Mineralogy of the pliocene trachyte and its carbonatitic minette inclusions in Ostrvica, FYR of Macedonia

The trachyte at Ostrvica hill (age 3.21±0.10 Ма) in Vardar zone is the most evolved volcan- ics of the ultrapotassic Pliocene-Quaternary series in F.Y.R. of Macedonia. It is aphyric, with clinopy- roxene and phlogopite microphenocrysts within a sanidine-anorthoclase groundmass. It contains inclu- sions of carbonatitic minette ranging in size from several mm to 6–7 cm. They are light coloured por- phyric 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 correla- tions are observed between Na and Fe3+, Al and Ti, and negative one – between Al and Si. In the inclu- sions 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 in- clusions two plagioclase generations (An41 and An25) exist. The estimated crystallization temperature of the minette clinopyroxenes is 1280–1180С, of plagioclase (An41) – 1130С and in the hosting trachyte – 1080С, at the pressures 6.9 and 7.7 kbar, respectively. The temperature of the feldspars crystalliza- tion (K-Na-feldspars and Pl24) in the minette groundmass is 809–878C. By analogy with other ultrapo- tassic volcanics from F.Y.R Macedonia it is suggested, that the discussed volcanics originated from phlogopite-bearing metasomatised mantle