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• Five stages of magmastism occurred along both sides of the BNS. • The Early Jurassic and the Late Cretaceous granitoids are produced by roll back and extension-collapse of the Tongka micro-block, separately. • The collision between Lhasa and Qiangtang terranes may continue to the latest Cretaceous (<74 Ma). • Ophiolites north of Amdo, Baqing and Dengqen were possiable lithospheric remanants of the LSO. The Tongka micro-block, located in the eastern section of the Bangong-Nujiang suture, and characterized by Mesozoic granitoids and high grade metamorphic rocks, holds the key to the evolution of the Bangong-Nujiang ocean (BNO). Here we present a systematic petrologic, geochemical, zircon U-Pb-Hf, and whole rock Sr-Nd-Pb isotopic investigation on six new granitoids in Tongka. The results suggest these granitoids yielded two stages of crystallization (171–177 Ma and 74 Ma) and two stages of metamorphism (201–215 Ma and 170–175 Ma). Combined with the existing work within BNS, the magmatism along the BNS can be divided into five stages: stage 1 (240–250 Ma), stage 2 (200–230 Ma), stage 3 (150–190 Ma), stage 4 (100–135 Ma) and stage 5 (73–100 Ma). Both the Early Jurassic and Late Cretaceous Tongka granitoids in this study are high-K calc-alkaline series, with enrichments in large ion lithophile elements (e.g., Cs, Rb, and K) and depletions of large ion lithophile elements (e.g., Nb, Ta, and Ti). The variable ε Hf (t) values (−17.3 to −7.6), ε Nd (t) values (−12.08 to −11.15) and ancient T DM2 (1.48–2.00 Ga) for the Early Jurassic granitoids and weakly negative ε Hf (t) values (−6.3 to −1.5), ε Nd (t) values (−8.75 to −8.60) and old T DM2 ages (1.07–1.33 Ga) for the Late Cretaceous granitoids, suggest that they are both mainly derived from the melting of ancient crust with minor mantle material input. However, the formation of the Early Jurassic and Late Cretaceous granitoids in Tongka is related to different tectonic settings: roll back after the Tongka micro-block subducted beneath the southern Qiangtang subterrane and extension-collapse after the Tongka micro-block collided with the northern Lhasa subterrane. The initial subduction of the BNO may start from 190 Ma, and the collision between the Lhasa and Qiangtang terranes may continute to the latest Cretaceous (<74 Ma). Ophiolites north of Amdo, Baqing and Dengqen were possiable lithospheric remanants of the Longmu Tso-Shuanghu ocean (LSO) rather than the BNO. [ABSTRACT FROM AUTHOR]

Highlights • Grenvillian orogeny of Rodinia in Oulongbuluke Block systematically investigated. • Metabasite, augen granitic gneiss, paragneiss ages of 1132 ± 33, 1118 ± 9, 1124 ± 12 Ma. • Peak metamorphic P-T conditions of 4.8–5.8 kbar and 652–700 °C. • Metabasites resemble the geochemical characteristics of Andean arc magmatism. • Oulongbuluke Block likely temporarily connected to Qilian Block and Tarim Craton. Abstract A systematic petrologic, geochemical, and zircon U-Pb and Lu-Hf isotopic investigation of an ∼1.1 Ga arc magmatism and metamorphism was studied to evaluate the Grenvillian orogeny and assembly of Rodinia in the Oulongbuluke Block in NW China. The peak metamorphic P-T conditions of the paragneiss were restricted to 4.8–5.8 kbar and 652–700 °C based on P-T pseudosection and conventional geothermobarometry. Zircon U-Pb ages of 1132 ± 33 Ma, 1118 ± 9 Ma, and 1124 ± 12 Ma were determined for the metabasite, augen granitic gneiss, and paragneiss, respectively, indicating magmatic intrusion and a metamorphism event in the late Mesoproterozoic. The metabasites exhibited enrichment of light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and depletion of heavy rare earth elements (HREEs) and high-field-strength elements (HFSEs), resembling the geochemical characteristics of Andean arc magmatism. The whole-rock geochemistry and in-situ zircon ε Hf (t) values of 4.3 and 11.2 suggested that the protolithic mafic magma originated from a subduction-modified lithospheric mantle. The low zircon ε Hf (t) values of −5.2 and −0.5 also suggested that the calc-alkaline augen granitic gneisses were derived from the partial melting of a late Paleoproterozoic crustal source. The ∼1.1 Ga arc-related magmatism and metamorphism in the Oulongbuluke Block indicated the occurence of a tectonic-thermal event in an Andean-type active continental margin in the late Mesoproterozoic. In this regard, the Oulongbuluke Block was likely temporarily connected to the Qilian Block and Tarim Craton, forming a single continuous block with similar Grenville age during the late Mesoproterozoic to early Neoproterozoic (1.1–0.9 Ga). [ABSTRACT FROM AUTHOR]

[Display omitted] • Early Paleozoic syn-collisional I-type granitoids in the North Qaidam orogen reflect crustal reworking rather than growth. • Early Paleozoic granitoid magmas originated from Proterozoic and Paleozoic crustal sources in the North Qaidam orogen. • Inhomogenous thicknesses of overriding plate triggered by syn-collision induced hot mantle convection and granitoid incubation. Granitoids constitute the major theme in evaluating the growth and reworking of continental crust on Earth. Here we present U-Pb geochronology, geochemistry and Sr-Nd-Hf isotopes of the early Paleozoic granitoids emplaced during pre- and syn-collisional stages in the North Qaidam orogen to gain insights on the granitoid typology, genetic mechanism, as well as the implications for the evolution of continental crust. The pre- and syn-collisional granitoids in this region all belong to I-type granite and are derived from different continental crustal sources including late Mesoproterozoic to early Neoproterozoic metamorphic crystalline basement, and juvenile continental crust formed during early Paleozoic oceanic subduction. Granitic magmas derived from the two sources underwent a series of magmatic processes such as mixing or assimilation, which lead to the transitional geochemical and isotopic features, suggesting that besides source components, magmatic processes from melt extraction to granitoid emplacement also exerted an important influence on the formation of these granitoids and crustal maturation. Although S-type granitoids are commonly taken as the fingerprint for continental collision, our study emphasizes that the role of I-type granites formed in syn-collisional setting should not be underestimated. Besides subducted slab processes such as roll-back, retreat, or break-off, we propose a novel geodynamic model which envisages that the lateral inhomogeneity in lithospheric thickness triggered by continental collision within the overriding plate above subduction zone controlled the magmatism. Both growth and reworking of continental crust occur during oceanic subduction, whereas syn-collision setting is dominated by continental crust reworking as in the North Qaidam orogen. [ABSTRACT FROM AUTHOR]

• Cretaceous granitic intrusions formed by three magmatic events in SE China. • The intensity of crust–mantle interactions increased gradually from 145 Ma to 86 Ma. • These granitic intrusions are related to Paleo-Pacific subduction or rollback. The geodynamic transformation of the Cathaysia Block and its relationship with the Cretaceous granitic intrusions have been controversial in recent decades. In this study, petrology, zircon U–Pb ages, Lu–Hf isotopic composition, whole-rock geochemistry and Sr–Nd isotopes were carried on Cretaceous granites that were collected from the Fujian province, SE China. Combining previously published ages with the new ages obtained in this study indicates that the magmatism in the study area occurred in three distinct stages at 145–137 Ma, 136–118 Ma and 107–86 Ma. The first stage I-type granites were intruded at 137 ± 1 Ma and 145 ± 1 Ma, which are characterized by low ε Nd (t) values of −7.3 to −6.8 and ε Hf (t) values of −12.5 to −2.5. It was formed by partial melting of Mesoproterozoic medium-to-high K basaltic rocks in the thickened crust of ~40 km without significant hybridization of mantle-derived magmas and fractional crystallization. The second stage highly differentiated I-type granites yield zircon U-Pb ages of 125 ± 1 Ma and 128 ± 1 Ma, which contain slightly negative ε Nd (t) values of −5.4 to −5.3 and ε Hf (t) values of −8.4 to −3.2. It was generated by partial melting of the Mesoproterozoic basement at the depth of 30–40 km triggered by underplating of a small amount of mantle-derived magma, with primary crustal melts undergoing plagioclase-dominated fractionation crystallization. The third stage I-type granites yield zircon U-Pb ages of 104 ± 1 Ma and 105 ± 1 Ma, which contain ε Nd (t) values of −1.7 to −7.3 and ε Hf (t) values of −6.0 to −0.5. It was derived from partial melting of Mesoproterozoic basaltic source at a depth of ~30 km. Besides, a mass of mantle-derived magma was mixed into the crustal melts, accompanied with plagioclase-dominated fractionation crystallization. Combining previously published data and those obtained in this study provides significant information for the tectonic evolution of Paleo-Pacific. In stage 1 (145–137 Ma), the existence of I-type granites and adakitic rocks suggests that the dip-angle subduction of the Paleo-Pacific Plate led to the partial melting of the thickened lower crust under the compressional environment. In stage 2 (136–118 Ma), the generation of I-type and A-type granitic magmas imply the thinning of the lithospheric crust under the extensional environment and the upwelling of the asthenospheric mantle, which are caused by slab rollback of Paleo-Pacific. In stage 3 (107–86 Ma), the appearance of much more I-type and A-type granites indicates that the break-off of the Paleo-Pacific plate led to much stronger crust–mantle interactions under the extensional environment. [ABSTRACT FROM AUTHOR]