Borgoiskoe Volcanic Field

The Borgoiskoe volcanic field consists of thick, more than 5 km, accumulations of effusive and sedimentary rocks into which laccoliths and sills have been emplaced. It is partly covered by recent sediments. Within the sedimentary rocks, which consist of conglomerates, siltstones, sandstones and limestone, occur basic tuffs and moderately alkaline basalt lavas which are of hawaiite type. The latter crop out extensively over the basin, where they form a cover 120 m thick. The lavas comprise dozens of flows of thicknesses up to about 10 m. The flows in the central part of the depression are horizontal. Approximately simultaneously with the eruption of the lavas laccoliths and sills of crinanite and glenmuirite were intruded. The laccoliths are clearly reflected in the topography, forming hills up to 100-150 m high. The crinanites are analcime dolerites containing olivine (4-11%), plagioclase (25-41% and An53-62), titanaugite (10-20%), analcime (up to 20%), alkali feldspar (up to 20%) and biotite (4-5%). In glenmuirites the biotite content rises sharply, up to 19%, and the clinopyroxene content falls to a maximum of 2%. The usual accessories in the crinanites and glenmuirites are apatite and Fe-Ti oxides. The compositions of rocks and minerals are given by Belov (1963).