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The study of the petrogenesis of some magmatic rocks with special geochemical attributes provides effective information for us to explore the deep geodynamic background of their formation. A series of granitic porphyry dykes have been found in the mélange zone of the Asa region in southern Tibet, whose genesis may be closely related to the evolution of the Meso-Tethyan Ocean. Regional geodynamic evolution is investigated by whole-rock geochemical analysis, zircon U–Pb dating and Lu–Hf isotopic analysis of two porphyritic granites. The Asa porphyritic granites have high SiO2 (74.29–78.65 wt %) and alkalis (Na2O + K2O = 6.51–9.35 wt %) contents, and low Al2O3 (11.60–14.51 wt %), CaO (0.04–0.19 wt MgO (0.01–0.10 wt %) contents. They are enriched in Zr, Nb, Ce, Y and Hf and depleted in Ti, Ba, Sr and P, consistent with A-type granites. The samples are relatively rich in LREEs, with LREE/HREE ratios of 1.73–3.04. They display negative Eu anomalies (Eu/Eu* = 0.24–0.28) and obvious Ce anomalies in some samples. Zircon U–Pb analyses show that the porphyritic granites formed in late Early Cretaceous time, 107.4 to 105.5 Ma. Zircon εHf(t) values are in the range of 6.9 to 12.0. These data indicate that the porphyritic granites were sourced from interaction between mantle-derived and juvenile lower crust-derived melts, with the addition of oceanic sediment-derived melts. This occurred when the subducting Bangong–Nujiang oceanic crust split to create a slab window. Rising asthenosphere triggered re-melting of lower crust basalts, resulting in the formation of the late Early Cretaceous A-type granites around Asa. [ABSTRACT FROM AUTHOR]

Intra-oceanic subduction is a fundamental process on Earth, the study of which can improve understanding of plate tectonic processes and the history of continental growth. Several studies of the intra-oceanic subduction of the Meso-Tethys Ocean have been published in recent years, particularly relating to the Bangong–Nujiang suture zone (BNSZ) and the Shiquanhe–Nam Tso mélange zone (SNMZ) in central Tibet. Here, we report newly discovered occurrences of Jurassic diorite (161.2–160.6 Ma) and plagiogranite (161.0 Ma) from the western part of the central SNMZ. The diorite samples have moderate SiO 2 and high MgO contents and Mg# values; high Ba/Th and Ba/La ratios; low K 2 O, TiO 2 , and REE contents; low (La/Yb) N , (La/Sm) N , and Th/Yb ratios; and positive ε Hf (t) values, similar to high-Mg diorite formed by partial melting of a subduction-modified mantle wedge. The plagiogranite intrudes gabbro of the ophiolite suite and has high SiO 2 and Na 2 O contents; low Al 2 O 3 , K 2 O, P 2 O 5 , TiO 2 , and total REE contents; and clear negative Eu anomalies, similar to shear-related oceanic plagiogranite, which can be formed by partial melting process of gabbro in the high temperature shear zone near the expansion center. The formation processes of Asa high-Mg diorite and shear-related plagiogranite are consistent with initial subduction within the Meso-Tethys ocean. Based on these data and those for sedimentary and Jurassic magmatic rocks of the region, we propose that, during subduction, the Northern Lhasa terrane was formed from a collage of oceanic island arcs, oceanic plateaus, and accretionary complexes. • Intermediate-acid magmatism in the SNMZ, central Tibet. • High-Mg diorites and shear-type plagiogranite with Late Jurassic ages (161–160 Ma). • The Jurassic intra-Meso-Tethys oceanic subduction event (165–160 Ma). • The generation of the Northern Lhasa terrane by accretion of intra-oceanic arcs. [ABSTRACT FROM AUTHOR]

The tectonic-thermal mechanism of crustal melting provides vital information on continental differentiation and orogenic evolution. This study aims to understand the evolution and tectonic setting of newly discovered lower crustal leucosome-rich granulites of the Qilian Orogen, NW China. Tonalitic leucosome within garnet amphibolite occurs in net-like arrays or as stringers and has diffuse boundaries with the host, with enriched LILE, and high Sr and Sr/Y ratios, similar to that of adakite, formed by water-fluxed melting. However, granitic leucosome within garnet granitic gneiss shows sharp contacts with the host, and has low Sr, Ba, K and Pb but high Rb, Rb/Sr, with negative Eu anomalies, consistent with dehydration melting. Petrographic observations, geochemical data and pseudosection modelling suggest that garnet amphibolite experienced water-fluxed melting and garnet granitic gneiss underwent dehydration melting, respectively. LA-ICP-MS zircon and titanite dating indicate the anatectic event proceeded at ∼480–470 Ma. Phase equilibrium modelling suggests the garnet amphibolite and garnet granitic gneiss share a similar P-T path involving decompression heating from ∼12.1 kbar/711–746 °C to reach peak temperatures of 798–836 °C at 10.4 kbar, and subsequently decompression cooling to 4.6–7.6 kbar/600–672 °C. The calculated P-T conditions suggest Barrovian-type metamorphism with thermal gradients of 24.2 °C/km for the Central Qilian lower crust. We interpret a retro−/back-arc setting resulting from the southward subduction of the North Qilian oceanic lithosphere. [Display omitted] • Water-fluxed melting of hornblende and dehydration melting of biotite in the crust root. • An anatectic event proceeded at ca. 480–470 Ma in the Central Qilian belt. • A hot retro−/back-arc setting is proposed to provide enough heat for anatexis and metamorphism. [ABSTRACT FROM AUTHOR]

Due to the unclear tectonic setting of the Jurassic ophiolites within the Bangong-Nujiang suture, how and when intra-oceanic subduction initiated within the Bangong Meso-Tethys remain controversial. Based on the new petrological, geochemical, and geochronological data reported in this paper, ∼190–180 Ma MORB-like ophiolites and OIB-like mafic rocks were identified from the Shiquanhe ophiolitic complex in the western Bangong-Nujiang suture. The MORB-like gabbros exhibit strong light rare earth elements (LREEs) and heavy rare earth elements (HFSEs) depletion and were derived from partial melting of a highly refractory depleted mantle source with limited slab-derived fluids. The OIB-like mafic rocks are enriched in LREEs and HFSEs and were originated from the melting of upwelling asthenospheric mantle without subduction additions. The data for the MORB-like ophiolites reported in this paper and previously published data demonstrate the formation of Early Jurassic fore-arc ophiolites in response to the Early Jurassic intra-oceanic subduction initiation within the Bangong-Nujiang suture. The Early Jurassic OIB-like rocks formed in the extensional tectonic setting, together with regional geological evidence, suggest that the Early Jurassic initial subduction event within the Bangong Meso-Tethys resulted in broad upper plate extension. Our study not only reveals the intra-oceanic subduction initiation age of the Bangong Meso-Tethys but also implies that subduction initiation can cause strong extension in a wider region of the upper plate in the Tethys oceanic system, not just in the fore-arc region. • Discovery of MORB-like fore-arc ophiolite (190–185 Ma) in the Shiquanhe area. • Identifying of OIB-like gabbro-monzonite (185–180 Ma) in the Shiquanhe melange. • Confirming Early Jurassic intra-oceanic subduction initiation in Bangong Ocean. [ABSTRACT FROM AUTHOR]

As a part of the Bangong-Nujiang suture, the Shiquanhe-Namco ophiolite mélange belt (SNM), together with the Bangong-Nujiang ophiolite mélange belt (BNM), records the evolutionary history of the Mesotethys. In terms of the closure of the Mesotethys, previous sedimentological research has focused on the BNM, while the SNM remains insufficient. On the basis of detrital zircon dating and sedimentologic data, in this study, we determined the existence of Aptian deep-marine turbidites in the Zhongcang and Lhakorco areas of the SNM. Sandstone petrographic and detrital zircon analysis combine to show that the Zhongcang turbidites provide records of the recycled from the accretionary complex and Triassic-Jurassic strata in the southern Qiangtang margin, with the minor addition of Qiangtang magmatic arc rocks, while sediments in the Lhakorco turbidites were derived mainly from recycled orogen source and Late Jurassic-Early Cretaceous magmatic arc in the Lhasa Terrane. Further analysis suggests that the Aptian turbidites were deposited in a deep-marine oceanic basin of Mesotethys and this oceanic basin prevented Qiangtang-derived sediments transported onto the Lhasa terrane. The Mesotethys experienced diachronic closure from east to west during the Early Cretaceous, and the final closure was most likely completed along the SNM after Aptian. • Discovery of the Aptian deep-marine fan turbidities in SNM • Featuring the Aptian Qiangtang and Lhasa source separation along SNM • Implications for diachronous closure of the Mesotethys [ABSTRACT FROM AUTHOR]

Although voluminous Jurassic ophiolites have been reported along the Bangong–Nujiang Suture Zone (BNSZ), the tectonic setting of ophiolites and the Jurassic evolution history of the Meso-Tethys Ocean remain mysterious. This study presents petrological, geochronological and geochemical data for newly discovered Jurassic ophiolitic complex in the Asa area, the Shiquanhe–Namco Ophiolite sub-belt (SNO sub-belt) of BNSZ. Detailed geological investigations and zircon U-Pb dating reveal that the Jurassic ophiolitic complex are composed of a suite of Mid-Late Jurassic (165–158 Ma) ophiolite and Early Jurassic gabbros (ca. 190 Ma). The Early Jurassic gabbros show affinities with island arc tholeiitic (IAT) basalts, which were derived from depleted mantle sources modified by oceanic slab-derived fluids and formed in a forearc settings. The Mid-Late Jurassic ophiolite mainly consists of peridotites, cumulate gabbros, isotropic gabbros, plagiogranites and greenschists. Whole-rock geochemical and zircon Hf isotopic data suggest that the Mid-Late Jurassic ophiolite formed in an intra-oceanicisland arc setting. Together, the Asa ophiolitic complex document magmatic processes from forearc extension to the development of intra-oceanicisland arc, confirming the occurrence of intra-oceanicsubduction system within the Meso-Tethys during the Jurassic. We tend to regard that both northward intra-oceanicsubduction and ocean–continent subduction were initiated during the Early Jurassic in the Meso-Tethys. During Mid-late Jurassic, a northward intra-oceanicarc–back-arc basin system developed in the Meso-Tethys, along with a continental forearc spreading basin resulting from northward ocean-continental subduction beneath the South Qiangtang terrane, producing the complex Jurassic ocean structure of the Meso-Tethys. [ABSTRACT FROM AUTHOR]

• A New Precambrian basement has been identified in Asa region, southern Tibet. • The emplacement age of gabbro intrusions in this basement is ca. 875 Ma. • These gabbro intrusions were formed in an intercontinental rift environment. • The North Lhasa terrane may originate from Australia. Due to the less exposure of Precambrian basement, the origin of ancient continental blocks in the Tibetan plateau and their paleogeographic locations in the Rodinia supercontinent remain enigmatic. Here we first report the newly identified Precambrian basement from the Asa region of the North Lhasa terrane (NL), southern Tibet. The zircon U–Pb age and whole-rock geochemical data on gabbro intrusions (known as Tugu gabbros) in this basement reveal an important tectonic evolution process of continental-oceanic transition experienced by the North Lhasa terrane in the early Neoproterozoic (ca. 900–875 Ma), which may be the prelude to the disintegration of Rodinia in China. Tugu gabbros (ca. 876–874 Ma) belong to the series of sub-alkaline tholeiitic basalt and have N-MORB geochemical affinities. In combination, the high (87Sr/86Sr) i ratios (0.709–0.711), positive ε Nd (t) values (2.8–3.9) and the relatively young Nd isotopic pattern age (t DM C = 1.2–1.3 Ga) indicate that it came from a depleted mantle source and were formed in an intercontinental rift environment on the northern margin of Rodinia. According to previous studies and data provided herein, the era and nature of Precambrian magmatic-metamorphic records in North and South Lhasa terranes cannot be compared, suggesting these may have been two separate terranes in the Precambrian. We propose that the South Lhasa terrane (SL) originated from India, while the North Lhasa terrane may originate from Australia. After splitting from Australia at ca. 930–875 Ma, the North Lhasa terrane continued to drift westward owing to the break-up of Rodinia. During the process of drifting, the North Lhasa terrane received sedimentary input from India and developed Andean-type arc magmatism with the influence of oceanic subduction on the northwest margin of supercontinent. [ABSTRACT FROM AUTHOR]