Zircon Hf isotope of Shennongjia volcanic rocks on the northern margin of the Yangtze craton

Based on documented geochronological and whole-rock geochemical data, this paper reports for the first time the zircon Hf isotopic composition of the Shennongjia andesitic ignimbrite, with the purpose of constraining the magmatic source and petrogenesis. The results obtained by the authors reveal that the Nd-Hf decoupling occurs in the rocks. Although the samples have relatively constant elemental geochemical characteristics and Nd isotopic composition, highly variable zircon εHf(t) values (-24.0~+0.3) are existent in a single sample, corresponding to zircon Hf isotope model age(TDM) of 2587~1608 Ma and suggesting that terrestrial clastic components contamination occurred during the generation and upwelling of magma. Such a component evidently modified part of the zircon Hf isotopic composition during magma crystallization but did not affect the whole geochemical and Nd isotopic composition, revealing that the zircon in-situ Hf isotope was more sensitive to the clastic contamination than whole-rock Nd isotope during the formation and evolution processes of magma. Combined with whole-rock geochemical features, this paper points out that the Shennongjia andesitic ignimbrite might have been derived from Proterozoic mafic lower crustal anatexis caused by upwelling of mantle-derived magma, and experienced the contamination of subducted clastic rocks during its upwelling.