Metamorphic evolution of ultramafic rocks of komatiitic composition from the Central Zone of the Limpopo Belt, South Africa

Abstract

A large number of published accounts are available on the metamorphic evolution of the Central Zone of the Limpopo Belt. However, most investigations on the P-T evolution of this zone are restricted to pelitic and calc-silicate rocks. In this paper we present for the first time a detailed mineralogical and petrological investigation on an ultramafic granulite of komatiitic composition, which occurs in the vicinity of Musina in the Central Zone. These rocks are composed mainly of magnesium-rich Ca-amphibole (pargasite: MgO up to 18 wt%), together with Mg-rich olivine (X Mg = 0.82 to 0.83) and orthopyroxene (X Mg = 0.82 to 0.84), Cr-rich spinel, magnetite and some sulfide minerals. This set of minerals represents the peak metamorphic assemblage formed during the early stage of the metamorphic evolution of the Central Zone and is different from the mineral assemblage of less altered komatiites. This assemblage was subjected to minor retrogression and alteration to chlorite and serpentine during the waning stage of the metamorphic evolution of the Central Zone. Thermodynamic calculations combining classical conventional methods as well as equilibrium assemblage modelling using Theriak-Domino, in conjunction with textural relationships and mineral chemistry, indicate that the studied rocks went through granulite facies metamorphic conditions at a temperature of up to 820°C and an intermediate pressure of up to 8.5 kbar. These P-T conditions are in good agreement with previous investigations of metamorphic conditions on other rock types (mainly metapelites) in the Central Zone of the Limpopo Belt.