Vishnevogorskii

The Vishnevogorskii intrusion forms the northern part of the Ilmenogorskii-Vishnevogorskii complex. This massif is about 25 km long and up to 4 km wide and is located in the axial part of the Urals anticlinal structure. The northern part of the massif is wider and has the form of a steeply dipping dome which extends under metamorphic rocks of Riphean age. The northern part the main intrusive body is accompanied by satellite intrusions of nepheline syenite which have thicknesses up to several 100s of metres. The intrusion has contacts which are conformable with the enclosing rocks. In the southern part the complex is represented by veins and dykes of miaskitic nepheline syenite. Generally the southern part of Vishnevogorskii closely resembles the northern part of the Ilmenogorskii complex (No. 6 and Fig. 59). The Vishnevogorskii complex is composed mainly of miaskitic nepheline syenites with biotite, but there are nepheline syenites with amphibole and intermediate varieties with biotite and amphibole. Small bodies of biotite-bearing alkaline syenites are also found. The alkaline rocks of the complex have gneiss-like structures and textures with a clear preferred orientation of the feldspar. The nepheline syenites are frequently greatly albitized and partly replaced by calcite, especially in the central part of the massif, and also in the satellite intrusion at the northern end. In some places stockworks of calcite carbonatites occur. Numerous alkaline pegmatites, which are predominantly developed within fenites and close to the outer contacts of the intrusion, are a characteristic feature of the Vishnevogorskii intrusion. Miaskitic nepheline syenites occupy about half of the area of the intrusion and vary from fine- to coarse-grained rocks with massive or banded structures which are reflected in the preferred orientation of biotite and other minerals. The main rock-forming minerals of the miaskites are nepheline, oligoclase, orthoclase, lepidomelane and magnetite. Secondary and accessory minerals include microcline, albite, biotite, pyrochlore, zircon, ilmenite, apatite, titanite, calcite, cancrinite, zeolites and sulphides. A highly variable mineralogy is typical of rocks near to the contact zone. Alkaline pegmatites are divided into two groups. Those of the first group are tabular in form, have thicknesses of up to 5-7 m and can be traced for distances of 10s or 100s of metres. They are generally located in the outer parts of the complex close to the contact and are composed of 95-98% microcline, albite, nepheline, biotite and natrolite. They also contain cancrinite, sodalite, analcime, calcite, magnetite, ilmenite, apatite, zircon and pyrochlore. The second group of pegmatites, which are situated in the central parts of the complex and cut across the gneissosity of the nepheline syenites, comprises microcline, nepheline, biotite, albite, natrolite, cancrinite and aegirine-augite which form 97-98% by volume. In the axial parts of the pegmatites nepheline predominates over microcline, while in the marginal parts microcline is predominant. Accessories include minerals of uranium, thorium and rare-earths, as well as vishnevite, for which this is the type locality, magnetite and calcite. Two stages of carbonatite emplacement have been identified. Early carbonatites, which constitute the main phase, are represented by calcite and silico-calcite types. They lie both within the complex and in the fenite aureole. Sometimes the bodies of carbonatite are concentrated to form complex zones 100-150 m across and stretching for many kilometres. In plan such zones consist of chains of flattened lens-like bodies which follow each other in an en echelon pattern. Albite-biotite-calcite and biotite-calcite varieties with apatite (up to 20%), ilmenite, titanite and zircon are the most abundant varieties. The veins which constitute the later carbonatite stage are narrow, up to 10 cm wide, and have variable compositions depending on the composition of the enclosing rock. For instance, in miaskites these veins contain calcite, apatite, ilmenite, orthite, betafite and thorite, whereas in fenites the veins consist of ankerite, calcite, aegirine, rutile, brookite and sphalerite. Ronenson (1966) suggested that the nepheline syenites are of anatectic origin being the final stage of a metasomatic process whereby the enclosing rocks were transformed by alkaline fluids. Rb-Sr and U-Pb isochron studies (Kramm et al., 1983; Chernyshev et al., 1987) have, however, apparently established the mantle origin of the nepheline syenites of the Vishnevogorskii massif (Rb-Sr ratio 0.70304±0.00060) and that there were two stages in the generation of the nepheline syenites. Calcite from carbonatites and alkaline rocks gave d18Oo/oo values of +6.4 to +7.5, with some values up to +8.0. Silicates from miaskitic nepheline syenites gave d18Oo/oo values of +5.4 to +5.5. Calcite from carbonatite gave dCo/oo of -7.1 to -7.5 and sulphide -1.6 to -2.9 (Dontsova et al., 1977; Kononova et al., 1979; Grinenko et al., 1970).