Petrogenesis And Deep Geological Processes Of Mesozoic Basalts From The Northern Great Xing'an Ridge

Northeast China is located between North China Craton and Siberian plate,belonging to eastern Central Asian Organic Belt.It has experienced complex and long-term multi-block collage and orogenesis.Recent years,geologists have gained great progress on the study of Mesozoic tectonic evolution history for the area in Northeast China.However,they haven't arrived at a common understanding of some scientific issues such as the origin and dynamic mechanism of Mesozoic volcanic rocks.In order to grasp the essentials of these issues,we analysed major and trace element compositions and Sr-Nd-Pb-Hf isotope compositions of bulk rocks,major and trace element compositions of olivine phenocrysts and Sr isotope compositions of plagioclase phenocrysts of Mesozoic basalts in the Northern Great Xing'an Ridge,northeastern China.Mesozoic volcanic rocks of the Great Xing'an Range consist of a series of intermediate-basic volcanic rocks,such as basalts,basaltic andesites,trachuandesites and so on.They are rich in Ba,Pb,Sr and lack of Nb,Ta,Ti,which are somewhat alike to the lower crust and differ from MORB and OIB.Theses differences indicate that Mesozoic volcanic rocks come from continental lithospheric mantle rather than asthenosphere.Mesozoic volcanic rocks have enriched(⁸⁷Sr/⁸⁶Sr)_ivalues and enriched to weakly depleted(¹⁴³Nd/¹⁴⁴Nd)_ivalues,most of which ploted into EM1 areas in(⁸⁷Sr/⁸⁶Sr)_ivs(¹⁴³Nd/¹⁴⁴Nd)_idiagram,revealing the importance of recycled crust in the source.In the article we analysed the influence of alteration,crustal contamination and crystallization on magma evolution.We found that Mesozoic volcanic rocks experienced different degrees of alteration,according to the weak correlation between fluid mobile elements and inactive elements.Nevertheless,we found that Mesozoic volcanic rocks experienced extensively distributed crustal contamination,as there exists positive correlation between LILEs/HFSEs,(⁸⁷Sr/⁸⁶Sr)_i and SiO₂ contents and basalts with high SiO₂ contents own plagioclase phenocrysts with high ⁸⁷Sr/⁸⁶Sr ratios.Meanwhile,Late Jurassic volcanic rocks were produced through crystallization of olivines and clinopyroxenes of the primary melts,early early Cretaceous volcanic rocks were generated after the seperation of Fe-Ti oxides and plagioclases of the primary melts while late early Cretaceous volcanic rocks were generated after the seperation of olivines,clinopyroxenes and plagioclases of the primary melts.The nature of magma source may be covered because of the influence of AFC processes.In hence,we selected basalts with high MgO contents to dig out more information of magma source.Olivines from late Jurassic basalts have sifnificantly higher Ni contents.As Fo of olivine pehnocrysts increase,Mn content decreases with Fe/Mn ratio increases,all of which indicate that a pyroxenite component contributes to late Jurassic source lithology.Meanwhile,Olivine phenocrysts from late early Cretaceous volcanic rocks have relatively low Ni,Cr and Ni/Co contents,which is similiar to peridotite source.Nevertheless,Mn contents and Fe/Mn ratios mostly fall into the range of peridotite source,suggesting peridotite may be the major component of late early Cretaceous source lithology.In the diagram of Pb/Zr vs Ba/Zr and?Th/Nb?_N vs Ba/Zr,late Jurassic and late early Cretaceous volcanic rocks show obvious positive correlation,proving that they all have been metasomatized by fluid in subduction belt.Olivine phenocrysts of two periods have relatively low V/Sc ratios?<3?,showing that they were formed in open environment.In conclusion,we come up with an assumption that Late Jurassic basalts formation is related to the subduction of Mongolia-Okhotsk ocean plate and late early cretaceous basalts formation is more likely to be influenced by the delamination of continental crust after the final closer of Mongolia-Okhotsk ocean.