Spatial distribution of zircon with shock microtwins in pseudotachylite-bearing granite gneisses, Vredefort impact structure, South Africa

Autor(en)

Elizaveta Kovaleva, Gerlinde Habler

Abstrakt

Mechanical microtwins in zircon is a characteristic shock feature and a potential tool for estimating shock pressure and/or shear stress. However, so far, zircon with microtwins has rarely been described in situ. The present study of pseudotachylite-bearing granitic gneisses from the Vredefort impact structure focuses on the spatial distribution of zircon with microtwins and other deformation features.

Zircon hosted by granitic gneiss does not reveal extensive shock deformation. With decreasing distance to pseudotachylite, zircon progressively develops (sub)planar fractures, planar deformation bands, low-angle boundaries, rarely thin {112} microtwin lamellae and, closest to the vein, it is fragmented. In contrast, zircon hosted by pseudotachylites contains thick and closely-spaced microtwin lamellae, which form up to three sets within individual grains. Microtwin lamellae are associated with planar {112} low-angle boundaries, and with overprinting crystal-plastic and brittle deformation. We suggest that planar {112} low-angle boundaries in zircon are indicative of shock deformation; and that microtwins in zircon within pseudotachylite veins are an indicator of the impact origin for the latter, as opposed to endogenic, seismically-induced friction melts. Locally-elevated shock pressure and shear stress induce the formation of pseudotachylites in the target rock as well as shock microtwins in zircon.

Organisation(en)

Department für Lithosphärenforschung

Externe Organisation(en)

University of the Free State

Journal

Journal of Structural Geology

Band

129

Anzahl der Seiten

16

ISSN

0191-8141

DOI

https://doi.org/10.1016/j.jsg.2019.103890

Publikationsdatum

12-2019

Peer-reviewed

Ja

ÖFOS 2012

105116 Mineralogie, 105101 Allgemeine Geologie, 105120 Petrologie

Schlagwörter

ASJC Scopus Sachgebiete

Geology

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/33c99615-f18d-4824-a549-1785a9c7c9a6