Yarrabubba - A Large, Deeply Eroded Impact Structure in the Yilgarn Craton, Western Australia

Abstract

Yarrabubba is a newly discovered impact structure situated within the complex granite^greenstone terrain of the Yilgarn Craton. Shock-metamorphic effects including shatter cones, planar deformation features in quartz grains, and pseudotachylites, were found in deeply eroded Archean granites near Yarrabubba station, southeast of Meekatharra, Western Australia. Aeromagnetic images reveal arcuate demagnetization features at diameters between 11 and 25 km, which roughly correspond to the outcropping of the Yarrabubba Granite, and are centered on a magnetic-high halo around the Barlangi Granophyre (119‡50PE, 27‡10PS). Rapid-quench textures in the Barlangi Granophyre and its inter-fingering relationships with pseudotachylites suggest that it is an impact melt that was injected into the Yarrabubba Granite and spread along fault discontinuities. Both the potassic Yarrabubba Granite and the felsic Barlangi Granophyre are atypical in the northern Yilgarn, as are the abundant fracturing and frictional melting within the local granitoids. These anomalous geological features associated with shock-metamorphic effects are indicative of a hypervelocity impact origin. The age of the impact is uncertain, but geological and geophysical relationships suggest that the Yarrabubba structure was formed during the early Proterozoic.