Melilitic and monticellite-bearing rocks of the Krestovskaya intrusion (northern Siberian Platform) and their genesis

The geologic structure of the Krestovskaya. intrusion, discovered in 1996, is discussed. Petrographic characteristics of melilitic and monticellite-be aring rocks, their mineralogy, and composition of minerals, including rare ones (kalsilite, combeite, rankinite, larnite, and djerfisherite), are repo rted. Studies of melt inclusions have shown that melilitic and monticellite -bearing rocks crystallized at the magmatic stage in the following sequence : perovskite (1250-1230 degreesC)--> melilite (1235-1160 degreesC)--> monti cellite (1090-990 degreesC). The melt during crystallization of perovskite was lowly siliceous (30-33 wt. % SiO2), depleted in Al2O3 (5-6 wt.%) and Mg O (6-8 wt.%), enriched in CaO (17-21 wt. %) and alkalies (totalling about 1 1 wt. % with predominance of K over Na), and contained 2-3% SO3 and, perhap s, some quantities of Cl and CO2, i.e., was alkaline-ultramafic, enriched i n Ca, and had K-Na or even Na-K type of alkalinity. During crystallization of perovskite, liquid silicate-salt immiscibility manifested itself. Crysta llization of melilite started from the same melt and proceeded under the co nditions of ongoing two-phase liquid immiscibility with a drastically chang ing oxygen fugacity. During crystallization and fractionation of melilite, the contents of Si and At in the melt strongly increased, the content of Fe sharply decreased, the amount of Na in the total of alkalies diminished an d that of K grew, and the quantities of Ca and Mg remained the same. Subseq uently the high content of K favored binding of Al-Si radicals into molecul es of phlogopite and kalsilite and crystallization of djerfisherite under r educing conditions. The extremely high activity of Ca favored crystallizati on of rather rare calcium silicates, larnite and rankinite, and binding of the rest amount of Na into combeite and pectolite. The silicate-salt melt, spatially separated as a result of liquid immiscibi lity, was compositionally close to highly siliceous carbonatitic alkaline m elt of Fort Portal in Uganda. It was responsible for autometasomatic transf ormation of olivine and pyroxene rocks, which resulted in monticellite and garnet at the magmatic stage. Among volatiles, CO2 and H2O were predominant in silicate-salt melts, while CO was present in minor amount.