Abstract

Archaean komatiites of the greenstone belt in Manica area, western Mozambique contains accessory amount of disseminated chromite grains. The komatiite rocks comprise peridotitic and basaltic komtiite. These komatiites are affected by regional metamorphism as all of the studied area. It is observed that only chromian spinel is preserved as the primary mineral and that all the other primary minerals in the komatiite samples are completely altered. The present work gives first time data on the effect of metamorphism on texture and composition of chromite enclosed in komatiite, Manica area, Mozambique. The primary chromian spinel has high Cr^# (Cr/(Cr+Al) atomic ratio; approximate value = 0.9, which is comparable with Cr^# value of chromian spinel in the worldwide komatiites. The chromian spinel have low Al and Mg value (up to 0.26 and 0.21 respectively, low Mg^# and high Cr^# and Fe^# values. Most of the studied spinel grains rimmed by ferritchromite which is transformed outwards to magnetite. Textural features and mineral chemistry of chromian spinel enclosed in these komatiites indicate postmagmatic alteration and metamorphism in greenchist to amphibolite transition facies. The metamorphism also involves reequilibration of the chromian spinel cores with the surrounding silicate assemblage.

Key words: Chromite, microprobe analyses, komatiite, Mozambique.