Direct evidence for Archean crust in the Western Domain of the Karagwe Ankole Belt, Rwanda: Implications for Neoarchean to Paleoproterozoic crustal evolution

Neoarchean orthogneiss, dated at 2629 +/- 9 Ma by SIMS zircon U-Pb, has been discovered in the Rusizian terrane of the Western Domain of the Karagwe Ankole Belt, Rwanda, redefining the NW margin of the Tanzania Craton. Metamorphism of the orthogneiss at 1959 +/- 4 Ma was associated with Paleoproterozoic granitic arc magmatism in the Rusizian terrane over the period ca. 2000-1840 Ma. The major and trace element composition of the peraluminous, calc-alkalic orthogneiss is consistent with magmatism in an arc setting at an active continental margin. Its Nd depleted mantle model age of ca. 2800 Ma is consistent with derivation by partial melting and reworking of older crust at the NW margin of the Tanzania Craton. The Paleoproterozoic thermal event was related to the accretional collision between the Congo and Tanzania Cratons during the assembly of the Nuna supercontinent. Future tectonic modelling of the Karagwe Ankole Belt must take into account the presence of Archean basement beneath the Western Domain.