Analysis Of Sedimentary Environment Of Paleogene Gonjo Formation In The Gonjo Basin Of Southeast Tibet

Gonjo Basin is located in the northern section of Sanjiang metallogenic Belt and the eastern part of Nanqianwusu Basin in eastern Xizang Province.Due to multiple tectonic movements,tectonic deformation is complex,mineral resources are abundant,and mineralization has obvious zonation characteristics.In recent years,many scholars have conducted in-depth studies on the tectonic evolution,material distribution and polymetallic mineralization of the basin,and have obtained fruitful results,which provide a lot of theoretical basis of the genetic study of polymetallic deposits in the basin.However,because Gonjo Basin is located in the eastern part of the East Tesla tectonic belt in the Sanjiang area,the tectonic movement is very complicated,and the Palaeogene deposits in the basin have multiple sources,Therefore,although a lot of previous studies have been done,it is still necessary to further study the filling sequence,sedimentary characteristics and sedimentary environment of the basin,in order to provide theoretical support for the study of metal mineralization during the sedimentary evolution of Gonjo Basin.Through detailed field investigation,profile survey and sample collection,this paper studies the sedimentary characteristics of the Paleogene Gonjo Formation in Gonjo Basin,southeast Tibet by means of mineralogy,petrology observation,heavy mineral analysis and geochemical analysis.The following understandings are obtained:（1）Petrographic characteristics and clastic analysis Q-F-R diagram shows that the sandstones of the Paleogene Gonjo Formation in the Gonjo Basin are feldspar and lithic sandstones,which are characterized by low compositional maturity and close to source location.According to Qm-F-L diagram,the provenance of the study area is from the continental provenance area of the transition area.The Qp-Lv-Ls,Lm-Lv-Ls and Qm-P-K diagrams further prove that the provenance of the study area is from the volcanic arc orogenic belt in the provenance area of continental block.（2）The heavy mineral hematite shows that the sandstones of Gonjo Formation in Gonjo Basin are in oxidizing environment.The study of heavy mineral characteristic index shows that the parent rock type of sandstone in the study area is mainly acid igneous rock,and there are also low grade metamorphic rock mixed in.The provenance direction of the study area is generally from the southeast direction,among which the provenance of Hajia County is not only from the southeast of Youzha township,but also from the Paleozoic strata in the upper wall of the eastern thrust fault.（3）The ratio of major elementn（Fe₂O₃）/n（K₂O）indicates that sandstone in the study area has experienced high weathering.The projection diagrams of Fe₂O₃+Mg O-AI₂O₃/SIO_2,Si O₂-K₂O/Na₂Oand Fe₂O₃+Mg O-AI₂O₃/（Ca O+Na₂O）show that the tectonic background of the Paleogene Gonjo Formation sandstone in the study area is active continental margin and continental island arc background.According to the F₁-F₂ projection diagram,the source rocks of Gonjo Formation in the study area are mainly felsic igneous rocks.（4）The characteristics of trace elements indicate that the spider map,La/Sc-Co/Th and TH-HF-CO diagrams of trace elements reveal that the sandstones of the Paleogene Gonjo Formation in the study area come from the upper crust.The characteristic ratio of trace elements indicate that the sedimentary environment of Gonjo Formation sandstone in the study area is warm and wet oxidized continental freshwater environment.The diagrams of Th-Co-Zr/10,La-Sc-Th and Zr-Th and the ratios of trace elements also indicate that the tectonic background of the study area is active continental margin and continental island arc.（5）The distribution pattern of rare earth elements shows a right-leaning pattern of relative enrichment of light rare earth elements and relative deficit of heavy rare earth elements,with obvious negative Eu anomaly and weak negative Ce anomaly.The REE characteristic ratio shows that the tectonic background of the study area is continental island arc.The La/Th-Hf diagram shows that the provenance of Gonjo Formation sandstone is from the acid island arc provenance area of the upper crust（6）The grain size analysis results show that the probability accumulation curve of the Paleogene Gonjo Formation sandstone in the study area is divided into three types:"one-jump and one-suspension type,""one-jump and one-suspension type,""rolling,jumping and suspension type,"and the sedimentary environment of the reaction is the sedimentary characteristics of the above-water distributary channel in the delta plain,the underwater distributary channel in the delta front,the estuary bar and the far-sand bar.