TRACE ELEMENT GEOCHEMISTRY OF ZIRCONS FROM THE VELYKA VYSKA SYENITE MASSIF, UKRAINIAN SHIELD
Authors: E.V. Levashova, H.O. Kulchytska, S.G. Skublov, I.M. Herasymets, S.G. Kryvdik, O.L. Galankina, M.E. Mamykina, D.S. Levashov
Number of views: 25
Zircon crystals from crushed syenite samples of individual intrusive bodies in the Velyka Vyska Massif were studied using the
SEM-EDS (BSE images and chemistry) and SIMS methods. The results obtained were compared with early published data for zircon from the Azov and Yastrubets syenite massifs, Ukrainian Shield that host Zr-REE mineralization published earlier. Most of the zircon crystals analyzed are chemically inhomogeneous, but either azonal or show poorly-defined regular zoning. A darker contrasting marginal zone is visible only in scarce crystals. The presence of solid silicate, sulphide, phosphate and carbonate inclusions support the assumption that the zircon crystallization medium was heterogeneous. The percentages of rare-earth, radioactive and other trace elements are below 1.6 wt.% and their mean value of 0.4% is smaller than in zircon from other massifs. The percentage of U + Th (average 0.07 wt.%) is too small for radioactive elements to be responsible for metamictization. Similar to zircon from other massifs, Y, which predominates over REE at a ratio of 1.3 : 1, plays the leading role. As the total amounts of Y and REE increase, REE composition becomes rich in LREE. Zircon from Velyka Vyska syenites has the highest HREE-content. The LREE/HREE mean ratio for this zircon is 0.06, while for Azov and Yastrubets zircons the ratio is 0.18 and 0.26, respectively. There are two types of REE distribution pattern for zircon crystals. Those of type I, characteristic of most crystals, are differentiated.
They are identical to the pattern of central areas of crystals from other massifs and are typical of zircon which has crystallized from melt. Flattened spectra of type II are characteristic of crystal margins with a well-defined zonal structure. They are identical to those for the marginal parts of zircon crystals from other massifs and are characteristic of zircon crystallization in which a fluid phase was involved. The estimated zircon crystallization temperature calculated by Тi-thermometry is 600-1000 °С, commonly 700—800 °С, which is slightly lower than the values expected.