Geochemical Characteristics And Paleoenvironmental Analysis Of Eocene-Miocene Carbonate Rocks In Laojunshan Area, Northwestern Yunnan

The closure of the Neo-Tethys Ocean and the subsequent Indian and Eurasian plate collisional orogeny directly triggered the exhumation and large-scale uplift of the Qinghai-Tibet Plateau,which led to a global changes in the geological environment and showed drastic spatial and temporal differences.Laojunshan area in northwest Yunnan is located at the southeastern margin of the Qinghai-Tibet Plateau.This area is not only accompanied by the differential blocks uplifting and subsidence caused by the integrallyplateau uplift,but also developed vast strike-slip related pull-part basins and thrust nappe structures.However,previous studies in this area have been more limited by the stratigraphic framework established by the 1:200000and 1:50 000 regional surveys.Recently,some new data have been obtained on account of the regional geology,deposit geology,especially zircon U-Pb dating of igneous and/or metamorphic rocks,and muscovite ⁴⁰Ar-³⁹Ar dating of tectonic slices.However,the explanation of its response to the uplift process of the plateau is still unreasonable,and often contradicts each other.During the 1:50000 regional geological supplementary investigation in the Laojunshan area by our research group,three sets of carbonate-clastic rock assemblages were newly identified in the original Cenozoic stratigraphic units?such as,Yunlong Formation/E₁y,Baoxiangsi Formation/E₂b?.Their stratigraphic horizons can respectively correspond to the bottom,middle and top strata of the Paleocene Baoxiangsi Formation?E₂b?,and shows clearly sedimentary relationship with overlying and underlying stratum.Among them,the lower rock assemblages and underlying strata?usually Triassic,Cretaceous or Paleocene?are unconformity contacts,while the upper rock assemblages and overlying strata?Shuanghe Formation/N₁s in the region?are continuous deposits.Therefore,the three sets of carbonate-clastic rock sedimentary assemblages belong to the Eocene-Miocene epoch.The information of regional geological evolution in the same period and its response to the uplift process of the Qinghai-Tibet Plateau should be recorded in detail.Obviously,it provides the best geological evolution marker and research site for studying the sedimentary response,paleoenvironmental change and its dynamic mechanism of the uplift process of the plateau.At the same time,whether the sedimentary environment of these carbonate rocks is the sedimentary environment of the continental lake basin commonly considered by predecessors is still controversial.Therefore,on the basis of a large number of relevant results and a detailed investigation of the typical geological route in the Laojunshan area are reviewed and analyzed,with the geological mapping of 1:50000,this paper chooses three relatively complete carbonate rock sections in Laojunshan area as the research object,using the comprehensive research methods of 1:5000 geological survey,stratigraphic profile measurement,mineral composition statistical analysis,C-O isotope tracing,element geochemical tracing,sedimentary environment restoration and paleoelevation estimation,with the investigation of the carbonate rocks of specific horizons and the terrigenous clastic rocks of upper and lower horizons,the sedimentary sequence of the sedimentary formation was established,the filling process and paleoenvironment of the inland lacustrine basin were ascertained,and the changes of sedimentary environment in the area and its response to the uplift of the Qinghai-Tibet Plateau were revealed.The main achievements and understandings of this research are as follows:1.Baoxiangsi Formation?E₂b?was widely distributed and developed in Laojunshan area.The formation is a set of red terrigenous coarse clastic deposits with medium-thick layered purple-red conglomerate and gravel-bearing medium-coarse sandstone at the bottom and medium-thin layered siltstone.The thick layered conglomerates consist of grey-purple and purple-red layered feldspar-bearing sandstone and quartz sandstone at the top,and volcanic intercalations can be seen in some areas.Carbonate rocks are commonly developed in the upper part of the conglomerates at the top and bottom.The rock characteristics of the conglomerates show obvious differences due to different sedimentary environments.2.Sedimentary Facies Analysis shows that the early Eocene of the Laojunshan area is the residual sea,and the sedimentary environment of the marine source continental carbonate which provided the provenance for open lakes was transformed into a shallow-semi-deep Lake marl sedimentary environment in the late Eocene.3.The C-O isotope and trace element geochemical characteristics of carbonate rocks and reconstruction of palaeoenvironments indicate that the carbonate rocks at the bottom of the Eocene Baoshangsi Formation in the Laojunshan area?Fujing and Maliqing sections?are oxygen-rich,high-temperature arid climate and semi-saline water sedimentary environment;the carbonate rocks at the top?Liziyuan and Shuanghe sections?are oxygen-poor,high-temperature,warm-humid climate and fresh water sedimentary environment.4.Excluding post-diagenetic reformation factors,combined with the temperature measured by predecessors?20.8??and the correction of Paleo-Sea water,and using the Rayleigh fractionation model in eastern Tibet for reference,the paleoelevation of the Eocene in the Laojunshan area is estimated by using the C-O isotope analysis of the carbonate rocks as follow:2636m in the Fujing,2609m in the Liziyuan and2675m in the Jinhua-Shuanghe section.This indicates that the Eocene depositional period has reached the present altitude.5.The study of the depositional clastic rocks mixed with carbonate rocks in BaoXiangsi Formation shows that the clastic composition is mainly monocrystalline quartz,and according to the value of Q/?F plus R?,it is worth knowing that its provenance may come from the west side.It is confirmed by the value of n?Al?/n?Al plus Fe plus Mn?that the brine during the sedimentary period is likely to be a provenance from the residual sea of Neo-Tethys.Combined with the main and trace geochemical characteristics of the rocks,it is concluded that the sedimentary period of the BaoXiangsi Formation was mainly formed by regional material with passive continental margin structural attributes.6.Comprehensive analysis shows that when the limestone deposits at the bottom of the Eocene Baoshangsi Formation in the Laojunshan area,the salinity of the lake is higher,the marine environmental characteristics are obvious,and the continental elements are involved in the sedimentary period.Its provenance should be the remaining residual marine deposits after Tethys closure.The regression event of the residual sea of the New Tethys responded to the collision process between the Indian plate and the Eurasian plate,and the persistent subduction of the collision directly controlled the regional uplift and sedimentation of the Qinghai-Tibet Plateau and the southeastern margin of Laojunshan Mountains.