Tectonic attribution of the Langshan area in western Inner Mongolia and implications for the Neoarchean–Paleoproterozoic evolution of the Western North China Craton: Evidence from LA-ICP-MS zircon U–Pb dating of the Langshan basement

Abstract

The Langshan area in western Inner Mongolia, China, is situated on the conjunction of the Alxa Block in the west, the northern North China Craton (NCC) in the east, and the Central Asian Orogenic Belt in the north, but its tectonic attribution has long been in debate, because the Alxa Block may be an independent Precambrian block from the NCC and the boundary between the two blocks may lie along the Bayanwulashan, to the west of the Langshan, instead of around the Helanshan area, to the east of the Langshan. In this study, new LA-ICP-MS zircon U–Pb ages of four gneisses, one amphibolite, and three granites, combined with previously published age data from Langshan basement rocks, indicate that the Langshan area may be subdivided into the Main Langshan, to the north of a series of NNW-trending faults, and the Southern Langshan in the south. The Main Langshan basement rocks were mainly formed during Neoarchean–Paleoproterozoic, with five major stages of ~2.7Ga (2686Ma), ~2.6Ga (2619–2563Ma), ~2.5Ga (2534–2491Ma), ~1.95Ga (1969–1938Ma), and ~1.7Ga (1672Ma). These stages are largely consistent with the major tectonothermal events of the northern NCC during Neoarchean–Paleoproterozoic but significantly different from the principal formation period (ca. 2.5–1.9Ga) of Alxa basement rocks. The Southern Langshan is characterized by the presence of late Paleoproterozoic (~1.9Ga) and Neoproterozoic (~0.9Ga) rocks, and the latter is consistent with the Neoproterozoic magmatism in central Alxa Block, which might be formed in a orogenesis-related compressional environment, in contrast to contemporaneous rift-related extensional tectonic setting within the NCC. Therefore, the Main Langshan and the Southern Langshan are most likely closely related to the northern NCC and the Alxa Block, respectively.