Ryabinovaya Pipe

The Ryabinovaya pipe cuts through a dense swarm of dykes the overall length of which is about 1 km. Individual dykes have a complex zonal structure with the marginal parts composed of a magmatic breccia which consists of a mixture of angular fragments of coarse-grained alkaline syenite in a fine-grained cement of olivine-diopside lamproite. The relative proportion of cement and syenite debris is highly variable. The width of the marginal contamination zone is usually 2-3 cm but varies depending on the thickness of the dyke. A sharp change in the appearance and composition of the rocks takes place in the central section of dykes which are more than 10 m thick. Xenoliths of syenite and lamproite olivine phenocrysts are now absent and the texture becomes medium-grained and slightly porphyrytic. Compositionally the rocks of this central zone correspond to phlogopite shonkinite porphyries and highly magnesian phlogopite minettes. The Ryabinovaya pipe proper is composed of autolith breccias with a magmatic cement and corresponding in composition with olivine- diopside lamproites (mg 73-75). The pipe rocks are fine-grained and porphyritic with large, first generation phenocrysts of olivine and smaller second generation olivine phenocrysts in the matrix. The olivine has a composition of Fo92-94, with a NiO content of 0.5%, and comprises 15-25% of the rock. Chromite inclusions with Cr2O3 values of 61% are present in the olivine. The fine-grained matrix consists of microlites of diopside, patches of feldspar, in which it is sometimes possible to recognise relict aggregates of pseudoleucite, and tiny crystals of dark-brown mica (1-3%).