Origin of the Mashava Igneous Complex, south central Zimbabwe: Evidence from Prince Mine chromite compositions

Abstract

Electron microprobe analyses were performed on chromites from Mashava Igneous Complex (MIC), a Neoarchean ultramafic complex located in the western extremity of the Masvingo greenstone belt (MGSB) in Zimbabwe. In backscattered electron images (BSEs), the massive, layered, and sometimes serpentinite hosted chromite crystals are rimmed by a thin lighter-colored zone enriched in Fe relative to the much larger cores. Chromium number, Cr#, and magnesium number, Mg#, from the darker grain cores range from values of 70.0 to 75.8 and 69.0 to 80.6, whereas the narrow lighter colored rims have chromium numbers that range from 90.7 to 99.6 and 58.5 to 69.3, respectively. These Mg#s are much higher than those typical of layered intrusions, and are interpreted to be a consequence of the initial high magnesium concentrations of the bulk ultramafic magma of the MIC.On a plot of Cr2O3 versus TiO2, MIC chromites plot within the field of komatiites, whereas on plots of Cr# versus Mg# the chromites plot in the stratiform field. However, on both TiO2 (wt%) and Al2O3 (wt%) versus Cr2O3 (wt%) plots, MIC samples fall within the fields of podiform chromites. The high Cr# of MIC chromite cores suggest arc affinities based on their inferred Al2O3-TiO2 contents of parental melts to the chromites. It is therefore proposed that MIC formed at a convergent boundary and was later accreted to its present position at the margin of the MGSB, or formed entirely due to subduction processes, which may also have formed this part of the MGSB. The ferritchromite rims likely formed during subsequent metamorphism under at least greenschist facies metamorphism, which likely also resulted in the formation of chrysotile asbestos.