Donzenac slate

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The Donzenac slate is a Cambrian formation of the French Massif Central . The formation forms part of the Thiviers-Payzac unit .

Etymology and designation

The Donzenac slate, French Schistes de Donzenac , is named after its type locality - the French commune Donzenac in the west of the Corrèze department . The same formation is called Semblat Shale north of Juillac and Excideuil Shale in the western section .

Geography and geology

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

The Donzenac shale is exposed in the Thiviers-Payzac unit, but is also located in the deeper subsurface of the Génis unit further south . In the basement of the Horst von Châtres northwest of Terrasson-Lavilledieu it reappears on the surface.

In the Thiviers-Payzac unit, the slate follows a slightly curved, approximately 40-kilometer-long band that extends from the Donzenac type locality to Lanouaille . The strike width of the formation is up to 3 kilometers. Shifted to the left by the Dussac fault , this band then continues to northeast of Thiviers .

Stratigraphically, the Donzenac schist usually overlays the Thiviers sandstone and is in turn covered by the Engastine Mafit . Its thickness reaches a good 400 meters. Since the Engastine-Mafit wedges north of the Puy-des-Âges syncline , the stratigraphically higher Puy-des-Âges quartzite follows the slate in the hollow axis . In the Génis syncline , in whose south leg the Donzenac slate is in turn wedged, the Génis porphyroids from the Génis unit already appear in its hanging wall .

Petrology

The Donzenac slate emerged from an alternating layer of former Siltite and Grauwacken . It has silky gray tones and consists mainly of phyllosilicates such as muscovite ( phengite ) and biotite or muscovite and chlorite , followed by quartz , acidic plagioclase and garnet from the almandine zone.

In addition to siltites, the slate contains occasional, relatively fine-grain, dark green, arenitic intermediate banks in the decimeter to meter range - these are likely to be former Grauwacken. These metagrauwacke have clasts of quartz, plagioclase and also epidote , which are surrounded by newly formed minerals (phyllosilicates, quartz and fine-grained albite ).

Siltite and Grauwacken usually replace each other, but the finer fraction of Siltite predominates in the hanging wall of the formation.

The Donzenac slate can also have very significant graphite layers .

Chemical composition

Oxide
wt.%		 Slate 1
at Juillac		 Slate 2 Slate 3
at Voutezac
SiO 2 58.60 62.00 69.00
TiO 2 0.96 0.86 0.89
Al 2 O 3 18.40 18.40 14.70
Fe 2 O 3 3.60 3.65 0.70
FeO 3.65 1.80 3.00
MnO 0.10 0.05 0.03
MgO 3.70 2.90 2.10
CaO 0.95 0.25 0.40
Na 2 O 3.10 2.00 2.00
K 2 O 2.30 3.70 3.85
P 2 O 5 0.15 0.17 0.17
H 2 O - 0.05 0.11
H 2 O + 4.90 4.80 2.80

The SiO 2 content of Donzenac shale is quite variable and fluctuates between 58.6 and 69 percent by weight, corresponding to andesites and dazites . The alkalis reveal a sub-alkaline character. The shale is also a calcareous high-K rock. In a way, its chemical composition shows a strong resemblance to the underlying Thiviers sandstone.

Magmatism

The Donzenac slate is touched by the Saut-du-Saumon orthogneiss along its northern boundary . In the lower Ordovician, this was intruded into the sequence of layers of the Thiviers-Payzac unit as a granitoid and then sheared ductile in the course of the carbonic deformations.

metamorphosis

The Donzenac slate suffered an epizonal regional metamorphosis . The biotite and almandine isograde were exceeded. The degree of metamorphosis increases towards the north-northeast, but mesozonal conditions have not been reached, as staurolite does not yet appear.

tectonics

The Donzenac slate is embedded in large folds . This very wide fold structure is in turn folded tightly and steeply, with the wavelength of the narrow fold structure being only 150 meters. Parallel to the OSO or SO-trending folds axis planes was a regional foliation . Continuous shear movements are responsible for the fold structures in the slate, which can thus be interpreted as tensile folds rotating in the maximum stretching direction in a transpressive , ductile shear zone .

The shear movements were originally shifting left (sinistral), but were later superimposed shifting right (dextral). Sigmoidal porphyroclasts from biotite with the associated pressure shadows made of quartz clearly show the sinistral sense of shear. The overlying dextral shear ligaments are marked by chlorite and reveal elongation and retromorphic conditions .

After the ductile deformation had ended, the slate was penetrated by transverse fractures in the low-temperature, brittle area. These strike mainly north-east, rarely north-north-east or north and show predominantly left-sided offset.

Age

Like the underlying Thiviers sandstone, the Donzenac slate is also included in the Cambrian.

See also

literature

  • Pierre-Louis Guillot and others: Feuille Juillac . In: Carte géologique de la France at 1/50 000 . BRGM, 1978.
  • Pierre-Louis Guillot and others: Feuille Thiviers XIX-33 . In: Carte géologique de la France at 1/50 000 . BRGM.
  • Pierre-Louis Guillot: La série métamorphique du Bas Limousin de la vallée de l'Isle à la vallée de la Vézère, le socle en bordure du bassin aquitain. (Doctoral thesis) . Univ. d'Orléans, France, 1981, pp. 391 .
  • JM Peterlongo: Massif Central . In: Guides Géologiques Régionaux . Masson, 1978, ISBN 2-225-49753-2 .

Individual evidence

  1. ↑ 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 .
  2. ↑ Pierre-Louis Guillot and others: Feuille Juillac . In: Carte géologique de la France at 1/50 000 . BRGM, 1978.
  3. ^ Berthé, D. and Brun, JP: Evolution of folds in the South Armorican Shear Zone . In: J. Struct. Geol. Band 2 , 1980, p. 127-133 .