Abstract
Electron microprobe analyses of phlogopites from five carbonatite intrusions (C1 oldest, to C5 youngest) constituting the carbonatite plug in the Jacupiranga complex support previous conclusions based on magnetite analyses that C2 to C5 represent a trend of differentiation, and that C1 has different chemical characteristics. All analyzed grains are zoned but no systematic zoning trends occur. The FeO/(FeO + MgO) ratio is very low (0.065 to 1.2), MgO is high (22.5 to 28.2%), TiO2 is very low (up to 0.44w%), BaO may be very high (up to 10.3 w%), and Na2O may be as high as 2.77 w%. With decreasing age from C2 to C5, MgO increases. Phlogopites from a banded reaction rock between carbonatite and jacupirangite are similar to, but not related to the phlogopites of the adjacent carbonatite. Compared with other carbonatite micas, the MgO-rich, TiO2-poor Jacupiranga phlogopites present one of the less evolved compositions, similar to those in mantle peridotites. Carbonatite micas are generally lower in TiO2 than BaO-rich micas from other rocks.
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