Zircon and baddeleyite in impact melt rocks of the Boltysh structure

UDC 549.3 : 523.681.8
Gurov Ye.P., Permyakov V.V.
Institute of Geological Sciences of the NAS of Ukraine
55-b, O. Honchar Str., Kyiv, Ukraine, 01601
E-mail: yevgeniy.gurov@gmail.com; 0673866227@ukr.net
Zircon and baddeleyite in impact melt rocks of the Boltysh structure
Mineralogical Journal 2016, 38 (1): 12-20

Abstract: Zircon is the only accessory mineral in impact melt rocks of the Boltysh structure, which had been preserved after the complete melting of the granitic target rocks of the crater basement. Zircon contains some information about influence of two main processes during the impact. First: morphology and structure of its grains witness about their intense resorption by impact melt. Second: zircon grains preserve some information about their shock metamorphism before entering into the impact melt. Crystals of zircon preserve their initial shapes only within the endomorphic zone of the melt sheet up to 10—15 m thick, while within the whole inner zone they are intensively rescored forming ellipsoidal and skeletal grains. Shock metamorphic effects in zircon are presented by planar microstructures, origin of diaplectic glasses and shock melting. Planar microstructures in zircon were formed at the lowest shock pressures, these correspond to I—III stages of shock metamorphism of granitic rocks. Reidite, the dense phase of ZrSiO4, that is stable at temperatures up to 1200 °С, was not determined in impact melt rocks of the Boltysh structure, while it was preserved in impact glasses of some impact craters. Zircon grains with a block structure and gas bubbles are the probable products of reverse transition of reidite. Diaplectic zircon glass originated at IV stage of shock metamorphism and later it was recrystallized forming microaggregates of zircon. The grains of melted zircon glass with gas microvoids were formed at temperatures >1900 °С. Euhedral crystallites of baddeleyite in impact melt rocks of the Boltysh structure were formed by crystallization from impact melt.

Keywords: impact structure, zircon, baddeleyite, shock metamorphism.


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