NEW PB-PB SINGLE ZIRCON AGE CONSTRAINTS ON THE TIMING OF NEOPROTEROZOIC GLACIATION AND CONTINENTAL BREAK-UP IN NAMIBIA

Dating of single zircons from low-grade metamorphosed rhyolites in the Rosh Pinah Formation of the Gariep Belt in southwestern Namibia, usin g the Pb evaporation technique, yielded a primary crystallization age of 741 +/- 6 Ma. Both the stratigraphic position and the geochemistry of the volcanic rocks indicate an early continental rift environment. The new data not only provide an age for the massive Zn-Pb-Cu sulfide mineralization associated with these volcanic rocks, but they also set a maximum age limit for Neoproterozoic continental break-up in southe rn Namibia. This age is statistically indistinguishable from a recentl y reported age of 748 +/- 3 Ma for stratigraphically equivalent volcan ic rocks in the northern rift of the Damara Belt, suggesting that the onset of the formation of the N-S-trending Adamastor ocean and of the NE-trending Khomas ocean in central and northern Namibia occurred at t he same time. The volcanic unit directly overlies, in both rift graben s, a diamictite horizon with glaciogenic features. Our new results fur ther constrain the age of this glacial epoch, which may be correlated with the Sturtian glaciation, to around 750 Ma.