| Nangqian Basin is belong to Qiangtang block,the main block of Qinghai-Tibet plateau.Research on Nangqian Basin is significant to Qiangtang block even Qinghai-Tibet plateau problems such as tectonic evolution,kinematics and dynamics.It's formation and evolution involve the Qinghai-Tibet plateau uplift and the evolution of Tethys Ocean.The salinities and distribution area of salt springs in the Nangqian Basin are all prominent in the world.High concentrations of saline spring is a rare geological geographical phenomenon nature.Geological fluid can provide the scientific basis for geochemical field,hydrodynamic field,geothermal field and the tectonic evolution.Therefore this article selected high concentrations of salt spring in Nangqian basin as research objects.The source of salt material and theirformation conditionswere studied here for the evolution of the basin provide geochemical evidence.Saline springs in Nangqian basin were investigated on May 2013 and May 2014 and June 2015 three times.According to the investigate 8 saline springs and the rivers around them were selected as the study objects,they are Dagai Spring,Ranmu Spring,Niuriwa Spring,Riazhong Spring,Duolunduo Spring,Baizha Spring,Gayang Spring,Chaha Spring.The element composition analysis and stable isotope analysis of the saline spring is completed in the test center,the Chinese Academy of Sciences institute of salt lake.The results show that the average salinity of salt springs in the Nangqian Basin is 254.6g/L,range from 150g/L~294g/L.The salinities and distribution area of salt springs in the Nangqian Basin are all prominent in the world.The hydrochemistry type of salt springs is sulfate type,with neutral p H value.High salinitity and element geochemistry characteristics indicate that there exist salt strata under the Nangqian Basin,which can provide rich materials to salt springs.The boron isotope studies indicated that the ?11B value in salt springs range from 3.55‰ to35.49‰,with a gradually decrease from northwest to southeast.In combination with the Br content of salt springs,we believe that the saliferous strata is continental origin.Diagram of cluster analysis for chemical Components of saline springsindicated that chemical elements can be divided into three types: Na-Cl-K-Br,B-Li-SO4-HCO3 and Ca-Mg.Triangular diagram of saline spring compositions in the Nangqian Basin show that Na+accounted for more than 90% of the total cation,Cl-accounted for more than 95% of the total anion.Then we suggest that genetic type of salt springs in the Nangqian Basin is dissolution of mainly haliteand a small amount oflimestone and gypsum rock.The salt spring Na/Cl value distribution around 1.0 or slightly greater than 1.0(>0.86 the dissolution brine;<0.86 the sedimentary brine),so it can further illustrate the salt spring genetic type is dissolution brine.In addition,an inverse relationship between boron isotopevalue and boron content in springs is displayed,with a two-end-members: high boron content verselow ?11B value and low boron content verse high ?11B value.Meanwhile,a same inverse relationship is also showed between boron content and p H value.In combination with regional geological studies,we therefore suggest that the large amount of igneous rocks exposed in the Salt Spring area maybe an important source of the boron in this region.On the basis of comprehensive comparison study,four conditions for salt spring formation in the Nangqian Basin are suggested here: 1)the saliferous strata is the source of salt materialsin salt springs;2)stable rainfall offer sustained water source;3)widely distributed faults supply channels for the springs;4)large topographicaltitude differences provide a stronger hydraulic gradient. |
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