PETROLOGICAL AND GEOCHEMICAL INVESTIGATION OF ROCKS FROM THE DAVIE-FRACTURE ZONE (MOZAMBIQUE CHANNEL) AND SOME TECTONIC IMPLICATIONS

The Davie Ridge is an important structural feature of the Mozambique Channel. It represents the actual configuration of an old fracture zone (Davie Fracture Zone) between Africa and Madagascar along which Madagascar drifted southward during Late Jurassic and Early Cretaceous times. This fracture zone is built of crystalline continental basement (granites, gneisses and meta-arkoses) locally covered by a deformed <<flysch>> sequence, by alkaline lavas, tuffs and breccias and by a thin succession of carbonate oozes ranging in age from Cretaceous to Pleistocene. The sedimentary terrigeneous products derived from the fractured and eroded African basement and were deposited during an initial rifting/transtension in the incipient Somali Basin. During the southwards displacement of Madagascar, the lowermost arkosic sediments suffered low grade metamorphism in a collisional setting. The peak metamorphic conditions reached 4.5-5 kb and 350-400 degrees-C. These conditions imply at least 10-12 km deep burial of sediments (in transtensional?) and subsequent uplift (in transpressional setting?) to the present surface. Deep fracturing along the Davie Fracture Zone allowed the emplacement of alkaline within plate basalts in a marginal basin setting. However, these basalts were emplaced only at die northern and southern extremities of the fracture zone, while the central parts remained, at least superficially, undisturbed. Thermobarometric calculations on fragments of the old African continental basement found in the meta-arkoses confirm that the crustal pile below the present central parts of the fracture zone underwent slow uplift and erosion. These features imply that, during Paleozoic and Mesozoic and before the symmetric rifting in the Somali and the Mozambique Basins, the crustal pile between Africa and Madagascar underwent slow uplift and erosion. Unlike the evolution of the Limpopo belt in southeastern Africa and the western India basement (where uplift patterns are steeper or disturbed), the crustal thickness between Africa and Madagascar seems to have inhibited the emplacement of huge volumes of magma along the Davie Fracture Zone. Although the amplitude of lithospheric thinning was the guiding mechanism for the volcanic activity along the fracture zone, both the basement reactivation and the volcanic activity along die Davie Facture Zone were controlled by the amplitude of uplift and erosion.