Tectonic Evolution Of The Middle-western Bangong-Nujiang Suture, Tibet

The ophiolite-bearing Bangong-Nujiang suture zone, traversing central Tibet from east to west, separates the Qiangtang block to the north from the Lhasa block to the south. Some key aspects about the Bangong-Nujiang suture zone, such as its nature of the ocean basin the suture represents, its evolution, subduction polarity, and closure mechanism, are still open to intense debate. The mantle geodynamics of the Bangong-Nujiang ophiolites, along with the role of the Nierong-Jiayuqiao metamorphic microcontinents in the central-eastern Bangong-Nujiang suture, also remain obscure. This has constituted a obstacle for understanding the evolution of the Tethys and the development of the Tibetan Plateau. This scientific issue is also critical to correctly assessing the structure and evolution of the Mesozoic-Cenozoic petroliferous basins on the suture zone.Intensive studies, including fieldwork, petrology, petrography, mineral geochemistry, geochemistry, Sr-Nd-Pb isotopes and zircon SHRIMP U-Pb and ⁴⁰Ar-³⁹Ar geochronology, were conducted on three key ophiolite profiles in the Bangong-Nujiang suture across Gaize, a famous town in western Tibet, where the ophiolites are well cropped out but little work has been covered so far. The rock assemblages, geochemistry, Sr-Nd-Pb isotopes, petrography, and spinel mineral chemistry of the Laguo Co, Dong Co and Chaerkang Co ophiolites profiles indicate that they were produced under environments of a supra-subduction zone. The metamorphic peridotites, cumulates, basalts and diabases from these three profiles all display negative TNT (Ti-Nb-Ta) anomalies, even though they belong to different tectonic units.The Laguo Co ophiolite in the northern margin of the Lhasa block geochemically show a distinct affinity with middle oceanic ridge basalts (MORB) and island arc tholeiites (IAT) and could possibly have been produced in an inter-arc basin above an intra-oceanic subduction zone. The mineral chemistry of spinels indicates that the Dong Co ophiolite within the suture exhibits MORB and IAT affinity, and the geochemistry of the cumulates shows that they could have formed under a middle oceanic ridge environments. Therefore, the Dong Co ophiolite perhaps was generated in an immature back-arc basin above the intra-oceanic subduction zone. In contrast, the Chaerkang Co ophiolite in the southern margin of the Qiangtang block geochemically displays characteristic features of MORB and IAT and could likely have been formed in an arc environment above the intra-oceanic subduction zone.The plagiogranites from the Laguo Co ophiolite were carefully studied for their genesis and geochronology. Our results indicate that these rocks could possibly have been derived from anatexis of hydrated gabbros by ductile shearing during transport of the oceanic crust. These plagiogranites have an age of 166±2.5 Ma by zircon SHRIMP U-Pb measuring. This age may represent the formation age of the ophiolite. indicative of that an oceanic basin could have existed in the western part of the Bangong-Nujiang zone in the Middle Jurassic.The arc-related rocks were identified the first time in this study in the southern margin of the Qiangtang block and on the north of the Chaerkang Co ophiolite. These magmatic rocks include rhyolites, andesites, and diorites, and have an age of 157.5±2.2 Ma based on zircon SHRIMP U-Pb method. Clearly they were a result of the northward subduction of the Bangong-Nujiang ocean under the southern Qiangtang block during the middle Jurassic.The Laguo Co and Chaerkang Co ophiolites are thrust over the Pre-Upper Jurassic shelf-facies sedimentary cover rocks, which are marked by -20 m-thick ductile faults filled by syn-tectonic felsic schists. Three samples of mica grains from these schists produce ages of 151-153 Ma by ⁴⁰Ar-³⁹Ar geochronology. Such bilaterally simultaneous obduction indicates that the Bangong-Nujiang oceanic basin must have closed just at this time. The closure is suggested to be a result of the low-angel subduction of the Neo-Tethys Ocean on the south of the Lhasa block, which drove the Lhasa block moved northward quickly and the Bangong-Nujiang ocean was closed.The rifting of the Bangong-Nujiang oceanic basin, the development of the MOR and SSZ oceanic basin, and the subduction of the oceanic crust developed earlier in the east and propagated westwards, showing a scissors-like opening and closure during a stage of -130 My. Two-stage remnant oceanic basins, present in eastern and western segments, respectively, were existed during the Permian (or Carboniferous) to late Triassic and the early to middle Jurassic (extended to late Jurassic in the western segment).The evolution of the Bangong-Nujiang suture zone, strictly speaking, controlled the sedimentation of the Late Jurassic peripheral foreland basin in the Lhasa block and the Early-Middle Jurassic back-arc basin in the Qiangtang block. A suite of Lower Cretaceous OIB-type volcanic rocks, with a ⁴⁰Ar-³⁹Ar age of 122.9 Ma, were recognized across Gaize and provided strong evidence that the thick Lower to Middle Cretaceous sedimentary rocks in the back-arc basin of the Gandese arc in central Tibetan Plateau should have been formed under an environment of rifting possibly owing to the roll-back of the Neo-Tethys oceanic crust after the low-angle subduction. The back-arc rifting of the Gandese arc could probably have extended to the Qiangtang block, and the uplifting time of the Tibetan plateau should not be earlier than the late Cretaceous.As the origin is concerned, the ophiolites across Gaize are characterized by the Indian Ocean-type MORB with Dupal Sr-Nd-Pb isotope anomalies. The Dong Co and Laguo Co ophiolites could geochemically be a result of mixture of DDM-type and EM II-type magmas. However, the Chaerkang Co ophiolite could geochemically have been generated from mixture of DMM-type and EM I-type magmas. These Dupal isotope anomalies are considered to have resulted from the early Permian (-280 Ma) mantle plume upwelling with Dupal isotopic features. This event possibly resulted in the rifting of the north margin of the Gondwana and initiated the Paleo-, Meso- and Neo-Tethyan Oceans.