Chakhyrtoi

Hedenbergite-nepheline syenites cut Upper Proterozoic marbles and form two main bodies. The southern one is represented by intensively albitized nepheline syenites and the northern one comprises numerous, sub-vertical injections of nepheline syenite intercalated with xenolithic marble blocks. The composition of the nepheline syenites is 35% nepheline, 35% microcline perthite and 15% Na-hedenbergite; there is some secondary alkali amphibole, rare plates of lepidomelane, calcite and typically accessory apatite and titanomagnetite. At the contact of nepheline syenites and marbles there are distinct reaction zones which vary in width from several centimetres to metres. The succession of zones are: marble - pyroxene-bearing recrystallized marble - pyroxenite-ijolite - ijolite with microcline - nepheline syenite. Chemical compositions of rocks, including rocks from the reaction zones, are available in Kononova (1964).