MAJOR PAN-AFRICAN IMPRINT IN THE UBENDIAN BELT OF SW TANZANIA - U-PB ON ZIRCON GEOCHRONOLOGY AND STRUCTURAL CONTEXT

A structural and geochronological (U-Pb on zicon) study of the Ubendian terranes of the Livingstone Mountains reveals an unexpected Pan-African imprint. In this area, the high-grade Precambrian basement is locally intensely sheared into thick mylonitic sequences in conformity with the surrounding basement rocks. Geochronological data from the late-mylonitic Kalunduru granite (724 +/- 6 Ma) and from the tonalitic non-mylonitic Ukenju gneiss (814 +/- 120 Ma-lower intercept) suggest that the mylonitic deformation is Upper Proterozoic in age. The upper intercept for the Ukenju gneiss (2,084 +/- 86 Ma) confirms on the other hand the Ubendia age for the protolith. These results suggest that the NW-SE structures held as characteristic for the Ubendian would be Pan-African (= Mozambiquian). Effects of Middle Proterozoic age (Kibaran) have not been recorded in the area.