The transition from oceanic to continental subduction and collision: A case study of the North Qaidam ultrahigh-pressure metamorphic belt, northern Tibetan Plateau. (February 2023)

• Record Type: Journal Article
• Authors: Zhang, Guibin; Han, Lei; Zhang, Lifei; Song, Shuguang; Liu, Shuaiqi
• Is Part Of: Journal of Asian earth sciences. Volume 242(2023)
• Publication Date: 2023-02
• Subjects: Coesite -- Eclogite -- Ultrahigh-pressure metamorphism -- North Qaidam -- Oceanic subduction -- Continental collision; Earth sciences -- Asia -- Periodicals; Sciences de la terre -- Asie -- Périodiques; Earth sciences; Asia; Periodicals; 555.05
• DOI: 10.1016/j.jseaes.2022.105488
• Languages: English
• ISSNs: 1367-9120

Abstract

Highlights: Coesite inclusion and Kfs − Qz pseudomorphs in Yematan eclogite indicate UHP metamorphism; An early Paleozoic ophiolitic belt was reconstructed in the north Dulan terrane; The Yematan eclogite records two stage of eclogite-facies metamorphism at ca. 440 Ma and ca. 420 Ma, respectively; The North Qaidam records a continuous process from oceanic to continental subduction and collision. Abstract: Ultrahigh-pressure (UHP) metamorphism in continental subduction zones initially occurs due to the pull of oceanic subduction, which leads to the deep subduction of continental material. However, petrological evidence of the preceding oceanic subduction is rare, which limits our understanding of the transition from oceanic subduction to continental collision. UHP eclogites with occurrence of coesite inclusions were collected in this study from the northern Dulan terrane in the North Qaidam orogen. These eclogites can be divided into three types according to their protolith features: (1) cumulate gabbro protoliths; (2) normal- to enriched-type mid-ocean ridge basalt; and (3) back-arc basalt contaminated by continental materials. Zircon U–Pb dating of igneous zircon cores shows that the eclogites have a protolith age of 495.9 ± 3.6 Ma. Two types of metamorphic zircon with omphacite inclusions have ages of 440 and 420 Ma, indicating two stages of eclogite-facies metamorphism. Based on these results, we suggest that the studied eclogites represent subducted relics of oceanic crust. Pressure–temperature calculations show that the oceanic eclogites experienced fast subduction during the prograde stage, which was followed by near-isothermal decompression with peak P–T conditions of 31–34 kbar and 635–665 °C. Therefore, the Dulan terrane preserves a continuous record from oceanic subduction to continental collision. The oceanic crust was subducted to a maximum depth at ca. 440 Ma and was then exhumed as a result of serpentinization. The exhumed oceanic crust entrained continental material and was overprinted by eclogite-facies metamorphism at ca. 420 Ma.