The timing and tectonic context of Pan-African gem bearing pegmatites in Malawi: Evidence from Rb-Sr and U-Pb geochronology

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 similar to 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 similar to 520 +/- 6 Ma and similar to 500-485 Ma. The youngest pegmatite intruded in the southern Malawian Ntcheu area in the Middle Ordovician at similar to 460 Ma. A large spread in Sr-87/Sr-86 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.

Automated summary

The Pan-African belts of Malawi contain a rich endowment of gem-bearing pegmatites, formed during different geological events. Some pegmatites are associated with the intra-continental rifting episode and the collision of Western and Eastern Gondwana, while most pegmatites formed during the collision of the northern and southern Gondwana cratonic entities. These pegmatites contain various gemstone species and show varying degrees of crustal affinity.