Corgnac granite

The Corgnac granite is a granite intrusion that was formed during the Ordovician in the basement of the north-western French Massif Central . In the course of the Variscan orogeny it was deformed and metamorphosed together with its rhyodacite host rocks in the Lower Carboniferous .

introduction

Geological overview map showing the location of the Thiviers-Payzac unit (in green); the Corgnac granite is on the top left (in pink).

The Corgnac granite forms an approximately 5 km long massif that penetrated north of Corgnac roughly parallel to the east-south-east trending foliage of the Thiviers-Payzac unit . Its spread across the strike direction is a maximum of 2 kilometers. The occurrence is not uniform, but is split into three narrow lobes by the Cambrian rhyodacites of the Thiviers sandstone . The southeast end of the intrusion is cut off by the Dussac fault - a significant left lateral shift. In several places the granite is transgressed and covered by Liass sediments of the Aquitaine basin .

That it was originally an intrusive granitoid can be seen from the relict-like, black, biotite-rich horn rocks . Similar to the Saut-du-Saumon orthogneiss - to which the Corgnac granite has great affinities - it was penetrated at the beginning of the middle Ordovician 470 million years ago in the Dapingium . In the course of the Variscan regional metamorphosis , it was then retrograde overprinted in the Tournaisium 350 million years ago (epizonal chlorite zone ). Its chemical composition is that of a calcareous to sub-alkaline monzogranite .

Petrology

The Corgnac granite has two very different facies :

cataclastic granite facies in the south.
porphyroclastic orthogneiss facies in the north.

The granite also secreted several smaller, porphyry micro-granite bodies. It is also believed that the main body is deeply associated with the Saut-du-Saumon orthogneiss.

mineralogy

The even-grained, sometimes slightly porphyry-like, gray to pink-colored, cataclastic granite facies consists of the following minerals:

Quartz - xenomorphic gusset filler
Plagioclase - automorphic, zoned, basic oligoclase
Microcline - subautomorphic and often perthitic
Biotite - large, frayed lamellae

Muscovite , zircon , apatite and opaque minerals are added as accessories . A granular, hypidiomorphic structure can be seen under the microscope.

The insignificant orthogneiss facies are derived from the granite facies, it was only significantly more ductile deformed. It therefore now forms a banded eye gneiss with almond-shaped, monocrystalline microcline and plagioclase eyes, surrounded by minerals of the foliation - fine-grained quartz, albite , granular clinozoisite and mica lamellae , the biotite of which is often chloritized . The more or less lepidoblastic muscovite gives the orthogneiss its typical banded appearance. The shear sense is unfortunately not clear, but it is likely to be on the right-hand side, as the granite belongs to the southern section of the Thiviers-Payzac unit, which is shifting to the right.

Chemical composition

Oxide wt.%	Corgnac granite	Saint-Mathieu leuco granite	Piégut-Pluviers-Granodiorite
SiO ₂	68.00	70.00	70.52
TiO ₂	0.54	0.43	0.46
Al ₂ O ₃	14.45	15.42	14.68
Fe ₂ O ₃	4.21 dead	0.72	2.30 dead
FeO		1.72
MnO	0.06	0.08	0.06
MgO	1.17	0.79	1.18
CaO	1.51	1.51	1.96
Na ₂ O	3.77	3.55	3.54
K ₂ O	3.01	4.20	3.86
P ₂ O ₅		0.12
H ₂ O ^-		0.15
H ₂ O ⁺			0.97

Compared to the neighboring Upper Carboniferous granitoids, the Corgnac granite is richer in total iron and TiO ₂ . Its alkalis are slightly less concentrated and emphasize sodium . The contents of SiO ₂ and Al ₂ O ₃ are somewhat lower overall.

Deformation and metamorphosis

The original mineral inventory of the Corgnac granite, including its micro -granites, was later changed to a green slate facially retromorphic. Quartz and biotite were z. B. broken cataclastically , the plagioclase of muscovite and clinozoisite infiltrated and rutile needles detached from the crushed biotite. The quartz also recrystallized as even-grained puzzle pieces, the feldspars were girdled by granulations and the plagioclase destabilized to form albite.

Age

The ages given in the introduction do not come from the previously undated Corgnac granite, but from the Saut-du-Saumon orthogneiss, which is considered to be of the same age. For this orthogneiss, Bernard-Griffith and colleagues (1977) found rubidium / strontium ages of 477 ± 22 million years as the time of intrusion and 361 ± 9 million years as the time of regional deformation. In the neighboring Estivaux granite, the argon method was used to measure regional deformations of 364.2 ± 3.8 and 346.2 ± 3.5 million years.

swell

P. - L. Guillot and others: Feuille Thiviers XIX-33 . In: Carte géologique de la France à 1/50000 . BRGM.
Peterlongo, JM: Massif Central . In: Guides Géologiques Régionaux . Masson, 1978, ISBN 2-225-49753-2 .
Roig, J.-Y., Faure, M. & 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 .

Individual evidence

^ J. Bernhard-Griffith, JM Cantagrel and JL Duthou: Radiometric evidence for an Acadian tectonometamorphic event in western Massif Central français . In: Contrib. Miner. Pet. Band 61 , 1977, pp. 199-212 .
↑ Roig, J.-Y., Faure, M. & 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 .

[1] J. Bernhard-Griffith, JM Cantagrel and JL Duthou: Radiometric evidence for an Acadian tectonometamorphic event in western Massif Central français . In: Contrib. Miner. Pet. Band 61 , 1977, pp. 199-212 .

[2] Roig, J.-Y., Faure, M. & 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 .