Late Neoarchean supracrustal rocks from the Anshan-Benxi terrane, North China Craton: New geodynamic implications from the geochemical record

Abstract

The late Neoarchean supracrustal rocks of the Anshan-Benxi (AnBen) terrane in the northeastern part of the North China Craton (NCC) have important regional tectonostratigraphic implications for the development of large-scale banded iron formations (BIFs). These supracrustal rocks underwent polyphase deformation and upper greenschist to upper amphibolite facies metamorphism and are subdivided into two main lithotectonic units in terms of differing ages, metamorphic grades, and lithological assemblages: the Benxi unit (∼2.56–2.53 Ga) composed of tholeiitic to transitional mafic rocks with minor siliciclastic sedimentary rocks, and the Anshan unit (∼2.53–2.51 Ga) consisting of siliciclastic sedimentary rocks with minor tholeiitic mafic rocks. In this study, we report the results of field investigations and petrographic, whole-rock major and trace element geochemical and Sm-Nd isotopic analyses, as well as zircon U-Pb-Hf-O isotopic analyses for these volcano-sedimentary rocks, and integrate the results with previously published data to provide a new geodynamic synthesis for the setting of the BIFs.

Three groups of amphibolites (meta-basalts) in the vicinity of BIFs were recognized in the AnBen area on the basis of their REE and HFSE characteristics: (a) Group 1 is characterized by nearly-flat to slightly depleted LREE patterns and an absence of significant Nb anomalies, similar to those of modern N-MORB, suggesting that their magmatic precursors were derived from partial melting of a strongly depleted mantle; (b) Group 2 displays relatively flat to slightly enriched LREE patterns and minor negative Nb anomalies, which are comparable to those of back-arc basin basalts and indicate that they were probably generated by partial melting of depleted mantle modified by slab-derived fluids; and (c) Group 3 exhibits LREE-enriched patterns and pronounced negative Nb anomalies, which are interpreted to be the result of a depleted mantle-derived melt interacting with older upper continental crust. The overall field and geochemical signatures suggest that these basaltic rocks were erupted in a back-arc basin within a continental volcanic arc setting with a protracted history, consistent with Nd isotope, inherited zircon age, and zircon Hf-O isotope data that require variable inputs of older crustal sources for their formation. Diagnostic geochemical features and detrital zircon age spectra further indicate that the source rocks for felsic to mafic detritus in the Anshan unit may have been derived dominantly from coeval volcanic rocks of the Benxi unit and/or distal volcanic arc edifices.

Collectively, the evidence indicates that the tectonic evolution of the AnBen terrane took place in two main stages: the first stage involved the generation of ∼2560 to 2530 Ma tholeiitic to transitional mafic rocks and associated siliciclastic sedimentary rocks of the Benxi unit, which is interpreted to have formed during opening of the back-arc basin, whereas the second stage corresponded to closure of the back-arc basin (∼2530–2510 Ma) and involved extensive chemical and siliciclastic sedimentary rocks with minor emplacement of tholeiitic mafic lavas.