Present Hydrothermal Activities During Collisional Orogenics Of The Tibetan Plateau

Being reputed as "the ridge of the world", the Tibetan plateau is the highest and the mostampliate plateau in the world. But its intensive uplifting only occurred during recent severalMillion years, and the enigma of the process and mechanism of lifting attracts many geologists topersevere in their's researches. Hydrothermal activities were very developed in the plateau,especially in the south Tibet. Out of question, the violent hydrothermal activities are induced bycollisional orogenics of the Tibetan plateau. This paper mainly deal the questions as follow: (1)the times and evolvements of hydrothermal events during the recent geological history. And then aframework of space-time's evolvement established. (2) the "heat bump" of hydrothermal systems.The research shows it is the "fractional melting layers" in the crust driving the hydrotherrnalactivities. Then this paper discussed the property, range and time of the molten layers. (3)enrichment processes of the metallogenetie element Cs during the uplifting of the plateau. (4) theformation fo an conceptional model for hydrothermal systems. (5) the uplifting process of theTibet plateau bulit based on the characteristic of the hydrothermal events.Four stages of hydrothermal events had been recognized in field work, and the ESR ages ofsinters showed that hydrothermal activities mainly had happened in four periods, and their ages is700～450ka.,400～350ka., 270～200ka., and 150ka～0 respectively. The earliest age of thehydrothermal activities is about 700ka., and then four hydrothermal events occurred. Based on theintensities of hydrothermal events, the four events can be divided into two cycles: the one includesthe early there hydrothermal stages during 700～450ka., and the other begins with 150ka., theforth hydrothermal stage. Thereinto, the activity of the first stage (which ages is 700～450ka.) ismost violence, and weakened in the second (the age is 400～350ka.) and third stages (the age is270～200ka.) in turn. Until the forth stage (start at 150 ka.), the intensity of hydrothermal activityincreased. The two strong hydrothermal events correspondent to the twice poignant tectogeneticmovements (the Kunhuang Movement and the Gonghe Movement) in time. On the basis of theresearch, this paper built the framework of space-time's evolvement. And more, the discuss aboutuplifting process also a content of the paper. The violent uplifting of the plateau occurring in600ka probably related to the large-scale upwelling of the deep asthenosphere. Upwelling heatcorrosion of the asthenosphere not only resulted in fractional melting of the lithosphere crust,which formed the low-speed and high-conduction layers, but provocated a certain scale ofhydrothermal activities draining to the surface along tensional fault and rift formed in the upliftingprocess. The other violent uplifting in 150ka possibly related to the large-scale fractional melting of the upper crust and thermal abnormity and thermal buoyancy. The crust fractional meltinglayers would heat up and drive the circulation of large-scale hydrothermal liquid.The analysis of the "heat bump" show that it is probably the fractional melting layers whichbehave as low-velocity and high-conductivity lying in the geophysical exploration in the crust.The fractional melting layers are likely silicate fused mass, other than liquid with main componentof water. The melting layers are not only providing heat energy for shallow hydrothermal system,but also supplying liquid and metallogenetic elements to them. The result of research suggest thatthe upper fractional melting layer has driven ampliate area's hydrothermal activities in the southTibet, whereas the near surface upwelling emplaced melting masses from the lower crust promotethe presenting of some high temperature geothermal fields. And thus, the distribution of fractionalmelting layers can be lined out according to the range of surface hydrothermal systems.The formation of the new type of Cs ore deposit——Cs—bearing geyserite deposit, whichonly be found in the Tibet plateau, is also an interesting hot point attracting many focuses. Thispaper discussed the enrichment of Cs in all kinds of geological actions occurring during thecollisional orogenics of the Tibet plateau in detail. The enrichment of Cs probably come formsome process as follow: The fractional melting of crust mass congregate the Cs of rocks firstly;Fractional crystallization of magma and separation of magmatic water drive a lot of Cs into theremnant magmatic melt and magmatic water; The extensive action of water and rocks during themigration upwards of liquid capture Cs from the wall rocks. All of the three process result in theawful enrichment of Cs in liquid.