Study On Lithologic Characteristics Of Color Hills In Linze County Of Gansu Province And Its Color Significance

Linze County,Zhangye City in Gansu Province,has developed gorgeous colorful hills with large area,and spectacular scenes.The hills are similar to Wucai Mountain in Aksu,Xinjiang,Badland national park in the U.S.and Rainbow Mountain in the Andes of Peru.The main colors of the hills are yellow,red,green and gray.The lithology of the strata is mainly mudstone,argillaceous sandstone and sandstone,and the thickness is about tens centimeter to a few meters.Due to the large color difference of interlayers,it gives people a strong visual impact.For many years,these colored hills are often mistakenly called"Danxia Formation"by some people because they are mostly developed in arid areas with red color.Danxia Formation has a strict scientific definition,and its research history is more than 90 years.The research content involves many aspects such as definition,morphology,classification,development and evolution.However,from the perspective of stratigraphic discipline system and the need to carry out international correlation research,there are still weak aspects in basic research of color hill strata and Danxia strata.At domestic and abroad,most of the research methods are more traditional,modern technology is less used,and most of the research focuses on macro and qualitative description and inference.The existence of the above problems directly leads to the lack of support for the definition of color hilly landform and Danxia landform,which cannot be distinguished for a long time.In addition,previous studies indicate that iron bearing minerals in the colored hill strata and Danxia strata may be an important cause of coloration,but the deportment of iron has not been discussed.In this study,X-ray X-ray diffraction（XRD）,X-ray fluorescence spectrometer（XRF）,Mossbauer spectrometer and other analytical instruments are used to carry out in-depth study on petrography,geochemistry and deportment state of iron in the colored hill strata of Linze County,Gansu Province,and compared with the typical Danxia formation in South China.The result is as following:（1）The main lithology of the colorful hills in Linze County,Gansu Province is mainly mudstone,argillaceous sandstone and fine sandstone,with a small amount of shale.The bedding structure is clear.Different color strata are interbedded,with thickness ranging from tens of centimeters to several meters.Rock strength is mostly weak,mainly argillaceous cementation.Whereas to the samples from Danxia Formation in Guangdong Danxia Mountain and Jiangxi Longhushan Mountain are mostly sandstone,containing a small amount of conglomerate.The rock is compact and hard,with a higher degree of consolidation,and most of them are iron cementation.（2）The results of optical microscope and XRD analysis showed that:The main mineral components of the colorful hills in Linze County are quartz（25-75%）and clay（5-40%）±feldspar（5-35%）,with small amount of zeolite,mica,dolomite,calcite,gypsum and sulfide.The yellow samples are mainly mudstone and sandstone;There are minor differences in mineral composition among grayish green sandy mudstone,grayish argillaceous sandstone and dark green shale;The mineral compositions of light red argillaceous sandstone and dark red argillaceous siltstone are quite different.The samples of Danxia from Guangdong Danxia Mountain and Jiangxi Longhu Mountain show that the main mineral composition is quartz and feldspar,with a small amount of clay and iron oxide,and the degree of consolidation is higher.And the four samples from typical Danxia area all contain iron oxide,indicating that the red color may be related to iron oxide.（3）The results of XRF analysis show that the geochemical composition of the colored hill strata can be divided into two groups:low silicon mudstone and high silicon argillaceous sandstone.The geochemical characteristics of the high silicon light red argillaceous sandstone and the cyan gray carbonaceous argillaceous sandstone are similar to those of the typical Danxia Formation in South China.Geochemical correlation analysis shows that Si O₂ has a high negative correlation with TFe₂o₃,Mn O,Mg O and LOI,and a high positive correlation with Al₂O₃ and Na₂O,indicating that the main mineral components of the colored hills are quartz,clay and feldspar.From the chemical weathering index CIA,there is no significant correlation between the CIA value of color hills and the color hue change of rocks.The chemical weathering index of typical Danxia in South China is higher,which indicates that the chemical weathering of typical Danxia in South China is more severe due to its subtropical humid climate,abundant heat and abundant rain.There is a high correlation between redox index and color hue,that is,the stronger the reducibility,the colder the hue.In addition,the total iron content and ferrous content of all samples have no linear relationship with red;The total iron content of red and yellow samples in Linze color hills is 6.51 wt%and 5.87 wt%,respectively.Whereas the total iron content of typical Danxia Formation in South China is only 0.81 wt%%,indicating that there is no simple correspondence between total iron and color,and it is necessary to analyze the occurrence state of elements.（4）In this paper,Mossbauer spectroscopy is used for the first time to analyze the deportment of iron in the colored hilly strata in Linze,Gansu Province.The results show that:the deportment of iron is mainly clay iron and oxide iron in the typical Danxia formation in South China.Deportment of iron in colored Hill Formation is more complex,mainly in the form of clay iron,oxide iron,carbonate iron and sulfide iron,which is consistent with the diversity of mineral composition.Among them,only clay mineral iron is observed in the cyan,grayish green and dark green samples,while the yellow samples are mainly clay mineral iron and sulfide iron.The light red samples contain both clay iron and oxide iron（25%）,and the dark red samples also contain carbonate iron（13.8%）.The grayish green formation is related to clay minerals,the yellow formation may be mainly related to iron sulfide,and the red formation may be related to iron oxide and iron-bearing calcite/dolomite.It is worth noting that the content of feldspar in yellow strata is high,and the influence of feldspar can not be ruled out.Comprehensively,the mineral composition of the strata with different colors is different,mainly quartz and clay minerals.The mineral assemblage is quartz（25-75%）and clay（5-40%）±feldspar（5-35%）±iron oxide±±zeolite±±mica±±dolomite±±calcite±±gypsum±±sulfide,which is basically consistent with the results of hand specimens and microscopic observation.Mossbauer spectrum shows that the occurrence of iron in the color hills is various.Although the contents of iron oxide（hematite）and carbonate minerals are not high,they may be the main cause of reddening;The dark green,gray green and cyan gray are mainly related to the existence of ferrous in clay minerals.Yellow may also be related to weathered feldspar and iron sulfide in rocks.