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The pressure–temperature (P‐T) path of blueschist is usually applied to understanding the evolution of the subducted oceanic crust. Garnet plays a key role in calculating the metamorphic conditions, but sometimes it is absent in the blueschists. Nevertheless, the high‐P experimental simulation may serve as a useful tool to constrain the P‐T conditions. In the middle of the Longmu Co‐Shuanghu Suture, North Tibet, blueschist blocks embedded in the marble were discovered, where garnet is abundant in the core but absent in the marble‐contacting margin. These two kinds of blueschist contain similar mineral assemblage (glaucophane, epidote, and phengite) and show comparable basaltic character. Pseudosection modelling was applied to the garnet‐bearing blueschist that calculated the peak metamorphic condition of ca. 1.9 GPa and 530°C. As for the garnet‐absent blueschist, experiments (1.2–2.3 GPa and 500–800°C) were performed on the designed mixture of 93 wt% basalt and 7 wt% calcite. The results show that the total Fe content in the glaucophane and barroisite is positively correlated with the pressure but not with temperature, which could be regarded as a geobarometer. Accordingly, the peak condition was constrained at 2.3 GPa and 500–600°C. Therefore, the equivalent metamorphic conditions of the blueschists indicate that the Palaeo‐Tethys oceanic crust subducted into at least 70 km. Furthermore, both the experiments and the T‐X (CaCO3) pseudosection, constructed based on basaltic lithology, support that the absence of garnet in the blueschist is predominantly caused by the mixing of calcite. In summary, this study provides a valid attempt in exploring metamorphic conditions by experiment simulation, and evaluates the profound influence of the interaction between the basaltic crust and overlying carbonate‐bearing sediments in the subduction zone. [ABSTRACT FROM AUTHOR]

The identification of a microcontinent in Bangong–Nujiang suture zone (BNSZ) is important to understanding the tectonic evolution of Bangong–Nujiang Tethys Ocean (BNTO). Subduction‐related ophiolite is one of the keys to discriminate a microcontinent in suture zones, which can be generated during the oceanic subduction beneath the microcontinent. Generally, ophiolites in the adjacent area of a microcontinent would show characteristics of a volcanic arc, while ophiolites in adjacent area of an ocean show characteristics of a fore‐arc. North Pengco ophiolites (NPOs) and South Pengco ophiolites (SPOs) were collected in the central section of BNSZ, and dated at 115.0–111.5 Ma by SHRIMP II. According to their trace element characteristics, NPOs were determined to have formed at a setting of subduction‐related volcanic arc and affected by slab‐derived fluids or melts. The diagram based on the Ce versus Ce/Y shows that NPOs originated from the partial melting of spinel peridotites. On the other hand, SPOs were generated from the partial melting of ultra‐depleted mantle at a setting of subduction‐related fore‐arc on the basis of extraordinarily low REEs summation and HFSEs (e.g., Zr, Ti, Nb and Ta), relatively high concentrations of compatible elements (e.g., Cr, Ni), large ratios of CaO/TiO2 (37.67–437.50) and Al2O3/TiO2 (20.78–424.50), which were also affected by slab‐derived fluids. Based on the relative location of NPOs and SPOs, it reasonably proposed that a northward subduction of a minor oceanic basin had occurred at the Early Cretaceous, trigged by the Dongkaco microcontinent (DMC). In reverse, these ophiolites provide evidence for the existence of DMC in BNTO. Combining with other subduction‐related ophiolites developed in BNSZ, BNTO might have comprised of several minor oceanic basins separated by some blocks like DMC in the central and west sections, rather than a unified ocean basin. [ABSTRACT FROM AUTHOR]

The Bangong‐Nujiang suture zone, separating the Lhasa and South Qiangtang terranes of the Tibetan Plateau, is marked by remnants of the Bangong‐Nujiang oceanic basin. In the Zanzong Co area of central Tibet, non‐marine molasse deposits of the newly identified Jingzhushan Formation record the eventual collision between these two terranes and subsequent ocean–continent transform along the middle segment of the Bangong‐Nujiang suture zone. In this contribution, it is observed that the Jingzhushan Formation, characterized by lacustrine facies and braided delta facies, rests unconformably on the underlying marine strata and ophiolite. Zircon U–Pb dating has been conducted with samples of four interlayer volcanic rocks and two sandstones from the Jingzhushan Formation. The interlayer volcanic rocks constrain the depositing age from ca. 115 to 72 Ma. Based on the youngest detrital zircon ages, the maximum depositional ages of the Jingzhushan Formation are suggested to be ca. 78 Ma. Petrographic and detrital zircon isotopic results indicate that sediments in the Jingzhushan Formation have mixed sources from the South Qiangtang terrane and the intervening Bangong‐Nujiang suture zone. Provenance analysis, together with regional data, suggests that the newly identified Jingzhushan Formation reflects continental molasse deposition in an intermontane basin during the post‐collisional stage. The development of the intermontane basin evidenced by molasse deposition of the Jingzhushan Formation reveals that the ocean–continent transform of the middle segment of the Bangong‐Nujiang Ocean had been completed in the late Early Cretaceous (ca. 115 Ma). [ABSTRACT FROM AUTHOR]

The Tiegelongnan Cu (Au) deposit is the largest copper deposit newly discovered in the Bangong-Nujiang metallogenic belt. The deposit has a clear alteration zoning consisting of, from core to margin, potassic to propylitic, superimposed by phyllic and advanced argillic alteration. The shallow part of the deposit consists of a high sulphidation-state overprint, mainly comprising disseminated pyrite and Cu-S minerals such as bornite, covellite, digenite, and enargite. At depth porphyry-type mineralization mainly comprises disseminated chalcopyrite, bornite, pyrite, and a minor vein molybdenite. Mineralization is disseminated and associated with veins contained within the porphyry intrusions and their surrounding rocks. The zircon U-Pb ages of the mineralized diorite porphyry and granodiorite porphyry are 123.1 ± 1.7 Ma (2σ) and 121.5 ± 1.5 Ma (2σ), respectively. The molybdenite Re-Os age is 121.2 ± 1.2 Ma, suggesting that mineralization was closely associated with magmatism. Andesite lava (zircon U-Pb age of 111.7 ± 1.6 Ma, 2σ) overlies the ore-bodies and is the product of post-mineralization volcanic activity that played a critical role in preserving the ore-bodies. Values of −4.6 ‰ to + 0.8 ‰ δ34S for the metal sulfides (mean − 1.55 ‰) suggest that S mainly has a deep magmatic source. The H and O isotopic composition is (δD = −87 ‰ to −64 ‰; δ18OH2O = 5.5 ‰ to 9.0 ‰), indicating that the ore-forming fluids are mostly magmatic-hydrothermal, possibly mixed with a small amount of meteoric water. The zircon εHf(t) of the diorite porphyry is 3.7 to 8.3, and the granodiorite porphyry is 1.8 to 7.5. Molybdenite has a high Re from 382.2 × 10−6 to 1600 × 10−6. Re and Hf isotope composition show that Tiegelongnan has some mantle source, maybe the juvenile lower crust from crust-mantle mixed source. Metallogenesis of the Tiegelongnan giant porphyry system was associated with intermediate to acidic magma in the Early Cretaceous (~120 Ma). The magma provenance of the Tiegelongnan deposit has some mantle-derived composition, possibly mixed with the crust-derived materials. [ABSTRACT FROM AUTHOR]

Magmatic flare-ups are well recognized as a fundamental feature of continental arc magmatism, but its driven mechanism still remains unresolved. Exploring this behavior is of significance in unraveling the deep dynamic processes related to plate subduction and crustal growth. In central Tibet, arc magmatism within the South Qiangtang terrane (SQT) caused by the northward subduction of the Bangong-Nujiang Ocean (BNO) is featured by a prominent high-flux event during Jurassic, but its details and corresponding geodynamic background are not determined. This paper reports geochronological and geochemical data of the granodiorites near the Rigen Co area in the middle part of SQT. These rocks were generated at ca. 150 Ma and display typical adakitic characters. Their low MgO (1.41–2.39 wt%), initial 87Sr/86Sr ratios (0.7055–0.7059) and slightly low ε Nd (t) values (−1.9 to −1.2), and high zircon ε Hf (t) (+1.9 to +8.4) and δ18O values (+6.6 to +9.2 ‰) suggest the adakitic granodiorites were derived from partial melting of subducted crust involving 10–20% inputs of overlying sediment. Our sample, together with previous reports delineates a distinct slab-window association (adakite and oceanic island basalt (OIB)-type diabase) along the southmost of SQT, which coincided with the magmatic flare-up at ca. 155 Ma as revealed by the age spectra of magmatic rocks and complementary detrital zircons. Furthermore, a trench-ward migration is recognized for the arc magmatism from ca. 170 Ma to 155 Ma, implying a slab-roll back process. Accordingly, this high-flux event with the slab-window formation is best explained by the trigger of ensuing slab break-off. These results, combined with other geological evidence, contribute new constraints on elucidating the Jurassic geodynamic evolution of the BNO subduction. • The Rigen Co granodiorites were generated by oceanic crust melting at ca. 150 Ma. • Jurassic arc magmatism in the South Qiangtang terrane shows a trench-ward migration. • The Late Jurassic magmatic flare-up was triggered by slab break-off. • A oceanic plateau-continent collision was proposed to explain these processes. [ABSTRACT FROM AUTHOR]