Geochronological systematics for the Chimoio-Macossa frontal nappe in central Mozambique: Implications for the tectonic evolution of the southern part of the Mozambique belt

Abstract

This work aims to acquire a better understanding of the tectonic evolution of the Mozambique Belt, for the region located in the central-western part of Mozambique that corresponds to the southernmost part of the East African Orogen, in Africa. The main objective was to make a robust geochronological study, by means of a series of U-Pb zircon ages, in order to establish some precise magmatic crystallization and/or metamorphic recrystallization ages, as well as to estimate provenance and maximum ages for meta-sedimentary units. Limited zircon Lu-Hf analyses and some Sm-Nd ages on metamorphic garnets were also obtained.Two large nappes, Macossa-Chimoio and Mungari, are envisaged in the study area, tectonically juxtaposed to the Zimbabwe Craton. The Macossa-Chimoio nappe comprises the high-grade paragneisses, granulites and migmatites characteristic of the Macossa and Chimoio Groups, which occur in close association with the pre-existent mesoproterozoic granitic rocks of the Báruè Complex. The metamorphic P-T conditions, estimated between 4 and 6 kb and 700–800 °C, and the tectonic unit was exhumed at different crustal levels. A few granitoids of the Báruè Complex yielded mesoproterozoic U-Pb zircon cristallyzation ages near 1100 M, but detrital zircons of some paragneisses yielded neoproterozoic maximum ages of deposition of 700–800 Ma. In addition, the main age of the very young cambrian metamorphism event was set by means of U-Pb dating of zircon overgrowths and Sm-Nd garnet-whole rock isochron at 500 Ma.The boundary between the Macossa-Chimoio and Mungari nappes is a thrust zone with a NW trending tectonic transport. It disappears to the NE below a Karoo sedimentary cover, but most likely reappears at the NE side of the basin as a prolongation of a long-lived tectonic boundary, the Lurio mega-shear belt of northeastern Mozambique which separates the Nampula tectonic block in the south from a northern block comprising the Marrupa and Unango complexes. Therefore, the Macossa-Chimoio Nappe would correlate closely with the Nampula Block, which includes mesoproterozóic orthogneisses with about 1100 Ma age, reactivated in the Cambrian.On the light of the U-Pb ages of overgrowth rims on zircon crystals and corroborative Sm-Nd isochron ages, the southern end of the Mozambique Belt in Africa was affected by a strong orogenic episode in the Cambrian. This confirms that the amalgamation of Gondwana was diachronous along the EAO, starting earlier in the north and cease later in the south. In the south, the orogenic event was coeval to the Kuunga Orogeny between Australia and Antarctica, usually constrained between 530 and 490 Ma.