Geochemistry of meta-sedimentary rocks associated with the Neoarchean Dagushan BIF in the Anshan-Benxi area, North China Craton: Implications for their provenance and tectonic setting

Abstract

The Anshan-Benxi area is situated in the northeastern part of the North China Craton (NCC) and consists predominantly of Neoarchean supracrustal rocks and plutons with minor Eoarchean to Mesoarchean rocks. These supracrustal rocks were originally considered to be formed synchronously (∼2.5 Ga) at different localities on an active continental margin. Recently, more detailed petrographic, geochemical, and geochronological analyses showed that the supracrustal strata in the Benxi area (2.56–2.53 Ga) were slightly older than those in the Anshan area (2.53–2.51 Ga), which further indicates that they were formed during opening and closure of the back-arc basin, respectively. However, the Dagushan banded iron formation (BIF) and associated supracrustal rocks in the Anshan area were interpreted to be deposited in Mesoarchean (∼3.10 Ga), which constructs a clear contrast with other supracrustal rocks in the Anshan-Benxi area and renders previous tectonic models controversial.On the basis of detailed field investigations in the Dagushan area, we present in situ zircon U-Pb-Hf isotopic, whole-rock geochemical and Sm-Nd isotopic data of a suite of meta-sedimentary rocks (i.e., meta-arenites, meta-shales, and high-Fe meta-shales), in order to provide constraints on their depositional age, provenance, and tectonic setting. The weighted mean U-Pb ages of the youngest group of detrital zircons for the meta-arenite in the Dagushan area, coupled with the maximum metamorphic age of ∼2.51 Ga in the Anshan-Benxi area, suggest that the Dagushan BIF was deposited between 2.53 and 2.51 Ga. Geochemical features such as the Al2O3/TiO2 and Th/Zr values, and REE patterns suggest that the meta-arenites and meta-shales were mainly derived from felsic igneous sources, whereas the high-Fe meta-shales were predominantly sourced from mafic rocks. The U-Pb age spectrum diagram of detrital zircons within meta-arenites further indicates that the dominant source rocks may be the coeval volcanic rocks in the Benxi area. Our zircon Hf isotopes, coupled with other previously published zircon Hf isotopic data from Neoarchean supracrustal rocks in the Anshan-Benxi area, suggest that major juvenile crustal growth and reworking events occurred at 2.9–2.7 Ga and ∼2.5 Ga, respectively. In all, the Dagushan meta-sedimentary rocks were probably deposited in a back-arc basin on the ancient continental margin.