Metamorphism and U-P Dating of the SASCA domain (South-West Ivory coast)
Résumé
The Sassandra-Cavally (SASCA) domain (SW Ivory Coast) is located in the transition zone between the Paleoproterozoic and Archean domains (Fig. 1). It is characterized by the coexistence of high-grade and low-grade metamorphic units and the existence of Archean relics in the Birimian formations. Heterogeneous gneiss formations comprising migmatites, orthogneisses, garnet-cordierite-sillimanite-garnet metasediment and staurolite-bearing micaschists are affected by three main deformation phases (D1-D3). Thermobarometric calculations using Theriak-Domino (de Capitani, 1994) suggest a retrograde evolution from the granulite facies to the amphibolite facies, from Grand Bereby to Sassandra. The metamorphism in the paragneisses of Grand Bereby and in the orthogneisses of San-pedro with quartz-plagioclase-biotite-garnet-sillimanite-ilmenite and quartz-garnet-biotite-ilmenite partial fusion, respectively, evolves at the transition of granulite and amphibolite facies with a maximum pressure of 10 kbar and a temperature of 820°C. It seems to characterize a decompression period in the granulite facies followed by cooling to the amphibolite facies with an apparent geothermal gradient of 40°C/km. However, the evolution of staurolite-bearing micaschistes occurred mainly in amphibolite facies with PT conditions corresponding to 3.5-6.6 kbar and 550-620°C. During this evolution, two metamorphic stages are distinguished: a prograde stage and a retrograde stage. The prograde path evolves according to a gradient of 30°C/km up to a peak of metamorphism estimated at 6.6 kbar-620°C in the amphibolite facies for the sample of Kounoukou. This path would correspond to a burial comparable to a thickening of the crust up to about 14 km in depth under an apparent geothermal gradient of 39°C/km. At this P-T path, the rock passes through a cooling phase with low decompression evolving according to a geothermal gradient of 40°C/km. This decompression would be associated with a cooling phase recorded in staurolite-bearing micaschist. U-Pb in situ dating of monazites gives ages of 2044 ± 11 Ma, 2003 ± 11 Ma, 1973 ± 11 Ma in the orthogneiss to quartz-garnet-biotite-ilmenite and 2000 ± 10 Ma for staurolite-bearing micaschists. These ages correspond to metamorphic ages in the SASCA domain and their meaning will be further investigated.
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