Paleoproterozoic high δ13Ccarb marbles from the Ruwenzori Mountains, Uganda: Implications for the age of the Buganda Group

Abstract

The Paleoproterozoic Buganda Group forms part of the arcuate Ruwenzori Fold Belt, in southern Uganda, separating the Archean granite–greenstone terranes of the Tanzanian craton from the Archean high-grade gneisses of the North Uganda terrane. It consists of a metasedimentary succession of mica schists, phyllites, quartzites, and rare marbles and calc-silicates, overlain by a thick succession of amphibolites and mafic metavolcanic rocks, including tholeiitic pillow lavas, that have a MORB-like composition. The volcano-sedimentary succession was once part of the ocean floor, and was later obducted onto the Tanzanian craton during the Ruwenzori orogeny at around 1.9Ga. The Ruwenzori Fold Belt and its rock protoliths are poorly dated, and the age of the volcano-sedimentary succession is only constrained between c. 2.13 and 1.99Ga. We report here on the stable isotope (C and O) geochemistry of marbles from the Ruwenzori Mountains in western Uganda, which has a bearing on the age of the Buganda Group.A suite of samples of impure dolomitic and calcitic marbles from the High Peaks Schist and the Kilembe Schist subgroups, Buganda Group, was analyzed. The marbles, which are metamorphosed to upper amphibolite grade, show a wide spread of δ13Ccarb values, ranging from +11.3 to −9.3‰ VPDB, and a range of δ18O values from −9.9 to −24.0‰ VPDB. There is a generally positive correlation between δ13C and δ18O values in the marbles as well as between δ13C and Mn/Sr ratios. Six of the 12 measured samples from the High Peaks Schist Subgroup have anomalously high δ13Ccarb values ranging between +11.3 and +10.0‰ VPDB.The Ruwenzori carbonate rocks are interpreted to have been deposited during the c. 2.22–2.06Ga Lomagundi positive carbon isotope excursion in seawater composition, and these rocks were subsequently affected by regional metamorphism during the c. 1.9Ga Ruwenzori orogeny, contributing to the wide range of δ13C and δ18O values found in this study. The Ruwenzori marbles, being interbedded in the lower part of the Stanley Volcanics Formation, also indicate an age for the volcanics between c. 2.22 and 2.06Ga, during the peak of the Lomagundi carbon isotope excursion. These age constraints are broadly consistent with a recently obtained age of c. 2.13Ga for a granulite in the basement for the Buganda Group.