Sources of the andesitic components in the Taveyannaz Sandstones and Champsaur Sandstones: implications for the Paleogene geodynamic evolution of the Alps

Paleogene magmatism is widespread in the western Alps and its origin is still a matter of debate. It consists of calc-alkaline and shoshonitie suites with mainly granodioritic intrusions in the Internal Zone (northern Italy), andesitic volcanism in the Delphino-Helvetic Zone (French-Swiss domain) and the Esterellite intrusions in Provence. In the External Alpine Zone, the Taveyannaz Sandstones and the Champsaur Sandstones preserve andesitic pebbles and mineral fragments dated at 32 Ma. On the basis of trace-element and isotopic geochemistry (Sr, Nd, Pb and O) of mineral separates (amphibole, pyroxene and plagioclase) and andesitic clasts, we characterise the nature of magmatic source(s) and compare the isotopic compositions of the andesites with those of well known Paleogene igneous suites. Geological setting. - Andesitic material in the Taveyannaz Sandstones and the Champsaur Sandstones consists of volcanic pebbles, clasts and isolated mineral fragments such as amphibole, clinopyroxene and plagioclase. These sandstones also contain, a variable but always low proportion of sedimentary, plutonic and metamorphic components Calcareous nannofossil associations place the Taveyannaz Sandstones in Zone NP23, which corresponds to an age of 32-29 Ma [Ruffini et al., 1994]. 40Ar/39Ar dates from amphibole separates cluster between 30.5 and 32.5 Ma, suggesting that volcanism occurred during early Paleogene [Ruffini et al., 1994; Feraud et al., 1995]. 40Ar/39Ar analyses of amphiboles from the Champsaur Sandstones yielded two plateau ages of 32.5 +/- 2.2 and 34.3 +/- 1.0 Ma [Feraud et al. 1995]. Paleogene magmatism is widespread in the western Alps and its origin is still a matter of debate. It consists of calc-alkaline and shoshonitie suites with mainly granodioritic intrusions in the Internal Zone (northern Italy), andesitic volcanism in the Delphino-Helvetic Zone (French-Swiss domain) and the Esterellite intrusions in Provence. In the External Alpine Zone, the Taveyannaz Sandstones and the Champsaur Sandstones preserve andesitic pebbles and mineral fragments dated at 32 Ma. On the basis of trace-element and isotopic geochemistry (Sr, Nd, Pb and O) of mineral separates (amphibole, pyroxene and plagioclase) and andesitic clasts, we characterise the nature of magmatic source(s) and compare the isotopic compositions of the andesites with those of well known Paleogene igneous suites. Geological setting. - Andesitic material in the Taveyannaz Sandstones and the Champsaur Sandstones consists of volcanic pebbles, clasts and isolated mineral fragments such as amphibole, clinopyroxene and plagioclase. These sandstones also contain, a variable but always low proportion of sedimentary, plutonic and metamorphic components Calcareous nannofossil associations place the Taveyannaz Sandstones in Zone NP23, which corresponds to an age of 32-29 Ma [Ruffini et al., 1994]. 40Ar/39Ar dates from amphibole separates cluster between 30.5 and 32.5 Ma, suggesting that volcanism occurred during early Paleogene [Ruffini et al., 1994; Feraud et al., 1995]. 40Ar/39Ar analyses of amphiboles from the Champsaur Sandstones yielded two plateau ages of 32.5 +/- 2.2 and 34.3 +/- 1.0 Ma [Feraud et al. 1995]. Paleogene magmatism is widespread in the western Alps and its origin is still a matter of debate. It consists of calc-alkaline and shoshonitie suites with mainly granodioritic intrusions in the Internal Zone (northern Italy), andesitic volcanism in the Delphino-Helvetic Zone (French-Swiss domain) and the Esterellite intrusions in Provence. In the External Alpine Zone, the Taveyannaz Sandstones and the Champsaur Sandstones preserve andesitic pebbles and mineral fragments dated at 32 Ma. On the basis of trace-element and isotopic geochemistry (Sr, Nd, Pb and O) of mineral separates (amphibole, pyroxene and plagioclase) and andesitic clasts, we characterise the nature of magmatic source(s) and compare the isotopic compositions of the andesites with those of well known Paleogene igneous suites. Geological setting. - Andesitic material in the Taveyannaz Sandstones and the Champsaur Sandstones consists of volcanic pebbles, clasts and isolated mineral fragments such as amphibole, clinopyroxene and plagioclase. These sandstones also contain, a variable but always low proportion of sedimentary, plutonic and metamorphic components Calcareous nannofossil associations place the Taveyannaz Sandstones in Zone NP23, which corresponds to an age of 32-29 Ma [Ruffini et al., 1994]. 40Ar/39Ar dates from amphibole separates cluster between 30.5 and 32.5 Ma, suggesting that volcanism occurred during early Paleogene [Ruffini et al., 1994; Feraud et al., 1995]. 40Ar/39Ar analyses of amphiboles from the Champsaur Sandstones yielded two plateau ages of 32.5 +/- 2.2 and 34.3 +/- 1.0 Ma [Feraud et al. 1995]. Paleogene magmatism is widespread in the western Alps and its origin is still a matter of debate. It consists of calc-alkaline and shoshonitie suites with mainly granodioritic intrusions in the Internal Zone (northern Italy), andesitic volcanism in the Delphino-Helvetic Zone (French-Swiss domain) and the Esterellite intrusions in Provence. In the External Alpine Zone, the Taveyannaz Sandstones and the Champsaur Sandstones preserve andesitic pebbles and mineral fragments dated at 32 Ma. On the basis of trace-element and isotopic geochemistry (Sr, Nd, Pb and O) of mineral separates (amphibole, pyroxene and plagioclase) and andesitic clasts, we characterise the nature of magmatic source(s) and compare the isotopic compositions of the andesites with those of well known Paleogene igneous suites. Geological setting. - Andesitic material in the Taveyannaz Sandstones and the Champsaur Sandstones consists of volcanic pebbles, clasts and isolated mineral fragments such as amphibole, clinopyroxene and plagioclase. These sandstones also contain, a variable but always low proportion of sedimentary, plutonic and metamorphic components Calcareous nannofossil associations place the Taveyannaz Sandstones in Zone NP23, which corresponds to an age of 32-29 Ma [Ruffini et al., 1994]. 40Ar/39Ar dates from amphibole separates cluster between 30.5 and 32.5 Ma, suggesting that volcanism occurred during early Paleogene [Ruffini et al., 1994; Feraud et al., 1995]. 40Ar/39Ar analyses of amphiboles from the Champsaur Sandstones yielded two plateau ages of 32.5 +/- 2.2 and 34.3 +/- 1.0 Ma [Feraud et al. 1995].