摘要
///This paper has reported zircon U-Pb dating,Hf isotopes,major and trace elements,and Sr-Nd isotope data of the volcanic rocks from the Laotudingzi Formation in the eastern margin of the Jiamusi Massif and the upper segment of the Baoquan Formation in the Xiao Hinggan Mountains,with the aim of constraining the regional tectonic evolution in eastern Heilongjiang Province.The former is composed mainly of rhyolite and subordinately of alkaline basalt,whereas the latter consists chiefly of rhyolite.Zircons from these volcanic rocks display striped absorption or fine-scale oscillatory growth zoning as well as high Th/U ratios(0.28~1.64),implying a magmatic origin.The dating results show that they were probably all formed in middle Devonian(a weighted mean 206Pb/238U age of 386 Ma).The mafic rocks from the Laotudingzi Formation in the eastern margin of the Jiamusi Massif have the data SiO2=50.40%~55.66%,Mg#=0.39~0.50,Na2O/K2O=2.88~4.99,Al2O3=15.66%~18.06%,(87Sr/86Sr)i=0.705 99,εNd(t)=+2.04,and show the enrichment of light rare earth elements(LREEs) and Cr,Co,Ni and Zr,and depletion of Nb,Ta,Ti,with high Zr/Y,Nb/U,Ta/U and Ce/Pb ratios,whereas the felsic rocks have the data SiO2=77.59%~78.75%,Na2O/K2O=0.32~0.36,Al2O3=11.62%~11.76%,(87Sr/86Sr)i=0.703 78~0.707 36,εNd(t)=-2.20~-4.15,εHf(t)=+1.8~+8.6,tDM1= 671~998 Ma,and display enrichment of Rb,Th,U,K and depletion of Nb,Ta,U,P,Ti.These data are typical of bimodal volcanism and indicate that the former might have been derived from the lithosphere mantle,contaminated by crustal material,whereas the latter probably originated from partial melting of the newly accreted crust.The rhyolite from the upper Baoquan Formation in the Xiao Hinggan Mountains is characterized by high SiO2(74.64%~76.09%),enrichment of K2O(Na2O/K2O=0.42~0.81),low Al2O3(12.56%~12.63%),enrichment of high field strength elements(such as Zr and Hf)and large ion lithophile elements(LILEs)such as Th,U and K,and strong depletion of Sr,Eu,P and Ti.These data are similar to features of A-type rhyolites.Besides,these volcanic rocks have εHf(t)=+2.5~+6.4,tDM1= 837~983 Ma,consistent with the data of Phanerozoic granitoids in the CAOB,suggesting that they might have originated from partial melting of the newly accreted crust.Combined with the regional geological data,it is inferred that there might have existed a passive continental-margin setting in the eastern margin of the Jiamusi Massif in middle Devonian,where the bimodal volcanic rocks were probably formed.Furthermore,the coeval A-type rhyolite of the upper Baoquan Formation in the Xiao Hinggan Mountains was probably formed in an intra-continental extensional environment.
