Saint-Silvain-Bas-le-Roc leuco granite
The Saint-Silvain-Bas-le-Roc leukogranite is a small intrusion of leukogranite that penetrated into the Marche Terran on the northern edge of the Massif Central at the turn of the Carboniferous / Permian . It is characterized by spectacular weathering forms .
etymology
The leuco granite is named after its type locality , the commune of Saint-Silvain-Bas-le-Roc in the Creuse department .
geography
The small, almost circular leukogranite intrusion, slightly elongated in the east-northeast direction, with a diameter of 4.7 to 6.5 kilometers, has its center 3.5 kilometers north of Toulx-Sainte-Croix and 3.5 kilometers south of Boussac . In the Pierres Jaumâtres it reaches an altitude of 581 meters.
geomorphology
The Saint-Silvain-Bas-le-Roc leuco granite weathered to form impressive, cubic meter-sized round shapes that form a sea of rocks in the Pierres Jaumâtres . Some of the blocks are covered by vertical channels or flutes, which are very reminiscent of channel carts in their shape . Pseudokarst forms such as Kamenitzas (bowl carts, circular basins created by precipitation) and their drainage channels can also be observed.
Due to the orogenesis of the Alps and the Pyrenees, the already leveled Massif Central was lifted out again at the beginning of the Tertiary , so that the erosion could start again. Favored by the prevailing subtropical climate, the weathering attacked the leuco granite along its fracture systems and penetrated several meters deep into it. Its feldspars decompose in situ into clay minerals. For the formation of the rocky seas, however, the cold periods of the Pleistocene were ultimately responsible, as their increased precipitation coupled with extreme temperature fluctuations (frost cracking) further broke the structure of the leucogranite, washed out the clay minerals and thus exposed the individual blocks. The formerly angular blocks then gradually rounded off. The last 100,000 years, ie the most recent Ice Age of the Würm, were particularly decisive for the formation process .
geology
The leuco granite cuts through the Jalesches granite and the Chanon granite with its three facies in the west and southwest , but is intruded in the south by the Toulx-Sainte-Croix leuco granite . Chanon granite appears again in the east. In the north it hides the Marche fault and then borders on the Boussac-Migmatit .
The Saint-Silvain-Bas-le-Roc leuco granite sits directly on the Marche Fault - the terrang boundary between the migmatitic Boussac terran in the north and the Marche terran in the south. It so to speak scarred the boundary of the terrangie and can therefore be viewed as stitching pluton . The Marche Fault is part of the Marche-Combrailles Fault Zone - a significant shear zone in the northern Massif Central and extension of a branch of the South Armorican Shear Zone (SASZ for short).
About 10 kilometers further east-northeast follows the geochemically quite comparable Grand Roche leukogranite - a circular intrusion 3 to 4 kilometers in diameter that also masks the Marche fault.
Petrology
The Saint-Silvain-Bas-le-Roc leuco granite is a mostly gray, occasionally pink, coarse-grained two-mica granite. The average grain sizes are between 2 and 5 millimeters, with feldspar phenocrystals between 2 and 3 centimeters in size. The structure is uniform.
mineralogy
The leuco granite is made up of the following minerals :
Apatite , zircon and opaque minerals act as accessories .
The idiomorphic alkali feldspar has large perthites , shows Carlsbad twins and contains inclusions of biotite and plagioclase. Small alkali feldspar crystals with microcline twins can be seen in the mesostasis of the rock . These crystals are sometimes surrounded by albite . The plagioclase forms crystals in the centimeter range. The hypidiomorphic quartz is sometimes slightly bluish and globular. The relatively rare biotite is light brown and only slightly pleochroic. It can appear as curved leaves up to one centimeter in size. It is often surrounded by muscovite. The muscovite lamellae are associated with biotite and can appear as inclusions or as an alteration product in feldspars. In leuco granite, the muscovitization of the alkali feldspar is often very advanced. The tourmaline appears in the contact area with the Toulx-Sainte-Croix leukogranite as a xenomorphic gusset filler. It shows no association with biotite.
Chemical composition
Listed for comparison are surrounding leuco granites such as Jalesches leuco granite, Grand Roche leuco granite and Toulx-Sainte-Croix leuco granite, all of which are significantly richer in SiO 2 .
The Saint-Silvain-Bas-le-Roc leukogranite has undergone a much stronger albitization and muscovization than the Jalesches leukogranite, which is even exceeded by the Toulx-Sainte-Croix leukogranite. The formation of tourmaline at the edge of the leukogranite was induced by the Toulx-Sainte-Croix leukogranite.
| Oxide wt.% |
Saint-Silvain-Bas-le-Roc leuco granite |
Jale's leuco granite |
Grand Roche leuco granite |
Toulx-Sainte-Croix leuco granite |
|---|---|---|---|---|
| SiO 2 | 68.83 | 72.90 | 74.29 | 74.73 |
| TiO 2 | 0.13 | 0.25 | 0.09 | 0.06 |
| Al 2 O 3 | 17.14 | 14.70 | 14.73 | 14.32 |
| Fe 2 O 3 | 1.55 | 1.97 dead | 1.00 | 0.72 dead |
| FeO | 0.93 | 0.72 | ||
| MnO | 0.05 | 0.05 | 0.02 | 0.06 |
| MgO | 0.21 | 0.39 | 0.18 | 0.07 |
| CaO | 0.15 | 0.54 | 0.18 | 0.14 |
| Na 2 O | 3.72 | 3.50 | 3.52 | 3.96 |
| K 2 O | 5.45 | 4.58 | 4.44 | 4.24 |
| P 2 O 5 | 0.49 | 0.44 | 0.42 | |
| dehydration | 1.51 | 1.49 | 1.27 | 0.92 |
The Saint-Silvain-Bas-le-Roc leukogranite is a quartz-normative and therefore silicon- saturated rock. In addition, it is corundum normative and saturated with aluminum, ie peraluminos and with an A '/ F = 3.04 at the same time hyperaluminos. The leuco granite is also depleted in total iron and very much depleted in MgO and CaO. The alkalis and also P 2 O 5 show quite high concentrations. The sum of the alkalis Na 2 O + K 2 O is quite high at 9.17 percent by weight and identifies the rock as alkali granite .
geophysics
Modeling based on gravity measurements (with a density of 2600 kilograms per cubic meter for the Saint-Silvain-Bas-le-Roc leuco granite) confirm the flattened teardrop shape of the intrusion. It appears rootless and begins at a depth of 3 kilometers. The contact to the Boussac-Migmatite dips relatively flat to the north, whereas the contact to the Toulx-Sainte-Croix-Leukogranite is steep in the south. The contact with the Chanon granite is steep and sharp.
The leuco granite produces a negative residual gravity anomaly of -7.5 milligals with an overall anomaly of -46 milligals - the lowest values in the Marche Terran.
tectonics
The Saint-Silvain-Bas-le-Roc leuco granite is practically non-ductile. However, it was stressed in the brittle area and is criss-crossed by numerous N 080 striking fractures. The fractures run more or less parallel to the Marche Fault and are most likely caused by late movements at this Terrang boundary (possible faults).
Age
JC Baubron was able to determine an age of 294 ± 19 million years on feldspars using the rubidium-strontium method - a sub-permean age from the sacmarium . The Saint-Silvain-Bas-le-Roc leukogranite is thus a very recent manifestation of the late to post-orogenic leukogranite generation of the Massif Central.
literature
- M. Cohen-Julien et al .: Feuille Boussac (618) . In: BRGM (ed.): Carte géologique de la France à 1/50 000 . Orléans 1991.
- JC Gauthier: Evolution granitique, développement des granites à deux micas et géochimie des alcalins dans la Marche orientale. 1st part: l'évolution granitique . In: Sci. de la Terre, Nancy . XVIII, 4, 1973, p. 318-351 .
- P. Isnard and H. La Roche: De la composition chimique à la composition minéralogique des granites à deux micas. Traitement par le calcul automatique dans les études en grande série. Application au massif de la Marche orientale . In: Sci. de la Terre, Nancy . XIII, 1, 1968, pp. 79-97 .
Individual evidence
- ↑ M. Cohen-Julien et al.: Feuille Boussac (618) . In: BRGM (ed.): Carte géologique de la France à 1/50 000 . Orléans 1991.
- ↑ Lemaire, D., Quenardel, JM and Debeglia, N .: Apport de la gravimétrie dans l'élaboration de modèles de mise en place des ensembles magmatiques du Nord du Massif central français. II - Le massif granitique de la Marche orientale . In: CR Acad. Sci., Paris, (II) . tape 305 , 1987, pp. 1073-1079 .
Coordinates: 46 ° 18 ′ 47.5 " N , 2 ° 13 ′ 35.5" E