The age of the Yuanjiacun banded iron formation in the North China Craton revisited

Abstract

Banded iron formations (BIFs) represent an iron-rich rock type that typifies Archean and Paleoproterzoic supracrustal successions, with nearly the absence of BIFs deposited during the Great Oxidation Event (GOE; 2.4–2.2 Ga). This paucity renders us difficult to apply BIFs to understand a shift of paleoenvironmental conditions in time and space and its specific control mechanism on BIF deposition. In this regard, we focus on the Yuanjiacun BIF in the Lüliang area of the North China Craton (NCC), because it is a potential BIF that might be deposited during the GOE. If the formation age of this BIF is confirmed, it would hold great potential as a good archive to record the redox conditions and transitions in the atmosphere–ocean system and reveal its genetic relationship to the paleoenvironmental evolution. Here, we present a detailed compilation of all previously published zircon U-Pb ages of metamorphosed sedimentary rocks within the Lüliang Group to assess the formation age of the Yuanjiacun BIF. In combination with our newly obtained SHRIMP zircon U-Pb ages of sericite-quartz phyllites intercalated with this BIF, we can further clearly recognize the major detrital zircon age distribution of the Lüliang Group to get a good understanding of its provenance. Results show that the youngest detrital zircon population of associated metamorphosed sedimentary rocks occurs between 2.23 and 2.20 Ga, representing the maximum depositional age of these rocks. This age, coupled with the formation ages of overlying volcanic rocks, further provides a well constraint on the depositional age of the Yuanjiacun BIF to ∼2.23–2.21 Ga, clearly during the GOE. In addition, the detrital zircon age distribution yields a dominant 2.5–2.6 Ga age population and subordinate age peaks at ∼2.2 Ga, 2.3–2.5 Ga, 2.6–2.8 Ga, and ∼3.0 Ga, suggesting that detrital zircons were sourced predominantly from weathering of Archean igneous rocks, possibly located in both Lüliang and adjacent Fuping-Wutai-Hengshan regions. By further constructing a comparision with other Paleoproterozoic strata in the NCC, we highlight the significance and uniqueness of the Yuanjiacun BIF. Its deposition is slightly older than nearly all Paleoproterozoic successions distributed in other regions of the NCC.