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1 Department of Earth Sciences, Boston University, Boston, Massachusetts 02215
The Archean Zimbabwe craton is made of a number of distinct tectonostratigraphic terranes assembled by plate tectonic processes. The central Tokwe terrane consists of 3.5–2.95 Ga gneissic rocks and structurally complex inliers of possibly older greenstone belts. These are overlain unconformably by a 2.9–2.8 Ga assemblage of mafic and felsic volcanic rocks and conglomerates, and a separate 3.0–2.7 Ga southeastward thickening platform sequence of sandstone, shale, and limestone. 2.7 Ga greenstone belts form two distinctly different domains flanking the central terrane. Northwest of the ancient gneissic terrane, ca. 2.7 Ga greenstone belts comprise a series of calc-alkaline lavas and intercalated sedimentary rocks intruded by syn-volcanic plutons. Southeast of the ancient gneissic complex, 2.7 Ga greenstone belts consist of thick piles of tholeiitic basalts overlying ultramafic lavas, resting allochthonously over the shallow-water platform sequence and older gneissic terrane. This division of the Zimbabwe craton is interpreted to show that the central Tokwe terrane had a continental magmatic arc built on its northwestern edge, as its southeastern margin rifted from another fragment, forming the Sea of Umtali. A passive-margin sedimentary wedge formed on the rifted southeastern edge of this ancient continent, and prograded onto the craton during sedimentary and tectonic loading of the craton margin. The southeastern greenstone belts formed as thick oceanic crust (oceanic plateau) in this back-arc basin, and were later obducted on to the rift and passive margin sequence as the Sea of Umtali closed ca. 2.7 Ga. This was followed by intrusion of granitic plutons of the Chilimanzi suite ca. 2.6 Ga in a tectonic regime of intracontinental strike-slip faulting, representing a response to the Zimbabwe-Kaapvaal continent-continent collision. Crustal and lithospheric thickening during intrusion of these late granites may have played a role in stabilizing the Zimbabwe craton and forming the lithospheric root.
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