DISTRIBUTION OF LANTHANIDES IN AMPHIBOLE AND PLAGIOCLASE FROM PLAGIOCLASITES OF THE RAY-IZ MASSIF (POLAR URALS)

Abstract

Relevance. This work shows the results of mineralogical, petrographic, and geochemical studies of plagioclasites localized among chromite-bearing ultramafic rocks of the Ray-Iz massif (Polar Urals). The spatial confinement of plagioclasites to the Tsentralnoye chrome ore deposit (corundum-bearing plagioclasites) and the Yugo-Zapadnoye IV ore occurrence (plagioclasites without corundum) determine the need for a detailed study of the rocks of the vein series of chromite-bearing sections. The purpose of the work. The study of mineralogical and petrographic characteristics, as well as the geochemistry of lanthanides in plagioclase and amphibole from plagioclasites of the Ray-Iz massif (Polar Urals). Results. In view of the results of microprobe analysis, it was found that the compositions of amphibole in plagioclasites vary considerably in accordance with the modern classification of calcium amphiboles, pargasite, edenite, magnesiohornblende and actinolite. Two types of zoning are determined in amphibole. Amphibole grains with zoning of the first type are characterized by an increase in the contents of Al2O3, TiO2, FeO, Na2O from the center toward the periphery of the grains; when zoning of the second type, on the contrary, there is a decrease in these components. The chemical composition of plagioclase corresponds to oligoclase An10-30.1. The decrease in the content of the anorthite molecule from the center to the edge of the grain was noted. The study of nature of the distribution of REE in rock-forming minerals made it possible to determine that amphibole is the main mineral concentrator of lanthanides in plagioclasites: the total REE content in the mineral is 250-450 ppm, while 16-18 ppm of lanthanides are concentrated in the plagioclase. It was established that the reason for the existence of two types of distribution spectra of lanthanides identified earlier is the ratio of the total amount of amphibole and plagioclase in the rock. The distribution of rare-earth elements in calcic amphiboles from plagioclasites is compared with the distribution of REEs in the amphiboles of the Nyurundukansky mafic complex in the North-Western Baikal region, the studies of which were carried out by S. G. Skublov. Using the amphibole-plagioclase geothermometer by T. Holland, J. Blundy and geobarometer by M. W. Schmidt parameters of amphibole plagioclasites formation were determined. Conclusion. The nature of the distribution of lanthanides in the main rock-forming minerals plagioclase and amphibole is found. The comparison of parameters and compositional characteristics of amphiboles allowed us to conclude that there is a direct relationship between temperature, the sum of REE and TiO2 in the mineral.