Magmatic Evolution of Pavin’s Group of Volcanoes: Petrology, Geochemistry and Modeling of Differentiation Processes. A Preliminary Study

Abstract

The volcanoes of the Pavin group (Montcineyre, Estivadoux, Montchal and Pavin) represent the most recent (~7 ky) volcanic activity in the Chaine des Puys s.l.) and also occupy a particular geographical and structural position in this volcanic chain. Petrology and geochemistry of the volcanic products (lava flows, scoriae, bombs and pumice clasts) of the different edifices show that they define a magma differentiation series from primary basalts (Montcineyre lava flow) to benmoreites (Pavin pumice clasts) with close similarities with that of the Chaine des Puys (stricto sensu). It is however characterized by some significant difference in the relative behavior of K2O and of numerous trace elements such as Sc, Co, Y, REE, Nb, Ta. These specific behaviors, in particular the relatively high bulk partition coefficients of Nb and Ta are characteristic of the fractionation of amphibole which is the only major mineral phase able to significantly fractionate these incompatible elements in basaltic to benmoreitic magmas. A quantitative modeling of the differentiation process using major and trace element partition coefficients is proposed which evidence the very early fractionation of amphibole from the most primary basaltic melts. This result is consistent with petrological observations and confirms the earlier and more efficient fractionation of amphibole at the expense of clinopyroxene in magmas of the Pavin group compared to those of the Chaine des Puys (stricto sensu). The results are used to discuss the influence of these specific differentiation conditions involving large H2O contents in primary melts likely contributing to the particularly high explosivity of the Pavin eruption which produced benmoreitic to trachytic magmas.