Mantle sources and magma genesis of Quaternary volcanic rocks in the Halaha River and Chaoer River area, Great Xing' an Range
大兴安岭哈拉哈河-绰尔河第四纪火山岩地幔源区与岩浆成因

The Halaha River and Chaoer River volcanic field(HC for short),middle of the Great Xing’an Range,is in the north of the North-South Gravity Lineament in eastern China.The volcanic rocks in HC,dominated by alkali olivine basalts in sodium series,is characterized by relative enrichment in large ion lithophile elements and light rare earth elements.The fractionation of rare earth element of the basalts is weak((La/Yb)N =8～12),They resemble alkali basalts in Datong,as shown by trace elements distribution patterns,and generally exhibit OIB-like characteristics.The basalts show nearly homogeneous Sr-Nd-Pb isotopic compostition similar to MORB source and present depleted mantle characteristics(εNd=4.8～5.9).All data show that bsalts of HC have a garnet lherzolite mantle source,low degree partial melting(8%～15%)in which results in the primitive magma.Crystal fractionation of olivine and pyroxene from the magma is weak and seldom contamination by the crust rocks happens during the magma ascending,which resulting the volcanic rocks with high MgO content(>9%),Ni content(>200×10-6)and Mg value(60～70).Regional extension triggers asthenospheric upwelling,which may lead to the genesis of magma and subsequent volcanism.