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The Riduo intermediate-felsic volcanic rocks are located in the eastern Lhasa terrane in southern Tibet. Zircon U–Pb dating, geochemical and Sr–Nd–Hf isotope analyses were carried out to constrain the age, petrogenesis and magma source of the Riduo magma suite, including andesite, dacitic tuff, and quartz diorite. Zircon U–Pb dating yielded Early Miocene ages of 19.5 ± 0.5 Ma (andesite), 17.6 ± 0.3 Ma (dacitic tuff), and 15.6 ± 0.5 Ma (quartz diorite). Geochemically, this early Miocene suite is adakitic with high SiO2 (55.76–69.46 wt.%), Al2O3 (15.21–16.61 wt.%), Na2O (3.82–4.83 wt.%), Sr (427–1033 ppm), and Sr/Y (24.5–145), but low Y (< 17.8 ppm) and Yb (< 1.82 ppm). The rocks also have low to high K2O (0.44–3.82 wt.%). They are enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs, e.g. K, Rb, Th and U), but depleted in high-field strength elements (e.g. Nb, Ta) and heavy rare earth elements (HREEs), and have weakly negative to positive Eu anomalies. Isotope analyses reveal that most samples have high whole-rock initial 87Sr/86Sr ratios (0.705611–0.708807), low εNd(t) (−2.25 to −1.15), and positive zircon εHf (t) (+1.29 to +6.49). We suggest that the early Miocene magmas in the eastern Lhasa terrane were likely sourced from post-collisional partial melting of the juvenile lower crust with minor mantle input. The magmas may have been crustal-contaminated during their ascent to the upper crust along the N-S-trending Cona-Riduo rift. [ABSTRACT FROM AUTHOR]

The Amdo Terrane, Central Tibet, was previously located in the middle of the Bangong–Nujiang Ocean (BNO), and is key to understanding the subduction history of the BNO. However, the high‐pressure (HP) mafic granulites of the Amdo Terrane remain poorly studied. Here, we report the petrology, geochemistry, and zircon U–Pb geochronology of the Amdo HP mafic granulites. The granulites occur mainly as enclaves or deformed dikes in granitic gneisses of the Amdo Terrane. Zircons from the granulites yield two clusters of ages, at 193–192 and 180 Ma. The former represents the best estimate of the age of peak metamorphism, and the latter represents the age of retrograde amphibolite‐facies metamorphism. Although the contents of mobile elements in the granulites, including the large‐ion lithophile elements (Rb, Sr, Cs, Pb, K), were significantly altered during metamorphism, the contents of the high‐field‐strength elements (HFSEs; Th, Nb, Zr, Ti) and rare earth elements (REEs) remained largely unchanged. The granulites belong to the tholeiitic basalt series, with SiO2 contents of 47.05–51.03% and Mg# of 35–50. The REE contents of the granulites are characterised by flat chondrite‐normalised patterns (LaN/YbN = 0.96–1.92; LaN/SmN = 1.0–1.59) and weak Eu anomalies (Eu/Eu* = 0.84–0.99). Most of the granulite samples have high TiO2 contents, clear negative Nb and Ta anomalies, and high Pb enrichment, with positive zircon εHf(t) values (+8.14 to +16.02). The granulites exhibit both normal mid‐ocean ridge basalt (N‐MORB) and island‐arc basalt (IAB) characteristics. Using the data presented here and previous data, we suggest that the protolith of the granulites were back‐arc basin basalts derived from high‐degree partial melting of depleted mantle in the spinel stability field, which records the early evolution of the BNO. Northward subduction of the BNO oceanic plate led to back‐arc spreading and the formation of back‐arc basin basalts on the northern margin of the Amdo Terrane. The back‐arc basin extended further to form an ocean basin, which separated the Amdo Terrane and the Southern Qiangtang–Baoshan (SQB) Terrane, resulting in the Amdo Terrane becoming an isolated microcontinent in the BNO. [ABSTRACT FROM AUTHOR]

The Duguer area represents one of the few occurrences of high-grade metamorphic rocks in the ‘Central Uplift’ zone of the Qiangtang terrane, central Tibet. The metamorphic rocks consist mainly of orthogneiss, paragneiss, and schist. To better understand the formation of these rocks, seven samples of gneiss and schist from the Duguer area were selected forin situzircon U–Pb analysis and Ar–Ar dating of metamorphic minerals. The results suggest two distinct metamorphic stages, during the Late Triassic (229–227 Ma) and Late Jurassic (150–149 Ma). These stages correspond to the closure of the Palaeo-Tethys Ocean and northward subduction of the Bangong–Nujiang Neo-Tethys oceanic crust, respectively. We suggest that the Late Triassic metamorphic rocks of the Duguer area in the central South Qiangtang subterrane provide evidence of continental collision between the North and South Qiangtang subterranes, following the subduction of oceanic crust. It is likely that deep subduction of oceanic crust occurred along the Longmu Co–Shuanghu–Lancangjiang suture zone (LSLSZ), which would have hindered exhumation owing to the high density of oceanic crust. Subsequent break-off and delamination of the subducted oceanic slab at ~220 Ma may have resulted in exhumation of high-pressure and high-grade metamorphic rocks in the South Qiangtang subterrane. The Late Jurassic ages of metamorphism and deformation obtained in this study indicate the occurrence of an Andean-type orogenic event within the South Qiangtang subterrane. This hypothesis is further supported by an apparent age gap in magmatic activity (150–130 Ma) along the magmatic arc, and the absence of Late Jurassic sediments. [ABSTRACT FROM AUTHOR]

In order to investigate the evolution of Shiquanhe-Yongzhu-Jiali ophiolitic mélange belt, the gabbros from new discovered Zhongcang ophiolitic mélange are studied through petrology, whole-rock geochemistry, zircon U-Pb dating and Lu-Hf isotope. The gabbros investigated in this paper contain cumulate gabbro and gabbro dike, and they have undergone greenschist-amphibolite facies metamorphism. The chondrite normalized rare earth element (REE) patterns of most of these rocks show flat types with slightly light REE (LREE) depletion and the N-MORB normalized incompatible elements diagrams indicate depletion in high field strength elements (HFSE) (Nb, Ta) and enrichment in large ion lithophile elements (LILE). These gabbros have island arc and mid-ocean ridge basalt affinities, suggesting that they were originated in an oceanic back arc basin. Whole rock geochemistry and high positive ɛ( t) values show that these gabbros were derived from ∼30% partial melting of a spinel lherzolite mantle, which was enriched by interaction with slab-derived fluids and melts from sediment. U-Pb analyses of zircons from cumulate gabbro yield a weighted mean age of 114.3±1.4 Ma. Based on our data and previous studies, we propose that an intra-oceanic subduction system and back arc basin operated in the Neo-Tethy Ocean of central Tibet during Middle Jurassic and Early Cretaceous, resembling modern active intra-oceanic subduction systems in the western Pacific. [ABSTRACT FROM AUTHOR]

To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the constraints on its tectonic evolution, here we report the results of zircon LA-ICP-MS U-Pb dating and geochemical features of Amdo gneiss in the Nyainrong microcontinent. The outcrops of Amdo gneiss is about 30 km south of Amdo County in northern Tibet. The field occurrence, mineral composition, textural characteristics, and whole-rock geochemical features of the four gneiss samples indicate the protolith of the gneisses is intermediate-acid intrusive rock. Gneiss zircon trace element tracing and genetic analysis shows that zircon has typical characteristics of magmatic zircon. The 206Pb/ 238U concordant age of zircon is 505–517 Ma, corresponding to the Middle-Late Cambrian, which is the formation age of the protolith. The samples have characteristics of high silicon, alkali-rich, alkalic rate AR =1.73–3.7, the differentiation index DI = 70.78–90.28; rock aluminum saturation index ranges from 1.02 to 1.05, FeO / MgO ranges from 2.63 to 4.50, 10000 × Ga/Al ranges from 2.12 to 2.41, and P 2O 5 and Al 2O 3 content decreased with SiO 2 increasing. Th and Y contents have a good positive correlation with Rb content; the genetic type of protolith of the gneiss is the differentiation of subalkaline over aluminum I-type granite. Combined with regional data, the tectonic setting of the Amdo gneiss protolith is closely related to the collision orogenic process. The preliminary view is that the Middle-Late Cambrian magmatic events developed on the microcontinent could be the result of Andean-type orogeny along the Gondwana super-continental margin after the end of the Pan-African orogeny. [ABSTRACT FROM AUTHOR]

The tectonic setting of the Tangjia–Sumdo metamorphic belt, which separates the North Lhasa Block (NLB) and South Lhasa Block (SLB), remains controversial. Moreover, the sedimentary evolution of the Tangjia–Sumdo metamorphic belt has been little studied, particularly during the early subduction evolution of the Sumdo Paleo-Tethys Ocean (SPTO). Here we present petrological, geochemical, and zircon U-Pb geochronological data on the clastic rocks and metabasaltic interlayers collected from the Sumdo Formation in the Sumdo area. The Sumdo Formation consists of metamorphosed sandstone, graywacke, phyllite, and interlayered metabasalts. Detrital zircon U-Pb dating of the metasandstone and quartz schist identified a youngest zircon age group of 389–316 Ma, which have negative ε Hf (t) values (−20.5 to −1.5) and old Hf crustal model ages (TC DM = 1450–2654 Ma). The zircon trace element features and Hf isotopic compositions show that the sedimentary detritus was sourced from the NLB, which indicates that this terrane experienced Late Devonian–early Carboniferous magmatism. The ages of the youngest detrital zircons (YC2σ age of 324 ± 13 Ma) indicate that the Sumdo Formation formed mainly during the late Carboniferous–early Permian. Combined with geochemical data of the metabasalts and other regional geological evidence, we conclude that the Sumdo Formation formed in an initial forearc basin. The Sumdo Formation was likely deposited along an active continental margin according to the in–situ suture model, where northward subduction of the Sumdo Paleo–Tethys oceanic lithosphere occurred beneath the NLB. As such, the Sumdo Formation provides a sedimentary record of the early subduction evolution of the SPTO. • The Sumdo Formation formed mainly in the late Carboniferous–early Middle Permian. • The Sumdo Formation formed in an inital forearc basin. • Upper Carboniferous–lower Permian sedimentary detritus was sourced from the North Lhasa Terrane. [ABSTRACT FROM AUTHOR]

Abstract: Previous investigations have indicated that the Changning–Menglian (CMSZ) and Longmu Co–Shuanghu (LSSZ) suture zones represent the Paleo-Tethys Ocean of northern Tibet and western Yunnan of China, respectively. However, our understanding of the Paleo-Tethys suture zone in eastern Tibet seems to be incomplete. The North Lancangjiang suture zone (NLSZ) has been hypothesized to mark the Paleo-Tethys Ocean in eastern Tibet. The present study reports new zircon U–Pb ages and Hf isotope data, and whole-rock major and trace element data from the gneissic granites from the Jitang complex in the Leiwuqi area of the NLSZ in order to verify this hypothesis. Six samples of gneissic granites yielded weighted mean 206Pb/238U ages of 249–237Ma. The geochemical data show that the gneissic granites are characterized by high SiO2 (70.79–77.75wt.%); K2O (2.00–6.13wt.%); low A/CNK values (<1.1); enrichment in Rb, Th, Ba, and Pb; depletion in Nb, Ta, Sr, Ti, and P; and negative Eu anomalies. These features suggest that the gneissic granites were high-potassium, calc-alkaline, and I-type granites. Moreover, the zircon εHf(t) values (from −2.7 to +2.5) and TC DM model ages (1447–1009Ma) indicate that they were generated by the partial melting of the Meso-proterozoic juvenile crust. These gneissic granites were interpreted to have formed in the post-collisional tectonic setting related to the closure of the Paleo-Tethys Ocean. By combining previously published and the new data, we propose that the NLSZ connects the LSSZ and CMSZ in the Tibetan plateau, and marks a Paleo-Tethys Ocean basin in eastern Tibet. [Copyright &y& Elsevier]

Highlights: [•] Cambrian metarhyolites emplaced in the central Lhasa subterrane of southern Tibet. [•] The metarhyolites were derived from anatexis of Proterozoic metasedimentary rocks. [•] The metarhyolites formed under a subduction environment. [•] The Gondwanan proto-Tethyan margin was interpreted as an active continental margin. [Copyright &y& Elsevier]