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The tectonic transition from oceanic subduction to continental collision is a fundamental process during orogenesis, yet the nature of such transition and associated deep geodynamic processes for the Tibetan Plateau are still controversial. The long‐lived magmatism of the Gangdese Belt of southern Tibet provides an excellent opportunity to study the tectonic transition from subduction of the Neo‐Tethys oceanic lithosphere to the continental collision between Indian and Asian plates. We present zircon U–Pb–Hf isotopes, whole‐rock geochemistry, and Sr–Nd isotopes for the intrusions from Maizhokunggar, southern Lhasa Terrane. The zircon U–Pb dating indicates that two episodes of magmatism occurred in the Maizhokunggar region, ca. 70 Ma diorites and ca. 60 Ma granitoids. The ca. 70 Ma diorites are medium‐K calc‐alkaline and characterized by pronounced high‐field‐strength element depletion and light rare earth element enrichment, similar to arc‐type rocks. They have homogeneous (87Sr/86Sr)i ratios (0.70378–0.70424) and εNd(t) values (+2.6 to +3.4), and εHf(t) (+5.9 to +13.0) with relatively young depleted mantle model ages (307–762 Ma), suggesting that they were generated from the juvenile mafic lower crust induced by the underplating of basaltic magmas from depleted mantle wedge. The ca. 60 Ma granitoids, Kajiacun (KJC) granodiorites, and Qulongxue (QLX) and Bangdacun (BDC) monzogranites are high‐K series and metaluminous to weakly peraluminous. The KJC granodiorites with high Sr/Y ratios (25–43) are adakitic and were likely derived from partial melting of a thickened lower crust. The BDC monzogranites have more evolved Sr–Nd–Hf isotopes and older Nd–Hf model ages than the QLX monzogranites. These geochemical variations were mainly controlled by the fractionation crystallization with assimilation of ancient crustal materials. We propose that at ca. 70 Ma, the subduction of the Neo‐Tethys oceanic slab triggered the partial melting of the juvenile lower crust and formed the diorites with arc affinity, and then the initial collision (>60 Ma) between the Indian and Asian plates caused locally thickened lower crust, resulting in adakitic granodiorites. Our study sheds light on the lithospheric scale magma evolution from the deep mantle to the shallow crust during the critical geodynamic transition to collision, and early uplift. [ABSTRACT FROM AUTHOR]

The subduction of the Proto-Tethyan Ocean is still a controversial subject. Here we report zircon U-Pb ages, geochemistry, and Hf isotopic compositions of the zircon in the Mengkete quartz diorite in the Buqingshan–A'nyemaqen Tectonic Mélange Belt, which lies at the southern margin of the East Kunlun orogenic belt on the northern Tibetan Plateau, to constrain the petrogenesis of subduction-related granitoids and to reconstruct the evolution of the Buqingshan–A'nyemaqen Ocean. Zircons U-Pb dating yields coeval ages of 441–436 Ma for Mengkete quartz diorite. Mengkete plutons have variable SiO2 (56.63–65.22%) and high Al2O3 (16.09–17.79%) contents and aluminous saturation indexes (A/CNK ratios) ranging from 0.77 to 0.96, which define their metaluminous and medium-K2O calc-alkaline signatures. The plutons have a low total rare-earth element (REE) content (45.49–168.31 ppm) and slightly positive Eu anomalies (Eu/Eu* in the range of 0.96 to 1.32). They are also enriched in large-ion lithophile elements (LILEs), such as Rb, Th, and Ba, but are depleted in high-field-strength elements (HFSEs), such as Nb, Ta, Zr, Hf, and Ti. Their zircon εHf(t) values ranging from 7.79 to 13.02, and the two-stage Hf (TDM2) model ages are in the range of 1130–657 Ma. These geochemical signatures indicate that the Mengkete quartz diorite was derived from partial melting of the mafic Meso-Neoproterozoic lower crust during the northward subduction of the Proto-Tethyan Ocean. The evolution of the East Kunlun Belt can be divided into five stages from the Neoproterozoic to late Middle Triassic: (1) an opend Buqingshan‒A'nyemaqen ocean during the Neoproterozoic up to 516Ma; (2) the continuous expansion of the Buqingshan‒A'nyemaqen ocean during the period 516 to 441Ma, along with the beginning of northward subduction in the Late Cambrian; (3) further subduction from 441 to 400Ma, accompanied by the production of large volumes of arc magmatic rocks; (4) the long period during which the Buqinshan–A'nyemaqen Ocean existed (400–240 Ma); and (5) the final closure of the Buqinshan–A'nyemaqen Ocean in the late Middle Triassic. [ABSTRACT FROM AUTHOR]

We report zircon U–Pb age data, Hf isotopes, and bulk-rock geochemical data for the Middle to Late Triassic granitoids in the Taerqi-Chabaqi-Kutihe region within the Xing’an Block of the central Great Xing’an Range, northeast China. Euhedral to subhedral zircon grains were extracted from the nine representative granitoids. These zircons exhibit oscillatory zoning typical of a magmatic origin. The zircon U–Pb determinations on the monzogranite, syenogranite and quartz diorite samples yielded ages between 244 Ma and 206 Ma, which essentially suggests that the magmatism in the Xing’an Block occurred during the Middle to Late Triassic period. Geochemically, these granitoids have SiO 2 = 62.97–76.31 wt%, A/CNK = 1.03–1.51, and K 2 O/Na 2 O = 0.40–2.39, we infer that they belong to high-K calc-alkaline series and are peraluminous I-type granites in nature. The major and trace element systematics indicate that these granitoids have different origins. Among them, the 244 Ma syenogranite of Taerqi and the 230–206 Ma monzogranite and syenogranite of Kutihe were probably generated from partial melting of pre-existing juvenile arc-type rocks in a relatively shallow crustal level. The 210 Ma quartz diorite of Chabaqi likely originated from the hybridization between a depleted mantle component and the juvenile subducted oceanic crustal materials that were buried to depths of the middle to lower continental crust. The 212 Ma monzogranite and syenogranite of Kutihe were probably generated from the partial melting of miscellaneous lower crustal materials at high pressure conditions. By combining these new data with regional geological data, we conclude that the early Mesozoic evolution of the central Great Xing’an Range was governed by two superimposed tectonic regimes, i.e., (1) post-orogenic extension due to slab break-off after the closure of the Paleo-Asian oceanic basin along the Hegenshan-Heihe suture belt, and (2) back-arc extension associated with the southward subduction of Mongol-Okhotsk oceanic plate. [ABSTRACT FROM AUTHOR]

[Display omitted] • Newly ca. 880 Ma gabbro-diorite and gabbro are identified in the Bikou Terrane. • Both the gabbro-diorite and gabbro were formed in a subduction-related tectonic setting. • The Bikou Terrane was a part of the subduction-related system during the early Neoproterozoic. The Neoproterozoic mafic intrusions in the Bikou Terrane along the northwestern margin of the Yangtze Block record crucial information on evaluating the tectonic evolution of the Yangtze Block, South China. However, their petrogenesis and tectonic significance are still poorly constrained. In this paper, we present the results of a comprehensive study of zircon U–Pb geochronology and whole-rock geochemistry from the gabbro-diorites and gabbros of the Bikou Terrane. Zircon U–Pb dating results indicate that these gabbro-diorite and gabbro samples were formed coevally at ca. 880 Ma. The gabbro-diorite samples are calc-alkaline in composition and have intermediate contents of SiO 2 , Mg#, Ni, and Cr. They are enriched in LREEs, and depleted in HFSEs (i.e., Nb, Ta, Zr, and Hf). They have relatively high ratios of Th/Yb, and low ratios of Nb/Yb, Nb/La, Th/Nd, Th/Zr, and U/Th, indicating that they were derived from partial melting of a mantle wedge source metasomatized by slab-derived melts. The gabbro samples are commonly classified as sub-alkaline tholeiitic rocks, and have constantly low to medium values of MgO, Mg#, Cr, and Ni, with distinct spikes at Th, U, Nb and Ta concentrations. They show an E-MORB-like enriched LREEs, low ratios of Nb/La, Ta/Yb, Th/La, and Nb/Zr, and medium ratios of Nb/Yb and Th/Yb, implying that they were most probably originated from the interaction between the upwelling enriched asthenospheric mantle and the mantle wedge peridotite. This mechanism may be triggered by the slab rollback process in a subduction-related environment. Our new investigations, along with available observations from other scholars, support the persistent subduction-related system along the northwestern margin of the Yangtze Block during the early Neoproterozoic. [ABSTRACT FROM AUTHOR]

The granodiorites, monzogranites and diorites are widely developed in the Balikun area of Eastern Tianshan Orogen. Of which, LA–ICP–MS zircon U‐Pb isotopic dating from the diorites revealed that they were emplaced at 327∼333 Ma, representing an important period of magmatism in the Early Carboniferous. Geochemically, they are characterized by moderate SiO2 (51.33–62.48 wt%), high but variable MgO (2.04–11.16 wt%, average 5.35), higher Mg# (40–73) and TiO2 (0.67–1.29 wt%), Na2O/K2O (1.39–2.95) as well as variable Cr (2.49–675 ppm) and Ni (1.31–174 ppm), showing a geochemistry similar to those of high‐Mg diorites or sanukitoids. In addition, they are enriched in the LILE, poor HFSE with an evident negative Nb anomalies and a REE pattern of moderate fractionator between LREE and HREE without or weak negative Eu anomalies. Their ∊Hf(t) are positive (+3.63–+15.65), suggesting a source from the depleted mantle. In addition, they have high TiO2 and Pb, and large quantity of amphibolite and biotite, indicating that they were most likely derived from the partial melting of depleted mantle metasomatized by the slab‐derived melt under a hydrous condition. Consequently, combined with the contemporary volcanics and granitoids formed in the island arc settings, we proposed that the subduction was continued till Early Carboniferous in the Bogda‐Harlik tectonic belt. After that, wide occurrence of the post‐collisional A‐type granites and mafic‐ultramafic intrusions indicate this tectonic belt entered the post‐collisional environment from Late Carboniferous to Permian. [ABSTRACT FROM AUTHOR]

The granodiorites, monzogranites and diorites are widely developed in the Balikun area of Eastern Tianshan Orogen. Of which, LA–ICP–MS zircon U‐Pb isotopic dating from the diorites revealed that they were emplaced at 327∼333 Ma, representing an important period of magmatism in the Early Carboniferous. Geochemically, they are characterized by moderate SiO2 (51.33–62.48 wt%), high but variable MgO (2.04–11.16 wt%, average 5.35), higher Mg# (40–73) and TiO2 (0.67–1.29 wt%), Na2O/K2O (1.39–2.95) as well as variable Cr (2.49–675 ppm) and Ni (1.31–174 ppm), showing a geochemistry similar to those of high‐Mg diorites or sanukitoids. In addition, they are enriched in the LILE, poor HFSE with an evident negative Nb anomalies and a REE pattern of moderate fractionator between LREE and HREE without or weak negative Eu anomalies. Their ∊Hf(t) are positive (+3.63–+15.65), suggesting a source from the depleted mantle. In addition, they have high TiO2 and Pb, and large quantity of amphibolite and biotite, indicating that they were most likely derived from the partial melting of depleted mantle metasomatized by the slab‐derived melt under a hydrous condition. Consequently, combined with the contemporary volcanics and granitoids formed in the island arc settings, we proposed that the subduction was continued till Early Carboniferous in the Bogda‐Harlik tectonic belt. After that, wide occurrence of the post‐collisional A‐type granites and mafic‐ultramafic intrusions indicate this tectonic belt entered the post‐collisional environment from Late Carboniferous to Permian. [ABSTRACT FROM AUTHOR]