Hidden Archaean basement beneath the southwestern Yangtze Craton: Evidence from 2.8 Ga xenocrystic zircons in an early Cretaceous dolerite dyke

Abstract

The distribution of Archaean basement rocks in the southwestern Yangtze Craton, China, is not well documented due to the scarcity of outcrops. In this study, we identified Meso–Neoarchaean zircons in a dolerite dyke that was emplaced in the Meso–Neoproterozoic Dongchuan Group in the southwestern Yangtze Craton. Ninety-seven analyses of zircons from the dolerite yielded two distinct U–Pb age groups. The younger group of zircons, characterised by euhedral crystals and relatively high Th/U ratios (0.15–2.03), has a weighted-mean age of 109.4 ± 3.0 Ma (n = 27), which is considered to be the emplacement age of the dolerite. The older group of zircons, with a weighted-mean age of 2831 ± 18 Ma (n = 33), crystallised during the Meso–Neoarchaean. These zircons might be xenocrysts captured during magma emplacement. Moreover, the zircons have ε_Hf(t) values of −24.59 to −4.16 for the young population and −9.80 to −3.86 for the old population, indicating the dolerite was derived by melting of Archaean crustal igneous rocks. These results demonstrate the existence of Archaean rocks in the southwestern Yangtze Craton and provide new evidence for the spatial distribution of Archaean basement.