Omolonskii

At Omolonskii a sequence of sedimentary volcanogenic rocks of Jurassic age is cut by numerous intrusive sheets, dykes, stocks and laccoliths, which extend over 20 km2. These occurrences are composed of teschenite, crinanite, essexite-diabase, essexite, solvsbergite and tinguaite. The intrusive bodies are in some places associated with small outcrops of extrusive alkali basalt and augitite. The crinanites are composed of plagioclase (An62-63), clinopyroxene, pseudomorphes after olivine, analcime, Ti-Fe oxide minerals and apatite. Accessory minerals include chlorite, calcite and biotite. The teschenites comprise considerable orthoclase (44%), hornblende (22%), analcime (7%), clinopyroxene of variable composition (4%), biotite (4%), plagioclase An47-48 (11%), apatite and Ti-Fe oxide minerals. The region is known for its numerous dykes of solvsbergite and tinguaite. The solvsbergites have trachytic textures and are characterised by the variability of the constituent aegirine and hastingsite. Alkali feldspar is the dominant phase (about 80%) and accessories are represented by apatite and zircon. Tinguaite comprises alkali feldspar (51%), nepheline (23%), analcime (6%) and aegirine (20%) as well as riebeckite, apatite, zircon, rutile and magnetite.