ALBERT

Beschreibung: We report new in-situ U-Pb zircon ages which were obtained from a variety of lithologies within the Dete-Kamativi Inlier of western Zimbabwe using both LA-ICP-MS and SIMS. These include igneous or metamorphic zircon grains from granitoids and granitic gneisses as well as detrital zircon grains from a number of paragneisses. The oldest ages are from two highly deformed migmatitic granitoid orthogneisses, which yielded Neoarchaean concordia ages of 2723 Ma and 2694 Ma, which are interpreted to be crystallisation ages. These are the first Archaean ages from the Dete-Kamativi Inlier, indicating that the Archaean Zimbabwe Craton extends much farther west than previously recognised. Our new maximum ages of deposition of 2.31, 2.0 and 2.07 Ga for the Malaputese, Inyantue and Tshontanda Formations, respectively, match closely with previously determined maximum ages of 2.29, 2.07 and 2.16 Ga for the Deweras, Lomagundi and Piriwiri Groups, thus supporting the lithostratigraphic correlation of the Dete-Kamativi Inlier with the Magondi Supergroup. Foliated and unfoliated K-feldspar megacrystic granites and granodiorites have yielded concordia ages ranging from 2.08 to 2.01 Ga. They contain abundant Archaean inherited zircon grains ranging in age from 3.34 to 2.66 Ga, implying the involvement of Archaean crust in their petrogenesis. These results suggest the formation of a calc-alkaline continental magmatic arc on the western edge of the Archaean Zimbabwe Craton, some 80 Ma prior to the onset of the Palaeoproterozoic Magondi Orogeny.

Beschreibung: The Pan-African belts of Malawi contain a largely unexplored endowment of gem bearing pegmatites. We present U–Pb in zircon (LA-ICPMS) and Rb–Sr mineral isochron geochronological and isotope data from pegmatites across Malawi. The pegmatites contain tourmaline, beryl, aquamarine, zircon, amethyst and sunstone as gemstone species. Two zircon bearing pegmatites in southern Malawi intruded early in the Pan-African orogenic cycle at 719 ± 5 Ma and 729 ± 4 Ma and are associated with the emplacement of alkaline rocks that formed during an intra-continental rifting episode in the eastern part of former Rodinia. One further zircon pegmatite containing inherited zircon of a similar age (746 ± 44 Ma) was emplaced at 598 ± 15 Ma, after the assembly of Western and Eastern Gondwana and the formation of the East African Orogen (EAO). The majority of the analysed pegmatites, however, are significantly younger. The ∼550 Ma pegmatites were emplaced during the Kuunga Orogeny, correlating with the collision of northern and southern Gondwana cratonic entities. During a prolonged post-collisional period, possibly related to crustal collapse and extension, further gem-mineralised pegmatites formed at ∼520 ± 6 Ma and ∼500–485 Ma. The youngest pegmatite intruded in the southern Malawian Ntcheu area in the Middle Ordovician at ∼460 Ma. A large spread in 87Sr/86Sr initial isotopic ratios between 0.70556 and 0.79018 suggests a variety of magma sources for the Kuunga-related pegmatites with a variably strong crustal affinity.