PETROGENESIS OF THE MASHAVA IGNEOUS COMPLEX IN SOUTH CENTRAL ZIMBABWE BASED ON A STUDY OF ITS SILICATE MINERAL COMPOSITIONS

An investigation of the economically important Mashava Igneous Complex (MIC) in south central Zimbabwe was carried out using mineral compositions in order to gain some insights into its formation. The MIC, a Neoarchean ultramafic complex located in the western extremity of the Masvingo greenstone belt (MGSB), is composed of amphibole, serpentine, epidote, chlorite, clinopyroxene, orthopyroxene, plagioclase, and olivine.

MIC amphiboles display a wide range in composition from the dominant tremolite-actinolite consistent with greenschist facies metamorphism, through magnesio-hornblende to tschermakite which are both consistent with amphibolite facies metamorphism. MIC serpentines also display wide ranges in both FeO and SiO₂ compositions, and the majority of them being of lizardite, chrystotile, and antigorite varieties. Clinozoisite is the only epidote type in the MIC, whereas chlorite varies widely in composition which reflects the varying concentrations in iron and silica. MIC chlorite types range from talc chlorite to pseudothuringite.

MIC Plagioclase also shows a wide compositional range from high albite to bytownite, with only two samples plotting in the alkali feldspar anorthoclase field. Pyroxene occurring in the MIC ranges in composition through enstatite, pigeonite, augite, and diopside whereas olivine in rich in MgO with forsterite contents ranging from 80 to 89. Application of the chlorite thermometry equation of Kranidiotis and MacLean (1987) yields temperatures ranging from ~200-320^oC whereas temperatures ranging from 130-445^oC are obtained using equations of Lanari et al. (2014).

Chlorites in the North West Arm of the MIC were formed at relatively lower temperatures (97-234^oC) than those which from the Central Sector (204-445^oC), supporting the conclusion that these MIC blocks have different origins. Pressure estimates for the metamorphism of the MIC range from 3.8-7.3 kbars, which corresponds to depths of ~13 - 25 km, respectively, likely during thrusting onto its present location.