Stratigraphy, Metamorphism and Tectonic evolution of the early proterozoic Usagaran belt, Tanzania

Detailed geological field work in the Isimani division, Tanzania followed by petrographic, structural geological, whole-rock geochemical and mineral chemical studies in the laboratory were carried out in order to establish the stratigraphy of the early Proterozoic Usagaran Orogen and its metamorphic and structural history. The Usagaran Belt is situated in the southeastern Tanzanian Craton (2570 Ma) and is composed of the following two major lithological units: (1) the gneissic-amphibolitic Isimani Suite and (2) the sedimentary-volcanic unconformably overlies the former. The Isimani Suite is made up of three lithodemic units. (1) The Mbunga River Lithodeme mainly composed of alternating bands of muscovite-feldspar gneiss, biotite gneiss and porphyroclastic microcline gneiss interlayered with minor bands of amphibolite, stauralite-bearing kyanite-garnet gneiss and grunerite-bearing iron formation. (2) The Iguruba Lithodeme containing only gneisses. (3)The Lluhomero Lithodeme made up of only amphibolite. The Konse Group is subdivided into six conformable formations: (1) the orthoquartzitic Mkulula Formation (the lowest), (2) the conglomeratic Ruaha River Formation, (3) the Kilimbe Formation of mica schists, (4) the volcanic Kikuyu Formation, (5)the Ihumbirisa Formation made up of dolomitic marble and (6)the Mhwana Formation containing quartzites, mica schists, iron rormations and Mnsilicate-bearing mica schists. The Isimani Suite was first metamorphosed (m1) to granulite upper amphibolite conditions (T= 780oC and P = 10 kbars) and during the D2/ Dk1 it underwent a retrograde (green schist-amphibolite facies) metamorphism (M2)(T = 500°C to 700°C, P = 6 kbar to 4 kbar). The Konse Group was also metamorphosed for the first time (Mki) during the D2/Dk1 under conditions similar to those of the M2. The Usagaran Orogeny is characterised by three phases of deformation. DI deformed the Isimani Suite alone and produced recumbent folds (FI) and the metamorphic bands (SI). The DI was shortly followed by the deposition of the Konse Group. Later the two units were deformed together. This was the second deformation event for the Isimani Suite (D2) but the first for the Konse Group (DkI). The D2/Dk1, which is the main deformation event of the Usagaran Orogen, is characterized by isoclinal folds ( F2 in the Isimani/FKI in the Konse) with axial plane foliation (S2 in the Isimani/SkI in the Konse). Numerous shear zones and faults parallel to the S2/SkI are associated with the D2/DkI. At this stage some blocks of the Isimani Suite were thrust onto the Konse Group and the entire Usagaran Belt was thrust against the Tanzanian Craton. Several synD2/DkI dolerite and gabbro bodies intruded the entire Usagaran Belt. Later the Usagaran Belt was again folded (D3 in the Isimani Suite and Dk2 in the Konse Group. The D3/Dk2 formed several sub-vertical conjugate faults striking ESE and NNW associated with large scale kink folds, warping and crenulations. The intrusion of post-orogenic granites and granodiorites about 1890-1740 Ma ago and the intrusion of post-orogenic dolerites of unknown age marked the end of the geological history of the Usagaran Belt. The geotectonic evolution of the Usagaran Belt is explained by the "rifting, delamination and A-subduction" model of Kroner (1983). In this model the Isimani Suite represents a volcanic sedimentary assemblage formed in a basin situated between the Tanzanian Craton to the north and the Zambian Craton to the south formed during the crustal extension stage. At the beginning of the collision stage, the Isimani Suite together with the Zambian Craton, overode the Tanzanian Craton leading to the flexuring of the latter to form a peripheral foreland basin (the Konse basin) on its southeastern margin. The Konse Group was then deposited in this basin. The total collision (the locking stage) is correlated with the main phase (D2/DkI) of deformation of the Usagaran Belt. After the locking stage, the SE-oriented I-principal stress axis of the D2DkI ceased. The I-principal stress axis of the D3/Dk2 was oriented NE-SW.