Thiviers-Payzac Unit
Encyclopedia
The Thiviers-Payzac Unit is a metasediment
ary succession of late Neoproterozoic
and Cambrian
age outcropping in the southern Limousin
in France
. The unit geologically
forms part of the Variscan
basement
of the northwestern Massif Central
.
and Payzac
, two small towns in the northeastern Dordogne situated within the unit's outcrop area. The term nappe
is somewhat misleading.
), a plateau peneplained during the Eocene
and whose elevation barely reaches above 400 metres. The unit starts just west of Thiviers in the northern Dordogne and then follows for 70 kilometres a semicircular arc segment, passing through Lanouaille
, Payzac, Orgnac
, Donzenac
and finishing just east of Brive in the Corrèze. In the beginning the unit follows a WNW-ESE strike
(N110), but then just north of Orgnac swings into a NW-SE course (N 135). On its north side the unit is separated by the Estivaux Fault, a left-lateral, ductile strike-slip fault, from rocks of the Upper Gneiss Unit. On its western end appear rocks of the Lower Gneiss Unit. In the southwest the unit is overlain by liassic
sediments of the Aquitaine Basin
. The South Limousin Fault, also a ductile wrench fault but with a right-lateral shear, separates the unit from the Génis Unit
in the south. The unit finally disappears in the southeast below some small occurrences of Pennsylvanian
sediments, yet the bulk of the unit is covered mainly by Permotriassic red beds of the Brive Basin. Following the Auvézère
the maximum width of the unit across strike is only 9 kilometers.
Just northwest of Terrasson
in the eastern Dordogne there is an upfaulted basement high, that is also included within the main unit. This outlier is about 10 kilometers long and also follows the ESE-direction; its width across strike is only 5 kilometers.
of the Thiviers-Payzac Unit shows the following succession (from top to bottom):
The unit hosts the Mississippian Estivaux granite and two Ordovician
granitoid
s, the Saut-du-Saumon orthogneiss and the Corgnac granite.
-bearing quartzite
and consequently forms erosion-resistant reliefs within the plateau of the Bas-Limousin. The formation crops out in a merely 200 meter wide band in the western and central part of the Thiviers-Payzac Unit. The quartzite shows similarities to the Puy-de-Cornut-Arkose of the Génis Unit. Even a relationship with the grès armoricain in Brittany
is taken into consideration. Its age is therefore most likely Ordovician (Tremadocian).
, magmatic rocks. They likewise appear only in the western and central Thiviers-Payzac Unit. In a 500 Meter wide strip they follow immediately to the south of the Puy-des-Âges quartzite. Near Juillac this strip widens to about 2 kilometers. The age of the maximally 500 meter thick mafic rocks is taken to be Cambrian. They consist of alternating greenschist
s and amphibolite
s in which are intercalated several layers of metadolerites and metagabbros
. The very fine grained greenschist of light green to dark green colour contains as major constituents the mineral
s plagioclase
(oligoclase
or andesine
), amphibole
(hornblende
) and epidote
(clinozoisite
). Biotite
is a minor constituent and quartz
, calcite
and opaques are accessories. The greenschist represents ancient subalkaline basalt
s. The metadolerites and the metagabbros are much more coarse-grained and consist mainly of hornblende and basic plagioclase that has undergone saussuritisation.
Donzenac
and extends to Lanouaille. Here the band is cut off by the left-lateral Dussac Fault, a major strike-slip fault, and offset by several kilometers to the southwest. The band then follows through to just northeast of Thiviers, where it ends. The Donzenac schist is also included in the Cambrian. The schist has silky hues of grey and is mostly made up of phyllosilicates like muscovite
and biotite
or muscovite and chlorite
. The phyllosilicates are accompanied by quartz, acid plagioclase and garnet
of the almandine zone
. The schist sometimes reveals relatively fine-grained, decimeter-sized, dark interlayers of arenitic
composition, most likely ancient greywacke
s. The arenitic interlayers show clasts of quartz, plagioclase and epidote surrounded by newly formed minerals like phyllosilicates, quartz and very fine-grained albite
.
Innumerable, meter-sized doleritic dike
s cut through the formation inducing local contact metamorphism.
The term “sandstone” is somewhat misleading, because the formation is clearly dominated by the rhyodacitic tuffs of volcanic origin, all the other facies merely being alteration products. The once sodium
-rich rhyodacitic tuffs have now become dark, massive and thickly bedded rocks. Lodged within a fine-grained matrix of chlorite, white mica, quartz and albite are millimeter-sized clasts of quartz, plagioclase (albite or oligoclase) and epidote. The following observations underline the explosive character of the volcanic rocks:
The greywackes are mineralogically
very similar, but richer in quartz phenocrysts and their matrix is enriched in phyllosilicates. They are probably derived from the rhyodacites. Likewise the chemical composition of the quartzites, which is almost identical to the ryhodacites!
The underlying formations of the Thiviers sandstone - plagioclase-rich paragneisses and micaschists - are nowhere exposed.
in the south and whose tentacles end in the Orgnac region near the river Loyre.
The orthogneiss is made up of eye-shaped, subcentimeter-sized alakali feldspar porphyroclast
s and is therefore an augengneiss. Its matrix consists of quartz and feldspars. The biotite has aligned itself parallel to sigmoidal shear bands. The porphyroclasts have been broken and sheared left-laterally along parallel fractures.
The entire body of the orthogneiss is intensively foliated
in a NW-SE fashion. The foliation planes are close to vertical and contain horizontal lineation
s also striking NW-SE. The rock is therefore a S-L tectonite. Along its borders with the enclosing formations (Thiviers sandstone and Donzenac schist) mylonite
s and ultramylonites with sinistral shear sense have formed. In its interior dextral and sinistral shearing interfere in a non-coaxial fashion.
The protolith of the orthogneiss was once a porphyritic
granitoid
which was later on (during the Variscan orogeny) deformed plastically. Its original cooling age has been determined as middle Ordovician (Acadian phase). According to Bernard-Griffiths (1977) the mylonitic deformations took place close to the Devonian/Carboniferous boundary about 361 million years ago.
es along its edges. Like the very similar Saut-du-Saumon orthogneiss the Corgnac granite also intruded during the middle Ordovician about 470 million years ago. The granite was overprinted at about 350 million years ago during the regional metamorphism under retrograde conditions (Chlorite zone). Its chemical composition defines it as a subalkaline monzogranite.
Two very different facies can be distinguished in the Corgnac granite:
The equigranular, sometimes porphyric, grey to rose-coloured granitic facies contains the following minerals:
Accessory minerals are muscovite, zircon, apatite
and opaques.
All these minerals were altered during the greenschist facies retromorphism. Biotite and quartz for instance were broken cataclastically, plagioclase was invaded by muscovite and clinozoisite and rutile
needles exsolved from biotite. The granitic facies has produced several smaller, porphyric, microgranitic apophyses.
The orthogneissic facies is derived from the granitic facies, it only underwent a stronger ductile deformation. The facies presents itself now as a banded augengneiss with amygdular eyes of microcline and plagioclase surrounded by foliation minerals like very fine quartz, albite, granular clinozoisite and mica lamellae. The biotite is often chloritized. The shear sense has not been determined, but most likely is right-lateral judging by the Corgnac granite being part of the southern section of the Thiviers-Payzac Unit.
. Like the Saut-du-Saumon orthogneiss its shape is also squid-like developing tentacular arms at its northern gneissic and also mylonitic end. The granite is about 8 kilometers long in the NW-SE direction and 3 kilometers wide across strike. In the northeast the left-lateral Estivaux strike-slip fault separates the massif from rocks of the Upper Gneiss Unit. In the southwest it is surrounded by the Thiviers sandstone. Its southeastern limited is outlined by the Clan river.
The granite develops four different facies:
Its mineralogy (in the melanocratic facies) shows the following composition:
The melanocratic facies contains lots of mafic enclaves and schlieren
. The white and the pink facies are a more fine-grained variation of the melanocratic facies, they also lack hornblende and sphene. Their mutual colour difference is due to the coloration of the feldspars, the pink facies most likely being richer in hematite
. The leucocratic facies
can be regarded as a strongly sheared leucogranite
that is very rich in muscovite.
The granite therefore possesses a pronounced gradient
in deformation and in mineral alignment as one progresses from west to east. Approaching the S-C mylonitic, sinistral Estivaux Fault the little deformed melanocratic facies yields to the strongly deformed leucocratic facies, at the same time the contents in microaplite (representing a residual melt) descend from 20 % to merely 5 %.
The shear sense in the Estivaux granite is uniformly sinistral.
. Similar as in the Génis Unit the folding is tight and upright and the wavelength even somewhat shorter (100 to 125 metres, but can increase in the south to about 200 metres). The more or less horizontal fold axes strike WNW-ESE (N 110, west of the Loyre river). The stratification
(S0) is steeply inclined (around 80 °) and north or south dipping. Parallel to the folds axial plane a recognisable schistosity (S1) has developed underlined by newly formed minerals. The tight folding is overprinted by a second fold generation of open folds that have generated a very long wavelength (about 2 kilometers) series of synclines and anticlines. The axis of the first syncline is situated right next to the South Limousin Fault, followed by the first anticline underneath Saint-Mesmin. The second, central syncline is outlined by the trace of the Puy-des-Âges quartzite and the second anticline runs through Saint-Cyr-les-Champagnes
.
A crenulation lineation has also formed which trends more or less parallel to the folds. Newly formed metamorphic minerals also align themselves preferably in this direction.
After reaching the Loyre river the Thiviers-Payzac unit makes a definitive right-hand turn and all the structural elements swing into the NW-SE direction (N 135). This new trend is followed till the unit finally disappears just east of Brive.
under low to medium grade. Its upper reaches show upper greenschist facies conditions, the lower sections reached already lower amphibolite facies conditions. The presence of chlorite and chloritized biotite in shear bands and in pressure shadows indicates retromorphism, which has been known in the southern Limousin for quite a while.
The following microtectonic methods underline the sinistral shear sense in the northern outcrop area of the Thiviers sandstone:
North of Saint-Cyr-les-Champagnes the neighbouring Upper Gneiss Unit also shows a left-lateral shear sense (sinistrally sheared quartz lenses).
In the Donzenac Schist, where both shear senses are present, one can observe how the dextral shearing is overprinting the sinistral shearing. The sinistral movements happened therefore later. On left-laterally sheared, sigmoidal porphyroblasts of biotite dextral shear bands are superimposed on; additionally retrograde chlorite
formed in these late shear bands.
The pervasive shearing is responsible for the folding in the Thiviers-Payzac Unit; the folds can be interpreted as tear folds that were rotated into the maximum stretching direction in a transpressive
, ductile shear zone.
The tectonic movements didn't stop at the close of the ductile deformations. For instance in the brittle realm lots of small, mainly NE-SW-striking strike-slip faults were initiated with left-lateral displacements in the order of about 500 meters – an exception being the Dussac Fault north of Lanouaille which has a left-lateral displacement of nearly 6 kilometers!
(Chantonnay Syncline in the Vendée
) and in the Rouergue. In the southern Armorican Massif the dextral shearing motions are timed as Namurian and Westphalian (Serpukhovian
till Moscovian
), i.e. 325 till 305 million years ago. One can therefore propose for the deformations in the Thiviers-Payzac Unit of the Bas-Limousin, which is regarded as the southeastern prolongation of the Vendée, a middle to late Carboniferous age. Similar ages for the leucogranites in the northern and central Limousin seem to support this assumption.
Yet some radiometric datings
using the argon
-method find much earlier Tournaisian ages for the intrusion of the Estivaux granite and for the motions along mylonite zones within the Saut-du-Saumon orthogneiss. These findings imply a tectonic phase in the southern Limousin already during Mississippian times (Bretonic Phase).
Metasediment
In geology, metasediment is sediment or sedimentary rock that shows evidence of having been subjected to metamorphism. The overall composition of a metasediment can be used to identify the original sedimentary rock, even where they have been subject to high-grade metamorphism and intense...
ary succession of late Neoproterozoic
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
and Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
age outcropping in the southern Limousin
Limousin (région)
Limousin is one of the 27 regions of France. It is composed of three départements: Corrèze, Creuse and the Haute-Vienne.Situated largely in the Massif Central, as of January 1st 2008, the Limousin comprised 740,743 inhabitants on nearly 17 000 km2, making it the second least populated region of...
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...
. The unit geologically
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
forms part of the Variscan
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:...
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...
of the northwestern Massif Central
Massif Central (geology)
The Massif Central forms together with the Armorican Massif one of the two big basement massifs in France. Its geological evolution started in the late Neoproterozoic and continues to this day. It has been shaped mainly by the Caledonian orogeny and the Variscan orogeny. The Alpine orogeny has...
.
Terminology
The Thiviers-Payzac Unit, sometimes still called Thiviers-Payzac Nappe or Bas-Limousin Group, was named after ThiviersThiviers
Thiviers is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-Personalities:It is notable as being the town in which Jean-Paul Sartre lived as a child. Painter Pierre Bouillon was born there.-References:*...
and Payzac
Payzac, Dordogne
Payzac is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-History:The commune was written as Peisac, Peyzac, Paysac and since the late-19th century: Payzac...
, two small towns in the northeastern Dordogne situated within the unit's outcrop area. The term nappe
Nappe
In geology, a nappe is a large sheetlike body of rock that has been moved more than or 5 km from its original position. Nappes form during continental plate collisions, when folds are sheared so much that they fold back over on themselves and break apart. The resulting structure is a...
is somewhat misleading.
Geography
Geographically the Thiviers-Payzac Unit belongs to the Bas-Limousin (southern LimousinLimousin (région)
Limousin is one of the 27 regions of France. It is composed of three départements: Corrèze, Creuse and the Haute-Vienne.Situated largely in the Massif Central, as of January 1st 2008, the Limousin comprised 740,743 inhabitants on nearly 17 000 km2, making it the second least populated region of...
), a plateau peneplained during the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
and whose elevation barely reaches above 400 metres. The unit starts just west of Thiviers in the northern Dordogne and then follows for 70 kilometres a semicircular arc segment, passing through Lanouaille
Lanouaille
Lanouaille is a commune in the Dordogne department in Aquitaine in southwestern France.-Geography:The commune is linked with the departmental road D704 . It is a small commercial town.-History:...
, Payzac, Orgnac
Orgnac
Orgnac is part of the name of several communes in France:* Orgnac-l'Aven, a commune in the Ardèche department* Orgnac-sur-Vézère, a commune in the Corrèze department...
, Donzenac
Donzenac
Donzenac is a commune in the Corrèze department in central France.-Population:...
and finishing just east of Brive in the Corrèze. In the beginning the unit follows a WNW-ESE strike
Strike and dip
Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented...
(N110), but then just north of Orgnac swings into a NW-SE course (N 135). On its north side the unit is separated by the Estivaux Fault, a left-lateral, ductile strike-slip fault, from rocks of the Upper Gneiss Unit. On its western end appear rocks of the Lower Gneiss Unit. In the southwest the unit is overlain by liassic
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...
sediments 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...
. The South Limousin Fault, also a ductile wrench fault but with a right-lateral shear, separates the unit from the Génis Unit
Génis Unit
The Génis Unit is a Paleozoic metasedimentary succession of the southern Limousin and belongs geologically to the Variscan basement of the French Massif Central...
in the south. The unit finally disappears in the southeast below some small occurrences of 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...
sediments, yet the bulk of the unit is covered mainly by Permotriassic red beds of the Brive Basin. Following the Auvézère
Auvézère
The Auvézère is a small river in the Dordogne department of France. It is a tributary of the Isle River, which is itself a tributary of the Dordogne River.-Geography:...
the maximum width of the unit across strike is only 9 kilometers.
Just northwest of Terrasson
Terrasson-Lavilledieu
Terrasson-Lavilledieu is a commune in the Dordogne department in Aquitaine in southwestern France.The Gardens of the Imagination —classified as a remarkable garden by the French Ministry of Culture—are situated in Terrasson...
in the eastern Dordogne there is an upfaulted basement high, that is also included within the main unit. This outlier is about 10 kilometers long and also follows the ESE-direction; its width across strike is only 5 kilometers.
Stratigraphy
The stratigraphyStratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
of the Thiviers-Payzac Unit shows the following succession (from top to bottom):
- Puy-des-Âges quartzite
- Engastine mafics
- Donzenac schist
- Thiviers sandstone
The unit hosts the Mississippian Estivaux granite and two Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
granitoid
Granitoid
A granitoid or granitic rock is a variety of coarse grained plutonic rock similar to granite which mineralogically are composed predominately of feldspar and quartz. Examples of granitoid rocks include granite, quartz monzonite, quartz diorite, syenite, granodiorite and trondhjemite. Many are...
s, the Saut-du-Saumon orthogneiss and the Corgnac granite.
Puy-des-Âges quartzite
The Puy-des-Âges quartzite on top is a very resistant, white, sericiteMuscovite
Muscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...
-bearing quartzite
Quartzite
Quartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...
and consequently forms erosion-resistant reliefs within the plateau of the Bas-Limousin. The formation crops out in a merely 200 meter wide band in the western and central part of the Thiviers-Payzac Unit. The quartzite shows similarities to the Puy-de-Cornut-Arkose of the Génis Unit. Even a relationship with the grès armoricain in Brittany
Brittany
Brittany is a cultural and administrative region in the north-west of France. Previously a kingdom and then a duchy, Brittany was united to the Kingdom of France in 1532 as a province. Brittany has also been referred to as Less, Lesser or Little Britain...
is taken into consideration. Its age is therefore most likely Ordovician (Tremadocian).
Engastine mafics
The underlying Engastine Mafics are a complex of maficMafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
, magmatic rocks. They likewise appear only in the western and central Thiviers-Payzac Unit. In a 500 Meter wide strip they follow immediately to the south of the Puy-des-Âges quartzite. Near Juillac this strip widens to about 2 kilometers. The age of the maximally 500 meter thick mafic rocks is taken to be Cambrian. They consist of alternating greenschist
Greenschist
Greenschist is a general field petrologic term applied to metamorphic or altered mafic volcanic rock. The term greenstone is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the...
s and amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
s in which are intercalated several layers of metadolerites and metagabbros
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
. The very fine grained greenschist of light green to dark green colour contains as major constituents the mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s plagioclase
Plagioclase
Plagioclase 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...
(oligoclase
Oligoclase
Oligoclase 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...
or andesine
Andesine
Andesine is a silicate mineral, a member of the plagioclase feldspar solid solution series. Its chemical formula is 4O8, where Ca/ is between 30%-50%...
), amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
(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....
) and 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...
(clinozoisite
Clinozoisite
Clinozoisite is a mineral, a complex sorosilicate of calcium and aluminium and is usually a grey green colour.Its formula is Ca2Al3[O|OH|SiO4|Si2O7]....
). Biotite
Biotite
Biotite 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...
is a minor constituent and quartz
Quartz
Quartz 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,...
, 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:...
and opaques are accessories. The greenschist represents ancient subalkaline basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
s. The metadolerites and the metagabbros are much more coarse-grained and consist mainly of hornblende and basic plagioclase that has undergone saussuritisation.
Donzenac Schist
Below the Engastine mafic complex follows the epizonal (low-grade) Donzenac Schist. The schist crops out in a 3 kilometer wide, slightly curved band that starts at the type localityType locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
Donzenac
Donzenac
Donzenac is a commune in the Corrèze department in central France.-Population:...
and extends to Lanouaille. Here the band is cut off by the left-lateral Dussac Fault, a major strike-slip fault, and offset by several kilometers to the southwest. The band then follows through to just northeast of Thiviers, where it ends. The Donzenac schist is also included in the Cambrian. The schist has silky hues of grey and is mostly made up of phyllosilicates like muscovite
Muscovite
Muscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...
and biotite
Biotite
Biotite 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...
or muscovite and 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:...
. The phyllosilicates are accompanied by quartz, acid plagioclase and garnet
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
of the almandine zone
Almandine
Almandine , also known incorrectly as almandite, is a species of mineral belonging to the garnet Group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asia Minor. Almandine is an iron alumina garnet,...
. The schist sometimes reveals relatively fine-grained, decimeter-sized, dark interlayers of arenitic
Arenite
Arenite is a sedimentary clastic rock with sand grain size between 0.0625 mm and 2 mm and contain less than 15% matrix. The related adjective is arenaceous...
composition, most likely ancient greywacke
Greywacke
Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
s. The arenitic interlayers show clasts of quartz, plagioclase and epidote surrounded by newly formed minerals like phyllosilicates, quartz and very fine-grained albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
.
Thiviers sandstone
The Thiviers sandstone is the lowermost formation in the Thiviers-Payzac Unit appearing at the surface, taking up about two thirds of its total surface area. The formation is a detrital, volcanic succession of late Neoproterozoic and Cambrian age. It can be subdivided into four different facies:- medium- to coarse-grained greywackeGreywackeGreywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
s. - sandy schists and rhyodaciticRhyodaciteRhyodacite is an extrusive volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of granodiorite. Phenocrysts of sodium rich plagioclase, sanidine, quartz, and biotite or hornblende are typically set in an aphanitic to glassy light to intermediate...
tuffTuffTuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
s. - polygenetic and intraformational conglomerateConglomerate (geology)A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
s. - quartziteQuartziteQuartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...
s formed from recrystallised sandstoneSandstoneSandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s.
Innumerable, meter-sized doleritic dike
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
s cut through the formation inducing local contact metamorphism.
The term “sandstone” is somewhat misleading, because the formation is clearly dominated by the rhyodacitic tuffs of volcanic origin, all the other facies merely being alteration products. The once sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
-rich rhyodacitic tuffs have now become dark, massive and thickly bedded rocks. Lodged within a fine-grained matrix of chlorite, white mica, quartz and albite are millimeter-sized clasts of quartz, plagioclase (albite or oligoclase) and epidote. The following observations underline the explosive character of the volcanic rocks:
- fragmented quartz crystals with sharp, pointed edges and tips.
- broken, pointed plagioclase.
- Rock fragments of once albite-rich, leucocratic lavaLavaLava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
.
The greywackes are mineralogically
Mineralogy
Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...
very similar, but richer in quartz phenocrysts and their matrix is enriched in phyllosilicates. They are probably derived from the rhyodacites. Likewise the chemical composition of the quartzites, which is almost identical to the ryhodacites!
The underlying formations of the Thiviers sandstone - plagioclase-rich paragneisses and micaschists - are nowhere exposed.
Intrusive rocks
As already mentioned before the Thiviers-Payzac Unit houses three different magmatic bodies:- Saut-du-Saumon orthogneiss.
- Corgnac granite.
- Estivaux granite.
Saut-du-Saumon orthogneiss
The prekinematic Saut-du-Saumon orthogneiss is a 15 kilometer long, very drawn out, squid-like, NW-SE-striking granitic body, whose tail starts at DonzenacDonzenac
Donzenac is a commune in the Corrèze department in central France.-Population:...
in the south and whose tentacles end in the Orgnac region near the river Loyre.
The orthogneiss is made up of eye-shaped, subcentimeter-sized alakali feldspar porphyroclast
Porphyroclast
thumb|350px|right|A [[mylonite]] showing a number of porphyroclasts: a clear red [[garnet]] left in the picture while smaller white [[feldspar]] porphyroclasts can be found all over...
s and is therefore an augengneiss. Its matrix consists of quartz and feldspars. The biotite has aligned itself parallel to sigmoidal shear bands. The porphyroclasts have been broken and sheared left-laterally along parallel fractures.
The entire body of the orthogneiss is intensively foliated
Foliation
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....
in a NW-SE fashion. The foliation planes are close to vertical and contain horizontal lineation
Lineation
In Western handwriting, the base line, the x-height or corpus size, the height of the ascenders and the bottom line of the descenders make up four horizontal lines which represent the lineation of handwriting...
s also striking NW-SE. The rock is therefore a S-L tectonite. Along its borders with the enclosing formations (Thiviers sandstone and Donzenac schist) mylonite
Mylonite
Mylonite 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...
s and ultramylonites with sinistral shear sense have formed. In its interior dextral and sinistral shearing interfere in a non-coaxial fashion.
The protolith of the orthogneiss was once a 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...
granitoid
Granitoid
A granitoid or granitic rock is a variety of coarse grained plutonic rock similar to granite which mineralogically are composed predominately of feldspar and quartz. Examples of granitoid rocks include granite, quartz monzonite, quartz diorite, syenite, granodiorite and trondhjemite. Many are...
which was later on (during the Variscan orogeny) deformed plastically. Its original cooling age has been determined as middle Ordovician (Acadian phase). According to Bernard-Griffiths (1977) the mylonitic deformations took place close to the Devonian/Carboniferous boundary about 361 million years ago.
Corgnac granite
The Corgnac granite is a 6.5 kilometer long massif within the Thiviers sandstone. It is aligned more or less parallel with the regional foliation. Across strike it is only 2.5 kilometers wide. Its intrusive granitic nature is demonstrated by occasional hornfelsHornfels
Hornfels is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat of intrusive igneous masses and have been rendered...
es along its edges. Like the very similar Saut-du-Saumon orthogneiss the Corgnac granite also intruded during the middle Ordovician about 470 million years ago. The granite was overprinted at about 350 million years ago during the regional metamorphism under retrograde conditions (Chlorite zone). Its chemical composition defines it as a subalkaline monzogranite.
Two very different facies can be distinguished in the Corgnac granite:
- cataclastic granitic facies in the south.
- porphyroclastic orthogneissic facies in the north.
The equigranular, sometimes porphyric, grey to rose-coloured granitic facies contains the following minerals:
- interstitial quartz
- plagioclase – often zoned, of basic composition
- microcline – often perthiticPerthitePerthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar . Typically the host grain is orthoclase or microcline, and the lamellae are albite...
- biotite
Accessory minerals are muscovite, zircon, 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...
and opaques.
All these minerals were altered during the greenschist facies retromorphism. Biotite and quartz for instance were broken cataclastically, plagioclase was invaded by muscovite and clinozoisite and rutile
Rutile
Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
needles exsolved from biotite. The granitic facies has produced several smaller, porphyric, microgranitic apophyses.
The orthogneissic facies is derived from the granitic facies, it only underwent a stronger ductile deformation. The facies presents itself now as a banded augengneiss with amygdular eyes of microcline and plagioclase surrounded by foliation minerals like very fine quartz, albite, granular clinozoisite and mica lamellae. The biotite is often chloritized. The shear sense has not been determined, but most likely is right-lateral judging by the Corgnac granite being part of the southern section of the Thiviers-Payzac Unit.
Estivaux Granite
The synkinematic Estivaux granite is a calc-alkaline massif of magmatic origin that was deformed heterogeneously about 346 million years ago during the TournaisianTournaisian
The Tournaisian is in the ICS geologic timescale the lowest stage or oldest age of the Mississippian, the oldest subsystem of the Carboniferous. The Tournaisian age lasted from 359.2 ± 2.5 Ma to 345.3 ± 2.1 Ma...
. Like the Saut-du-Saumon orthogneiss its shape is also squid-like developing tentacular arms at its northern gneissic and also mylonitic end. The granite is about 8 kilometers long in the NW-SE direction and 3 kilometers wide across strike. In the northeast the left-lateral Estivaux strike-slip fault separates the massif from rocks of the Upper Gneiss Unit. In the southwest it is surrounded by the Thiviers sandstone. Its southeastern limited is outlined by the Clan river.
The granite develops four different facies:
- melanocratic facies.
- leucocratic facies.
- white facies.
- pink facies.
Its mineralogy (in the melanocratic facies) shows the following composition:
- As porphyroclasts:
- 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...
– can attain a grain size of 4 millimeters, the porphyroclasts are partially broken and filled in by microapliteApliteAplite 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...
.
- orthoclase
- Essential minerals in the submillimeter to millimeter range are:
- quartz
- microcline
- plagioclase (albite/oligoclase)
- biotite – also as porphyroclasts, with inclusions of zircon
- muscovite
- green hornblende – also as porphyroclasts
- Accessory minerals are
- sphene
- 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...
- myrmekiteMyrmekiteMyrmekite 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...
The melanocratic facies contains lots of mafic enclaves and 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...
. The white and the pink facies are a more fine-grained variation of the melanocratic facies, they also lack hornblende and sphene. Their mutual colour difference is due to the coloration of the feldspars, the pink facies most likely being richer in 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...
. The leucocratic facies
can be regarded as a strongly sheared leucogranite
Leucogranite
In geology, leucogranites are amongst the youngest intrusions related to anatexis of continental crust anywhere in the world. Leucogranites are commonly found in deformed metapelitic/metagraywacke sequences that have been thrusted over basements during crustal thickening associated with continental...
that is very rich in muscovite.
The granite therefore possesses a pronounced gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
in deformation and in mineral alignment as one progresses from west to east. Approaching the S-C mylonitic, sinistral Estivaux Fault the little deformed melanocratic facies yields to the strongly deformed leucocratic facies, at the same time the contents in microaplite (representing a residual melt) descend from 20 % to merely 5 %.
The shear sense in the Estivaux granite is uniformly sinistral.
Structural organisation
The entire Thiviers-Payzac Unit is intensively foldedFold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
. Similar as in the Génis Unit the folding is tight and upright and the wavelength even somewhat shorter (100 to 125 metres, but can increase in the south to about 200 metres). The more or less horizontal fold axes strike WNW-ESE (N 110, west of the Loyre river). The stratification
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
(S0) is steeply inclined (around 80 °) and north or south dipping. Parallel to the folds axial plane a recognisable schistosity (S1) has developed underlined by newly formed minerals. The tight folding is overprinted by a second fold generation of open folds that have generated a very long wavelength (about 2 kilometers) series of synclines and anticlines. The axis of the first syncline is situated right next to the South Limousin Fault, followed by the first anticline underneath Saint-Mesmin. The second, central syncline is outlined by the trace of the Puy-des-Âges quartzite and the second anticline runs through Saint-Cyr-les-Champagnes
Saint-Cyr-les-Champagnes
Saint-Cyr-les-Champagnes is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-External links:* *...
.
A crenulation lineation has also formed which trends more or less parallel to the folds. Newly formed metamorphic minerals also align themselves preferably in this direction.
After reaching the Loyre river the Thiviers-Payzac unit makes a definitive right-hand turn and all the structural elements swing into the NW-SE direction (N 135). This new trend is followed till the unit finally disappears just east of Brive.
Metamorphism
The Thiviers-Payzac Unit experienced regional metamorphismMetamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
under low to medium grade. Its upper reaches show upper greenschist facies conditions, the lower sections reached already lower amphibolite facies conditions. The presence of chlorite and chloritized biotite in shear bands and in pressure shadows indicates retromorphism, which has been known in the southern Limousin for quite a while.
Structural evolution
Like the Génis Unit the Thivier-Payzac Unit was also affected by upright ductile shearing. But unlike the Génis Unit it does not possess a uniform shear sense. In the south the unit shows the same dextral shear sense as the Génis Unit right to the anticline at Saint-Mesmin. Farther to the northeast this is superseeded by a mixed zone where both shear senses are present. When finally the northern outcrop band of the Thiviers sandstone is reached only sinistral shearing is preserved in the rocks. This sinistral shear sense becomes clearly evident close to the Estivaux Fault. Here the shear coefficient γ takes on the value of 5.4 which represents an accumulated left-lateral displacement of about 30 kilometers. As regards the granitoids the Saut-du-Saumon orthogneiss is affected by two shear senses whereas the Estivaux granite has been deformed solely left-laterally.The following microtectonic methods underline the sinistral shear sense in the northern outcrop area of the Thiviers sandstone:
- analysis of quartz
axes – displaced in the direction of shearing – sinistral. - interpretation of asymmetric quartz pressure shadows around garnet – clearly sinistral.
- interpretation of porphyroclasts of the σ-type (in the Thiviers greywacke).
- interpretation of shear bands - clearly sinistral (in the Donzenac schist near AllassacAllassacAllassac is a commune in the Corrèze department in central France.-Population:...
). - interpretation of quartz pressure shadows on biotite porphyroblastPorphyroblastA porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape....
s – sinistral.
North of Saint-Cyr-les-Champagnes the neighbouring Upper Gneiss Unit also shows a left-lateral shear sense (sinistrally sheared quartz lenses).
In the Donzenac Schist, where both shear senses are present, one can observe how the dextral shearing is overprinting the sinistral shearing. The sinistral movements happened therefore later. On left-laterally sheared, sigmoidal porphyroblasts of biotite dextral shear bands are superimposed on; additionally retrograde 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:...
formed in these late shear bands.
The pervasive shearing is responsible for the folding in the Thiviers-Payzac Unit; the folds can be interpreted as tear folds that were rotated into the maximum stretching direction in a transpressive
Transpression
Transpression is a geological term used to describe a region of the Earth's crust that experiences strike-slip shear and a component of shortening, resulting in oblique shear. Transpression typically occurs at a regional scale, such as plate boundaries that have an oblique convergence. More...
, ductile shear zone.
The tectonic movements didn't stop at the close of the ductile deformations. For instance in the brittle realm lots of small, mainly NE-SW-striking strike-slip faults were initiated with left-lateral displacements in the order of about 500 meters – an exception being the Dussac Fault north of Lanouaille which has a left-lateral displacement of nearly 6 kilometers!
Timing of the deformations
The timing of the tectonic movements is based mainly on comparisons with lithologically and structurally similar terranes in the Armorican MassifArmorican Massif
The Armorican Massif is a geologic massif that covers a large area in the northwest of France, including Brittany, the western part of Normandy and the Pays de la Loire. Its name comes from the old Armorica, a Gaul area between the Loire and the Seine rivers...
(Chantonnay Syncline in the Vendée
Vendée
The Vendée is a department in the Pays-de-la-Loire region in west central France, on the Atlantic Ocean. The name Vendée is taken from the Vendée river which runs through the south-eastern part of the department.-History:...
) and in the Rouergue. In the southern Armorican Massif the dextral shearing motions are timed as Namurian and Westphalian (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...
till Moscovian
Moscovian (Carboniferous)
The Moscovian is in the ICS geologic timescale a stage or age in the Pennsylvanian, the youngest subsystem of the Carboniferous. The Moscovian age lasted from 311.7 ± 1.1 to 306.5 ± 1.0 Ma, is preceded by the Bashkirian and is followed by the Kasimovian...
), i.e. 325 till 305 million years ago. One can therefore propose for the deformations in the Thiviers-Payzac Unit of the Bas-Limousin, which is regarded as the southeastern prolongation of the Vendée, a middle to late Carboniferous age. Similar ages for the leucogranites in the northern and central Limousin seem to support this assumption.
Yet some radiometric datings
Radiometry
In optics, radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which characterize the light's interaction with the human eye...
using the argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
-method find much earlier Tournaisian ages for the intrusion of the Estivaux granite and for the motions along mylonite zones within the Saut-du-Saumon orthogneiss. These findings imply a tectonic phase in the southern Limousin already during Mississippian times (Bretonic Phase).
Sources
- Peterlongo, J.M. (1978). Massif Central. Guides Géologiques Régionaux. Masson. ISBN 2-225-49753-2
- Roig, J.-Y., Faure, M. & Ledru, P.(1996). Polyphase wrench tectonics in the southern French Massif Central: kinematic inferences from pre- and syntectonic granitoids. Geologische Rundschau, 85, pp. 138–153