The geology and geochemistry of the Espungabera Formation of central Mozambique and its tectonic setting on the eastern margin of the Kalahari Craton

Abstract

Whole rock major and trace element chemistry as well as radiogenic isotope data from the Espungabera Formation of central Mozambique are compared with published data from the Umkondo Formation lavas in SE Zimbabwe and Straumsnutane Formation lavas in western Dronning Maud Land, Antarctica. These formations form part of the ∼1100Ma Umkondo Igneous Province in southern Africa and are now preserved on the Grunehogna (in Antarctica) and Zimbabwe (in Zimbabwe) Cratons.The chemical data indicate that the Espungabera Formation lavas are dominantly tholeiitic and basaltic to basaltic andesitic in composition. The Espungabera lavas are dominated by plagioclase, clinopyroxene and Fe–Ti oxides. Metamorphic mineral assemblages indicate the lavas have been metamorphosed under mid-greenschist facies on a retrograde path to prehnite-pumpellyite facies conditions. The decrease in FeOt with increasing MgO content in the Espungabera lavas and the slight decrease in TiO2 with increasing MgO indicates fractionation of Fe–Ti oxides. The lavas are characterised by negative Nb anomalies; enriched LILE’s and high 87Sr/86Sr isotopic ratios. The 87Sr/86Sr data calculated at 1100Ma suggest contamination by continental crust during the petrogenesis of the lavas. The Espungabera volcanics have negative εNd values (−2.83 to −3.49) also suggesting that the magma was contaminated by older crust.Comparison of the chemical data from the Espungabera Formation with data from the Umkondo Group basalts from SE Zimbabwe and the Straumsnutane Formation lavas from Dronning Maud Land, Antarctica shows that they are similar. These similarities, along with similarities in the available geochronological data suggest that these rocks are comagmatic. Both units are also geochemically similar to some rock units that form part of the Umkondo Large Igneous Province (i.e. Zimbabwe basalts that were regarded as Umkondo basalts by Munyanyiwa (1999), Waterberg sills, Umkondo sills and Type III Mutare and Guruve dykes identified by Ward (2002)), and therefore we conclude that the Espungabera lavas in Mozambique also form part of the Umkondo Igneous Province.The craton-based tholeiitic Umkondo Igneous Province is broadly coeval with tonalitic calc-alkaline and granitic gneisses in the Nampula and Maud Terranes in Mozambique and Antarctica respectively, immediately east of the Kalahari Craton in a reconstructed Gondwana. These data can be interpreted to indicate that the Espungabera and Straumsnutane lavas form part of a back-arc complex, west of a volcanic arc/subduction zone along the eastern margin of the Kalahari Craton at ∼1100Ma.