MG-AL RICH ZONE IN THE CONTACT BETWEEN MANTLE ROCKS (PERIDOTITES) AND GRANULITES FACIES ROCKS IN THE BENI BOUSERA MASSIF (INTERNAL RIF, MOROCCO)

The Beni Bousera peridotite massif (Internal Rif , Morocco) is formed in a major part of Spinel-bearing lherzolite rimed by a layer of garnet-bearing peridotite which is in direct contact with HP-HT granulite metamorphic rocks (16Kbar, 860°C). A shearing contact between these two formations shows the presence of metamorphic ultramafic intercalations underneath the deformed granulites.

Foliation is well distinct and allocates in both rocks with a succession of metamorphic micro-zones with very diverse mineral assemblages. It consists on the spatial arrangement of 3 centimetric zones separating garnet-spinel bearing peridotites from garnet-kyanite bearing granulites:

- Phlogopite, orthopyroxene, spinel zone.

- Corundum, sapphirine, spinel zone.

- Sillimanite, kornerupine zone.

Geochemistry of the different phases shows peraluminous (corundum, kornerupine, sapphirine, spinel) and magnesian (phlogopite, enstatite) assemblages; the P-T conditions estimated using THERMOCALC are 9 to 10Kbars and 700 to 950°C in the saphirine – sillimanite – corundum stability domain.

These thermobarometric conditions reveal that these rocks -forming the top of the peridotite massif- have incurred a metamorphic evolution at high temperatures, which is related to an isothermal decompression after or during the setting up of these rocks at the base of the crust (60 km thick). However, the kornerupine could have formed sequentially under nearly constant P–T conditions (sillimanite stability field) during the infiltration of fluid in the rocks. These hydrothermal processes are also involved and drive the peridotites to a metasomatic contamination at the contact with fluid-rich crustal rocks that leads to crystallization of phlogopite and amphibole. This contact is possibly a shear zone crossing the limit crust-mantle.

The data show that, in the Rif Mountains, the granulites and formerly diamond bearing ultramafic rocks, are uplifted in the same geodynamic environment and give very strong constraints on the Alpine metamorphic evolution of the whole Alboran domain.