Petrogenesis and tectonic significance of the late Middle Triassic I-type granite in the eastern section of the North Qilian tectonic belt: Constraints on the initiation of post-collision between North China Block and South China Block

Abstract

A significant number of Triassic granitic plutons are exposed at the junction of the Qinling and Qilian Orogenic Belts. The Early Triassic to early Middle Triassic granites are distributed in a linear or zonal fashion, trending northwest–southeast, which aligns with the regional tectonic lines. In contrast, the late Middle Triassic to Late Triassic granites exhibit a more diverse distribution, appearing as isolated points or planes that are not constrained by tectonic unit boundaries. These granites provide essential insights into the tectonic evolution of the Qinling Orogenic Belt and the junction of the Qinling and Qilian Orogenic Belts during the Triassic. In this study, we conducted petrological, geochemical, zircon UPb dating, and LuHf isotope analyses for Longyang I-type monzogranite located in the eastern section of the North Qilian Tectonic Belt. The results indicate that the crystallization age of the Longyang pluton is 237.6 ± 1.3 Ma, corresponding to an emplacement age in the late Middle Triassic. The Longyang pluton belongs to metaluminous (A/CNK = 0.88 to 0.97) and high-K calc-alkaline series. The pluton displays an enriched large-ion lithophile elements and light rare earth elements, depleted high field strength elements and heavy rare earth elements pattern, with no significant Eu anomaly. The Hf isotopic composition of the Longyang pluton reveals ε_Hf(t) values ranging from −5.4 to −0.8, with a two-stage model age (T_DM2) between 1291 and 1580 Ma, indicating that its magmatic source is derived from the partial melting of the Mesoproterozoic thickened lower crust. Combined with the mineralogy, geochemical features and regional tectonic background, we propose a post-collision setting. These findings indicate that the subduction of the A'nimaque–Mianlue Ocean had completed around 237 Ma, marking the onset of a comprehensive transition from compression to extension stage for the North China Block and the South China Block. Furthermore, they suggest that the eastern section of the North Qilian Tectonic Belt entered the post-collision initiation stage in the late Middle Triassic.