Termination of the Southern Irumide Belt in Tanzania: Zircon U/Pb geochronology

• The Irumide Belt extends into S-Tanzania.
• Magmatic ages of 1000–1055 Ma and a metamorphic overprint of 563–616 Ma is found.
• The Irumide Belt is separated from Western Granulites/Usagaran Belt by the Mwembesi Fault system.
• Eastern Granulites/Cabo Delgado Complex is a large scale nappe structure.

Distinctly different crustal blocks converge in southern Tanzania which are thought to belong to the Paleoproterozoic Usagaran/Ubendian Belt, the Mesoproterozoic Irumide Belt and the Neoproterozoic East African Orogen. Zircons from thirteen samples were dated by LA-SC/MC-ICPMS in order to assign the different crystalline basement rocks to one of the above mentioned geological units based on the age of metamorphism and formation of the igneous precursor rocks. The mainly migmatic gneissic samples were taken along a traverse from Mbamba Bay, Lake Malawi to Masasi, S-Tanzania. Zircon cores and rims trace magmatic protoliths between 1000 ± 9 and 1055 ± 6 Ma and metamorphism between 563 ± 10 and 616 ± 12 Ma. Ages belonging to the Usagaran/Ubendian event as well as the 640 Ma EAO event were only found in the westernmost and easternmost part of the transsect, respectively. Magmatic ages of 724 ± 6 and 764 ± 4 Ma were determined in locally confined undeformed granitoid bodies. Mostly similar ages as well as lithological units can be found in the Unango and Marrupa Complexes in N-Mozambique which are considered as part of the Southern Irumide Belt. The exact location of terrane boundary between the Southern Irumide Belt and the Western/Eastern Granulites and the Usagaran/Ubendien Belts in southern Tanzania remains uncertain. With aid of digital elevation data, satellite images and airborne geophysical data we suggest that a combination of fault systems is responsible for the distribution of the different units. The northeastern limit of the Southern Irumide Belt (Unango and Marrupa Complexes) can be traced along a major tectonic boundary, the Mwembesi Fault system, that extends from Malawi to the south of the Mahenge Mountains in Tanzania.