Ediacaran iron formations from the North Qilian Orogenic Belt, China: Age, geochemistry, Sm–Nd isotopes and link with submarine volcanism

Abstract

Iron formations (IFs) are relatively rare in the Neoproterozoic, and a few examples are closely related to the Cryogenian (∼717–635 Ma) Snowball Earth glaciations. In this study, we investigated the rare IFs formed during the Ediacaran period (635–541 Ma) which are hosted in volcano-sedimentary successions belonging to the Zhulongguan Group in the North Qilian Orogenic Belt in northwestern China. Detrital zircon U–Pb data from the tuffaceous breccia associated with the Kawa IF in this region show a large variation in ages between 749 Ma and 2706 Ma. However, two ferruginous tuff samples yielded zircon 206Pb/238U ages of 601.9 ± 6.1 Ma and 598.2 ± 5.5 Ma, respectively. These ages indicate that the Kawa volcano-sedimentary successions and associated IF were deposited at ∼ 600 Ma. Banding/lamina are not well developed in the IF and the rock is dominantly composed of hematite and jasper, with minor clay minerals and breccia. The IF shows high TFe2O3 and SiO2 contents (TFe2O3 + SiO2 = 83–90 wt%) with relatively elevated Al2O3, TiO2, HFSEs (e.g., Nb, Hf and Th) and rare earth element (REE) concentrations. The geochemical features are consistent with chemical sediments with significant clastic material input (∼20%). The rocks also show light REE depletion [(La/Yb)PAAS = 0.23–0.34], slightly positive Eu anomalies (Eu/Eu*=1.03–1.12), low Y/Ho ratios of 27.17–29.53, and slightly negative εNd(t) values (−2.03 – −0.70), indicating a submarine hydrothermal source for the iron with dilution by detrital components and seawater. The lack of negative Ce anomalies and redox-sensitive trace element enrichments (e.g., Mo, U, and V), the presence of chamosite, and the relatively high TFe2O3 contents in the ferruginous tuff and slate suggests anoxic and ferruginous conditions. Our results link the post-Cryogenian Kawa IF with submarine volcanism and hydrothermal activity, in response to the breakup of Rodinia.