Nyazepetrovsk

This complex consists of intrusive and extrusive peralkaline and mildly alkaline rocks. Nepheline and peralkaline syenites are encountered as small lens-like bodies 0.2-10 m long. The extrusive rocks are represented by trachybasalts and trachyandesites, including tuffs, trachytes and perhaps phonolites. The intrusive and extrusive rocks of Nyazepetrovsk are situated in a zone of approximately north-south-trending faults which cut sedimentary and volcanogenic rocks of Ordovician age. Related to the faults also are small areas and massifs of earlier dunite, peridotite, pyroxenite and gabbroic rocks. Cutting through pyroxenite and peridotite are nepheline and peralkaline syenites which are enriched in K-feldspar and nepheline and vary in composition to shonkinites and ijolites. Adjacent to veins of alkaline syenites cutting gabbroic rocks are reaction zones containing blue-green amphibole, K-feldspar, zeolites and calcite. Not infrequently the nepheline and peralkaline syenites have porphyritic textures with a trachytic groundmass. Feldspar (orthoclase and albite) comprises 65%, and spreustein and liebnerite, which have replaced nepheline, 27% of the nepheline syenites. Dark coloured minerals are represented by hastingsite, andradite-grossular, aegirine-salite and sometimes by biotite. Accessory minerals include titanite, apatite, fluorite, pyrite, chalcopyrite, magnetite, zircon and pyrochlore. Chemically, the predominance of potassium over sodium is usual. Shonkinites and ijolites have spotted textures and banded structures with K-feldspar developing large porphyroblasts in the former. Clinopyroxene, olivine, magnetite, amphibole and biotite are typical dark coloured constituents of these rocks. The peralkaline syenites are mineralogically similar but for the absence of nepheline. The petrochemistry of the rocks is described by Levin (1974) and Svyazhin and Levin (1971).