Kosistekskii

The Kosistekskii complex is situated to the northeast of the town of Aktyubinsk. It is highly irregular in shape with a separate part of the complex lying several kilometres to the southeast; it has an area of about 60 km2. The marginal contacts have steep dips of 70-90° and it is probably a thick, sheet-like intrusion the upper parts of which have not been eroded extensively.Alkaline rocks occupy about two thirds of the area of the complex the principle part of which is composed of quartz, biotite, biotite-amphibole and pyroxene granosyenites and alkaline granites. Nepheline syenites, which are divided into fine-, medium- and coarse-grained varieties, contain sodalite, which has replaced nepheline, while the feldspars are extensively albitized.