A revised method of plotting analyses of igneous rock suites is described, using mafic and felsic indices. The felsic index (F) is the ratio between (Na 2 O + K 2 O) X 100 and CaO + Na 2 O + K 2 O. The mafic index (M) is the ratio between (FeO + Fe 2 O 3 ) X 100 and MgO + FeO + Fe 2 O 3 . A composite M-F diagram has advantages over the triangular FMA diagram. It has been used to analyze the differentiation trend of the tholeiitic series of the Okonjeje complex in South-West Africa.

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