The Pan-African reconstruction of NW Angola: petro-structural and temporal constraints

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, wedeveloped 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 andwestward. 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.