Canaveilles group

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The Canaveilles Group is a metasedimentary series of rocks of Neoproterozoic and Cambrian ages that occurs in the Pyrenees . It represents the oldest sequence of purely sedimentary origin in this area.

etymology

The Canaveilles group, also Canaveilles series , was named after its type locality Canaveilles in the Pyrénées-Orientales department .

Distribution area

The Canigou from the northwest. The summit consists of orthogneiss, the upstream ridges are built by the Canaveilles group.

The distribution area of ​​the Canaveilles group extends to the eastern Pyrenees. In the Montagne Noire further north , the southernmost foothills of the French Massif Central , there is a comparable sequence, the la Salvetat-Saint-Pons series . The upper Alcudium of the Iberian Peninsula also has similarities with the Canaveilles group. The focus in the spread of the group lies in the area surrounding the type locality and frames the Canigou summit structure . But it is also found on the Spanish side of the eastern Pyrenees in the Cadí Nappe.

stratigraphy

In the vicinity of the type locality, the marine Canaveilles group reaches a thickness of up to 3000 meters, otherwise it varies between 2000 and 4000 meters. At Canigou, their base overlies leptynite transitional gneisses with the eye gneisses below . The group goes back to the Ediacarium around 580 million years ago. It mainly carries slate clays, as well as subordinate black slate and greywacke with carbonate and rhyodacite inclusions. Archaeocyathid limestone also occurs in the area of ​​the Cadí Nappe in the Lower Cambrian . The Canaveilles group is replaced on the border with the Central Cambrian by the already somewhat weaker metamorphic (green schist facies, chlorite zone ), flyschoid Jujols group , more precisely by the basal, olistocurrent- rich Tregurà formation of the Jujols group.

Carbonatic interferences

At the type locality within its slate, the Canaveilles group contains four sequences of carbonatic intercalations that have been metamorphosed into marbles and calcareous silicate stones (from hanging to lying ):

Basal limestone marbles

The basal limestone marbles ( French: marbres de base ) are up to 150 meters thick. They contain 5 layers of limestone marble, some of which are massive, as well as a layer of impure marbles formed from calcarenites . The latter layer also has gneiss-like intermediate layers in the decimeter range, which consist of calcium silicate and probably arose from marls .

Dolomite marbles

The fine-grained, gray-beige colored dolomite marbles develop the mineral chondrodite . Clinochlor and phlogopite can also occur in some areas .

There are several quartzite and greywacke levels between the dolomite marbles and the limestone marbles above .

Lime marbles

The white, sometimes gray-banded limestone marbles are very strongly crystallized and are subject to strong fluctuations in thickness. Their thickness, which is normally around 20 meters, can grow up to 180 meters in places. These strong fluctuations suggest a rezifalen close origin, probably is former bioherms .

Calcium silicate rocks

The very fine-grained calcium silicate rocks are formed like gneiss. They emerged from former, very potassium-rich marls. They can take on the character of multi-colored banded (light shades, greenish) horn rocks and contain the minerals diopside , tremolite , clinozoisite , basic plagioclase , microcline and microscopic biotite .

At the type locality the calcareous silicate rocks appear in the form of sandy calcareous slates .

Rhyodacite

The former rhyodacites, possibly also rhyodacite tuffs , were metamorphosed into fine-grained leptynites. Stratigraphically they follow in the hanging wall of the basal limestone marbles, sometimes only above the limestone marbles. Radiometric dating of the rhyodacites showed 581 million years, confirming the ediacaric age of the Canaveilles group.

Emergence

The Canaveilles group came to be deposited on the former northern edge of Gondwana during the Neoproterozoic and the Lower Cambrian. The sediments are of marine origin and were probably deposited on the continental margin. Possible bio-thermal baths in the carbonate inclusions and the archaeocyathid collections in the Cadí blanket in the Lower Cambrian indicate shelf or shelf edge deposits (reefs). The Rhyodacites suggest the presence of an island arch , so the Canaveilles group could well have been deposited in a backarc location. This is supported by its relationship to the central Iberian Alcudium, a roughly 15 kilometers (!) Thick, strongly subsidiary , neoproterozoic sediment series, which was deposited on the active northern edge of Gondwana, probably along a transform fault .

metamorphosis

During the Variscan Orogeny in Pennsylvania , the sediments of the Canaveilles group were metamorphosed under mesozonal conditions (lower amphibolite facies ) around 310 million years ago . The clayey parts of the sequence became mica schists of the cordierite, andalusite and sillimanite zone in the lower section of the group , and further up to green slate facial phyllites of the biotite zone . The carbonic inclusions turned into marbles and calcareous silicate stones. In contact close to the transition gneiss is first of andalusite - Isograd and then the cordierite -Isograd passed. This indicates an intrusive origin of the gneisses as former granitoids .

Magmatism

The sediment sequence of the Canaveilles group is often traversed by two-mica granite dikes and associated pegmatites , which belong to the deep-seated granite of the Canigou. The magmatites predominantly intrude the basal limestone marbles and dolomite marbles, but are also found in higher elevations. Diorite and quartz diorite can also penetrate the lower layers . The granitization took place after the ceiling structures were laid towards the end of the Variscan orogeny.

tectonics

The Canaveilles group was not only metamorphically shaped in the course of the Variscan mountain formation , but also heavily tectonically stressed. On the Canigou, for example, it was deformed together with the gneisses and Augengneisen (former orthogestics) into a huge horizontal isoclinal fold. Due to the shearing off of the legs, two ceiling units were formed, which were later plastically deformed (internal folding) and also bulged in an anti-clinical way. When the brittle state was reached, thrusts up and back occurred with progressive narrowing (on the southern flank of the Canigou) .

Since several revisions of the intrusion age of the orthogneiss have found a sub-Ordovician age of around 474 million years for the last crystallization on zirconia , the idea of ​​a Cadomian basement can no longer be maintained and the isoclinal fold structure has therefore also become questionable. The ceiling tectonics and later deformations are not to be questioned.

Another consequence of the intrusive character of the orthogneiss is that the paragneisseries, which were also considered Cadomic and originated from Grauwacken, must now also be included in the Canaveilles group, because they were only more strongly overprinted during the Variscan orogeny.

Individual evidence

  1. ↑ A. Cocherie et al .: U-Pb zircon (ID-TIMS and SHRIMP) evidence for the early Ordovician intrusion of metagranites in the late Proterozoic Canaveilles Group of the Pyrenees and the Montagne Noire (France) . In: Bulletin de la Societé Géologique de France . tape 176 , 2005, pp. 269-282 .
  2. ↑ B. Laumonier, A. Autran, P. Barbey, A. Cheilletz, T. Baudin, A. Cocherie, C. Guerrot: Conséquences de l'absence de socle cadomien sur l'âge et la signification des séries pré-varisques ( anté-Ordovicien supérieur) du sud de la France (Pyrénées, Montagne Noire) . In: Bull. Soc. géol. Fr. 175, n ° 6, 2004, p. 643-655 .

literature

  • M. Jaffrezo: Pyrénées Orientales Corbières . In: Guides géologiques régionaux . Masson, 1977, ISBN 2-225-47290-4 .