GEOCHEMISTRY OF MELT INCLUSIONS IN ZIRCON FROM AZOV Zr-REE DEPOSIT (UKRAINIAN SHIELD)

Abstract

A local (ion microprobe) study of the geochemistry of primary melt inclusions in zircon from the Azov Zr-REE deposit was carried out for the first time. Among them there are recrystallized inclusions that are predominant and cracked inclusions of glass. The latter were formed under the influence on the rock by the flow of CO2-fluids that characterizes by high RT-parameters. The melt inclusions in zircon can be classified into two groups relative to time of their capture by the crystal, that are different in content of the major, trace and rare earth elements. The early inclusions are acid and late inclusions are basic in composition. Early inclusions of the acid composition contain much less Fe, Ca, REE, Y, Zr, P, Th by compared with late inclusions. The minimal content of trace elements is established for early inclusions of natural quenching, which have lost their tightness under the influence of flows of high-temperature CO2-fluids. The composition of the recrystallized inclusions corresponds to the composition of potassium feldspar with minor admixtures of albite and femic mineral. Enrichment of late melt inclusions with fluorine, their oversaturation with REE, Zr and Y reflects the processes of liquation, which led to the separation of the syenite melt into two immiscible liquids-rich in iron, incompatible elements and fluorine, and rich in silica. The zircon/melt distribution coefficients Kd for rare-earth elements are determined for both groups of melt inclusions. The Kd values for zircon from the Azov deposit are significantly (ten to hundred times) lower than published data for zircon from magmatic acid rocks. It was revealed that at the last stages of formation of the Azov deposit, when the syenite melt being oversaturated with incompatible elements (REE, Zr, Y), zircon follows the composition of the melt-REE spectra of zircon and syngenetic primary glass inclusion are similar and differs only in their content.