Zircon U-Pb age, Hf isotopes and geochemistry of Shuichang Algoma-type banded iron-formation, North China Craton: Constraints on the ore-forming age and tectonic setting

摘要

The Shuichang iron deposit is an Algoma-type BIF in the North China Craton. The Shuichang BIF is interlayered with plagioclase-hornblende gneiss, indicating that these two lithologies are temporally closely associated. We use the content of major elements and their ratios in order to define the protolith of the plagioclase-hornblende gneiss. SIMS U-Pb dating of zircons from the plagioclase-hornblende gneiss suggests that the Shuichang BIF was formed at ca. 2547±7Ma and was subjected to metamorphism at 2513±4Ma. The major elements and REE data of the ore, as well as individual magnetite and chert bands indicate that the Shuichang BIF precipitated from hydrothermal fluids discharging on the sea floor and that iron and silica may be associated with submarine volcanic activity. In-situ Lu-Hf analysis of zircons from the plagioclase-hornblende gneiss yielded initial ~(176)Hf/~(177)Hf ratios from 0.280903 to 0.281197 and ε_(Hf)(t) values from -10.5 to 0.4 with a weighted mean value of -8.72±0.38. The single stage Hf model age ranges from 2836Ma to 3249Ma. We infer that the protolith of the plagioclase-hornblende gneiss originated from the enriched mantle mixed with a small amount of subducted crust materials. The Shuichang BIF formed at 2.5Ga and is coeval with other BIFs in North China and in other cratons in the world. We propose that the Shuichang BIF was formed in a subduction-related back-arc basin or intra-arc basin, based on the geochemical signatures of the wall rocks. We further point out that the 2.5Ga global BIF formation peak not only predates the amalgamation of the North China Craton, but probably also record an ancient supercontinent event.