Igneous and detrital zircon U-Pb and Lu-Hf geochronology of the late Meso-to Neoproterozoic northwest Botswana rift: Maximum depositional age and provenance of the Ghanzi Group, Kalahari Copperbelt, Botswana and Namibia

摘要

New igneous and detrital zircon laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb geochronology and Lu-Hf isotopic data are presented for the Mesoproterozoic Kgwebe Formation and the unconformably overlying Ghanzi Group in northwestern Botswana. The Makgabana Hills porphyritic rhyolite flow from the Ghanzi area yielded a U-Pb concordia age of 1085.5 +/- 4.5 Ma and provides a new maximum depositional age for the unconformably overlying Ghanzi Group. Detrital zircon (n = 448) from the Ghanzi Group yielded a (207)pb/Pb-206 age distribution with a dominant (70 to 90%) Mesoproterozoic population (similar to 1450 to similar to 1050 Ma), a smaller (5 to 20%) Paleoproterozoic (similar to 2200 to similar to 1700 Ma) population, and a few (n = 4) older (similar to 3000 Ma to similar to 2450 Ma) grains. A maximum depositional age constraint of similar to 1060 to similar to 1050 Ma was obtained for middle and upper Ghanzi Group based on the weighted-mean Pb-207/Pb-206 age of the youngest clusters of overlapping zircon ages for each sample.