Chetlasskii

This occurrence is composed mainly of peralkaline syenites. They include alkali feldspar (25-30%), oligoclase (3-6%), amphibole (30%), which is variable in composition (barkevikite, hastingsite, arfvedsonite, riebeckite), and aegirine (15%). Accessory minerals are apatite, titanite, rutile and monazite. Dykes and veins of lamprophyre and carbonatite cut the peralkaline syenites. The lamprophyres are much altered and divided into two groups according to their age. The earlier group is represented by small porphyry dykes and veins the phenocrysts including intensively hydrated biotite or phlogopite and aggregates of serpentine and talc, which apparently replaced olivine. The groundmass is brown and intensely turbid and only small plates of much altered mica can be identified. Accessories include apatite, rutile, chrome spinels, epidote and corundum.The lamprophyres of the later group are ultrabasic rocks (SiO2 about 40%) with rather high contents of alkalis (K2O + Na2O about 2-3%) and they form dykes 3-8 m thick and 0.8-6.2 km long and also stock-like bodies. Again these rocks are highly altered but fresh samples of a green colour can be found occasionally. Lamprophyres of this group have a brecciated structure, with xenoliths of the country rocks, and variable texture (fine- and medium-grained, porphyritic and poikilitic). Poikilitic amphiboles are up to 0.5 cm diameter and enclose diopside-augite and low-Fe biotite crystals, while aggregates of talc, serpentine and calcite pseudomorph olivine. The groundmass is composed of clinopyroxene, biotite, epidote, serpentine and chlorite. Accessories comprise chrome spinel, magnetite, rutile and anatase. Many veins of carbonatite of variable composition are known and include phlogopite-calcite and calcite-feldspar types with alkaline amphibole and aegirine.