Spinel to plagioclase reaction in high heat flow mantle domains entrapped in uprising lavas: the case study of mantle xenoliths from Azrou-Timahdite, Middle Atlas, Morocco

Mantle xenoliths (lherzolites and a wehrlite) found in Cenozoic alkaline lavas from Azrou-Timahdite (Middle Atlas, Morocco) were petrographically studied with a particular emphasis on mineral chemistry. The most striking feature of these rocks is that plagioclase formed as a result of Al-rich spinel destabilization. This reaction is recorded in different steps and determined a variety of microstructures that allowed to identify four groups of xenoliths. Spinel chemistry generally becomes more refractory approaching the magma-xenolith interface. Geothermobarometry of both xenoliths and host lavas based on pyroxene equilibria and Forsterite-Anorthite-Ca Tschermak-Enstatite (FACE) geobarometer is consistent with a scenario in which spinel to plagioclase reaction was triggered at 0.7-1 GPa by hot (> 1100 degrees C) upraising magma in a mantle already characterized by a high heat flow (> 80 mW/m(2)). The entrapment of the xenoliths by the host magma en route to the surface caused a chemical interaction resulting in orthopyroxene destabilization with the formation of anorthoclase. The high geothermal gradient outlined by the mineral chemistry of the xenoliths is consistent with the geodynamic setting of the area, where the lithosphere has been thinned due to the upraising of a mantle plume active since Mesozoic and producing the Mesozoic Central High Atlas basalts.