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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]

Tonalitic–trondhjemitic–granodiorite (TTG)-like leucosomes, which are formed from anatectic melts, record crustal growth and evolution and thus provide constraints on the process of anatexis. However, potential Mg isotope disequilibrium during crustal anatexis makes interpreting the genesis of anatectic melts challenging, thereby potentially hindering the application of the Mg isotope system in investigations of crustal evolution and crustal anatexis. A key factor in Mg isotope fractionation during anatexis is the behaviour of garnet, which becomes preferentially enriched in light Mg isotopes compared with other Mg-hosting minerals during partial melting processes. In this study, a combination of petrography, whole-rock geochemistry, and mineral microanalysis reveals that garnets in leucosomes from the Dulan area of the North Qaidam orogen, were formed by dehydration melting of hornblende. Garnet-bearing leucosomes yield low δ26Mg values (−0.70‰ to −0.33‰) compared with garnet-free leucosomes (−0.19‰ to −0.16‰) and global arc rocks (−0.35‰ to 0.06‰). The different Mg isotopic compositions are attributed to the effect of peritectic garnets on the Mg isotope system in anatectic melts. Quantitative modelling further indicates that the lower Mg isotopic values of garnet-bearing leucosomes require the incorporation of ∼5% peritectic garnets into anatectic melts during magma ascent. Our results demonstrate the influence of peritectic garnets on the Mg stable isotope system and highlight that this influence on the Mg isotopic compositions of peritectic minerals during crustal anatexis must be considered in interpretations of crustal evolution using the Mg isotope system. • The Grt-bearing TTG-like leucosomes show low δ26Mg values than Grt-free ones. • The Grt-bearing leucosomes could be explained by incongruent melting and garnet entrainment. • Garnet entrainment is first-order factor of δ26Mg values of TTG-like melts. [ABSTRACT FROM AUTHOR]

Magma mingling between mafic and felsic magmas is a plausible mechanism for the formation of mafic microgranular enclaves (MMEs) which are common worldwide. Through dissecting the MMEs with parentage of magma mingling, it can be ascertained how two types of magmas with contrasting physicochemical properties interplay and what implicit information magma mingling records in crust-mantle interaction. Here we present petrography, geochronology, major and trace elements, Sr-Nd-Hf isotopes, and mineral geochemistry for rock associations from the North Qaidam orogen, which provides new insights into the processes of magma mingling, petrogenesis of large-volume host plagiogranites and crust-mantle magmatic interaction. MORB (mid-ocean ridge basalt)-like and arc-like magmas can be distinguished from mantle-derived mafic samples with zircon U Pb ages ranging from 480 to 465 Ma. MORB-like magmas originated from a MORB-type asthenospheric mantle, whereas arc-like magmas were derived from an ultra-depleted asthenospheric mantle as a result of consecutive extraction of MORB-like magmas and late-stage metasomatism by subduction-related aqueous fluids. Given the content discrepancies for major oxides, gabbro-dioritic MMEs are formed by incipient mechanical mingling between gabbro-diorite magmas and felsic plagiogranite magmas. MMEs and host plagiogranites yield coincident crystallization ages of 478 Ma and overlapped whole-rock Sr Nd ((87Sr/86Sr) i = 0.7044–0.7052; ε Nd (t) = +4.6 ∼ +5.6) and zircon Hf isotopes (ε Hf (t) = +13 ∼ +16). The host plagiogranite magmas are most likely derived by partial melting of pre-existing analogues of gabbro-diorite intrusions under the condition of consecutive underplating of different batches of cognate magmas over short time scales, which in turn determine the consistent isotope compositions between MMEs and host plagiogranites. As for mineral geochemistry, the plagioclases with the highest An% value and the hornblendes implying the highest melt water content, appear within the MME-bearing plagiogranites next to the chilled margin of MMEs, illustrating that aqueous fluid and heat transfer play a pioneering but finite role in incipient magma mingling. The consecutive underplating of cognate mantle-derived magmas as an ultimate engine leads to the partial melting of newly emplaced basites to form massive plagiogranite magmas and geochemically stratified crust. With magma ascent into a shallow level of crust, magma mingling between underplated mafic magmas and resultant plagiogranite magmas is finally realized. • Enclaves originated from an ultra-depleted mantle metasomatized by subduction fluids. • Finite transfer of fluid and heat appeared from enclaves to hosts in magma mingling. • Consecutive underplating of cognate magmas resulted in overlapped isotopic signatures between enclaves and hosts. • Large-volume host plagiogranites derived from partial melting of antecedent basites. [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]

• The Baibuziwan area has experienced three stages of metamorphism and anatexis at ~1.95 Ga, ~1.90 Ga, ~1.85 Ga, respectively. • It is confirmed for the first time that Baibuziwan migmatites were formed via Late-Paleoproterozoic fluid-fluxed melting. • The lithosphere after collisional orogeny of the Khondalite Belt may have experienced repeated asthenospheric upwelling. The metamorphism and anatexis of the Khondalite Belt may provide key information for the Late-Paleoproterozoic collisional orogeny of the North China Craton (NCC). Migmatites formed by anatexis are widely developed in the Daqingshan Complex, central Khondakite Belt. However, the timing and anatectic regime of these migmatites have not been constrained. In this paper, we present systematic petrographic, whole-rock geochemical, Sr-Nd isotopic, zircon U-Pb and Hf isotopic investigation of the Baibuziwan stromatic migmatite and different types of leucosome in the northwestern Daqingshan Complex. The results show that: (1) the metamorphic rocks in Baibuziwan underwent in-situ to in-source partial melting, and the anatectic melts migrated and gathered in different degrees; (2) the ∼2.37 Ga Bt-Hbl-Pl gneiss in the Daqingshan Complex is possible protolith of the Baibuziwan migmatites; (3) the Baibuziwan migmatites have experienced three stages of metamorphism and anatexis in the Late-Paleoproterozoic, i.e., ∼1.95 Ga, ∼1.90 Ga, ∼1.85 Ga; (4) the anatectic regime of the Baibuziwan migmatites is fluid-fluxed melting, which has been proposed for the first time in Daqingshan Complex. Combined with previous studies, we suggest that following the collisional orogeny, the thinned lithosphere of the Khondalite Belt may have experienced asthenospheric upwelling. The lower crust of the Daqingshan complex was heated and dehydrated repeatedly, and the generated fluid poured into the overlying crust, which eventually led to the episodic metamorphism and fluid-fluxed melting in the Baibuziwan area. [ABSTRACT FROM AUTHOR]