Mineralogy of the Pliocene Trachyte and its Carbonatitic Minette Inclusions in Ostrvica, F.Y.R. Macedonia

The trachyte at Ostrvica hill (age 3.21±0.10 Ма) in Vardar zone is the most evolved volcanics of the ultrapotassic Pliocene-Quaternary series in F.Y.R. Macedonia. It is aphyric, with clinopyroxene and phlogopite microphenocrysts within a sanidine-anorthoclase groundmass. It contains inclusions of carbonatitic minette ranging in size from several mm to 6–7 cm. They are light coloured porphyric rocks, rich in vacuoles, composed of phlogopite and completely altered olivine(?) phenocrysts amongst acicular clinopyroxenes within a feldspar–calcite groundmass with abundant Fe-oxides and acicular apatite microlites. The inclusions are rimmed by a mm thick mixing zone composed of the same minerals but with intermediate composition between that of minette and trachyte. The clinopyroxenes are mainly diopside-augite with low Ti and Al content (with 6Al only in the minette). Positive correlations are observed between Na and Fe³⁺, Al and Ti, and negative one – between Al and Si. In the inclusions phlogopites the negative correlation between Mg# and 4Al is found. The feldspars in the trachyte and minette inclusions are Ca-sanidine to Ca-anorthoclase, in the mixing zone – sanidine only. In the inclusions two plagioclase generations (An₄₁ and An₂₅) exist. The estimated crystallization temperature of the minette clinopyroxenes is 1280–1180°С, of plagioclase (An₄₁) – 1130°С and in the hosting trachyte – 1080°С, at the pressures 6.9 and 7.7 kbar, respectively. The temperature of the feldspars crystallization (KNa- feldspars and Pl₂₄) in the minette groundmass is 809–878°C. By analogy with other ultrapotassic volcanics from F.Y.R Macedonia it is suggested, that the discussed volcanics originated from phlogopite-bearing metasomatised mantle.