Abstract
The Kaleybar nepheline syenite intrusion forms the largest silica undersaturated alkaline exposure in northwestern Iran. It consists of various rock types ranging from nepheline syenite to nepheline diorite that were emplaced during Eocene-Oligocene times, corresponding to the Alpine orogeny. The essential rock-forming minerals in nepheline syenite are plagioclase, K-feldspar, nepheline and amphibole. Clinopyroxene is the dominant phase in nepheline diorites. Titanian garnet occurs as an uncommon accessory phase forming reddish to deep brown individual grains. Chemically it is intermediate between Ti-andradite (67 to 78 mole %) and grossular (21 to 33 mole %) with TiO2 contents ranging from 1.5 to 5.0 wt %. Stoichiometry and R-mode factor analysis on garnet chemistry show that the dominant exchange vectors are Si-Ti and Al-Fe substitutions in the tetrahedral and octahedral crystal sites, respectively. A magmatic origin of the investigated Ti-garnet is suggested on the basis of mineralogical criteria and chemical properties.
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