Geochronological and geochemical studies of the metasedimentary rocks and diabase from the Jingtieshan deposit, North Qilian, NW China: Constraints on the associated banded iron formations

Abstract

The Jingtieshan banded iron formations (JBIFs), located in the western segment of the North Qilian area, are one of the major iron deposits in NW China. The BIF occurs within the Jingtieshan Group intercalated with metasedimentary rocks which have undergone lower greenschist-facies metamorphism, and were intruded by a series of diabase sills/dykes. Here we present U–Pb data on detrital zircons from the metasedimentary rocks and the results show ages ranging from 1331 to 2919Ma. Zircons from two diabase samples yield emplacement ages of 1236±3Ma (n=10, MSWD=0.26) and 1234±7Ma (n=19, MSWD=0.55), respectively. These new data constrain the depositional age of the JBIF at 1.23–1.33Ga. The geochemical characteristics (e.g., CIA, PIA, Al2O3/TiO2, trace element ratios and REE patterns) of the metasedimentary rocks indicate that the protolith sediments were mainly sourced from mafic and intermediate igneous rocks, followed by post-depositional K-metasomatism. The prominent age peak at 1747Ma obtained from detrital zircons in the metasediments suggests that the zircons were mostly sourced from the underlying Zhulongguan Group in North Qilian area. The subordinate population of Early Mesoproterozoic detrital zircons was probably sourced from the North Qilian area. The minor population of zircons with Archean ages was derived from the Archean basement in the neighboring old craton. The diabase samples are characterized by slight LREE enrichment, display no distinct Eu and Ce anomalies, and show enrichment in Rb, Ba, K, and Pb and depletion in Nb, Ta, and Sr. These features suggest that the primary magmas of the diabases were likely generated by partial melting of the transitional mantle. The data presented in this study suggest that the JBIF formed in a continental margin sea within an extensional setting.