Khodzhaachkan

The 50 km2 intrusion lies in the core of a very large east-west-trending anticline, has vertical contacts and sends numerous apophyses into the surrounding schists. Syenite gneisses, so called because of their gneissose structure, monzonites and alkaline syenites as well as melanite-nepheline syenite, shonkinite, cancrinite syenite and amphibole-nepheline syenite grade into each other and comprise the first intrusive phase. Aegirine-augite-nepheline syenites occur in the eastern part of the intrusion and melanite syenites and shonkinites form schlieren some metres in length. Quartz syenites, of which there are biotite-, biotite-amphibole- and pyroxene-bearing varieties, occur as dyke-shaped bodies in the southwestern part of the intrusion and cut across the nepheline syenites and country rock schists. Fine-grained granites form sills and dyke-shaped bodies up to 3 m thick in the central part of the complex. Among the granites there are biotite- and tourmaline-bearing varieties and they are cut by dykes of syenite aplite and albitite, and pegmatites and veinlets of calcite and wollastonite. In the marginal contact zones the schists are intensively altered, including albitised, and in places changed into syenite gneisses; limestones are changed to wollastonite skarn.