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May 18, 2014Keywords:
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Ti-bearing aenigmatite from Djinga Tadorgal (Adamawa plateau) and Sao Tomé (Cameroon Line) phonolites: geochemical implications and application of the QUILF thermobarometer for the crystallization conditions
Abstract
The chemical and structural compositions of aenigmatite from Djinga Tadorgal and Sao Tomé phonolites were compared to those previously described in the felsic lavas and nephelinite from the Adamawa Plateau and Cameroon Line, in order to determine their crystallization temperatures and pressures. Chemical analysis results and the application of QUILF thermobarometer, indicated the equilibrium temperatures between 655 °C and 791 °C, oxygen fugacity (fO2) below the FMQ buffer, pressures near 0.1 GPa and silica activities less than 1.0 for Djinga Tadorgal and Sao Tomé phonolites. The Ti-bearing aenigmatite from Djinga Tadorgal and Sao Tomé phonolites are a late and accessory mineral, resulting from the reaction between Tirich magnetite and Na2O-rich magmatic liquid.
Keywords: Peralkaline lavas, mineral chemistry, silica activity, thermodynamic conditions
