Petrology of the alkaline magmatism from the Cretaceous North-Pyrenean rift zone (France and Spain)

The Cretaceous alkaline magmatism from this zone consists of a variety of rock types: basalts, trachytes, lamprophyres, gabbros, teschenites, ultramafic cumulates and nepheline syenites. EPMA values for pyroxenes and amphiboles are used to trace the magmatic differentiation. In alkali basalts and monchiquites, the composition of early pyroxenes and amphiboles suggests fractionation in upper-mantle to lower-crustal conditions, whereas pyroxene phenocryst rims and microphenocrysts are Al- and Ti-rich, consistent with lower-P crystallization. Modelling of major and trace elements suggests that trachytes and non-peralkaline feldspathoidal syenites could be extracted from mafic parents by 70-80% crystallization of a gabbroic cumulate. The mantle source appears homogeneous in its REE contents throughout the Pyrenees and is shown to be LREE-enriched with (La/Yb) (sub N) approx 5.