Cratère de Beaunit

Cratère de Beaunit
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height	785  m
location	Puy-de-Dôme department , France
Mountains	Chaîne des Puys , Massif Central
Coordinates	45 ° 54 '15 "  N , 2 ° 58' 0"  E Coordinates: 45 ° 54 '15 "  N , 2 ° 58' 0"  E
Type	Maar and cinder cone
rock	Traxhybasalt and trachyandesite
Age of the rock	Around 50,000 years of BP

The Cratère de Beaunit is a composite volcano near Charbonnières-les-Varennes , in the French department of Puy-de-Dôme , Auvergne-Rhône-Alpes . It is the northernmost volcanic center in the main chain of the Chaîne des Puys and was formed in the late Pleistocene around 50,000 years ago BP .

geography

The Cratère de Beaunit (also Maar de Beaunit ) is located 2 kilometers west of the center of Charbonnières-les-Varennes on the D 138. The former maar is traversed by the Ruisseau l'Ambène in an easterly direction. It now forms an almost circular leveling area at around 785 meters above sea level, which girdles the eponymous hamlet of Beaunit. The crater is completely filled with sediment. South of Beaunit is the Puy de Beaunit (also known as Puy Gonnard ), a cinder cone that was later created on the southern edge of the maar , which covers a large part of the southern sector of the maar and culminates at 861 meters. Immediately to the west of Beaunit, the lava flow from the south-facing Puy Thiolet (or Puy Thiollet) came to a standstill, and beneath it is the north-western sector of the maar.

geology

introduction

The Chaîne des Puys is the youngest volcanic district in France with typical alkaline intraplate volcanism. It can boast over 100 volcanic centers that were created over an arched eyrie of the Variscan basement , the Plateau des Dômes . This plateau consists of rhyolite tuffs of the Viséum in the north , but further south of peralkalic granitoids of the Upper Carboniferous and migmatic gneisses of the early Paleozoic. It is bounded to the east by the Limagne Trench and to the west by the Olby Trench (Fossé d'Olby). The volcanoes are lined up over a distance of 30 kilometers in a 3 to 4 kilometer wide band in north-south direction and thus run parallel to the edge fault of the Limagne Trench.

Volcanism began around 100,000 years BP and lasted up to 5,000 years BP, with the majority of the volcanic centers being formed between 13,500 and 7,000 years BP, with a maximum of 10,000 years BP for the basalts. It is questionable whether it is now completely extinguished. The extracted magmas form a series of magmas differentiated by fractional crystallization , which ranges from potassium-rich alkali basalts to mugearite and benmoreite (rarely) to trachytes . A total of around 8 cubic kilometers of basaltic lavas and 1 to 2 cubic kilometers of trachy derivatives as viscous lava domes were formed.

description

Three volcanological units meet at the crater. The ejecta of the maar with basaltic magma components has accumulated a crescent-shaped wall over 1 kilometer long in the northeast, which reaches 835 meters at its southeast end. It consists of two units: in the lying position, the layered gray explosive breccia of the maar, which has a sandy aspect. In addition to basaltic bombs with a "cauliflower structure" (created by deterrence), it contains granitic basement inclusions of all sizes, down to the smallest lapilli. It is interesting to note that even enormous blocks of granite did not deform their respective base. In the hanging wall of black Schweißschlackenlapilli derived from an outbreak of the Puy Beaunit follow.

The lava flow of the Puy Thiolet consists of leuco basalt (basaltic trachyandesite, also known as labradorite), which is quite rich in bubbles and dark gray in color, but appears lighter than normal basalt.

The strombolian cinder cone of the Puy Beaunit on the southern edge of the maar was created later, as its black pozzolans in the hanging wall of the ejection wall clearly demonstrate. It also contains large blocks of granite. For geological research, however, its wealth of inclusions in the earth's mantle ( peridotites ) is of utmost importance. Highly metamorphic rocks of the lower crust are also represented, such as Charnockite and Granulite .

The either homogeneous or composite mantle xenolites can reach up to 30 centimeters in size. The homogeneous foreign inclusions are either spinel peridotites such as lherzolite , harzburgite and dunite or pyroxenites such as websterite , olivine websterite and orthopyroxenite . The composite rocks are banded and are made up of ultramafic and mafic layers, in which chromite-rich horizons are interposed.

Age

The ejecta of the maar was sampled by Miallier and colleagues in 1997 and, by means of thermoluminescence, showed an age of 54,000 ± 7,000 years BP. However, a year earlier Rosseel found only 43,950 ± 5,100 years BP. The lava flow from the Puy Thiolet in turn produced 15,910 ± 250 years BP, thus a minimum age for the formation of the maar. In 1986 Juvigné and colleagues were able to use ¹⁴ C to date a phreatic tephra of unknown origin ( Lac Pavin ?) In the maar to 5840 ± 120 years BP, a very young age.

Individual evidence

1. ↑ Villemant, B. et al .: Cristallisation fractionnée d'un magma basaltique alcalin: la série de la Chaîne des Puys (Massif Central, France). II. Geochimie. In: Bulletin de Minéralogie . tape 103 , 1980, pp. 267-286 .
2. ↑ Boivin, P. et al.: Volcanologie de la Chaîne des Puys, Massif Central Français (58 edition) . Parc Naturel Régional des Volcans d'Auvergne, Clermont-Ferrand 2009, p. 196 .
3. ↑ Peter Longo, JM: Massif Central. Limousin, Auvergne, Velay. In: Guides Géologiques Régionaux . Masson, Paris, New York, Barcelona, ​​Milan 1978, ISBN 2-225-49753-2 .
4. ↑ Féménias, Olivier et al.: Petrologie des xénolithes ultramafiques du Puy Beaunit (Massif central français): un gisement atypique du manteau sous-jacent . In: CR Acad. Sci. Paris, Sciences de la Terre et des planètes . tape 332 , 2001, pp. 535-542 .
5. ↑ Miallier, D. et al.: Datation par Thermoluminescence du Maar de Beaunit (Puy-de-Dôme) . In: Internal PCCF report . RI 97-03. Université Blaise Pascal, Laboratoire de Physique Corpusculaire 1997.
6. ↑ Rosseel, J.-B .: Synthèse chrono-magmatologique de la Chaîne des Puys. Unpublished diploma thesis (DEA) . Univ B Pascal, Clermont-Ferrand II 1996, p. 50 .
7. ^ Brousse, R. and Lefevre C .: Le Volcanisme en France . Masson (Guides Géologiques Régionaux), Paris 1990.
8. ↑ Juvigné, E., Bastin B. und Gewelt, M .: Découverte de retombés volcaniques d'age Holocène dans la Chaine des Puys septentrionale (Massif Central, France) . In: Revue des Sciences Naturelles d'Avergne . tape 52 , 1986, pp. 33-36 .