Earth and Planetary Science

Shock‑Related Microstructural Deformation in Zircon from the Colônia Impact Crater, Brazil

Victor Velazquez Fernandez

São Paulo University

Jose Maria Azevedo Sobrinho

Environmental Research Institute, Secretariat for the Environment, Infrastructure and Logistics

Rodrigo Ferreira Lucena

School of Arts, Sciences, and Humanities, University of São Paulo

Alethéa Martins Sallun

Environmental Research Institute, Secretariat for the Environment, Infrastructure and Logistics

William Sallun Filho

wsallun@gmail.com

DOI: https://doi.org/10.36956/eps.v5i1.3117

Received: 31 January 2026; Published: 26 March 2026

Abstract

Microdeformation features in zircon grains from borehole samples of the Colônia impact crater were characterized by transmitted-light petrography. High-resolution polarized-light images of 40 zircon grains (>50 µm), selected from 200 polished thin sections, were used to determine crystallographic orientations, classify textural domains, and characterize stress-induced microstrain features. Zircon morphologies comprise 67% euhedral, 30% subhedral or fragmented, and 3% anhedral or rounded crystals, with sizes ranging from 30 to 150 µm. Euhedral prismatic crystals, elongated parallel to the c-axis, display aspect ratios of 3:1–4:1; rounded grains yield lower ratios approaching 2:1. Microstructural analysis of six representative grains—three euhedral and three subhedral—reveals five distinct deformation types: (i) recrystallization, (ii) crystallographic misorientation, (iii) growth-zone disorder, (iv) planar deformation features (PDFs), and (v) granular textures. Euhedral grains with metamorphic relict cores predominantly exhibit recrystallization and crystallographic misorientation, consistent with intense thermomechanical overprinting. Subhedral crystals of igneous origin, by contrast, are dominated by granular textures and PDFs, recording dynamic recrystallization and shock metamorphism under high-strain conditions. The presence of PDFs unequivocally establishes a shock origin; the remaining microstructures record progressive deformation across a range of pressure–temperature conditions. These microdeformation patterns reflect anisotropic strain imposed by impact-generated shock waves, which preferentially affected the mechanically weaker prismatic faces {100} and {110}. Taken together, the results confirm the reliability of zircon as a recorder of extreme tectonothermal events and provide crystallographic constraints for post-impact structural reconstructions.

Keywords: Colônia Impact Crater, Zircon Crystal, Microdeformation Features, Shock Metamorphism

References