Metasomatism in the Lithospheric Mantle beneath Middle Atlas (Morocco) and the Origin of Fe- and Mg-rich Wehrlites

摘要

Mantle xenoliths enclosed in the Plio-Quaternary alkaline basalts from Bou Ibalrhatene, Middle Atlas, Morocco, are characterized by a wide range of lithological and chemical heterogeneity, consistent with metasomatism of their lithospheric mantle source. Subordinate porphyroclastic to protogranular spinel-lherzolites associated with websterites exhibit major and trace element signatures, along with a depleted mantle isotopic affinity, that testify to ancient melt extraction processes, possibly during Neo- to Paleoproterozoic times. These samples show large ion lithophile element (LILE) enrichments that have been imparted a long time after the accretion of their protoliths to the lithosphere, induced by alkaline melts rising from upwelling HIMU-like asthenosphere since the late Cretaceous or Eocene. Extensive sectors of the percolated lherzolite and harzburgite mantle progressively approached chemical equilibrium with the migrating melts, forming porphyroblastic amphibole-rich ultramafic rocks with strong LILE enrichment, highly variable (enriched to strongly depleted) high field strength element (HFSE) abundances and HIMU-like isotopic signatures. As a result, the older depleted lithosphere was progressively refertilized. The presence among the Bou Ibalrhatene ultramafic xenoliths of both Fe- and Mg-rich wehrlites is a rare occurrence in xenolith suites. The Fe-rich wehrlites formed by the migration through dunitic channels, which opened during earlier reactive porous flow, of larger and larger volumes of rising alkaline melts, becoming progressively less evolved in composition and more similar to the erupted lavas. In contrast, subordinate Mg-rich wehrlites characterized by the presence of apatite, extreme LILE enrichment and HFSE depletion testify to the localized chemical effects of either carbonatite melts related to the oldest (i.e. Eocene) volcanic activity in the area or, more probably, to highly evolved SiO₂-saturated silicate melts that vanished through interaction with the lithosphere. As a whole, the textural, petrographic and chemical characteristics of the Bou Ibalrhatene xenoliths suggest prolonged thermo-chemical erosion of an originally heterogeneous lithosphere with a dominant depleted mantle isotopic signature by melts rising from a HIMU-like asthenospheric mantle source in response to intraplate tectonic reactivation and rifting of the Pan-African basement. The overall scenario envisaged for lithosphere–asthenosphere interaction beneath the Middle Atlas is similar to that inferred for other Cenozoic Central–Northern African volcanic centres, such as Hoggar (Algeria) and Gharyan (Libya). The majority of the mantle sectors beneath the NE–SW volcanic alignment associated with the Trans-Moroccan fault system consist of rejuvenated lithosphere formed by small-scale, shallow-mantle upwelling and accretion of asthenospheric material since late Cretaceous or Eocene times.