87Rb-87 Sr constraints on the genesis and evolution of the Cantal continental volcanic system (France)

The authors have investigated 24 whole rocks and mineral separates of five different rock types from the Cantal shield volcano in France, applying high-precision Rb-Sr techniques. The chemical and isotopic systematics suggest the distinction of two series throughout the different rock classes, one practically uncontaminated, the other seriously influenced by wall rock assimilation. The first group comprises basalts and intermediate rocks with 87Sr/86Sr = 0.70340-0.70382. The second group in addition includes rhyolites and the corresponding 87Sr/86Sr ratios vary between 0.70421 and 0.71270. The data of mineral separates support the hybridization hypothesis and possibly suggest an original 87Sr/86Sr ratio of 0.7028 for the magma source region. Moreover they provide internal isochron ages which place a period of extensive volcanic activity at 8.1-8.8 m.y. ago in accord with K-Ar ages of volcanic rocks from the center of the Cantal volcano. (Auth.)