Petrogenesis of the Katangan mafic rocks, Zambia: Insights from lithogeochemistry, large igneous province classification, and petrological modeling

Abstract

The Neoproterozoic Katangan mafic rocks are composed of gabbro and basalt sills, dykes, and flows. They are characterized as a large igneous province distributed throughout the 450 km long and 600 km wide arcuate Central African Copperbelt (CACB), and are in southeastern Democratic Republic of Congo and northwestern Zambia. Understanding the petrogenesis of these mafic intrusive rocks is vital to assist future studies, as examples regarding their relation to the vast Cu and Co deposits in the Central African Copperbelt or consequences of Rodinia's break up. Lithogeochemical data suggest that these rocks were formed due to an extensional regime developed during the rifting stage of the CACB, which resulted in the upwelling of the asthenospheric mantle. Consequently, petrological modeling demonstrated how these rocks derived from primitive magmas and evolved chemically through fractional crystallization, assimilation, and magma mixing, generating a range of compositions with close chemical affinity. Based on this analysis, we have subdivided them into two main groups with distinct sources each further subdivided into two subgroups. A voluminous group with ocean island basalt (OIB) affinities is widespread throughout the basin and can be modeled as the product of crustal assimilation by a picritic magma, concomitant with fractional crystallization. Magma types of this group have interacted with upper crust to develop a characteristic high Th/Ti trace element character. A less common magma represents the product of interactions between a tholeiite with enriched mid-ocean ridge (E-MORB) affinity with metasomatized subduction zone-modified lithospheric mantle (SZLM) to generate an array of moderately to extremely potassic magmas, which we classify as shoshonitic to ultrapotassic.