Syn- and post-collisional mafic igneous rocks from the East Kunlun Orogenic Belt: Implications for recycling of subducting continental crust during the Silurian-Devonian

Abstract

Syn- to post-collisional mafic igneous rocks are widely distributed along continental collision orogenic belts and can be used to constrain the nature of the orogenic mantle and relevant chemical geodynamic processes during orogenesis. However, there is uncertainty about the mechanisms of recycling of continental crustal materials in continental subduction channels. We report chronological and geochemical data for syn- and post-collisional mafic dikes from the East Kunlun Orogenic Belt (EKOB), western China, which provide insights into the processes of geochemical transfer and geodynamic evolution of this fossil orogen. Zircon UPb dating of the mafic dikes yields ages of 433 Ma and 409–407 Ma, corresponding to syn- and post-collisional magmatism, respectively. The rocks exhibit typical arc-like trace-element characteristics and weakly enriched Sr–Nd–Hf isotopic compositions, as well as variable zircon O isotopic compositions. These geochemical characteristics are comparable to those of Neoproterozoic paragneiss influenced by Early Paleozoic metamorphism associated with high–ultrahigh-pressure eclogitization in the EKOB and indicate that paragneiss-derived hydrous melts were likely involved in the mantle source of the studied mafic igneous rocks during continental collision. This qualitative interpretation is supported by quantitative modeling of the geochemical transfer in the continental subduction zone. We propose that the syn-collisional mafic igneous rocks were formed during the continental collision between Central Kunlun and South Kunlun Belts, whereas the post-collisional mafic igneous rocks formed in an extensional regime associated with orogenic lithosphere thinning and asthenospheric upwelling.