Merlis serpentinite
Coordinates: 45 ° 45 ′ 0 ″ N , 0 ° 49 ′ 56 ″ E
The Merlis serpentinites are a cluster of scattered serpentinite deposits on the northwestern edge of the Massif Central in France . They emerged from mantle peridotites .
Type locality
The serpentinites are named after their type locality , the hamlet of Merlis (also spelled Merly ) belonging to the municipality of Vayres ( Haute-Vienne ) .
Geography and geology
The main occurrence is the serpentinite east of Merlis , which was previously mined in two quarries for gravel extraction. On the map, this deposit has a crescent-shaped shape that is open to the west and is a little over a kilometer long in the north-south direction, but just under 400 meters in the east-west direction.
Starting from it, several smaller occurrences in the 100 meter range follow in a garland-like direction - 3 smaller occurrences immediately north of the main mass and the hectometric occurrences of Puytreillard (west of Vayres) ( 1 ), Les Soumagnes ( 2 ) and Bonnefont (northwest of Vayres) ( 3 ) , Bellevue (southwest of Saint-Gervais ) ( 4 ), Gélisant (southeast of Verneuil ) ( 5 ) and Le Cluzeau (near Massignac ) ( 6 ).
The serpentinite bodies of Saint-Bazile and Oradour-sur-Vayres ( 7 ), Champagnac-la-Rivière (with 6 individual occurrences) ( 9 ), La Martinie (south of Champagnac) ( 10 ), La Boissonnie and Lageyrat line up in a south-easterly direction (west of Châlus ) ( 11 ) and La Rougerie (east of Dournazac ) ( 12 ). Separated from this branch somewhat to the northwest are the two occurrences of Cussac ( 8 ). The southeast branch can very probably still be traced back to the occurrences at Chevalerie (west of La Coquille ) ( 13 ), Comboux (southwest of Saint-Jory-de-Chalais ) ( 14 ) and Le Suquet (northwest of Saint-Martin-de-Fressengeas ) ( 15 ) continue to the south-southwest. The possible end point of the garland is then the metaharzburgite from La Rebière (municipality of Saint-Martin-de-Fressengeas) ( 16 ), which is the only occurrence that still reveals the original mineral inventory including the igneous foliation in places.
The western garland stretches for around 13 kilometers, the arched southern garland is around 38 kilometers to the metaharzburgite of La Rebière .
The serpentinite bodies are all located in the lower gneiss cover of the north-western Massif Central.
The main occurrence of Merlis and the western branch are in the lying area of the plagioclase- bearing paragneiss - an exception, however, are the two occurrences of Les Soumagnes and Bonnefont , which are located a little further north and which already occur in leptynite gneisses . All of these deposits are located in the immediate or relative vicinity of Chéronnac , which lies a little further to the south and is oriented in a west-east direction - Leukogranite ( CL ) - a leucogranite of Pennsylvania with very clear foliation. The occurrences of the southeast branch are consistently associated with the leptynitic eye gneisses , which are folded into the paragneiss and form the slope of which they form.
Lithology and mineralogy
The Merlis serpentinites are ultra- basic rocks with a very low SiO 2 content of around 40% and a very high MgO content of around 35%. They are likely to have emerged from mantle rocks , most likely from Harzburgites or Lherzolites . Depending on the degree of metamorphism and tectonic deformation, the rocks can be formed quite differently lithologically . Accordingly, two varieties can generally be distinguished as end links:
- massive type
- schisty type
Massive serpentinites
The massive, black to dark green serpentinites come closest to their original peridotic parent rock. For example, the metaharzburgite from La Rebière , which has the orange-brown weathering color characteristic of peridotites, still shows the minerals typical of a hypersthene peridotite olivine , orthopyroxene ( hypersthene ), spinel and clinopyroxene with a salitic-diopside composition. Massive serpentinites further north contain bronzite and augite and a brown spinel.
The serpentinite rocks, which often crumble in blocks, are bounded by straight or curved surfaces that are coated with silky shimmering serpentine minerals. These interfaces often show signs of armor stripes , which is why they represent tectonic movement surfaces . Inside, black, sometimes gray vein networks can be seen that have a metallic sheen due to their magnetite content . In between, light green, mother-of-pearl, rectangular to oval, 2 to 10 millimeters in size, consisting of completely bastitized pyroxene, stand out . Furthermore, 0.5 to 3 millimeters long, gray-black spinels can be distinguished, which are surrounded by light gray magnesium chlorite.
Very strongly serpentinized / tectonized rocks have a marble-like habit that is created by the change from dark green / black to light green zones. Running veins are mostly filled with serpentine minerals, magnetite and transversely fibrous chrysotile .
The following minerals can be seen under the microscope:
- Very rare, relic olivine
- Antigorite
- Chrysotile
- Colorless, magnesium- rich chlorite (Pennine)
- Opaque magnetite
Porphyroblasts of the parent rock can only be seen in phantom, they have meanwhile been replaced by a fiber network of tremolite actinolite , magnesium chlorite, serpentine minerals and talc (bastitization). Furthermore, the former chrome spinel was replaced by magnetite.
The Puytreillard deposit has a banded serpentinite as a special feature. In it, centimeter-thick dark and light layers alternate with one another. The dark layers consist of completely pseudomorphosed olivine (replaced by antigorite, red-spotted iddingsite , Mg-chlorite and magnetite), the light layers mainly of tremolite fibers, Mg-chlorite and magnetite. The parent rock of this type of band is probably a former pyroxenite in which accumulated layers of dunite were embedded.
Slate serpentinites
When the serpentinization becomes noticeable , knot-like agglomerations of colorless, magnesium-rich chlorite , reticulated serpentine minerals , a felt work of colorless amphiboles ( tremolite ), talc , anthophyllite and pargasite appear .
In the schisty serpentinites, the newly formed chlorite is becoming increasingly important. It forms large, leaf-like layers that run parallel to the regional foliation. The formation of chlorite has progressed in places to the formation of independent chlorite slates (for example in the deposits of La Rougerie , Cussac, Lageyrat , La Boissonie and Champagnac). The chlorite slate can be interspersed with asbestos veins and coated with silica-rich impregnations made from chalcedony or quartz .
Parent rocks
The following original rocks of the Merlis serpentinites can be reconstructed:
- Peridotites (Harzburgites and Lherzolites)
- Magnesium-rich (allivalitic) peridotites
- Magnesium-rich gabbros (also allivalitarian)
Occasionally, a stratigraphic sequence can also be made out within the parent rock, starting from peridotites in the horizontal (now massive serpentinites) to allivalitic peridotites (now retromorphosed into chlorite or tremolite schist) and gabbros in the hanging wall (now as medium-grain amphibolites ) runs.
It is possible that the parent rocks were overprinted by postmagmatic alteration before the regional metamorphosis began.
Chemical composition
Here is the mean chemical composition of the massive serpentinites including significant trace elements :
| oxide | Weight% | Trace elements | ppm |
|---|---|---|---|
| SiO 2 | 39.58 | Ba | 25th |
| TiO 2 | 0.05 | Co | 99 |
| Al 2 O 3 | 1.26 | Cr | 2641 |
| Fe 2 O 3 | 1.11 | Hf | 0.1 |
| FeO | 7.36 | Li | 10 |
| MnO | 0.11 | Nb | 0.15 |
| MgO | 35.71 | Ni | 2084 |
| CaO | 0.32 | Rb | 1 |
| Na 2 O | 0.20 | Sr | 8.5 |
| K 2 O | 0.05 | Ta | 0.1 |
| P 2 O 5 | 0.05 | Th | 0.55 |
| dehydration | 13.33 | U | 0.1 |
| V | 56 | ||
| Y | 1.1 | ||
| Zr | 2.5 |
Striking is the high magnesium content and, among the trace elements, chromium and nickel .
metamorphosis
The serpentinization of the parent rocks of the Merlis serpentinites took place in the course of the Variscan regional metamorphosis (Mediovariszischer Barrow type , period 400 to 350 million years BP ), which took place in the Limousin under medium pressures and at medium to high temperatures - in other words an MP / MT -Metamorphosis, which caused the formation of regional foliation. The water-containing mantle rocks that formed under high temperatures and relatively high pressures experienced a multi-phase retromorphism . The end product of this development are the chlorite slates already mentioned. The general chloritization can also be observed in other deep rocks of the Limousin.
Regional geological connection
The individual Merlis serpentinites are embedded more or less concordantly as shear lenses in the foliation of the paragneiss, leptynite gneisses and eye gneisses.
In general, the strike directions of the individual serpentinite deposits in the northern section move between ESE and SSE (N 120 to N 150) with an average angle of incidence (30 to 60 °) to NE. In the southern section the foliation then bends in the northeast direction (N 045) (it follows the La Coquille fault zone here ) and shows a general, relatively shallow dip (by 30 °) to the southeast.
The deposits in the northern section were all heavily tectonically stressed - the only exception is the metaharzburgite from La Rebière in the extreme south, which is possibly associated with the much lower metamorphic Thiviers-Payzac unit in the immediate vicinity .
As already mentioned, what is striking about the Western Garland is its spatial association with the Chéronnac leuco granite. In the hanging wall, the southern garland follows the leptynite eye gneisses, which in turn drive over the mica slate of the parautochthonous mica slate unit or the Saint-Mathieu leukogranite .
Overall, therefore, all serpentinite deposits surround the so-called Saint-Mathieu-Dom - a dome-like bulge of the deeper basement in the northwestern Massif Central. This bulge pushes the parautochthonous mica slate unit - the structurally deepest ceiling unit in the Massif Central, which still comes to lie under the lower gneiss ceiling - up on its eastern edge.
Serpentinites are tectonically very mobile rocks due to their high water content (in this case 13% by weight). Their spatial arrangement therefore underlines significant tectonic movement paths along which the mantle rocks were digged into the crustal area.
swell
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