Abstract

The Keban syenitic intrusive is composed of porphyric syenite, monzonitic-syenite and monzonite. These rocks were emplaced in a shallow level and display a phaneritic texture with euhedral K-feldspar megacrystals (Kfms) with simple twinning and growth structures. These Kfms contain mafic and felsic inclusions. The fact that the inclusions are usually placed along the longer axis of the K-feldspar crystals and their marginal zones indicates that the amount of melt of the magma was still high towards the end of the cooling. The petrographic properties indicate that the Kfms were formed as a result of multi–phase crystallization and continued to grow during alkali feldspar phenocrystals crystallization period. The contact of the megacrystals with the matrix, their simple twinning and their relationships with the inclusions, indicate that the megacrystals were formed in a subsolidus environment and in the liquid as individual (not in situ) crystals and in a dynamic magma. The major element content indicates that megacrystals are orthoclase. Major element whole-rock geochemistry data reveal a high-K, metaluminous character, with a low aluminum saturation index (ASI) value. These rocks are rich in Rb, Ba, Cs, Th and light rare earth element (LREE), but are poor in heavy rare earth element (HREE), MgO (0.52%wt) and Ni (<20 ppm).

Key words: Keban syenitic intrusive, K-feldspar megacrystal, mineral chemistry, syenite-monzonite porphyry, Eastern Turkey.