The unit is separated from the upper gneiss cover on its north side by means of the Estivaux Fault , a left-hand, ductile leaf displacement . In the far west it encounters rocks from the lower gneiss cover . Its southern boundary is formed by the overlying Liass sediments of the Aquitaine Basin in the west . In the central section, the southern limousine fault - also a ductile lateral shift, but with a right-hand sense of movement - separates it from the Génis unit in the south. In the eastern part it dives under the Permian red sediments of the Briver Basin . The maximum width of the unit across the strike direction in the Auvézère Valley is only about 9 kilometers.

Immediately northwest of Terrasson in the east of the Dordogne is the Horst von Châtres - a basement cantilever that is still part of the Thiviers-Payzac unit. For about 10 kilometers it has the same strike direction (WNW-ONO) as the main unit; across the strike, this opening is only about 5 kilometers wide.

Stratigraphic structure

The Thiviers-Payzac unit shows the following stratigraphic structure (from hanging to lying ):

The unit is also intruded by the Lower Carboniferous Estivaux granite as well as the Saut-du-Saumon orthogneiss and the Corgnac granite , Ordovician granitoids .

Puy-des-Âges quartzite

Puy-des-Âges quartzite

The uppermost Puy-des-Âges-quartzite is a very resistant, white, sericite- bearing quartzite , which forms hardened ridges in the terrain, which protrude over the clod of the Bas-Limousin that was leveled in the course of the Eocene . It occurs in a strip just under 200 meters wide in the western and central parts of the Thiviers-Payzac unit. It shows affinities to the neighboring Puy-de-Cornut-Arkose of the Génis unit. A relationship to quartzites in the Rouergue, in the Monts-de-Lacaune , in the Montagne Noire , and to the Grès Armoricain of Brittany is being considered. The Puy-des-Âges quartzite should therefore have an Ordovician age ( Tremadocium ).

Engastine Mafit

The Engastine Mafit below is a complex of dark, mafic rocks of igneous origin. It too only reaches the surface in the western and central parts of the unit. In an approximately 500-meter-wide band, which extends north of Juillac to 2 kilometers, it follows immediately south of the Puy-des-Âges quartzite. Its age is classified as Cambrian . The Mafit, which becomes a maximum of 500 meters thick, is in the form of alternating green slate and amphibolite layers, in which several metadolerite or metagabbrol layers are interposed. The very fine-grained, light to dark green slate contains plagioclase ( oligoclase / andesine ) and a little biotite, mainly amphibole ( hornblende ) and epidote ( clinozoisite ), quartz , calcite and opaque minerals can also be added as accessories . The green schist emerged from former sub-alkaline basalts . The metadolerites or metagabbros , on the other hand, are coarse-grained (er) and consist of green hornblende and basic, saussuritized plagioclase.

Donzenac slate

The epizonal Donzenac slates lie beneath the Mafit . Their smear width is up to 3 kilometers. They follow a slightly curved band that extends from their type locality Donzenac to Lanouaille. Shifted to the left by the Dussac fault , this band then continues to the northeast of Thiviers. The Donzenac slates are also included in the Cambrian. The slates have gray, silky tones and consist mainly of phyllosilicates such as muscovite and biotite or muscovite and chlorite , followed by quartz, acidic plagioclase and garnet from the almandine zone. The slates contain occasional, relatively fine-grained, dark, arenitic intermediate banks in the decimeter range; these are likely to be former Grauwacken. The arenites have clasts of quartz, plagioclase and also epidote, which are surrounded by newly formed minerals (phyllosilicates, quartz and fine-grained albite ).

Thiviers sandstone

At Saint-Mesmin , the Auvézère cuts through rocks from the Thiviers sandstone

The Thiviers sandstone forms the deepest surface formation of the Thiviers-Payzac unit and takes up about two thirds of its surface area. It is a volcanic-detritic sequence of the Late Neoproterozoic to Cambrian Age, which can be divided into four facies:

The Thiviers sandstone is also interspersed with countless dolerite tunnels in the meter range.

The term “sandstone” is somewhat misleading, since the facies of the rhyodacite tuffs are clearly primarily decisive for the formation and all other facies are transformation products of the same. The sodium- emphasized, former rhyodacite tuffs are now available as dark, massive or thick banked (meter range) rocks. In a fine-grained matrix of chlorite, light mica, quartz and albite, quartz, plagioclase (albite or oligoclase) and epidote stand out as millimeter-sized clasts. The following phenomena underline the explosive character of the volcanic parent rock: broken, angular, pointed-pointed quartz, angular plagioclase and, above all, incorporated rock fragments rich in albite, leucocratic lavas . The Grauwacken have a very similar mineralogical structure, but they are more quartz-accentuated and their matrix is ​​richer in phyllosilicates. They are likely to have emerged from the Rhyodacites. The chemical composition of quartzites is similar, it is also identical to that of rhyodacites.

The deeper subsoil of the Thiviers sandstone, consisting of plagioclase-rich paragneiss and mica schists , is not exposed in the Thiviers-Payzac unit.

Igneous intrusive rocks

As mentioned above, the Thiviers-Payzac unit is penetrated by three very different intrusive bodies:

geochemistry

The following analyzes are intended to illustrate the chemical composition of rocks from the Thiviers-Payzac unit:

Oxide
wt.%		 Thiviers sandstone 1 Thiviers sandstone 2 Rhyodacitic
metatuffe		 Donzenac slate 1 Donzenac slate 2 slate Dolerite Engastine amphibolite Engastine Greenschist Corgnac granite Saut-du-Saumon orthogneiss
SiO 2 69.00 66.30 66.60 62.00 58.60 57.50 54.10 51.20 49.50 68.00 71.60
TiO 2 0.69 0.73 0.68 0.86 0.96 0.90 1.55 1.71 2.10 0.54 0.44
Al 2 O 3 13.50 14.00 14.55 18.40 18.40 18.10 14.50 13.40 16.04 14.45 13.50
Fe 2 O 3 0.45 1.85 0.90 3.65 3.60 3.70 1.80 1.85 10.60 dead 4.21 dead 0.27
FeO 4.75 3.60 3.80 1.80 3.65 3.90 8.55 5.80 3.10
MnO 0.10 0.09 0.12 0.05 0.10 0.10 0.20 0.14 0.17 0.06 0.05
MgO 2.70 2.50 2.25 2.90 3.70 3.65 6.30 4.30 11.04 1.17 0.90
CaO 1.75 3.30 3.10 0.25 0.95 1.10 7.15 9.10 1.03 1.51 1.65
Na 2 O 3.25 3.95 4.60 2.00 3.10 2.00 2.90 4.10 2.41 3.77 3.50
K 2 O 2.40 1.45 1.60 3.70 2.30 3.70 1.45 0.63 0.99 3.01 3.65
P 2 O 5 0.16 0.16 0.14 0.17 0.15 0.17 0.18 0.27 0.20
H 2 O - 0.10 0.05 0.10 0.25 0.25 0.30
H 2 O + 1.65 2.10 1.60 4.80 4.90 4.70 1.55 3.45 1.68 0.40

The Thiviers-Payzac unit can be subdivided into three chemical groups according to its SiO 2 content - the mafic Engastine rocks, the intermediate Donzenac shales and the acidic rhyodacites plus intrusiva. The dolerites in the Thiviers sandstone also tend to have a mafic tendency, manifested in their very high TiO 2 , total iron, MnO, MgO and CaO content. Conversely, these levels are quite low for intrusives. The Donzenac slates are extremely aluminum-rich rocks. The sum of the alkalis Na 2 O and K 2 O fluctuates within the unit between 3.4 and 7.15 percent by weight and shows a sub-alkaline main sequence character. The sometimes quite high Na 2 O concentration indicates a spilitization . The K 2 O concentration finally refers to the overall calcareous character of the unit (middle and high K rocks).

Structural structure

The entire Thiviers-Payzac unit is intensely folded . Similar to the Génis unit, this is a fairly close, upright fold structure with a usually slightly shorter wavelength of 100 to 125 meters (but can grow to 200 meters in the south). The fold axes scattering slightly around the horizontal line strike OSO-WNW (N 110, west of the Loyre). The strata surfaces (S 0 ) mostly show a very steep dip (by 80 °) to the north or south. A distinct foliation , recognizable by newly formed minerals, has formed parallel to the fold axis planes (S 1 ). The close-fitting fold structure is dominated by a second open fold, which has deformed the entire unit into a long-wave sequence (wavelength about 2 kilometers) of synclines and anticlines . The first syncline is located directly at the Südlimousin fault, followed by an anticlinory below Saint-Mesmins, a central syncline marked by the course of the Puy-des-Âges quartzite and an anticline near Saint-Cyr-les-Champagnes .

There have Kleinfältelungslineare ( engl. Crenulation lineation ) is formed, which extend more or less parallel to the Faltenbau. Elongated, newly formed, metamorphic minerals have also preferred to line up in this direction.

When the Loyre is reached, however, all structural elements generally swivel in the NW-SE direction (N135). The unit then follows this trend until it expires east of Brive.

metamorphosis

The Thiviers-Payzac unit was regionally metamorphosed under epi- to mesozonal conditions ; in its upper areas it is present as upper green schist facies , in lower areas it is already present as lower amphibolite facies . The presence of chlorite and chloritized biotite in shear bands and pressure shadows also suggests retrograde processes, which have been known for a long time in the southern limousine. In contrast to the underlying gneiss cover, it was not affected by the original high-pressure metamorphosis D 0 in the period 420 to 400 million years.

The regional metamorphosis of the Barrow type can be associated with the deformation phase D 2 in the Massif Central. The latter is generally characterized by ceiling movements pointing to the northwest. The physical pressure-temperature conditions progressed progressively and in the Thiviers-Payzac unit reached their maximum 0.4 to 0.6 GPa and 400 to 500 degrees Celsius. Bellot and Roig (2007) found slightly higher values ​​of 0.5 to 0.8 GPa and 460 to 670 degrees Celsius. The comparable Leyme unit further south, however, with 0.66 to 0.9 GPa ± 0.12 GPa and 615 to 655 ± 35 degrees Celsius, was lowered significantly lower and heated up more.

Structural development

The Thiviers-Payzac unit, like the Génis group, was subject to a ductile, steep shear . In contrast to the Génis unit, however, it does not have a uniform sense of shear . Its southern part up to the anticlinorium near Saint-Mesmin still shows the same right-hand shear sense that is also decisive in the Génis unit. A mixed zone then follows towards the northeast, in which both directions occur. When the northern outcrop area of ​​the Thiviers sandstone is reached, only the left-hand shear sense prevails, which is particularly evident in the Estivaux fault that delimits the unit. The shear coefficient γ reaches a value of 5.4 along this fault; This results in an overall left-hand offset of around 30 kilometers for the zone. The Saut-du-Saumon orthogneiss still has both shear criteria, while the Estivaux granite is only deformed sinistrally.

The following microtectonic processes and structures underline the sinistral shear sense in the northern outcrop area of ​​the Thiviers sandstone:

As well as in the Donzenac slate (to be observed at Allassac ):

  • sinistral shear ligaments.
  • Sinistral quartz pressure shadows on biotite porphyroblasts.

The upper gneiss blanket also shows left-hand shear north of Saint-Cyr-les-Champagnes (quartz lenses sheared on the left).

In the Donzenac slate - in the mixed area of ​​the two shear senses - it can be observed that the right-hand shear overprints the sinistral shear sense. It therefore took place later. Sigmoidal biotite porphyroblasts that were originally sheared on the left side are overlaid by right-side shear bands in which retrograde chlorite has formed.

The continuous shear movements are responsible for the fold structures of the Thiviers-Payzac unit, which can thus be interpreted as tensile folds rotating in the maximum stretching direction in a transpressive , ductile shear zone.

However, the tectonic strains in the Thiviers-Payzac unit did not stop with the end of the ductile deformations . In the brittle area , for example, the unit was offset on the left by numerous smaller, mostly NE-SW-oriented lateral shifts, with offset amounts of 500 meters. An exception is the Dussac fault north of Lanouaille with a left-hand displacement amount of almost 6 kilometers.

Timeframe

The temporal classification of the tectonic shear movements is based on comparisons with lithologically and structurally similar terrains in the Armorican Massif (Chantonnay Synclinorium in the Vendée) and in the Rouergue. In the southern Armorican massif, the right-hand shear movements occurred in the Namur and Westphal ( Serpukhovian to Moskowian , 325 to 305 million years ago). Analogous to this, a mid to late Carboniferous age of the shear deformation can be assumed for the Thiviers-Payzac unit of the Bas-Limousins ​​(which is viewed as the southern extension of the Vendée). This is further supported by the comparable age of the synkinematic leukogranites in northern and central Limousin.

In contrast to this, however, the age information obtained from the Tournaisium using the argon method stands for the intrusion age of the Estivaux granite and for the mylonitic movements on the Saut-du-Saumon-Orthogneiss. They imply an older tectonic process in the early Lower Carboniferous for the southern limousine (deformation stage D 2 or Breton phase ). Correspondingly, a period of 360 to 350 million years is estimated for the regional metamorphosis. Dating of Monazite by J. Melleton and colleagues confirm an age of 360 million years for this synkinematic MP / MT metamorphosis.

See also

credentials

  1. ↑ A. Bitri, C. Truffert, J.-P. Bellot, V. Bouchot, P. Ledru, J.-P. Milési and J.-Y. Roig: Imagerie des paléochamps hydrothermaux As-Sb d'échelle crustale et des pièges associés dans la chaîne varisque: sismique réflexion verticale (GéoFrance 3D: Massif central français) . In: CR Acad. Sci. tape 329 . Paris 1999, p. 771-777 .
  2. ↑ J.-P. Floc'h, JM Joubert, J. Constans and G. Maurin: Notice explicative de la feuille au 1/50000 de Bellac . BRGM, Orléans 1993, pp. 78 .
  3. ↑ J.-P. Bellot: La structure de la croûte varisque du Sud-Limousin (Massif central français) et ses relations avec les minéralisations aurifères tardi-orogéniques: apports des données géologiques, gitologiques, géophysiques et de la modélisation 3D. Thèse de 3ème cycle. Université Montpellier II, 2001, p. 320 .
  4. ↑ Ledru et al .: Ou sont passées les nappes dans le Massif Central français? In: Bull. Soc. Geol. Fr. Band V , 1989, p. 605-618 .
  5. ↑ J.-P. Bellot and J.-Y. Roig: Episodic exhumation of HP rocks inferred from structural data and PT paths from the southwestern part of Massif Central (Variscan belt, France) . In: J. Struct. Geol. Band 29 , 2007, p. 1538-1557 .
  6. ↑ Manuel Duguet, Nicole Le Breton and Michel Faure: P – T paths reconstruction of a collisional event: The example of the Thiviers-Payzac Unit in the Variscan French Massif Central . In: Lithos . tape 98 , 2007, p. 210-232 .
  7. ↑ Roig, J.-Y., Faure, M. and Ledru, P .: Polyphase wrench tectonics in the southern French Massif Central: kinematic inferences from pre- and syntectonic granitoids . In: Geologische Rundschau . tape 85 , 1996, pp. 138-153 .
  8. ^ Berthé, D. and Brun, JP: Evolution of folds in the South Armoricain Shear Zone . In: J. Struct. Geol. Band 2 , 1980, p. 127-133 .
  9. ↑ Duthou, JL et al .: Paleozoic granitoids from the French Massif Central: age and origin studied by 87 Rb / 87 Sr system . In: Phys Earth Planet Interiors . tape 35 , 1984, pp. 131-144 .
  10. ↑ J. Melleton, M. Faure and A. Cocherie: Monazites U-Th / Pb chemical dating of the Early Carboniferous syn-kinematic MP / MT metamorphism in the Variscan French Massif Central . In: Bull. Soc. géol. Fr. t. 180, no 3, 2009, p. 283-292 .

swell

  • BRGM: Feuille Thiviers . In: Carte géologique de la France à 1/50000 .
  • Peterlongo, JM: Massif Central . In: Guides Géologiques Régionaux . Masson, 1978, ISBN 2-225-49753-2 .
  • Roig, J.-Y., Faure, M. and Ledru, P .: Polyphase wrench tectonics in the southern French Massif Central: kinematic inferences from pre- and syntectonic granitoids . In: Geologische Rundschau . tape 85 , 1996, pp. 138-153 .
  • Santallier, D .: Les roches basiques de la série métamorphique du Bas Limousin (Massif Central, France). - Thèse d'état, Orléans . 1981, p. 340 .