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Unusual Ti and Zr aegirine-augite and potassic magnesio-arfvedsonite in the peralkaline potassic oversaturated Buhovo-Seslavtzi complex, Bulgaria

¹ Department of Mineralogy, Petrology and Economic Geology, Sofia University "St. Kliment Ohridski", 15 Tzar Osvoboditel, Sofia 1000, Bulgaria
² Laboratoire de Pétrologie et Volcanologie, Université de Paris-Sud, Bat. 504, 91405 Orsay, France

^* Corresponding author, e-mail: momchil{at}gea.uni-sofia.bg

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.

Key-words: potassic peralkaline syenites and granites, Ti and Zr aegirine-augite, potassic magnesio-arfvedsonite.