Depositional environment of the Kasama Group, northeastern Zambia: Evidence from detrital zircon U-Pb-Hf isotopic compositions

The Bangweulu Block of northeastern Zambia which consists mainly of basement and sedimentary rocks is affected by northeastern Paleoproterozoic Ubendian Belt, northwestern Mesoproterozoic Kibaran Belt, southeastern Mesoproterozoic Irumide Belt and southwestern Pan African Lufulian Belt. The basement rocks are mainly composed of schist, andesitic-rhyolitic metamorphic volcanic rocks, granitoids and other intrusions. From lower to upper part the sedimentary rocks are mainly composed of the Mporokoso Group, the Kasama Group, the Katanga supergroup and Cenozoic fluvial and lacustrine sediments. The authors systematically studied the geological characteristics of the Kasama Group, U-Pb-Hf isotopic compositions and geochemical characteristics of detrital zircon from the Kasama Group in Kasama area and obtained 32 detrital zircons U-Pb ages of the purplish red siltstone from the Kasama Group. Specifically, their ages mainly vary from (1849±23) to (1993±21) Ma, suggesting that the Kasama Group was formed younger than (1434±14) Ma and should belong to Mesoproterozoic. (¹⁷⁶Hf/¹⁷⁷Hf)_i values of 22 zircons are 0.281049-0.281618 and ε_Hf(t) values are-12.8--1.6 with corresponding TDM2 ages of 2406-3487 Ma. The REE and trace elements of sandstones from Kasama Group were analyzed. The REE have obviously negative δEu anomalies and the points mainly fall in the sedimentary area; the trace elements characteristics of sandstones show that sandstones formed in the passive continental margin.