Variations of Sr–Nd–Hf–O isotopes and redox conditions across a Neoproterozoic arc magmatic belt in the western margin of the Yangtze craton

Abstract

We use zircon trace elements and HfO isotopes, plus whole-rock chemical and SrNd isotope compositions to investigate the controls on the chemistry of Neoproterozoic arc magmas. The samples are from the gabbrodiorite intrusion of the Gaojiacun mafic-ultramafic complex and from the Tongde diorite batholith of the Panxi Neoproterozoic magmatic belt in the western margin of the Yangtze craton, western China. Zircon UPb ages reveal that both intrusions are coeval, emplaced at ∼820 Ma. Major element compositions of whole rocks and major silicate minerals indicate that the Tongde batholith formed from more evolved magma than the parental magma for the Gaojiacun intrusion. The mantle-normalized trace element patterns of whole rocks from both intrusions are similar, showing light REE enrichments and pronounced negative NbTa anomalies. The SrNd isotopic compositions of the whole rocks are also similar, plotting within or very close to the mantle array, indicating no to minor contamination with the upper crust. Zircon HfO isotopes indicate that the Gaojiacun magma has much higher ƐHf (6.26 ± 0.52) and δ18O (7.0 ± 0.3 ‰) than the Tongde magma (ƐHf = 4.81 ± 0.39; δ18O = 5.7 ± 0.2 ‰). The δ18O values of the Gaojiacun zircons are also higher than the normal mantle (δ18O = 5.3 ± 0.6 ‰). Zircon trace element compositions show that, despite their close spatial association (<10 km), the coeval intrusions have contrasting redox states, with the estimated fO2 values of ΔFMQ+1.0 and ΔFMQ-0.7 for Tongde and Gaojiacun, respectively. The former is within the range of modern arc basalts, whereas the latter is lower than the average value of MORBs. The very low fO2 of the Gaojiacun magma could be due to the presence of subducted oceanic sediments enriched in organic matter (OM) in the source or due to contamination with OM-rich sedimentary rocks in the upper crust. Since such crustal rocks have not been reported for the region, we prefer the former explanation. The preferred model is also supported by Sr–Nd–Hf–O isotopes. The results from this study reveal that both reduced and oxidized magma could be produced simultaneously in a subduction zone.