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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 εHf(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]

• The South Orogen underwent early post-collision during 1.90–1.85 Ga, followed by late post-collision during 1.80–1.73 Ga. • The North Orogen experienced early post-collision at 1.80–1.73 Ga and then entered the late post-collision at 1.67–1.60 Ga. • Finally, the South Orogen and North Orogen simultaneously entered the initial breakup stage during 1.55–1.47 Ga. The Tarim Craton located in the northwest of China, where the Paleo-Mesoproterozoic extensional magmatism related to the Columbia supercontinent is widely developed. However, the process and time of the extension and initial breakup of the Tarim Craton are still controversial during Paleo-Mesoproterozoic era. In this contribution, we present a systematic petrographic, geochemical, and zircon U-Pb-Hf investigation on Aketashitage granite dykes, Wulan granitic gneisses and Astingbulake metadiabases from the Tarim Craton. Results from our study indicate that the ca. 1.88–1.86 Ga Aketashitage granite dykes belong to I-type granites, which were probably derived from low degree partial melting of thickened Archean crust during early post-collisional setting. The ca. 1.55 Ga Wulan granitic gneisses also show the characteristics of I-type granites, which mainly came from partial melting of mafic lower crust with minor mantle-derived magma input during the rift setting. The ca. 1.55 Ga protoliths of Astingbulake metadiabases show affinity to ocean island basalts and/or continental flood basalts and were produced by partial melting of enriched continental lithospheric mantle under continental rifting setting. In conjunction with previous studies, it can be concluded that the Tarim Craton experienced episodic extension to initial breakup: the South Orogen underwent early post-collisional extension during ca. 1.90–1.85 Ga, followed by late post-collisional extension during ca. 1.80–1.73 Ga; the North Orogen experienced early post-collisional extension at ca. 1.80–1.73 Ga and then entered the late post-collisional extension at ca. 1.67–1.60 Ga; finally, the South Orogen and North Orogen simultaneously entered the initial breakup stage during ca. 1.55–1.47 Ga. Based on a comparison of the global Mesoproterozoic (ca. 1.58–1.45 Ga) magmatic activities as a response to the initial break up of the Columbia supercontinent, we suggest that the Tarim Craton (including the Oulongbuluke Block) and North China Craton were close to India, Australia, Yangtze, Siberia, Congo, São Francisco and West African. [ABSTRACT FROM AUTHOR]

Compared with the statement of the mantle plume hypothesis for large-scale plus high-volume intraplate magmatism, a series of OIB-like mafic rocks which are low-volume and spatially closely related to subduction zones still remain mysterious and inapposite to be explained by using traditional mantle plume hypothesis. In the North Qaidam, most previous studies have focused on ultra-high pressure metamorphic rocks and have paid little attention to mafic magmatism as a mechanism of generating magma with a mantle signature. The Kendelong (KDL) metagabbro with consistently intrusive ages of 485–490 Ma within the Olongbuluke microblock that is overriding plate above the subduction zone is analogous to Oceanic Island Basalts (OIBs) in geochemistry and formed contemporaneously with oceanic subduction. Analyzed samples of the KDL metagabbro have sporadic (87Sr/86Sr) i ratios of 0.7008–0.7063 that have been influenced by paulopost alteration, relatively uniform positive ε Nd (t) values of +0.3 to +1.4, zircon ε Hf (t) values of +1.5 to +6.8, and coupled Nd–Hf isotopes that lie on the terrestrial array. After ruling out the possibility of contributions from subducted enriched components and mantle metasomatites, geochemical features and isotopic compositions indicate that the magmas were derived from a relatively primitive asthenosphere without prior large-volume melt extraction. Rare earth element modeling shows that a low degree of partial melting (<10%) of an asthenospheric mantle source in the garnet stability field should be responsible for the magmatic origin of the KDL OIB-like metagabbro. Considering the temporal and spatial correlations between the KDL metagabbro and other types of mafic rock in adjacent regions, we propose that decompression melting of upwelling asthenosphere triggered by rollback of subducted oceanic slab is a plausible regime to explain the petrogenesis of the early Paleozoic KDL OIB-like metagabbro and to trace a fortuitous tectonic transition event. Moreover, even if the OIB-like magmatism is spatially related to subduction zones, the participation of subduction materials could not be an exclusive prerequisite to sentence its genesis. [Display omitted] • OIB-like magmation is reported in the North Qaidam. • OIB-like magmation in the North Qaidam is related to plate tectonics. • Magmas were derived from decompression-induced melting of upwelling asthenosphere. • Slab only provided geodynamic regime for the Kendelong OIB-like metagabbro. [ABSTRACT FROM AUTHOR]