Early Carboniferous within-plate magmatic rocks related to Proto- and Paleo-Tethys evolution in western Yunnan, SW China: Geochronological and geochemical constraints

Western Yunnan in SW China preserves abundant rock records related to the eastern Proto- and Paleo-Tethys oceans, yet the transition between the two oceans remains poorly understood. This paper presents new zircon U–Pb ages and Lu–Hf isotopic data, together with whole-rock elemental and Nd isotopic data for the magmatic rocks from Mengding and Menglong in western Yunnan. The Mengding basaltic rocks show oceanic island basalt geochemical features and have similar whole-rock Nd isotopic compositions with ε_Nd(t) values of +3.5–+4.4 and uniform zircon Lu–Hf compositions with ε_Hf(t) values of +6.1–+8.0, similar to the modern Hainan oceanic island basalts. These rocks yielded a zircon U–Pb age of 341 ± 2 Ma, indicating their eruption in a passive continental rift basin setting during the early Carboniferous. The Menglong mafic rocks yielded a similar zircon U–Pb age of 343 ± 3 Ma. The Menglong mafic–ultramafic rocks display E-MORB geochemical signatures, suggesting a within-plate extensional setting. They exhibit uniform Nd isotopic compositions with ε_Nd(t) values of +4.1–+4.4, whereas their zircon Ɛ_Hf(t) values vary from −19.5 to +11.1, indicating a mixed magma source with contributions from asthenosphere mantle and decompression melting of the metasomatized mantle wedge. Our results, together with the regional geological and stratigraphical evidence, suggest that the Menglong mafic–ultramafic rocks are comparable with the Late Devonian–early Carboniferous magmatic rocks in northern Tibet, eastern Myanmar and northern Thailand, likely formed in a post-collision extensional setting in the aftermath of Proto-Tethys closure. Taking account of the zonal distribution of the Late Devonian–early Carboniferous magmatic rocks, we propose that slab detachment of the Proto-Tethys led to an overall extensional tectonic setting, resulting in a series of rift basins in western Yunnan during the Late Devonian to early Carboniferous. The Paleo-Tethys probably opened on one of the rift basins of the Proto-Tethys.