| The India-Asia collision has shaped the basic modern Asian tectonic landscape,and led to major changes in Asian river systems,monsoons,and ecological landscape.A series of Paleogene sedimentary basins are distributed on the southeastern margin of the Tibet Plateau.The sedimentary records filled in the basins truly record the early collision and uplift history of the Plateau.Therefore,tracing the sedimentary environment and sources of basin sediments has great significance for analyzing the basin-mountain coupling relationship and revealing the evolution process of the Tibet Plateau structure and geomorphology.Gonjo basin is typical of these Paleogene sedimentary basins in the southeastern margin of Tibet Plateau.Because of its location,complete Paleogene sediments and weak tectonic activity,Gonjo basin has become an excellent place for sedimentary environment analysis and provenance research.On the basis of previous research results,this study carries out the research on sedimentary facies and environment,detrital components,heavy minerals,detrital zircon U-Pb age and paleocurrent of the Paleogene sedimentary strata of Gonjo basin,restores the provenance change history,establishes the source-sink system,and discusses the basin-mountain coupling process and regional evolution.The main conclusions are as follows:(1)The Paleogene of Gonjo basin is divided into Gonjo and Ranmugou formation from bottom to top.The Gonjo formation mainly developed braided river flood plain deposits,while the Ranmugou formation developed braided river lake deposits.(2)The analysis of paleocurrent and gravel components shows that the paleocurrent in braided fluvial deposits of the lower Gonjo formation is divergent,and the main source of the conglomerate may be the Paleozoic or Triassic marine strata source area of the hanging wall of the thrust fault on the east.The paleocurrent in the middle and lower Ranmugou formation is mainly in the southeast,indicating that the main source is from the north of the basin.The provenance of lacustrine sediments in the upper Ranmugou formation is consistent with that in the middle and lower.(3)The analysis of sandstone clastic components shows that the mainly sandstone type of Gonjo and Ranmugou formation are lithic sandstone and sub lithic sandstone.The source rocks of Gonjo formation is dominated by sedimentary rocks.In the middle and lower Ranmugou formation,a large amount of materials from the source area of the old basement metamorphic rocks were added.The increase of volcanic debris in the upper Ranmugou formation may indicate the active of magmatic in the late filling period of the basin.(4)Heavy mineral analysis shows that most of the heavy minerals in Gonjo basin are extremely stable and stable.The high content of authigenic mineral hematite limonite indicates that the sedimentary environment is mainly oxidation environment.The high content of extremely unstable minerals in the upper Gonjo formation indicates that from the near source materials.The heavy mineral assemblage shows the characteristics of mixed source area,and the source rocks are mainly sedimentary rocks,followed by magmatic rocks and metamorphic rocks.(5)The U-Pb ages of zircon from Gonjo and Ranmugou formation in Gonjo basin can be divided into five groups:200-350 Ma,350-500 Ma,700-1000 Ma,1700-2000 Ma and 2400-2600 Ma.The peak age of 203-275 Ma is closely related to the Mesozoic volcanic eruption event(about 205 Ma)in the North Qiangtang block and the Indosinian granite(205-243 Ma)widely distributed in Songpan-Ganzi block.The 350500 Ma age group may be from the sedimentary recycle material of Songpan-Ganzi block or metamorphic zircon records from the North Qiangtang block;The 700-1000 Ma age group is consistent with the age of the Proterozoic basement of the SongpanGanzi block;the 1700-2000 Ma and 2400-2600 Ma Paleoproterozoic and Archean zircons may be derived from the Ningduo group in the metamorphic basement of the North Qiangtang block,or from the source basement records of the Songpan-Ganzi block.The lower Gonjo formation has the mixed source area of North Qiangtang block and Songpan-Ganzi block,and Songpan-Ganzi block is the main source area.From the upper Gonjo formation to the lower Ranmugou formation,the proportion of ancient zircon increases,which indicates that more metamorphic basement materials of the North Qiangtang block were added in this stage.In the middle and upper Ranmugou formation,the Songpan-Ganzi block is more active and provides more material for the basin.Zircon of<60 Ma appears in the upper Ranmugou formation,which may be the in-situ denudation deposit of volcanic rocks in the basin.(6)The comprehensive provenance tracing results of Gonjo basin show that there are stages in the evolution of the basin and the tectonic activity in the source area:when the basin received the deposition of the lower Gonjo formation,the Songpan-Ganzi block provided part of the material;at the same time,because the basin margin was controlled by strong tectonic activity in the early stage of the basin development,the local small internal rivers provided the material around the basin;when the basin developed to the late stage of the Gonjo formation,the India-Asia collision reactivated the large-scale structural deformation of Songpan-Ganzi orogenic belt in Indosinian,and provided a large amount of material to the basin as the main source area;during the sedimentary period of the lower and middle Ranmugou formation in late Paleocene early Eocene,the thrust fault in the eastern margin of Gonjo basin was active to a certain extent,which made the ningduo group in the Qiangtang metamorphic basement denuded and transported to the Gonjo basin.During this period,the Songpan-Ganzi block continued to be active and remained the main source area of the basin;in the late Early Eocene,the upper Ranmugou formation was deposited.At this stage,the Qiangtang block was uplifted due to the India-Asia collision,and the denudation of the Songpan-Ganzi orogenic belt was weakened,so the sedimentation of the basin ended,and the Gonjo basin gradually uplifted and reached the peak so far.At the end of the basin filling deposition or after,high-K magmatism and compressional strike slip tectonic movement were active.The uplift of the basin and the high-K magmatism in the basin may have responded to the peak of the collision between the India-Asia collision. |
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