Nepheline-bearing alkali feldspar syenite (pulaskite) in the Khibina pluton, Kola Peninsula, NW Russia: petrological investigation

The geological position and probable origin of nepheline-bearing alkaline s yenite (pulaskite) found within foyaite of the Khibina pluton, Kola Peninsu la, Russia, are discussed. A large body of pulaskite (4.5 km(2)) was identi fied under the Quaternary deposits in the eastern part of the pluton as a r esult of gravimetric surveying and drilling. Genetic relations between pula skite and the host foyaite were examined by whole-rock chemistry, including major and trace elements, the composition of the rock-forming clinopyroxen es and the diagram of phase equilibria in the system NaAlSiO4-KAlSiO4-SiO2- H2O. The results regarding mafic silicates show that pulaskite may constitu te the less evolved member of a pulaskite-foyaite series, which suggests th at it was formed by feldspar fractionation from an almost eutectic phonolit ic melt, the parental one for both foyaite and pulaskite. The geochemical d ata, however, also enable us to assume the probability of additional petrog enetic mechanisms. The trace element abundance patterns may provide prelimi nary evidence for a, process of crustal contamination of a mantle-derived p honolite magma. This process may have contributed to the genesis of the pul askite. (C) 2000 Elsevier Science Ltd. All rights reserved.