Merlis Serpentinites

45.750.83222222222222Koordinaten: 45 ° 45 '0 "N, 0 ° 49' 56" E

The Merlis - serpentinites are a bevy of dispersed Serpentinitvorkommen in the northwestern Massif Central of France. They have emerged from Mantelperidotiten.

Type locality

The serpentinites are ordered by their type locality, the municipality of Vayres (Haute -Vienne ) belonging hamlet Merlis (also written Merly ) named.

Geography and Geology

Main occurrence is the serpentinite east of Merlis, who was formerly mined in two quarries for gravel extraction. This occurrence has on the chart a west open, half-moon shape and is from north to south about one kilometer long, east-west direction, however, only about 400 meters.

Starting from it follow in West direction garland like several smaller deposits in hundred meters range - 3 smaller deposits immediately north of the main mass and the hektometrischen occurrence 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 ( at Massignac ) ( 6).

In south-easterly direction to line up the Serpentinitkörper of Saint- Bazile and Oradour -sur- Vayres ( 7), Champagnac -la -Rivière ( with 6 single occurrence) (9 ), La Martinie (south of Champagnac ) (10 ), La Boissonnie and Lageyrat (west of Châlus ) (11) and La Rougerie (east of Dournazac ) ( 12). Deducted from this branch a bit to the northwest, the two occurrences of Cussac (8). The southeast branch can be very likely still on the occurrence in chivalry (west of La Coquille ) (13), Comboux (southwest of Saint- Jory- de -Chalais ) (14) and Le Suquet (northwest of Saint -Martin -de- Fressengeas ) (15) to continue after south-southwest. Possible endpoint of the garland is then the Metaharzburgit of La Rebière ( commune of Saint -Martin -de- Fressengeas ) (16 ), which is the only occurrence in places yet to recognize the original mineral constituents together with the magmatic foliation.

The West Garland stretches over 13 kilometers, the arcuate Südgirlande is up to Metaharzburgit of La Rebière after all, around 38 kilometers.

The Serpentinitkörper are all located in the Lower Gneiss ceiling of the north-western Massif Central.

The main occurrence of Merlis and the West branch are eingeschuppt in the footwall of the plagioclase -bearing paragneiss - but with the exception of the two occurrences of something Les Soumagnes and Bonnefont that are already occurring in Leptynitgneisen more northerly. All of these deposits are located in the immediate vicinity of the relative or a little further to the south pending, oriented in east-west direction Chéronnac - Leukogranits (CL ) - a leucogranite of Pennsylvaniums with very distinct foliation. The deposits of the southeast branch are all folded with the paragneiss associated leptynitischen eyes gneisses whose Hangendgrenze form.

Lithology and mineralogy

The Merlis - serpentinites are ultrabasic rocks with a very low SiO2 content of about 40% and a very high MgO content of about 35%. You are likely to have resulted from mantle rocks, most likely from Harzburgiten or Lherzolithen. The rocks can be formed lithologically quite different depending on the metamorphosis and tectonic deformation degree. Consequently, can generally be distinguished as end members two varieties:

• Massive type
• Schistose type

Massive serpentinites

The massive, black to dark green serpentinite come peridotischen their original parent rock closest. Thus, for example the Metaharzburgit of La Rebière which has the for peridotites characteristic orange-brown weathering color, sometimes even typical of a hypersthene - peridotite minerals olivine, orthopyroxene ( hypersthene ), spinel and clinopyroxene with salitisch - diopsidischer composition. Massive serpentinites to the north perform bronzite and augite and a brown spinel.

The often blocky decaying Serpentinitgesteine ​​bounded by straight or curved surfaces that are covered with silky shimmering Serpentinmineralen. These interfaces often show Harnisch Trie Mung, they therefore represent tectonic movement areas dar. Inside can be seen black, sometimes gray wire networks, which have a metallic shine because of their magnetite content. In between, light green, pearlescent, rectangular to oval, 2-10 mm wide receipts lift, which consist of fully bastitisierten pyroxenes. Furthermore, 0.5 to 3 millimeters long can distinguish gray and black spinels, which are surrounded by light gray magnesium chlorite.

Very strong serpentinisierte / tektonisierte rocks have a marble-like habit, caused by the change from dark green / black with light green zones. Overhauling veins are usually filled by Serpentinmineralen, magnetite and transversal fibrigem chrysotile.

Under the microscope, the following minerals indicate:

• Very rare, reliktueller olivine
• Antigorite
• Chrysotile
• Colorless, magnesium - rich chlorite ( Pennine )
• Opaque magnetite

Porphyroblasts the parent rock are only phantom to realize they are now replaced by a fiber mesh of tremolite - actinolite, Mg - chlorite, talc and Serpentinmineralen ( Bastitisierung ). In addition, former chromium spinel was replaced by magnetite.

The occurrence of Puytreillard has a special feature on a banded serpentinite. Switch to him centimeter-thick dark and light layers from each other. The dark layers consist of fully pseudomorphosiertem olivine (replaced by antigorite, rotfleckigem Iddingsit, Mg - chlorite and magnetite), the bright layers mainly of tremolite, Mg - chlorite and magnetite. In the parent rock bands of this type it would probably be a former pyroxenite, were incorporated into the Kumulatlagen of dunite.

Schistose serpentinite

In noticeable expectant serpentinization nodular aggregates of colorless, magnesium-rich chlorite, reticular Serpentinminerale, a feltwork colorless amphiboles appear ( tremolite ), talc, anthophyllite and pargasite.

In the schistose serpentinites of the newly emerged chlorite wins watching important. It forms large, sheet-like layers that run parallel to the regional foliation. The Chloritbildung is sometimes up to the formation of discrete chlorite progressed ( for example, in the presence of La Rougerie, Cussac, Lageyrat, La Bois Sonie and Champagnac ). The chlorite can riddled with asbestos veins and are coated by silica- rich impregnations of chalcedony or quartz.

Parent rocks

The following output of the rocks Merlis - serpentinite can be reconstructed:

• Peridotites ( harzburgites and lherzolites )
• Magnesium - rich ( allivalitische ) peridotites
• Magnesium - rich gabbros ( also allivalitisch )

Occasionally, even a stratigraphic sequence are identified within the parent rocks, which, starting from peridotites in the footwall (now massive serpentinite ) to allivalitischen peridotites ( now chlorite or Tremolitschiefern retromorphosiert ) and gabbros in the hanging wall ( there are now a medium -grained amphibolites ago) towards runs.

Perhaps the parent rocks were overprinted before the start of the regional metamorphism of a post- magmatic alteration.

Chemical composition

Below is the average chemical composition of massive serpentinite including significant trace elements:

Strikingly, the high content of magnesium and trace elements of chromium and nickel.

Metamorphosis

The serpentinization of the parent rocks of Merlis - serpentinite occurred during the Variscan regional metamorphism ( mediovariszischer Barrow- type, period 400 bis 350 million years BP), which expired in Limousin at intermediate pressures and at moderate to high temperatures - in other words, an MP / MT metamorphism, which resulted in the formation of the regional foliation. The resulting under high temperatures and relatively high pressures hydrous mantle rocks experienced this a multiphase Retromorphose. End product of this development are the already mentioned chlorite. The general chloritisation can also be observed in other plutonic rocks of the Limousin.

Regional geological context

The individual Merlis - serpentinites are embedded as shear lenses more or less concordant to the foliation of the paragneiss, Leptynitgneise eyes and gneisses.

In general, the strike directions of the individual Serpentinitvorkommen move in the northern section between OSO and SSO (N 120 to N 150 ) with mitllerem angle of incidence (30 to 60 °) to the NE. In the southern sector of the foliation then bends in the northeast direction (N 045) a ( it follows here the La Coquille fault zone ) and shows a general, relatively shallow dip ( 30 °) to the southeast.

The deposits in the northern section were all tectonically highly stressed - the only exception is the Metaharzburgit of La Rebière in the far south, which is probably associated with the much lower metamorphic, forthcoming in the immediate vicinity Thiviers - Payzac Unit.

It is notable for the West garland, as already mentioned, their spatial association with the Chéronnac - leucogranite. The Südgirlande follows in the hanging wall of the leptynitischen eyes gneisses, which in turn run over the mica-schists of mica slate Parautochthonen unit or the Saint -Mathieu- leucogranite.

Overall, therefore, surround all Serpentinitvorkommen the so-called Saint -Mathieu dome - a dome-like swelling of the deeper bedrock in the northwestern Massif Central. This bulging presses the Parautochthone mica schist unit - the structurally deepest ceiling unit in the Massif Central, which comes to lie still under the Lower Gneiss ceiling - up to its eastern edge.

Serpentinites are (in this case 13 % by weight ) tectonically very mobile rocks due to their high water content. Therefore, their spatial arrangement underlines significant tectonic movement paths along which the mantle rocks were scraped into the crustal field.

Swell

• Briand, Bernard et al: Feuille Châlus. In: BRGM (ed.): Carte de la France à géologique 1/50 000 Orléans.
• Chevremont, P. et al: Feuille Rochechouart. In: BRGM (ed.): Carte de la France à géologique 1/50 000 Orléans.
• Floc'h, J.-P. inter alia: Feuille La Rochefoucauld. In: BRGM (ed.): Carte de la France à géologique 1/50 000 Orléans.
• Guillot, P.-L. inter alia: Feuille Thiviers. In: BRGM (ed.): Carte de la France à géologique 1/50 000 Orléans.
• Wimmenauer, Wolfhard: Petrography of igneous and metamorphic rocks. Enke, Stuttgart 1985, ISBN 3-432-94671-6.

• Metamorphic Rock
• Geography (France)