DEFORMATION ASSOCIATED WITH MELT PERCOLATION IN THE UPPER MANTLE: FIELD, STRUCTURAL AND PETROLOGICAL EVIDENCE FROM THE BENI BOUSERA ULTRAMAFIC MASSIF (NE MOROCCO)

In the Beni Boussera Ultramafic massif evidence indicates that melt flow an deformation occurred simultaneously: In the field we identified tens to hundreds of meters wide melt percolation zones (MPZ). MPZ are characterized by replacive dunite, harzburgite, and “secondary” lherzolites, within a heterogeneous “primary” lherzolite. Pyroxenite dykes are not homogenously distributed throughout the massif. Few centimeter-thick pyroxene dykes are sporadically present throughout the massif, (garnet-) augite pyroxenite and Cr-diopside pyroxenite occur in increased frequency (up to ~ 40-80 vol% of single outcrops) in the restricted MPZ. Distinct difference in mineral chemistry, texture and deformation fabric is observed in the MPZ and surrounding lherzolite. In the MPZ, a linear and planar fabric defined by the orientation of spinel and pyroxenes is developed within peridotites. Sharply defined pyroxenite dykes are generally parallel to foliation. Locally observed isoclinal recumbent folds and sheared folds of those dykes exhibit an axial planar foliation parallel to the foliation in the surrounding peridotites. The axes of these folds are generally parallel to the mineral lineation observed in the peridotites. In rare cases (garnet-) augite pyroxenite oblique to the foliation is observed. By contrast, Cr-diopside dykes, are generally significantly less deformed than augite pyroxenite from the same outcrop indicating that deformation was contemporaneous with the formation of the Cr-diopside patches. Microstructural observations indicate that beside the main peridotite foliation a second oblique foliation defined by the shape preferred orientation of olivine is developed. Using this oblique fabric to determine the sense of shear indicate generally Top-to-the-South directed shear but an change of shear sense is observed across the northern main MPZ. Lattice preferred orientation (LPO) of olivine crystals from a sample transect across one MPZ was analyzed and compared with LPO from the surrounding host lherzolite and harzburgite. The LPO patterns are generally weak and are slightly but systematically different for peridotites within the MPZ compared with those outside the zone; across the MPZ, orientation of the olivine [100] axis with respect to the mineral foliation changes asymmetrically.