Composition of mafic minerals from peralkaline potassic syenites-granite association from Bulgaria

S

Several potassic-alkaline Variscan plutons (330 – 305 Ma) outcrop in Kraishte and Stara planina regions in Bulgaria: Lutskan, Svidnya, 7^th Prestola Monastery, Buhovo-Seslavtsi, potassic syenites west of Shipka and Shipka (from west to east). These magmatic bodies have intermediate to acid compositions and evolve toward peralkaline syenites-granite residual varieties. They present a broad diversity in rock-forming mineralogy reflecting the variations of magma chemistry and conditions of crystallization. Evolution of mafic silicates in the plutons show unique features which enable to discriminate the trend of mineral evolution in each magmatic complex.

Pyroxenes from Svidnya pluton are presented in all facial types. Its compositions cover the entire spectrum from calcic to sodic varieties as pyroxenes evolve from diopside to aegirine. The clinopyroxenes from peralkaline syenite porphyries in Buhovo-Seslavtsi pluton belong to Ca-Na pyroxenes and their compositions vary from Wo₂₅-En₁₃-Fs₁₃-Ac₄₂- to Wo₁₁-En₆-Fs₂-Ac₆₅. Pyroxenes from potassic syenites west of Shipka display limited range and belong to pure diopside, whereas pyroxenes in the peralkaline dykes from Shipka pluton are aegirine-augites. Also, aegirine-augite and aegirine from Svidnya and Buhovo-Seslavtsi are enriched in Ti (TiO₂ up to 6.5 wt. %), while aegirine-augite from Shipka shows high Zr content (ZrO₂ up to 2.9 wt. %), as Ti and Zr enter pyroxene structure via Na(Mg,Fe²⁺)_0.5(Ti, Zr)_0.5Si₂O₆ molecule.

Amphiboles from Lutskan and 7^th Prestola Monastery are low temperature, reflecting their near-solidus stage of crystallization or postmagmatic reequilibration due to the circulation of deuteric fluids. Their composition is winchite - riebeckite, and winchite – barroisite, respectively. In turn, amphiboles from Svidnya complex display a narrow compositional variation from richterites to magnesio-arfvedsonite, and rarely eckermanite. Amphiboles in Buhovo-Seslavtzi complex show broad diversity in their composition as they belong to sodic-calcic and sodic groups. They evolve from ferrobaroisite, ferrowinchite to richterite and potassic-magnesio-arfvedsonite with [A]-site filled by K. Amphiboles from the potassic syenites outcropping west of Shipka are arfvedsonite, characterised with elevated Ti content (up to 4.4 wt. % TiO₂).

Micas from all complexes show limited evolution. In Svidnya, Buhovo-Seslavtsi, Shipka and 7^th Prestola Monastery only biotite is present. Characteristic feature of biotites from Shipka is the elevated fluorine content (up to 5 wt. % F) which coupled with presence of fluorite implies on the F-domination in the fluid phase during the crystallization of the rocks. In Lutskan and in the syenites west of Shipka micas show broad range of variation from phlogopite to biotite.

 

Acknowledgements: The financial support provided by the NSF (Ministry of Education and Science of Bulgaria) through DH 14/8 project is acknowledged.