| Bangong-Nujiang suture zone is an important part of the Tethyan tectonic domain, which is more than2000km in length across the entire Tibetan plateau from east to west. Since this suture zone records spatial-temporal transform structure of the Bangong-Nujiang ancient ocean basin and ocean-continent the suture zone is interested by the geologists around the world. The study area is located at Bangong Lake region, the western part of Bangong-Nujiang suture zone, where the Bangong Lake ophiolite belt runs through the study area in NWW direction. Here the Mesocenozoic marine and continental sedimentary strata are well exposed, hence Bangong-Nujiang suture zone recorded the Tethyan Ocean formation and evolution of Bangong-Nujiang zone. Therefore the area becomes one of the ideal places to study ocean-continent transform and Bangong-Nujiang suture zone, intracontinental tectonic structure during the conversion process. This paper studies Early-Late Cretaceous strata and magmatic and tectonic characteristics of Late Cretaceous at Bangong Lake in the western part of Bangong-Nujiang suture zone as a main object. Using structural geology, geochronolog, sedimentology and geochemistry, combined with previous geological survey and research about the eastern and central part of Bangong-Nujiang suture zone, this paper will investigate sedimentary-magma-tectonic records and transform time of the western part of the Bangong-Nujiang suture zone during the ocean-continent transformBased on the research about early Cretaceous strata in the Bangong Lake region, the western part of Bangong-Nujiang suture zone, it is suggested that the Bangong Lake is a residual sea basin. Duoni Formation(K1d) is a coal-bearing clastic rock, black shale, marine-terrigenous facies sediment, which developed in the study area is, indicating that the residual basin has entered the final phase of contraction. The north of Bangong-Nujiang belt is Qiangtang block, an arid subtropical and tropical climates to OLi group (K1o) as the representative of the latter due to the shrinking basin sediment become more and more terrestrial environment to alluvial fan deposition is characterized, until the end of deposition. Upper Cretaceous Jingzhushan Formation is characterized by continental molasse, its lithology are conglomerates and sandstones, with evolution from early river facies to late lake facies. It is continental mountain basin at closure of the Tethys Ocean after the ocean-continent transform. In addition to the study area, we also did geological survey and section measurement about Jingzhushan group or equivalent strata at Gaize, Nima, Dangxiong, Bianba and other places in Bangong-Nujiang suture zone, summed up its structure, construction, temporal time and spatial characteristics. It is the first time that we did in the study area Jingzhushan group of science, magnetic stratigraphy ESR’s, obtained in the study area Jingzhushan group began depositing at about92-90Ma. By Jingzhushan group structure, construction, time, space summary characteristics, this paper argues Jingzhushan group is an important symbol for Bangong-Nujiang suture zone to complete the ocean-continent conversion and go into intracontinental tectonic environments, tectonic regime of this period is crustal extension, EW trending graben (or rift) control Jingzhushan group and corresponding period magmatic rocks formation and distribution.Zircon LA-ICP-MS U-Pb age of Risong tonalite at south of Bangong Lake ophiolite belt region is106Ma in the late Early Cretaceous, zircon LA-ICP-MS U-Pb ages of Jiawei granodiorite veins and granodiorite porphyry dikes are91Ma and83Ma respectively, both of them are the late Cretaceous magmatic products. Granodiorite and granodiorite porphyry of Late Cretaceous are typical adakitic rocks. The discriminant diagrams of its tectonic environment show that they were formed in continental extensional environment after the closure of the Meso-Tethys Ocean. Probably they were the products of mantle asthenosphere diapirs and related crustal thermal extensional tectonic basin-mountain backdrop role, synchronous formed graben which controlled Jingzhushan group. The Adakite appeared in this period suggests that Bangong Lake region has entered an inner plate tectonic regime in the91Ma.This provides reliable information for the evolution of time about Tethys in Bangong Lake region.Before the Ocean-continent transform is completed, compressure is main structural feature of tectonic deformation, most of them are in EW direction, indicating the role of nearly north-south squeezing, squeezing deformation mostly distribute all in the region, from the center to the sides gradually weakened, indicating that the overall structure of the background for the extrusion, compression stress center strongest sides weaker. After ocean-continent transform is completed, nature of the study area was turn to extensional, the main structural features are normal faults and detachment faults, the fault plane strikes nearly east-west, indicating north-south extension. Since the Late Cretaceous there no longer have been compressional deformations, implying that the study area converts to extensional tectonic setting during the Late Cretaceous. Therefore, this paper suggests that Jingzhushan Formation thickens continental molasse which distributed along Bangong-Nujiang suture zone are formed in continental linear stretch negative landforms (graben or rift) tectonic and sedimentary environment, thereby reflecting the space-time structure and tectonic are difficult to associated with the orogenic belt collision; combined with the regional characteristics of pollen assemblage of Jingzhushan group, suggesting that the regional sea level should be lower. Paleocene-Eocene continental sedimentary Niubao Formation is angular unconformity at the top of the Jingzhushan group, indicating the Paleocene-Eocene still negative topography. Today relatively negative and two sides positive topography of Bangong-Nujiang suture zone topography may be formatted at the beginning of the Late Cretaceous. |
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