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Geochemistry of the high-Mg andesites at Zhangwu, western Liaoning: Implication for delamination of newly formed lower crust

引用

Science China Earth Sciences 2007年第12期50卷 1773-1786页

作者： HUANG Hua1,GAO Shan1,2,HU ZhaoChu1,LIU XiaoMing2 & YUAN HongLing2 1 State key laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Wuhan 430074,China 2 State key laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi’an 710069,China State key laboratory of Geological Processes and Mineral Resources China University of Geosciences Wuhan China State key laboratory of Continental Dynamics Department of Geology Northwest University Xi’an China

Ten volcanic samples at Zhangwu,western Liaoning Province,North China were selected for a sys-tematic geochemical,mineralogical and geochronological study,which provides an opportunity to ex-plore the interaction between the continental crust and mantle beneath the north margin of the North China *** one basalt sample(SiO2= 50.23%),the other nine samples are andesitic with SiO2 contents ranging from 53% to 59%.They have relatively high MgO(3.4%―6.1%,Mg#=50―64) and Ni and Cr contents(Ni 27×10?6―197×10?6,Cr 51×10?6―478×10?6).Other geochemical characteristics of Zhangwu high-Mg andesites(HMAs) include strong fractionation of light rare earth elements(LREE) from heavy rare earth elements(HREE),and Sr from Y,with La/Yb greater than 15,and high Sr/Y(34― 115).Zircons of andesite YX270 yield three age groups with no Precambrian age,which precludes ori-gin of the Zhangwu HMAs from the partial melting of the Precambrian *** oldest age group peaking at 253 Ma is interpreted to represent the collision of the Siberia block and the North China block,resulting in formation of the Central Asian orogenic belt by closure of the Mongol-Okhotsk *** intermediate age group corresponds to the basalt underplating which caused the wide-spread coeval granitoids in the North China craton with a peak 206Pb/238U age of 172 *** youngest age group gives a 206Pb/238U age of 126±2 Ma,which is interpreted as the eruption age of the Zhangwu *** high 87Sr/86Sri(126 Ma)>0.706 and low εNd(t)= ?6.36―?13.99 of the Zhangwu HMAs are distinct from slab *** common presence of reversely zoned clinopyroxene phenocrysts in the Zhangwu HMAs argues against the origin of the Zhangwu HMAs either from melting of the water saturated mantle or melting of the lower *** light of the evidence mentioned above,the envisaged scenario for the formation of the Zhangwu HMAs is related to the basaltic underplating at the base of the crust,which led to the thickening of the lower crust and

关键词： Zhangwu HMAs, basalt underplating, newly formed lower crust, delamination, North China craton

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