Affiliation:
1. College of Earth Science and Engineering Shandong University of Science and Technology Qingdao 266590 China
2. Nanjing Center, China Geological Survey Nanjing 210016 China
3. Key Lab of Submarine Geosciences and Prospecting Techniques, MOE, Institute for Advanced Ocean Study, College of Marine Geosciences Ocean University of China Qingdao 266100 China
4. Sinopec Qingdao Liquefied Natural Gas Co., Ltd. Qingdao 266400 China
Abstract
AbstractPost‐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 was identified in the North Qilian Orogenic Belt (NQOB). This paper carries out 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 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 ɛHf(t) values (‐5.7 to 2.1), which agree well with those of negative ɛNd(t) values (‐6.4 to ‐2.9) for the whole‐rock samples, indicating derived from partial melting of Mesoproterozoic crust. Besides, low Sr/Y ratios (1.13‐21.28) and high zircon saturation temperatures (745 to 839 °C with majority being >800°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 be produced by 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, combining with previous studies, provide more information about the tectonic processes after the closure of the North Qilian Ocean. The tectonic transition from continental collision to post‐collisional delamination could be constrained at ∼430 Ma, which is provided by the sudden decrease of Sr/Y and La/Yb ratios and increase of zircon ɛHf(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.