Geochronology of the Zambezi Supracrustal Sequence,Southern Zambia:A Record of Neoproterozoic Divergent Processes along the Southern Margin of the Congo Craton

摘要

The Zambezi supracrustal sequence (ZSC) of southern Zambia comprises a metasedimentary package of clastics and carbonates,with a thick sequence of basal volcanics and lavas.The sequence has traditionally been interpreted as a Neoproterozoic continental rift succession,but the lack of reliable age constraints hinders any tectonic interpretation.In this article,we date magmatic and detrital zircons using the U-Pb SHRIMP method in order to better constrain the timing of rifting,volcanism,and basin deposition.The basal volcanoclastic Kafue Rhyolite and Nazingwe formations were erupted at ca.880 Ma,and the sequence was intruded by the Lusaka Granite at ca.820 Ma,providing lower and upper limits on the age of sedimentation.Whole-rock Nd isotopic signatures of these volcanics indicate that they formed as a result of assimilation and recycling of basement gneisses,probably during crustal thinning and extension.We uphold the correlation between the ZSC and the Roan Group in the Zambian Copperbelt and suggest that both successions formed in discrete rift basins along the southern margin of the Congo-Tanzania-Bangweulu (CTB) Craton,however,extension at this time probably did not result in complete continental separation.If the CTB Craton were an integral part of Rodinia,then rifting at ca.880 Ma would represent one of the first known records of attempted breakup of the supercontinent.