The Pan-African reconstruction of NW Angola: petro-structural and temporal constraints
Résumé
At the end of Neoproterozoic times, assembly of the Gondwana supercontinent resulted in the closure of several oceanic domains and accretion of large cratons. Various tectono-metamorphic belts developed et the margins of these cratons during the Panafrican orogeny. During this work, we
developed a study combining petro-structural and geochronological investigations on the West Congolian belt (NW Angola) resulting from the collision between the Congo and Sao Francisco cratons. Two main tectono-metamorphic units have been recognized in the studied area, namely eastern and western internal units, and show a westward increase of deformation and metamorphic grade. The WIU consists of high-grade gneisses and migmatites with intercalation of amphibolites, quartzites and pegmatites. This sub-unit experienced metamorphic conditions that increase upward and
westward. Maximum P-T conditions for high-grade gneisses have been estimated at 10-12 Kbar and 600-650 °C. One garnet amphibolite intercalated with gneisses has been dated and yields ages of 539±7 Ma and 498±5 Ma for U-Pb and Ar-Ar methods, respectively. A pegmatitic dyke concordant to the regional foliation provides a concordant U-Pb age of 544±13 Ma taken as our best estimate for pegmatite emplacement. Two paragneisses have been also dated with ages ranging from 589±12 Ma to 678±48 Ma suggesting a detrital origine and indicating that detritus was derived from Neoproterozoic source material. At least monazites from one of this gneiss show a complex U-Pb age distribution (three batches between 560-490 Ma) and Ar-Ar biotite age of 487±5 Ma. All this age underlines the predominance of the Pan-African deformation and metamorphism in the construction of this belt.