Évolution métamorphique et tectonique d’une portion de croûte du craton du congo remobilisée à l’éburnéen dans la région de lolodorf (complexe du nyong, sw cameroun)

Abstract

This research work focuses on the Lolodorf region delimited by longitudes 10°40’ to 10°55’ East and latitudes 3°10’ to 3°25’ North, in the South-West of Cameroon, in the Nyong Complex. The calibration of geodynamic events associated with the Paleoproterozoic in this region was conducted using a multidisciplinary approach involving petrography, chemico- mineralogy, thermobarometry, and whole-rock geochemistry. In addition, the patterns of finite deformation were established from morphostructural and tectonic analysis data. The very insufficient qualitative and quantitative data on the Eburnean basement of the Lolodorf region allow us to question the problem of reconstructing its metamorphic and tectonic history. The results obtained make it possible to constrain and propose a geodynamic evolution model which increases understanding of the mechanisms of emplacement of the rocks of the Nyong Complex. Petrological studies reveal that the Lolodorf region is formed from terrains of varied origin and lithology (charnockitic gneisses, clinopyroxene metasyenites and metamafites). These formations were metamorphosed into the transitional amphibolite-granulite facies (805°-890 °C, 12.19 - 13.97 kbr) associated with anatexis. The retromorphic phase occurred under the conditions of the high-grade amphibolic facies (601-795 °C, 1.30-9.69 kbr) and a fugacity between -19.63 and -12.47 atm. The geotectonic contexts of the N-MORB, P-MORB type were created by divergence of plates in an intra-plate zone. On the other hand, intra-plate alkaline basalts were created by convergence of plates in an ocean-continent transition zone of a torn margin. These formations evolved in two active margin settings that recorded a retrograde decompressive isothermal polyphase metamorphic and tectonic evolution in the transitional amphibolite-granulite facies (HP/MP-HT). The Eburnean convergence implemented a reworking and the emergence of juvenile magmatic crusts. In this environment, high pressure and temperature (HP-HT) metamorphic rocks are coupled with intense partial melting that favored a thickening of the crust and a subsequent rapid exhumation of the crust during tectonic events. The geomorphology highlights two plateaus of unequal importance separated by a large depression delimited by the escarpments (faults) of Ngovayang and Lokoundjé. Lithology imposes its mark on morphology through differential erosion due to high to moderate tectonic activity. The climate controls the fluvial incision and the sculpture of the elements of the morphology of the region. The region bears the imprints of three ductile deformation phases D -D and an 1 3 essentially brittle phase D4. The structural elements present a geometry and an orientation in agreement with two tectono-metamorphic episodes D and D compressive and strike-slip E- 1 3 W and NW-SE separated by an extensive tectonic phase D of N-S direction. In the region, 2 the NE-SW-directed D extensive deformation phase closes the Proterozoic episode. This 4 tectonics is induced by the E-W trans-Amazonian and NW-SE trans-Saharan-Nigerian- Congolese collisional systems and their subsequent relaxation.