Petrogenesis and Tectonic Implications of the Early Triassic Nianzi Adakitic Granite Unit in the Yanshan Fold and Thrust Belt: New Constraints from U‐Pb Geochronology and Sr‐Nd‐Hf Isotopes

Abstract

The Nianzi granite unit, which includes the Nianzi, Xiaolianghou, and Xiawopu granite intrusions, is a significant component in the northern part of the North China Craton (NCC) and is situated in the Yanshan Fold and Thrust Belt (YFTB). However, there is still debate regarding the tectonic evolution history of the YFTB during the late Permian to Triassic period, specifically regarding the timing of subduction and collision between the NCC and Paleo‐Asian ocean. The Nianzi granite unit exhibits unique petrologic, geochronologic, and geochemical signatures that shed light on the tectonic evolution history of the YFTB. This study presents detailed petrology, whole‐rock geochemistry, and Sr‐Nd isotopic, zircon U‐Pb dating, and Lu‐Hf isotopic data of the granites within the Nianzi granite unit. Our findings demonstrate that the granites primarily comprise subhedral K‐feldspar, plagioclase, quartz, minor biotite and hornblende, and accessory titanite, apatite, magnetite, and zircon. Zircon U‐Pb dating indicates that the Xiaolianghou granite was emplaced at 247.5 ± 0.62 Ma. Additionally, the adakitic characteristics of the Nianzi, Xiawopu, and Xiaolianghou granite intrusions, such as high Sr and Ba contents and high ratios of Sr/Y and (La/Yb)N, combined with negative Sr‐Nd and Lu‐Hf isotopes (87Sr/86Sri = 0.705681–0.7057433, ɛNd(t) = −20.97 to −21.98, zircon ɛHf(t) = −9.92 to −20.26), as well as I‐type granite features of high SiO2, Na2O, and K2O/Na2O ratios, enriched Rb, K, Sr, and Ba, and depleted Th, U, Nb, Ta, P, and Ti, suggest that the Nianzi granite unit was mainly derived from the partial melting of a thickened lower crust containing hydrous, calc‐alkaline to high‐K calc‐alkaline, mafic to intermediate metamorphic rocks. In light of these parameters, we further integrate our data with previous studies and conclude that the Nianzi granite unit was generated in a post‐collisional extensional environment during the Early Triassic.