The pressure–temperature–time–deformation history of the Beni Mzala unit (Upper Sebtides, Rif belt, Morocco): Refining the Alpine tectono-metamorphic evolution of the Alboran Domain of the western Mediterranean

The structural and thermal relaxation overprint associated with the Neogene Alboran rifting have obscured the early Alpine tectono-metamorphic evolution of the Alboran Domain, representing the metamorphic core of the Betic–Rif orogen of the western Mediterranean region. This study focuses on the Beni Mzala unit, forming the lower and deeper structural level of the Alpine metamorphic nappe stack (Upper Sebtides) in the Moroccan Rif. Meso- and micro-scale structural investigations are carried out on high-P aluminum silicate (Ky-bearing)-quartz segregations that occur as boudins within the main retrogressive syn-greenschist foliation (S2/D2) and assumed to preserve the early M1 HP metamorphism associated with the Alpine orogenic construction in the Alboran Domain. These boudins host an early crenulated high-P foliation (S1, D1/M1) made of quartz–kyanite–white mica–rutile. A large spread in white mica composition is documented, with the highest Si content per formula unit (up to 3.18 apfu) preserved along the S1 foliation and the lower Si content observed in the white mica marking the S2 retrogressive foliation (D2/M2) and the rim of S1 mica. Microtextural evidence documents post-tectonic andalusite growth and static recrystallization of the quartz microlithons. Inverse (Zr-in-Rt thermometry) and forward modelling thermobarometry are integrated with Ar–Ar white mica geochronology to define the peak and exhumation pressure–temperature–time (P–T–t) path of the Beni Mzala unit. Minimum thermo-baric estimates for the M1 event are ~1.4 GPa and 600°C, corresponding to a metamorphic gradient of ~11°/km, consistent with subduction zone metamorphism. Exhumation is constrained by re-equilibration of the white mica composition (from high to low celadonite) between c. 29 and 22 Ma, during a nearly isothermal retrogressive path, with final equilibration at high-T/low-P conditions within the andalusite stability field (~0.2–0.3 GPa and 500°C). A minimum late Oligocene age is proposed for the Alpine D1 tectono-metamorphic stage in the Rif, suggesting as feasible the previously proposed Eocene timing for the subduction-zone metamorphism of the Alboran Domain. Conclusive evidence is provided to link the early Miocene tectono-metamorphic event to a late thermal perturbation that affected the Alboran Domain at shallow crustal conditions, post-dating the almost complete exhumation of the deep roots of the Alpine belt in the western Mediterranean.