Tholeiitic magmatism associated with continental rifting in the Lufilian Fold Belt of Zambia

Abstract

Metabasic rocks form a small but geologically important component of the geology of the base metal-rich Neoproterozoic Copperbelt of central Africa. The disposition of the metabasic rocks follows the 150 km long, arcuate, structural trend of the thrust belt. Despite their structural disposition, the metabasic rocks played a passive role during the thrusting due to their rigidity and lithological contrast with the host sedimentary rocks. The main thrust horizons are located along evaporite layers below the position of the metabasic rocks. The metabasic rocks form part of an allochtonous unit overlying para-autochtonous rocks of the Upper and Lower Katangan sequences. A petrological and geochemical study of the metabasic rocks indicates that they crystallised from genetically related, tholeiitic magmas. Fractionation of olivine, pyroxene and plagioclase played an important role in the generation of the range of mafic compositions. The rocks are enriched in incompatible trace elements with chondrite-normalised (La/Yb)(N) values ranging from 4 to 7. Incompatible trace element ratios (e.g. La/Nb 1.5; La/Ta = 16) indicate little interaction between the protolith to the metabasic rocks and continental crust. The trace element data instead suggest that melting of an enriched asthenospheric source produced the magmas. A comparison of the studied metabasic rocks with those from other parts of the Lufilian Belt shows an overall geochemical similarity of the magmas. All the metabasic rocks from the Katangan succession show a close chemical affinity with intraplate magmas.