Structural evolution of the outer arc of sukumaland greenstone belt in north-western Tanzania

The Sukumaland Greenstone Belt lies in the south of Lake Victoria in northwestern part of Tanzania forming part of the Lake Victoria gold field of Tanzania which is the most gold producing area of the country. Several geological studies have been carried out in the area but the compilation of the geological map of the area by Bath (1990) showed that outcrops of the Achaean supracrustal rocks of the area form two arc patterns (outer and inner arcs) with E-W trending limbs that close to the east. From this study it has been shown that the rock units and stratigraphy of the northern limb of the proposed outer arc are totally different form those of the southern limb of the arc. The basal member of the Geita — Kitongo Belt is dominated by patches of mafic metavolcanics (greenschists) whose geochemistry (mostly Fe- rich quartz tholeiites) confirms their lateral continuity. Contrary to this, the southern limb of the arc (the Siga Hill Belt) is dominated by thick sequences of Banded Iron Formation (BIF) intercalated with thin units of tuff and clastic sediments (mostly silty-mudstones). Due to these differences in the lithotypes and stratigraphy of the two parts of arc, it is concluded here that the two parts of the initially proposed arc represents different stratigraphic piles deposited in isolated basins and it is most likely that their geometry represent the geometrical pattern of the two depositional basins and not due to folding of an originally continuous lithological units. The presence of numerous primary structures in the supracrustal rocks suggests that the rocks were not highly strained by deformation. The absence of a clear evidence of systematic vergence variation across the proposed, clear structures to support vertical movement during the deformation arc and high straining features in the rocks exclude the possibility that the arc is due to deformation with N-S oriented compression stresses. It is therefore concluded here that most likely the arc pattern is inherited from the geometry of the primary depositional basins. It is proposed that there were at least two separate depositional basins for different parts of the arc.