VOLCANIC CLAST-RICH TURBIDITES OF THE TAVEYANNE SANDSTONES FROM THE THONES SYNCLINE (SAVOIE, FRANCE) - RECORDS FOR A TERTIARY POSTCOLLISIONAL VOLCANISM

To investigate the nature and the provenance of the volcanic clasts of the Taveyanne sandstones from the Thones syncline (Savoie, France), m odal petrographic, mineralogical and geochemical analyses were perform ed on selected samples in some stratigraphic sections. The content of the volcanic materials ranges from 65 to 92% by volume. Non volcanic c omponents include plutonic/gneissic rock fragments and intrabasinal an d extrabasinal carbonates. Lithic clasts of volcanic origin range from basaltic-andesites to dacites and minor rhyolites, with a predominanc e of an desites. Detailed modal petrographical investigations on two s elected stratigraphical sections have shown significant differences in the ratio mafic/acid volcanic lithics. Whole-rock analyses of samples with more than 90% by volume of volcanic clasts combined with micropr obe analyses of the main mineral phases of the volcanic clasts (clinop yroxene, plagioclase, amphibole, biotite) suggest a derivation from an orogenic, medium to high K calc-alkaline volcanism. The proved Lower Oligocene age of the Taveyanne volcanics and their orogenic character strongly suggest that the Taveyanne volcanism is related to the high-K igneous activity proving rocks which crop out in a more internal part of the Alpine belt (andesitic to ultrapotassic dikes, plutons and rar e volcanic flows). It is proposed that all these high-K magmatic manif estations are genetically linked; they do not correspond to an Upper C retaceous island are system as previously suggested by other authors, but they represent the record of the Tertiary postcollisional Alpine m agmatic activity.