Puy de Chalard

Puy de Chalard
The Puy de Chalard
height	840  m
location	Puy-de-Dôme department , France
Mountains	Chaîne des Puys , Massif Central
Coordinates	45 ° 57 '33 "  N , 2 ° 58' 27"  E Coordinates: 45 ° 57 '33 "  N , 2 ° 58' 27"  E
Type	Cinder cone
rock	Trachy basalt
Age of the rock	51,050 ± 7,900 years BP

The Puy de Chalard is an extinct volcano in the French department of Puy-de-Dôme , which belongs to the Chaîne des Puys . It is made of potassium-rich trachy basalt .

geography

The Puy de Chalard, also Puy du Chalard or just Puy-Chalard or Puy Chalard , is located in the area of ​​the municipality of Manzat , around 40 kilometers northwest of Clermont-Ferrand , around 7 kilometers west of the marginal fault of the Oligocene Limagne Rift. After the Gour de Tazenat , which joins 2.5 kilometers to the north-northeast, it is the second northernmost volcanic structure in the Chaîne des Puys. The Maar de Beaunit is seven kilometers south . This distance between volcanic centers, which is quite large for the Chaîne des Puys, is interrupted halfway by two small basalt deposits (ejecta from a basaltic maar) at the Puy de Roche Noire .

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

The Puy de Chalard from the northwest

The northern section of the Chaîne des Puys is characterized by phreatomagmatic volcanic constructs. These include the Puy de Chalard, which sits on a hidden maar, the Gour de Tazenat, the Maar de Beaunit and the Maar de Laty . All of these structures are located in a fault zone , which runs through a bulge in the predominantly granitic basement.

The Puy de Chalard is a composite volcano as it occurs quite often in the Chaîne des Puys, Devès and Cézallier . It consists of a phreatomagmatic maar and an igneous, strombolian cinder cone of trachybasaltic composition. An explosion subsequently blasted out part of its northwest flank and sent a likewise trachybasaltic lava flow to the northwest. The maar was formed first and then almost completely covered by the cinder cone building up above it.

Not much of the original maar has survived, but its dimensions can still be identified geomorphologically by means of an almost circular depression in the area, in the center of which the later cinder cone was raised. The edge of the maar is also marked by basement outcrops.

To the east of the hamlet of Touzet (located south of the Puy de Chalard), ejecta breccia from the maar can still be seen. They have a thickness of 6 to 7 meters and dip slightly to the northwest , probably because of the existing palaeo-relief . The upcoming breccias consist of angular fragments of granite , micro- granite , rhyolite tuffs of the Viséum , as well as lapilli and basalt blocks , including some with a "bread crust structure".

Petrology

The leaked lava flow is a potassium-rich trachy basalt. It has clinopyroxene , olivine and plagioclase as phenocrystals . In its matrix there are microcrystals of plagioclase, magnetite and apatite .

The following chemical composition was determined for the trachy basalt:

Oxide wt.%	Trachy basalt	Trace element	ppm
SiO 2	47.76	Sc	25.9
TiO 2	2.19	V	243
Al 2 O 3	16.01	Cr	114
Fe 2 O 3	11.56	Co	34
MnO	0.19	Ni	54
MgO	6.37	Zn	79
CaO	9.40	Rb	50
Na 2 O	3.50	Sr	690
K 2 O	1.72	Zr	225
P 2 O 5	0.57	Ba	546
H 2 O	0.39	La	48.7

Age

By taking a sample on the northwest flank of the volcano, Guérin (1983) was able to determine an age of 51,050 ± 7,900 years BP using thermoluminescence .

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. Jump up ↑ Jeambrun, Michel: Le Puy Chalard et le maar de Laty, appareils complémentaires du système phréatomagmatique développé au Nord de la Chaîne des Puys (Massif Central Français) . In: Bull. Soc.géol. France . (7), 1, XXV, n ° 2, 1983, p. 273-275 .
4. ↑ Guérin, G .: La thermoluminescence des plagioclases, méthode de datation du volcanisme. Applications au domaine volcanique français: Chaîne des Puys, Mont Dore et Cézallier, Bas-Vivarais. (PhD thesis) . Université Pierre et Marie Curie - Paris VI, Paris 1983.