Geochronology and Geochemistry of Paleozoic to Mesozoic Granitoids in Western Inner Mongolia, China: Implications for the Tectonic Evolution of the Southern Central Asian Orogenic Belt

Abstract

Situated between the South Tianshan suture zone to the west and the Solonker suture zone to the east, the Yagan and Zhusileng-Hangwula arcs (YZHAs) in western Inner Mongolia in China occupy a critical place to investigate the tectonic history of the middle segment of the southern Central Asian Orogenic Belt (CAOB). In this work, field-based petrological studies and zircon U-Pb dating results reveal several episodes of granitic magmatism from 400 to 230 Ma in the YZHAs. Whole-rock geochemical and zircon Lu-Hf isotopic data indicate that all the 400–230 Ma granitoids underwent intensive fractional crystallization and were generated by magma mixing involving different proportions of mantle- and crust-derived materials. The ∼400 Ma monzogranites show (high-K) calc-alkaline affinities, akin to Stype granitoids. They were most likely generated in a postcollisional setting, corresponding to the assembly of the YZHAs before the Early Devonian. The 298–290 Ma granitoids belong to transitional I/S-type to A-type, whereas the 280–277 Ma granitoids are typical I-type. These Permian granitoids show increasingly evolved zircon εHf(t) values and formed from crust-mantle magma mixing, suggesting an advancing subduction setting. The ∼230 Ma monzogranites exhibiting fairly positive zircon εHf(t) values (16.26 to 110.49) and high contents of mafic compositions and transition elements probably formed in a postcollisional setting after the assembly of the YZHAs and the Alxa Terrane. We infer that the final assembly of the middle segment of the southern CAOB probably occurred in the Early–Middle Permian.