Guruve and Mutare dykes: preliminary geochemical indication of complex Mesoproterozoic mafic magmatic systems in Zimbabwe

Abstract

The northeast-trending Guruve and southeast-trending Mutare mafic dyke swarms of northern Zimbabwe are broadly contemporaneous with Mesoproterozoic Kibaran supracrustal rocks and granitoids of southern Africa which formed during the formation and breakup of the Rodinia Supercontinent. As such, they represent an important magmatic component of the Mesoproterozoic events of southern Africa. Despite their intracratonic setting and overall continental tholeiitic character, slight differences in the abundances of certain elements indicate that the two swarms are not absolutely geochemically equivalent to each other. Each of the swarms comprises dykes with subtly but significantly different compositions, particularly in terms of their rare earth element (REE) and other incompatible element geochemistry. Three subgroups of Mutare dykes (M1, M2 and M3) and less well-defined Guruve dyke subgroups (G1, G2 and G3) are indicated by their variously fractionated REE distributions. Chondrite-normalised La/Lu ratios ( La N Lu N ) vary from 2.0–6.6 in the M1 to M3 Mutare dykes, and from 1.5–6.3 in the G1-G3 Guruve dykes. Simple gabbro fractionation, crystal accumulation, variable partial melting, or crustal contamination models can not alone account for the different REE fractionation profiles, and individual geochemical evolution pathways become apparent for the subgroups once they have been identified from their REE geochemistry. Other incompatible element (Zr, Y, Ti, Hf, Th, Ta, La, Nb) ratios equally suggest different subgroups of both dyke sets. The geochemical complexity of the two dyke swarms indicates that neither can have been derived from simple evolving continental tholeiitic mafic magma batches, but that they developed from rather more complex magma systems, at different stages of the Mesoproterozoic tectonomagmatic event.