Ba- and Ti-rich oxymica from nephelinites in the Middle Atlas Volcanic Province, northern Morocco

Abstract

A Ba- and Ti-rich mica (up to 14.0 wt% BaO and 13.1 wt% TiO2) occurs in nephelinites from the Middle Atlas Volcanic Province, Morocco. The rocks show a porphyritic texture composed of olivine, Ti-rich augite, nepheline and Ti-rich magnetite. Oxyphlogopite is found in secondary assemblages consisting of zeolites, clinopyroxene and Ti-oxides. This mineral occurs mostly as euhedral to subhedral crystals lining the walls of zeolitic veinlets and cavities in the nephelinites. The coupled substitutions of (1) Ba for K in the interlayer site and Si for Al in the tetrahedral site and (2) Ti4+ for Mg in the octahedral site and O2− in the (OH, F) site are major mechanisms of Ba and Ti incorporation in these micas. The average structural formula, calculated on the basis of 12 (O, OH, F, Cl), is (Si2.515-Al1.482-Fe3+0.003)4.000 (Al0.050-Ti0.661-Fe2+0.798-Mn0.012-Mg1.500)3.021 (Ca0.014-Ba0.398-Na0.066-K0.502)0.980 O10 (F0.308-Cl0.009-O1.319-OH0.365)2.001, suggesting a combination of oxyphlogopite and oxykinoshitalite, with contribution from the hypothetical end-member “Ti-eastonite”. Ba- and Ti-rich oxymica in the nephelinites may have crystallized under the influence of Ba-rich fluids derived from nephelinitic melts during late magmatic stages. The Ba enrichment in the host nephelinites indicates a lithospheric mantle source enriched by the circulation of metasomatic fluids.