THE GEOLOGICAL AND GEOCHEMICLA CHARACTERISTICS OF THE JIFENG KOMATIIITESIN THE DA HINGGAN LING MOUNTAINS

Field geological survey and geochemistry indicate that the metamorphic ultrabasic rock in Jifeng area of the north Da Hinggan Ling Mountains is komatiite with spinifex texture. The komatiite series consists of the ultramafic komatiite, basaltic komatiite, tholeiite and gabbro. The komatiite displays a geochemical trend from ultramafic to mafic,and the tholeiite shows a geochemical trend from rich Mg to rich Fe. The felsic igneous rock overlying the komatiite series displays a calc alkaline trend. The komatiite REE patterns show smooth or slightly rich in LREE, similar to that from South Africa. The eight samples composed of komatiite, tholeiite and gabbro yield a reasonable isochron age of 1727Ma±74.7Ma with I_Nd=0.510725±0.0000798, ε_Nd (t)=6.94±1.56. It is suggested that the Jifeng komatiites were derived from the depleted rheosphere mantle during the early Mesoproterozoic. This crustal accretion event might be related to the splitting of the Songnen Massif from the southorn continental margin of the Siberia Massif.