Tuchinskii

The Tuchinskii massif, of 2.3 x 0.7 km, lies between porphyritic and medium-grained leucocratic granites to the northwest and marbles with hornfelsed sandstones of Lower Cambrian age to the southeast. About 70% of the massif is composed of nepheline syenite and hastingsite and pyroxene alkaline syenites. The alkaline syenites are developed principally along the periphery of the massif, forming an external zone up to 300 m wide. Urtites (monmouthites) are found among the nepheline syenites in the form of small lens-like bodies up to 160 x 80 m. A characteristic feature of the intrusion is the interleaving of alkaline and nepheline syenites. Hastingsite syenites are present in the zone of nepheline syenites, where they form sheet-like lenses 60-70 m thick. Nepheline syenites, also in the form of lenses, occur in the peralkaline syenite of the peripheral zones of the massif, while small bodies of juvite, monmouthite and urtite up to 100 x 200 m are also found. The massive abounds in hornstone xenoliths. Moreover, from the preserved layering in the abundant xenoliths, it is possible to determine their original stratigraphic positions. In the vicinity of the main Tuch massif there are two smaller intrusions, the northern one of which is composed of massive nepheline and alkaline syenites, amongst which numerous limestone xenoliths have been preserved.