Geochemistry and petrogenesis of Eastern Pyrenean peridotites

Abstract

The high-temperature peridotite bodies of the Eastern Pyrénées (France), which are composed of spinel peridotites containing bands of pyroxenites and veins of amphibole-bearing ultrabasic rocks, have gone through a multi-stage evolution. The peridotites underwent partial melting in the stability field of garnet resulting in major variations of Mg, Al, Ca, Na, Ti, Sc, V, Ni and HREE. Then the peridotite residue was invaded by basaltic melts. The pyroxenite bands in the peridotites are high-pressure crystal segregates from these melts. Subsequently, after cooling in subcontinental lithospheric conditions, the peridotites interacted with alkali magma which was probably associated with the Cretaceous alkali magmatism of the Pyrénées. In addition to the crystallization of amphibole-rich ultrabasic rocks in vein-conduits and the re-equilibration of the wall-rock peridotites leading to LREE, Ti and Fe enrichments, this event was accompanied by extensive metasomatic processes. The metasomatism locally affected lherzolites, producing an increase of the modal proportions of clinopyroxene (± amphibole) (Caussou massif). The metasomatism was more widespread in the harzburgites where it produced an enrichment of LREE relative to HREE without a significant change in the modal composition.