Genesis Study On Cenozoic High Mg～# Potassic Volcanic Rocks In Qiangtang, Northern Qinghai-Tibet Plateau

The formation and evolution of volcanic rocks occurred in Qiangtang, NorthernQinghai-Tibet Plateau had gone through a very long geological time since the collisionbetween India continent and Eurasia one 60~50Ma ago. This volcanic rocks were thoughtas important window or geological probe for researching the material composition of thelithosphere, the structure and interaction of the crust-mantle in the plateau and themechanism of the plateau uplift. They have been followed with great interest bygeologists for a long time.The Mesozoic high Mg# potassic volcanic rocks in the Qiangtang, NorthernQinghai-Tibet Plateau were researched in the paper. The author has made systemicinvestigation on petrology, major element, trace element, REE and isotope geochemistryof high Mg# potassic volcanic rocks through comprehending plenty data and productionsof some scholars, and have carried comprehensive analysis and discussion on the genesisof high Mg# potassic volcanic rocks through investigation of the melting experimentationof high temperature and high pressure and dating of zircon. It can provide petrologyrestriction for the structural evolution of the lithosphere in the Northern Qinghai-TibetPlateau.The Cenozoic high Mg# potassic volcanic rocks are divided into high Mg# and Kcalc-alkaline series and high Mg# shoshonitic series in chymical composition. The highMg# and K calc-alkaline series were found chiefly at Cuoni, Duogecuoren, zuerken, wulahill, Dongyue lake, Yanzi lake and Zhentouya of the northern Qiangtang and Nadingcuoo,Zougouyouchacuoand Shuang lake of the southern Qiangtang and so on. The main rockconsist of latite, trachyte, dacite, trachytedacite and a little rhyloite. The high Mg#shoshonitic series were found chiefly in the northern Qiangtang, presently, at the west ofYudai hill, Taiping lake-Botao lake, the south of Haobo lake and Bandao lake-Ruejinla,and a little at Shuanghu of the southern Qiangtang, otherwise in the east of Fenghuo hill.The main rock compositions consit of tephrite, shoshonitic-latite and trachyte. Thesevolcanic rocks showed lava bed, and covered unconformitable the Paleogene Penna lakegroup or Jurassic Yanshiping group, which were formed by rapid cooling of lava. Thelandscape was remnant volcanic landform.The data of Ar-Ar isotopic dating of the Cenozoic volcanic rocks in Qiangtangindicated that the Cenozoic volcanic action in Qiangtang began with sodic alkaline basaltseries(60~44Ma ago), high Mg# and-K calc-alkaline potassic volcanic rocks have beenthe main part 42Ma ago, the peak value age was 42~37Ma. The main activie time of highMg# shoshonitic series was 36~32Ma, and then were superseded by the peralkalinepotassic-ultrapotassic series, and the Miocene Epoch shoshonitic neuter acidic volcanicaction was found in the south part of Zhentouya.The sodic alkaline basalt series hade weak fractional distillation in heavy REEs , theCenozoic high Mg# and K calc-alkaline series were characterized by richment of LREE,and depletion in heavy REE. The fractionation between LREE and HREE weresignificant, indicating the derivation from very deep source with garnet in the residue.The δEu values of the high Mg# and-K calc-alkaline series and the high Mg# shoshoniticseries did not change with the increase of SiO2 content. It had on or weak negative Euanomaly, indicating that the source region did not happen equilibrium with plagioclaseduring the course of evolvement of magma. The wider range of variations in compositionof the two series indicated that they were formed from the partial melting of the materialin thicked crust, or from mantle magma migmatization with crustal rocks or crustal melt .The higher composition of K2O and the higher Mg# value of the Mesozoic high Mg#potassic volcanic rocks hade important restriction of source magma. The experimentalresearch on water loss melting of calc-alkaline gabbro dioritic rock, which approachedthe general composition of low crustal rocks indicated that : (1) the melting degree wereincreased with the temperature in the same pressure, the melt composition was changedfrom basic to intermediate with the increase of melt, but the K2O composition of meltwasn't change. (2) Because the PH2O controlled the property of melt mainly, the changerule of the melt composition in the low pressure and high pressure wasn't obvious at thesame temperature, Because of the experiment system at half close and half open, the PH2Ofrom the hornblende dehydration was invariable during the course of melting. (3) themineral composition have important influence to the melt composition, besides thetemperature and PH2O, the melt nearby hornblende was enriched in Mg,Fe,Ca and Ti,the melt nearby plagioclase was enriched in Al and Na. (4) the melting degree wasincreased with the stress velocity at the same temperature.The result of experiments indicated that the water loss melting of calc-alkalinegabbro dioritic rock which approached the general composition of low crustal rocks couldproduced the basic-intermediate potassic melt, but the compositions of K2O and the Mg#value were lower than these of the Mesozoic high Mg# potassic volcanic rocks, Thecontent of K2O and the Mg# value of the melt were restricted by the sample ofexperiment . Not only the water loss melting but also dehydration melting experiments,the sample with basaltic composition could not produced the melt that were providedwith the composition of the high Mg# potassic volcanic rocks in the Qiangtang.During melting experiment, the melt glass happened to segregate phenomena whenthe temperature was more than 1000℃,the pressure was 2.4GPa and the melting degreewas more than 80%. The segregation resulted in the form of enriched iron oxide withspheroid and irregular bands, and leaded to Mg# value of silicate melt to increaseevidently. Then the segregation of enriched iron oxide from basic silicate melt couldproduced high Mg# andesitic melt, but the silicate melt have higher content of Al2O3. Thesegregation of enriched iron oxide occurred in the experiment provided importantexperiment proof for the forming mechanism and condition of enriched iron slurry in thenature.Trace element composition of the high Mg# potassic volcanic rocks was similar tothat of the intra-plate alkali basalt. The later from the mantle , which was extremelyenriched in large iron elements, also it shares the geochemical properties withactive-continent margin and island-arc basalts, which are extremely enriched in Nb, Ta,Ti high-field strength elements. Incompitable element ratios were characterized by lowerLa/Rb,Zr/Rb,Nb/Ba and higher K/La,Rb/Nb,Th/Nb,K/ Nb,Pb/La,which were similarto the character of incompitable element ratios of island-arc volcanic rocks in Aleutianislands and the quaternary ultrapotassium volcanic rocks in Taipei. The tectonicdiscrimination diagrams of high-field strength elements indicated that the departure of Yrelative to Ti, Zr, Nb occurred during the course of produce of pro-magma, and then thegeochemical characteristic of magma was deficient Y relative to Ti, Zr, Nb, the characterwas different from that of the island arc and active-continent margin basalts, the Sr,Nd,Pb isotope showed that the source magma hade the character of EM2 mantle . The abovecharacters and the higher abundance of Cr,Ni,Co indicated that the pro-magma werefrom the partial melting of the garnet monpyroxenes peridotite enriched lithospheremantle, which had ever been metasomatized by fluid from subducted slab.The results of SHRIMP and LA-ICPMS zircon dating gave that the ages of zirconwere often proterozoic in the high Mg# and-K calc-alkaline series in Duogecuoren andZougouyouchacuo and Leucite pollenite in Yulin hill. Both the ages of zircon and thecharacter of Sr,Nd,Pb isotope indicated that happened to mix and contaminate withcrustal material during underplating the mantle source magma.Synthesis before-mentioned analysis, the paper offered the following genesis modelof the high Mg# potassic volcanic rocks:(1) The Tethyan oceanic crust subducted toward the Eurasian continent, and led tothe upwelling asthenosphere of Qiangtang , then it formed the sodic alkaline basalticmagmatic activity 60Ma ago, in Qiangtang which were from asthenosphere melting .(2) After the collision between India continent and Eurasia one, the hysteresis ofoceanic crust thrusting led to the enriched lithosphere mantle, which was metasomatizedby fluid from subducted slab, and then it formed the high Mg# and-K calc-alkalinebasaltic magma, during magma underplating, happened the mixing between crustal andmantle magma , and magmatic fractional crystallization, at last ,formed the combinationof the intermediate high Mg# potassic volcanic rocks.(3) The subducted slab breakoff 30Ma ago, led to the upwelling of asthenosphere inthe core of Plateau, in the condition of asthenosphere fluid, so the ancient enrichedlithosphere mantle happened partial melting and formed the volcano activity of theperalkaline potassic-ultrapotassic series.