A Petrographic, Chemical and Paleomagnetic Study of the Significance of Pseudotachylites Associated with the Sudbury Structure

Abstract

<p> Pseudotachylites within the Levack Gneisses of the North Range Sudbury Structure were studied, with an emphasison petrography, major oxide chemistry, and paleomagnetism. </p> <p> The pseudotachylites are present as dark greyish green veins and larger scale breccia zones. The matrix is glassy and aphanitic and the fragments, mostly quartz and feldspar are subangular to subrounded. The larger fragments and the wall rock contain kink bands in biotites and planar features in feldspars and quartz. The planar features are defined by rows of parallel inclusions and are diagnostic of shock metamorphism when parallel to specific crystallographic orientations of quartz. The major oxide chemistry shows the pseudotachylites are enriched in total iron, magnesia and lime. This corresponds to other impact-generated pseudotachylite chemistries. Thus, these rocks are not a product of pure wall rock and either the mafics were selectively melted out or added from an external source. </p> <p> Paleomagnetic analysis confirms the age of the pseudotachylite is approximately the same as the North Range of the Sudbury Structure, the least deformed component. Thus whatever the event was it also formed the pseudotachylite. The fact that the pseudotachylite contains shock metamorphic features, supports that the event was likely an impact, as of yet the only known process capable of producing the required pressures, temperatures and strain rates. </p>