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

Metabasic rocks form a small but geologically important component of the ge ology of the base metal-rich Neoproterozoic Copperbelt of central Africa. T he disposition of the metabasic rocks follows the 150 km long, arcuate, str uctural trend of the thrust belt. Despite their structural disposition, the metabasic rocks played a passive role during the thrusting due to their ri gidity and lithological contrast with the host sedimentary rocks. The main thrust horizons are located along evaporite layers below the position of th e metabasic rocks. The metabasic rocks form part of an allochtonous unit ov erlying 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. Fractionatio n of olivine, pyroxene and plagioclase played an important role in the gene ration of the range of mafic compositions. The rocks are enriched in incomp atible trace elements with chondrite-normalised (La/Yb)(N) values ranging f rom 4 to 7. Incompatible trace element ratios (e.g. La/Nb1.5; La/Ta = 16) i ndicate 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. Al l the metabasic rocks from the Katangan succession show a close chemical af finity with intraplate magmas. (C) 1999 Elsevier Science Limited. All right s reserved.