Changing provenance of late Neoarchean metasedimentary rocks in the Anshan-Benxi area, North China Craton: Implications for the tectonic setting of the world-class Dataigou banded iron formation

摘要

The Anshan-Benxi (AnBen) area is situated in the northeastern part of the North China Craton and is considered to be the most important iron metallogenic province in China. The giant Dataigou banded iron formation (BIF) has the potential to be the largest iron deposit in this area, even in Asia. Here, we present in situ zircon U-Pb-Hf-O isotopic and whole-rock geochemical data for metasedimentary rocks (metapelites and metaarenites) and trondhjemitic gneiss associated with this BIF, in order to provide constraints on the setting of the deposit, with implications for geological and exploration models. SIMS zircon U-Pb dating results indicate that the trondhjemitic gneiss crystallized at 3450 +/- 19 Ma, and thus it is the oldest rock so far indentified in the Benxi area. The youngest group of detrital zircons from the metaarenite samples constrains their maximum-depositional age at similar to 2.54 Ga. In combination with the earlier metamorphic age of similar to 2.51 Ga for the metaarenites, the deposition age of the Dataigou BIF can be constrained between 2.54 and 2.51 Ga. The metasediments have undergone varying degrees of source weathering. Source rocks of the metapelites have undergone moderate to severe chemical weathering, whereas those of the metaarenites have subjected to relatively weak chemical weathering. Diagnostic geochemical features such as the Al2O3/TiO2 values, trace element ratios, and REE patterns suggest that the metaarenites were predominantly derived from felsic igneous sources, whereas the metapelites were sourced mainly from mafic tholeiitic rocks. The dominant late Neoarchean detrital zircons further indicate that they were most likely sourced from almost synchronous volcanic rocks in the AnBen area. Taking into account the lithostratigraphic, geochemical, and geochronological data, the Dataigou BIF is interpreted as having been deposited in a back-arc basin, most probably on the margin of an ancient continental crust. Continuous provenance variations of these metasediments during highstand times and transgression can be ascribed to changes in natures of nearby volcanisms. Integrated with previous zircon Hf isotopic data from other supracrustal rocks in the AnBen area, our Hf-O data reveal major juvenile crustal growth stages at 3.0 Ga and during 2.8-2.7 Ga, and a crustal reworking event with minor juvenile addition at similar to 2.5 Ga in the studied area. (C) 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.