Yarrabubba, Western Australia: a Large, Deeply Eroded, Ancient Impact Structure in the Yilgarn Craton

Introduction: Shock-metamorphic effects including shatter cones, planar deformation features (PDFs) in quartz grains, and both E-and S-type pseu-dotachylites were discovered in deeply eroded Ar-chean granites near the Yarrabubba homestead in Western Australia (119°50'E, 27°10'S). Preliminary estimates indicate that the Yarrabubba impact structure is of a Paleoproterozoic age and at least 30 km in diameter. Geological Observations: No circular crater-forms can be discerned either from the ground, on aerial photographs or from satellite images. Considering the lack of topography, the Yarrabubba Granite is remarkably well exposed in the central region of the structure. The Yarrabubba Granite is a pale pink, medium to coarse-grained monzogranite, consisting of quartz, albitic plagioclase and microcline, with subordinate muscovite and biotite. Aside from samples near the contact with the Barlangi Granophyre (impact melt), in which the quartz has been recrystalized, all of the samples of Yarrabubba Granite that we studied contained PDFs and planar fractures (PFs) in quartz, testifying to its shocked nature. Biotite is commonly altered to a mixture of chlorite and iron oxides, and it also appears that some muscovite may have formed from the alteration (impact-metasomatism?) of biotite. Shatter cones are also present in finer grained varieties of the Yarrabubba Granite to at least 4 km north of Barlangi Rock (the center of the structure). The Yarrabubba Granite contains many melt rocks along faults, ranging from a few millimeters to over a meter thick. The thicker bodies are generally a flinty green aphanitic felsic rock with inclusions of granulated quartz. The fault melts often harbor mylonites and are baked when in contact with the Barlangi Granophyre; and thus, they appear not to be impact melt dikes, but rather, E-type pseudotachylites. In the northern Yarrabubba Granite outcrops, sub-millimeter S-type pseudotachylite veins occur that display the typical " cobweb " pattern [1]. These can be observed to at least 5 km from the center, but their distribution appears to be sporadic. The Barlangi Granophyre was originally mapped at Barlangi Rock and an outcrop to the southwest, where it was described as soda-rhyolite hypabyssal granophyre, with bladed silica rather than cuneiform intergrowths [2]. Our field work revealed that there is substantially more of the Barlangi Granophyre in the area, as it stretches for nearly 10 km 2 in map view. Where contacts are exposed in the north and the west, the granophyre has an intrusive, shallow, sill-like rela