The monzogranite of El Fereyid is one of the rare metal-rich granites, where zircon is one of the ore minerals. Thus, studying zircon here is of vital importance. Most of the studied zircon grains are metamict and thus the loss of radiogenic Pb is detected for them. Nevertheless, our study allowed us to obtain the U-Pb (SHRIMP-II) age of magmatic crystallization for the El Fereyid monzogranite: 626 ± 13 Ma. These data allowed the correction of earlier determined age (K-Ar system in biotite) for the El Fereyid massif.Zircon grains of El Fereyid monzogranite demonstrate heterogeneous structure in CL images, they are rich in rare earth elements (REE, concentration 3000-22500 ppm), trace elements (U: 2000-14800 ppm, Th: 300-2500 ppm), and has low Th/U ratios (average 0.18). Most zircon grains exhibit multiple internal oscillatory zoning in CL images, indicating a typical magmatic origin. The core of zircon grain has a dark tone on CL imaging and magmatic-type REE spectra. Zircon cores are enriched in REEs, U, Th, and Y, thus recording that the magma was rich in incompatible elements. Light CL rims of zircon grains are depleted, relative to the cores in most of the trace elements, except for P, Ca, and Ti. REE spectra in rims show similar patterns that are more often demonstrated by hydrothermal zircon grains. This explains their crystallization in the late-magmatic stage when magma was rich in fluids but depleted in most of the trace elements.
The article presents the results of studying the rocks of the pyroclastic facies of the Mriya lamproite pipe, located on the Priazovsky block of the Ukrainian shield. In them the rock's mineral composition includes a complex of exotic mineral particles formed under extreme reduction mantle conditions: silicate spherules, particles of native metals and intermetallic alloys, oxygen-free minerals such as diamond, qusongite (WC), and osbornite (TiN). The aim of the research is to establish the genesis of volcaniclastic rocks and to develop ideas of the highly deoxidized mantle mineral association (HRMMA), as well as to conduct an isotopic and geochemical study of zircon. As a result, groups of minerals from different sources are identified in the heavy fraction: HRMMA can be attributed to the juvenile magmatic component of volcaniclastic rocks; a group of minerals and xenoliths that can be interpreted as xenogenic random material associated with mantle nodules destruction (hornblendite, olivinite and dunite xenoliths), intrusive lamproites (tremolite-hornblende) and crystalline basement rocks (zircon, hornblende, epidote, and granitic xenoliths). The studied volcaniclastic rocks can be defined as intrusive pyroclastic facies (tuffisites) formed after the lamproites intrusion. Obviously, the HRMMA components formed under extreme reducing conditions at high temperatures, which are characteristic of the transition core-mantle zone. Thus, we believe that the formation of primary metal-silicate HRMMA melts is associated with the transition zone D".
The present study contains the detailed ion microprobe data on trace and rare earth elements distribution in the large zircon crystal about 10 × 6 mm in size with distinct growth and sector zonings from Ilmen Mountains feldspathic pegmatite. The zircon crystal morphology is a combination of a prism {110} and a dipyramid {111}. It is found out that the growth sector of the prism {110} generally contains higher concentrations of Th, U, REE, Y, and Nb and exhibits a more gently sloping HREE distribution pattern and a steeper LREE distribution pattern, in contrast to zircon from the growth sector of the dipyramid {111} development. Such a sector zoning pattern was formed at a late stage in crystal growth, when the prism {110} began to prevail over the dipyramid {111}. The zircon studied displays the growth zoning formed of alternating bands in back-scattered electron (BSE) image: wide dark and thin light bands. The last ones contain elevated Th, U, REE, Y, Nb, and Ti concentrations, Th/U ratio and Ce/Ce*. This growth zoning is most probably due to simultaneous crystallization of other minerals that concentrate trace elements, e.g., apatite and monazite, and the lack of equilibrium between zircon and fluid (melt).
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