Piégut-Pluviers Granodiorite
Encyclopedia
The Piégut-Pluviers Granodiorite is situated at the northwestern edge of the Variscan Massif Central
in France
. Its cooling age has been determined as 325 ± 14 million years BP (Upper Mississippian, Serpukhovian
).
, a small town in the Arrondissement of Nontron
in the northern Dordogne
. Its outline is in the shape of an inverted comma pointing north with a nearly squarish main southern part turned into the NE-SW direction. The main body measures 15.5 kilometers in the NE-SW direction and 15 kilometers in the NW-SE direction and terminates in a tapering triangular section. This section continues farther north in an almost ten kilometer long appendix that is separated from the main body only by a very thin layer of migmatitic
gneiss
. The granodiorite takes up about 250 square kilometers in surface area. Its lowest point has an elevation of 135 meters above sea level along the western edge; the highest point in the northeast rises to about 375 meters above sea level. Geomorphologically the granodiorite forms a tabular slab, that is gently inclined to the southwest without any major topographic differences.
, Bashkirian
). In the northwest and southeast
it is bordered by various paragneisses (unit bk). Along the southwestern and western border the granodiorite is transgressed by lias
sic arkose
s (unit j1) belonging to the sedimentary infill of the Aquitaine Basin
. Together with the leucogranite the granodiorite forms a domal swell in the basement − the Saint-Mathieu Dome. The contact relationships with the paragneisses are not always clear-cut, sometimes there is a diffuse border region of several hundred meters where granodiorite and paragneiss interfinger. This indicates that the paragneiss is the host rock of the granodiorite and casts doubts on a truly intrusive origin.
The western Massif Central
is a nappe-stack of several basement
slivers in the following spatial arrangement (from top to bottom):
This arrangement is only spatial and does not reflect temporal relationships. Major thrust
contacts are situated between the Parautochthonous Micaschist Unit and the Lower Gneiss Unit and in between the two gneiss units.
deviation, so these values should be used with caution.
Accessories are zoned allanite
, apatite
, epidote
, occasionally green hornblende
, zircon
and zoisite
. An opaque mineral is pyrite
.
The coarse-grained facies sometimes contains dark, fine-grained rounded to subrounded inclusions that cover the centimeter to decimeter size range. The long axes of the feldspar grains show a preferred orientation in some places.
This facies is normative in quartz (oversaturated in SiO2) and in corundum
(peraluminous). It is also subalkaline. In the alphabet-soup classification it represents an I-type granitoid of calcalkaline characteristics belonging to the K-series. Compared with average granodiorites, this rock type has a higher SiO2 value and approaches the composition of monzonitic
granite
s.
.
river southeast of Nontron at the southern edge of the Piégut-Pluviers Granodiorite. This facies gradually develops from the common facies by a decrease in grain size. Mineralogically and chemically its composition is also very similar, the only difference being an increase in hornblende and a slight decrease in alkali feldspar. The fine-grained facies also occurs in a narrow strip along the northeastern and the northwestern margin of the granodiorite and in isolated patches in the interior, which might represent a roof facies of the granodiorite.
. In the past this facies has been extensively quarried (Tabataud quarry south of Nontron). Associated with this facies are several NW-SE striking mineralised veins that have been mined for lead
, silver
and zinc
. Mining activity stopped in 1939. The Cantonnier vein is famous for very rare mineralisations. Besides baryte, calcite
, chalcedony
, dolomite
, galena
, marcasite
, pyrite
and sphalerite
occur very rare minerals like anglesite
, cerussite
, crocoite
, dundasite
, embreyite, hisingerite
, leadhillite
, mimetite
, ozokerite
(pseudomineral), pyromorphite
, native silver
, vauquelinite
and wulfenite
.
The following chemical compositions are averages (14 analyses for the common facies, 2 analyses for the porphyric facies, 3 analyses for the fine-grained facies and 3 analyses for the fine-grained hornblende-bearing facies):
The magnesium numbers Mg # spread between 0,55 and 0,59 and are somewhat elevated compared with an average granodiorite. The anomalous fine-grained border facies shows an exceptionally low value of 0,47. The aluminosity (A'/F) values take up quite a large range with a tendency to peraluminous
compositions. According to the alphabet-soup classification, the Piégut-Pluviers Granodiorite is a border case of an I-type granitoid. Again, the fine-grained border facies tends towards an S-type, thus documenting a contamination by the metasedimentary paragneisses.
gneiss layer. Genetically it is probably related to the main granodiorite. The microgranite is porphyritic
with a very fine groundmass. It develops two different facies, a light grey facies in its interior and a dark facies along the border. Mineralogically and chemically the microgranite very much resembles the fine-grained hornblende-bearing facies of the main massif. The phenocrysts can reach 12 millimeter in size and consist of quartz, plagioclase and biotite. Alkali feldspar is found only in the groundmass, sometimes associated with myrmekite
. Besides the common accessories chlorite, epidote and zircon occurs titanite
. The dark facies is rich in green hornblende.
Rocks compositionally very similar (but texturally different) to the light grey facies can also be found in isolated kilometer-sized enclaves within the main granodiorite (near Saint-Barthélemy-de-Bussière
and south of Marval
. They show intrusive contact relationships with the main body.
dikes. These usually upright dikes can occasionally attain one kilometer in length, their maximum thickness varying between one and ten meters. They are mainly oriented North-South and in many places follow a cross pattern formed by the N 020 and the N160 directions. Mineralogically the fine-grained phenocrysts consist of quartz, feldspars and biotite. The plagioclase phenocrysts occasionally reach a grain-size of 10 millimeters. The alkali feldspar is confined to the groundmass.
Associated with the pink aplite dikes is a coarser-grained red facies occurring in two larger outcrops near Ballerand and near Fargeas (commune of Abjat-sur-Bandiat
). This facies contains also micropegmatites and pegmatitic
geodes. In some places it also carries enclaves of quartz diorite
and monzogabbro.
The reddish colours of the aplites and the red facies rocks is due to hematite
invading the plagioclase and staining it. These rocks therefore underwent Fe-metasomatism
(see thin section at left). Most of the biotite has been altered to chlorite
(chloritization) indicating retromorphism under greenschist facies conditions.
s do occur as well, mainly as dikes; in some places as amygdules with beautiful smoky quartz
(rare).
dikes are fairly common in the metamorphic country rocks of the granodiorite, but extremely rare in the massif itself. Unweathered lamprophyres have a dark green to green colour; they are very dense fine-grained rocks that weather in creamy colours. Amongst all the different facies they have the lowest SiO2-content and approach dioritic compositions (quartz-bearing microdiorites with tonalitic
affinities).
The aplites are clearly peraluminous and belong to the S-type and are very similar to the fine-grained border facies. Microgranites and lamprophyres show similarities and belong to the I-type; they are hypaluminous to normal aluminous rocks and most likely have originated from an independent magma pulse or batch.
of tectonic origin, that is clearly evident in more weathered superficial outcrops. This indicates that the granodiorite was continuing to be deformed during the Variscan orogeny
even in the subsolidus stage together with the country rocks. The stereo net
on the right shows the spatial organisation of the granodiorite. One can discern a crossed pattern formed by the NW-SE and the NE-SW direction, very typical for this part of the Massif Central. This pattern can be interpreted as follows:
This implies, that the granodiorite was situated in regional shear zone
or Riedel zone, in which the main material transport was directed to the Southeast (under extensional flow conditions) complicated by a simultaneous material transport under compression to the Southwest.
With the gradual cooling of the Variscan orogen the ductile movements came to an end. Accumulated strains in the granodiorite were now released in a brittle manner by faults, fracture
s and joint
s. Into these late orogenic zones of material weakness aplite
s, pegmatite
s and lamprophyre
s intruded. At the end of the Pennsylvanian
hot hydrothermal solutions deposited lead
, zinc
and silver
in veins, and in a second pulse a suite of rare arsenic
-molybdenum
mineralization was created.
in the QAPF diagram
yet are in close vicinity of the granite
field. Chemically they are granites; to be more precise they are adamellites. Likewise in the TAS diagram
they plot as rhyolite
s.
At a first glance the rocks of the massif appear rather homogeneous, but closer inspection reveals several different petrologic facies. Chemical analyses indicate the existence of SiO2-poorer rock types, i.e., the fine-grained hornblende-bearing border facies, the microgranites of the northern apophysis, the lamprophyres and the dark inclusions. The clear-cut distinction of the microgranites and the lamprophyres from the other facies argues for the existence or the hybridisation of two separate magma
s.
Besides purely magmatic structures like schlieren
and the alignment of feldspars due to viscous flow one can discern several tectonic structures in the granodiorite:
The occasional appearance of myrmekite
(mainly in the microgranites) and the hematitization of the plagioclase in the aplites and in the red facies rocks points to metasomatic
processes.
The chloritization of the biotite is a clear sign for retrograde metamorphic overprinting under greenschist facies conditions. This fate the Piégut-Pluviers granodiorite shares with many other crystalline basement rocks in the Massif Central.
The massif therefore bears witness of multiple processes affecting it underlining the complexity in the formation of granitoids. Already during its initial magmatic stage the solidifying crystal mush continued being deformed in a ductile manner. Even after solidification deformations didn't stop but kept on transforming the rocks in a brittle fashion and allowing the still very hot metasomatic fluids to perform their alterations on the massif.
had chosen several granitoids to be investigated − amongst them the Piégut-Pluviers Granodiorite. This project found fierce opposition amongst the inhabitants of the Dordogne and after several demonstrations it was abandoned. After that research activities shifted to the granitoid of Civray
-Charroux
in the Vienne, which is covered by Jurassic sediments of the Seuil du Poitou
. At the moment ANDRA seems to favour the Bure
site in the Meuse in eastern France as the final repository − in Mesozoic clays 500 meter below the surface.
Massif Central
The Massif Central is an elevated region in south-central France, consisting of mountains and plateaux....
in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
. Its cooling age has been determined as 325 ± 14 million years BP (Upper Mississippian, Serpukhovian
Serpukhovian
The Serpukhovian is in the ICS geologic timescale the uppermost stage or youngest age of the Mississippian, the lower subsystem of the Carboniferous. The Serpukhovian age lasted from 328.3 Ma tot 318.1 Ma...
).
Geographical situation
The Granodiorite was named after Piégut-PluviersPiégut-Pluviers
Piégut-Pluviers is a commune in the Dordogne department in Aquitaine in southwestern France.The commune is located from Angoulême, from Périgueux and Limoges and from Bordeaux.-Population:-History:...
, a small town in the Arrondissement of Nontron
Arrondissement of Nontron
The arrondissement of Nontron is an arrondissement of France, located in the Dordogne department, in the Aquitaine region. It has 8 cantons and 80 communes.-Cantons:The cantons of the arrondissement of Nontron are:# Bussière-Badil# Champagnac-de-Belair...
in the northern Dordogne
Dordogne
Dordogne is a départment in south-west France. The départment is located in the region of Aquitaine, between the Loire valley and the High Pyrénées named after the great river Dordogne that runs through it...
. Its outline is in the shape of an inverted comma pointing north with a nearly squarish main southern part turned into the NE-SW direction. The main body measures 15.5 kilometers in the NE-SW direction and 15 kilometers in the NW-SE direction and terminates in a tapering triangular section. This section continues farther north in an almost ten kilometer long appendix that is separated from the main body only by a very thin layer of migmatitic
Migmatite
Migmatite is a rock at the frontier between igneous and metamorphic rocks. They can also be known as diatexite.Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally...
gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
. The granodiorite takes up about 250 square kilometers in surface area. Its lowest point has an elevation of 135 meters above sea level along the western edge; the highest point in the northeast rises to about 375 meters above sea level. Geomorphologically the granodiorite forms a tabular slab, that is gently inclined to the southwest without any major topographic differences.
Geological overview
The Piégut-Pluviers Granodiorite is surrounded in the north, northeast and east by the Saint-Mathieu Leucogranite and its eqivalents (unit 17 on the geological map), which has a slightly younger age of 315 ± 17 million years BP (PennsylvanianPennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...
, Bashkirian
Bashkirian
The Bashkirian is in the ICS geologic timescale the lowest stage or oldest age of the Pennsylvanian, the youngest subsystem of the Carboniferous...
). In the northwest and southeast
it is bordered by various paragneisses (unit bk). Along the southwestern and western border the granodiorite is transgressed by lias
Lias Group
The Lias Group or Lias is a lithostratigraphic unit found in a large area of western Europe, including the British Isles, the North Sea, the low countries and the north of Germany...
sic arkose
Arkose
Arkose is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose....
s (unit j1) belonging to the sedimentary infill of the Aquitaine Basin
Aquitaine Basin (geology)
The Aquitaine Basin is after the Paris Basin the second largest Mesozoic and Cenozoic sedimentary basin in France, occupying a large part of the country's southwestern quadrant. Its surface area covers 66,000 km2 onshore. It formed on Variscan basement which was peneplained during the Permian...
. Together with the leucogranite the granodiorite forms a domal swell in the basement − the Saint-Mathieu Dome. The contact relationships with the paragneisses are not always clear-cut, sometimes there is a diffuse border region of several hundred meters where granodiorite and paragneiss interfinger. This indicates that the paragneiss is the host rock of the granodiorite and casts doubts on a truly intrusive origin.
The western Massif Central
Massif Central
The Massif Central is an elevated region in south-central France, consisting of mountains and plateaux....
is a nappe-stack of several basement
Basement
__FORCETOC__A basement is one or more floors of a building that are either completely or partially below the ground floor. Basements are typically used as a utility space for a building where such items as the furnace, water heater, breaker panel or fuse box, car park, and air-conditioning system...
slivers in the following spatial arrangement (from top to bottom):
- Upper Gneiss Unit
- Lower Gneiss Unit
- Parautochthonous Micaschist Unit (unit bo) containing the Saint-Mathieu Leucogranite
- Piégut-Pluviers Granodiorite
- paragneisses
This arrangement is only spatial and does not reflect temporal relationships. Major thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
contacts are situated between the Parautochthonous Micaschist Unit and the Lower Gneiss Unit and in between the two gneiss units.
Age
Field observations don't allow a decision on the relative ages of the two granitoids. Radiometric ages seem to indicate an older age for the granodiorite which has so far yielded 325 ± 14 million years BP and 314 ± 14 million years BP, whereas the leucogranite was dated at 315 ± 17 million years BP and at 304 ± 17 million years BP. Yet there is considerable overlap in these Rb-Sr-data and also a high standarddeviation, so these values should be used with caution.
Petrological facies
The Piégut-Pluviers Granodiorite is not homogenous, but consists of several petrological facies:- coarse-grained (common) facies
- coarse-grained porphyric facies
- fine-grained facies
- fine-grained hornblende-bearing facies
Coarse-grained facies
The coarse-grained or common facies takes up the largest surface area. Grain sizes vary in general between 2 and 6 millimeter with roughly isometric grains. The fresh rock has a grey colour; weathered outcrops can take on a brownish-reddish stain. It comprises the following minerals:- quartzQuartzQuartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
- rounded, globular grains of 1 to 3 millimeter in diameter, can form agglomerations with 1 to 3 centimeters in diameter − 26 volume percent - plagioclasePlagioclasePlagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
− often displays normal zoning, with calcic cores An 33-35 (andesineAndesineAndesine is a silicate mineral, a member of the plagioclase feldspar solid solution series. Its chemical formula is 4O8, where Ca/ is between 30%-50%...
) and more sodic rims An25 oligoclaseOligoclaseOligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite and anorthite . The albite:anorthite molar ratio ranges from 90:10 to 70:30.Oligoclase is a high sodium...
); some euhedral grains reach 10 millimeters and more − 42 volume percent - orthoclaseOrthoclaseOrthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
− perthitic, Carlsbad twinning, corroded by quartz, some euhedral crystals can reach more than 10 millimeters in grain size − 18 volume percent - biotiteBiotiteBiotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
− millimeter grain size, brass coloured, with zirconZirconZircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
inclusions and often chloritizedChloriteThe chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.-Oxidation states:...
− 10 volume percent
Accessories are zoned allanite
Allanite
Allanite is a sorosilicate group of minerals within the broader epidote group that contain a significant amount of rare earth elements. The mineral occurs mainly in metamorphosed clay rich sediments and felsic igneous rocks...
, apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
, epidote
Epidote
Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often...
, occasionally green hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
, zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
and zoisite
Zoisite
Zoisite is a calcium aluminium hydroxy sorosilicate belonging to the epidote group of minerals. Its chemical formula is Ca2Al3O...
. An opaque mineral is pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
.
The coarse-grained facies sometimes contains dark, fine-grained rounded to subrounded inclusions that cover the centimeter to decimeter size range. The long axes of the feldspar grains show a preferred orientation in some places.
This facies is normative in quartz (oversaturated in SiO2) and in corundum
Corundum
Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...
(peraluminous). It is also subalkaline. In the alphabet-soup classification it represents an I-type granitoid of calcalkaline characteristics belonging to the K-series. Compared with average granodiorites, this rock type has a higher SiO2 value and approaches the composition of monzonitic
Monzonite
Monzonite is an intermediate igneous intrusive rock composed of approximately equal amounts of sodic to intermediate plagioclase and orthoclase feldspars with minor amounts of hornblende, biotite and other minerals...
granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
s.
Coarse-grained porphyric facies
This facies is very similar to the common facies mineralogically and chemically, the difference being an increase in grain-size of the feldspars (can reach 1 to 4 centimeter; in some places even 6 centimeter). The groundmass minerals usually have a grain size of 5 millimeter, which is slightly higher than in the common facies. The transition from the common to the porphyric facies is gradual. Major outcrops are centered on Lacaujamet near Piégut (old abandoned quarry for lintels and facing stones) and on Puybégout near AugignacAugignac
Augignac is a commune in the Dordogne department in Aquitaine in southwestern France.Augignac is located between Nontron and Piégut-Pluviers in the heart of the Parc Naturel Régional de Périgord-Limousin-Population:-External links:* *...
.
Fine-grained facies
The fine-grained facies can be found mainly along the BandiatBandiat
The Bandiat is a small river that is 91 km long, a left tributary of the Tardoire. It flows through the Haute-Vienne, Dordogne and Charente departments in France.-Geography:...
river southeast of Nontron at the southern edge of the Piégut-Pluviers Granodiorite. This facies gradually develops from the common facies by a decrease in grain size. Mineralogically and chemically its composition is also very similar, the only difference being an increase in hornblende and a slight decrease in alkali feldspar. The fine-grained facies also occurs in a narrow strip along the northeastern and the northwestern margin of the granodiorite and in isolated patches in the interior, which might represent a roof facies of the granodiorite.
Fine-grained hornblende-bearing facies
These fine- to medium-grained rocks have a rather dark appearance, they are quite rich in green hornblende but nearly devoid in alkali feldspar (less than 10 volume percent). Their SiO2-contents are fairly low, so they compositionally approach dioritic rocksDiorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
. In the past this facies has been extensively quarried (Tabataud quarry south of Nontron). Associated with this facies are several NW-SE striking mineralised veins that have been mined for lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
and zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
. Mining activity stopped in 1939. The Cantonnier vein is famous for very rare mineralisations. Besides baryte, calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
, chalcedony
Chalcedony
Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic...
, dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
, galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
, marcasite
Marcasite
The mineral marcasite, sometimes called white iron pyrite, is iron sulfide with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Both structures do have in common that they contain the disulfide...
, pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
and sphalerite
Sphalerite
Sphalerite is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and other sulfides...
occur very rare minerals like anglesite
Anglesite
Anglesite is a lead sulfate mineral with the chemical formula PbSO4. It occurs as an oxidation product of primary lead sulfide ore, galena. Anglesite occurs as prismatic orthorhombic crystals and earthy masses, and is isomorphous with barite and celestine. It contains 74% of lead by mass and...
, cerussite
Cerussite
Cerussite is a mineral consisting of lead carbonate , and an important ore of lead. The name is from the Latin cerussa, white lead. Cerussa nativa was mentioned by Conrad Gessner in 1565, and in 1832 F. S. Beudant applied the name cruse to the mineral, whilst the present form, cerussite, is due to...
, crocoite
Crocoite
Crocoite is a mineral consisting of lead chromate, PbCrO4, and crystallizing in the monoclinic crystal system. It is sometimes used as a paint, being identical in composition with the artificial product chrome yellow. It was discovered at Berezovsky deposit near Ekaterinburg in the Urals in 1766;...
, dundasite
Dundasite
Dundasite is a rare lead aluminium carbonate mineral. The mineral is named after the type locality, Dundas, Tasmania, Australia. The mineral was first discovered in the Adelaide Proprietary Mine. Dundasite was first described by William Frederick Petterd in 1893.Dundasite is an uncommon secondary...
, embreyite, hisingerite
Hisingerite
Hisingerite is an iron phyllosilicate mineral with formula . A black or dark brown, lustrous secondary mineral, it is formed by the weathering or hydrothermal alteration of other iron silicate and sulfide minerals....
, leadhillite
Leadhillite
Leadhillite is a lead sulfate carbonate hydroxide mineral, often associated with anglesite. It has the formula Pb4SO422.Leadhillite crystallizes in the monoclinic system, but forms pseudo-hexagonal forms due to crystal twinning. It forms transparent to translucent variably colored adamantine crystals...
, mimetite
Mimetite
Mimetite, whose name derives from the Greek Μιμητής mimethes, meaning "imitator", is an arsenate mineral which forms as a secondary mineral in lead deposits, usually by the oxidation of galena and arsenopyrite. The name is a reference to mimetite's resemblance to the mineral pyromorphite...
, ozokerite
Ozokerite
Ozokerite or ozocerite is a naturally occurring odoriferous mineral wax or paraffin found in many localities.-Sources:...
(pseudomineral), pyromorphite
Pyromorphite
Pyromorphite is a mineral species composed of lead chlorophosphate: Pb53Cl, sometimes occurring in sufficient abundance to be mined as an ore of lead. Crystals are common, and have the form of a hexagonal prism terminated by the basal planes, sometimes combined with narrow faces of a hexagonal...
, native silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, vauquelinite
Vauquelinite
Vauquelinite is a complex mineral with the formula CuPb2CrO4PO4OH making it a combined chromate and phosphate of copper and lead. Discovered in 1818 in the Beryozovskoye deposit, Urals, Russia, it is named after Louis Vauquelin , a French chemist....
and wulfenite
Wulfenite
Wulfenite is a lead molybdate mineral with the formula PbMoO4. It can be most often found as thin tabular crystals with a bright orange-red to yellow-orange color, sometimes brown, although the color can be highly variable. In its yellow form it is sometimes called "yellow lead ore".It crystallizes...
.
The following chemical compositions are averages (14 analyses for the common facies, 2 analyses for the porphyric facies, 3 analyses for the fine-grained facies and 3 analyses for the fine-grained hornblende-bearing facies):
| Oxide Weight % | Common facies | Porphyric facies | Fine-grained facies | Fine-grained Hbl facies | CIPW-Norm Percent | Common facies | Porphyric facies | Fine-grained facies | Fine-grained Hbl facies |
|---|---|---|---|---|---|---|---|---|---|
| SiO2 | 70,49 | 69,98 | 71,83 | 64,43 | Q | 28,78 | 25,84 | 31,04 | 21,55 |
| TiO2 | 0,44 | 0,42 | 0,30 | 0,61 | Or | 22,45 | 24,34 | 24,58 | 16,54 |
| Al2O3 | 14,69 | 15,42 | 14,95 | 15,90 | Ab | 29,77 | 31,55 | 29,94 | 27,91 |
| Fe2O3 | 0,93 | 0,51 | 0,43 | 0,82 | An | 9,69 | 9,31 | 5,44 | 17,08 |
| FeO | 1,38 | 1,65 | 1,48 | 3,42 | C | 1,23 | 1,41 | 2,62 | 1,17 |
| MnO | 0,06 | 0,04 | 0,04 | 0,08 | Hy | 4,56 | 4,91 | 3,66 | 10,33 |
| MgO | 1,18 | 1,21 | 0,74 | 2,49 | Mt | 1,68 | 0,78 | 0,69 | 1,54 |
| CaO | 2,02 | 1,89 | 1,32 | 3,60 | Il | 0,83 | 0,82 | 0,56 | 1,15 |
| Na2O | 3,52 | 3,73 | 3,54 | 3,30 | Ap | 0,11 | 0,02 | 0,39 | 0,27 |
| K2O | 3,80 | 4,12 | 4,16 | 2,80 | |||||
| P2O5 | 0,05 | 0,01 | 0,17 | 0,12 | |||||
| H2O- | 0,08 | 0,12 | 0,09 | ||||||
| H2O+ | 0,90 | 0,86 | 0,87 | 1,78 | |||||
| Mg# | 0,59 | 0,56 | 0,47 | 0,55 | |||||
| A'/F | 0,34 | 0,31 | 0,62 | 0,12 | |||||
| Al/K+Na+Ca | 1,08 | 1,10 | 1,17 | 1,05 |
The magnesium numbers Mg # spread between 0,55 and 0,59 and are somewhat elevated compared with an average granodiorite. The anomalous fine-grained border facies shows an exceptionally low value of 0,47. The aluminosity (A'/F) values take up quite a large range with a tendency to peraluminous
Peraluminous
Peraluminous rocks are igneous rocks that have a molar proportion of aluminium oxide greater than that of sodium oxide, potassium oxide and calcium oxide combined. Compare with peralkaline, metaluminous, and subaluminous. Examples of peraluminous minerals include biotite, muscovite, cordierite,...
compositions. According to the alphabet-soup classification, the Piégut-Pluviers Granodiorite is a border case of an I-type granitoid. Again, the fine-grained border facies tends towards an S-type, thus documenting a contamination by the metasedimentary paragneisses.
Microgranite
The microgranite crops out in an apophysis north of the main granodiorite. It is separated from the main massif along the Trieux river only by a very thin migmatiticMigmatite
Migmatite is a rock at the frontier between igneous and metamorphic rocks. They can also be known as diatexite.Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally...
gneiss layer. Genetically it is probably related to the main granodiorite. The microgranite is porphyritic
Porphyritic
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...
with a very fine groundmass. It develops two different facies, a light grey facies in its interior and a dark facies along the border. Mineralogically and chemically the microgranite very much resembles the fine-grained hornblende-bearing facies of the main massif. The phenocrysts can reach 12 millimeter in size and consist of quartz, plagioclase and biotite. Alkali feldspar is found only in the groundmass, sometimes associated with myrmekite
Myrmekite
Myrmekite describes a vermicular, or wormy, intergrowth of quartz in plagioclase. The intergrowths are microscopic in scale, typically with maximum dimensions less than 1 millimeter. The plagioclase is sodium-rich, usually albite or oligoclase. These quartz-plagioclase intergrowths are associated...
. Besides the common accessories chlorite, epidote and zircon occurs titanite
Titanite
Titanite, or sphene , is a calcium titanium nesosilicate mineral, CaTiSiO5. Trace impurities of iron and aluminium are typically present...
. The dark facies is rich in green hornblende.
Rocks compositionally very similar (but texturally different) to the light grey facies can also be found in isolated kilometer-sized enclaves within the main granodiorite (near Saint-Barthélemy-de-Bussière
Saint-Barthélemy-de-Bussière
Saint-Barthélemy-de-Bussière is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-References:*...
and south of Marval
Marval
Marval is a commune in the Haute-Vienne department in the Limousin region in west-central France.Inhabitants are known as Marvalais.-References:*...
. They show intrusive contact relationships with the main body.
Aplites
The Piégut-Pluviers-Granodiorite is crosscut by many grey, in some places pink microgranitic apliteAplite
Aplite in petrology, the name given to intrusive rock in which quartz and feldspar are the dominant minerals. Aplites are usually very fine-grained, white, grey or pinkish, and their constituents are visible only with the help of a magnifying lens...
dikes. These usually upright dikes can occasionally attain one kilometer in length, their maximum thickness varying between one and ten meters. They are mainly oriented North-South and in many places follow a cross pattern formed by the N 020 and the N160 directions. Mineralogically the fine-grained phenocrysts consist of quartz, feldspars and biotite. The plagioclase phenocrysts occasionally reach a grain-size of 10 millimeters. The alkali feldspar is confined to the groundmass.
Associated with the pink aplite dikes is a coarser-grained red facies occurring in two larger outcrops near Ballerand and near Fargeas (commune of Abjat-sur-Bandiat
Abjat-sur-Bandiat
Abjat-sur-Bandiat is a commune in the Dordogne department in Aquitaine in southwestern France. The commune was simply known as Abjat until 1975-Population:-Sport:Every year, since the 1990s, the locals compete with the conkers championship...
). This facies contains also micropegmatites and pegmatitic
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
geodes. In some places it also carries enclaves of quartz diorite
Quartz diorite
Quartz diorite is an igneous, plutonic rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase with 10% or less potassium feldspar. Quartz is present at between 5 to 20% of the rock. Biotite, amphiboles and pyroxenes are common dark accessory...
and monzogabbro.
The reddish colours of the aplites and the red facies rocks is due to hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
invading the plagioclase and staining it. These rocks therefore underwent Fe-metasomatism
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
(see thin section at left). Most of the biotite has been altered to chlorite
Chlorite
The chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.-Oxidation states:...
(chloritization) indicating retromorphism under greenschist facies conditions.
Pegmatites
PegmatitePegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
s do occur as well, mainly as dikes; in some places as amygdules with beautiful smoky quartz
Smoky quartz
Smoky or smokey quartz is a brown to black variety of quartz. Like other quartz gems, it is a silicon dioxide crystal. The smoky colour results from free silicon, formed from the silicon dioxide by natural irradiation.-Morion:...
(rare).
Lamprophyre
LamprophyreLamprophyre
Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions...
dikes are fairly common in the metamorphic country rocks of the granodiorite, but extremely rare in the massif itself. Unweathered lamprophyres have a dark green to green colour; they are very dense fine-grained rocks that weather in creamy colours. Amongst all the different facies they have the lowest SiO2-content and approach dioritic compositions (quartz-bearing microdiorites with tonalitic
Tonalite
Tonalite is an igneous, plutonic rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.In older references tonalite is...
affinities).
| Oxide Weight % | Microgranite | Aplite | Lamprophyre | CIPW-Norm Percent | Microgranite | Aplite | Lamprophyre |
|---|---|---|---|---|---|---|---|
| SiO2 | 63,70 | 72,00 | 60,00 | Q | 18,30 | 32,92 | 14,45 |
| TiO2 | 0,56 | 0,28 | 0,79 | Or | 15,98 | 27,71 | 13,30 |
| Al2O3 | 15,74 | 14,20 | 15,10 | Ab | 28,83 | 27,06 | 21,51 |
| Fe2O3 | 1,04 | 0,45 | 0,90 | An | 20,25 | 4,39 | 28,84 |
| FeO | 3,60 | 1,30 | 4,63 | Di | 0,14 | 0,45 | |
| MnO | 0,08 | 0,04 | 0,10 | Hy | 13,29 | 3,60 | 16,95 |
| MgO | 3,55 | 0,88 | 5,04 | Mt | 1,87 | 0,84 | 2,52 |
| CaO | 4,21 | 0,90 | 4,25 | Il | 1,07 | 0,55 | 1,56 |
| Na2O | 3,36 | 3,20 | 3,44 | Ap | 0,26 | 0,28 | 0,42 |
| K2O | 2,67 | 4,60 | 2,17 | C | 2,65 | ||
| P2O5 | 0,12 | 0,12 | 0,17 | ||||
| H2O- | 0,05 | 0,08 | 0,16 | ||||
| H2O+ | 0,87 | 1,10 | 1,85 | ||||
| Mg# | 0,63 | 0,54 | 0,65 | ||||
| A'/F | 0,02 | 0,63 | -0,01 | ||||
| Al/K+Na+Ca | 0,98 | 1,19 | 0,96 |
The aplites are clearly peraluminous and belong to the S-type and are very similar to the fine-grained border facies. Microgranites and lamprophyres show similarities and belong to the I-type; they are hypaluminous to normal aluminous rocks and most likely have originated from an independent magma pulse or batch.
Structures
Despite a rather homogenous appearance the Piégut-Pluviers Granodiorite carries a foliationFoliation
In mathematics, a foliation is a geometric device used to study manifolds, consisting of an integrable subbundle of the tangent bundle. A foliation looks locally like a decomposition of the manifold as a union of parallel submanifolds of smaller dimension....
of tectonic origin, that is clearly evident in more weathered superficial outcrops. This indicates that the granodiorite was continuing to be deformed during the Variscan orogeny
Variscan orogeny
The Variscan orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica and Gondwana to form the supercontinent of Pangaea.-Naming:...
even in the subsolidus stage together with the country rocks. The stereo net
Stereographic projection
The stereographic projection, in geometry, is a particular mapping that projects a sphere onto a plane. The projection is defined on the entire sphere, except at one point — the projection point. Where it is defined, the mapping is smooth and bijective. It is conformal, meaning that it...
on the right shows the spatial organisation of the granodiorite. One can discern a crossed pattern formed by the NW-SE and the NE-SW direction, very typical for this part of the Massif Central. This pattern can be interpreted as follows:
- a relatively flat, nearly symmetrical waveWaveIn physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
pattern in the NW-SE direction whose angles of incidence do not surpass 30 ° and whose wavelength varies from tens to hundreds of meters. Shear bandShear bandA shear band is a narrow zone of intense shearing strain, usually of plastic nature, developing during severe deformation of ductile materials....
s of the C'-type can be discerned. - an asymmetric wave structure in the NE-SW direction with rather steep NE-facing limbs. Shear bands are of the C-type.
This implies, that the granodiorite was situated in regional shear zone
Shear zone
A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening blocks stay relatively unaffected by the deformation...
or Riedel zone, in which the main material transport was directed to the Southeast (under extensional flow conditions) complicated by a simultaneous material transport under compression to the Southwest.
With the gradual cooling of the Variscan orogen the ductile movements came to an end. Accumulated strains in the granodiorite were now released in a brittle manner by faults, fracture
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
s and joint
Joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
s. Into these late orogenic zones of material weakness aplite
Aplite
Aplite in petrology, the name given to intrusive rock in which quartz and feldspar are the dominant minerals. Aplites are usually very fine-grained, white, grey or pinkish, and their constituents are visible only with the help of a magnifying lens...
s, pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
s and lamprophyre
Lamprophyre
Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions...
s intruded. At the end of the Pennsylvanian
Pennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...
hot hydrothermal solutions deposited lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
and silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
in veins, and in a second pulse a suite of rare arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
-molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
mineralization was created.
Final conclusion
Modally the rocks of the massif plot as granodioriteGranodiorite
Granodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...
in the QAPF diagram
QAPF diagram
A QAPF diagram is a double triangle diagram which is used to classify igneous rocks based on mineralogic composition. The acronym, QAPF, stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid ". These are the mineral groups used for classification in QAPF diagram...
yet are in close vicinity of the granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
field. Chemically they are granites; to be more precise they are adamellites. Likewise in the TAS diagram
TAS classification
The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali content and the silica content. These chemical parameters are useful, because the relative proportions of alkalis and silica play an important role in...
they plot as rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
s.
At a first glance the rocks of the massif appear rather homogeneous, but closer inspection reveals several different petrologic facies. Chemical analyses indicate the existence of SiO2-poorer rock types, i.e., the fine-grained hornblende-bearing border facies, the microgranites of the northern apophysis, the lamprophyres and the dark inclusions. The clear-cut distinction of the microgranites and the lamprophyres from the other facies argues for the existence or the hybridisation of two separate magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
s.
Besides purely magmatic structures like schlieren
Schlieren
Schlieren are optical inhomogeneities in transparent material not visible to the human eye. Schlieren physics developed out of the need to produce high-quality lenses devoid of these inhomogeneities. These inhomogeneities are localized differences in optical path length that cause light deviation...
and the alignment of feldspars due to viscous flow one can discern several tectonic structures in the granodiorite:
- foliationFoliationIn mathematics, a foliation is a geometric device used to study manifolds, consisting of an integrable subbundle of the tangent bundle. A foliation looks locally like a decomposition of the manifold as a union of parallel submanifolds of smaller dimension....
- shear zoneShear zoneA shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening blocks stay relatively unaffected by the deformation...
s with C/S fabric and sbc fabric (shear band cleavage) - myloniticMyloniteMylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...
granitoids along the southern edge of the massif
The occasional appearance of myrmekite
Myrmekite
Myrmekite describes a vermicular, or wormy, intergrowth of quartz in plagioclase. The intergrowths are microscopic in scale, typically with maximum dimensions less than 1 millimeter. The plagioclase is sodium-rich, usually albite or oligoclase. These quartz-plagioclase intergrowths are associated...
(mainly in the microgranites) and the hematitization of the plagioclase in the aplites and in the red facies rocks points to metasomatic
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
processes.
The chloritization of the biotite is a clear sign for retrograde metamorphic overprinting under greenschist facies conditions. This fate the Piégut-Pluviers granodiorite shares with many other crystalline basement rocks in the Massif Central.
The massif therefore bears witness of multiple processes affecting it underlining the complexity in the formation of granitoids. Already during its initial magmatic stage the solidifying crystal mush continued being deformed in a ductile manner. Even after solidification deformations didn't stop but kept on transforming the rocks in a brittle fashion and allowing the still very hot metasomatic fluids to perform their alterations on the massif.
Repository for nuclear waste
In the search for a suitable site as a repository for nuclear waste from the French nuclear power stations at the end of the 1990s ANDRAAndra
Andra is a 1971 science fiction novel, the first novel by English writer Louise Lawrence. The book was set 2000 years from now, after the world was destroyed by war leaving the earth knocked off its rotation and the ground above to become a desolate frozen wasteland with everyone that survived...
had chosen several granitoids to be investigated − amongst them the Piégut-Pluviers Granodiorite. This project found fierce opposition amongst the inhabitants of the Dordogne and after several demonstrations it was abandoned. After that research activities shifted to the granitoid of Civray
Civray, Vienne
Civray is a commune in the Vienne department in the Poitou-Charentes region in western France....
-Charroux
Charroux, Vienne
Charroux is a commune in the Vienne department in the Poitou-Charentes region in western France.The remains of the Benedictine Charroux Abbey, founded in the 8th century, are preserved in the town.-Demographics:-References:*...
in the Vienne, which is covered by Jurassic sediments of the Seuil du Poitou
Seuil du Poitou
The Seuil du Poitou is a geological denomination for an area in western central France where the Paris and Aquitaine sedimentary basins meet, and which also is a gap between the ancient mountain ranges Massif Armoricain and the Massif Central .Situated to the south of Poitiers, the area is the...
. At the moment ANDRA seems to favour the Bure
Bure, Meuse
Bure is a commune in the Meuse department in Lorraine in north-eastern France.It hosts the Meuse/Haute Marne Underground Research Laboratory for radioactive waste storage....
site in the Meuse in eastern France as the final repository − in Mesozoic clays 500 meter below the surface.
Literature
- Carte géologique de la France au millionième (1996). 6ème édition. Éditions BRGM. Service Géologique National. ISBN 2-7159-2128-4
- Geological maps of the BRGM, scale 1/50000. Sheets Châlus, Nontron, La Rochefoucauld, Montbron and Thiviers.
- Passchier, C. W. und Trouw, R.A.J. (1998) Microtectonics. Springer Verlag. ISBN 3-540-58713-6