Composition and correlation criteria of the rhyolitic ignimbrites in the Monts Dore volcanic field (France)

Abstract

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.