U-Pb zircon ages from leucogneiss in the Etheridge Group and their significance for the early history of the Georgetown region, north Queensland

Abstract

The Einasleigh Metamorphics form part of the Etheridge Group and are interpreted as the lowest exposed part of the Georgetown Inter of north Queensland. Due to a dearth of interbedded felsic igneous rocks that would be suitable for isotopic dating, the depositional age of the Etheridge Group has long been uncertain. This study presents U-Pb SHRIMP dating of zircons from leucogneiss in the Einasleigh Metamorphics. Based on field and geochemical evidence, and the nature of the contained zircons, the leucogneiss is interpreted as having a sedimentary origin derived from igneous sources, Old, poorly defined zircon components (mostly between ca 1800 and 2900 Ma) in most of the samples reflect detrital inheritance. Younger, well-defined, zircon populations in two samples of leucogneiss indicate that they were primarily derived from a possibly contemporaneous 1706 ± 6 Ma volcanic or volcano-plutonic terrane. High-grade metamorphism of these rocks occurred ∼ 150 million years later, and resulted in extensive zircon recrystallisation at 1562 ± 4 Ma. It is not clear whether this metamorphism and zircon crystallisation marks D1/M1 tectonothermal activity or the onset of the second major episode (D2/M2) of regional metamorphism and deformation, which peaked at co 1552 Ma. If the latter alternative is correct, D2/M2 lasted for at least 12 million years, and D1/M1 is likely to have occurred ∼ 20 million years earlier at ca 1580 Ma Two of the leucogneiss samples display evidence of isotopic resetting as young as 1500 Ma.