Abstract

The influence of parental melts on the chemical composition of rock-forming minerals of alkaline lamprophyres (monchiquites) referred to as the volcanic series of porphyritic potassic alkaline-ultrabasic rocks was studied in the Tomtor alkaline-ultrabasic carbonatite massif (Yakutia) hosting a unique deposit of Sc–REE–Y–Nb ores. Previous fluid–melt inclusion study showed that these rocks formed from two mixing alkaline-basic melts of different alkalinity. A detailed study of the chemical composition of minerals revealed a repeated irregular zoning in most of phenocrysts, which reflects the influence of different parental magmas and their mixing. It was established that the cores of diopside phenocrysts (Di I) with inclusions of Na–Fe-rich silicate melts have a low Mg-number and low contents of Ti, Al, and Ca and high contents of Na and Mn. The intermediate zones of phenocrysts (Di II) containing inclusions of K–Mg-rich silicate melts show a high Mg-number and are rich in Ti and Al and poor in Mn and Na. Groundmass grains and rims (and, sometimes, intermediate zones) of diopside phenocrysts often have a mixed Di I–Di II composition with slightly elevated contents of Mg, Ti, and Al. Amphibole phenocrysts, like the diopside ones, have both zones with low Mg contents and high Na/K ratios and Mn contents and zones with high Mg contents, low Na/K ratios, and low Mn and elevated Ti contents. Phlogopites are also of two varieties: highly magnesian, with high contents of Si and K and low content of Mn, and lowly magnesian, with low contents of Si and K and high content of Mn. Ilmenite, titanomagnetite, and fine grains of femic minerals are mostly of mixed varying composition. The chemical composition of rock-forming minerals, especially zoned phenocrysts, evidences that they crystallized with the participation of two alkaline-basic melts: Na–Fe-rich silicate melt enriched in Mn and K–Mg-rich silicate melt enriched in Ti but depleted in Mn.

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