Ba-enrichment in phlogopite of a nephelinite from Bohemia

Abstract

Ba-rich phlogopite occurs in nephelinite from Komorni hurka (Kammerbuhl), Czech Republic. The mica usually forms euhedral microphenocrysts and subhedral groundmass crystals. Its BaO content ranges from 13 to 16 wt%, at about 5 to 6 wt% TiO 2 . The average mineral formula is (Ba 0.48 K 0.46 Na 0.04 (Ca 0.02 )(Mg 2.22 Fe 0.40 Ti 0.32 Mn 0.01 )[(Si 2.54 Al 1.46 )O 10 ](F 1.39 O 0.61 ). It shows the presence of a considerable «oxy-component». Rarely, phlogopite of markedly power Ba and higher Ti contents was found included in olivine. This mica was probably formed earlier, under higher temperatures and Ti contents in the melt, than the more Ba-rich, relatively Ti-poor phlogopite. The Komorni hurka mica is comparable to Ba-rich micas from several other localities. In particular, the present report complements the study of Edgar (1992), concerning Ba-rich micas in nephelinitic rocks from the West Eifel, Germany. The high Ba contents in phlogopite from Komorni hurka contrast with the Ba-poor phlogopite from the neighbouring Zelezna hurka (Eisenbuhl) volcano, although the host rocks have similar geochemistry, age, and tectonic position. Differences in mica composition between nephelinites from both volcanoes are not a question of the amount of Ba supplied by the melt, but of different petrogenetic conditions