Detrital zircon U-Pb and Hf isotopes study of the Longshoushan Belt in the southwestern margin of the Alxa Block: Constraints on the tectonic evolution and affinity of the Alxa Block

     The tectonic evolution and affinity of the Alxa Block has long been controversial. The NW-SE trending Longshoushan Belt is in the southwestern margin of the Alxa Block, separated the Qilian Block. In this study, we present zircon U-Pb and Hf-isotope data of the middle and eastern Longshoushan, which could constrain the provenance and formation age of the Longshoushan Belt, and further constrain the tectonic evolution and affinity of the Alxa Block. The U-Pb ages of the detrital zircons from the amphibolite-facies metamorphosed volcanic-sedimentary rocks of the middle Longshoushan range from 3006 to 1981 Ma (peak at 2010 Ma), which were consistent with the Alxa Block and the western North China Craton, indicating that the middle Longshoushan was deposited in the Palaeoproterozoic, not in the Archean, and had tectonic affinity with the Alxa Block and the western North China Carton. Combined with the identical crustal growth events at 2.4-2.5 Ga of the middle Longshoushan, the Alxa Block and the western North China Craton, the Alxa Block was an integrated part of the Western Block of the North China Craton. The U-Pb ages of the detrital zircons from the greenschist-facies metamorphosed volcanic-sedimentary rocks of the eastern Longshoushan range from 3389 to 529 Ma (peak at 2.5 Ga and 1.0 Ga), which were highly consistent with Hexi Corridor, indicating that the eastern Longshoushan was deposited in the Cambrian, and had an affinity with the Hexi Corridor. In the Early Palaeozoic, the North Qilian Ocean subducted the Alxa Block and formed a typical trench-arc-basin system. With the closure of the North Qilian Ocean, the Central Qilian Block collided with the Alxa Block, formed the eastern Longshoushan, which was a foreland basin in the Hexi Corridor.