Secular mantle wedge evolution due to variable slab fluid metasomatism: Evidence from Permian-Triassic mafic igneous rocks in the Paleo-Tethys tectonic regime

Orogenic mafic magmatism provides us an excellent opportunity to study the temporal and compositional evolution of the mantle wedge above the subducting oceanic slabs. Here, we synthesize geochemical data for Permian-Triassic mafic igneous rocks from the East Kunlun Orogenic Belt (EKOB) in Western China, which provides new insights into the secular evolution of the mantle wedge during the Paleo-Tethys oceanic subduction in the Paleo-Tethys tectonic regime. We find that the Permian-Triassic mafic igneous rocks exhibit systematic changes in their geochemical compositions with temporal evolution. The decreasing Ba/Th (7.6-572), Pb/Ce (0.07-1.9) and Sr/Nd (2.8-130) ratios and increasing La/Sm (1.3-9.8) and Th/Yb (0.1-6.1) ratios of these Permian-Triassic mafic igneous rocks correspond with the temporal evolution. More importantly, the radiogenic Nd-Hf isotopes (epsilon Nd(t) = - 7.9-4.2, epsilon Hf(t) = - 12.3-18.9) show a progressively enriched trend with time. These data indicate that their mantle sources were continuously metasomatized by the subducting slab-derived fluids, whose compositions are generally transitional from igneous crust-derived aqueous solutions to sediment-derived hydrous melts. Meanwhile, the mantle source composition is generally transitional from hydrous peridotite to pyroxenite. Therefore, these Permian-Triassic mafic igneous rocks not only record the long-term evolution of mantle wedge properties, but also infer the continuous slab fluid evolution and associated crust-mantle interaction during the Paleo-Tethys oceanic subduction.