In Stara planina Mts., Bulgaria the Buhovo-Seslavtzi pluton is built up of potassic monzonites and quartzsyenites, which contain calcic pyroxenes and amphiboles, plagioclase and potassium feldspar. The plutonic rocks are accompanied by dykes of different composition: lamprophyres, syenite-aplites, syenite-porphyries, bostonite-porphyries and peralkaline syenite and granite porphyries. In the dykes of bostonite porphyries and peralkaline syenite and granite porphyries calcic pyroxenes and amphiboles are replaced by alkaline mafic phases: aegirine-augites, sodic-calcic and sodic amphiboles. Aegirine-augites are Ti-rich up to 5.95 wt. % TiO2 and Zr-rich up to 0.83 wt. % ZrO2. Ti and Zr present antipathetic relationships in aegirine-augite, as their repartition depends on the agpaicity. Ti enters pyroxene structure under strongly agpaitic conditions, whereas Zr enters aegirine-augite in mildly agpaitic conditions. Composition of sodic-calcic and sodic amphiboles evolves through ferrowinchite, ferrorichterite, richterite toward magnesio-arfvedsonite. Evolution ends with crystallization of potassic magnesio-arfvedsonite, which contains up to 4.67 wt. % K2O, as K totally fills [A] site. Pyroxenes and amphiboles show persistent magnesian character during the evolution toward sodic-calcic and sodic varieties. This trend is not common for silica-oversaturated rocks, and indicates high fO2 conditions and crystallization between NNO and HM buffers. The high activities of alkalis and silica are considered as the most important factors leading to the formation of these peculiar aegirine-augite and potassic magnesio-arfvedsonite.

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