Reconstructing The Late Mesozoic Closing Process Of The Middle And Western Segments Of The Bangong–Nujiang Ocean In Space And Time

The east–west trending Bangong–Nujiang Suture Zone(BNSZ), which crosses the central Tibetan Plateau and bounds the Gangdese plate to the south and the Southern Qiangtang–Baoshan plate to the north, provides an ideal natural laboratory for studying the geological history of the Tethys Ocean in China. In recent years, surveys of natural resources have indicated that the BNSZ is not only a key suture zone, but also an important metallogenic belt. Therefore, in addition to obtaining information on the evolution of the Tethyan tectonic domain and the Tibetan Plateau, further investigation of the BNSZ would provide a better understanding of mineral resources in the region. Since the 1980 s, many researchers have provided detailed information on the ophiolites, strata, and volcanic rocks in the BNSZ. However, the difficulties in undertaking fieldwork in the harsh climate of the region, combined with the complex geological evolution of the Tibetan Plateau, means that many aspects of the evolution of the suture zone remain poorly known. The processes involved in the closure of the middle and western segments of the Bangong–Nujiang Ocean during the late Mesozoic remain to be elucidated.Defining the closing time is key to reconstructing the closing process of the Bangong–Nujiang Ocean. At present, there are two opposing views regarding the closing time. First, most workers advocate that the ocean closed before the Early Cretaceous, as inferred from an unconformity between sedimentary rocks(including the Late Jurassic–Early Cretaceous Shangmuluo and Dongqiao formations) and remnants of oceanic material(including ophiolite and flysch deposits), and some Early Cretaceous granitoids derived from thickened crust. Second, the presence of some Early Cretaceous oceanic material(including ophiolite and ocean island) has led other workers to suggest that the Bangong–Nujiang Ocean was still developing during the Late Cretaceous and therefore must have closed after the late Early Cretaceous. As a result of these opposing views on the timing of closure, different models have been proposed for the closing process of the Bangong–Nujiang Ocean.Ophiolites, ocean islands, flysch deposits, and radiolarian cherts provide important clues to the evolution of ancient oceans, because they represent remnants of ancient oceanic crust and sedimentary basins. Magmatic rocks record the motion and activity of tectonic plates, and can be used to understand the geological evolution of a region. The sedimentary stratigraphy within and surrounding a suture zone, particularly the unconformity between sedimentary units and remnants of oceanic material, provides a direct record for the evolution and subsequent closure of the ocean. In this paper, we present a detailed study and review for the Cretaceous ophiolites, ocean islands, and flysch deposits in the middle and western segments of the BNSZ, and the Cretaceous volcanic rocks, and the late Mesozoic stratigraphy and sedimentary unconformity within and surrounding the BNSZ. We aim to reconstruct the spatial–temporal patterns of the closing process of the middle and western segments of the Bangong–Nujiang Ocean.Previous and our studies have shown that in the middle segment of the BNSZ, the Zhonggang ocean island, located in Dong Co Township of Gerze County, and the Tarenben ocean island, located in Duoma Township of Shuanghu County, preserve a double-layered structure comprising a basaltic basement(basalt, diabase, and gabbro) and an oceanic sedimentary cover sequence(conglomerate, limestone, and chert). This rock assemblage is similar to that of ocean islands in modern ocean basins. The basalts and gabbros of the Zhonggang and Tarenben ocean islands have high TiO2 contents, and show light rare earth element(LREE)-enriched chondrite-normalised REE patterns, and are enriched in high field strength elements(e.g., Nb, Ta, and Ti), yielding primitive-mantlenormalised trace element variation patterns that are similar to those of ocean island basalts. The petrology and geochemistry of samples from the Zhonggang and Tarenben ocean islands indicate they are typical ocean islands that formed in an ocean basin. Both the Zhonggang and Tarenben ocean islands formed during the late Early Cretaceous(108–123 Ma). In the middle segment of the BNSZ, the Kangqiong ophiolites, located in Zhongcang Township of Nyima County, and the Dong Co ophiolites, located in Dong Co Township of Gerze County, consist of mantle peridotite, mafic–ultramafic cumulates, mafic dykes, pillow basalts, and minor chert. This rock assemblage is similar to that of the modern oceanic lithosphere, suggesting that they represent fragments of the Bangong–Nujiang oceanic lithosphere. It has been suggested that the Kangqiong ophiolites formed in a super-subduction zone during the late Early Cretaceous, whereas the Dong Co ophiolites represent a multi-stage tectonic mélange that formed during the Jurassic to Early Cretaceous and which is the product of interaction between the mid-ocean ridge and mantle plume. In the western segment of the BNSZ, large numbers of late Early Cretaceous radiolarian cherts in the Qunang and Bangong Co ophiolites indicate that these ophiolites formed during the late Early Cretaceous. In the middle segment of the BNSZ, the Early Cretaceous bimodal rocks of the Zhaga Formation, located in Gerze County, formed during the subduction of the ridge. On the Southern Qiangtang–Baoshan plate, the bimodal rocks of the Early Cretaceous Maierze Formation, located in Wuma Township of Gerze County, formed during the initial development of a back-arc basin.In summary, the Early Cretaceous age of ocean islands, ophiolites, flysch deposits, and volcanic rocks within and surrounding the middle and western segments of the BNSZ suggest that the Bangong–Nujiang Ocean remained at least partially open during the Early Cretaceous, with ocean development and subduction continuing during the late Early Cretaceous. Therefore, the closure of the ocean must have occurred after the late Early Cretaceous. A comparison of the Early Cretaceous Zhaga Formation and flysch deposits of the Mugagangri Group shows that both sedimentary units contain the same clastic rock fragments and sedimentary structures. From this comparison, we are able to infer that the Zhaga Formation represents the Early Cretaceous section of the Mugagangri Group. Therefore, the continuous deposition of the Mugagangri Group from the Jurassic to Early Cretaceous suggests that the evolution of the Bangong–Nujiang Ocean was also ongoing during that time.Based on the regional geology, we infer that the Late Jurassic–Early Cretaceous regional unconformity between sedimentary rocks(including the Late Jurassic–Early Cretaceous Shangmuluo and Dongqiao formations) and remnants of oceanic material(including ophiolite and flysch deposits) in the middle and western segments of the BNSZ does not represent the final closure of the Bangong–Nujiang Ocean. Instead, this unconformity records the timing of arc–arc and arc–continent collision at the northern margin of the Bangong–Nujiang Ocean during the Late Jurassic–Early Cretaceous. We advocate that the end of the Early Cretaceous regional unconformity between the Qushenla Formtion and oceanic material represents the final closure the Bangong–Nujiang Ocean.We conclude that closure of the middle and western segments of the Bangong–Nujiang Ocean started in the Late Jurassic and mainly finished at the end of the Early Cretaceous. Both ―longitudinal diachronism‖ from north to south, and ―transverse diachronism‖ from east to west, were occurred during the closure of the Bangong–Nujiang Ocean. The late Mesozoic closing process of the middle and western segments of the Bangong–Nujiang Ocean is summarised below: Subduction and the development of the Bangong–Nujiang Ocean were ongoing prior to the Late Jurassic. After the Late Jurassic, the Bangong–Nujiang Ocean began to close because of the compressional regime surrounding the BNSZ. Along the northern margin of the Bangong–Nujiang Ocean, the arc, back-arc basin, and marginal basin of the multi-arc basin system firstly collided during the Late Jurassic–early Early Cretaceous, resulting in regional uplift and unconformity along the northern margin of the Bangong–Nujiang Ocean and the Qiangtang Basin north of the Bangong–Nujiang Ocean. However, the closure of the Bangong–Nujiang Ocean cannot be attributed to this arc–arc and arc–continent collision because subduction and the development of the Bangong–Nujiang Ocean continued until the late Early Cretaceous. The gradual closure of the Bangong–Nujiang Ocean was diachronous from east to west after the late Early Cretaceous, and was mostly complete by the end of the Early Cretaceous. The BNSZ and its surrounding areas were in a phase of orogenic uplift during the Late Cretaceous.