Carbonated alkali-silicate metasomatism in the North Africa lithosphere: Evidence from Middle Atlas spinel-lherzolites, Morocco

Mantle xenoliths from Pliocene-Quaternary volcanic diatremes in the Azrou area (Middle Atlas, Morocco) include protogranular to porphyroclastic spinel lherzolites with superimposed metasomatic textures involving pyroxenes and spinel as the main reacting phases. Thermobarometric estimates on these xenoliths show pressure (P)–temperature (T) equilibrium conditions in the range 1.1–1.4 GPa and 900–1090 °C. Bulk rocks have flat heavy rare earth element (HREE) patterns (0.6–2.2 times chondrite) and are variably enriched in light REE (LREE; LaN/YbN between 1.1 and 15.6). The constituent clinopyroxenes are characterized by flat HREE distributions (5.1–11.9 times chondrite) and variable LREE enrichment with LaN/YbN from 0.4 to 25, which generally conform to the bulk rock chemistry. Trace elements characteristics of the metasomatised clinopyroxenes suggest that the metasomatasing agents were highly alkaline carbonate-rich melts such as nephelinites/melilitites or, as extreme, silico-carbonatites. Sr–Nd isotopic composition analyses carried out on clinopyroxene separates yield 87Sr/86Sr = 0.70243–0.70335, 143Nd/144Nd = 0.51273–0.51325. The helium isotope composition of olivines ranges from 6.2 to 6.8 Ra. These values, coupled with the radiogenic lead isotopic composition available in the literature, suggest that the Middle-Atlas lithospheric mantle interacted with HIMU-like metasomatic components. These are in turn related to local mantle upwellings along pre-existing tectonic lineaments – located at the northern border of the West African Craton – that were reactivated as far-field foreland reaction of the Africa-Europe collisional system