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 (S-2/D-2) and assumed to preserve the early M-1 HP metamorphism associated with the Alpine orogenic construction in the Alboran Domain. These boudins host an early crenulated high-P foliation (S-1, D-1/M-1) 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 S-1 foliation and the lower Si content observed in the white mica marking the S-2 retrogressive foliation (D-2/M-2) and the rim of S-1 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 M-1 event are similar to 1.4 GPa and 600 degrees C, corresponding to a metamorphic gradient of similar to 11 degrees/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 (similar to 0.2-0.3 GPa and 500 degrees C). A minimum late Oligocene age is proposed for the Alpine D-1 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.