Petrography and mineral chemistry of the pelitic and semi-pelitic gneisses of the merelani tanzanite mining area, northeastern Tanzania

Sillimanite/kyanite + K-feldspar + quartz ± garnet + biotite ± Fe-Ti oxide-bearing rocks occur associated with calc-silicate rocks in the Merelani tanzanite mining area north-eastern Tanzania. The whole rock chemistry shows varying rock oxidation ratios, which are reflected in the mineralogical assemblages. Almandine garnet is oxidized to mixtures of hematite, quartz, and hydrous aluminosilicates in oxidized rocks and is absent in highly oxidized rocks. In the latter, ilmenite is oxidized into mixtures of rutile and hematite. Such relatively high temperature alteration/oxidation effects could have occurred with the influx of water. Absence of staurolite in these aluminium-rich rocks is indicative of high fugacity of oxygen during metamorphism. These rocks were regionally metamorphosed to the high-grade granulite facies with estimated formation temperatures and pressures of 596 – 726o C and 7.7 – 9.1 kbars and later, underwent retrograde metamorphism to the amphibolite facies. Tanzania Journal of Science Vol. 31 (2) 2005: pp. 81-9