The volcanic rocks of Benbatu Formation in Sonid Youqi mainly consist of basaltic-andesite and an-desite with a little dacite. Zircon U-Pb isotopic dating reveals the andesite formed in 300.9 ± 1.6 Ma. The volcanic rocks generally belong to the tholeiitic series with low K and comparatively high Fe in geochemical characteristics. Trace elements are characterized by REE weak fractionation, incompatible elements (Th, U, La, Ce) enrichment and HFSE (Nb, Ta, Ti) depletion, which is similar to the St. Kitts island arc volcanic rocks. The rocks of Benbatu Formation show Eu in negative anomaly and Sr in low content, indicating the magma evolution experienced plagioclase fractional crystallization, which also led to the rocks poor in Al and rich in Na. The volcanic rocks of Benbatu Formation in Sonid Youqi area originated from the partial melting by the subduction of mantle wedge, and the formation of the rocks is related to the subduction of Paleo-Asian Ocean plate.%苏尼特右旗地区本巴图组火山岩主要由玄武安山岩及安山岩组成,含少量英安岩.安山岩的锆石U-Pb年龄为300.9±1.6 Ma.火山岩的化学成分总体具有拉斑系列的低钾、相对富铁的特征,微量元素组成与圣基茨岛弧火山岩十分相似,以轻重稀土的弱分馏、富集不相容元素、相对亏损高场强元素为特征.本巴图组火山岩具Eu的负异常和贫Sr特征,指示岩浆演化过程中经历过以斜长石为主的分离结晶作用,这一作用也导致了岩石贫铝、富钠.本巴图组火山岩初始岩浆起源于俯冲地幔楔的部分熔融,其形成与古亚洲洋板块的俯冲作用有关.
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