Evidence for discrete Archean microcontinents in the Yangtze Craton

Abstract

It is unclear whether most Archean continents have always existed in their present-day size, or were initially microcontinents or supercratons. The Archean evolution of the Yangtze Craton in South China provides an ideal opportunity to reveal Archean continental architecture. Archean rocks in the Yangtze Craton have been documented in seven separate areas prior to this study. In this paper, we report the first discovery of Archean rocks in a new area in the southern Dabie orogenic belt, which we term the Xishui gneiss. The protolith of the granitic gneiss is strongly peraluminous with high A/CNK ratios of 1.10–1.23 and K2O/Na2O ratios of 0.70–1.72, belonging to the high-K calc-alkaline series. Three samples of the gneiss from different localities yielded protolith ages of 2.51, 2.47, and 2.47 Ga (zircon U–Pb weighted mean ages). Xenocrystic zircons with magmatic texture from the gneiss yielded U–Pb ages of 3.57–2.41 Ga, and records four phases of magmatism and crustal reworking at 3.56, 3.34, 3.18, and 2.51–2.47 Ga. The history of crustal growth recorded by the gneiss can be traced back to ~3.80 Ga, with peaks at 3.8–3.7 and 3.5–3.4 Ga. A comparison of the crustal evolution in the eight exposed Archean terranes of the Yangtze Craton suggests they were independent microcontinents during the Archean and that the present-day Yangtze Craton was amalgamated at 2.0–1.9 Ga during assembly of the Columbia supercontinent.