Petrology of the alkaline magmatism from the Cretaceous North-Pyrenean Rift Zone (France and Spain)

Abstract

The Cretaceous alkaline magmatism from the North-Pyrenean Rift Zone consists of a variety of effusive and intrusive rocks: basalts, trachytes, lamprophyres, gabbros, teschenites, ultramafic cumulates and nepheline syenites. An extensive survey of the magmatic mineralogy in the different rock-types is presented, and pyroxene and amphibole compositions are used to trace the magmatic differentiation. In alkali basalts and monchiquites, the composition of early pyroxenes (coloured Fe-rich xenocrysts and Cr-rich phenocryst cores) and amphiboles suggests fractionation in upper-mantle to lower-crustal conditions, whereas pyroxene phenocrysts rims and microphenocrysts are Al- and Ti-rich, consistent with crystallization at lower pressure. Modelling of major and trace elements suggests that trachytes and non-peralkaline feldspathoidal syenites could be extracted from mafic parents by 70-80% crystallisation of a gabbroic cumulate but the model fails to identify parental magmas of peralkaline nepheline syenites. The mantle source appears homogeneous in its REE contents over the Pyrenees and is shown to be LREE-enriched with a (La/Yb) N ratio close to 5