Spatio-temporal evolution of the Paleo-Tethys in western Yunnan: Insights from mafic rocks in the Lancang tectonic belt

The Sanjiang orogenic belt in the southeastern Tibetan Plateau provides an excellent record of the Paleo-Tethys tectonic evolution. This study introduces new constraints on the spatio-temporal evolution of the Paleo-Tethys through zircon UPb ages, in-situ LuHf isotope data, and analyses of whole-rock major oxides, trace elements, and Sr-Nd-Pb isotopic compositions from Late Paleozoic mafic rocks in the Lancang tectonic belt. The mafic rocks from the Xiaoheijiang, Banpo, and Yakou areas were dated to approximately 281–267 Ma, 295–292 Ma, and 293–291 Ma, respectively. The Xiaoheijiang mafic rocks exhibit geochemical signatures resembling those of fore-arc basalt, characterized by low (La/Sm)_N, relatively flat rare earth element (REE)-normalized patterns, positive ε_Nd(t) (+5.6 to +10.1), and zircon in-situ ε_Hf(t) values (+10.0 to +14.9). These features indicate an origin from a depleted mantle source with minor contributions from slab-derived components. The Banpo and Yakou mafic rocks display geochemical affinities to back-arc basin basalt, similar to the Okinawa Trough back-arc basin basalt. Their ε_Nd(t) values range from +5.6 to +9.9 and ε_Hf(t) values from +9.5 to +15.0, suggesting derivation from a mantle wedge source modified by slab-derived fluids or melts. In combination with available geochronological data concerning the Paleo-Tethys evolution along the Lancang tectonic belt, our findings support the hypothesis that the slow-speed, low-angle subduction of the Paleo-Tethys Ocean led to the formation of a forearc accretionary complex. The slab-derived fluids metasomatized the mantle wedge as subduction depth and angle increased, facilitating the development of the Lincang arc magmatism and the opening of the Banpo-Yakou back-arc basin. Continuous subduction promoted slab retreat under gravitational forces, inducing the upwelling of depleted mantle and the forming of forearc magmas.