Study On The Lacustrine Exhalative Rocks In The Suonahu Formation Of Eastern Gaize,Tibet

1)According to the features of the rock's assembles in Suonahu Formation,it was divided into 3 members:the low member was brick-red conglomerate which mainly contained limestone.The middle member of Suonahu Formation was calcareous mud-stone,marl with sandy fine conglomerate.The grain of quartz and the leaves of the paleoplants were found in the marl.The leaves-fossole was identified as Quercus pseudosemicarpifolia.Theup member was the red conglomeraticmudsand-st one and conglomerate.The conglomerate mainly contains limestones,and chert gravel can be found sometimes.On the whole,the cementation degree of the sedimentary rocks was bad,mainly poor-consolidated,and the rock assemblages indicated that Suonahu Formation was a set of lacustrine sediments.we defined the age of Suonahu Fomation as in earlyMiocene-Early Pleistocene by searching and comparing the paleo fossils.The sedimentary sequenc-e of the Suonahu Formation was coarse clastic rocks-carbonate rocks-coarse clastic rocks and the grain changed at the same time which from coarse to fine,then to coarse,meanwhile,the climate also differed from hot-dry to wet-warm,to hot-dry again.They represented a evolutionary process:fluvial facies-lacustrine facies-fluvial facies,and among the sedimentary process,hydrothermal liquids formed theexhalative rocks.2)The hydrothermal minerals in the white smoke type hydrothermal sedimentary rock of Suonahu Formation were mainly opal,chalcedony,quartz,dolomite and little metallic minerals.Four paragenetic associations of hot water minerals in the hydrothermal sedimentary rocks of the Suonahu Formation were recognized.Opal was homogeneous,with oval shape and other shapes;chalcedony had a comb,fibrous patterns which distributed around the quartz and opal;the quartz was microscopic crystalline which filled in the cracks of the hydrothermal sedimentary rocks whose forms were reticulated vein and the recrystallized.quartz was surrounded by the crusty structures.The crusty structures,formed with different times of deposits,represented the conditions of low-temperature liquids.The hydrothermal passages were seen with field outcrops.The marls with siliceous veins,siliceous rocks,quartzite,Dolomite were also seen outdoor.On the whole,they were characterized by siliceous rocks-carbonate rocks.3)The test results of carbon and oxygen isotope on the hydrothermal sedimentary rocks of Suonahu Formation showed that the average value of ¹⁸O,¹³C were-15.2‰,-3.79‰,With the data we concluded that the period of the sediment of Suonahu Formation was an open lacustrine environment,and by using?¹⁸O and experienced equations,it turned out that the hydrothermal minerals were formed at a relatively high temperature,which belonged to the hydrothermal sedimentary cause.The rocks formed at the temperature of 92.10?,owing to white smoke type hydrothermal sedimentary rocks.4)From the analyse results of the trace elements,we can found that the elements of Li?Sc?Rb?Hf?Cd?In?Cs?Pb?Bi were relatively low.From the trace elements spider graphic,we could see that the samples kept unchanged,parallel and the Ba?U?Sr?Zr?Sm were relatively enriched relatively,while Th,Nb were the opposite.The enrichment of Ba?U?Sr?Zr?Sm showed that the hydrothermal sedimentary character,and the deep materials had attended the sedimentary activity at the same time.U/Th had an average level of 19.35 in the hydrothermal sedimentary rocks which represented the hydrothermal sedimentary geochemistry characters.On the whole,the value of U/Th?V/Cr and V/Sr reflected anoxic diagenetic sedimentary environment.Co/Ni is 0.23 in the samples.Furthermore,it proved the construction of the hydrothermal sedimentary.Sr was high in the samples,indicating that the hydrothermal activity at the bottom of the lake had an great influence on it.5)From the REE analysis results,we found that the average of?REE is 11.11×10-6with abasal characteristics of hydrothermal depositional formation of?LREE>?HREE.REE distribution pattern proved that the weak negative Ce anomalies and the positive Eu anomalies.Y/Ho and negative Ce anomalies reflected the reducing of the hydrothermal liquids.Ba and?Eu had a positive correlation which indicating that both were synchronous enrichment.Positive?Eu directly represented the hydrothermal sedimentary construction.,meanwhile,infered that the hydrothermal water was strong reductive.6)Suonahu Formation“white smoke”exhalative rock was divided into four types:brecciate hydrothermal sedimentary rock in main exhalative mouth,network vein hydrothermal sedimentary rock in branch exhalative mouth,clastic hydrothermal sedimentary rock near main and branch exhalative mouth,hydrothermal sedimentary rock diffuse type hydrothermal sedimentary rock.7)The Ba of the“white smoke”exhalative rocks in Suonahu Formation,which was the abyssal element,had a clearly positive correlation with?Eu,they enriched synchronously,and the ore-forming material sources of the volcanics in Shipingding Formation were the mixture of the mantle and crust.On the whole,these evidences indicated that the hydrothermal liquids were sources of the mixture of the mantle and crust from out-basin.8)We builded the sediment model of the white smoke type of hydrothermal sedimentary rocks in the Suonahu Formation.In the research field,On the backup of the Qinghai-Tibet uplift,after the lacustrine carbonate rocks having formed,influenced by Cenozoic faulted structure,hydrothermal liquids spilled out,and the hydrothermal acticity influenced and changed carbonate rocks into exhalative rocks with the assembles of siliceous rocks-carbonate rocks.The siliceous and brecciated marl formed as a result of the invasion of the hydrothermal liquids,which damaged the surrounding rocks during its migration;the brecciated siliceous cement marl was the results of hydrothermal liquids'remodeling and damage on its surrounding rocks during its migration;weakly silicified marl veins formed in the branch channel,its influences were weak when compared with theinfluence in main channel;the siliceous phenomenon and the assembles of siliceous rocks-carbonate rocks didn't distribute widely,so it indicated that this siliceous activity were not regional,we inferred that this hydrothermal origin exhalative rocks was"point"event.