The tectonic history of a crustal-scale shear zone in the Tanzania Craton from the Geita Greenstone Belt, NW-Tanzania Craton

In this contribution, we present for the first-time field based evidence of a crustal scale shear zone from the southern margin of Geita Greenstone Belt. The Geita Shear Zone is a broad (-800 m wide) ductile, high-strain, deformation zone that can be traced for at least 50 km along the southern margin of the Geita Greenstone Belt. It is near vertical, trends similar to E-W and separates the mafic volcanics of the Kiziba Formation, to the north, from the TTG gneisses that crop out south of the shear zone. The shear zone is hosted almost entirely by the TTG gneisses and is characterised by a well-developed mylonitic foliation near the greenstone margin that transitions into a gneissic foliation and eventually becomes a weakly developed foliation further south. It contains approximately equal amounts of dextral, sinistral, and asymmetric shear sense indicators suggesting that the shear zone accommodated mainly flattening strain while the mineral stretching lineation defined by quartz and feldspar ribbons and stretched biotite selvages plunges shallowly W. A series of younger, sub-vertical, NW trending brittle-ductile, strike-slip shear zones truncate and displace the Geita Shear Zone with dextral displacement in the order of 2-4 km. Deformed tonalite interpreted to predate the shear zone yielded U-Pb zircon ages of similar to 2710 Ma while synshearing granodiorite samples have zircon ages between 2680 Ma and 2660 Ma. The similar to 2630 Ma age of the undeformed Nyankumbu granite is interpreted to mark the minimum age of movement on the shear zone. The presence of 3000 Ma and 3200 Ma zircon xenocrysts in the tonalite and granodiorite opens the possibility that older basement rocks underlie the greenstone belts in the northern half of Tanzania Craton. Whether or not the greenstone belts were erupted on older basement, thrusted on top of older basement rocks or incorporated older basement fragments has profound implications for the tectonic framework and evolution of the Tanzania Craton.