Multi-phase Paleozoic magmatism in the North Qaidam ultrahigh-pressure metamorphic units, NW China: implications for transition from continental collision to extensional collapse.

The North Qaidam subduction-collision zone is a typical Alpine-type ultra-high pressure (UHP) metamorphic zone due to the Early Palaeozoic subduction of the Qaidam Block beneath the Oulongbuluke Block, with an integrated record of oceanic subduction, continental subduction, collision, and exhumation to post-collisional extensional collapse. In this study, systemic petrology, zircon U-Pb geochronology, Hf isotopic data and whole-rock geochemical analyses have been carried out for granitoids in the North Qaidam UHP metamorphic units to determine their petrogenesis and further reveal the tectonic evolution of the continental collision orogen. Zircon U-Pb dating yields weighted mean ages of 433 ± 2 Ma, 351 ± 8 Ma, 416 ± 4 Ma, 389 ± 3 Ma and 383 ± 1 Ma, with the ε_Hf(t) values of the latter three ages ranging from −2.4 to 3.3, −5.9 to 1.1 and −3.3 to 2.4, respectively. Based on field relationship, petrological, geochronological, and geochemical results, these granitoids could be divided three groups. The early group granodiorites (~433 Ma) show a marked chemical resemblance to adakites: (1) high La/Yb and Sr/Y ratios, with low Y and HREE values; (2) high Al₂O₃ and low Mg^#, Cr, and Ni values with obvious positive Eu anomalies. Petrological and geochemical characters indicate that the early group adaktic granodiorites may be derived from partial melting of metabasite in a thickened mafic lower crust. The middle group granites (~416 Ma) have an affinity to I-type granitoids, characterized by high Na₂O contents (>4.18 wt.%) and low A/CNK values, which were probably generated by partial melting of felsic gneiss of subducted continental crust. The late group granites (389 ~ 351 Ma) are also I-type granitoids, which were originated from partial melting of Mesoproterozoic crust, with minor mantle material input. According to a comprehensive study of these granitoids, along with regional geological background and HP-UHP metamorphism, a four-stage evolutionary process of the North Qaidam UHP metamorphic belt from oceanic subduction, continental subduction, collision to extensional collapse is proposed. After the final closure of the Proto-Tethys Ocean, the subducted continental crust experienced HP-UHP eclogite-faces metamorphism (460 ~ 430 Ma), and thickening of the lower crust lead to generation of adakites and S-type granites. Following break-off of the subducting oceanic slab (425 ~ 400 Ma), the reduction in negative buoyancy caused the exhumation of deeply subducted continental crust accompanied by associated anataxis and syn-collisional I-type granitoids, representing the transition in tectonic regime from compression to extension. Due to the tectonic transition, mantle-derived intrusions and post-collisional granitoids were emplaced during 400 ~ 350 Ma. Simultaneously, the pronounced decreasing in La/Yb and Sr/Y ratios of magmatic rocks (age of 460 ~ 350 Ma) suggested that the crustal thickness decreased gradually from syn-collisional to post-collisional stage. [ABSTRACT FROM AUTHOR]