The Malagarazi supergroup of southeast Burundi and its correlative Bukoba supergroup of northwest Tanzania: neo- and mesoproterozoic chronostratigraphic constraints from Ar-Ar ages on mafic intrusive rocks

Abstract

Neo- and Mesoproterozoic subtabular sedimentary units with associated mafic volcanic and shallow depth intrusive rocks are exposed in southeastern Burundi and northwestern Tanzania along the border of the Tanzania Craton. This paper presents the first attempts to date these mafic intrusive rocks by the Ar-Ar technique. The 40Ar- 39Ar step-wise heating results obtained on mineral separates of five (micro)gabbros provide temporal constraints for the two episodes of magmatism previously considered as Neo- and Mesoproterozoic on the basis of lithostratigraphic arguments. The Ar-Ar results reveal the presence of excess Ar in samples formerly attributed to the Neoproterozoic magmatic episode (intrusive mafic rocks within the Musindozi and Kavumwe Groups in Burundi and their correlatives, the ‘Kabuye-Gagwe amygdaloidal lavas’ exposed in Burundi and Tanzania) and yield a better lower age estimate of 795 ± 7 Ma compared to the formerly reported K-Ar ages. Two distinct Mesoproterozoic magmatic episodes characterised by the emplacement of shallow depth intrusive mafic rocks have been recognised. One yields a range of comparable cooling ages of 1340 ± 9 Ma in the Kavumwe Group (Burundi) and of 1379 ± 10 Ma and 1355 ± 10 Ma in the Bukoba Sandstone Group (Tanzania). The second is recorded in the Nkoma Group (Burundi) with the emplacement of a dyke at 1282 ± 5 Ma. The Mesoproterozoic ages correspond to minimum ages for the deposition of the sedimentary country rocks belonging to these groups and elucidate former uncertainties regarding their chronostratigraphic position. These data confirm a previously proposed Kibaran foreland setting for the Nkoma, Kavumwe and Bukoba Sandstone Groups and establish that the extent of the Neoproterozoic Malagarazi (Burundi) and Bukoba (Tanzania) Supergroups is restricted to a more confined area than previously admitted.