Rhyolitic pyroclastic deposits, mostly ignimbrites (ash-and-pumice flow deposits) from plinian eruptions belonging to the early activity (> 2.5 Ma) of the Plio-Quaternary Monts Dore volcanic field (Massif central, France) are documented in order to provide lithofacies and compositional data that could serve as a rational basis for ignimbrite discrimination and tephrostratigraphic correlations. We define or redefine six major pyroclastic units (five ignimbrites and one pumice fallout deposit), based mainly on pumice texture and mineral assemblage. Pumices and quartz-hosted glass inclusions are rather uniform compositionally, being high-silica rhyolites with very low CaO, MgO and FeO contents. Juvenile mineral assemblages are K-feldspar ± quartz ± plagioclase, with rare biotite ± amphibole + accessory phases (mostly sphene, zircon, oxides, apatite). The feldspars, biotite and amphibole compositions of all units strongly overlap and poorly constrain ignimbrite discrimination. These data provide clues to some volcanological issues under debate in the Monts Dore volcanic field. The major rhyolitic pyroclastics found on the Monts Dore margins, e.g. the conspicuous « Grande Nappe » unit of previous authors, are here interpreted as stratigraphical units beneath those exposed in proximal areas, and concealed in the central structural depression (caldera) suggested by previous gravimetric studies. The poorly constrained caldera geometry should be envisioned as a compound, polygenic structure related to several ignimbrite eruptions spanning at least 350 ky, whose vent locations may have shifted with time. The very scarce Fe-Mg silicate phases, biotite and amphibole found in the pumices and ignimbrite matrices show complex compositional populations which can be explained only partly by xenocrystic contamination and require further petrological investigations.
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December 01, 2016
Composition and correlation criteria of the rhyolitic ignimbrites in the Monts Dore volcanic field (France)
Jean-Louis Bourdier;
1
Institut des Sciences de la Terre d’Orléans (ISTO), UMR 7327 – CNRS/Université d’Orléans/BRGM, 1A Rue de la Férollerie, 45071 Orléans cedex 2, France*
Corresponding author : jean-louis.bourdier@univ-orleans.fr
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Marion Auxerre;
Marion Auxerre
1
Institut des Sciences de la Terre d’Orléans (ISTO), UMR 7327 – CNRS/Université d’Orléans/BRGM, 1A Rue de la Férollerie, 45071 Orléans cedex 2, France
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Laurent Arbaret;
Laurent Arbaret
1
Institut des Sciences de la Terre d’Orléans (ISTO), UMR 7327 – CNRS/Université d’Orléans/BRGM, 1A Rue de la Férollerie, 45071 Orléans cedex 2, France
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Michel Pichavant
Michel Pichavant
1
Institut des Sciences de la Terre d’Orléans (ISTO), UMR 7327 – CNRS/Université d’Orléans/BRGM, 1A Rue de la Férollerie, 45071 Orléans cedex 2, France
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Bulletin de la Société Géologique de France (2016) 187 (6): 261–273.
Article history
received:
02 May 2016
accepted:
26 Oct 2016
first online:
14 Jul 2017
Citation
Jean-Louis Bourdier, Marion Auxerre, Laurent Arbaret, Michel Pichavant; Composition and correlation criteria of the rhyolitic ignimbrites in the Monts Dore volcanic field (France). Bulletin de la Société Géologique de France 2016;; 187 (6): 261–273. doi: https://doi.org/10.2113/gssgfbull.187.6.261
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Index Terms/Descriptors
- Cenozoic
- Central Massif
- correlation
- Europe
- France
- igneous rocks
- ignimbrite
- mineral assemblages
- mineral composition
- Mont-Dore France
- Neogene
- Pliocene
- Puy-de-Dome France
- pyroclastics
- Quaternary
- rhyolitic composition
- tephrostratigraphy
- Tertiary
- textures
- volcanic fields
- volcanic rocks
- Western Europe
- Grande Nappe
- Monts Dore volcanic field
Latitude & Longitude