P_{CO_2 } -T-Evolution and origin of zoning in melilite during the regressive stage of contact metamorphism in carbonate-bearing rocks

The article describes the thermal metamorphism of siliceous carbonate rocks near the dolerite intrusive body in Eastern Siberia. The mineral associations at the immediate contact with dolerite are the following: wollastonite+rankinite, rankinite+spurrite (+melilite?), spurrite+melilite+merwinite+calcite and merwinite+monticellite+melilite+calcite. The melilite in these associations is usually unzoned; its composition being essentially gehlenitic. During the regressive stage of contact metamorphism new akermanite-rich melilite and calcite were formed by replacement of merwinite and earlier gehlenitic melilite through participation of CO₂. The newly forming melilite grains have sharp compositional zoning. The origin of zoning was connected with the fall of temperature and decrease of the mole fraction of CO₂ in the fluid equilibrated with the minerals.