Parautochthonous mica slate unit

from Wikipedia, the free encyclopedia

The parautochthonous mica slate unit is the deepest tectonic cover unit of the northwestern Massif Central in France . It consists mainly of mica schists of the Neoproterozoic , which were deposited as slate clays on the northern edge of Gondwana .

etymology

The term Parautochthonous Mica Slate Unit , English Parautochthonous Micaschist Unit or PMU for short , is linked to the fact that its rocks are part of the parautochthonous Saint-Mathieu-Dom . The exact position of this basement bulge is not clear and its autochthonous character is questionable. This uncertain state of affairs is expressed in the prefix Para . Parautochthon thus means belonging to the autochthon or located in the vicinity of the autochthon .

introduction

Overview geological map of Saint Mathieu Cathedral

The Variscan Orogen has developed in several stages and is welded together from several crustal segments. In general, two cycles can be distinguished. After the initial subduction of the Massif Central Ocean with high pressure metamorphism, the continental collision finally occurred between Peri-Gondwana in the south, Laurussia in the north and the intervening microcontinents Armorica and Avalonia .

Like the rest of the Saint-Mathieu-Dom, the parautochthonous mica slate unit belongs to the Ligero-Arverni zone of the north-western Massif Central. This is equivalent to the Moldanubian zone of Germany (e.g. Black Forest ) and represents the highly metamorphic central part of the Variscikum. Characteristic for this central zone are four or five ceiling units , which 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 parautochthonous mica slate unit is shown on the opposite geological map with PMU . It differs from the two overlying gneiss blankets, as the first deformation stage D 1 of the Massif Central (subduction and high pressure metamorphosis) during the Silurian and Lower Devonian (430 to 390 million years) is not registered in it. Subsequently, both gneiss blankets experienced a high-grade regional metamorphosis with migmatite formation in the Central Devon .

In the Upper Devonian and the earliest Lower Carboniferous, the second deformation stage D 2 covered the entire pile of nappes (390 to 360 million years ago), with the three lowest nappes being metamorphosed to a high degree by amphibolite facies. The rocks received a generally south-easterly directed stretching lineation, which reflects the hanging-north-west shear direction. In the Lower Carboniferous ( Tournaisian ) between 360 and 350 million years ago, the upper crossed the lower gneiss blanket 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. This was followed by the thermal event of the Viseum between 350 and 325 million years ago , which can be detected as a Monazital age in the mica schists. From 325 million years onwards, expansion tectonics and transpressive lateral shifts dominated the limousin's pile. Due to the resulting pressure relief, there was anatexis and the intrusion of predominantly leukogranites in the Upper Carboniferous.

distribution

Mica slate from Pensol - silky, shiny facies

The parautochthonous mica slate unit is widespread over the entire Massif Central. It not only occurs on the eastern edge of the Saint-Mathieu-Dom, but also appears further east in the frame of the Millevaches massif , for example in the Sussac-Dom . Other deposits can be found in the Monts d'Ambazac and on the Plateau d'Aigurande . East of the Sillon houillier - a major north-northeast-trending side shift with normal offset, which cuts through the entire Massif Central and separates the western section of the volcanic central part - it appears again in the area Sioule in Sioule Dom (in La Bosse-anticlinorium and in the southern section east of the Pont-de-Menat synclinory ), which is also a bulge in the basement. The Lot slate and the mica slate in the Cevennes are very large deposits, both of which are also included in the parautochthonous mica slate unit, but besides mica slate also contain other siliciclastic lithologies - for example quartzite and rare orthogneiss . The parautochthonous mica slate unit also appears in the south of the Massif Central in the Rouergue and in the central section of the Montagne Noire .

Petrology

Petrologically, the parautochthonous mica slate is mainly composed of mica slates of medium and high degree of metamorphosis (amphibolite facies). Two facies can be distinguished - a leafy and a silky, shiny facies. The leafy facies are more coarsely crystalline and show relatively thick layers of mica that enclose thin quartz bands . The silky, glossy facies are much finer-grained and owe their slightly bluish sheen to the clearly developed foliation surfaces . The leafy facies are narrowly slate with partly finely pleated slate surfaces. Their structure is lepidoblastic or grano-lepidoblastic or almond-shaped. It occurs only in the northern section on the eastern edge of the Saint-Mathieu-Leukogranits . The structure of the silky, shiny facies is similar, but is more prone to banding.

mineralogy

Detail of the mica slate from Pensol, Haute-Vienne - silky, shiny facies

The very fine-grained and clearly schisty rocks are clearly dominated by mica minerals . They contain the minerals muscovite , biotite , feldspar and quartz . Accessories are zircon , apatite , ilmenite , rutile and graphite . As metamorphic formations appear on the foliation planes needles of staurolite and sillimanite , very rarely, kyanite and small garnets ( almandine ). The grenade can, however, become much larger in the leafy facies and reach up to 1 centimeter. Andalusite , cordierite and needle-shaped tourmaline are induced by contact metamorphosis . Chlorite is an alteration mineral and can replace biotite and garnet pseudomorphically.

Muscovite of large grain size, often formed as sericite, predominates. Quite large leaves of muscovite and biotite usually penetrate one another and make up most of the mica layers. Due to the incorporation of ilmenite needles, the mica structure can also appear fibrous. The quartz is often concentrated in an almond shape (exudation quartz), but occasionally also shows tectonically elongated stems; ribbon-like layers can also be finely pleated. The plagioclase is either an oligoclase or an albite (An 6-10 ). Together with quartz it usually forms small, millimeter-sized, elongated eyes (ocelles), which in the leafy facies reach significantly larger dimensions. The eyes can also be helical .

geochemistry

The following analyzes should clarify the chemical composition of the mica slate:

Oxide
wt.%		 Mica schist 1 Mica slate 2 Mica slate 3 Quartz mica slate
SiO 2 60.20 58.30 56.40 65.05
TiO 2 0.83 0.95 1.04 0.82
Al 2 O 3 19.20 20.90 21.60 17.00
Fe 2 O 3 2.10 2.46 2.60 2.32
FeO 5.55 5.40 4.25 4.24
MnO 0.38 0.39 0.17 0.32
MgO 2.15 2.20 2.40 2.15
CaO 1.20 1.06 0.45 1.27
Na 2 O 1.30 0.91 1.05 1.87
K 2 O 3.75 4.15 5.10 3.43
P 2 O 5 0.23 0.13 0.09 0.22
H 2 O - 0.05 0.06 0.20 0.03
H 2 O + 2.35 2.60 3.25 1.63

The SiO 2 content of mica schist fluctuates between 56 and 60 percent by weight, quartz-rich rocks reach 65 percent by weight. This corresponds to the chemistry of andesites or diorites . The high Al 2 O 3 content of 17 to 22 percent by weight is striking , but also the very high iron oxide content of 6.5 to 7.8 percent by weight. TiO 2 and MgO also show quite high values . The alkalis, especially Na 2 O, are generally depleted .

metamorphosis

Vergent folded mica slate of the leafy facies

The parautochthonous mica schists have experienced both a regional metamorphosis and a contact metamorphosis. Metamorphic mineral new formations of regional metamorphosis are muscovite, biotite, garnet, staurolite, sillimanite (fibrolite) associated with muscovite and thistle. The garnet can have been formed before, with and after the formation of foliage. It can be zoned, with almandine-rich edges and Spessartine in the core. Staurolite is a late formation. The occurrence of disthene generally indicates pressure conditions of at least 0.4 GPa to 0.6 GPa and staurolite indicates temperatures of over 600 ° C. However, the pressure was probably much higher - for example, conditions of 0.8 GPa were achieved in the Sioule mica schists.

The contact metamorphosis caused by the Saint-Mathieu leukogranite led to the formation of new poikiloblastic cordierite and andalusite, both of which disturb / overlay the foliation, but also of alkali feldspar . In addition, there was a continuous tourmalinitization and at the same time garnet was replaced by biotite and chlorite or by biotite, muscovite and quartz pseudomorphically.

Anatexis

The parautochthonous mica slate unit is closely associated with the Saint-Mathieu leuco granite and also the Chéronnac leuco granite. Numerous inclusions of mica schist can be found in both leuco granites , some of which can be formed as kilometer-long trains. Conversely, leucogranite lenses and apophyses are found in the mica schists. It can therefore be assumed that the mica schists were anatectically melted under pressure relief and secreted a leucogranitic magma . The Saint-Mathieu leuco granite is dated 315 million years ago, so the anatexis took place during the Upper Carboniferous in the Bashkirium .

tectonics

Wrinkle with closely spaced crenulation in leafy facies

Due to their lowering into the ductile central crustal area , the former clay sediments received a continuous flattening and the resulting flat foliage due to the enormous load, partly due to the tectonics. By crawling along the slate surface, small, quartz-feldspar-rich almond and eye structures were created, which sometimes also reveal the sense of shear. Extended quartz stems also indicate the direction in which the rock was stretched. Tectonic constrictions locally led to folds in the centimeter range (occasionally also isoclinal and laid out with the foliation), which in turn can be characterized by very close crenulations in the millimeter range. In some places, late kink folds and very complex fold patterns, which indicate intensive deformations (interference patterns), are observed. With the emergence of the Saint-Mathieu-Dom and the ceilings above (leaving the ductile area), the mica slate was then only brittle. Residual stresses in the rock could be reduced through faults and fissures .

meaning

Parautochthonous mica slate from the Cevennes of Saint-André-de-Valborgne - with very fine crenulation

The importance of the parautochthonous mica slate unit is based on the fact that, thanks to its fine-grain, clay-like lithology, it represented an ideal lubricant for the higher ceilings - which were able to advance on their back with greatly reduced frictional resistance to the west and south-west while simultaneously flowing north-west-south-east. Their deep-lying tectonic position (as a deck sole) is therefore not surprising.

See also

literature

Individual evidence

  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. ↑ M. Faure et al .: Devonian geodynamic evolution of the Variscan belt, insights from the French Massif Central and Massif Armoricain . In: Tectonics . tape 27 , 2008, doi : 10.1029 / 2007TC002115 .
  3. ↑ A. Cocherie et al .: Electron microprobe dating as a tool for understanding closure of U-Th-Pb system in monazite from migmatite . In: Am. Mineral. tape 90 , 2005, pp. 607-618 .
  4. ↑ 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 .
  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. ↑ 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 .