Résumé
The geochemistry of high-Mg pyroxenite dykes in the Ronda peridotite (S Spain) shows that the subcontinental lithospheric mantle in the westernmost Mediterranean evolved above a subducting slab shortly before its intracrustal emplacement. We present new SrNd-Pb isotopic data of crustal rocks in the western Betics that might account for the isotopic signature of Ronda high-Mg pyroxenite. We show that lower crustal garnet granulites from the Alpujarride complex, which overly the peridotites, account for the characteristic relatively high 207Pb-208Pb/204Pb and low 206Pb/204Pb subductioncomponent of Ronda high-Mg pyroxenite. The involvement of fluids/melts derived from lower crust in the genesis of high-Mg Ronda pyroxenite indicates that the peridotites in the westernmost Mediterranean were placed in a forearc position above a newly initiated subduction zone, followed by underthrusting of the lower crust. This possibly occurred during the late Oligocene due to the inversion of an extensional back-arc continental basin.As subduction progressed, the thinned subcontinental mantle was emplaced over the foreland following its collision with the trench.