Crust-Mantle Interaction During Continental Deep Subduction: Evidence From Light Mo Isotopes in Post-Collisional Mafic Igneous Rocks From the East Kunlun Orogen

It is intriguing whether and how crust-mantle interaction would proceed in continental subduction zones. Here, we report zircon U-Pb ages and Hf isotopes, major and trace elements, and Sr-Nd-Mo isotopes for Late Paleozoic mafic igneous rocks from the East Kunlun orogen (EKO). Zircon U–Pb dating of these rocks yields concordant ages of 384–387 Ma, corresponding to post-collisional magmatism in the EKO. The mafic rocks are characterized by arc-type trace element distribution patterns and slightly enriched Sr-Nd isotopes, indicating their derivation from a fertile mantle source with the involvement of crustal material. Their high Th/La ratios (0.28–0.77) are inconsistent with the recycling of typical oceanic crust but can be related to continental crust-derived melts with residual allanite during crustal anatexis. This is further strengthened by the similarity in Sr-Nd isotopes and zircon Hf isotopes between these mafic rocks and gneiss that represent ancient continental crustal components in the EKO. Notably, the post-collisional mafic rocks have variably low δ⁹⁸Mo values of −0.64‰ to −0.07‰. Combined with the enriched Sr-Nd isotopes and high Th/La features, it is inferred that the light Mo isotopic signatures are inherited from the deeply subducted continental crust. Given that continental crust is generally characterized by high δ⁹⁸Mo values of 0.10–0.40‰, the light Mo isotope compositions of the mafic rocks should be inherited from the previously dehydrated continental crust-derived melts. Our study highlights that the light Mo isotope features can be used to decipher the recycling of continental crust in collisional orogens.