Piégut-Pluviers Granodiorite

The Piégut -Pluviers granodiorite is a Granodioritmassiv on the northwestern edge of the Variscan Massif Central in south-western France. Its cooling age was 325 ± 14 million years BP dated ( Lower Carboniferous, Upper Mississippian accurate, Serpukhovian ).

• 6.1 Aplite
• 6.2 pegmatites
• 6.3 lamprophyre

Geography

The Granodioritmassiv is named after Piégut -Pluviers, a municipality in the north of the arrondissements Nontron in the Dordogne, which is in the solid middle. It has roughly the shape of an inverted commas with a nearly square central part of the south, which is rotated in the southeast-northwest direction. This central part has a triangular, pointed northward extension. The central part measures in northeast-southwest direction maximum of 15.5 km, in southeast-northwest direction 15 kilometers. The extension is about 10 km, but is separated from the actual solid even by a very thin, about 100 meters Gneisseptum powerful ( around 100 meters thick intermediate layer of migmatitischem paragneiss ). The Granodioritmassiv claimed some 250 square kilometers of surface area. Its lowest point is open-minded to 135 meters on the western edge and its culmination at around 375 meters above sea level in the northeast. It therefore represents an easy incident to Southwest desk plaice, which shows only relatively small differences in relief in their interior.

Geological Survey

In the east, northeast and north is the Granodioritmassiv from Saint -Mathieu- leucogranite and its equivalents girded ( unit N ° 17 on the geological map ), which with 315 ± 17 million years BP is slightly younger ( Pennsylvanian, Bashkirian ). In the southeast and northwest of the massif limit various paragneisses ( unit bk ). On its south and south-west side it is largely transgrediert of sediments of the Lias ( unit j1), which belong to the Aquitaine basin. Together with the Saint -Mathieu- leucogranite forms the solid parautochthone a structural bulge in the basement, known as the Cathedral of Saint-Mathieu. Among the paragneisses of the granodiorite shows often no sharp contact conditions, it is rather a diffuse transition region, which may extend over several hundred meters and in which granodiorite and gneiss layers repeated dovetail. This suggests that the paragneiss represent the output and host rocks of the granodiorite.

Structurally can be in this section of the western Massif Central, the following construction recognize (from top to bottom):

• Upper Gneiss ceiling
• Lower Gneiss ceiling
• Parautochthoner mica leucogranite with
• Granodiorite
• Paragneiss

Age

Based on field observations, it can not decide the age distribution position of the two granitoids. Radiometric measurements seem to argue for a higher age of Piégut -Pluviers - granodiorite are present in the values ​​of 325 14 and 314 ± 14 million years. The Saint -Mathieu- leucogranite was dated 315 ± ± 17 million 17 and 304 years. The high standard deviation in these Rb -Sr values ​​but urges caution. Readings with a more reliable U -Pb method are still lacking.

Facies

The Granodioritmassiv is not homogeneous, but has several Petrofazies. Among them:

• Coarse-grained facies
• Coarse-grained porphyritic facies
• Fine-grained facies
• Fine-grained, hornblende leading facies

Coarse-grained facies

The coarse-grained facies occupies most of the Granodioritmassivs to complete. The grain size ranges generally 2-6 mm, the individual grains are approximately equal ( isometric ). When fresh, the rock has a gray color. It contains the following minerals:

• Quartz - 1 to 3 millimeters rounded grains also clusters 1-3 inches - 26 percent by volume
• Orthoclase - perthitisch, Carlsbad twins, corroded quartz, scattered euhedral crystals in the centimeter range - 18 percent by volume
• Plagioclase - often zoned andesine core with AN33 -35, AN25 with oligoclase rim, scattered euhedral crystals in the centimeter range - 42 percent by volume
• Biotite - 1 millimeter in size, bronze, with Zirkoneinschlüssen and often chloritisiert - 10 percent by volume

Accessory minerals are zoned allanite, epidote, apatite, and occasionally green hornblende, zircon and zoisite. As Opakmineral acts pyrite. This facies contains occasionally dark, fine-grained, centimeter - to dezimetergroße, rounded dioritic inclusions. The Feldspatlängsachsen often show a Einregelung.

The coarse-grained quartz- normative Granodioritfazies is ( SiO2 oversaturated ) and also korundnormativ, therefore it has peraluminosen character. It is also subalkalisch. In the typology of the granitoids is this is a kalkalkalisches rock I-type, which belongs to the K- series. Compared to average granodiorite this rock type has a higher SiO2 content and approaches the composition of monzonitic granites.

Coarse-grained facies porphyroids

This Faziestypus mineralogically and chemically differs only slightly from the coarse-grained facies, but it is much larger feldspars from ( grain size usually 1-4, sometimes even up to 6 inches reaching ). The grain size of the groundmass minerals is on average 5 mm, so is also somewhat higher than in the Normalfazies. The transitions to Normalfazies are fluid. Main distribution areas are Lacaujamet at Piégut (former quarry for facade window and door stones) and Puybégout at Augignac.

Fine-grained facies

The fine-grained facies is mainly located on the southern edge of the Granodioritmassivs on Bandiat at Nontron. Again, the transitions to Normalfazies are fluid - it is a gradual reduction of the grain sizes instead. Mineralogy and chemical composition are also very similar, the only difference is a slightly stronger presence of hornblende and a lower content of alkali feldspar. Other occurrences of fine-grained facies are located in a thin band on the north-east and north-west edge of the massif, and in small isolated areas in the interior. The latter may have originated in relative proximity of the roof area.

Fine-grained, hornblende leading facies

This is a fine to medium grained rock of dark color, containing little more alkali feldspar (less than 10 percent by volume ), but is quite rich in green hornblende. It has a substantially lower SiO2 content and approaches in its composition dioritic rocks. This facies was formerly mined in Tabataud quarry in Bandiattal south of Nontron as a building block and as gravel. With her ​​southeast-northwest trending veins are associated, was where mined for lead, silver and zinc. The reduction, however, was set in 1939. The Cantonnier -speed has become known for rare and very rare minerals. In addition to barite, galena, calcite, chalcedony, dolomite, marcasite, pyrite and sphalerite are found, for example, anglesite, cerussite, Dundasit, Embreyit, Hisingerite, Krokoit, Leadhillit, Mimetite, ozokerite, pyromorphite, sleek silver, Vauquelinite and Wulfenite.

The following chemical compositions are averages of 14 analyzes for the coarse-grained facies, 2 analyzes for Porphyrfazies, 3 analyzes for the fine-grained facies and 3 analyzes for the fine-grained hornblende leading facies:

The magnesium Mg # numbers move from 0.55 to 0.59, they are slightly increased compared to an average granodiorite. An exception is the very anomalous fine-grained Marginal Facies with a very low value of 0.47. The Aluminosität scatters quite strong with a tendency to Peraluminosität. Type default can not clearly define it is a limiting case of a I- intrusive type of Piégut -Pluviers granodiorite. Here, too, shows the fine-grained Marginal Facies strong deviations towards a S- type, which was obviously contaminated by the metasedimentary paragneisses.

Micro granite massif

The satellitäre micro granite massif in the north of the Granodioritmassivs is separated only by a very thin Paragneisseptum in the valley of Trieux. Genetically it is likely but are directly related to the main massif. This is a porphyry with microscopically fine groundmass. It can be distinguished two facies, a facies with a light gray base and a Marginal Facies with very dark base. The rock is similar in mineralogical structure and chemical composition very much the fine-grained, hornblende leading facies of the main massif. The phenocrysts can reach up to 12 millimeters and consist of quartz, plagioclase and biotite. The alkali feldspar is found in the matrix and may have as a special training Myrmekit. Besides the usual accessory minerals chlorite, epidote and zircon and titanite occurs. The dark Marginal Facies contains green hornblende.

Porphyritic granites of the micro lighter type also occur in the main massif, where they remain limited to the smaller deposits in the kilometer range in Saint- Barthélemy- de Bussière and south of Marval. They show intrusive character here.

Dike rocks

Aplite

The Granodioritmassiv is penetrated by numerous gray, often pink, mikrogranitischen Aplitgängen. The gears can be up to several kilometers long and up to 1 meter, sometimes even up to 10 meters are powerful and are usually more or less vertical. The phenocrysts are quartz, feldspar and biotite; they usually have only small grain sizes, but can reach 10 millimeters in the feldspars. The alkali feldspar is also located at the Aplitgängen in the matrix. The corridors are largely north-south oriented with a typical crossed transit network consisting of the directions N 020 and N 160

Associated with pink aplites is a coarse-grained Rotfazies that occurs in two larger bodies at Ballerand and Fargeas (municipality Abjat -sur- Bandiat ). It leads Mikropegmatite and pegmatitic geodes. It also includes occasional inclusions of quartz diorite and Monzogabbro.

The red coloration of the Aplitgänge Rotfazies and is due to the intensive Hämatitisierung plagioclase. The rocks were therefore exposed to a Fe - metasomatism (see adjacent thin section ). The biotite are strongly chloritisiert, this suggests a transformation under retrograde conditions.

Pegmatites

Also pegmatites occur occasionally, usually in the form of corridors, rare as drusen with beautiful smoky quartz in cavities formed.

Lamprophyre

Lamprophyrgänge are not uncommon in the metamorphic rocks of the neighboring Granodioritmassivs, but in the granodiorite itself, they are extremely rare. The lamprophyres are freshly battered condition dark gray -green, very dense, fine-grained rocks, the beige weather. Among all facies types have the lowest SiO2 content and approach strongly the dioritic pole ( quartz Leading Mikrodiorite, relationship to tonalite ).

The Aplitfazies is clearly peraluminos and the S- type, it has strong similarities to the fine-grained Marginal Facies. Micro- granite and lamprophyre are all very similar and distinct from the I- type; they are hypaluminos to normal Aluminos and obviously represent an independent Magmentyp or Magmenabsonderung dar.

Tectonics

The most important tectonic foliation in fact is a ubiquitous Granodioritmassiv. This is mostly already seen with the naked eye, especially at superficially weathered parts. This means that the solid was further deformed kinematic Variscan orogeny in the course of together with its neighboring rocks. The opposing stereographic imaging allows a clear spatial organization in the granodiorite recognize. Significantly enters via cruising patterns from the northeast-southwest and the southeast - northwest direction in appearance - typical for this section in the Massif Central. The pattern can be interpreted as follows:

• A relatively flat, almost symmetric wave structure in South- northwest direction, the angle of incidence does not exceed 30 degrees - with a wavelength in the Deka and Hektometerbereich. Field observations can shear bands from the C'- type detect.
• An asymmetric wave structure in northeast-southwest direction, the East side can be erected very steep. Shear bands from the C- type.

This means that the Granodioritmassiv was embedded in a supra-regional shear zone or Riedel with a general material removal under expansion ( outflow ) to southeast with the same erfolgendem under compression material transport to the southwest.

With the gradual cooling of the Variscan basement and the ductile deformation came to an end. Accumulated stresses were degraded in Granodioritmassiv in the form of brittle faults, fractures and joint sets later. In the late orogenic fracture zones aplites, pegmatites and lamprophyres invaded. Hydrothermal solutions formed towards the end of the Carboniferous veins with lead - zinc-silver, but also of rare arsenic -molybdenum mineralization.

Final considerations

According to its modal fraction, ie, the classification according to the principal minerals in QAPF diagram, the rock of the massif is just addressed as granodiorite, but is in the immediate vicinity of the granite field. Chemically, but it is already granite, specifically a Adamellit; in the TAS diagram, for example, it falls into Rhyolithfeld R.

Clean the surface Granodioritmassiv appears to be quite homogeneous. But on closer inspection, numerous different facies can be distinguished. Chemical analyzes point to the existence of SiO2 significantly poorer rock types (fine -grained hornblende leading Marginal Facies, microgranite extension, lamprophyre and dark inclusions ). The clear delineation of Mikrodiorit and the Lamprophyrfazies of the remaining Gesteinsfazies in the rock parameters can the presence or hybridization of two Magmentypen suspect.

In addition to purely magmatic structures such as streaks and regulation of the feldspars ( viscous flow texture ) but also clearly tectonic structures in Granodioritmassiv can be seen: foliation, shear zones with C / S structure and sbc - structure ( engl. shear band- cleavage ). On the southern edge of the massif can also find mylonitisierte rocks.

The occasional occurrence of Myrmekit and Hämatitisierung of plagioclase are evidence of metasomatic processes. The chloritization of biotite indicates a retrograde, grünschieferfazielle overprinting of Granodioritmassivs. This Retromorphose is found in many parts of the Massif Central.

The massif is therefore arose from a complex interplay of magmatic, metasomatic and ultimately tectonic processes.

The Granodioritmassiv as a repository

In connection with the search for a suitable repository for radioactive waste from French nuclear power plants ANDRA had adopted a number of granite massifs in the eye end of the nineties, including the Piégut -Pluviers granodiorite. This project has been met but then with the local population on large resistance (several large-scale demonstrations, including Piégut -Pluviers ), so that was the location Piégut finally moved away. In addition to further research in granitoids (eg in Jura hidden under solid sediments from Civray - Charroux, Vienne ) in the Seuil du Poitou ANDRA is now clearly focused on the repository in Bure in the Meuse ( clay sediments ).

Swell

• Géologique Carte de la France au millionième (1996). 6ème édition. Éditions BRGM. Service Géologique National. ISBN 2-7159-2128-4
• Geological maps of the BRGM in 1 / 50,000. Leaves Châlus, Nontron, La Rochefoucauld, and Montbron Thiviers.
• Passchier, C. W. and Trouw, R.A.J. (1998) Microtectonics. Springer Verlag. ISBN 3-540-58713-6

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• Geography (France)