Geochemical Characteristics Of The Middle To Late Jurassic Sediments In The Qiangtang Basin And Its Implications

The potassium resources mainly beared in the oceanic lithostratigraphy are being in serious shortage in our country.From detailed geologic observations,Qiangtang basin is rich in Jurassic oceanic lithostratigraphy,meanwhile,lies at the same tectonic unit with eastern-Asian Karakum basin which bears large Jurassic potassium deposits.Whether the Qiangtang basin is qualified to be a perfect place to form potassium deposits or not has led a generation of scholars pay attention to this area.In this contribution,under high-resolution paleomagnetic age control of Qiangtang basin,we report on a detailed comprehensive geochemical study of well exposed 3100 m Yanshiping section along Kunlun Shan in the Qiangtang basin for the first time.Combined with the sedimentary environments analysis,Middle-Late Jurassic litofacies and tectonic setting of Qiangtang basin to reveal sea level fluctuations of Tethys Ocean,paleoclimate changes and eventually evaluate potential of potassium deposit formation in Qiangtang basin.This paper has made the main conclusion and progression as follow:(?)The Fe/Mn ratios and CaCO3 content in the bulk samples indicate that this area suffered two complete circles of oceanic transgression-regression at Middle-Late Jurrasic time.The Quemocuo formation consists of mixed color detrital sediments,which was interpreted as delta fan and tidal-flat fan,with low carbonate content and high Fe/Mn,was in the regression stage.The Buqu formation consists of micritic limestone and biogenic limestone interlayered with siltstone,which was interpreted as carbonate plateform fan,the carbonate content increase sharply reach at the maximum value,the Fe/Mn decrease at this time interval reached at the minimum value.The Xiali formation consists of mixed color fine-grained detrital sediments interlayered with gypsum and limestone,which is interpreted as tidal-flat fan,the carbonate content decrease and the Fe/Mn increase obviously,was in the regression stage.The Suowa formation consistes of mainly micritie limestone and biogenic limestone interlayered with siltstone,which was interpreted as carbonate plateform fan,the carbonate increase sharply and the Fe/Mn decrease obviously.(?)The chemical weathering proxies based on the oxides content in the silicate minerals,e.g.,the ratio of Na2O/Al2O3,and Na2O/Al2O3 in the acid-insoluble fraction and chemical index of alternation(CIA),exhibit that this area suffered from a series of paleoclimate transitions:humid-arid-humid-arid at Middle to Late jurassic.The Quemocuo formation and Xiali formation display relatively low Na2O/Al2O3?Na2O/Al2O3 and high CIA,indicates relatively humid climate with strong chemical weathering.The Buqu formation and Suowa formation display high Na2O/Al2O3?Na2O/Al2O3 and low CIA indicates relatively arid climate with weak chemical weathering.(?)Utilizing the sensitivity of trace element plays an important role in inversing paleoclimate and tracing provenance.Ce/Ce*in acid-soluble fraction shows coupling relationship with CIA,higher Ce/Ce*correspond with lower CIA.For this reason,we propose that the ratio of Ce/Ce*can be treated as a paleoclimate proxy.The high Ce/Ce*represent weak chemical weathering and arid climate,the high Ce/Ce*represent strong chemical weathering and humid climate.(?)The K+ and Na+ content in water soluble salts,which are sensitive proxies of salt formation,show the much higher concentrations in the Xiali formation and low concentrations in other formations,this conclusion shows that in the upper Xiali formation,the sedimental supply was relatively abundant.Summarized by provenance,tectonics,paleoclimate,and palegeographical conditions,the results indicate Xiali formation has the great potential of potassium deposits lies in the Qiangtang basin.Because Tethys Ocean retreated from the east to the west of the Qiangtang basin in Jurassic and Yanshiping locates in the east margin of the basin,there is no potassium deposits formation but some gypsum outcrop in this area.The further work of potassium deposits exploration should focus on the Xiali formation in the central and western part of the Qiangtang basin.