Wide distribution of 3.46–3.1 Ga rocks in the Kongling Complex of the Huangling area, Yangtze Craton, south China: Petrogenesis and significance

Abstract

The early evolution of the Earth is one of the most fundamental issues in Earth science. Here, we present whole-rock geochemistry, SHRIMP UPb zircon dating, and Hf isotopic analysis of newly-discovered Paleoarchean-Mesoarchean rocks in the Kongling Complex of the Huangling area at the northern margin of the Yangtze Craton, south China. Paleoarchean (3.46 Ga) supracrustal rocks (biotite-plagioclase gneisses) have been identified for the first time, with the rocks having low ΣREE (total REE) contents of 65.9 ppm, a low (La/Yb)_n value of 6.4, and magmatic zircons with ε_Hf(t) values from −6.22 to −1.31 and t_DM2(Hf) model ages of 4.11–3.82 Ga. Trondhjemitic gneisses with zircon ages of 3.3–3.1 Ga were identified in four areas, and xenocrystic zircons of similar age occur in widespread 3.0–2.9 Ga trondhjemitic gneisses. All the trondhjemitic gneisses are similar in major elements but show large variations in trace element compositions, with ΣREE, (La/Yb)_n and Eu/Eu* being 17.87–295.04 ppm, 14.22–80.50 and 0.55–5.22, respectively. They contain magmatic zircons with ε_Hf(t) values from −5.06 to −0.35 and t_DM2(Hf) model ages of 3.91–3.56 Ga. They plot in the low-K mafic rock and tonalite fields in the Al₂O₃/(FeOt+MgO)–3CaO–5(K₂O/Na₂O) diagram. It is concluded that the Paleoarchean-Mesoarchean trondhjemitic rocks were formed either by partial melting of basaltic rocks under low- to medium-pressure conditions or by partial melting of earlier TTG rocks that formed under these conditions. The Kongling Complex formed the continental nucleus to the Yangtze Craton, with most crustal materials being derived from the mantle at pre-3.6 Ga.