Magnesian skarns from the sites of layer-by-layer injections of basic magma into the evaporites of the platform's sedimentary cover

Contact-reaction magnesian skarns formed at the magmatic and postmagmatic s tages have been found, in addition to the earlier revealed autoreaction ska rns, in the Angara-Ilim volcanotectonic structures in the intracontinental paleorift zones of the Siberian Platform. The spinel-forsterite magnesian s karns of the magmatic stage are confined to the overdome parts of doleritic bodies and are the result of the interaction of massive dolomites with flu ids released from liquid magma. The magnesian skarns of the postmagmatic st age are localized in the marginal parts and on the front (outwedging) of do leritic sills, apophyses, and the branches of intrusive bodies in a carbona te-evaporite series. The skarns resemble gravel conglomerates with carbonat e cement and have a banded structure. It has been established that the foun d fragments are globules of disintegrated doleritic porphyrites, completely or partially substituted by zonal magnesian skarns. The cryptocrystalline mesostasis and phenocrysts of olivine, plagioclase, and pyroxene had underg one dispersion and substitution. The unaltered core is in contact with a fa ssaite zone, which passes first Into a spinel-fassaite zone and then into a forsterite-magnetite zone and calciphyre. In paleoisland-arc and margin-co ntinental regions, these skarns have no analogs in texture-structural pecul iarities and the mechanism of deposit formation. Their origin was caused by the speciality of intrusion and solidification of basic magma in chemicall y active carbonate-evaporite series.