The Research For Mesozoic Volcanic Stratigraphic Age And Causes In The Zhalantun Area,the Great Xing'an Mountains

The Great Xing'an Range exposed a lot of volcanic rocks,covering the whole Great Xing'an Range,study area is located in the middle Zhalantun of the Great Xing'an Range,exposed Manitu,Baiyingaolao and Meiletu.This paper summarizes the data available through the system research,was measured volcanic rocks stratigraphic section of the study area,and collected a large number of samples,made a detailed microscopic thin section,geochemical analysis and zircon U-Pb isotopic dating,obtained as follows four-point understanding:(1)The rock association types of Manitu volcanic rocks includes trachyandesite,trachyte,andesite and andesitic pyroclastic rocks;Baiyingaolao volcanic rocks includes rhyolite and rhyolitic pyroclastic rocks;The rock association types of Meiletu volcanic rocks are symbiotic overflow phaseManitu volcanic rocks in the study area is mainly terrigenous sedimentary face tuffaceous sandstons and outbreak face andesitic tuff,clip a small amount of overflow face andesite;Baiyingaolao volcanic facies compare complex,mainly for overflow face,outbreak face,volcanic channel face and subvolcanic rock face.Meiletu is overflow face,no other type of lithofacies characteristics.(2)Manitu?Baiyingaolao and Meiletu measured sections were selected unaltered or only slightly altered andesite,rhyolite and basaltic andesite rock samples,choose better crystal and oscillation growth Endless belt magmatic zircons for U-Pb isotopic dating.The results show Manitu?Baiyingaolao and Meiletu volcanic rocks were formed in the Early Cretaceous,ages were 128.3±0.8Ma ? 128.0±1.3Ma and 128.1±1.2Ma.(3)In the Research area,Manitu?Baiyingaolao and Meiletu volcanic rocks the overall showed LREE enrichment,HREE is depletion,Are enriched in Rb,Ba,K and other large LILE,Loss Nb-Ta,Zr,and Ti(ie,abnormal TNT)high field strength elements(HFSE),reflecting the terrestrial arc magmatic rocks geochemical characteristics.Different with Manitu and Meiletu,Baiyingaolao appeared weak positive Eu anomaly,it indicates that such rhyolite different from the general sense of rhyolite from the crust.(4)Through a comprehensive study petrology and geochemistry suggest that the study area Manitu?Baiyingaolao and Meiletu.volcanic rocks mainly are Sub-alkaline potassium calc-alkaline rock series,part of Meiletu and Manitu belong to calc-alkaline rock series,only a small amount of Baiyingaolao and Meiletu's sample belong to the shoshonite series,Baiyingaolao and Meiletu trend to shoshonite series.They were formed in continental margin arc extensional tectonic environment background.Among them,Manitu rock produced in the crust mafic rocks partial melting,Baiyingaolao was andesitic magma crystallization differentiation product,Basaltic andesite(adakite)derived from subducted oceanic slab partial melting;And niobium-rich basaltic is formed in adakitic melt explained to the mantle wedge partial melting.