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Migmatites can be produced in different stages of orogeny and thus have a great significance to constrain the evolution of an orogenic belt. Migmatites widely occur in the Tongbai orogen, but their formation ages and geological significance have not been well constrained. In this study, a combination of cathodoluminescence (CL) imaging, U-Pb-Hf-O isotopes and trace elements of zircons were carried for migmatites in the Tongbai orogen. Most inherited cores in a melanosome show variable U Pb ages and Hf isotope compositions with high δ18O values, suggesting they might be detrital zircons from its protolith. Whereas other relict cores in this sample have nebulously zoning or no zoning, flat HREE patterns with insignificant Eu anomalies and relatively low formation temperatures, indicating their formation under eclogite-facies conditions. They yield U Pb ages of 228 ± 3 Ma, providing direct evidence for the involvement of Triassic subducted continental materials. The overgrowth rims and some new grown grains in the migmatites show euhedral shapes, low CL luminescence, high Th/U ratios, clear negative Eu anomalies and steep HREE patterns, suggesting they are anatectic zircons formed during partial melting. They yield U Pb ages from 139 ± 2 to 132 ± 1 Ma. Combined with previous studies, we suggest that migmatization mainly occurred at 139–130 Ma in the Tongbai orogen. The anatectic zircons in the leucosomes show much lower δ18O values (5.79 ± 0.23‰ vs. 8.94 ± 0.16‰) and higher 176Hf/177Hf ratios (0.282302–0.282648 vs. 0.281728–0.282504) than the zircons in adjacent melanosomes, suggesting that the leucosomes might be externally sourced. According to a compilation of the geochronological information of migmatite and magmatism, the Tongbai-Dabie-Sulu orogens have a spatial and temporal variation of partial melting and magma activities in the post-collisional stage. These might result from the diachronical tectonic switch and collapse of the orogenic lithospheric keels caused by the subduction and rollback of the Paleo-Pacific slab. • The Tongbai migmatites formed at ca. 139–130 Ma. • There exist spatiotemporal variations of partial melting and magmatism in the Tongbai-Dabie-Sulu orogen. • A diachronous tectonic switch has been documented. [ABSTRACT FROM AUTHOR]

The geodynamic evolution of the Himalayan–Tibetan orogen during the transition from Neo-Tethyan oceanic to Indian continental subduction remains unclear. Therefore, we conducted a new geochronological, geochemical, and Sr–Nd–Pb–Hf isotopic study of post-collisional diabase dikes in the central segment of the southern Lhasa subterrane. The diabase dike samples yield zircon U–Pb ages of 51–49 Myr, consistent with the timing of the Gangdese magmatic flare-up (ca. 53–50 Myr). In addition, the diabase dikes exhibit a geochemical intraplate affinity and depleted Sr–Nd–Pb isotopic compositions. Their variable Nb contents allow, the dikes to be divided into low-Nb (<8 ppm) and high-Nb (>9 ppm) types, which represent the melting products of slab fluid-modified Gangdese lithospheric mantle and a mixed mantle source comprising Gangdese lithospheric and deep asthenospheric mantle, respectively. In addition, the wide variation in εHf(t) values (−10.0 to +12.3) of zircons from these diabase dikes is consistent with the Gangdese zircon Hf isotopic 'pull-down' at ca. 50 Ma. Integrating our new data with published results suggests that the zircon Hf isotopic 'pull-down' was a direct consequence of the involvement of subducted Indian continental components in the formation of the Gangdese continental lithosphere. The combined data further suggest that upwelling of the asthenosphere, triggered by breakoff of the Neo-Tethyan oceanic slab, resulted in extensive mantle and crustal melting in the Lhasa terrane and the Tethyan Himalaya in southern Tibet. • The diabase dikes in the Pengcuolin–Dazhuqu region were emplaced at ca. 50 Ma. • Indian continental components were involved in the Gangdese lithosphere at ca. 50 Ma. • The breakoff of the Neo-Tethyan slab is believed to have occurred at ca. 53–50 Ma. [ABSTRACT FROM AUTHOR]

Characterization of the magmatic response to scissor-like oblique continental collision and of the associated geodynamic mechanisms and crustal evolution is crucial to understanding the formation of collisional orogens. This paper presents geochronological, elemental, and isotopic data of three Triassic mafic–felsic intrusions formed during scissor-like continental collision in the Qinling orogenic belt (QOB), China. The biotite pyroxenite, gabbro, quartz monzonite, and syenite of the Jiuzigou (JZG) mafic–felsic intrusive complex were emplaced at 233–231 Ma and have extremely high Ba and Sr contents, fractionated rare-earth elements, insignificant Eu anomalies, and enriched Sr-Nd isotopes. A negative relationship between ε Nd (t) values and SiO 2 content suggests that the JZG complex was formed by assimilation and fractional crystallization (AFC) processes of enriched-mantle-derived melts. The Pangjiahe (PJH) and Laowangou (LWG) granite porphyries were emplaced at 224 Ma and 215 Ma, respectively, and are characterized by enriched large-ion lithophile elements, depleted high-field-strength elements, and enriched Sr and Nd isotopic compositions (0.70565–0.70648 and − 3.22 to −5.33 respectively), which indicate that these granite porphyries were sourced from mafic lower crust of the South Qinling terrane. The relatively high Mg numbers (45.8–49.4) might indicate mixing of crustal melt and minor mafic magma for the PJH granite porphyries. In contrast, the low Mg numbers (24.8–41.4) and obvious negative Eu anomalies of the LWG granite porphyries suggest an origin by fractional crystallization of crustal melts. The spatial–temporal distribution of the Triassic intrusions emplaced during the continental collision is proposed to have been produced by southeastward slab tearing in the QOB. Slab tearing may be a common geodynamic process during oblique collision generally. The origin and evolution of Triassic mafic and felsic intrusions in the QOB reveal that this oblique-collisional orogenic belt likely underwent a transition from net crustal growth to crustal reworking as collision progressed. • Mafic-felsic complex emplaced at 233–231 Ma was derived from an enriched mantle. • Granite porphyry emplaced at 224 Ma was formed by crustal melting with input of mantle. • Granite porphyry emplaced at 215 Ma was formed by fractionation of crustal melts. • Slab tearing is likely a regime inducing magmatism during oblique continental collision. • The oblique collision is characterized by a transition from crustal growth to reworking. [ABSTRACT FROM AUTHOR]

In this paper, we present zircon U Pb dating, elemental geochemistry, and Sr–Nd–Hf isotopic compositions for Late Carboniferous intrusive rocks in the northern Great Xing'an Range, northeast China. We decipher the mechanisms which triggered lithospheric thinning under microblock-microblock collision and provide a new perspective on the Late Paleozoic evolution of the eastern Central Asian Orogenic Belt. Zircon U Pb dating indicates that Jifeng gabbroic diorites and Xinlin granites were formed during a short time interval at 322–321 Ma, whereas the Xinlin gabbros were crystallized at 300 ± 2 Ma. The Jifeng gabbroic diorites exhibit typical arc characteristics, depleted in HFSEs and enriched in Pb and have positive ε Nd (t) (+1.88 to +2.00) and ε Hf (t) (+6.6 to +10.7) values, suggesting they originated from the partial melting of depleted lithospheric mantle modified by slab fluids. The Xinlin gabbros belong to alkaline series and are characterized by the enrichment in HFSEs with positive Nb and Ta anomalies and depleted Nd Hf isotopic compositions (zircon ε Hf (t) = +12.5 to +18.9, whole rock ε Nd (t) = +8.0 to +8.1). High Nb concentrations and low La/Nb (0.81–0.83) and La/Ta (12.50–14.61) ratios of the Xinlin gabbros indicate an origin from a depleted asthenospheric mantle. The Xinlin granites have high-K calc-alkaline compositions, belonging to I-type granites. These granites have positive ε Hf (t) values (+0.2 to +15.1) and juvenile two-stage model ages (1314–506 Ma) with low Sr and high Yb contents, indicating they were formed by the partial melting of a juvenile lower crust under low-pressure conditions. These data, together with the temporal and spatial distribution and the geochemistry characteristics of the Carboniferous igneous rocks in the Great Xing'an Range, suggest that the amalgamation between the Erguna–Xing'an and Songnen blocks occurred before the Late Carboniferous. The occurrence of asthenospheric mantle-derived mafic rocks indicates localized thinning process of lithosphere. Late Carboniferous magmatism on the Erguna–Xing'an Block attributed to dripping or piece removal of the lithosphere due to asthenospheric fluctuation triggered by slab break-off. • Lithospheric and asthenospheric sources of mafic rocks represent the different stage of post-collisional magmatism. • The amalgamation of the Xing'an and Songnen blocks occurred before late Early Carboniferous. • Lithospheric dripping triggered by slab break-off generated the planar Late Carboniferous post-collisional magmatism. [ABSTRACT FROM AUTHOR]

The Central Asian Orogenic Belt (CAOB), characterized by its significant Phanerozoic accretion, develops tremendous post-Carboniferous granites in its southern part and their origins are still unclear, hindering our understanding on the CAOB late-staged evolution, especially its crust growth history. Herein, we presented zircon U-Pb-Hf-O and whole-rock geochemical and Sr Nd isotopic data for the granitic rocks of early Permian (minor late Carboniferous) from the Khan-Bogd area in southern Mongolia. These early Permian granitic rocks formed at ∼280–290 Ma, together with the coeval previously-studied A-type granites there, are mainly high temperatures (∼800–960 °C) granitic rocks and have contributions of deep substances such as relicts of oceanic slab, lithospheric or asthenospheric mantles, as suggested by their zircon Hf O, and whole-rock Sr Nd isotopes. Similar cases occurred commonly in other areas of southern CAOB and we infer that this is related to a significant shift of the early Permian non-subduction-related magmatism from the earlier arc magmatism. Combined with the accompanying significant contraction of the Paleo- Asian Ocean (PAO), we concur the occurrence of the closure of multiple secondary oceans of the PAO and later accompanying slab-breakoff occurred commonly after the Carboniferous in the southern CAOB. Thus, we propose that the relicts of oceanic slabs, as an important magma origin of these post-Carboniferous granites, were reasonably caused by the commonly-occurred slab breakoff and therefore, it is of great importance for the vertical accretion of the post-Carboniferous evolution of the CAOB, rather than mantle plume and other models. [Display omitted] • The studied early Permian granites have contributions of relicts of oceanic slab. • The relicts of oceanic slab probably resulted from oceanic slab breakoff. • The slab breakoff is an important vertical accretion mechanism there. [ABSTRACT FROM AUTHOR]

The petrogenesis of adakite and adakitic rocks and their tectonic settings are debated. Adakites may be derived by many processes which include: partial melting of subducted oceanic slab or lower crustal material; fractional crystallization of basaltic magma; magma mixing processes associated with arc-related and non-arc tectonic settings. It may be inaccurate to assume that adakitic rocks are solely derived from subducted oceanic slabs. The development of accretionary orogens, as it relates to adakite formation, is a topic of significant interest. Geochemical data coupled with field observations are vitally important to address the interplay between tectonics and magma genesis. In this comment, we discuss the petrogenetic processes and possible tectonic setting of the "adakite" in the southern Beishan region of the Central Asian Orogenic Belt (CAOB), to provide insight on the use of geochemical data to discriminate the tectonic setting of their formation. Based on our field observations, we disagree with the model presented by Zheng et al. (2020) as it pertains to the existence of an "ophiolitic mélange" and the evidence of a Permian arc system but rather, advocate that the adakitic rocks at Beishan are generated at an extensional setting. [ABSTRACT FROM AUTHOR]

Abstract: The Baishiquan and Pobei Early Permian mafic–ultramafic intrusions were emplaced into Proterozoic metamorphic rocks in the Central Tianshan and the Beishan Fold Belt, northern Xinjiang, NW China. The Baishiquan intrusion comprises mainly gabbro, and mela-gabbro sills occurring within and along the margins of the gabbro body. In the Pobei intrusion, two distinct gabbroic packages, a lower gabbro and the main gabbro, are intruded and overlain by small cumulate wehrlite bodies. Both intrusions are characterized by enrichments of large ion lithophile elements and Th and U relative to the high field strength elements, and show strong negative Nb and Ta anomalies and positive K and Pb anomalies, leading to higher Th/Yb and Nb/Yb than in mid-ocean ridge basalt and ocean island basalt. These features are comparable with subduction-related mafic rocks and post-collisional magmas. Geological and geochemical considerations indicate that the parental magmas of the two intrusions were derived from decompression melting of ascending asthenosphere and reacted with overlying subduction-modified lithospheric mantle. We believe that these parental magmas were generated by post-collisional extension along the Chinese Tianshan, perhaps triggered by slab break-off or delamination of thickened lithosphere. Relatively lower (143Nd/144Nd)i and higher (87Sr/86Sr)i than other Permian mafic–ultramafic intrusions in the eastern Chinese Tianshan indicate that the parental magmas of these two intrusions experienced significant contamination by old crustal rocks. [Copyright &y& Elsevier]

Accretionary orogens are the primary sites for continental growth, but the rate and amount of crust generation throughout its evolution are poorly constrained. In this contribution, our new results about the genesis of two intermediate-felsic plutons are combined with a compiled granitoid dataset to evaluate the amount and rate of crustal growth throughout the evolution of the Western Kunlun orogen, which is a typical accretionary orogen associated with the consumption of the Proto-Tethys during the early Paleozoic. The ca.446 Ma Sanshili pluton was formed through interactions between metasomatized mantle wedge-derived oxidized magmas and the lower arc crust, as indicated by high whole-rock Mg#, high Ce4+/Ce3+ ratios (308–861) of ca.446 Ma zircons, and the existence of inherited zircons with ages of 546–472 Ma. The Yirba dioritic to granodioritic pluton was emplaced at 474 ± 3 Ma. Samples from the Yirba pluton are characterized by high K 2 O content, higher Mg# (40–49) than pure crustal melts, slightly higher Y + Nb concentrations, and high Th/Nb ratios, and slightly enriched to depleted Hf–Nd isotopes. Combined with the presence of the 502–531 Ma inherited zircons, the Yirba pluton is suggested to from through differentiation of the metasomatized lithospheric mantle derived-magmas beneath the juvenile intra-oceanic arc in combination with crustal reworking during regional extensions. The Yirba pluton, together with contemporary A 1 -type granites, thus marks an extension event at ca. 475 Ma in the Western Kunlun orogen. The compiled dataset reveals three magmatic flare-ups at 530–500 Ma, 480–470 Ma, and 445–430 Ma, corresponding to two slab rollback events and the slab break-off after the final closure of the Proto-Tethys. The three episodes of more intensive magmatism are associated with more radiogenic Hf–Nd isotopes and increased Nb/La and Nb/Y ratios, indicating more contributions from intraplate-like sources during lithospheric extensions. Meanwhile, element ratios (La/Yb, Sm/Yb, and Sr/Y) that are sensitive to crustal thickness are also elevated. These phenomena are consistent with rapid juvenile crust generation during extensional stages of accretionary orogens. Our study has also shown that the rate of new crust production is quite uniform for different extensional events. The crustal generation rate during slab break-off is much higher than that during slab rollback, although the proportions of juvenile inputs in granitoids formed during slab rollback are relatively higher. This may reflect extra inputs from partial melting of oceanic slabs and subducting sediments like those in continental collision zones and/or rapid asthenospheric upwelling coupled with enhanced crust reworking during slab break-off. • The Yirba pluton was formed at ca.475 Ma during regional extensions. • There are three magmatic flare-ups in the Western Kunlun orogen. • The crust generating rate is uniform for different extensional events. [ABSTRACT FROM AUTHOR]

Distinct types of mafic rocks, including gabbro-norite, gabbro-diorite, and monzogabbro-diorite, are exposed in the southern Chinese Altai. Zircon U-Pb dating of these intrusions reveals that they were emplaced at ca. 283–275 Ma. Geochemical data show that these intrusions are characterized by low Ce/Pb (3.1–10.3), low Nb/U (5.5–38.9), and high Th/Yb (0.4–2.4) ratios. They are slightly LREE enriched, with selective enrichments of LILE and LREE over HFSE and significant Nb-Ta and Zr-Hf depletions. These rocks have depleted Nd isotopic compositions (εNd(t) = 1.8–5.0). In addition, the monzogabbro-diorite samples contain abundant primary amphibole and biotite and show high Rb/Y ratios. The above geochemical characteristics and mineral associations indicate that their parental magmas were hydrous and likely resulted from partial melting of subduction-metasomatized sub-continental lithospheric mantle (SCLM). In contrast, the orthopyroxene-bearing gabbro-norite and gabbro-diorite intrusions show anhydrous and high-T mineral assemblages, implying that the hot asthenosphere must have been also involved in their magma genesis. Given the absence of oceanic island basalt (OIB)-like signals in our samples, the contribution of the coeval Tarim mantle plume can be precluded. A slab break-off scenario is proposed for the Chinese Altai after its collision with the East Junggar in Permian, with products of mafic magmas generated by the interaction between partial melts of SCLM and upwelling depleted asthenospheric mantle. Apart from the Altai Range, the coeval mafic-ultramafic magmatism in the Beishan and East Tianshan orogens share comparable petrogenesis, indicating a general tectonic regime of the western CAOB during this transitional period. • Permian mafic intrusions in the Chinese Altai were formed in an extensional setting. • Partial melting of the SCLM and asthenosphere gave rise to the mafic magmatism. • Upwelling of the asthenosphere was triggered by slab break-off. • A post-collisional tectonic regime prevailed in the western CAOB during the Permian. [ABSTRACT FROM AUTHOR]

The slab dynamic processes, such as roll-back during oceanic subduction and break-off during collision orogenesis, would affect the movement of mantle material and the interaction between crust and mantle, accompanied by significant amounts of magmatism. Providing accurate depictions for these processes are extremely difficult, especially in the case of ancient orogens. Here, we present an integrated study of petrology, geochronology, geochemistry and Sr–Nd–Hf isotopes for Early Paleozoic granitoids from the Duojielaka, Ledu and Binglingsi plutons of the southeastern Qilian orogen. Our LA-ICP-MS U Pb zircon data show that these granitoids were emplaced at 476–432 Ma. The petrological and geochemical data suggest that the ~476 Ma Duojielaka porphyritic syenogranites formed by feldspar-dominated fractionation from relatively K-rich basaltic rock-derived adakitic parental magma. The ~454 Ma Duojielaka biotite monzogranites are highly fractionated I-type granites and were derived from basement orthogneisses mixed with minor mafic rocks. The Ledu fine- and medium-grained quartz diorites (~446 and ~452 Ma) are high-K calc-alkaline to shoshonitic and were produced by partial melting of relatively old K-rich mafic crust followed by minor mixing with mantle-derived magmas. The ~432 Ma Binglingsi biotite monzogranites with adakitic characteristics most likely originated by partial melting of thickened mafic lower crust. Based on the new data, in combination with previously published data, we suggest that the roll-back of the South Qilian oceanic slab occurred during the Middle Cambrian to early Middle Ordovician. The Duojielaka porphyritic syenogranites represented crustal anatexis in an arc front associated with slab roll-back. The Late Ordovician–early Middle Silurian intrusive rocks probably resulted from diachronous slab break-off in the post-collisional stage. The rocks record the opening of the break-off window from east to west and the gradual heat propagation from the central part to the southern and northern margins of the eastern Central Qilian due to the widening of break-off window and asthenospheric upwelling. We propose that complex geodynamic processes and their thermal effects can be traced by a comprehensive consideration of the petrogenetic, temporal and spatial evolution of related magmatism. • Genesis of Early Paleozoic granitoids (476–432 Ma) in the eastern Qilian orogen. • Magmas were predominantly derived from diverse crustal sources. • Magma generations were related to slab roll-back and diachronous break-off. • Orogenic geodynamic processes and their thermal effects can be traced by magmatism. [ABSTRACT FROM AUTHOR]

Abundant Permian magmatism is exposed in the southern Beishan Orogenic Collage, and records key details on the closure of the Paleo-Asian Ocean. The Xiaoxigong granites, exposed in the southern Beishan orogen, exhibit middle Permian zircon U Pb ages (269–267 Ma). They are sodium-rich and exhibit adakite-like geochemical characteristics, such as high Sr content and low Yb and Y contents, coupled with high Sr/Y (99–120) and (La/Yb) N (41–66) values. The Xiaoxigong granites are depleted in whole-rock Nd isotopes [ε Nd (t) = +4.2 to +4.5] and zircon Hf isotopes [ε Hf (t) = +4.3 to +11.4]. Geochemical compositions show that the Xiaoxigong granites formed by partial melting of the subducted Liuyuan oceanic slab, and subducted sediments also played important roles. Two types of dikes were identified in the early Permian host granite (~280 Ma) in the Yinwaxia area. The Group I dikes (267 Ma) are calc-alkaline gabbros, exhibit some subduction-derived geochemical imprints, and are interpreted as derived from an E -MORB-like source that was metasomatized by subduction-related fluids; the Group II dikes are sodium-rich, calc-alkaline diorites, and exhibit high MgO (5.45–6.73 wt%) and compatible element contents, and depleted isotopic compositions. They are sanukitic high-Mg diorites derived from interactions between melts of subducted sediments and overlying mantle peridotites. The association of contemporaneous high-Mg dioritic dikes, arc-like gabbroic dikes, and adakitic granites indicate middle Permian oceanic slab subduction in the southern Beishan. Furthermore, we propose that a subducting oceanic slab window can account for Permian magmatism in the Shibanshan unit, and that the final termination of the Paleo-Asian Ocean in the southern Beishan occurred after the middle Permian. • Middle Permian adakites, high-Mg diorites and arc-like gabbros are reported. • A middle Permian oceanic slab subduction is proposed in the southern Beishan. • The Permian magmatism was formed in an oceanic slab window in the southern Beishan. [ABSTRACT FROM AUTHOR]