Berdyaush

This intrusion of 3.5x10 km lies within Precambrian limestones. Three intrusive phases comprise the complex: these are rapakivi granite, syenite and nepheline syenite. The intrusion has a concentric zonal structure with the nepheline syenites located in the centre (Fig. 55). However, mapping and prospecting work in the past few years, the map from which is not yet available, has indicated that the nepheline syenites do not form a continuous body, but comprise a series of dykes 0.4-0.5 km thick with lengths of up to 2.5 km. The nepheline syenites are fine-grained, bluish-grey rocks which are usually porphyritic and with a trachytic groundmass. All the main minerals are present as phenocrysts. The dark-coloured minerals are represented mainly by hastingsite and aegirine-salite with small quantities of magnetite and late biotite. On average the nepheline syenites contain 72% feldspar, 21% nepheline and 7% dark-coloured minerals. Not infrequently the nepheline syenites are altered and replaced by albite, zeolites and sericite. The marginal zones of nepheline syenite dykes often grade into alkaline syenite, but the alkaline syenites also form independent dykes. The alkaline syenites consist predominantly of alkali feldspar with small amounts of oligoclase or andesine also present. Mafic minerals comprise amphibole, biotite and aegirine-salite and accessories, which are more diverse in the alkaline syenites than the nepheline syenites, are titanite, orthite and zircon. The genetic role of the rapakivi granites in the complex is a matter of discussion.