New data on the deposition age of the volcano-sedimentary Chela Group and its Eburnean basement : implications to post-Eburnean crustal evolution of the SW of Angola

The Chela Group comprises a volcano-sedimentary sequence deposited in a widespread epicratonic basin that extends to southern of the Lubango region in SW of Angola. The basement rocks of the Chela Group integrate the Eburnean belt, consisting mainly of gneisses and migmatites, syntectonic foliated peraluminous granites, and late-tectonic metaluminous granites of the Chela and Gandarengos Mountains. Rb/Sr radiometric age of 2.1 ± 0.1 Ga was determined for the syntectonic granites whereas the late-tectonic Chela granite yielded a zircon U-Pb age of 1947 ± 5 Ma. This isotopic dating allow us to take it as a maximum age for the Chela Group. Moreover, the porphyritic calc-alkaline granite of the Gandarengos Mountain and a rhyolitic mass, spatially related to this granite, revealed U-Pb zircon ages of 1810 ± 11 Ma and 1814 ± 88 Ma, respectively. The rhyolite is intrusive into Tundavala Formation, base of the Chela Group, and should be the source of the pyroclastites composing the Humpata Formation. The time interval between 1814-1810 Ma obtained for the volcano-plutonism intrusive into the Chela Group restrains its minimum age and permits to define the interval 1947-1810 Ma as the main period of deposition of this sedimentary sequence in a post-orogenic environment. After the deposition of the Chela Group, succeeds a period of crustal accretion marked by the installation of leucocratic peraluminous granites and an extensional Mesoproterozoic (1.4-1.1 Ga) bimodal magmatism, followed during the Neoproterozoic by deposition of the Damara Supergroup and development of the Pan-African belt. At that period, any tectonometamorphic event reportable to Grenville-Kibaran orogeny was not recognized in SW Angola