Biotite mineral chemistry of the Bozqush and Kaleybar alkaline igneous intrusions, NW Iran

Mineral chemistry of micas from the subduction-related mafic and felsic alkaline rocks of the Kaleybar alkali-gabbro and the Bozqush nepheline syenite was determined. The micas generally occur as large poikilitic grains with Fe-Ti oxides and mafic minerals and give the rocks a spotted texture. Micas in the alkali-gabbro sometimes display a distinct lamination. Based on the International Mineralogical Association (IMA) classification scheme, the mica compositions plot between the siderophyllite and eastonite end members and show high Fe/(Fe + Mg) ratio (>0.33); therefore they are biotites. Chemical composition of the biotites varies from Mg-biotite in the Kaleybar alkali-gabbro through Fe-biotite in the Bozqush nepheline syenite. Based on their FeO, MnO, MgO, TiO2, and AlVI content, they are primary magmatic biotites. Despite the alkaline affinity of the biotite host rocks, they plot in the calc-alkaline orogenic suite field on FeO*, MgO, and Al2O3 geochemical diagrams.