Chronology and geochemistry of Neoarchean BIF-type iron deposits in the Yinshan Block, North China Craton: Implications for oceanic ridge subduction

Abstract

The Dongwufenzi iron deposit is a typical Algoma-type Banded Iron Formation (BIF) in the Yinshan block of the North China Craton. The iron orebodies occur as layers interlayered with plagioclase amphibolite. LA–ICP-MS U–Pb dating of zircons from the plagioclase amphibolite yielded protolith crystallization age of 2538±9Ma (MSWD=1.01), followed by metamorphism at 2452±7Ma (MSWD=0.68). The geochemical characteristics of the ores including low Al2O3, TiO2 and HFSE contents, pronounced positive Eu anomalies, and the Y/Ho ratios (26–37) suggest that the BIFs precipitated from hydrothermal fluids discharged on the seafloor. The geochemical features of the plagioclase amphibolite suggest the protolith magma to be high-Fe basalt. Sm–Nd analyses of the plagioclase amphibolite yielded εNdt (t=2.54Ga) and single-stage depleted mantle Nd model ages (TDM) in the range of ca. +1.8 to +3.0 and 2.68 to 2.81Ga, respectively, suggesting an enriched mantle source. We link the source of Fe in the BIFs to the komatiitic complex in the same greenstone belt, and the source of Si with the high-Fe basalt. Integrated geochronological and geochemical data from the other Archean rocks in this area suggest that the Dongwufenzi BIF formed at 2.5Ga, coeval with the other BIFs in the North China Craton (NCC) as well as similar occurrences in other parts of the world. We propose a geodynamic model involving ridge subduction and slab window for the formation of the BIFs and the associated rock suites in the greenstone belt.