Puy de la Nugere

Puy de la Nugere
Eastern flank of the Puy de la Nugère with the Pozzulane quarry
height	994  m
location	Puy-de-Dôme department , France
Mountains	Chaîne des Puys , Massif Central
Coordinates	45 ° 51 '52 "  N , 2 ° 59' 4"  E Coordinates: 45 ° 51 '52 "  N , 2 ° 59' 4"  E
Type	Cinder cone
rock	Trachy basalt and trachyandesite
Age of the rock	14,000 to 10,900 years BP

The Puy de la Nugère is an extinct volcano ( cinder cone ) in the French department of Puy-de-Dôme ( Auvergne-Rhône-Alpes region ), which belongs to the Chaîne des Puys .

geography

The composite volcano Puy de la Nugère is located around 20 kilometers northwest of Clermont-Ferrand , on the western part of the municipality of Volvic . Among the large volcanic complexes of the Chaîne des Puys, it is the most northerly (there are, however, several smaller, more insignificant volcanic centers even further north). It sits on a plateau at 800 meters above sea level and reaches 994 meters. Since it is structured quite complex - it consists of several nested cinder cones that go back to different eruption stages - it occupies a significant surface. Its main crater is 82 meters deep and pierced to the east.

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 at the beginning of the Würm Ice Age around 100,000 years BP and lasted up to almost 4,000 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 via mugearite and benmoreite (rare) to trachytes and rhyolites (rare). 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 la Nugère is a composite volcano made up of several ash rings, strombolian cinder cones, six old lava flows and a solidified lava lake. The chemical composition of the lavas ranges from basalts to trachytes.

The volcano consists of a perforated crater that was occupied by a temporary lava lake of trachyandesite composition. Its cooled remains can in some places reach a thickness of up to 40 meters.

In the breakthrough zone, four strombolian cones have formed, from which lava flows emanate. On the eastern flank, two older, partly buried cones can also be seen, which, in addition to basalt and trachytic ashes, also produced torn basement fragments, which were ejected during an initial, very explosive phase.

A total of six lava flows had emerged, all of which flowed over the valley in the east towards Volvic. The longest of them reached the Limagnegraben near Marsat . The streams are now stacked on top of each other and separated by layers of ash and lapilla. Their maximum total thickness was drilled at 184 meters.

The two oldest are compact, dark gray trachybasalts. They contain up to 3 millimeters in size phenocrystals of olivine , clinopyroxene ( augite ) and plagioclase ( bytownite ), as well as microliths of the same minerals and volcanic glass . The next higher two streams are quite compact gray basalts, the augite phenocrystals of which reach a size of up to 5 millimeters; they also contain phenocrystals of olivine and bytownite. In addition to microliths of the three mentioned minerals, small apatite needles can also be seen in the microscope . Phenocrystals and microliths by far exceed the glass fraction. The two lava flows in the hanging wall consist of trachyandesite and are popularly referred to as "Andesite of Volvic" (which is strictly not correct) or as "Stone of Volvic" ( French Pierre de Volvic ). These are gray, low-phenocrystalline volcanic rocks which, due to their abundance of bubbles (interspersed with many small, irregularly shaped vacuoles), appear almost pumice-like . Occasionally, rare crystals of hornblende ( amphibole ) and plagioclase ( andesine ) can be seen with the naked eye. Two generations of microliths can be distinguished under the microscope: hornblende and andesine as a coarser fraction, among which smaller microliths of oligoclase and pyroxene , themselves surrounded by light-colored glass, have mixed.

Course of the eruption

Volcanological investigations revealed the following course of the eruption at the Puy de la Nugère:

• Stage of the “Old Nugère” - two very explosive strombolic cinder cones arise, which eject trachytes and then trachyandesites together with fragments of the crystalline basement.
• Stage of the "Great Nugère" - a very large strombolic cinder cone builds up, which partially overlays its two predecessors. The bottom four lava flows emanate from it (trachy basalts followed by basalts).
• Stage of the “New Nugère” - during an explosive phase, an explosion crater measuring 950 × 800 meters is created; at the same time a ring of ashes is thrown up, covering the west side of the Nugère. A lava lake collects in the newly formed crater, which then overflows on the east and north side and solidifies as a result.
• Stage of the “Puy de la Louve” - four strombolic cinder cones appear on the solidified lava lake, including the Puy de la Louve . They send out the last two lava flows with a trachyandesite composition, which, according to the current state of knowledge, represent the last secured volcanic activity on the Puy de la Nugère.

Age

Some tephra layers are known from the Puy de la Nugère , including several from Allerød . For example, the tephrene complex CF1a / CF1b ( Creux-Rouge-Tephra ) covers the period 14,000 to 13,000 years BP. The La-Moutade-Tephra could be set at 13,700 ± 400 years BP (calibrated). The Cellule-Tephra is counted to the Elder Dryas (13,540 to 13,360 years BP). A tephra (calibrated) from 13,405 to 13,090 years BP, also from the Puy de la Nugère, was found in a bog in the Monts du Forez and in lakes in the French Jura . In the Jura it occurs as two layers, which are estimated at 13,420 to 13,010 and 13,300 to 12,880 years BP. The 12.010 ± 150 year old Les Roches Tephra may also come from the Puy de la Nugère.

In connection with an investigation by Pierre de Volvic, the last eruptive stage was dated using thermoluminescence on plagioclase. It resulted in 10,900 ± 1,200 years BP. Several tephra layers in the Cézallier are assigned to this last stage of the Puy de la Nugère , such as "Tephra 4" from Godivelle-Nord 50 kilometers further south, which in turn could be dated to 10,300 years BP and thus a good temporal correspondence with the last lava flow of the Stage 4 has.

Overall, it can be seen that the volcanic activities on the Puy de la Nugère began no later than 14,000 years BP and lasted at least up to 10,300 years BP.

Petrology

Mineral inventory

The trachyandesite Volvic stone, a potassium-bearing Benmoreite , has clinopyroxene , plagioclase , iron-titanium oxide, amphibole , apatite and zirconium as phenocrystals . The microliths are olivine , alkali feldspar and pigeonite .

Chemical composition

The following geochemical analyzes were carried out on the rocks of the Puy de la Nugère:

Oxide wt.%	New Nugère initial phase	New Nugère final phase	Volvic stone K-Benmoreit	Old Nugère Mugearit	Old nugère trachyte	Cellule-Tephra Mugearite	Les Roches Tephra Benmoreit	Creux Rouge Tephra Trachyandesite	Godivelle tephra
SiO 2	62.45	56.98	57.10	52.20	65.05	51.96	53.60	60.60	55.48
TiO 2	1.01	1.53	1.12	1.65	0.50	1.62	1.86	1.30	1.67
Al 2 O 3	17.51	17.56	17.89	17.65	19.65	16.95	18.50	17.10	19.36
Fe 2 O 3	2.39	6.21	6.83	5.85	3.25	8.24	5.80	6.10
FeO	1.81	0.91		4.50
MnO			0.19	0.20	0.15	0.19	0.13	0.08	0.11
MgO	1.11	2.41	1.94	3.45	0.75	3.27	1.99	3.70	1.82
CaO	3.11	5.16	4.53	7.15	1.25	7.36	7.83	2.90	6.07
Na 2 O	5.35	4.68	5.42	4.30	5.05	4.50	4.75	2.50	4.46
K 2 O	3.71	3.06	3.40	2.10	3.90	2.39	2.80	6.60	4.86
P 2 O 5	0.21	0.23	0.55	0.60	0.10
H 2 O	0.69	0.50	0.05	0.10	0.50	1.56	1.88	0.08

Rock compositions from the Puy de la Nugère in the TAS diagram . Blue crosses are tephren. The compositions realized throughout the Chaîne des Puys are highlighted in yellow.

The Volvic stone is undersaturated with quartz and normatively does not contain nepheline , but olivine . The following normative components can be determined for him: or 20.42%, from 46.60%, an 14.69%, where 1.88%, di 1.90%, en 2.88, fs 4.32, ol 2.36, mt 1.50%, il 2.16% and ap 1.33%. The rock is thus in the border area between the SiO ₂ -undersaturated and the SiO ₂ -saturated members of the alkaline fractionation series in the Chaîne des Puys - the undersaturated members due to normative nepheline and the oversaturated members due to the occurrence of tridymite or cristobalite and normative Olivine to be labeled.

Individual evidence

1. ↑ Maurice Krafft and François-Dominique de Larouzière: Guide des volcans d'Europe et des Canaries . Paris 1991, ISBN 2-603-00813-7 , pp. 455 .
2. ↑ 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 , p. 267-286 .
3. ↑ 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 .
4. ↑ Vernet, G .: Les dépôts pyroclastiques distaux: caractérisations, établissement de téphrostratigraphies de référence, taphonomie et relations homme / volcan. Mémoire d'Habilitation à Diriger des Recherches . Université Blaise Pascal, Clermont-Ferrand, France 2011.
5. ↑ Vernet, G. et al .: La retombée de la Moutade, marqueur stratigraphique de l'Alleröd en Limagne septentrionale (Massif Central, France) . In: CR Acad. Sci. Paris . 310, series II, 1990, pp. 1077-1082 .
6. ↑ Etlicher, B., Janssen, CR, Juvigné, E. et al: Le Haut Forez (Massif Central, France) aprés le pléniglaciaire worms: environnement et téphra du volcan de la Nugère . In: Bulletin AFEQ . tape 4 , 1987, pp. 229-239 .
7. ↑ Guérin, G .: Thermoluminescence des plagioclases. Méthode de datation du volcanisme. Applications au domaine volcanique français: Chaîne des Puys, Mont Dore et Cézallier, Bas Vivarais. Thèse d'Etat . Université Pierre et Marie Curie, Paris 1983, p. 253 .
8. ↑ Juvigné, E., Bastin, B. and de Goër de Hervé, A .: Nouvelles données sur les tephras et l'environnement tardiglaciaires du Cézallier et de l'Artense (Massif Central, France) . In: Annales de la Société géologique de Belgique . 117 fascicule 2, 1994, p. 321-332 .
9. ↑ Maury, RC et al .: Cristallisation fractionnée d'un magma basaltique alcalin: la série de la Chaîne des Puys (Massif Central, France). I. Petrology . In: Bull. Minéral. tape 103 , 1980, pp. 250-266 .