Structural Geology of the Umkondo Group in Eastern Zimbabwe and Geochronology of Associated Mafic Rocks and Possible Correlatives in Zimbabwe.

Abstract

Metasedimentary rocks of the Mesoproterozoic Umkondo group in eastern Zimbabwe record the presence of an epicontinental basin situated near the eastern margin of the Kalahari craton. The Umkondo group is subdivided into the Zimbabwe- and Mozambique facies. The Mozambique facies consists of highly deformed and strongly metamorphosed, greenschist- to amphibolite facies grade psammites and metapelitic schists that were clearly thrust westwards during the Mozambique orogeny ~ 550 Ma ago. The Zimbabwe facies comprises weakly to nonmetamorphosed strata of pebbly basal arkoses that are succeeded by shallow-marine stromatolitic limestones, cherts and cross-bedded arkoses that are interbedded with basalts and mafic and felsic tuffs. Rocks of the Zimbabwe facies were metamorphosed to greenschist- to prehnite-pumpellyite facies grade. Tuffs record input of pyroclastic material into the Umkondo basin and possibly reflect explosive volcanic activity related to the Umkondo igneous event. The Zimbabwe facies is locally deformed by north-south-trending, upright folds with wavelengths of up to tens of metres and east dipping small-scale thrusts that indicate a west-directed sense of movement.

Until now, there has been controversy as to whether the Zimbabwe facies rests nonconformably or tectonically on the Archaean Basement. West-verging small scale folds and small-scale thrusts are evidence that the Zimbabwe facies has been locally deformed. However, the pebbly basal arkose of the Zimbabwe facies rests depositionally on the Archaean Basement. Some Umkondo sills were emplaced along the nonconformity and their contacts with the Archaean Basement and overlying Zimbabwe facies strata are clearly intrusive. Most importantly, some Umkondo sills transgress the Archaean Basement/Zimbabwe facies contact and bedding without break. These observations, together with the lack of tectonic discontinuity at the Archaean Basement/Zimbabwe facies contact, and the occurrence of undeformed stromatolites in limestones near the contact, suggest that the contact is a nonconformity. The Zimbabwe facies is therefore authochtonous on a large scale and the west verging small-scale structures are indicative of localized movement that probably accommodated displacements on a maximum scale of hundreds of metres.

Dolerites intrude the Archaean Basement and the overlying Umkondo strata as both dykes and sills. U-Pb analyses of 5 single grain baddeleyite fractions from a granophyre that intrudes the lower Umkondo group yielded a crystallization age of 1108.5 ± 0.4 Ma. Nine grains from another sill that intrudes the upper levels of the Umkondo group yielded dates that are 2 to 7% discordant. A York (1969) regression through six analyses yielded an upper intercept of 1109.0 ± 2.2 Ma and lower intercept of 211 ± 41 Ma. The upper intercept is interpreted as the crystallization age, and hence the emplacement age of the sill, while the lower intercept age of 211 ± 41 Ma may be indicative of episodic Pb loss event that was probably associated with Karoo reheating throughout the Zimbabwe craton. These age data, combined with the Hanson et al. (1998) age of 1105 ± 2 Ma for another sill in the Chimanimani area show that the Umkondo sills in eastern Zimbabwe were emplaced in different stratigraphic levels of the Zimbabwe facies of the Umkondo group within a period of 4 Ma, i.e. between 1109 and 1105 Ma.

Precise age for the Guruve, Kamativi and Mutare dykes remain speculative since only one north northwest trending dyke in eastern Zimbabwe yielded baddeleyite grains. U-Pb analyses of 3 fractions of single baddeleyite grains and 2 multi-grain fractions from this dyke are concordant to slightly discordant. Four least discordant analyses yield a Pan-African age of 724 ± 2.1 Ma, which is interpreted to be the crystallistion age of the dyke and is nearly 400 Ma younger than the Umkondo sills. Thus the Mutare dykes are strictly not a swarm since a swarm comprises of dykes that are contiguous, contemporaneous and consanguineous (Ward et al., 1999, 2000). Emplacement of dykes in eastern Zimbabwe was structural controlled by pre-existing faults since the dykes and faults patterns are similar and some dykes occur along sections of faults. Assuming minimum magma pressure required to open north northeast trending fractures, the maximum principal stress (s1) during the emplacement of the dyke was north northwest-south southeast and is consistent with contractional deformation in the Zambezi belt at the time of dyke emplacement. The dyke may be part of a Pan African tectonomagmatic event that may have affected the entire Zimbabwe craton and was associated with collision of the Congo and Kalahari cratons. It is inferred that dykes that fed Umkondo sills do exist in eastern Zimbabwe, but farther geochronological work is necessary to identify them, and to establish the ages of the Guruve and Kamativi dykes.