Decoupling of isotopes between magmatic zircons and their mafic host rocks: A case study from the ca. 830 Ma Jiabang dolerite, South China

Abstract

Zircon is widely used to constrain the crystallization age and magma source of the host rocks. However, in recent years an increasing number of studies have demonstrated that Hf-O isotopes of zircon grains are not always in equilibrium with the whole-rock isotopes of the host rocks. Here we present a case from the Jiabang dolerite from southeastern Guizhou Province, South China, to demonstrate the decoupling of isotopes between magmatic zircons and their mafic host rocks as a result of limited crustal contamination. In-situ SIMS U-Pb zircon dating shows that the Jiabang dolerite formed at 838 ± 7 Ma. The Jiabang dolerite has low SiO2 of 47.6 – 54.2 wt%, high MgO of 4.93 – 15.6 wt%, and Fe2O3 of 8.51 – 11.9 wt%, typical of mantle-derived magma. All whole-rock samples in this study have depleted Nd-Hf isotopes, with εNd(t) values ranging from + 2.6 to + 3.8 and εHf(t) values from + 6 to + 10, with whole-rock εNd(t) values similar to those of contemporary OIB-like mantle-derived melts in South China. Such whole-rock chemical features imply that the source of the Jiabang dolerites is mantle-dominated and that any crustal contribution should be insignificant. However, zircons from the dolerite display negative εHf(t) values of −6 to −1 and high δ18O values of 9.7–11.5 ‰ that are similar to those of the Guibei granitoids in the study area, suggesting a crustal origin. Our AFC simulation based on Magma Chamber Simulator (MCS) demonstrates that AFC played a limited role in the generation of the Jiabang dolerite and indicates that the decoupling between zircon Hf-O and the whole-rock Nd-Hf isotopes likely reflects a contribution from wallrock-derived melts during emplacement of the dolerites. We propose that the zircons crystallized in interstitial liquids that were contaminated by crustal-derived fluids/melts, resulting in decoupling between the whole-rock isotopes and the magmatic zircon isotopes. Due to mass balance requirements, the crustal melts/fluids would have insignificant effect on the whole-rock isotopes, but will strongly modify the Hf-O isotopes of zircons that crystallized in the interstitial liquids. This case study shows that caution needs to be applied when comparing zircon Hf-O isotopes from mafic intrusive rocks to constrain their source.