Genesis Of Quaternary Fossil Travertines In Langpu Reshuitang,Tengchong,Yunnan

As an important part of a spring system,a smooth slope is often widely distributed in the spring system.Previous studies involved with smooth slope travertines show that most of these travertines were deposited in low temperatures.High-temperature smooth slope travertines were rare studied in the past and their characteristics and genesis are still unclear.Considering that 1)modern springs in Langpu Reshuitang are featured with high temperatures?T?70??,2)fossil travertines formed on a smooth slope surface in Langpu Reshuitang are full composed of abiotic crystalline crusts,and 3)no records about plant activities are found in the fossil travertines,the fossil travertines might be smooth slope travertines deposited in high-temperature environments.A detail study about these travertines can help us better understand genesis of smooth slope travertines and factors controlling the petrological and mineralogical composition of smooth slope travertines.Basing on specific investigations about the chemical composition and cooling and evaporation models of waters from modern springs in Langpu Reshuitang,this study identified the sedimentary facies type and the petrological facies type of the studied fossil travertine and modern travertines from two active springs in Langpu Reshuitang.Mineralogical characteristics,crystal morphological features,and stable carbon and oxygen isotope composition are also checked.The results show:1)modern springs in the studied area discharge waters with high temperatures?T?70??,near-neutral pH?6.3–8.34?,and strong precipitation potentials of aragonite and calcite,2)amorphous SiO₂ will be formed during the cooling or evaporation processes of the modern spring waters,while other minerals,such as halite,mirabilite and Mn oxides,can be only formed after spring waters have been completely evaporated,3)the fossil travertines mainly composed of abiotic crystalline crusts belong to smooth slope facies,4)calcite is the predominate mineral in the fossil travertines,whereas trace Mn-Fe oxides,halite,thenardite,and allogenic clastics were also found,5)feather-like crystals are the most common calcite crystals in the fossil travertines,and some fan-like crystals and platy crystals and rare spike crystals were also developed,6)two types of non-calcified microbes?filamentous microbes and bean-like microbes?are developed in the studied fossil travertines,and 7)?¹³C?-1.59‰--0.87‰?and?¹⁸O(?¹⁸O=-22.42‰--23.70‰)of the fossil travertines are stable and no noticeable changes are found.Fan-like crystals,feather-like crystals,and platy crystals in the fossil travertines were precipitated in non-equilibrium conditions and their formation is possibly related to intense CO₂ degassing of spring waters.However,spike crystals are the products of calcite dissolution.The formation Mn-Fe oxides in the Langpu Reshuitang fossil travertines has close relation with microbial activity,but halite and thenardite were formed by full evaporation of low-temperature spring waters with poor SiO₂.The C-O isotopic results show that compared with most of the travertines in Tengchong,the studied fossil travertines were formed in a high-temperature environment?T>80??;and the parent CO₂ of the studied fossil travertines was derived from mantle,only minor of the parent CO₂ was from the Permian carbonates.Comparison between the smooth slope travertines in Langpu Reshuitang and other smooth slope travertine in other areas shows that the petrological and mineralogical characteristics in these slope deposits are strictly controlled by temperature.In high-temperature smooth slope environments,microbial activity is limited,while CO₂ degassing is strong,resulting in the formation of abiotic crystalline crusts.On the contrary,many microbes will thrive in low-temperature enviroments,causing strong microbial activity,and only slight CO₂ degassing occurs in these environments.As a result,lots of microbiogenic materials?e.g.micrite and bacterial shrubs?,instead of abiotic materials?e.g.fan-like crystals?,will be formed in these low-temperature smooth slope environments.The formation of the smooth slope travertine deposits at Langpu Reshuitang was influenced by a deep-seated magma chamber.The high-temperature magma chamber released much mantle-derived CO₂ and heated cold groundwater to produce high-temperature fluids.These fluids migrated along faults or fractures to a mountain slope at Langpu Reshuitang to form a high-temperature spring and water from this spring flew on the smooth surface of the mountain slope to form a hot spring fan.After these high-temperature fluids discharged on the Earth's surface and in the flowing process of the hot spring water,these fluids with high CO₂ contents occurred strong CO₂ degassing,resulting in the formation the smooth slope travertine deposits mainly composed of feather-like calcite crystals at Langpu Reshuitang.