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 eompostition similar to MORB source and present depleted mantle characteristics(eNb =4. 8 ~5. 9). All data show that bsalts of HC have a garnet Iherzolite 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 ( > 200x10 "6)and Mg value (60-70). Regional extension triggers asthenospheric up welling, which may lead to the genesis of magma and subsequent volcanism.%哈拉哈河-绰尔河第四纪火山地处重力梯度带上的大兴安岭中段.火山岩主要类型为钠质系列碱性橄榄玄武岩.火山岩大离子亲石元素和轻稀土元素相对富集,轻重稀土分异程度弱((La/Yb)N=8～12),稀土元素和微量元素配分曲线与大同碱性玄武岩平行,总体上表现出与OIB相似的特征.在Sr-Nd-Pb同位素组成特征上表现出亏损地幔的特点(εNd=4.8～5.9),接近MORB的源区范围.哈拉哈河-绰尔河第四纪火山岩岩浆由轻稀土富集的石榴子石二辉橄榄岩低程度(8％～15％)部分熔融产生,火山岩高MgO(＞9％)、Ni( ＞200×10-6)和Mg*(60 ～70),表明它们是较原始的岩浆,岩浆上升过程经历了橄榄石和辉石为主的弱分离结晶作用,没有受到地壳物质明显混染.区域伸展作用引发软流圈地幔上涌是哈拉哈河-绰尔河第四纪火山的岩浆成因.
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