L'aire géothermale de Chassolle est dans un environnement volcanique comprenant des volcans anciens (plus de 3 Ma) et modernes (200 000 ans environ). Dans les deux cas, il s'agit d'un volcanisme basaltique de volume modeste, à dynamisme effusif dominant, mais quelques maars sont aussi présents. L'étude pétrographique des laves indique qu'il n'y a pas de réservoir magmatique dans la région de Chassolle et que le système éruptif visible ne peut pas constituer la source de la chaleur dispensée dans l'aire géothermale. D'autres sources hypothétiques doivent être prises en considération.
Mots-clés: Roche volcanique, Néogène, Quaternaire, Composition alcaline, Origine, Flux chaleur, Puy-de-Dôme (Région Chassolle).
Chassolle geothermal area is located in a region of volcanic rocks that range in age from Pliocene to Pleistocene.
The older volcanic rocks are aphyric to slightly porphyritic basanite lava flows that contain olivine, clinopyroxene, Ti-magnetite and plagioclase phenocrysts. The lavas were erupted from fissures. Cinder cones situated at the periphery of the Cezallier volcanic massif mark vents that fed lava flows which filled a Pliocene fluvial valley system. The Pliocene volcanic formations are strongly eroded; cinder cones are almost entirely devoid of tephra, and intracanyon lava flows form plateaus.
Initial topography can be inferred from the character of contacts between the lava flows and their basement, and a tentative Pliocene map is presented.
Older volcanic products are generally undifferentiated, with the exception of some lava flows displaying amphibole megacrysts. Thus, they have not evolved in magma chambers. The exceptional Signal du Luguet differentiated lavas were produced in one magma chamber, located 10 km west of Chassolle. We believe, therefore, that the Pliocene volcanic activity, because it is too old (3 m.yr.) and undifferentiated, cannot be responsible for the present heat flow in Chassolle area.
The younger activity, about 200 000 years old, is restricted to four volcanic centres. Their history generally began with hydromagmatic explosive events, followed by cinder cone building synchronous with lava flows filling valleys. Present fluvial erosion locally cuts across lava flows, as exemplified by Mazoires-Rentieres area, but as a rule is restricted to flow morains, resulting in 40 m high cliffs over the stream bed. As volcanic products are chiefly composed of undifferentiated alkali basalts, this activity cannot be responsible of the present heat flow in Chassolle area.
We propose, as working hypotheses, that heat flow is produced either by a late-Quaternary intrusive body, or by an incipient magma chamber, which could result in future volcanic activity. However, neither geological nor seismological data support these hypotheses. Therefore, we favour the model of heat flow emanating from a diapiric asthenospheric source.
Dernière mise à jour le 26.02.2019