Zircon U-Pb geochronology and Lu-Hf isotope of the Jinghong Paleng pluton in southern Lancangjiang tectonic belt, and its constraints on the tectonic evolution of the Paleo-Tethys

Outcropped in the western Damenglong Town of Jinghong City, the Paleng pluton is lithologically dominated by diorite in which several small gabbro, amphibolite, pyroxenite and hornblende peridotite plutons are emplaced. By means of LA-ICP-MS zircon U-Pb method, gabbro, tonalite and amphibolite in the Paleng pluton were dated, and yielded crystallization age of 252.5±1.6 Ma, 264.3±2.0 Ma and 273.6±1.9 Ma, respectively, which indicates that the Paleng pluton was emplaced in Middle-Late Permian. Compared with other plutons along this tectonic belt, it was emplaced relatively late, and might be formed from weaker differentiation in the late stage of ocean basin subduction. The study of whole rock major and trace elements shows that both the diorite and gabbro have the geochemical characteristics of typical island-arc magmatism, indicating that the pluton was formed in the oceanic island arc environment related to plate subduction. The initial Hf isotope ratios(¹⁷⁶Hf/¹⁷⁷Hf)_i of the zircons from amphibolites range from 0.282833 to 0.282858, with corresponding ε_Hf(t) values of 8.8~13.2. These zircons have depleted mantle Hf model ages(t_DM1) of 416~499 Ma, with an average of 470 Ma, obviously older than the crystallization of the original rocks(273.6 Ma). Combined with the analysis of regional geological data, the subduction of the ancient Tethys Ocean(Changning-Menglian Ocean) in western Yunnan began in the Late Carboniferous, and the ocean basin closed and entered the arc-continent collision orogenic stage until the end of the Late Permian.