BARIUM-RICH PHLOGOPITE AND BIOTITE FROM SOME QUATERNARY ALKALI MAFIC LAVAS, WEST EIFEL, GERMANY

Barium-rich phlogopite and biotite occur in leucite nephelinites and nephelinites from several quarries northeast of Gerolstein in the West Eifel Quaternary province of Germany. The micas occur usually as groundmass minerals but also as microphenocrysts. The degree of Ba-enrichment in the micas is variable, and reaches a maximum of 14 wt.% BaO. With increasing BaO, there is a decrease in K2O and SiO2 and increase in Al2O3 for all micas whereas TiO2 and F tend to increase. Compared to Ba-rich micas from other localities, the West Eifel micas exhibit very comparable substitutions, with the Ba in the interlayer site and with a coupled substitution K + 3(Mg, Fe) + 3Si = Ba + 2Ti + 3Al accounting for the mica chemistry. The micas in this study have the characteristic low Al2O3 content of micas in igneous rocks with only (IV)Al, relative to Ba-rich micas from metamorphic rocks that have both (IV)Al and (VI)Al. Differences between the West Eifel micas and those from less Ba enriched micas in lamproites and those produced in high pressure-temperature experiments are discussed. The Ba content of the West Eifel lavas is much less than that of most lamproites although the Ba contents of the West Eifel micas are much greater than those of most micas in lamproites. In the latter, Ba may partition preferentially into minerals other than mica. In the West Eifel region phlogopite is possibly an unimportant reservoir as a single mantle-derived phlogopite xenocryst has much lower BaO, TiO2 and F relative to micas in the lavas.