Geochemistry and U–Pb zircon dating of the high-K calc-alkaline basaltic andesitic lavas from the Buanji Group, south-western Tanzania

Abstract

SHRIMP zircon U–Pb and Sm–Nd isotopic data together with major and trace elements data are presented for the basalt andesitic lavas from the Buanji Group, south-western Tanzania in order to establish their emplacement age, ascertain their mantle sources and processes responsible for their generation. Zircon U–Pb data shows that the Buanji Group lavas were emplaced at 1674 ± 15 Ma and do not therefore belong to the Neoproterozoic Bukoban Supergroup. Having erupted on top of sedimentary rocks, the 1674 ± 15 Ma age of the volcanic rocks provides the younger limit to the age of deposition of underlying un-dated sediments. The Bunaji Group lavas are amygdaloidal and contain euhedral phenocrysts of plagioclase + orthopyroxene + clinopyroxene as well as subhedral quartz and opaque minerals mainly magnetite with quartz filling in the interstices. As a suite, they are compositionally uniform and are classified as high-K calc-alkaline basaltic andesites. The samples display...