Geochemical study of the Umkondo dolerites and lavas in the Chimanimani and Chipinge Districts (eastern Zimbabwe) and their regional implications

The Umkondo Group is a supracrustal sequence cropping out in eastern Zimbab we in the Nyanga, Chimanimani and Chipinge Districts. In these areas the se quence has been divided into a weakly metamorphosed and deformed unit of ar gillaceous, arenaceous and carbonate rocks (Zimbabwe facies) in the west, a nd a strongly deformed and medium- to high-grade metamorphosed sequence of mainly quartzites and metapelites (Mozambique facies) in the east. The two sequences were tectonically juxtaposed during the Neoproterozoic Pan-Africa n Mozambique Belt deformation. The Zimbabwe facies sedimentary rocks are intruded by extensive dolerite si lls and minor interlayered basalt flows. The mafic rocks are sub-alkaline c ontinental tholeiites. They have low mg numbers associated with low Cr, Cu, Ni and Co, which indicate that the parental magma underwent some different iation processes en route to the surface. They are LREE enriched with (La/Y b)(N) = 5.0-7.6, high Ce/Yb (>10) and La/Nb (>0.5) values, and exhibit trou ghs at Nb, Sr, Ti and P on a MORB-normalised, multi-element spider diagram. These chemical characteristics, together with the large areal extent of th e Umkondo dolerites and basalts, suggest that the Umkondo mafic igneous sui te was once widespread and formed part of a continental flood basalt provin ce. This is supported by the depositional environment (shallow water platfo rm type setting) of the sedimentary sequence into which the mafic rocks wer e emplaced. The widespread occurrence of the Umkondo igneous event is furth er supported by the similarity in palaeomagnetic poles of a number of mafic units in southern Africa. (C) 1999 Elsevier Science Limited. All rights re served.