Geology of the Limousin
The geology of the Limousin is an integral part of the geology of the French Massif Central . It is limited mainly to the west of the Sillon Houiller upcoming Variscan plot Mountain Terran .
geography
The historical region of the Limousins consists of the three departments Corrèze , Creuse and Haute-Vienne . Geologically , it borders in the north on the sediment backfill of the Paris Basin , in the northwest and west on the sediments of the Seuil du Poitou and in the southwest on the sediments of the Aquitaine and Briver basins . To the east, the terran is cut off from the rest of the Massif Central by the significant north- northeast trending , crustal fault of the Sillon houiller.
Lithologies encountered
The geology of the Limousin is clearly dominated by granitoids . The second most important rock group are gneisses , followed by mica schists , rare serpentinites , amphibolites and very rare eclogites . Sedimentary rocks are only found sporadically in the interior of the Limousin - they can be found, for example, in the coal basins of the Stefanium near Aubusson , Bourganeuf and on the south-eastern edge near Bort-les-Orgues . A tertiary burglary basin exists near Gouzon in the Creuse department.
The ejecta breccia of the Rochechouart-Chassenon crater , which were created 214 million years ago by the fall of a meteorite , are a unique rarity in France .
Structural structure
After the completion of the subduction in the Lower Devonian and the onset of a continental collision at the beginning of the Middle Devonian, a pile of roofs formed in the Limousin around 380 million years ago . This is built up from the hanging wall to the lying wall as follows:
- Génis unit and Mazerolles unit
- Thiviers-Payzac unit , Leyme unit and Gartempe unit
- Upper gneiss cover
- Lower gneiss cover
- Parautochthonous mica slate unit
The individual ceilings show different degrees of metamorphosis . The highest degrees of metamorphosis (upper amphibolite facies ) were reached in the upper gneiss cover. The Génis Unit and the Mazerolles Unit, however, only experienced the low-grade conditions of the Greenschist facies . It is assumed that the upper gneiss cover once covered the entire Limousin as a closed cover, but is now only present in larger, separated clods. The two gneiss blankets had pushed themselves flat over the parautochthonous mica slate unit. The higher units are partly steep and can be found on the edge of the gneiss cover. The parautochthonous mica slate unit forms dome-like bulges in the basement, which are intruded by granitoids. Examples are the Millevaches massif , the Saint Mathieu cathedral and the Sussac cathedral . The two gneiss blankets come to rest in the recesses in between.
None of the listed units remained in the original, autochthonous association . Even the basal parautochthonous mica slate unit is very likely foreign.
Granitoids
The granitoids are undoubtedly the most characteristic rocks of the Limousin. They emerged in several phases over a period of around 370 to 290 million years. Its oldest representatives are the calcareous intrusives of the Limousin tonalite line ( LTL for short ), which were located mainly in the upper gneiss cover between 370 and 350 million years ago. These are quartz diorites , tonalites and diorites . It is possible that these granitoids formed the root area of an Andesite island arc .
The intrusives of the tonalite line, based on crust shortening, were then replaced by the generation of peraluminous granitoids of the Guéret type from the turn of the Upper Devonian / Lower Carboniferous, when the orogen was gradually relieved of pressure. The best example of this is the 356 million year old Guéret granite from which it is named - a flat, layered intrusive body with a large surface area. Also worth mentioning is the 352 million year old Vaulry-Cieux granite . The Guéret granitoids survived until the end of the Viseum , their last representative being the calcareous Piégut-Pluviers granodiorite , which emerged 325 million years ago.
From the Namurian onwards , the orogen was recorded by strong elongation and as a consequence, a generation of leukogranites was created . Examples are the 324 million year old Saint-Sylvestre leuco granite , the 319 million year old blond granite and the 315 million year old Saint-Mathieu leuco granite , which in their spatial configuration were mainly linked to distensive lateral shifts and normal faults . The leuco granites were also accompanied by Shoshonite and very potash-rich calcareous-alkali stones, such as the 315 million year old Esse-Hiesse granite or the 305 million year old Chirac-Étagnac granite .
The granitoids can be developed very differently petrologically. This is made clear by color differences, such as the shimmering blue Aureil granite south of Limoges or the pink Pérols-sur-Vézère granite in the Corrèze. The grain sizes can also vary significantly; very fine-grained representatives can be found at Péret-Bel-Air in the Corrèze and coarse-grained pegmatites at Treignac (also Corrèze).
Gneiss
Gneisses are the second most common type of rock in the Limousin, mainly paragneiss , orthogneiss and eye gneiss . They appear in the two gneiss covers.
The structurally lower lower gneiss cover is made up of paragneiss, into whose original sediments calcareous granitic magmas were injected in places during the Ordovician . The latter are now available as ortho or eye gneisses. The parent rocks of the paragneiss were mainly Grauwacken , which had been deposited on the northern continental margin of Gondwana . The parent rocks, together with the intruded granites, were then subjected to the Variscan regional metamorphosis over the course of the Middle Devonian (in the period from 390 to 375 million years), and the intrusives contained therein were orthogneissified. The Thaurion orthogneiss near Limoges, for example, is dated 375 million years ago and reflects the time of the regional metamorphosis. In contrast, the orthogneiss from Meuzac and from Saint-Yrieix-la-Perche were found to be 468 million years old - which corresponds to the original intrusion time in the Middle North Dovician. Eclogites and garnet peridotites with relics of high pressure metamorphosis can occur in the hanging wall of the lower gneiss ceiling - examples are Sauviat in Haute-Vienne and La Faurie near Uzerche in Corrèze.
The upper gneiss cover appears in the north on the Plateau d'Aigurande , northwest of Saint-Junien , south of Limoges and in the Bas Limousin northwest of Tulle . In the Limousin it is generally assigned an age of 440 to 400 million years. It does not contain an orthogneiss, but instead leptynites (former rhyolite volcanic rocks). As already mentioned at the beginning, the granitoids of the Limousin tonalite line occur in it, which cross the entire western section of the Limousin in a south-easterly strike direction.
Ophiolites of the Limousin
Squeezed between the two gneiss blankets are the remains of former ultramafites (peridotites) and gabbros , which are now available as serpentinites and are interpreted as former ophiolites of the so-called Massif Central Ocean . Finds are located south of Limoges in the heathland around La Roche-l'Abeille in the Lande de la Flotte et du Cluzeau and in the Lande de Saint-Laurent (both Haute-Vienne).
Low metamorphic units
The epizonal, relatively low metamorphic units of the Thiviers-Payzac, Gartempe, Génis and Mazerolles units form the hanging wall of the ceiling pile and consist predominantly of schisty rocks . Their parent rocks include volcanic rocks and their derivatives ( rhyodacites and ignimbrites ).
Parautochthonous mica slate unit
The parautochthonous mica slate unit is the deepest unit of the nappes in the Limousin, but it has not been metamorphosed the most. In addition to the dominant mica schists , it contains quartzites , leptynites and, very rarely, orthogneiss (for example in the valley of the Dronne and in the Thaurion arch ). It usually frames basement bulges and their leucogranite massifs (Aigurande, Brâme , Millevaches, Saint-Goussaud , Saint-Mathieu and Saint-Sylvestre). The age of the unit is not certain, but it is usually considered older than 540 million years and placed in the Neoproterozoic (so-called Brioverian ).
Tertiary volcanic rocks
In the east of the Corrèze and in the immediate vicinity of the Sillon houiller, the Limousin still touches the foothills of the Tertiary volcanism of the Auvergne . Examples can be found at Bort-les-Orgues with phonolite columns , at Liginiac at the Puy de Manzagol and at Sérandon at the Puy d'Enval and at the Puy du Verdier . Volcanic deposits are also found near Saint-Privat in the north and east of the Xaintrie Blanche plateau (sites near Auriac , Bassignac-le-Haut , Darazac and Rilhac-Xaintrie ).
Sediments
In addition to the aforementioned coal basins of Aubusson, Bourganeuf and Bort-les-Orgues, in addition to the Ahun-Lavaveix coal basin in the Creuse, two other smaller deposits appear on the south-south-east-trending Argentat Fault ( Argentat Basin and L'Hôpital Basin to the northeast from Tulle). In the Leyme unit at Figeac there is also another brazier - the Saint-Perdoux-Lacapelle-Marival brazier .
More important limestone clusters are found near Chasteaux , Lissac-sur-Couze and Saint-Robert in the Corrèze. But they already belong to the Briver basin.
Natural resources
gold
The Limousin, like the rest of the Massif Central, is quite rich in natural resources . Their mining goes back to antiquity . As early as the 5th century BC under the Gauls Gold mined at numerous sites . The mining concentrated mainly on the area of Saint-Yrieix-la-Perche and on the Monts d'Ambazac . With the conquest of Gaul by the Romans , gold mining came to a standstill. It did not experience a new beginning until the 19th century, but it already declined significantly after the end of the Second World War. The last existing gold mine, Bourneix , was closed in 2002.
The gold is bound to fault zones in the basement and is found finely distributed in mostly cataclastic quartz .
Other metals
In addition to gold, the Gauls dug for tin in the Monts de Blond and silver was extracted from galena . During this time, iron was first mined at Ussel and Bort-les-Orgues, which was then continued under the Romans in an industrial style. The Romans also recognized the benefits of mercury (near Peyrat-le-Château ), silver-containing lead, as well as barite ( Mercœur and Les Farges near Ussel) and copper (near Ayen ).
From the 18th century lead was mined near Saint-Germain-les-Belles and a little later lead- zinc in Nontronnais . In the 20th century came antimony (as antimonite near Glandon in Haute-Vienne, Mérinchal in Creuse and Chanac-les-Mines in Corrèze), tungsten (found in Puy-les-Vignes near Saint-Léonard-de-Noblat ) and temporarily also beryllium (as beryl at Lussac-les-Églises ) and fluorine .
uranium
The Limousin had a total of 40 uranium mines in the 20th century , the most productive of which were in the north of the Haute-Vienne and the Monts d'Ambazac. The degradation began after the Second World War, when France for nuclear power rise and the peaceful development of nuclear energy drove forward. In the nineties, a severe decline in production set in, which finally led to the closure of the last existing mine at Jouac in May 2001 .
The uranium minerals autunit , chalcolite and pitchblende were mined . The most important host rock for uranium mineralization is the Saint-Sylvestre leuco granite. The production rate was considerable - for example, 1000 tons of yellow cake were extracted per year in 1977 and the chemical enrichment to uranium concentrate, which took place in Bessines , turned over 200,000 tons of ore annually.
The environmental impact of uranium mining was considerable and continues to pose a threat to the city of Limoges' drinking water supply. For example, epidemiological studies by the Université de Limoges in the Gartempe valley - on which mining was concentrated - found an increase in the risk of cancer in 2012 .
coal
There are also smaller coal deposits in the Limousin , which were mined from the 17th century. The deposits are spread over the western Briver basin, the Argentat basin (both Corrèze), the Aubusson basins, Bort-les-Orgues, Bourganeuf and, above all, the Ahun-Lavaveix coal basin in the Creuse. The deposits are now exhausted or unprofitable. The last pits were shut down at the end of the sixties. A total of 12 million tons of coal had been mined in the Ahun Lavaveix coal basin.
kaolin
Another important raw material of the Limousin is kaolin , which is of great importance for the local porcelain manufacture . Kaolin was first discovered in the Limousin in 1767 near Marcognac near Saint-Yrieix-la-Perche. The feldspar, which is also urgently needed for the production of porcelain, comes from pegmatites of the Monts de Châlus and the Monts d'Ambazac. Other kaolin mining areas are on the Bénévent-l'Abbaye plateau and in the Massif des Monédières in the Corrèze.
Individual evidence
- ↑ Babuska, V., Plomerova, J., Vecsey, L., Granet, M. and Achauer, U .: Seismic anisotropy of the French massif central and predisposition of Cenozoic rifting and volcanism by Variscan suture hidden in the mantle lithosphere . In: Tectonics . tape 21 (4) , 2002, pp. 1-20 .
- ↑ Kelley, Simon P. and Spray, John G .: A late Triassic age for the Rochechouart impact structure, France . In: Meteoritics . vol. 32, 1997, pp. 629-636 .
- ↑ Michel Cuney, Marc Brouand, Jean-Marc Stussi and Denis Virlogeux: Le complexe plutonique de Charroux-Civray (Vienne): témoin du magmatisme infra-carbonifère dans le segment occidental de la chaîne varisque européenne . In: Géologie de la France . n ° 1-2, 2001, p. 143-166 .
- ↑ Santallier, D. and Floc'h, JP: Les éclogites de La Faurie (Bas-Limousin, feuille d'Uzerche à 1/50000) . In: Bull. BRGM, Sect. 1: Géologie de la France . n ° 2, 1979, p. 109-119 .
- ↑ Roig, JY, Calcagno, P., Bouchot, V., Maluski, H. and Faure, M .: Modelisation 3D du paleochamp hydrothermal As + Au (330-300 Ma) le long de la faille d'Argentat (Massif Central français) . In: Chron. Calc. Min. Volume 528 , 1997, pp. 63-69 .
- ↑ Genna, A., Roig, JY, Debriette, PJ and Bouchot, V .: Le bassin houiller d'Argentat (Massif Central français), consequence topographique d'un plissement de son substratum varisque . In: CR Acad. Sci. Paris . tape 327 , 1998, pp. 279-284 .
- ↑ Vetter, P .: Géologie et paleontologie des bassins houillers de Decazeville, de Figeac et du détroit de Rodez . In: Houillères du Bassin d'Aquitaine . Albi 1968, p. 1-637 .
- ↑ Christian Belingard: Cancers et mines d'uranium: enquête en Limousin . In: france3-regions.francetvinfo.fr . 2012.