Zircon geochronology and partial structural re-interpretation of the late Archaean Mashaba Igneous Complex, south-central Zimbabwe
Ion microprobe (SHRIMP) U-Pb zircon dating has determined the age of a gabbroic enclave within the supraregional intrusive komatiitic Mashaba Igneous Complex (MIC) to be 2743 ± 11 Ma and that of an interpreted comagmatic thick differentiated komatiitic flow within the adjacent Mashava-Masvingo greenstone belt to be 2754 ± 13 Ma. Recent geological observations support a revised structural interpretation of the MIC as an assemblage of three separate horizontal layered sills emplaced almost coevally at two different crustal depths: the chromititiferous Prince Sill intruded concordantly, and prior to regional folding, into the basal sedimentary rocks of the late Archaean Upper Bulawayan Supergroup, the relatively thin and evolved West Sill within early Archaean greenstone and gneissic basement, and the major Northwest Arm-Main Sill mainly within adjacent basement gneiss. The sill assemblage was vertically linked via ultramafic dykes and, as previously proposed, was fed through the dyke-like Northeast Arm, whose overlying layered rocks are here interpreted as correlatives of the Northwest Arm-Main Sill. The new precise age for the MIC is 50 Ma years older than the former geological estimate of ~2.7 Ga. The comparable date for the volcanic phase supports the interpretation of both as component phases of a putative Masvingo W komatiitic sill-flow complex but is incompatible with interpretation of the late Archaean lithostratigraphy of the Mashava-Masvingo belt as a thrust-stacked package of unrelated continental and oceanic rocks.