TECTONIC SETTING AND TERRANE ACCRETION OF THE ARCHEAN ZIMBABWE CRATON

The Archean Zimbabwe craton is made of a number of distinct tectonostr atigraphic 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 overlai n unconformably by a 2.9-2.8 Ga assemblage of mafic and felsic volcani c rocks and conglomerates, and a separate 3.0-2.7 Ga southeastward thi ckening platform sequence of sandstone, shale, and limestone. 2.7 Ga g reenstone belts form two distinctly different domains flanking the cen tral terrane, Northwest of the ancient gneissic terrane, ca, 2.7 Ga gr eenstone belts comprise a series of calc-alkaline lavas and intercalat ed sedimentary rocks intruded by syn-volcanic plutons, Southeast of th e ancient gneissic complex, 2.7 Ga greenstone belts consist of thick p iles of tholeiitic basalts overlying ultramafic lavas, resting allocht honously over the shallow-water platform sequence and older gneissic t errane, This division of the Zimbabwe craton is interpreted to show th at the central Tokwe terrane had a continental magmatic are built on i ts northwestern edge, as its southeastern margin rifted from another f ragment, 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 o ceanic crust (oceanic plateau) in this back-are basin, and were later obducted on to the rift and passive margin sequence as the Sea of Umta li closed ca, 2.7 Ga, This was followed by intrusion of granitic pluto ns of the Chilimanzi suite ca, 2.6 Ga in a tectonic regime of intracon tinental strike-slip faulting, representing a response to the Zimbabwe -Kaapvaal continent-continent collision, Crustal and lithospheric thic kening during intrusion of these late granites may have played a role in stabilizing the Zimbabwe craton and forming the lithospheric root.