Kontozerskii (Kontozero)

Morphologically the Kontozerskii complex forms a caldera. It lies within the Lovozerskii graben, which is a part of the Kola aulacogen. The caldera has an area of about 55 km2 and, as was pointed out by Kirichenko (1970), is a unique structure without an analogue in the adjacent regions of Russia. The caldera is circumscribed by arcuate fractures along which are breccia zones. The thickness of the sedimentary and volcanogenic rocks preserved within the caldera is over 2 km (Pyatenko and Osokin, 1988) and there are two distinct series, although Pyatenko and Osokin (1988) distinguish three. The first, and lower, one, which can be correlated with the Devonian rock series of the Lovozero complex, is represented by picrites, alnoites, augitites, nephelinites and limburgites which are interlayered with tuff breccias, tuffs, conglomerates and shales. The second, and upper series, is made up of sheets of lava breccia, lavas and tuffs of melilitite, melanephelinite, melilitic picrites and carbonatites which are intercalated with tuffs, shales, tuffaceous sandstones and limestones. The thickness of individual sheets varies between 0.1 and 0.5 m up to 20 m. About 10% of the succession is composed of carbonatite tuffs and lavas which are intercalated with bedded carbonate tuff siltstones, limestones and dolomites. The picrite-alnoite series are porphyritic rocks with phenocrysts of olivine, pyroxene and biotite set in a groundmass of olivine, augite-diopside, biotite, melilite, titanomagnetite and perovskite. Limburgites have phenocrysts of augite and serpentinised olivine with a groundmass of augite, magnetite, perovskite, serpentine, chlorite and glass. Augite phenocrysts 3-10 mm in diameter occur in the augitites and are set amongst small grains of augite (0.1-1 mm), magnetite, biotite and glass, the last occupying 40-50% byvolume; rarely in the groundmass there is analcime and apatite. The nephelinites are composed of nepheline, aegirine-augite, melanite, biotite, an opaque phase and some carbonate. Melilite picrites and melilitites are strongly carbonated rocks containing phenocrysts of altered melilite and olivine in a groundmass of augite aggregates, melilite, apatite, magnetite and carbonated glass. The volcanic carbonatites are lava breccias, lavas and tuffs with calcite carbonatite predominating (Pyatenko and Saprykina, 1976 and 1981). They contain calcite phenocrysts together with olivine and biotite and fine-grained magnetite and ilmenite. Less common dolomite-calcite carbonatites contain a little albite, zeolite, chlorite and pyrrhotite. Carbonatites also occur in necks, which cut the stratified series, and are formed after picrite breccias, which they replace and cement. Pyatenko and Osokin (1988) give analyses of the minerals of the carbonatites, including rare elements, and chemical data, including rare earths, for numerous rocks. Intrusive rocks are subordinate to volcanogenic sedimentary formations. They extend along the western margin of the Kontozerskii depression, where they form an upland area of 4x0.8 km. The intrusive rocks comprise pyroxenites, melteigites, nepheline syenites and alkaline syenites. The pyroxenites consist of augite-diopside, biotite, titanomagnetite, perovskite and minor melanite, titanite and apatite. The principal minerals in the melteigites are nepheline and zoned pyroxene with some biotite, titanite, apatite, titanomagnetite and amphibole. The nepheline syenites comprise K-feldspar, nepheline, aegirine and sodalite with apatite, analcime, albite, biotite, titanite and alkali amphibole. Finally, the alkaline syenites contain both plagioclase (30-40%) and alkali feldspar (60-70%), chlorite, which has probably replaced aegirine, and accessory zircon. The volcanic and sedimentary rocks adjacent to the contact are fenitized.