Phlogopite as an indicator of magmatic differentiation in the Amba Dongar carbonatite, Gujarat, India

Brown micas (phlogopite, phlogopitic biotite and tetraferriphlogopite) acco unt for 4 to 15 % of the sovites and alvikites of the Amba Dongar carbonati te-alkalic complex. In order of emplacement sovite is an earlier phase than the alvikites which intrude the former as thin dikes. Sovitic micas are Mg -rich and compositionally homogeneous whereas alvikitic micas show variatio n from moderately Mg-rich to Fe-rich and are strongly zoned from Mg-rich co res to Fe-rich rims. Their Al content decreases distinctly from early sovit ic to late alvikitic micas. The most Al-poor micas (usually the outermost r im of the strongly zoned alvikitic mica) are compositionally "tetraferriphl ogopite". This indicates that Al in the tetrahedral site is increasingly re placed by Fe3+ which reflects concomitant Fe-enrichment in the carbonatite magma as fo(2) also increased. The TiO2 content of all these micas is highl y variable; the lowest TiO2 content is found in the most magnesian micas. T his study shows that mica was on the liquidus throughout the crystallizatio n of calcitic carbonatite magma at Amba Dongar, and chemical variation with in the micas can be used to deduce the differentiation history of the carbo natite magma at Amba Dongar and thought to be of general application to oth er carbonatite suites.