Early Devonian Post-collisional Granitic Magmatism in the North Qilian Orogenic Belt, Western China: Insights into Lithospheric Delamination and Orogenic Collapse

Post-collisional magmatism contains important clues for understanding the reworking and growth of continental crust, as well as lithospheric delamination and orogenic collapse. Early Devonian magmatism has been identified in the North Qilian Orogenic Belt (NQOB). This paper reports an integrated study of petrology, whole-rock geochemistry, Sm-Nd isotope and zircon U-Pb dating, as well as Lu-Hf isotopic data, for two Early Devonian intrusive plutons. The Yongchang and Chijin granites yield zircon U-Pb ages of 394-407 Ma and 414 Ma, respectively. Both of them are characterized by weakly peraluminous to metaluminous without typical aluminium-rich minerals, LREE-enriched patterns with negative Eu anomalies and a negative correlation between P2O5 and SiO2 contents, consistent with geochemical features of I-type granitoids. Zircons from the studied granites display negative to weak positive epsilon(Hf)(t) values (-5.7 to 2.1), which agree well with those of negative epsilon(Nd)(t) values (-6.4 to -2.9) for the whole-rock samples, indicating that they were derived from the partial melting of Mesoproterozoic crust. Furthermore, low Sr/Y ratios (1.13-21.28) and high zircon saturation temperatures (745 degrees C to 839 degrees C, with the majority being >800 degrees C) demonstrated a relatively shallow depth level below the garnet stability field and an additional heat source. Taken together, the Early Devonian granitic magmatism could have been produced by the partial melting of ancient crustal materials heated by mantle-derived magmas at high-temperature and low-pressure conditions during post-collisional extensional collapse. The data obtained in this study, when viewed in conjunction with previous studies, provides more information about the tectonic processes that followed the closure of the North Qilian Ocean. The tectonic transition from continental collision to post-collisional delamination could be constrained to similar to 430 Ma, which is provided by the sudden decrease of Sr/Y and La/Yb ratios and an increase in zircon epsilon(Hf)(t) values for granitoids. A two-stage tectonic evolution model from continental collision to post-collisional extensional collapse for the NQOB includes (a) continental collision and crustal thickening during ca. 455-430 Ma, characterized by granulite-facies metamorphism and widespread low-Mg adakitic magmatism; (b) post-collisional delamination of thickened continental crust and extensional collapse of orogen during ca. 430-390 Ma, provided by coeval high-Mg adakitic magmatism, A-type granites and I-type granitoids with low Sr-Y ratios.