Using multiple methods to better constrain the depositional age of a meta-sedimentary succession: An example of the Laoling Group from the northeastern North China Craton

Abstract

The youngest detrital zircon (DZ) or groups of zircons age is usually interpreted as the maximum depositional age (MDA) and the oldest metamorphic zircon since deposition is used to resolve the minimum depositional age of a meta-sedimentary succession. However, DZ grains with multi-cycle or fingerprinting proximal sources may cause the MDAs older than the true depositional age, whereas post-depositional resetting has the opposite effect. Here, we present DZ U-Pb, Lu-Hf isotopic and trace elemental data, supplemented by whole-rock geochemistry, from the greenschist-facies metamorphosed Laoling Group consisting of clastic and carbonate rocks in the northern margin of the Jiao-Liao-Ji belt (JLJB) in the North China Craton to investigate the use of multiple methods to constrain its true depositional age. The dominant late Neoarchean and middle Paleoproterozoic DZ age peaks from the meta-sandstones, meta-siltstone, meta-mudstone, and meta-carbonate rocks are interpreted to be sourced from the Southern Jilin Complex and the JLJB, respectively. The low Th/U ratios (<0.18) and Ce/Nd ratios (<2.5) of the 1.96–1.77 Ga metamorphic zircons, which is consistent with the monazite ages of 1.95–1.76 Ga, are used to filter the possibly altered detrital magmatic zircons and the source-normalized α-dose metrics bigger than ca. 0.8 imply a predominant first-cycle origin, therefore the true depositional age of the group is constrained at 2.08–2.03 Ga. Additionally, in consideration of the strike of the basin parallel to the boundary between the JLJB and the Longgang Block, two periods of regional extensional tectonism, bimodal provenance, and depositional age coeval with the second magmatic event in the JLJB, we suggest that the Laoling Group most likely deposited in a back-arc basin in the northern margin of the JLJB. Finally, the temporal changes of zircon Hf isotope and Eu/Eu* in zircon crustal thickness proxy, especially the coeval excursion towards more radiogenic Hf signals and crustal thinning at 2.17–2.13 Ga, is consistent with the back-arc basin opening and closure model rather than the intra-continental rift model.