On the Stratigraphy of the Palaeoproterozoic Deweras and Lomagundi Groups (Magondi Supergroup) in Zimbabwe

Abstract

The Magondi Belt at the northwest rim of the Archaean Zimbabwe Craton extends over a length of 250 km from the Munyati River area in the south to the Shamrocke Mine area in the north. It is composed of the rocks of the Palaeoproterozoic Magondi Supergroup, which is subdivided into the Deweras, Lomagundi and Piriwiri Groups. The predominantly continental sediments and basalts of the Deweras Group were deposited in a rift-related basin margin environment marked by crustal instability. The depositional environment of the overlying Mcheka and Nyagari Formations of the Lomagundi Group is more stable and reflects shelf margin conditions. The Deweras sediments and basic volcanic rocks occurring in the southern part of the belt show a good stratigraphic correlation with those of the Silverside Mine area located between the middle and the northern part of the belt. A correlation of the Silverside Deweras rocks with the successions of the northern part of the belt (Mhangura and Doma West areas) becomes more difficult due to facies change, the presence of two impersistent amphibolite sills in different stratigraphic positions of the Deweras Group, and the distinct transgressive character of the marine Lomagundi rocks. Based on field relations, deformation and similar chemical composition it is suggested that the amphibolite sills of the Doma West area are equivalents of the Deweras metabasalts occurring in the Silverside area and in the southern part of the belt. The amphibolite sills were folded together with the enclosing Deweras sediments during the Magondi Orogeny that occurred between 2000 and 1800 Ma. Refolding of the Deweras rocks took place around 530 Ma during the Pan-African Orogeny. In the northernmost part of the belt (Shamrocke and Makuti-East areas), deformed amphibolites are also found in the Lomagundi Group. Using the immobile trace elements Zr/TiO 2 versus SiO 2 and Nb/Y versus SiO 2 of whole rock geochemical data the Deweras metabasalts of the Silverside area were classified as subalkaline basalts. The plot coincides with that of Deweras basic lavas of the Munyati, Mupfure and Alaska areas (southern part of the belt) and amphibolite sills occurring in the Deweras Group of the Doma West area (northern part of the belt). The metabasalts of the Silverside, Munyati, Mupfure and Alaska areas are characterised by a similar trend of their primitive-mantle normalised element patterns.