Engastine Mafit
The Engastine-Mafit is a Cambrian formation of the French Massif Central . The formation forms part of the Thiviers-Payzac unit .
etymology
The Engastine-Mafit, French Complexe basique d'Engastine , is named after its type locality - the hamlet of Engastine in the French commune of Allassac in the west of the Corrèze department .
Geography and geology
The Engastine-Mafit, which can reach a maximum of 500 meters, is only unlocked in the Thiviers-Payzac unit. It emerges in two separate outcrop areas, which are separated from each other by flat sediments of the Upper Carboniferous . In its northwestern occurrence, it follows a 10 km long and approximately 800 meter wide band, which, however, widens to 1,600 meters north of Juillac . This band begins at Auvézère south of Payzac and ends east of Juillac. In its southeastern occurrence, from the Loyre to the southeast of Vignol, it follows a band around 11 kilometers long and a maximum of 1200 meters wide to immediately south of Donzenac .
Stratigraphically , the Mafit usually superimposes the Donzenac slate into which it is folded in the Puy-des-Âges syncline . But it can also reach down to the Thiviers sandstone , for example east of Allassac. Due to the synclinal folding, the Engastine Mafit is followed again by the Donzenac slate and then the Puy des Âges quartzite . The latter can also lie directly over the Mafit in places.
Petrology
The Engastine-Mafit is available as alternating green slate and amphibolite layers in which several metadolerite or metagabbrol layers are interposed. In the footwall seem greywacke and in the hanging wall to sporadic chlorite set.
The very fine-grained, light to dark green slate ( grain size 100 to 150 μ) emerged from a former orthoprasinite and, in addition to plagioclase ( oligoclase / andesine ) and a little biotite, mainly amphibole ( hornblende ) and epidote ( clinozoisite ), quartz and calcite can be accessories and opaque minerals are added. The fibrous or radiant hornblende is rich in iron and magnesium. The uniformly grained plagioclase, which tends to be polygonal, occurs xenomorphically and is more or less flattened in the plane of the foliage. The relatively rare biotite is brown-red and is occasionally in the form of chloritized spindle-shaped lamellae.
The metadolerites or metagabbros , on the other hand, are coarse-grained (er) and consist of green hornblende and basic, saussuritized plagioclase. Original plagioclase ridges with a diablastic structure can still be recognized as a relic - which indicates the doleritic or gabbroid character.
Millimeter-sized quartz crystals can be seen with the naked eye in the Grauwacken. Under the microscope, clasts of quartz and plagioclase of different grain sizes appear, to which hornblende, epidote and calcite can join. The fine-grained matrix is made up of quartz, feldspar, chlorite and epidote. The silky, shiny, fine-grain chlorite slates are green or reddish in color, their lepidoblastic structure consists mainly of quartz and chlorite.
Chemical composition
| Oxide wt.% |
Greenschist | Amphibolite 1 | Amphibolite 2 | Amphibolite 3 |
|---|---|---|---|---|
| SiO 2 | 49.50 | 49.50 | 51.20 | 53.00 |
| TiO 2 | 2.10 | 2.70 | 1.71 | 1.59 |
| Al 2 O 3 | 16.04 | 16.45 | 13.40 | 16.50 |
| Fe 2 O 3 | 10.60 dead | 12.10 dead | 1.85 | 2.65 |
| FeO | 5.80 | 6.30 | ||
| MnO | 0.17 | 0.18 | 0.14 | 0.15 |
| MgO | 11.04 | 6.34 | 4.30 | 5.20 |
| CaO | 1.03 | 7.56 | 9.10 | 5.45 |
| Na 2 O | 2.41 | 2.16 | 4.10 | 4.65 |
| K 2 O | 0.99 | 0.90 | 0.63 | 1.05 |
| P 2 O 5 | 0.27 | 0.16 | ||
| H 2 O - | 0.25 | 0.20 | ||
| H 2 O + | 1.68 | 2.01 | 3.45 | 2.70 |
The SiO 2 content of the mesocratic Engastine Mafit fluctuates around 50 percent by weight and reveals the basaltic character of the rock. The Mafit is also sub-alkaline based on the alkalis. The concentration of Na 2 O can be quite high and indicates a spilitization of the rock. The K 2 O content of 1 percent by weight indicates a calcareous character (medium K). Overall, the Engastine-Mafit is likely to be composed predominantly of tholeiitic tuffs - which can still be recognized as such in less metamorphically stressed sectors.
literature
- Pierre-Louis Guillot and others: Feuille Juillac . In: Carte géologique de la France at 1/50 000 . BRGM, 1978.
- Pierre-Louis Guillot and others: Feuille Thiviers XIX-33 . In: Carte géologique de la France at 1/50 000 . BRGM.
- Pierre-Louis Guillot: La série métamorphique du Bas Limousin de la vallée de l'Isle la vallée de la Vézre, le socle en bordure du bassin aquitain. (Doctoral thesis) . Univ. d'Orléans, France, 1981, pp. 391 .
- JM Peterlongo: Massif Central . In: Guides Géologiques Régionaux . Masson, 1978, ISBN 2-225-49753-2 .