Emplacement age of the Tshibwe kimberlite, Democratic Republic of Congo, by in-situ LAM-ICPMS U/Pb dating of groundmass perovskite

Abstract

Kimberlitic perovskites have been investigated in order to constrain the emplacement age of the Tshibwe kimberlite. U–Pb isotopic analyses of the perovskite grains from the Tshibwe kimberlite samples yielded an age of 85 ± 10 Ma, which is in agreement with stratigraphic and other circumstantial geological evidence. This Late Cretaceous age coincides with a sharp decrease of spreading rates at the southern Mid-Atlantic Ridge at about 84 Ma. This probably coincided with a change in the direction and velocity of African plate motion. Previous studies have proposed that such changes may provide a trigger for kimberlite magmatism. The Kasai craton appears to have been affected by two main pulses of kimberlite magmatism, an earlier pulse at 130-113 Ma resulting in kimberlites of the NE-SW-oriented Lucapa Corridor in Angola, and a later pulse of roughly 85-70 Ma responsible for the Tshibwe, Mbuji Mayi and possibly the kimberlites of the Kabinda field in the DRC, all of which are in clusters oriented roughly E-W. Mbuji-Mayi lithospheric mantle appears to be warm as the one beneath Kabinda kimberlites, whereas Tshibwe appears to have a cool lithospheric mantle. We suggest that the Tshibwe kimberlite was emplaced before a regional lithospheric heating event occurred that affected the mantle beneath Mbuji Mayi and (most likely) Kasendou. Therefore, the most consistent interpretation is that Tshibwe is the oldest of the three and all three kimberlite clusters are likely of Late Cretaceous age. Their apparently similar age and orientation suggests that they were emplaced along an east-west oriented zone of lithospheric weakness in the northeastern Kasai craton, tentatively termed the “Kasai corridor”.