Mid to late Archaean (3.3–2.5 Ga) tonalitic crustal formation and high-grade metamorphism at Mt. Riiser-Larsen, Napier Complex, East Antarctica

Abstract The Archaean Napier Complex consists of felsic orthogneiss, mafic granulite and a variety of paragneisses metamorphosed in the granulite facies, in part at ultrahigh-temperature (UHT, >900 °C). Orthopyroxene-bearing felsic orthogneiss of tonalitic-granodioritic composition, the highly recrystallized Archaean tonalite-trondhjemite-granodiorite (TTG) crust, is a major constituent of the complex, especially at Mt. Riiser-Larsen, which is located within the UHT region. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses of zircon grains from three different layers of orthopyroxene-bearing felsic orthogneiss were carried out. Two of the analyzed orthogneiss samples yield 3270±12 Ma and 3267±5 Ma concordia ages from igneous zircon cores. Another sample shows slightly scattered and discordant ages (∼3350 Ma) with a concordia age of 3073±12 Ma. These data suggest that the initial TTG crust of the Mt. Riiser-Larsen was formed at ∼3270 Ma, and perhaps at 3070 Ma. These ages are considerably younger than the formation of tonalitic crust at Mt. Sones, Gage Ridge, and at Fyfe Hills (3840–3770 Ma), but older than that at Proclamation Island (2980 Ma) and several other areas at ∼2840 Ma. Two of the orthogneiss samples yield subordinate 2850–2790 Ma zircon concordia ages that are considered to result from a high-grade metamorphic event, which may also relate to 2830 Ma granitic magmatism recorded in the area. Zircon grains from the three analyzed orthogneiss samples yield an intense 2520–2450 Ma age cluster that probably reflect the UHT metamorphic event in the area. The ca. 3300 Ma age for TTG magmatism and ca. 2500 Ma age for high-grade metamorphism are comparable to the events in South India, which is thought by some authors to have been continuous with the Napier Complex from the Mid Proterozoic until Gondwana break up.