A comparison of U-Pb and Hf isotopic compositions of detrital zircons from the North and South Liaohe Groups: Constraints on the evolution of the Jiao-Liao-Ji Belt, North China Craton

摘要

The Jiao-Liao-Ji orogenic belt is one of the important Paleoproterozoic mobile belts in the North China Craton (NCC) and its central segment consists of the North and South Liaohe Groups and the Liaoji Granitoids. The North and South Liaohe Groups consist of sedimentary and volcanic successions metamorphosed from greenschist to lower amphibolite facies, and the Liaoji granitoids are divisible into 2.2-2.1 Ga (pre-tectonic) monzogranitic gneisses and 1.88-1.85 Ga (post-tectonic) porphyritic monzogranites and alkaline syenites. Combined with cathodoluminescence (CL) imaging, we report LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) U-Pb and Hf isotopic data for detrital zircons from the North and South Liaohe Groups. Zircons grains from both groups have igneous zircon cores that yielded two U-Pb age populations at 2.0-2.2 Ga (major) and similar to 2.5 Ga (minor), indicating that their provenances were similarly dominated by Paleoproterozoic Liaoji Granitoids and late Archean basement rocks. Some structureless zircon rims in both groups yield concordant or upper intercept ages similar to 1.9 Ga, interpreted as the peak metamorphic age of the two groups. These results suggest that both groups were deposited in the period 2.0-1.9 Ga, shortly after the emplacement of the Liaoji Granitoids. Moreover, zircons from both groups have similar Hf model age peaks (T-DM(C) peaks at 2.66-2.75 Ga and 2.80-3.00 Ga) and epsilon Hf values (-5 to +9 and -7 to +5). Based on these striking similarities, we propose that the protoliths of the North and South Groups may have formed simultaneously, and that the basement rocks underneath the two groups belong to the same Archean continental block rather than two different blocks as previously considered. The Archean basement underwent Paleoproterozoic rifting, accompanied by the emplacement of the Liaoji granitoids at 2.2-2.1 Ga and the deposition of the North and South Liaohe Groups at 2.1-1.9 Ga, and the rift was subsequently closed by similar to 1.9 Ga.