Petropavlovskii

This stock of subalkaline gabbro is injected by intrusions of theralite, feldspathic ijolite-urtite and foyaite. The theralites are porphyritic rocks with nepheline phenocrysts. The groundmass is composed of nepheline (15%), plagioclase (41%), clinopyroxene (7%), hornblende and biotite; apatite, titanite and magnetite occur as accessory phases. The feldspathic ijolites, apart from nepheline (42-60%) and clinopyroxene (30%), contain a little microcline and andesine. In nepheline syenites the proportions of the minerals vary greatly, notably nepheline (22-42%), feldspar (46-52%) and aegirine-augite (2-18%); hornblende and biotite are also present and the accessory minerals are titanite, apatite and Fe-Ti oxide minerals. In the central and northern parts of the complex small bodies and veins of carbonatite are present which, as a rule, have sharp contacts. In some places the adjacent rocks are altered with the development of garnet, idocrase, apatite, magnetite and calcite. The carbonatite mineralogy is 20-70% calcite, 1-5% clinopyroxene, 7-60% monticellite, 5-20% apatite, 1-10% magnetite, 0-50% phlogopite. Homogenization temperatures have been determined for micro-inclusions in carbonatite minerals and gave up to 890°C for monticellite, 890-700°C for clinopyroxene, 700°C for phlogopite, 650-550°C for apatite and 590-400°C for calcite.