Davidki

The Davidki intrusion is located on the northeastern outskirts of the Korosten pluton in the area of intersection between the east-west-trending Ovruch basin and the north-south-trending Vilcha basin. The massif has a rounded outline, a concentric zonal structure and occupies an area of about 30 km2. The periphery is composed of gabbroic rocks of which a narrow strip, 150 to 300 m wide, seems to encircle the massif completely; there is a fine-grained chilled zone. Syenites occupy almost the whole central area of the massif and at the present level of erosion predominate. The thickness of the syenites in the centre, from drilling data, is over 360 m, decreasing to 30 m towards the periphery. Plagioclasites probably form a continuous ring around the syenites of the central part. The dips of the contacts of the intrusion are inwards, generally at angles of 40-50°. The Davidki intrusion is considered to be layered (Krivdik et al., 1986) with syenites and plagioclasites at the centre forming the main layered series, while the gabbroic rocks of the periphery form a marginal layered group. The marginal layered group shows a distinct rhythmic layering which manifests itself in the alternation of leucocratic and melanocratic gabbros, troctolites and gabbro-diabase. The drilling also indicates the presence of cryptic layering which is represented by an upward increase in the Fe:Mg ratio of titanaugite (from 40 to 45), and a change in the plagioclase from An45-50 to An28-30. In syenites the Fe:Mg ratio of clinopyroxene (titaniferous ferrohedenbergite and hedenbergite) increases upward in a borehole from 80 to 92; aegirine is sometimes present. Further, transverse-columnar rocks (crescumulates) similar to those of the Skaergaard intrusion have been found in gabbroic rocks and syenites (Krivdik et al., 1986).