Génis Greenschist
The Génis Greenschist is a geological formation from the Lower Devonian of the French Massif Central . The formation forms part of the Génis unit .
etymology
The Génis green slate is named after its type locality , the French community Génis in the northeast of the Dordogne department .
Geography and geology
The Génis green slate (on the opposite geological map in light green) is only exposed in the heart of the Génis syncline . The outcrop area forms a pointed triangle 6 kilometers long and 3 kilometers wide, opening to the south-southeast. The green slate overlays the Génis sericite slate (in yellow) in the syncline . The formation dips to the south-southeast under Permian red sediments of the Briver Basin . To the north-east of Anlhiac it is covered by the Lias of the Aquitaine Basin or it rises in the syncline axis west of the Auvézère opposite the Serizitschists.
Petrology
The Génis Greenschist is a very varied formation made up of orthoprasinites , metadolerites , gabbros , tuffs and silica slates .
The orthoprasinites make up the bulk of the green slate. They are very massive, fine-grained, sometimes schisty rocks with a greenish to purple color. Its former volcanic character can still be recognized in places by bubbles, variola and pillow lavas . The original magmatic structure is aphyric. Microliths made from plagioclase form a network in which chlorite , actinolite and, more rarely, epidote and calcite can be recognized. However, this magmatic structure was largely destroyed by the metamorphosis. The metamorphic paragenesis now consists of small-grain albite , packet-like chlorite, occasionally zoned epidote, radiant actinolite, pumpellyite , calcite and opaque minerals. The metamorphic structure is fine-grained and granoblastic, sometimes also banded in layers, recognizable by the preferred orientation of the opaque minerals.
The metadolerites are very hard, fine to medium-grain rocks with a dark green color. In the coarser varieties, prismatic pyroxene appears in the millimeter range. The mineral composition of the fine-grained varieties - diabase-like dolerite - can only be seen under the microscope. Igneous remains are light pink augite (in the vicinity of the diopside or salite ) and plagioclase, which is saussuritized as rectangular slats. Within the formation, a development of the dolerite towards schisto amphibolite rocks with a nematoblastic structure can be observed. The resulting foliation is underlined by the alignment of the actinolite needles, the chlorite leaves, the epidote sections and the fine-grained albites.
The Metagabbros are very massive, dark green to black rocks. They appear only as smaller bodies within the orthoprasinites and metadolerites, the largest of which is on the right side of the Auvézère valley below Moulin du Pont . Their grain size varies from millimeters to centimeters. The minerals are white or greenish plagioclase and dark green to black pyroxene. The structure is granular, the plagioclase is saussuritized and the composition of the pyroxene is identical to the pyroxene of the metadolerite. The metamorphic paragenesis is also the same as that of the metadolerites.
The tuffs are schisty and very rich in albite. In the case of Sarrazanas , for example, they show a slate matrix of silky sheen and greenish to purple hues. Small feldspar eyes stand out, which are colored pink or white and are only the size of the head of a pin. In some places the tuffs are brecciated. Under the microscope, the matrix consists of sericite and chlorite and the eyes of hypidiomorphic to idiomorphic, sericized plagioclase prisms (almost pure albite).
The deep red silica slates are very hard and extremely fine-grained rocks. They can only be found within the formation as small, lenticular bodies in the meter range. These are jasper-like micro quartzites, which are composed of a mosaic of tiny quartz grains of 10 to 20 μ. Some samples show streaks of fibrillar silica. To the south of the hamlet of Vieillecroze , remains of circular to elliptical radiolarians were discovered in a meter-sized lens . However, their poor state of preservation prevented them from being assigned to a species.
Chemical composition
| Oxide wt.% |
Greenschist 1 | Greenschist 2 | Greenschist 3 | Greenschist 4 | Greenschist 5 |
|---|---|---|---|---|---|
| SiO 2 | 43.80 | 45.60 | 46.20 | 47.20 | 48.40 |
| TiO 2 | 2.21 | 1.62 | 2.47 | 2.03 | 2.28 |
| Al 2 O 3 | 16.00 | 17.10 | 15.05 | 14.65 | 14.80 |
| Fe 2 O 3 | 5.25 | 2.80 | 2.40 | 3.85 | 2.35 |
| FeO | 8.35 | 8.55 | 8.90 | 8.65 | 6.95 |
| MnO | 0.21 | 0.18 | 0.18 | 0.18 | 0.16 |
| MgO | 6.70 | 7.85 | 7.60 | 6.05 | 5.65 |
| CaO | 10.00 | 9.10 | 9.45 | 9.95 | 11.05 |
| Na 2 O | 2.70 | 3.45 | 3.20 | 3.05 | 4.20 |
| K 2 O | 0.25 | 0.06 | 0.15 | 0.18 | 0.05 |
| P 2 O 5 | 0.21 | 0.16 | 0.23 | 0.19 | 0.25 |
| H 2 O - | 0.05 | 0.15 | 0.07 | 0.05 | 0.20 |
| H 2 O + | 4.00 | 3.85 | 3.70 | 3.30 | 2.80 |
The mafic green schists are oceanic tholeiites due to their chemistry - predominantly basaltic , but occasionally also andesitic . They show a very strong secondary enrichment in Na 2 O and are therefore classified as Spilite . This spilitization is also present in other metavolcanites in the Bas-Limousin and is probably of deep hydrothermal origin.
tectonics
The Génis greenschists is like the other formations of génis unit within the Génis-syncline additional secondary verfaltet . The folds are upright, narrow and have a wavelength of around 150 meters. The vertical fold axis planes strike east-southeast (N 110). The fold axes descend at 10 ° to ESE. This ductile fold structure, interpreted as tension folds, is left brittle by a north-east trending transverse fracture near Paquetie .
Overall, the Génis unit with its layer package can be viewed as part of a transpressive, dextral shear zone .
See also
literature
- P. - L. Guillot et al.: Feuille Juillac . In: Carte géologique de la France at 1/50 000 . BRGM, 1978.
- JM Peterlongo: Massif Central . In: Guides Géologiques Régionaux . Masson, 1978, ISBN 2-225-49753-2 .
Individual evidence
- ↑ P. - L. Guillot and others: Feuille Juillac . In: Carte géologique de la France at 1/50 000 . BRGM, 1978.