Limousin ophiolite

The Limousin ophiolite is an ophiolithic rock association of the Massif Central , which is inserted in the Limousin below the upper gneiss cover . It is considered a remnant of the former Massif Central Ocean .

geography

The Limousin ophiolite forms a discontinuous band starting from La Porcherie in the southeast of the Haute-Vienne department to near Ladignac-le-Long in the southwest. The Merlis-serpentinite at Vayres further west are strictly speaking not expect more to the actual Limousin ophiolite because they are taking a different tectonic position and along the base of the lower gneiss ceiling occur.

The individual massifs begin 6.5 kilometers east of La Porcherie south of the hamlet of Chassagnas , immediately followed by a massif near Le Raineix . About 1.7 kilometers southeast of La Porcherie, three small deposits are lined up around the hamlet of La Roche . To the southwest of the La Porcherie train station is another, somewhat more important, single massif. To the west follow the La Plagne deposits 3 kilometers southwest of Saint-Germain-les-Belles , then the large serpentinite area of the Landes de la Flotte et du Cluzeau , the serpentinites of the Lande de Saint-Laurent and finally the occurrence at Le Buisson 3 Kilometers northeast of Ladignac-le-Long.

geology

Upcoming serpentinite in the Lande de Saint-Laurent

The Limousin ophiolite is squeezed in as a tectonic blanket in the border area of the two gneiss blankets . Due to the thrust movements and later lateral shifts in the Upper Carboniferous , the originally connected lithological bond was broken up into several smaller individual massifs. These individual massifs, which are embedded in a huge tectonic mélange, are usually not much larger than a kilometer, with a maximum length of 5 kilometers. Their thickness ranges from several hundred meters to one kilometer. The shear zones surrounding them are usually very difficult to see in the terrain. These are very chlorite-rich mylonites , which are now almost completely decomposed into clayey material. The metamorphic sliding sole of the ophiolites has not yet been found anywhere.

stratigraphy

The reconstructed stratigraphic sequence of the ophiolite from the lying to the hanging wall is as follows:

layered and isotropic gabbros plus amphibolites
Wehrlite and a few troctolites
Dunite with pockets of chromite
Harzburgite
Diopside- bearing Harzburgite with rare Lherzolites

In the military of Israel, and gabbros few mafic could transitions are discovered - massive dolerite and basaltic pillow lava , however, are completely absent.

Petrology

Massive serpentinite from the Landes de la Flotte et du Cluzeau

The stratigraphy reveals an ultramafic peridotite sequence of the oceanic mantle, now present as serpentinites , which assumes an allival character in the middle part (plagioclase appears as anorthite ) and only changes into mafic troctolites, gabbros and amphibolites of the oceanic crust in the hanging wall , with the mafic rocks throughout Amphibolites are present.

Serpentinites

The serpentinites can appear either solid or foiled. The massive serpentinites still show the minerals olivine (rich in magnesium), enstatite (orthopyroxene), augite (clinopyroxene) and brown spinel . Veins and networks of serpentine minerals ( antigorite , chrysotile and lizardite ), colorless to slightly greenish chlorite , magnetite , pargasite and calcite , sometimes also zoisite and prehnite appear as metamorphic neoplasms . The foiled serpentinites are characterized by their astonishing development of large coplanar chlorite crystals, which are regulated parallel to the regional foliation. The chloritization that has taken place can progress to the formation of real chlorite slate with colorless chlorite, pargasite, green spinel and opaque minerals.

Allival serpentinites

The allival serpentinites are also massive (predominantly) or foiled. The former are homogeneous rocks with scattered plagioclase crystals in the millimeter to centimeter range. The plagioclase is either fresh or chlorinated. The latter have plagioclase in the centimeter range, which are amphibolitized and / or chloritized. They show an alternation of thin gabbro and serpentine bands. They are characterized by a variety of metamorphic parageneses, for example with pargasite, anthophyllite , gedrite, spinel, corundum, serpentine minerals and chlorite. Reaction zones around plagioclase are also common.

Overall, both ultramafic serpentinite types are resin burgite and lherzolite cumulates, which were deposited at the very beginning of the differentiation process of a tholeiitic magma .

Amphibolites

The amphibolites of the hanging wall can be classified into two types:

Amphibolites with rich relic minerals
Amphibolites with little or no relic minerals.

In the first type, gabbros, troctolites and also rare eucrites can be clearly identified through the relic minerals as parent rocks . The dark-green to gray-green gabbros are fine to medium-grain rocks with igneous deposits in the millimeter range and a clear cumulative structure . Alternating positions in the centimeter to decimeter range with intermediate ultramafic layers are common. Relic minerals are augite and hypersthene , plagioclase ( labradorite or bytownite ), brown spinel and occasionally brown hornblende in centimeter-sized prisms. Additional secondary parageneses are pargasite, diopside , labradorite and titanite . The troctolites differ from the gabbros by their wealth of corona structures. Orthopyroxene, clinopyroxene, amphibole and green spinel grew up on olivine cumulates. The eucrites with primary bytownite are also rich in corona structures.

The second type consists of common amphibolites (with a light green hornblade) derived from gabbros and more rare pargasitites . The amphibolites are rich in plagioclase (primary bytownite and especially secondary labradorite) and also contain diopside, rarer zoisite, titanite and prehnite in cracks. Pargasitites consist of 50 to 70 percent by volume of pargasite. They are light green, massive and homogeneous rocks. Pargasite is also joined by colorless chlorite, green spinel and occasionally plagioclase. The latter appears primarily as well as secondary (as andesine ) and forms parallel layer textures with corundum , thistle and sometimes also gedrit , zoisite and late prehnite.

The chemical composition of the ordinary amphibolites corresponds to tholeiitic basalts, the apparently metasomatically formed pargasitites to alkali basalts or olivine-containing tholeiites.

metamorphosis

The Limousin ophiolite only registered a static metamorphosis , which transformed the gabbros of the hanging wall into undeformed amphibolite and the igneous ultramafite of the lying wall into serpentinite and / or pargasite-bearing chlorite . Berger and colleagues (2005) were able to show with various thermobarometric methods that the different amphibole varieties experienced a low-pressure metamorphosis of 0.2 GPa, with temperatures covering the entire spectrum - from the green slate or zeolite facies to high-temperature late magmatic conditions.

Among the Western European ophiolites of Variscides thus represents the Limousin ophiolite a very exceptional case in that was not concerned by a subduktionsbedingten HP metamorphism nor a orogenetischen medium-pressure regional metamorphism - unlike other Variscan Ophiolithvorkommen in Germany ( Münchberger mass ), Austria (Plankogel and Speik in the Eastern Alps) and Spain ( Cabo Ortegal ). Only the Chamrousse ophiolite in the French western Alps is comparable .

Emergence

It is believed that the Limousin ophiolite formed near a former, slowly spreading, oceanic ridge and was hydrothermally metamorphosed on the ocean floor. Speak for it

the frequent occurrence of undeformed metamorphic rocks - characteristic of the oceanic lower crust
a calcium - aluminum - and magnesium - metasomatism recognizable by the growth of calcium aluminum silicates (such as Zoisit, prehnite, - Grossular and hydrogrossular completely replaced), which occurred in veins or the primary-magmatic minerals
the reconstructed pressure-temperature conditions and ultimately
the innumerable similarities with the oceanic crustal rocks and other ophiolite occurrences around the world that were drilled by the Ocean Drilling Program .

literature

M. Chenevoy et al. a .: Notice explicative de la feuille Nexon à 1/50 000 . In: Éditions du BRGM . Orléans 1990.

Individual evidence

↑ ^a ^b J. Berger, O. Femenias, JCC Mercier and D. Demaiffe: Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker . In: Journal of Metamorphic Geology . tape 23 , 2005, pp. 795-812 , doi : 10.1111 / j.1525-1314.2005.00610.x .
^ G. Dubuisson, JCC Mercier, J. Girardeau and JY Frison: Evidence for a lost ocean in Variscan terranes of the western Massif Central, France . In: Nature . tape 337 , 1989, pp. 729-732 .
↑ RP Menot, JJ Peucat, D. and M. Scarenzi Piboule: 496 Ma age of plagiogranites in the Chamrousse ophiolitic complex (external crystalline massifs in the French Alps): evidence of a Lower Paleozoic oceanization . In: Earth and Planetary Science Letters . tape 88 , 1988, pp. 82-92 .

[Berger-1] J. Berger, O. Femenias, JCC Mercier and D. Demaiffe: Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker . In: Journal of Metamorphic Geology . tape 23 , 2005, pp. 795-812 , doi : 10.1111 / j.1525-1314.2005.00610.x .

[2] G. Dubuisson, JCC Mercier, J. Girardeau and JY Frison: Evidence for a lost ocean in Variscan terranes of the western Massif Central, France . In: Nature . tape 337 , 1989, pp. 729-732 .

[3] RP Menot, JJ Peucat, D. and M. Scarenzi Piboule: 496 Ma age of plagiogranites in the Chamrousse ophiolitic complex (external crystalline massifs in the French Alps): evidence of a Lower Paleozoic oceanization . In: Earth and Planetary Science Letters . tape 88 , 1988, pp. 82-92 .