A low delta Li-7 lower crustal component: Evidence from an alkalic intraplate volcanic series (Chaine des Puys, French Massif Central)

摘要

The intraplate volcanic suite of the Chaine des Puys (French Massif Central) shows a complete petrologic range, from alkali basalts to trachytes. The significant variations of trace elements and radiogenic isotopes along the series strongly support the occurrence of crustal assimilation associated with fractional crystallization (AFC). The least contaminated basalts are clearly related to a HIMU-type reservoir (Pb-206/Pb-204>19.6; Sr-87/Sr-86<0.7037; epsilon(Nd)>+4). The behavior of radiogenic isotopes suggests that the most likely crustal contaminants are meta-sediments located in the lower crust. The Li isotopic compositions of the lavas range from high delta Li-7 (> + 7 parts per thousand) in basalts to lighter values in more evolved lavas (down to delta Li-7 approximate to 0 parts per thousand). The mantle component, expressed in the least evolved lavas, has a heavy Li isotopic signature, in good agreement with previous delta Li-7 measurements of OIB lavas with HIMU affinities. The evolution of Li isotopic compositions throughout the volcanic series is in agreement with the AFC model suggested by the Sr-Nd-Pb isotopic systems. Although the behavior of Li isotopes during assimilation processes is currently poorly constrained, our calculations suggest that at least a portion of the lower crust beneath the Chaine ties Puys is characterized by a light Li isotopic composition (delta Li-7<-5 parts per thousand).