Subducted oceanic crust in the source of post-collisional magmatism: Geochemical constraints from the Early Devonian volcanic rocks in the East Kunlun Orogenic Belt, Northern Tibetan Plateau

The post-collisional volcanic rocks are widely distributed in orogenic belts and may preserve the geochemical signatures of previous crust-mantle interaction between mantle-wedge peridotite and subducting oceanic slab. However, uncertainties persist regarding the mechanisms by which subducted oceanic crustal materials participate in the formation of post-collisional magmas. The nature and quantity of recycled crustal materials in the mantle source of the post-collisional magmas are not well constrained. Here, we present an integrated study of whole-rock major and trace elements and Sr-Nd-Hf isotopes together with zircon UPb dating for the Dagangou volcanic rocks from the East Kunlun Orogenic Belt (EKOB). Our results demonstrated that the Dagangou volcanic rocks from the EKOB were formed at 409.2 ± 3.5 Ma, coeval with the regional post-collisional magmatism. These volcanic rocks were divided into two magma series: calc-alkaline and tholeiitic, with the calc-alkaline lavas erupting slightly earlier than the tholeiitic lavas. The calc-alkaline volcanic rocks display relatively high SiO₂ (51.3–59.9 wt%), La/Sm (3.6–6.1), Th/La (0.2–0.5), ⁸⁷Sr/⁸⁶Sr_i (0.7070 to 0.7234), and low FeOT (5.6–9.0 wt%), Nb/U (2.9–7.8), ε_Nd (−8.85 to −1.17) and ε_Hf (−3.78 − +2.60). In contrast, the tholeiitic volcanic rocks exhibit relatively low SiO₂ (49.3–55.3 wt%), La/Sm (2.0–4.2), Th/La (0.1–0.4), ⁸⁷Sr/⁸⁶Sr_i (0.7043 to 0.7111), and high FeOT (8.0–10.2 wt%), Nb/U (3.3–14.8), ε_Nd (−3.19 − +2.86) and ε_Hf (−0.10 − +8.30). Our modeling calculations suggest that the enriched pyroxene-rich metasomatites in the mantle wedge have played a dominant role (~60–80 %) in the generation of the calc-alkaline volcanic rocks. In contrast, both the depleted peridotite in the asthenospheric mantle and the enriched metasomatites in the mantle wedge have made significant contributions to the formation of tholeiitic volcanic rocks. The Dagangou volcanic rocks from EKOB were generated by partial melting of the depleted peridotite in the asthenospheric mantle and the pyroxene-rich metasomatites in the lithospheric mantle, which was triggered by the detachment of the Proto-Tethyan slab and subsequent upwelling of the asthenospheric mantle.