Geochemistry And Tectonic Setting Of The Early-Cretaceous Intermediate Volcanic Rocks In Zhalantun Area,Inner Mongolia

Mesozoic volcanic rocks are widely outcropped in the Great Xingan Mountain.According to the age,geochemical characteristics,geological origin and tectonic setting of volcanic rocks in Great Xingan Mountain,the studies had been still a hot debate previously.On the basis of the regional geological survey?1:50000 mapping?of Zhalantun area,The author has carried out a comprehensive study of the strata,volcanic rocks,intrusive rocks,their tectonic characteristics and settings in the study area.Especially,taking the aim of the andesites from Manitu and Longjiang Formation that had not been studies previously,the paper has reached several conclusions as following by means of typical profile measure in the field work,geological characteristics and geochronology U-Pb dating.1.The Manitu Formation is composed predominately of the andesite,andesitic clastic rocks and can been classified into the overflow facies mainly and explosion facies minor with greygreen and grey-black color.The Longjiang Formation is composed predominately of tuffaceous sandstone,tuff,bearing-breccia crystal tuff characterized by the explosion–sedimentary,explosion,minor overflow facies,the upper layer consists of minor andesite,dacite with grey-purple and yellow color.2.The Manitu trachyandesites have SiO₂ concentrations range from 56.41% to 62.17%;They show higher Na₂O/K₂O ratios of 0.88-1.49?average 1.29?;The REE concentrations of 117.25ppm-187.08 ppm.On a chondrite-normalized REE diagram,these trachyandesites exhibit fractionated of LREE/HREE with?La/Yb?_N ratios of 3.83-16.00,enrichment in LREE??LREE=104.23ppm-171.51ppm?and large ion lithophile element?LILE?,depletion in high field strength element?HFSE?.They also display somewhat Eu anomalies??Eu=0.71-1.30,average 0.94?.The geochemical characteristics indicate that they are mixing magma of crust-mantle,which were derived from lower crust and minor mantle components.They were produced at the setting of lithosphere stretching and thinning,which was associated with close of Mongolia-okhotsk ocean and later post-collisional extension.They may be derived from the partial melting of lower crust which dominated by garnet-bearing materials.The Manitu trachyandesites were formed in early stage during Early Cretaceous.3.Longjiang formation andesites have a variable and higher SiO₂ concentrations of 59.73%-67.20%;They display relative higher Na₂O/K₂O ratios of 0.91-2.19?average 1.45?;The REE concentrations of 83.24ppm-147.13 ppm with an average of 119.40 ppm.On a chondrite-normalized REE diagram,these andesites exhibit high?La/Yb?_N ratios of 8.80-15.91,enrichment in LREE?average 109.07 ppm?and large ion lithophile element?LILE?,depletion in high field strength element?HFSE?,and somewhat Eu negative anomalies??Eu=0.94-1.17?.They were probably produced from the partial melting of lithosphere mantle which was metasomatized by fluids.Also,they were contaminated by crustal materials during ascent.The Longjiang andesites were formed in late stage during Early Cretaceous.4.The tectonic implication for the the Manitu Formation and Longjiang Formation could be resulted from the transition process from Collisional-orogeny to Post collisionalorogeny that could be caused by the scissors-like closure of Mongol-Okhotsk Ocean as the result of the distant effect of Subducion-Collision-extension in the different areas far away from the suture Zone.