A Ti-, Zr-, Th-, and U-Rich Hydrothermal Assemblage Associated with the Precambrian Yarrabubba Impact Structure (Western Australia).

The deeply eroded Yarrabubba impact structure in the Archean (~3.0–2.65 Ga) granite-greenstone terrain of the north-central Yilgarn Craton of Western Australia is among the oldest impact structures on Earth. Impact lithologies include the ‘Barlangi Granophyre’ (BG) interpreted as an injection of impact melt into the crater basement (i.e., an impact melt rock that locally contains granitic xenoliths); fault-related pseudotachylites; monomict granitederived lithic breccias; and shatter-coned granite that also contains shocked quartz [1,2]. The BG is well exposed at ‘Barlangi Rock’, a monolith accessible from the Meekatharra–Sandstone Road (27°11'S, 118°50'E). The original size of the Yarrabubba impact structure is poorly constrained [1]. Extrapolation of a ~25 km-diameter magnetic anomaly that coincides with the impact structure suggests an original diameter of ~30–70 km [2]. Likewise, the age of the Yarrabubba impact is only loosely bracketed by the age of the youngest shocked target rock, the ~2.65 Ga Archean ‘Yarrabubba Granite’; dolerite dikes possibly as young as ~1.2 Ga [3], which seem to postdate the impact structure [1,2]; and a Mesoproterozoic alteration (minimum) 40Ar/39Ar age of ~1.1 Ga for pseudotachylites [4]. Uranium-lead geochronology of zircon grains recovered from the BG predominantly yielded ages between 2.79 Ga and 2.65 Ga [5,6]. Post-impact K-metasomatic and hydrothermal alteration of the Yarrabubba impactites produced veins of bladed calcite locally replaced by silica, K-feldspar veins, veinlets of fluorite, biotite, chlorite, prehnite, and caused micaceous overprint of the shocked target granite [2,7]. The present study focuses on newly discovered late-stage zircon and aggregates of zircon intergrown with thorite (henceforth thorite–zircon) found in the BG alongside secondary TiO2.