THE PALAEOPROTEROZOIC UBENDIAN SHEAR BELT IN TANZANIA - GEOCHRONOLOGYAND STRUCTURE

The Ubendian belt is a linear, NW-SE trending orogenic belt in western Tanzania. It is part of a larger Palaeoproterozoic orogen, developed around the west and south-western margin of the Archaean Tanzanian cra ton. The Ubendian Belt has experienced several periods of reactivation since the Palaeoproterozoic, acting as a zone of displacement during successive orogenic and rift-forming events. The Ubendian Belt is char acterized by an early deformation and granulite-facies metamorphism, i sotopically dated at 2100-2025 Ma, and marked by an E-W to ESE-WNW tre nding foliation. This phase also affected the adjacent Usagaran (and B angweulu?) Belt and is interpreted as a product of collisional orogeny along the SW margin of the Tanzanian, and possibly Congo, cratons. A second phase of deformation, apparently restricted to the Ubendian Bel t, is characterized by large, NW-SE trending, dextral shear zones. Thi s phase is responsible for the creation of the eight crustal blocks de veloped throughout the belt and overprints much of the earlier deforma tion fabric. This second deformation phase is terminated by late- to p ost-kinematic calc-alkaline granitic batholiths (ca 1860 Ma). A phase of tectonic reactivation occurred locally at ca 1725 Ma. Notable by it s absence, is any evidence of Kibaran isotopic ages in the Ubendian Be lt. Major Kibaran orogenic belts occur to the north (Burundi) and sout h (Zambia) of the Ubendian Belt, although they are apparently not isot opically recorded within it. A third phase of deformation is character ized by Neoproterozoic (ca 750 Ma) reactivations of the Ubendian shear zones. This deformation is marked by the development of sinistral, br ittle-ductile shear zones, displaying retrograde metamorphic mineral a ssemblages, and intruded by alkaline plutons. These late shear zones w ere the preferential locus for the brittle rift faults of the western branch of the East African Rift.