Saint-Mathieu leuco granite

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The Saint-Mathieu-Leukogranit is located in France on the northwestern edge of the Variscan Massif Central . It originated in the Upper Carboniferous .

geography

The leuco granite is named after Saint-Mathieu , a small town in the south of the Haute-Vienne department . The massif is about 30 kilometers long and a maximum of 8 kilometers wide. From its northern end, a 14-kilometer-long, only up to 3-kilometer-wide band extends to the southwest. The massif takes up about 150 square kilometers of surface area. Its lowest point is 108 meters above sea level near Montbron , its highest point is 470 meters above sea level east of Milhaguet .

geology

Overview geological map of Saint Mathieu Cathedral

The leuco granite massif (unit SML on the geological map) frames the Piégut-Pluviers-Granodiorite ( PPG ) in the north and east, with which it forms an arch in the basement , the so-called Saint-Mathieu-Dom . It can be divided into several sub-areas:

  • Central section, actual leuco granite, consisting of normal facies and fine-grain facies ( Champniers-et-Reilhac-leuco granite ) in the northeast and east of the Piégut-Pluviers-Granodiorite.
  • Southern section ( Saint-Saud-Leukogranit ) with separate apophysis.
  • Northwestern and northern framing of fine-grain facies and C / S facies ( Roussines granite ), very heavily tectonically stressed.

The central and southern sections are in turn bounded in the east by the parautochthonous mica slate unit (unit PMU ). With the mica schists, structurally the lowest-lying unit in the ceiling of the Massif Central, the leuco granite has a close genetic connection. In places, hectare to kilometers of mica slate lenses and lines can be found in the leuco granite. Conversely, isolated occurrences of leucogranite can still be found relatively far in the adjacent mica slate band to the east. The leucogranite should therefore represent an anatectic differentiation product of the mica schist.

For the general situation, the ceiling stack in the western Massif Central is briefly listed (from structurally higher to structurally lower):

The northwestern band is linked to very strong shear movements, but these are not horizontal movements, but rather the relatively steep Montbron Fault , which in turn is associated with the Cordelle Fault , a displacement with a sinistral component. In the northwestern band, very clear coplanar structures have developed in some cases.

Age

The cooling ages of the Saint-Mathieu leukogranite were dated to 315 ± 17 and 304 ± 17 million years BP using the rubidium-strontium dating ( Bashkirian and Kasimovian ). The leuco granite thus seems to be a little younger than the Piégut-Pluviers granodiorite. However, the contact conditions between the two granitoids do not allow a final judgment about the age. Both rock types form a fine-grained facies in the contact area.

Petrological facies

The Saint-Mathieu leukogranite is not homogeneous, but is composed of several types of facies. The following types of rock can essentially be distinguished:

  • coarse-grained normal facies
  • fine-grained facies
  • coplanar fine-grained facies

In this context, the Roussines granite, which has a clear, tectonically caused C / S structure and is closely related to the coplanar fine-grain facies, as well as the Cheronnac leukogranite ( CL ) in the north with clear foliation, should also be mentioned .

Coarse-grained normal facies

Coarse-grained normal facies of the Saint-Mathieu-Leukogranite
Coarse-grained facies, river scree of the Saint-Saud-Leukogranite from the Dronne

This facies is the actual Saint Mathieu leuco granite. In terms of area, it takes up the largest part of the massif, especially if the Saint-Saud leuco granite is included, which is only a slightly different variety. The rock is coarse-grained, the grain sizes range between three and four millimeters, the distribution of the grains is usually uniform, but can be regulated in areas of movement. The normal facies has the following modal mineral inventory:

  • Quartz - 33 percent by volume - polycrystalline aggregates up to five millimeters in size
  • Orthoclase - 28 percent by volume - perthitic , phenocrystals can reach three to five centimeters, sometimes up to ten centimeters, zoned inclusions of quartz, plagioclase and mica
  • Plagioclase - 26 percent by volume - oligoclase , discreetly zoned, core 15–20 , margin 9–12 , often sericized
  • Muscovite - 9 percent by volume - diamond-shaped and hexagonal crystals, sometimes epitaxial adhesions with biotite and symplectic adhesions with quartz
  • Biotite - 4 percent by volume - often chloritized

Accessory occur:

The rock is oversaturated with SiO 2 , corundum normative and peraluminous . It belongs to the calcareous series and shows a monzonite tendency. The Saint-Saud leuco granite takes on a porphyry character near Lacoussière .

Fine-grain facies

Fine-grain facies of the Saint-Mathieu-Leukogranite

The transition from the coarse-grained to the fine-grained facies, often also referred to as Champniers-et-Reilhac leukogranite , is progressive. The fine-grained facies form a relatively thin, deca to hectometric contact zone with the Piégut-Pluviers granodiorite, it is andalusite-bearing. In the southern part it also forms the western limit of the massif, but here it is in contact with paragneiss. Around Champniers-et-Reilhac it experiences a greater expansion (known as the sillimanite-bearing Champniers-et-Reilhac leuco granite ). In the northwestern band it is very closely associated with the Roussines granite . The fine-grained facies can occasionally have a structural adjustment. Dark, mica-rich inclusions are relatively rare.

The grain sizes are generally between one and two millimeters, but there are definitely transitions to the coarse-grained facies, and isolated porphyry passages can also be observed. The fine-grained facies contain the following minerals:

  • Quartz - 30 percent by volume - scattered individual grains or as a gusset filler
  • Orthoclase - 32 percent by volume - perthitic, allotriomorphic
  • Plagioclase - 30 percent by volume - albite or oligoclase , partly idiomorphic, partly allotriomorphic
  • Biotite - 4 percent by volume - millimeter-sized sequins
  • Muscovite - 3 percent by volume - diamond-shaped, associated with biotite, often as a gusset filler, with inclusions of quartz and aluminum silicate

Accessory occur:

  • Andalusite - greyish-pink, only occasionally occurring, partly muskowitized or sericitized, inclusions in muskowite
  • Sillimanite - as fibrolite needles, associated with biotite (epitaxy), prismatic inclusions in plagioclase, muscovite or quartz, often serialized
  • Cordierite - very rare, mostly pinitized
  • Corundum - very rare
  • Apatite
  • Tourmaline
  • Zircon

The fine-grain facies are oversaturated with SiO 2 , and are corundum normative and hematite normative . It belongs to the calcareous series with an alkaline, monzonitic tendency. The rock is peraluminos and an S-type granitoid.

Coplanar fine-grained facies

The anatectic Roussines granite. Below the tape measure, tectonically caused SC structure with leucogranitic and paragneissic layers, interspersed with deferment creeps. Above a larger leucogranitic lens.

The coplanar fine-grained facies is bound to the Montbron Fault and occurs only in the northwestern band. It has a very pronounced, northeast-southwest-trending structure regulation. It also alternates with trains of Roussines granite. The SC structure in the Roussines granite reveals a right-shifting shear sense. Since it its part of C 'areas of a Abschiebungskrenulation ( English extensional crenulation cleavage is) passes, one more transtensive strain has the shear zone at a late stage have experienced.

Chemical composition

The chemical composition of the individual petrological facies is illustrated by the following analysis average values:

Oxide
weight percent
Coarse-grained
facies
Fine-grain
facies
Saint-Saud
porphyry facies
Roussines
granite
CIPW norm
percent
Coarse-grained
facies
Fine-grain
facies
Saint-Saud
porphyry facies
Roussines
granite
SiO 2 72.82 73.34 70.22 71.52 Q 32.12 31.30 28.13 29.04
TiO 2 0.35 0.17 0.33 0.17 Or 26.05 29.22 25.91 30.84
Al 2 O 3 15.44 15.08 15.50 15.26 From 29.63 30.05 29.49 31.34
Fe 2 O 3 0.59 0.93 0.73 1.12 On 4.84 3.34 7.49 3.33
FeO 1.10 0.52 1.74 0.53 C. 3.13 2.65 2.40 2.13
MnO 0.04 0.03 0.05 0.04 Hy 2.46 1.70 4.49 0.95
MgO 0.68 0.68 0.86 0.38 Mt 1.06 1.16 1.33 1.18
CaO 1.05 0.69 1.57 0.81 Il 0.67 0.32 0.62 0.32
Na 2 O 3.50 3.55 3.46 3.65 Ap 0.13 0.03 0.13 0.26
K 2 O 4.41 4.95 4.36 5.13 Hm 0.35 0.58
P 2 O 5 0.06 0.01 0.06 0.11
Mg # 0.53 0.72 0.49 0.58
A '/ F 1.07 1.37 0.62 1.55
A / Na + K + approx 1.24 1.21 1.17 1.13

All facies are very strong peraluminos or hyperaluminos (the CIPW standard leads corundum and A '/ F> 0.33) and clearly belong to the S-type (A / Na + K + Ca> 1.1).

The strong chemical similarity of the coarse-grained facies with the Saint-Saud leuco granite on the one hand and the fine-grained facies with the Roussines granite on the other hand is striking. The reason for this is likely to be found in different parent rocks.

Dike rocks and pegmatites

Microgranite dikes

The Saint-Mathieu leuco granite is penetrated by numerous grayish-pink micro-granite veins. The corridors can be several kilometers long. They are mostly aligned N 000, N 020 or N 160, more rarely also N 045 or N 135. Their comparatively younger age is underlined by the fact that they pass through facies boundaries and even the contact with the Piégut-Pluviers granite without changing direction.

Lamprophyre ducts

Similar to micro-granites, lamprophyres also penetrate the leuco-granite massif as elongated, but only a few meters thick corridors. They usually only occur in the southern section, in the Saint-Saud leuco granite. These are kersantites and minettes , which have enriched uranium . Their strike direction is mostly north-south.

Quartz veins

The lamprophyre ducts are occasionally replaced by quartz ducts.

Pegmatites

Font granite from pegmatites in the Saint-Mathieu-Leukogranit

Some pegmatite occurrences also occur in the leuco granite . They mostly only contain the common minerals quartz, feldspars and mica. Very beautiful pegmatitic font granite near Lac de Saint-Mathieu is worth mentioning .

Contact metamorphosis and tourmalinization

The Saint-Mathieu leukogranite caused a contact metamorphosis in the parautochthonous mica slate unit and in the structurally higher leptynites and eye gneiss of the lower gneiss cover . In the immediate vicinity of the leucogranite, the minerals cordierite and andalusite were newly formed in the contact aureole , a little further away biotite and muscovite were recrystallized (up to a distance of five kilometers).

In addition, the mica schists in the contact area of ​​the leucogranite experienced a very strong tourmalinization in places (for example east of Milhaguet ).

tectonics

Foliation data of the Saint-Mathieu-Leukogranit in the Stereonet
The west portal of the church of Saint-Mathieu was built predominantly from the leuco granite of the same name

Similar to the Piégut-Pluviers-Granodiorit, the Saint-Mathieu-Leukogranite also reveals a foliation of tectonic origin on closer inspection . In the northwestern band along the Montbron Fault, this foliation becomes pervasive and eventually leads to the formation of a coplanar structure. Usually the foliation lies flat, the angles of incidence of the undulation usually do not exceed 35 °. The steeper angles of incidence fall into the NNW. This anisotropy indicates an internal C / S structure with plastic flow in a south-south-east to south-east direction. In the northern section, the structures pivot first to the north and then in the band to strictly north-westerly directions. This documents the strong influence of the Montbron Fault on the leuco granite that was ductile sheared into it. The angles of incidence in the shear zone are generally around 60 ° to the northwest, but are often saiger as well.

After the ductile phase had subsided and as the cooling progressed, fracture tectonics began in the leukogranite , which manifested itself in faults and a pronounced fracture system . In this late phase, there were also east-west stretching phenomena in the massif, which enabled the numerous microgranite, lamprophyte and quartz veins to take their place in the leuco granite. A large part of the steep faults and crevices follow the same directions as the dikes (maxima at N 020 and N 170), more flatter structures mainly strike N 045 and N 135. The crevices also share these directions, but also show their main maximum at N 105, which should have its origin as P-shearings in a northwest-southeast-oriented Riedel zone . This shows that the deformations continued even in the brittle state.

Final consideration

The Saint-Mathieu leukogranite is undoubtedly of magmatic origin, as evidenced by the contact aureole, by pegmatite containing script granite and, above all, by the anatectic Roussines granite. The tourmalinization indicates contact metasomatic processes. The starting rock of the coarse-grained facies and the Saint-Saud-Leukogranite in the eastern part is obviously the parautochthonous mica schist. The fine-grain facies and the Roussines granite in the western section, on the other hand, are likely to have emerged from the paragneiss. The influence of the Variscan tectonic deformations on the leuco granite can be seen everywhere, very clearly on the Roussines granite and on the coplanar facies along the Montbron fault. The tectonic movements continued into the late orogenic expansion phase of the massif and beyond.

The Saint-Mathieu leukogranite is not an isolated case in the Limousin , but belongs to a series of similar leukogranites further northeast, such as the Cognac-la-Forêt leukogranite , which penetrated along the Cordelle Fault, the Peury leukogranite , associated with the Cordelle Fault and the Nantiat fault , and the large-scale and very uranium-rich Saint-Sylvestre leukogranite north of Limoges . All of these leuco granites follow a deep crustal fracture zone with a northeast-southwest strike direction .

Photo gallery

See also

literature