Structural and geochronological constraints on the evolution of the eastern margin of the Tanzania Craton in the Mpwapwa area, central Tanzania

A study of the position, nature and geochronology of the eastern margin of the Tanzania Craton near Mpwapwa yields new constraints on Archaean to Neoproterozoic orogenesis of central Tanzania. The eastern part of the craton comprises typical Neoarchaean grey granodioritic orthogneisses dated with the LA-ICP-MS method by U-Pb zircon at ca. 2.7 Ga. A gradual eastward increase in strain in these rocks culminates in a 1-2 km wide, locally imbricated, north-south-trending, ductile thrust/shear zone with an oblique top-to-the-NW sense of movement. East of the craton-edge shear zone, high-grade supracrustal rocks are termed the Mpwapwa Group in view of uncertain regional correlations. There is an apparent lithological zonation of the Mpwapwa Group parallel to the craton margin shear zone. In the west, the group consists of typical shelf facies metasedimentary rocks (marbles, calc-silicates, quartzites, etc.). U-Pb dating of detrital zircons from two quartzites reveal only Archaean detritus, constraining their maximum depositional age to ca. 2300 Ma) with a strong metamorphic overprint at ca. 1960 Ma (zircon), confirmed by a metamorphic titanite age of ca. 1990 Ma, again believed to date the initial phase of craton-margin shearing and juxtaposition of the Archaean crustal blocks. The role of the Neoproterozoic East African orogeny in the evolution of the craton margin is unclear, but the geometry of the shear zone, the presence of Neoproterozoic zircon rims in the Palaeoproterozoic granite and published studies from nearby, all suggest that the latest movements on the shear zone may be Neoproterozoic in age and that the structure may represent the local western front of the East African Orogeny. (c) 2012 Elsevier B.V. All rights reserved.