Sandyk (Chechekty)

The Sandyk massif is situated in the D'umbgolsky Mountain Range 90 km to the west of the Kyzyl-Ompul intrusion (No. 4) in a northwesterly-trending fault zone. It extends over about 80 km2 but much of the northern part is obscured by recent deposits. The country rocks are Caledonian granitoids of the Susamyrskii batholith. The complex is composed of both subalkaline and alkaline rocks, the most extensive being rocks of the subalkaline series, which occupy 75% of the area, which are monzonites and mesocratic and leucocratic syenites. These are coarse-grained, porphyritic and sometimes trachytic textured rocks containing alkali feldspar, basic plagioclase, pyroxene and biotite. Through the series from monzonite to leucocratic syenite the plagioclase decreases from 40% to 5%, diopside-augite from 50% to 5% and biotite from 15% to 2-3% while the content of orthoclase increases from 30% to 90%. Some syenites contain 5-10% quartz and 2-10% hornblends; accessory minerals are titanite, zircon, magnetite and apatite. The rocks of the alkaline series, which are located mostly in the northern part of the complex in a centre known as Chechekty (Fig. 90), cut across the subalkaline rocks. They are coarse-grained, leucocratic nepheline syenites, composed mostly of orthoclase (60-80%) and nepheline (10-30%) with hornblende, augite and biotite; accessories include titanite, zircon, magnetite and thorianite. There is some pseudoleucite, which displays dactylotypic textures, in the nepheline syenites. The presence of up to 10% sodic plagioclase and the prevalence of ferrohastingsite over the other dark-coloured minerals is typical for the alkaline syenites.