Detrital Zircon Records of the Banxi Group in the Western Jiangnan Orogen: Implications for Crustal Evolution of the South China Craton

Abstract

AbstractThe Neoproterozoic evolution of the Jiangnan Orogen is important for understanding the tectonic history of South China. As a volcanic‐sedimentary sequence developed in the Nanhua rift, the Banxi Group preserves the records of important magmatic and tectonic events linked to the assembly and breakup of the Rodinia supercontinent. In this study, we report the results from whole‐rock major‐ and trace‐element concentrations, with zircon LA‐(MC)‐ICP‐MS U‐Pb ages, trace elements and Lu‐Hf isotopic compositions of sandstones from the Banxi Group. The rocks are characterized by high SiO2 (65.88%–82.76%, with an average of 75.50%) contents, moderate (Fe2O3T + MgO) (1.81%–7.78%, mean: 3.79%) and TiO2 (0.39%–0.54%, mean: 0.48%), low K2O/Na2O (0.03–0.40, mean: 0.10) ratios and low Al2O3/SiO2 (0.11–0.24, mean: 0.15) ratios. The sandstones have high ΣREE contents (mean: 179.1 ppm), with chondrite‐normalized REE patterns similar to the upper crust and PAAS, showing enriched LREE ((La/Yb)N mean: 14.85), sub‐horizontal HREE curves and mild Eu (Eu/Eu*: 0.75–0.89, mean: 0.81) negative anomalies. Their geochemical characteristics resemble those of passive continental margin sandstones. Most of the zircons are magmatic in origin and yield a U‐Pb age distribution with three peaks: a major age peak at 805 Ma and two subordinate age peaks at 1990 Ma and 2470 Ma, implying three major magmatic sources. The Neoproterozoic zircons have ɛHf(t) values ranging from –47.4 to 12.4 (mostly –20 to 0), suggesting a mixture of some juvenile arc‐derived material and middle Paleoproterozoic heterogeneous crustal sources. The Hf model ages of middle Paleoproterozoic zircons (∼1990 Ma) with negative ɛHf(t) values (–12.65 to –6.21, Ave. = –9.8) concentrated around the Meso‐Paleoarchean (mean TDMC = 3.3–3.1 Ga). For late Neoarchean detrital zircons (∼2470 Ma), ɛHf(t) values are divided into two groups, one with negative values (–9.16 to –0.6) with model ages of 3.5–2.9 Ga, the other featuring positive values (1.0 to 3.9) with model ages of 2.9–2.7 Ga, recording a crustal growth event at ∼2.5 Ga. Neoproterozoic zircons show volcanic arc affinities with partly intraplate magmatic features. We propose that the Banxi Group formed in a rift basin within a passive continental margin setting, which derived detritus from felsic to intermediate rocks from the Yangtze Block and a small amount of arc volcanic rocks. The middle Paleoproterozoic detrital zircon data suggest Columbia‐aged basement lies beneath the western Jiangnan orogen.