The record of geological processes in zircon from polymetamorphosed orthogneisses from the Napier Mountains, Napier Complex, East Antarctica

The Napier Complex in Enderby Land and western Kemp Land is a unique component of the East Antarctic Shield because it records a timeline of crustal growth from the Eo- to Neoarchean. It is mainly composed of enderbitic and charnockitic gneisses and granulites that were metamorphosed at ca. 2.5 Ga, and locally at ca. 2.8 Ga, under high- to ultra-high-temperature conditions. Despite generating scientific interest for several decades, the geological history of the complex has not been well constrained. In this study, samples from the Napier Mountains were selected for zircon imaging and U-Pb dating by Secondary Ion Mass Spectrometry. They record metamorphic growth, recrystallization, and modification of zircon at 2800-2770, 2740-2720 Ma and 2490-2460 Ma. For the first time, fluid-related alteration at around 2730 Ma is evident in a granitic gneiss from Grimsley Peaks. At the similar time, dioritic gneiss was formed at Mount Marr. Tonalitic and granitic gneisses from Grimsley Peaks yield protolith crystallization ages of around 3210 and 2825 Ma, respectively. The generation of granitic gneiss was coeval with ∼2.8 Ga metamorphism in the area. These new data from this little-known part of the complex provide a better understanding of the crustal evolution of the Napier Complex.