Bolshetagninskii (Tagninskii, Tagna)

This complex has a circular outline with a diameter of about 3.6 km. It is a stock-like or a pipe-like body cutting Riphean phyllitic schists and was emplaced in several phases. The earliest rocks are ijolites and melteigites which have been preserved as large blocks, up to 1 km2, within peralkaline and nepheline syenites. The ijolite occupies about 20% of the total area of the complex. The ijolites are composed mainly of nepheline and diopside-augite, with lesser amounts of biotite, feldspar, schorlomite, titanite and apatite. The second intrusive phase is represented by aegirine/hedenbergite-nepheline syenites, cancrinite syenites and aegirine syenites. These were followed by the intrusion of leucocratic syenites, which cover about 30% of the complex. The syenites consist predominantly of microcline and biotite; nepheline, cancrinite, calcite and pyroxene are present in small amounts. The syenites are cut by arcuate dykes of porphyritic picrite, alnoite and damkjernite. The nucleus of the massif is composed of carbonatites, more than 80% of which are calcite carbonatites. At a later stage fluorite-ankerite-calcite carbonatites were emplaced. The calcite carbonatites are massive, occasionally banded, medium- and fine-grained calcite rocks containing biotite, pyroxene, apatite and accessory pyrite, pyrochlore and zircon. Sr, C and O isotopic data for primary calcite in foyaite are avaliable in Konev et al. (1984).