Geochemistry And Petrogenesis Of Mesozoic Volcanic Rocks In The Geological Corridor Of Western Liaoning

Geological corridor of western Liaoning located in the northern margin of the North China Craton(NCC),had successively experienced tectonic superpositions of Tethys Ocean Plate,Paleo Asian Ocean Plate and Pacific Ocean Plate since the Phanerozoic.Mesozoic magmatism is well developed in this area,and a large scale of volcanic rocks exposed.This thesis studies on the Mesozoic volcanic rocks in the geological corridor.LA-ICP-MS zircon U-Pb dating results indicate that Mesozoic volcanism in the geological corridor can be subdivided into three stages: including early Jurassic(182~178 Ma),late Jurassic(160 ~149 Ma)and early Cretaceous(131~112 Ma).Based on the mineral analysis,whole-rock major and trace element analysis,zircon Hf isotope analysis,together with their rock associations,spatial distributions,and regional tectonic events,this thesis discusses rock petrogenesis and tectonic setting of different stages of Mesozoic volcanic rocks.Combined with rock association,spatial and temporal relationship,geochemistry data and isotopic data of Mesozoic intrusive rocks,this thesis aims to establish the chronological framework of magmatism in the corridor,to reveal the relationships of Mesozoic magmatism and tectonic evolutions in the corridor,to discuss the constraints of Mesozoic Magmatic events on the destruction of the North China Craton.The volcanic rocks of early Jurassic consist of trachy andensite,trachy dacite and a small amount of rhyolite,most of them belong to calc-alkaline rocks,and a few are alkaline series rocks.Trachy andensite and trachy dacite have the feature of adakitic rock.Pyroxene phenocrysts in rocks have multiple cycles of crystallization and positive rhythm zonal texture,meanwhile rocks have high content of Ni,Cr and Mg# value,are enriched in light rare-earth elements(LREEs)and large ion lithophile elements(LILEs,eg.,Ba,Rb,K and Pb),and depleted in heavy rare-earth elements(HREEs)and high field-strength elements(HFSEs,eg.,Nb,Ta,P,Ti,Y).Moreover,their zircon ?Hf(t)values of-16.0 ~-8.7.These characteristics indicate that the primary magma was derived from the partial melting of ancient lower crust.Rhyolite rocks also have the feature of adakitic rock,and have similar geochemical features to that of trachy andensite.Additionally,their zircon ?Hf(t)values of-11.6 ~-7.4.All of the facts indicate that the primary magma was derived from the partial melting of ancient lower crust.The early Jurassic volcanic rocks are formed in the transformation stage that from intraplate tensile into intracontinental orogenic extrusion state,which is considered to cause partial melting of the mantle lithosphere,and then lead to basaltic magma underplating event.The volcanic rocks of late Jurassic consist of basaltic trachy andensite,trachy andensite,trachy dacite,rhyolite and a small amount of basalt.They are calc-alkaline rocks or alkaline series rocks.Basalt rocks have low content Cr,Ni,Sc,Co and Mg# value,which indicate that the magmatic source maybe basaltic lower crust with contamination of middle-upper crustal material.Compared with basalt rocks,basaltic trachy andensite rocks have lower Mg# value,MgO,Cr,Ni,Co,Sc and HREEs content,but higher LREEs content,which indicate that they are derived from basaltic magma.The major of trachy andensite and trachy dacite have the similar geochemical characteristics to that of the early Jurassic rocks with the feature of adakitic rock.The linear correlation between the major elements and SiO2 sugesst rocks underwent a certain extent of fractional crystallization.In addition,their zircon ?Hf(t)values of-21.9~-7.9,suggest that the magma originated from the partial melting of the lower crust,which could be have minor contribution of mantle material.Rhyolite belongs to calc-alkaline rocks.Rocks have higher contents of Th and U,but lower values Sr/Y.Meanwhile,they are enriched LREEs and LILEs,and depleted HREEs and HFSEs.In addition,they have an Eu negative anomaly and zircon ?Hf(t)values of-23.4 ~-17.8.All of facts indicate that magma source region may be the partial melting of upper parts of lower crust that composed of felsic granulite or gneiss rocks.The late Jurassic volcanic rocks may be formed in an active continental margin environment of the subduction of the Paleo Pacific plate.Basaltic magma underplating caused inhomogeneous structure of crust mantle,which could explain some inner areas of the NCC that underwent an extensional environment.The volcanic rocks of early Cretaceous consist of basalt,basaltic trachy andensite,trachy andensite,trachy dacite and a small amount of rhyolite,most of them are alkaline series rocks.The values of content Cr,Ni and Sc are between the content of lower crust and mantle,indicating a contribution of mantle material.The pyroxene phenocrysts have multiple cycles of crystallization and reverse zonal texture,and the Mg# values of them vary in the larger range,suggesting the magma source region may be from lithospheric mantle,the basaltic lower crust or the crust mantle transition zone.The rare earth elements(REEs)of basaltic trachy andensite are similar to that of basalt,but with a higher fractional degree of HREEs and LREEs,enriched LREEs and LILEs,and depleted HREEs and HFSEs.All indicate magma source region is from an ancient lower crust that may be the result of further evolution of basaltic magma.Trachy andensite and trachy dacite have similar geochemical characteristics.The linear correlation between the major elements(eg.,MgO,TFeO,TiO2)and SiO2 indicate that magma experienced crystallization differentiation.Meanwhile,they have more extensive REEs content and higher degree of LREEs fractionation,which suggest that it may be derived from basaltic trachy andensite magma after further crystallization differentiation.In addition,multiple cycles of crystallization or zonal texture can't be found in the rocks of pyroxene phenocrysts,which may be suggest that there is no basaltic magma mixing in the later stage of magma evolution.The REEs content of rhyolite are far less than that of basaltic or intermediate volcanic rocks,and rocks have a lower fractional degree of HREEs and LREEs,which suggest that it is not a possible result from the fractional crystallization of basaltic or intermediate magma.Meanwhile,rocks have the similar geochemical characteristics to that of late Jurassic rhyolite with low values of Sr/Y,and their zircon ?Hf(t)values range from-20.9 to-15.4.All of those facts indicate the primary magma was derived from the partial melting of upper parts of lower crust that composed of felsic granulite or gneiss rocks,and then experienced fractional crystallization during eruption process.The Triassic granitic rocks are the product of partial melting of ancient lower crust with a small amount contribution of mantle materials,which indicate the lithosphere might start thinning process in the late Triassic.The scale of magmatic activity inplies lithosphere thinning was limited,and no evident craton destruction occurred.The NCC undergoes a tectonic transformation stage during the the Jurassic period.The early Jurassic magmatic rocks are derived from the partial melting of lower crust,while the outlier of Hf isotope indicates that there may be input of mantle material,but its contribution was limited.The scale and occurrence of magmatic rocks,north-south nappe structures suggested that it may be a collective remote effect of both Mongolia Okhotsk ocean closure and the subduction of the Paleo Pacific plate.The effect is considered to cause the partial melting of the enriched lithospheric mantle to form basaltic magma,and then the underplating caused the partial melting and accretion of the ancient lower crust.However,it did not result significant thinning of the lithosphere.The late Jurassic magmatic rocks are derived from the partial melting of Archean or Proterozoic lower crust with mantle material contribution.The scale of magmatic activities,the occurrence of alkaline series rocks and regional metamorphic core complexes,implied that the continuous subduction of the Paleo Pacific plate not only resulted in partial melting of the lithospheric mantle and the ancient crust,but also altered the physical properties of the lithosphere mantle.There is a certain degree of thinning of lithosphere,and some areas are in a state of tension.All facts imply that a prerequisite for craton destruction have been prepared.The early Cretaceous magmatic rocks have multiple source regions,which imply that stronger crust-mantle interaction occurred.The scale of magmatic rocks,regional metamorphic core complexes,large scale alkaline series rocks and “A” type granites,suggestting that the lithosphere was in a strong tensile state.The thickness of lithosphere is greatly thinning,and the craton destruction reaches its peak.It is considered that high angle orthogonal subduction or retreat of Paleo Pacific plate caused upwelling of massive asthenosphere material,and then the lithospheric mantle completely lost its stability.