Surtekin

The Surtekin intrusion is situated on the southern slope of the Atbashi mountain ridge of the Tyan-Shan. It has an area of 20 km2 and lies within schists and limestones which are metamorphosed up to 400 m from the contact. The contacts dip steeply at 40-80° towards the centre. The intrusion was formed in four phases involving (1) monzonite, shonkinite and essexite, (2) peralkaline syenite, (3) nepheline syenite and (4) quartz syenite. Monzonite and essexite are concentrated in the southeastern contact zone. Nepheline syenite occupies 75% of the area of the intrusion with biotite-amphibole-bearing varieties predominant; many are trachytic in texture. Volumetrically of less importance are urtite and aegirine-nepheline syenite. Alkaline syenites are intimately connected with the nepheline syenites the intrusive contacts between them being generally gradational. The marginal parts of the nepheline syenites are strongly albitized such that sometimes they are altered into pure albitites. Quartz syenites (shonkinites), in the form of dyke-shaped and stock-like bodies, cut the nepheline syenites. Dykes of nepheline syenite pegmatite and tinguaite are concentrated in the marginal areas of the intrusion where they are sometimes albitized.