The composition and structure of zircon from the harzburgite inclusion in granulites of the Pobuzhsky complex, Ukrainian Shield

S. B. Lobach-Zhuchenko, S. G. Skublov, Yu. S. Egorova, D. V. Prishchepenko, O. L. Galankina

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2018. V. 147. N 6. P. 22-40

https://doi.org/10.30695/zrmo/2018.1476.01

Full text is available on eLIBRARY.RU

Language: Russian 

Abstract

The multilateral investigation (electron microscopy, mass-spectrometry of secondary ions, Raman spectroscopy) has been carried out for zircon from the harzburgite inclusion in granulites of the Pobuzhsky complex in the Ukrainian shield. This zircon is divided, by a series of its features, into three groups. Zircon of the first group has the magmatic genesis, it has crystallized 2.8 Ga ago at the temperature about 650 °C from the melt with orthopyroxene composition. The second group zircon was hydrothermally altered about 2.75 Ga at the temperature near 750 °C. The third group zircon distinguishes by several geochemical characteristics (the higher Lu/Hf ratio, etc), it was formed before the hydrothermal process, but its composition has been altered by the fluid effect. Conditions of the magmatic zircon (1st group) crystallization were characterized by the higher fO2 value, in comparison with the later hydrothermal alteration. The hydrothermal process provided the distortion of the zircon crystal lattice, which is reflected in Raman spectra of the mineral, and formation of amorphous structure in the uranium-rich zircon of the 3rd group. Difference between zircons of 1st and 2nd groups, from one side, and the 3rd group zircon by contents of trace elements indicates for different initial melts of their origin (melt for the 3rd group zircon was more mafic). Obtained results support the notion about hybrid character of the melt and formation of the rock in two main stages — magmatic one and the fluid alteration, divided in time and with different oxidation-reduction conditions.

Key words: zircon, rare earth elements, Raman spectroscopy, harzburgites, Pobuzhsky complex, Ukrainian shield.