Abstract
The Northern Marginal Zone of the Limpopo Belt is an Archean high-grade metamorphic terrane in Zimbabwe. It is mainly composed of charnockites and opx-free quartzofeldspathic gneisses (biotite-gneiss) with minor amounts of mafic and pelitic gneisses. Temperature conditions of both the charnockite and the biotite-gneiss are estimated to be about 500-680°C from the compositions of coexisting magnetite and ilmenite. Oxygen barometry (fo2) of the charnockite, estimated from ilmenite-magnetite and opx-magnetite-quartz assemblages, is higher and the range smaller than that of the biotite-gneiss. It suggests that the increase in the fo2 level of quartzofeldspathic rocks above the QFM-buffer brought about charnockitization: The increasing fo2 appears to be responsible for lowering a H2O or elevating a CO2, because of breakdown reaction of biotite and/or hornblende to form opx.
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