Petrologic Features,Geochemical Characteristics And Genetic Significance Of Mesozoic Volcanic And Intrusive Rocks In Zhenglan Banner,Inner Mongolia

Zhenglan Banner area is located in the transition area of the north Hebei and west Liaoning volcanic rock belt and Great Khingan in the northern margin of North China Craton of Inner Mongolia.It is an important part of the volcanic rock belt in eastern China.Mesozoic igneous rocks are exposed extensively and the lithologies and volcanic rock facies are very rich.The igneous rocks include granite porphyry,porphyroclastic lava,ignimbrite,rhyolite,trachyte,quartz gabbro,etc.In this paper,through the field geological investigation,LA-ICP-MS zircon UPb age dating,major and trace elements component analysis,Sr-Nd-Hf isotopic tracer and so on,we studied the igneous rock facies classification,formation time,evolution of the magma,source of the magma and tectonic background.Through field geological and indoor work under the polarizing microscope,the dome of porphyroclastic lava can be divided into ignimbrite,felsitic porphyroclastic lava,granular porphyroclastic lava and granite porphyry from outside to inside.The porphyroclastic lava body overlap in the effusive rhyolite-trachyte,which show that the extrusive dome should be later than the effusive rocks.Porphyroclastic lava dome consistent within the error range for age 141.6±0.6?139.4±0.7 Ma.The formation age of the rhyolite was 141.4±0.7 Ma.The formation age of Porphyroclastic lava dome and effusive rocks within the error range are the same.So they should produced during the same period at the early Cretaceous.The geochemical characteristics of them are consistent and all show the wealthy silicon and alkali,especially rich K;poor Ca,Mg and Al,enrichment of Rb,Th,U and other large ion lithophile elements,and strong loss of Ba,Sr,Ti,P;Enrichment of light rare earth elements(LREE).The Eu negative anomaly is intense.And they obviously experienced crystallization differentiation.Extrusive and intrusive porphyroclastic lava dome and the effusive alkaline rhyolite and trachyte present different evolution curves in MgO;CaO,K2O,Co,Ba,Sr,Zr with SiO2.And they ?Hf(t)and ?Nd(t)are different.So they should form in two different stages for the same period magmatism.And the ?Nd(t)and SiO2 has good linear relationship,which means the magma may experienced assimilation and contamination of North China Craton ancient crystalline basement.The Si02 content of porphyroclastic lava dome reduce gradually from ignimbrite to central intrusion(from edge to center).It illustrates that the acidic magma in the roof of magma chamber formed the ignimbrite and the basic magma at the bottom of magma chamber formed the granite porphyry.Granitic igneous rocks in Zhenglan Banner area possess the geochemical characteristics of A1 type and A1-A2 transition granite.Hf isotopic projects between the evolution area of Mesoproterozoic lower crust.And Sr-Nd isotopic projects between the depeted mantle and lower and upper crust of North China Craton and part of the samples distribute in the scope of the earth's crust basic granulite inclusions of Hannuoba.They have isotopic loss characteristics significantly.Part inherited zircons project Precambrian rocks area of the northern margin of North China Craton in the same period magmatic rocks.So the acidic igneous rocks may derived from partial melting of intensely transformed lower crust of the North China Craton.Rock association of Zhenglan Banner in Mesozoic is alkaline rhyolite,trachyte and basaltic porphyrite,which shows bi-model volcanic and intrusive rocks.Combined with aegirine and aegirine-augite trachyandesite of zhangjiakou Formation in Duolun area,they formed peralkaline and alkaline series and no calcium alkaline igneous rocks associated.So they reflect the continental rift environment,which is consistent with the tectonic environment discrimination diagrams of trace element of granite.The value of ?Hf(t)is gradually reduced from Huanggangliang region-Zhenglan Banner areaTaipusi Banner area-Zhangjiakou region,which means the influence of ancient continental crust material becomes more and more strong.In other words,it also means the decreasing influence of new crustal material or transformation degree of mantle materials on the North China Craton.And it may reflects the startup time of craton destruction on the northern margin of North China Craton(Zhenglan Banner and Taipusi Banner region)-was earlier than the central(zhangjiakou area).