Abstract: The main types of alkaline rocks in Gejiu, Yunnan are alkaline and feldspathoid syenites. The feldspathoid syenite contains many feldspathoid minerals including nepheline, sodalite and alkaline dark minerals. In this paper, we further divide the feldspathoid syenite into four categories according to its mineral composition and characteristics: melanite nepheline sodalite, nepheline sodalite, nepheline and sodalite nepheline syenites. Geochemical analysis revealed peralkaline characteristics with very high K2O＋Na2O content. The rocks also show potassium characteristics. The high rock differentiation index (DI) indicates high degree evolutionary differentiation of magma at its origin. The rare earth element (REE) patterns show enrichment in light rare earth elements (LREE) and depletion in heavy rare earth elements (HREE). The rocks are generally enriched in high field strength elements (HFSE) Zr and Hf but depleted in Ta and Ti; they are enriched in large ion lithophile elements (LILE) Th and U but depleted in Ba and Sr, and P. They have low Cr, Co and Ni contents and moderate negative Eu and weak negative Ce anomalies. These petrographic and geochemical features are obvious indication that feldspathoid syenite underwent fractional crystallization during the magmatic evolution. The temperature of the magma origin was about 835 ℃ and the origin laid relatively deep. The rock subtype is A1 of the A type suites. We suggest that the Gejiu feldspathoid syenite may be the product of high fractional crystallization from enriched mantle of amphibole or phlogopite facies at high temperatures and that it was formed in the late Yanshanian extensional tectonic setting with the mixing of continental crust components.

Key words: feldspathoid syenite, petrography, petrogeochemistry, Gejiu, Yunnan