Ion microprobe U-Pb ages on detrital zircon grains from the Ghanzi Group: implications for the identification of a Kibaran-age crust in northwest Botswana

Ion microprobe U-Pb ages of 1104+/-16 Ma, 1363+/-11 Ma and 1748+/-13 Ma hav e been obtained on detrital zircons from the siliciclastic rocks of the Gha nzi Group intersected in borehole CKP4 in northern Botswana. Available data show that the time of deposition of the Ghanzi Group is bracketed between 1106+/-2 and 627+/-6 Ma. These ages permit constraints to be placed on the source of the sediments, it is shown that the main component of the sedimen ts is sourced locally, from the Kibaran-age Kgwebe igneous rocks. These dat a are taken to indicate that the Kibaran orogenic event affected northwest Botswana. The Kibaran-age lithological units of northern Botswana represent a link between the Kibaran-age belts of central-eastern Africa and souther n Africa. The other two populations of zircons are probably exotic. One is linked to the ca. 1400-1300 Ma rocks known within the Kibaran orogenic syst em in central and southwest Africa, possibly the Choma-Kalomo Block in Zamb ia, whereas the ca. 1750 Ma detrital zircons were probably sourced from Pal aeoproterozoic granitoids exposed in northern Namibia and southern Angola. (C) 2000 Elsevier Science Limited. All rights reserved.