Cryptic geological histories accessed through entombed and matrix geochronometers in dykes

Abstract

Deep geology of ancient continental crust can be difficult to access, with direct observation restricted to limited exposures. The age and composition of hidden geology can be gleaned from indirect isotopic modelling or via detrital minerals within overlying basins. Here we present an alternative, where direct grain sampling of ancient components within the South West Terrane, Yilgarn Craton, by a Proterozoic dyke evidences deep intact, or detritus from, Paleoarchean crust. U–Pb geochronology on this dyke reveals c. 3440 Ma zircon inclusions within titanite. This zircon was protected from overprinting fluids that obliterated unshielded crystals. Similar ancient zircon is present within recent sediment from the Swan-Avon river, which drains the terrane. The most parsimonious interpretation is that the dyke is 1390 Ma. Sequential overprinting is also recorded, with titanite preserving primary crystallization and c. 1000 Ma Pinjarra Orogeny-related overprinting. In contrast, apatite preserves c. 210 Ma ages, correlated with denudation of sedimentary cover.