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.

► Eastern margin of Tanzania Craton is a ca. 1960 Ma top-to-the-NW ductile thrust zone.
► The orthogneisses of eastern part of craton were dated by U–Pb zircon at 2674 ± 73 Ma.
► High grade paragneisses to east (Mpwapwa Group) are also Archaean (>ca. 2.6 Ga) and thus do not belong to the Usagaran belt, but to the “Western Granulite” of the East African Orogen.
► Charnockites in south have been dated at 2705 ± 23 Ma and point to a third Archaean assemblage.
► A post-Usagaran granite dated at 1871 ± 35 Ma shows Neoproterozoic zircon overgrowth.