Textural Identification of Polycrystalline Magmatic, Tectonically-Deformed, and Shock-Related Zircon Aggregates

Abstract

Zircon with polycrystalline or polygranular appearance is either produced in the magmatic environment through crystallization, or due to deformation in metamorphic settings (including regional metamorphism and ductile shear zones), or as a result of shock-induced recrystallization. All three types can be easily confused and potentially lead to incorrect interpretations, especially if the crystallographic orientation analyses of zircon are not conducted. It is particularly important to establish the difference between tectonically-deformed polygranular zircon and shock-induced polygranular zircon because the latter serves as an indicator of shock event and is often used for dating asteroid impacts. In this paper, a series of polycrystalline zircon grains from ductile shear zones and metamorphic rocks are analyzed using a combination of techniques (BSE, CL, orientation contrast, EBSD, and microprobe mapping), and their properties are compared to reported polycrystalline zircons from magmatic and impact settings. This work shows how appearance, crystallographic orientation, and CL signature of “granules” differ between the different types of deformed zircon.

Shock deformation in zircon grains from the Mien impact structure, Sweden

“…2015; Montalvo et al. 2019; Kovaleva 2020), EBSD analysis of granular grains can reveal whether or not the granules formed as a response to a back‐transformation from reidite to zircon (Erickson et al. 2017; Timms et al.…”

“…2017; Timms et al. 2017) or if the texture formed from tectonic deformation (e.g., Kovaleva 2020), which means that it is possible to detect prior occurrences of reidite. The general criteria to establish the former presence of reidite are that neoblasts occur in two or more orthogonal 90° sets, forming high misorientation angles and with coincidence between poles for (001) and {110} (Cavosie et al.…”