Petrogenesis and tectonic regime transition of Late Palaeozoic to Early Mesozoic multiphase alkaline rocks in Central Jilin Province, NE China

The Central Jilin region located within the tectonic belt between the eastern Central Asian Orogenic Belt (CAOB) and the northern North China Craton (NCC), records multiple stages of tectonic overprinting by Palaeo-Asian Ocean and Palaeo-Pacific systems, which generated abundant alkaline rocks. Despite significant progress, two main issues remain unresolved: (1) the petrogenetic mechanisms and magma source characteristics of alkaline rocks, and (2) the controversial timing of the tectonic transition from Palaeo-Asian Ocean closure to Palaeo-Pacific Ocean subduction. This study analyzes nine Late Palaeozoic-Early Mesozoic alkaline granites through zircon U-Pb dating, whole-rock geochemistry, and Hf isotopes. LA-ICP-MS dating identifies three emplacement phases: Permian (261.0–251.6 Ma), Middle Triassic (245.4–237.0 Ma), and Middle Jurassic (174.0 Ma). Geochemical data confirm that all samples are A-type granites or adakite, with εHf(t) values ranging from −1.91 to +9.82, indicating they originated from Meso-Neoproterozoic mafic, thickened lower crust through partial melting under high-temperature ( > 650°C), high fO2 (ΔFMQ = −0.56 to +4.83), and low-H2O (average 2.91 wt%) conditions. Permian intrusions formed in a Palaeo-Asian Ocean subduction-related back-arc extensional setting, while Middle Triassic intrusions reflect a tectonic transition where early-stage rocks (245.4–261.3 Ma) exhibit extensional features, but terminal-phase intrusions (around 237 Ma) show signs of compressional collisional orogeny, marking the onset of orogenic collision. By integrating regional high-pressure metamorphism and sedimentary hiatuses, this study constrains the final closure of the Palaeo-Asian Ocean in the Central Jilin to the Middle Triassic. Notably, the Middle Jurassic back-arc affinity (174.0 Ma) confirms significant influence from Palaeo-Pacific Plate subduction by the Early Jurassic, demonstrating the initiation of Palaeo-Pacific Plate subduction during this period.