Pesochnyi (Kholodnoye Ozero)

This is an irregularly-shaped, poorly exposed massif covering about 6 km2. It is intruded into granite gneisses of Archaean age and is composed mainly, in order of decreasing age, of peridotite, pyroxenite, ijolite-urtite, urtite, apatite-amphibole rocks, carbonatites, alkaline and nepheline syenites, augitites and amphibole monchiquites. Ultrabasic rocks compose about 90% of the total area of the complex, of which pyroxenites occupy no less than 80%. The peridotites generally occur in the marginal parts of the intrusion or between pyroxenites which form bodies of irregular form. The contacts between the peridotites and pyroxenites are generally observed to be gradational. Thepyroxenites are coarse- and gigantic-grained rocks, which are sometimes enriched in apatite and kaersutite. The alkaline rocks are situated in the central part of the complex and form two separate bodies and some veins. They are represented by ijolite-urtite, urtite and ijolite which almost everywhere are carbonated. The mineralogy of these rocks is aegirine-augite and nepheline with subordinate hastingsite, titanite, apatite, calcite, natrolite and phlogopite. Alkaline pyroxenites are observed among the carbonated alkaline rocks. To the east of the main body of alkaline rocks giant-grained ijolite-urtites and urtites form distinct units. The carbonatites occur as cross-cutting, vein-like bodies with widths of 0.5-1.2 m and as nests and stockworks which, by gradual transition, grade into pyroxenites. The carbonatites consist of calcite, aegirine-diopside and apatite with minor amphibole, titanite, zircon, baryte, ankerite, dolomite, pyrrhotite, galena and francolite. The peridotites, pyroxenites, ijolite-urtites, urtites, carbonated rocks and carbonatites are cut by dykes of nepheline syenite, augitite and amphibole monchiquite.