stripes
The Srednetatarskii intrusion cuts limestones in which are beds of coal and clay. In plan it is approximately circular and stock-like and predominantly comprises foyaite and ijolite with an area of muscovite syenite in the north, which is unlikely to be consanguineous with the Srednetatarskii alkaline occurrence. Country rock xenoliths are widespread within the intrusion. The foyaites are leucocratic medium-grained nepheline syenites comprising nepheline (26-8%), microperthite and other feldspars (50-65%), aegirine (2-10%), titanite (0.4%), fluorite (0.1-0.6%) and lavenite (0.1-2.2%). In a narrow zone near the contact the proportion of dark coloured minerals increases up to 10-15%. Lying beneath the ijolites in the central part of the occurrence foyaites have been encountered at a depth of 50-200 m. The ijolites are likely to form a subhorizontal body overlying the foyaites. As well as the ijolites, urtites and melanocratic juvites are also present. Their mineral composition is variable. Feldspar-free varietes include nepheline (65-85%), aegirine (13-32%), titanite (2-3%) and lavenite (0.5%) and the juvites nepheline (10-20%), microcline and microperthite (up to 26%), aegirine (14-23), titanite (1-2%), fluorite (0.2-0.3%), apatite (0.1-0.2%) and lavenite (0.5%). Various types of pegmatite andpostmagmatically altered rocks, which have undergonemicroclinization, albitization or calcite-cancrinite hydrothermal alteration, are present. In the pegmatites astrophyllite, eudialyte, zircon and other rare minerals are found.
SVESHNIKOVA, E.V. SEMENOV, E.I. and HOMYKOV, A.P. 1976. The Transangarsky alkaline massif, its rocks and minerals. Nauka, Moscow, 80 pp.
YASHINA, R.M. 1984. Magmatic series of the alkaline intermediate (syenoids) rocks from the shield and the consolidated folded areas. In V.A. Kononova (ed) Magmatic rocks: Alkaline rocks 138-84. Nauka, Moscow.