Tectonic evolution of the Alxa Block and its affinity: Evidence from the U-Pb geochronology and Lu-Hf isotopes of detrital zircons from the Longshoushan Belt

摘要

The NW-SE trending Longshoushan Belt is a significant transitional belt between the Alxa and Qilian blocks. Its basement rocks were predominated by greenschist- to amphibolite-facies metamorphosed volcanic-sedimentary rocks, which were named the Longshoushan Complex and sporadically exposed in the middle and eastern Longshoushan Belt. The U-Pb ages of detrital zircons from the middle Longshoushan are scattered in the period of 3006-1981 Ma, predominated by Paleoproterozoic ( > 95%) with a single peak of 2.0 Ga. These zircons have the Hf-176/ Hf-177 ratios of 0.281150-0.281687, corresponding to hafnium model ages (T-DM(c)) of 3.04-2.15 Ga. These age features are in accordance with those of the Alxa Block and the western North China Craton. In contrast, the protoliths of the eastern Longshoushan reveal a wide age range from 3389 Ma to 529 Ma, which can be apparently divided into four age populations of 2.5 Ga, 1.6 Ga, 1.0 Ga and 0.8 Ga. This is consistent with recent reported age data of the Hexi Corridor, indicating that the eastern Longshoushan is not a part of the Longshoushan Complex, but has affinity with the Hexi Corridor. The cumulative probability curves of measured crystallization ages and depositional ages of detrital zircons imply that the middle Longshoushan could be deposited in a convergent basin (e.g., back-arc basin) in the Paleoproterozoic, but the eastern Longshoushan was typical of tectonic setting of a collisional basin (e.g., a foreland basin) in the Cambrian. The closure of the North Qilian Ocean formed the North Qilian Orogen between the Central Qilian and Alxa blocks, resulting to a foreland basin in the Hexi Corridor (i.e., eastern Longshoushan).