Upper gneiss cover

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The upper gneiss blanket , in French Unité Supérieure des Gneiss , is a widespread tectonic blanket unit in the French Massif Central . It represents the structurally higher of the two gneiss blankets .

Structural structure

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

The allochthonous upper gneiss cover, English Upper Gneiss Unit or UGU for short , 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):

The upper gneiss cover is shown on the opposite geological map (in dark blue) with UGU .

Introductory characterization

In the upper gneiss cover serpentinite deposits at La Flotte south of Limoges

Although the upper gneiss cover lies over the parautochthonous mica slate unit and the lower gneiss cover, it has been metamorphosed the most of all the covers . It consists mainly of gneiss ( paragneiss and orthogneiss ) and migmatites (migmatitic paragneiss) and is similar in its rock composition to the lower gneiss cover.

At its base is the so-called leptinite amphibolite complex - a bimodal igneous association of acidic lavas and tuffs with mafic rocks such as basalts , gabbros and rare ultramafites .

The upper gneiss cover also contains scaling of ophiolithic oceanic crustal rocks - tectonic remnants of the former Massif Central Ocean (or Central European Ocean ) - now mostly present as serpentinitized gabbros and ultramafites.

In the paragneiss there are also remains of eclogites and granulites (granulitic orthogneiss), which indicate a high pressure metamorphosis. The zösit eclogite facies were reached in places, but blue slate is very rare.

Schedule

The Eovariszian rock scraps found in the paragneiss were formed during a subduction process in the outgoing Silurian and Lower Devonian between 420 and 400 million years (with a maximum in the Pragian around 410 million years). The subduction corresponds to the first deformation stage D 0 in the Massif Central .

During their ascent again, these high-grade rocks, which now build up the hanging wall of the ceiling, experienced isothermal decompression, which led to the melting of the clay-rich, but also the quartz-feldspar-rich layers. It includes amphibolite blocks that were created from retrograde, but not melted, eclogite blocks. The age of the anatexis is classified as Upper Devonian ( Frasnium - 385 to 375 million years) (first generation of migmatites - Migmatite I ). It corresponds to the deformation event D 1 with a ductile shear directed to the south-west. It was responsible for the sliding of the upper gneiss cover over the lower gneiss cover. In the north of the Massif Central, the upper gneiss cover in the Middle Devon is covered for the first time by sediments from the Somme unit .

The entire upper gneiss blanket then later experienced an MP / MT regional metamorphosis of the Barrow type in the Tournaisium between 360 and 350 million years ago, during which amphibolite facial conditions were reached. The associated deformation event D 2 is characterized in the upper gneiss cover by a ductile shear directed to the north-west, recognizable by the elongation linear. In the Bas Limousin, the Thiviers-Payzac unit pushed north-west over the upper gneiss cover. The Brévenne basin closed in the northeast .

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 between 333 and 325 million years ago a second generation of migmatites - Migmatite II .

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 again anatexis and the intrusion of predominantly leukogranites in the upper carbon. The third generation of migmatite ( Migmatite III ) around 300 million years ago was limited to the Velay Cathedral . 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 ).

stratigraphy

In the Monts du Lyonnais, the upper gneiss cover can be divided into three, but this cannot be applied to the entire Massif Central. So here follow a bimodal quartz-feldspar-rich series (with the leptynite-amphibolite complex at the base) paragneiss, which enclose blocks of eclogites and peridotites. The hanging wall is then formed by mafic rocks, which, unlike the central section, no longer contain any high pressure residues.

In the Limousin, the Limousin ophiolite is followed in the lying position by means of thrust contact, partly isoclinally folded paragneiss with amphibolis. Eclogite lenses can be enclosed in the wrinkle legs of the amphibolite. Orthogneiss then appear further in the hanging wall and are also crossed by amphibolites.

Petrology

Metamorphites

The upper gneiss cover is mainly made up of gneiss, including paragneiss (migmatitic, amphibolic and leptinitic paragneiss ). Are found within the ceiling - especially at their base - tectonically eingeschuppte bands and lenses of amphibolites (former gabbros and basaltic tuffs) and serpentinite (former ultramafic peridotites - harzburgites , dunites , Defense Lite and Troktolithe ). These mafic rocks are interpreted as ophiolites and thus as remnants of the former Massif Central Ocean , although radiolarites and silica slate are missing. On the southern edge of the upper gneiss cover in the Limousin, they reach a thickness of up to 1000 meters as the Limousin ophiolite and are therefore often viewed as a separate tectonic cover unit. This is known as the Middle Allochthon , as it is tectonically located between the lower and upper gneiss cover. Graphite-containing slates and migmatites (metatexites) also occur in the lying position.

Paragneiss is the most common type of rock. 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 amphibole-carrying paragneiss are inserted into the normal facies as bands and lenses up to a maximum of 1 kilometer in length. The migmatitic paragneiss also contain boudins made of eclogites, lenses with mafic and felsic granulites and orthogneiss.

The light-colored leptinitic rocks are formerly acidic volcanic rocks or their weathering products - predominantly rhyolites and dazites . They are usually associated with amphibolite lenses and form within the paragneiss along their base the so-called leptinite-amphibolite complex , French Complexe leptyno-amphibolique or CLA for short , which characterizes the beginning of the upper gneiss cover.

Migmatites are characterized by the leukosome formation that usually occurs parallel to foliation (metatexites). The leukosome ligaments are usually quite thin (millimeter range), but can swell up to 10 centimeters thick, granular lenses. With advanced melting, nebulitic to very massive rocks (diatexites) are formed.

Igneous rocks

Numerous granitoids have also penetrated the upper gneiss cover. Particularly noteworthy are the tonalite , diorite and quartz diorite bodies belonging to the Limousin tonalite line - for example, smaller quartz diorites south of Saint-Junien , the 360 ​​million year old Saulgond quartz diorite , the Saint-Jean-de-Ligoure diorite , the Aixette -Nexon quartz diorite and the Uzerche quartz diorite ; Granites and granodiorites such as the 352 ± 12 million year old Glane granite , the young, 305 ± 11 million year old Chirac-Étagnac granite and the Nexon-Les-Cars granite . The upper gneiss layer can also be cut off by young leukogranites such as the 308 million year old, myrmekite- bearing Cognac-la-Forêt leukogranite , which penetrated along the ductile, northeast-trending Cordelle fault .

mineralogy

In addition to the mica biotite and muscovite, the paragneiss also contains the minerals plagioclase ( oligoclase ) and quartz . Alkali feldspar ( microcline ) is also usually present in various forms. Metamorphic formations are garnet , thistle and sillimanite (fibrolite). The garnets are rich in pyrope on their edges and rich in almandine in the cores. They are suitable for determining the pressure-temperature path of metamorphosis.

Zircon , apatite and opaque oxides appear as accessories . The amphibole- bearing paragneiss also contains iron-rich Ferrotscher makite , as well as allanite , ilmenite and rare sulfides. The leptinitic paragneiss have a very similar mineralogy, but with them biotite is replaced by lepidomelan . Their plagioclase is far richer in albite and their garnet is heavily enriched in the almandin component. Allanite is often present.

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.

geochemistry

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

Oxide
wt.%		 Paragneiss 1 Paragneiss 2 Paragneiss 3 Amphibolic
paragneiss 1		 Amphibolic
paragneiss 2		 Leptinite 1 Leptinite 2 Amphibolite Serpentinite
SiO 2 64.30 70.60 75.40 56.80 70.40 75.17 77.20 49.65 43.00
TiO 2 0.67 0.42 0.10 0.98 0.31 0.13 0.12 2.20 0.67
Al 2 O 3 16.50 14.95 12.70 16.50 13.80 12.84 11.90 14.65 9.10
Fe 2 O 3 5.10 dead 4.46 dead 2.20 dead 14.20 dead 4.41 dead 1.71 dead 1.30 dead 13.49 dead 11.60 dead
FeO
MnO 0.07 0.08 0.02 0.26 0.07 0.05 0.04 0.21 0.17
MgO 2.13 1.31 0.49 0.20 1.04 0.20 0.20 5.45 23.60
CaO 0.58 0.38 0.24 6.35 3.66 0.71 0.58 9.24 6.40
Na 2 O 3.64 3.25 2.79 4.35 2.87 3.19 3.25 2.37 0.20
K 2 O 4.17 1.89 3.73 0.84 1.93 4.78 4.50 0.70 0.05
P 2 O 5
H 2 O -
H 2 O + 2.80 2.96 1.79 0.23 0.82 0.44 0.34 0.69 5.90

The SiO 2 content of the quartz feldspar rocks shows large fluctuations from 56.8 to 77.2 percent by weight. The rocks can therefore be described as intermediate to acidic. Your alkalis (Na + K) vary between 4.8 and 8.0 percent by weight. The Al 2 O 3 contents are generally high (11.9 to 17.5 percent by weight) and reveal the rocks as originally sandy-clayey, siliciclastic sediments (rich in quartz and phyllite) or as rhyolite erosion products.

Separated from this are the ultrabasic to basic oceanic crustal rocks with 43 to 50 percent by weight SiO 2 . Their oceanic character is underlined above all by very high iron-magnesium values. CaO is greatly increased in the amphibolites, whereas the alkalis Na 2 O and K 2 O are very to extremely low in both rock groups.

Occurrence

In addition to its widespread distribution in the Limousin - southern, central and northwestern Limousin - the upper gneiss cover can be found in many sections of the Massif Central. It can be found, for example, on the Aigurande plateau on the northern edge, on the Sioule , in the Artense , in the Haut-Allier , in the Margeride ( Marjevols cliff ), in the Monts du Lyonnais , in the Vivarais on the eastern edge, near Najac , in the Vibal and Lévézou cliffs in the Rouergue in the southern section.

The upper gneiss blanket appears not only in the Massif Central, but also in the southern section of the Armorican Massif . Here, also known as the Upper Allochthon , it is found in the Champtoceaux complex and further south-west in the Les Essarts complex in the Vendée .

metamorphosis

Pressure-temperature diagram of the upper gneiss cover on the Sioule, Massif Central, France. The course of two samples is shown.

The high pressure metamorphosis (deformation stage D 0) in the upper gneiss cover can be dated to a period of around 415 million years. Its maximum was at a temperature of 750 ° C between 2.8 and 3.0 GPa, which corresponds to a depth of up to 90 kilometers - to be recognized by the occurrence of coesite in eclogite lenses of the Monts du Lyonnais. In the Limousin, pressures of 2.9 GPa were achieved at temperatures of 660 to 700 ° C.

The deformation stage D 1 took place between 385 and 375 million years in the granulite facies. With partial melting, a first generation of migmatite formed, which was formed under pressures of 0.8 to 1.0 GPa and temperatures of 800 to 850 ° C. (Monts du Lyonnais). In the Limousin the pT conditions were slightly lower - 0.75 to 0.9 GPa and 750 to 800 ° C. The entire bottom of the upper gneiss cover was affected, which probably represented a fluidal zone (English flow channel ) (and thus probably facilitated the pressing of the previously subducted eclogites and also the ceiling sliding).

The pT conditions achieved between 360 and 350 million years in the regional metamorphosis associated with D 2 were somewhat higher in the upper gneiss cover than in the lower gneiss cover. In particular, the metamorphosis in the upper gneiss cover was at a higher temperature.

The maximum pressures were z. B. at the Sioule at just under 1.2 GPa and the temperatures at 760 ° C. In comparison, the lower gneiss cover reached 1.0 GPa and 600 ° C.

This maximum was followed by almost isothermal pressure relief outside the staurolite field to 750 ° C. and 0.6 GPa with subsequent cooling to 650 ° C. and 0.5 GPa.

The path of the lower gneiss blanket is more complicated in comparison and the decompression moved within the staurolite field up to 0.2 GPa.

The final cooling of the two gneiss blankets to 400 ° C then took place during the late Viseum between 337 and 320 million years ago.

tectonics

The upper gneiss cover, like the lower gneiss cover beneath it, is internally not homogeneous, but rather heavily tectonically stressed. Evident are Inter thrust faults and folds . The wavelengths of the folds can extend into the tens of kilometers. For example, the upper gneiss cover south of Limoges forms a huge syncline , the Saint-Germain-les-Belles syncline , and even further south in the Bas Limousin, the Uzerche syncline . On the Sioule, too, it is folded into a large syncline structure - the Pont-de-Menat syncline . The general strike direction of these structures is southeast-northwest.

Oceanic crust remnants may have flaked up at internal thrusts. The internal ceiling / fold construction is also offset by lateral shifts, which mainly offset the structures on the left in a north-east-south-west direction.

Age

The following radiometric age information has been published so far from the upper gneiss cover, including U-Pb data from para and orthogneiss. For example, the Ceaulmont orthogneiss lasted 349 ± 14 million years. This corresponds to the deformation stage D 2 of the Tournaisium. D 1 is also documented in the Monts du Lyonnais as 384 million years old. The eclogitic high pressure metamorphosis in the Limousin (D 0) dates back to an age of 412 million years. Orthogneiss gave Ordovician ages of 467 million years in the Lyonnais, 478 to 487 million years in the Marjevols cliff, and 475 to 489 million years in the Limousin. These ages are associated with the formation of the leptino-amphibolite complex. Much older ages come from the Sereilhac paragneiss , which registered 523 ± 4 and 555 ± 7 million years. These ages of the Lower Cambrian and Ediacarian should represent the deposition period of the metasediments on the northern edge of Gondwana. Ediacaric ages are also present in the Ceaulmont-Orthogneiss (574 ± 28 million years) and in paragneiss from the Plateau d'Aigurande (558 ± 9 million years). Even higher ages, measured in zirconia, were 710 and 763 million years ( Cryogenium ), 1700 and 2400 ( Paleoproterozoic ) and even 2700, 2800 and 3100 million years ( Archean ).

See also

literature

Individual evidence

  1. ^ M. Faure, E. Bé Mézème, A. Cocherie, P. Rossi, A. Chemenda, and D. Boutelier: Devonian geodynamic evolution of the Variscan Belt, insights from the French Massif Central and Massif Armoricain . In: Tectonics . tape 27 , 2008, p. 19 , doi : 10.1029 / 2007TC002115 .
  2. ↑ M. Faure, C. Leloix and JY Roig: L'évolution de la chaîne polycyclique hercynienne . In: Bull. Soc. Geol. France . tape 168 , 1997, pp. 695-705 .
  3. ^ S. Costa: East-west diachronism of the collisional stage in the French massif Central: implications for the European Variscan Orogen . In: Geodin. Acta . tape 5 , 1991, pp. 51-68 .
  4. ↑ P. Ledru et al: Ou sont les nappes dans le Massif Central français? In: Bull. Soc. géol. Fr. 8, V, 1989, pp. 605-618 .
  5. ^ 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 .
  6. ↑ C. Cartannaz, P. Rolin, A. Cocherie, D. Marquer, O. Legendre, CM Fanning and P. Rossi: Characterization of wrench tectonics from dating syn-to postmagmatism in the north-western French Massif Central . In: Int. J. Earth Sci. 2006, doi : 10.1007 / s00531-0066-0101-y .
  7. ^ M. Faure: Late orogenic carboniferous extensions in the Variscan French Massif Central . In: Tectonics . tape 14 , 1995, pp. 132-153 .
  8. ↑ E. Be Mezème, A. Cocherie, M. Faure, O. Legendre and P. Rossi: Electron microprobe monazite geochronology: a tool for Evaluating magmatic age domains. Examples from the Variscan French Massif Central. In: Lithos . tape 87 , 2006, pp. 276-288 .
  9. ↑ P. Ledru, G. Courrioux, C. Dallain, JM Lardeaux, JM Montel, O. Vanderhaeghe, and G. Vitel: The Velay dome (French Massif Central): melt generation and granite emplacement during orogenic evolution . In: Tectonophysics . tape 342 , 2001, p. 207-237 .
  10. ↑ Michel Faure, Jean-Marc Lardeaux and Patrick Ledru: A review of the pre-Permian geology of the Variscan French Massif Central . In: Comptes Rendus Géoscience . tape 341 (2-3) . Elsevier Masson, 2009, p. 202-213 , doi : 10.1016 / j.crte.2008.12.001 .
  11. ↑ G. Dubuisson, J.- C. Mercier, J. Girardeau and J.- Y. Frison: Evidence for a lost ocean in Variscan terranes of the western Massif Central, France . In: Nature . tape 337 , 1989, pp. 729-732 .
  12. ^ MT Peiffer: La signification de la ligne tonalitique du Limousin. Son implication dans la structuration varisque du massif Central français. In: Comptes Rendus Acad. Sci. 303 II. Paris 1986, p. 305-310 .
  13. ↑ C. Pin and J.- J. Peucat: Ages des épisodes de métamorphisme paléozoïques dans le Massif central et le Massif armoricain . In: Bull. Soc. Géol. France . Paris 8 1986, p. 461-469 .
  14. ↑ 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 .
  15. ^ A b J. Berger et al.: New occurrence of UHP eclogites in Limousin (French Massif Central): Age, tectonic setting and fluid-rock interactions . In: Lithos . 2010.
  16. ↑ JM Lafon: Geochronology U-Pb appliquée à deux segments du Massif Central français. Le Rouergue oriental et le Limousin central (doctoral thesis) . Montpellier, France 1986, p. 1152 .
  17. ↑ O. Vanderhaeghe: Migmatites, granites and orogeny: Flow modes of partially-molten rocks associated with melt / solid segregation in orogenic belts . In: Tectonophysics . tape 477 (3-4) , 2009, pp. 119-134 .
  18. ↑ Bernhard Schulz: EMP-monazite age controls on PT paths of garnet metapelites in the Variscan inverted metamorphic sequence of La Sioule, French Massif Central . In: Bull. Soc. géol. Fr. t. 180, no 3, 2009, p. 271-282 , doi : 10.2113 / gssgfbull.180.3.271 .
  19. ↑ JL Duthou, M. Chenevoy and M. Gay: Age Rb / Sr Dévonien moyen the migmatites à cordierite du Lyonnais (Massif Central francais) . In: CR Acad. Sci. tape 319 , 1994, pp. 791-796 .
  20. ↑ JL Feybesse et al. a: Carte géologique de la France (1/50000), feuille Saint Symphorien (721). Feuille explicative. BRGM, Orléans 1995, p. 110 .
  21. ↑ J. Melleton et al .: Precambrian protoliths and Early Paleozoic magmatism in the French Massif Central: U-Pb data and the North Gondwana connection in the west European Variscan belt . In: Gondwana Research . tape 17 (1) , 2010, pp. 13-25 .