Evidence for a shock-metamorphic breccia within a buried impact crater, Lake Raeside, Yilgarn Craton, Western Australia

ABSTRACT

Mineral exploration drilling 60 km west of Leonora in 2008 intersected >95 m of poorly consolidated granitoid-dominated breccia at the base of a Cenozoic paleochannel beneath Lake Raeside. The breccia, initially interpreted as a kimberlite, is composed of poorly consolidated fragments of granitic gneiss, felsite and metamorphosed mafic rock within a matrix of fine to medium-grained breccia. Microscopic examination revealed quartz grains displaying well-developed planar deformation features (PDFs) dominated by the ωʹ {1013} planar set, diaplectic silica glass and diaplectic plagioclase glass. These features constitute the diagnostic hallmarks of shock metamorphism owing to high-velocity impact of a large meteorite or asteroid. The PDFs in quartz grains of the breccia are distinctly different from metamorphic deformation lamellae produced tectonically or in diatremes. Airborne total magnetic intensity data suggest an outline of an 11 km-diameter crater, consistent with the significant thickness of the shock-metamorphosed breccia at >95 m, suggestive of the existence of a large impact structure.

KEYWORDS:

• Impact breccia
• silica glass
• plagioclase glass
• planar deformation features
• Lake Raeside
• Yilgarn Craton

Editorial handling:

• Graziella Caprarelli

Acknowledgements

Core used in this study was obtained from the Kalgoorlie Core Library of the GSWA. Wayne Hitchcock is thanked for his logistic support prior to and during inspection of the core. John Vickers prepared sections for petrography and analysis, and assisted with core photography. The SEM–EDS analyses were performed by David DiBugnara. We are grateful to Tara Webster and Bill Pappas for whole-rock analyses, Stephen Wyche for advice and Harry Kokonnen for assistance in setting up the Universal stage. Arthur Hickman publishes with permission of the Director of the GSWA. We thank Graziella Caprarelli, Bob Pidgeon and anonymous referees for their review comments and suggestions.

Editorial handling: Graziella Caprarelli

Disclosure statement

No potential conflict of interest was reported by the authors.