Lower gneiss cover

The lower gneiss cover , in French Unité Inférieure des Gneiss , is a widespread tectonic cover unit in the French Massif Central . It represents the structurally deeper of the two gneiss blankets .

introduction

Structural structure

Geological overview map of the Saint-Mathieu-Doms with the surrounding ceilings

The lower gneiss cover belongs to the Ligero-Arverni zone of the Massif Central. This is equivalent to the Moldanubian zone of Germany (e.g. Black Forest and Bavarian Forest ) and represents the highly metamorphic central part of the Variscikum. Characteristic for this central zone are four or five ceiling units that were stacked on top of each other during the Variscan continental collision from the Upper Devonian and during the Lower Carboniferous .

In the Limousin , the following pile of roofs can be observed (from structurally higher to structurally lower):

Génis unit
Thiviers Payzac Unit ( TPU )
Upper gneiss cover ( UGU )
Lower gneiss cover ( LGU )
Parautochthonous mica slate unit ( PMU )

The lower gneiss cover is shown on the opposite geological map (in light blue) with LGU .

Timeframe

Leptynite from Tuquet near Châlus with isoclinal folds

The Variscan Orogen has developed in several stages and is welded together from several crustal segments. In general, two major cycles can be distinguished. After the initial Eovarisian subduction of the Massif Central Ocean (or the Central European Ocean ) to the north with associated high pressure metamorphosis (stage D 0 ), there was finally a continental collision between Peri-Gondwana in the south, Laurussia in the north and the intervening microcontinents Armorica and Avalonia .

The lower gneiss ceiling ( English Lower Gneiss Unit or abbreviated LGU ) has registered in the Massif Central with the Upper Gneiss ceiling, the first deformation stage D 0 caused by subduction and high / ultrahigh-pressure metamorphism HP / LT during the Silurian and Devonian (the period 440/430 up 390 million years). In the Lower Devonian, the Rhine Ocean also closed by subduction, but this time in a southerly direction. After this subduction event, both gneiss blankets experienced a high-grade regional metamorphosis from the Middle Devon with migmatite formation in the frasnium (385 to 375 million years ago), due to reappearance (pressure relief) of the subducted crustal sections. The maximum pressure-temperature conditions in the lower gneiss cover were 1.2 GPa and 500 ° C. The associated deformation level D 1 caused the general sense of shear hanging to the southwest.

Deformation level D 2 then covered the entire pile of nappes from the Upper Devonian and the earliest Lower Carboniferous including the parautochthonous mica slate unit (period 380 to 350 million years), whereby the three lowest nappes were metamorphosed to a high degree amphibolite facially (metamorphosis of the Barrow type MP / MT with 0.9 GPa and 600 ° C in the lower gneiss cover). In the Tournaisium between 360 and 350 million years ago, the rocks were given a south-easterly directed stretching lineation, which reflects the ductile hanging-north-west shear sense. In this context, the upper crossed the lower gneiss cover with the underlying parautochthonous mica slate unit in a northwest direction. As the upper gneiss blanket had previously been metamorphosed to a greater extent, this simulates a "reverse" metamorphosis.

The thermal event of the Viseum (stage D 3 ) followed between 350 and 325 million years ago and can be traced back to the Monazital age . The central area of the Massif Central was subject to southeast directed expansion and transtension, while the outer areas experienced a northeast directed narrowing. The event brought about widespread magmatism between 350 and 340 million years ago ( Guéret granite etc.), the volcanism of the Tufs Anthracifères around 330 million years ago and a second generation of migmatites between 340 and 325 million years ago.

From 325 million years onwards, expansion tectonics, but also transpressive lateral shifts, dominated the limousine's nappe pile (stages D 4 and D 5 ). Due to the resulting pressure relief, there was anatexis and the intrusion of predominantly leukogranites in the Upper Carboniferous. The still syn-orogenic stage D 4 (325 to 315 million years) took place with stretching in the southeast-northwest direction, D 5 (305 to 275 million years) was already post-orogenic and showed stretching from north to northeast (with rift formation ).

Petrology

Basal eye gneiss of the lower gneiss cover near Mialet , Dordogne

Metamorphites

The lower gneiss cover is mainly made up of gneiss, including paragneiss , eye gneiss and leptynitic eye gneiss. Leptynites and very rarely tectonically scaly bands and lenses of amphibolites and serpentinites are also encountered within the nappe . The latter are interpreted as ophiolites and thus as remnants of the former Massif Central Ocean. Migmatite also occurs.

Paragneiss is the most common type of rock. Two facies can be recognized - a two- mica facies with biotite and muscovite and a pure biotite facies. The paragneiss was deposited at the end of the Neoproterozoic ( Ediacarium ) and the beginning of the Cambrian on the northern edge of Gondwana as Pelite and Grauwacken .

The eye gneisses, which are clearly foiled and provided with a lineation, usually form the basis of the lower gneiss cover. They emerged from sheared granitoids. Your feldspar eyes are usually between 0.5 and 2 centimeters in size.

The leptynitic rocks are formerly acidic volcanic rocks or their weathering products - predominantly rhyolites and dazites .

The amphibolite lenses are regarded as former gabbros . The serpentinites are former peridotites , probably Harzburgites .

Migmatites are characterized by the leukosome formation that usually occurs parallel to foliation (metatexites). With advanced melting, nebulitic to very massive rocks (diatexites) are formed.

Igneous rocks

The Mazières quartz diorite

The enamel formation went beyond the migmatite stage and so the metamorphic rocks of the lower gneiss cover were intruded by several generations of magmatites from around 360 million years on . In the western Limousin the following intrusive bodies can be distinguished:

the Exideuil quartz diorite and Mazières quartz diorite belonging to the limestone Limousin tonalite line

the 352 million year old Oradour-sur-Glane-Granite , an aluminum-rich, mesocrate two-mica granite

the calcareous Chirac-Étagnac granite (305 million years old) and Saint-Gervais granite

the Nexon-les-Cars-Granite , a hybrid biotite granite, with its southern branch of Vieillecour

the generation of aluminum-rich and leucocratic leukogranites such as the Brame leukogranite (325 million years old), the Peury leukogranite , the Cognac-la-Forêt leukogranite (308 million years old), the Chéronnac leukogranite and the Saint-Mathieu leukogranite (315 million years).

The magmatic activities ended with sub-alkaline micro-granites, lamprophyres and an extremely rare ultrapotassic and alkaline quartz syenite near Oradour-sur-Vayres .

The intrusions also changed the lower gneiss cover in part with metamorphic contact. New biotite, cordierite and occasionally tourmaline were formed - this can be seen in the contact of the Saint-Mathieu leukogranite.

mineralogy

Amphibolite from Champagnac-la-Rivière , encrusted into the base of the lower gneiss cover

The two- mica facies of the paragneiss contain not only mica but also the minerals plagioclase ( oligoclase ) and quartz . The alkali feldspar microcline acts as an antiperthite . Metamorphic formations are garnet ( almandine ), thistle , staurolite and sillimanite . The biotite facies have practically an identical mineralogy, but almost without muscovite. It is much finer-grained and has a more massive structure.

The eye gneisses contain biotite, muscovite, albite , partly perthitic alkali feldspar (microcline) and quartz. Accessories are apatite , zircon and magnetite . Garnet is added as a metamorphic mineral.

Quartz, microcline, oligoclase, biotite and muscovite appear in the leptynites. Apatite, zircon and the oxides ilmenite and magnetite act as accessories . A special feature is the formation of myrmecite at the edges of the oligoclase, which are surrounded by microcline. Garnet is present again as a metamorphic neoplasm. In some Leptyniten occurs as mica lepidomelane up as amphibole appear Ferrohastingsit and Allanit and rare Titanit may be encountered.

The scaled amphibolites and serpentinites differ completely in their mineralogy from the surrounding quartz-feldspar rocks due to the occurrence of amphibole or serpentine minerals.

The migmatites resemble leukogranites or leukogranodiorites in their mineral composition. They too are distinguished by the presence of myrmecite. The metamorphic mineral is garnet again.

geochemistry

The following analyzes are intended to illustrate the chemical composition of rocks from the lower gneiss cover of the western Limousin:

Oxide wt.%	Paragneiss 1	Paragneiss 2	Paragneiss 3	Paragneiss 4	Eye gneiss	Leptynitic eye gneiss	Leptynite 1	Leptynite 2	Amphibolite	Serpentinite
SiO ₂	62.00	68.10	70.60	77.60	71.03	74.68	77.69	73.60	49.35	40.29
TiO ₂	0.77	0.61	0.58	0.00	0.33	0.20	0.11	0.05	0.52	0.06
Al ₂ O ₃	17.30	15.15	13.10	11.65	14.60	13.45	11.70	14.60	16.44	2.00
Fe ₂ O ₃	1.95	1.50	4.52 dead	1.17 dead	2.44 dead	1.91 dead	1.57 dead	1.03 dead	7.21 dead	8.49 dead
FeO	4.30	4.45
MnO	0.10	0.10	0.66	0.02	0.33	0.03	0.11	0.07	0.13	0.10
MgO	3.20	2.90	1.77	0.34	0.51	0.35	0.11	0.09	9.79	34.72
CaO	1.35	1.30	1.18	0.39	0.53	0.75	0.22	0.34	12.82	0.53
Na ₂ O	2.70	3.20	2.50	2.30	3.61	2.67	2.55	4.00	1.74	0.20
K ₂ O	3.10	2.40	2.80	4.47	5.12	5.37	4.08	4.30	0.66	0.05
P ₂ O ₅	0.18	0.15							0.06	0.05
H ₂ O ^-	0.10	0.05					0.10
H ₂ O ⁺	2.75	2.00	1.61	1.72	1.18	0.78	1.30	1.53	1.39	12.97

The SiO ₂ content of the quartz feldspar rocks shows large fluctuations from 62 to almost 78 percent by weight. However, the rocks can still be described as generally acidic. Your alkalis (Na + K) vary between 4.8 and 8.7 percent by weight. The Al ₂ O ₃ contents are generally high (11 to 17 percent by weight) and reveal the rocks as originally sandy-clayey, siliciclastic sediments (rich in quartz and phyllite) or as rhyolite erosion products, which can be seen clearly in diagram Q versus A of distinguish more mafic granitoids.

Separated from this are the ultrabasic to basic oceanic crustal rocks with 40 to 50 percent by weight SiO ₂ . Their oceanic character is underlined above all by their very high iron and magnesium values. CaO is greatly increased in the amphibolites, whereas the alkalis Na ₂ O and K ₂ O are very low.

tectonics

The lower gneiss cover is internally not homogeneous, but rather heavily tectonically stressed. You can see internal thrusts and folds , sometimes also isoclinal folds . Oceanic crust remnants may have flaked up at internal thrusts. The folds usually show a direction of vergence, for example to the west in the western Limousin. The internal ceiling / fold structure is also offset by lateral shifts, which usually penetrate the structures in a northeast-southwest direction.

Occurrence

In addition to its widespread distribution in the Limousin , the lower gneiss cover can be found in almost all sections of the Massif Central. It can be found, for example, in the Tulle anticline , in the area around Bellac , on the Aigurande plateau in the north, on the Sioule , near Ussel , in the Margeride , in the Rouergue and in the northeastern Cevennes .

Meteorite impact

A rarity is the Rochechouart-Chassenon crater , which was formed around 200 million years ago as a result of a meteorite impact in the lower gneiss cover.

Natural resources

The lower gneiss cover of the Limousin is known for its gold deposits , which have been mined since the times of the Gauls . The rocks also show selective enrichments in uranium , lead and zinc . Another important raw material is kaolin for making porcelain.

literature

CW Passchier and RAJ Trouw: Microtectonics . Springer Verlag, 1998, ISBN 3-540-58713-6 .

Service Géologique National: Carte geologique de la France au millionième - 6 ^ème édition . Editions BRGM, ISBN 2-7159-2128-4 .

Geological maps of the BRGM on a scale of 1/50000. Leaves Châlus , La Rochefoucauld , Montbron , Rochechouart and Thiviers .

Individual evidence

↑ J.-M. Lardeaux et al: The Variscan French Massif Central - a new addition to the ultra-high pressure metamorphic “club”: exhumation processes and geodynamic consequences . In: Tectonophysics . tape 332 , 2001, pp. 143-168 .
↑ J.-Y. Roig and M. Faure: La tectonique cisaillante polyphasée du Sud-Limousin (Massif central français) et son interprétation dans un modèle d´évolution polycyclique de la chaîne hercynienne . In: Bull. Soc. géol. Ms. Band 171 , 2000, pp. 295-307 .
^ M. Faure et al.: Late Visean thermal event in the northern part of the French Massif Central: new 40Ar / 39Ar and Rb-Sr isotopic constraints on the Hercynian syn-orogenic extension . In: Int. J. Earth Sci. (Geol. Rundsch.) . tape 91 , 2002, p. 53-75 .

[1] J.-M. Lardeaux et al: The Variscan French Massif Central - a new addition to the ultra-high pressure metamorphic “club”: exhumation processes and geodynamic consequences . In: Tectonophysics . tape 332 , 2001, pp. 143-168 .

[2] J.-Y. Roig and M. Faure: La tectonique cisaillante polyphasée du Sud-Limousin (Massif central français) et son interprétation dans un modèle d´évolution polycyclique de la chaîne hercynienne . In: Bull. Soc. géol. Ms. Band 171 , 2000, pp. 295-307 .

[3] M. Faure et al.: Late Visean thermal event in the northern part of the French Massif Central: new 40Ar / 39Ar and Rb-Sr isotopic constraints on the Hercynian syn-orogenic extension . In: Int. J. Earth Sci. (Geol. Rundsch.) . tape 91 , 2002, p. 53-75 .