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Geochronology, Geochemistry and Petrogenesis of the Hongqiang Basalts from Northeast Qinghai-Tibetan Plateau
详细信息   
摘要
West Qinling orogenic belt located at the northeastern margin of Tibetan Plateau is a transform junction belt among the Paleo-Asian tectonic domain,the Tethyan tectonic domain and the Pacific tectonic domain,and is a key region in the division of the geographical and geological structures between the eastern and western China.A large amount of Mesozoic volcanic rocks are widely distributed in West Qinling.However,there are some debatable issues on petrogenesis and geodynamic settings of the Mesozoic volcanic rocks in this region,because of poor basic geological research in the past,especially lacking reliable geochronological and geochemical data of the volcanic rocks,which had affected,the comprehensive understanding of the tectonic setting and evolutionary history of West Qinling.In this paper,we present petrographic,geochemical and isotopic chronological data of Mesozoic basalts in Hongqiang Area of Xiahe County in West Qinling,Gansu Province,and discuss its petrogenesis and geodynamic setting.The LA-ICP-MS zircon U-Pb age of the basalt is(104.8±0.99)Ma(MSWD=0.63) which is consistent with its field relationships,and indicates that the magma activity of the Hongqiang basalts occurred mainly in the Early Cretaceous.The petrographic study shows that the basalts are characteristic of occurring plagioclase phenocrysts with large amount of plagioclase microlites and micro-phenocrysts in the groundmass.In petrochemistry the volcanic rocks are characteristic of higher SiO2,Al2O3,Na2O and lower CaO contents demonstrating that the basalts belong to a sodic-alkaline magma series which is different from the Cenozoic ultrapotassic volcanic rocks from Lixian-Dangchang in the eastern part of the study area.As to geochemistry,the Hongqiang basalts are rich in REE and HREE(Nb,Ta,Zr,Hf,Ti) and slightly depleted in LILE(Rb,K),and have positive εNd(t) values(5.9-7.5),low initial 87Sr/86Sr ratios(0.703 3-0.705 6) and Pb isotopic ratios are 206Pb/204Pb(t)=17.707-18.319,207Pb/204Pb(t)=15.398-15.626 and 208Pb/204Pb(t)=37.266-38.454 3 respectively.All of these mentioned above indicate that the Early Cretaceous basalts of Hongqiang have the geochemical characteristics similar to continental OIB,and their source may be a depleted asthenosphere mantle with mixing characteristics of DMM and PREMU.Combined with a comprehensive analysis of the geotectonic setting of the Hongqiang basalts and contrastive research between Hongqiang Early Cretaceous basalts and Lixian-Dangchang Cenozoic ultrapotassic volcanic rocks,we suggest that the Early Cretaceous Hongqiang basalts may be a product of continental rift magmatism,its petrogenesis and dynamic background are strongly associated with the nature and evolution of North-South Tectonic Belt since Mesozoic.The differences on the petrography,whole rock geochemistry,magma series and types of the volcanic rocks between Early Cretaceous sodic-alkaline basalts and Cenozoic ultrapotassic volcanic rocks in the E-W segment of West Qinling may be related to the evolution of the asthenospheric mantle source during different periods and the depth of magmagenesis as well as the degree of partial melting.

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