The Proterozoic Choma-Kalomo Block, SE Zambia: Exotic terrane or a reworked segment of the Zimbabwe Craton?

Abstract

The Choma-Kalomo Block in south-eastern Zambia was hitherto considered as solely Mesoproterozoic in age based on granitoids which were previously dated at 1.37 and 1.18Ga respectively. The Choma-Kalomo Block was therefore thought to constitute an exotic terrane with respect to the neighbouring Palaeoproterozoic Magondi Belt and Archaean Zimbabwe Craton. This study of the Choma-Kalomo Block presents new U-Pb SIMS age data for zircons collected from previously undated metasedimentary rocks, revealing an abundant Palaeoproterozoic component (2040–1861Ma). Palaeoproterozoic (2040±5Ma) xenocrystic zircons in a Mesoproterozoic (1370±3Ma) leuconorite point to reworking of older crustal material, and suggest that the Choma-Kalomo Block is not a juvenile Mesoproterozoic terrane. Our U-Pb age data on columbite-tantalite fragments from tin-bearing pegmatites in both the Choma-Kalomo Block and the Dete-Kamativi Inlier (which is part of the Magondi Belt) indicates that Sn-(Ta-Nb-W-Li-Be) mineralisation within the two terranes occurred c. 1030 to 920Ma. New 40Ar-39Ar dating, confirms previous data, and indicates that a thermal event affected the region between 1020 and 980Ma. The similarities between the Choma-Kalomo Block and the Dete-Kamativi Inlier imply that these two terranes have a common history, at least as far back as the Palaeoproterozoic, but were certainly juxtaposed by the late Mesoproterozoic era. A large difference in lithospheric thickness between the Choma-Kalomo Block and the adjacent Zimbabwe Craton could explain the different rheological behaviour of the Choma-Kalomo and Dete-Kamativi areas, with the former being strongly deformed during the Neoproterozoic (Pan-African) orogeny. Our data indicate that the Choma-Kalomo Block is not an exotic terrane, rather it may represent a reworked portion of the Zimbabwe Craton, however what is unclear is whether the subcontinental lithosphere has been thinned, or the Choma-Kalomo Block represents a terrane with an originally thinner lithosphere.