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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 ~ 2.7 Ga (2686 Ma), ~ 2.6 Ga (2619–2563 Ma), ~ 2.5 Ga (2534–2491 Ma), ~ 1.95 Ga (1969–1938 Ma), and ~ 1.7 Ga (1672 Ma). 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. 2.5–1.9 Ga) of Alxa basement rocks. The Southern Langshan is characterized by the presence of late Paleoproterozoic (~ 1.9 Ga) and Neoproterozoic (~ 0.9 Ga) 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. [ABSTRACT FROM AUTHOR]

The Langshan area is the northeastern part of the Alxa block and adjacent to the Inner Mongolia-Da Hinggan Orogenic Belt (IMDOB) to the north, and geochronological and geochemical studies of the Langshan plutons would be helpful for unraveling the relationship of the magmatism in the Langshan area with that in the IMDOB. Based on zircon U–Pb ages presented in this study and in published papers, five magmatic stages are recognized in Langshan area: Late Silurian (∼418 Ma), Carboniferous (328–304 Ma), Early Permian (294–272 Ma), Late Permian (260–254 Ma) and Middle-Late Triassic (245–227 Ma). The Late Silurian two-mica granite is peraluminous and has a source from ancient continental crust, indicating a collision event. The late Early Carboniferous mafic plutons are subalkaline, show appinitic affinities, and may originate from the subduction-modified continental lithospheric mantle. The late Early Permian granodiorite is weakly peraluminous and high-K calc-alkaline and may be derived from ancient continental crust, while the late Early Permian alkaline gabbro–diorite displays EM I-like Sr–Nd isotopic signatures, indicating an enriched subcontinental lithospheric mantle source. The Late Permian granodiorite is calc-alkaline and peraluminous and may result from the partial melting of ancient lower continental crust, whereas the Late Permian leuconorite belongs to low-K tholeiitic series and has a cumulate origin from an enriched mantle source with the input of juvenile mantle materials. The Paleozoic–Triassic magmatic stages in the Langshan area are consistent with the main magmatic periods in the northern NCC, which are the Late Silurian to Middle Devonian (∼418–386 Ma) and Early Carboniferous to Late Triassic (∼342–211 Ma, with three magmatic stages of Carboniferous, Permian and Triassic). Especially, the Carboniferous to Late Triassic magmatic period in the northern NCC are closely comparable with that in the IMDOB, indicating the two domains might be involved in similar tectonic processes during Late Paleozoic. [ABSTRACT FROM AUTHOR]