Refertilization Processes in the Subcontinental Lithospheric Mantle: the Record of the Beni Bousera Orogenic Peridotite (Rif Belt, Northern Morocco)
Résumé
Correlations between major and minor transition elements in tectonically emplaced orogenic peridotites have been ascribed to variable degrees of melt extraction and melt–rock reaction processes, leading to depletion or refertilization. To elucidate how such processes are recorded in the subcontinental lithospheric mantle, we processed a large geochemical dataset for peridotites from the four tectono-metamorphic domains of the Beni Bousera orogenic massif (Rif Belt, northern Morocco). Our study reveals that variations in bulk-rock major and minor elements, Mg-number and modal mineralogy of lherzolites, as well as their clinopyroxene trace element compositions, are inconsistent with simple partial melting and mainly resulted from different reactions between melts and depleted peridotites. Up to 30% melting at < 3 GPa and cryptic metasomatism can account for the geochemical variations of most harzburgites. In Grt–Sp mylonites, melting and melt–rock reactions are masked by tectonic mixing with garnet pyroxenites and subsolidus re-equilibration. In the rest of the massif, lherzolites were mostly produced by refertilization of a refractory protolith (Mg-number ∼ 91, Ol ∼ 70%, Cpx/Opx = 0·4) via two distinct near-solidus, melt–rock reactions: (1) clinopyroxene and orthopyroxene precipitation and olivine consumption at melt/rock ratios <0·75 and variable mass ratio between crystallized minerals and infiltrated melt (R), which are recorded fairly homogeneously throughout the massif; (2) dissolution of orthopyroxene and precipitation of clinopyroxene and olivine at melt/rock ratios ≤1 and R = 0·2–0·3, which affected mainly the Ariègite–Seiland and Seiland domains. The distribution of secondary lherzolites in the massif suggests that the first refertilization reaction occurred prior to the differentiation of the Beni Bousera mantle section into petro-structural zones, whereas the second reaction was associated with the development of the tectono-metamorphic domains. Our data support a secondary, refertilization-related origin for most lherzolites in orogenic peridotite massifs.