Evidence for an Extended Post-Collisional
Calc-Alkaline Volcanic System along the Alpine Belt:
the Characterization of Volcanism through Clinopyroxene and Amphibole Composition

Meinert Rahn Institut für Mineralogie, Petrologie und Geochemie, University of Freiburg, Germany


Raffaella Ruffini CNR - Geodinamica Catene Collisionali, Torino, Italy

and Institut de Minéralogie, University of Lausanne, Switzerland

David Schmidt Mineralogisch-Petrographisches Institut, University of Basel, Switzerland


In the northern external parts of the Swiss and French Alps, detrital volcanic material can be found within several flysch units demonstrating volcanic activity during ongoing orogeny. The Taveyanne Formation is the best known host rock of volcanic material, not simply because in this formation the amount of volcanic material makes up to 90% of the turbiditic flysch sandstones (Vuagnat, 1952), but also because Taveyanne sandstones occur for several hundreds of km from eastern Switzerland (Glarus Alps) to the Western Alps in western Switzerland and France (Haute Savoie; Stalder (1979)). The lower Oligocene age of the volcanic material (Ruffini et al., 1995) suggested a close relation to the contemporaneous Tertiary Periadriatic intrusions (Dal Piaz and Venturelli, 1983). The volcanic material was eroded and mixed with material of mainly sedimentary origin, and underwent Alpine very low-grade to low-grade metamorphism (Rahn et al., 1994; Schmidt et al., 1995). Thus, the use of whole-rock data to clarify the origin of these volcanic rocks is limited by several assumptions (Rahn et al., 1995).

Remaining Volcanogenic Minerals

The aim of the study is to compare the mineralogy of Taveyanne volcanic rock fragments along the whole Alpine arc in order to verify if magmatic characteristics of the volcanism change laterally. For this purpose compositional data on primary magmatic clinopyroxenes, amphiboles, some plagioclase and biotite are presented. Relicts of clinopyroxenes are ubiquituous and show a restricted chemical variation within the field of diopsides and augites, a slight increase in Ca relative to Fe and Mg is observed in the more acid lava clasts. Amphiboles are restricted to the western areas, and include magnesio-hornblende and edenites within intermediate lava clasts, but are pargasitic within dacitic clasts. Plagioclases are rarely preserved from alteration into albite. Primary grains exhibit anorthite ranging from labradiorite to andesine, becoming more albite-rich in dacitic clasts. Remnants of biotite are Ti-rich. The chemical composition of the main phases in the lava fragments strongly suggest a high-K calc-alkaline magmatic source.

Magmatic and Tectonic Setting

Various discrimination diagrams have been proposed for cpx in order to discriminate between different magmatic and tectonic settings (Le Bas, 1962; Leterrier et al., 1982). In these diagrams all areas along the Alpine arc show the same pattern, referring to calc-alkaline magmatism. Minor element distributions and eigenvector calculations (Nisbet and Pearce, 1977; Morris, 1988) point to orogenic andesites. The calculation of crystallization temperatures out of the relict magmatic phases (e.g. Blundy and Holland, 1990; Kretz, 1982; Schmidt, 1992) yields pressures of around 9 kbar and temperatures close to 900°C, which are consistent with melt generation at a lower crust level, suggesting a subduction-related context for the magma generation. The Oligocene age and the chemical characteristics of the Taveayanne volcanic rocks suggest that the emplacement of the volcanism was essentially post-collisional.


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