长白山望天鹅火山双峰式火山岩的成因演化

The Genesis and Evolution of Bimodal Volcanic Rocks in Wangtian′e Volcano, Changbaishan

望天鹅火山活动从早到晚分为三期：①长白期以粗面玄武岩为主，少量碱性玄武岩和玄武质粗安岩；②望天鹅期又分早晚两个亚期，早期玄武质粗安岩和粗面岩，晚期碱性流纹岩；③红头山期碱性流纹岩。望天鹅火山岩的岩石学、ＲＥＥ、不相容元素和Ｎｄ、Ｓｒ、Ｐｂ同位素（１４３Ｎｄ／１４４Ｎｄ、８７Ｓｒ／８６Ｓｒ、２０６Ｐｂ／２０４Ｐｂ分别变化于０．５１２５６７～０．５１２６４２、０．７０５１９８～０．７１６７４２和１７．４３４～１７．９３４），揭示了岩浆源区明显受洋壳的俯冲和壳幔混合混染，三期火山岩具有共同的岩浆源区和紧密的成因演化关系，分离结晶作用控制了岩浆的演化过程。原始碱性系列玄武质岩浆经橄榄石、辉石的结晶分异，向粗面玄武岩演化；粗面玄武岩经斜长石强烈的结晶分离作用，还有辉石、钛铁氧化物和磷灰石的分离结晶作用形成具显著Ｅｕ、Ｓｒ、Ｂａ、Ｔｉ、Ｐ负异常的碱性流纹岩，导致本区火山岩缺失中性（ＳｉＯ２＝５７％～６３％）的安山岩和粗安岩，呈现双峰式火山岩组合的特点。

The activities of Wangtian′e volcano lasted for three periods:①The Changbai period volcanic rocks consisting mainly of trachybasalt with small amount of alkalic basalt and basaltic trachyandesite;② The Wangtian′e period can be further divided into early and late periods, the early period volcanic rocks consisting of basaltic trachyandesite and trachyte, the late period volcanic rocks cosisting of alkalic rhyolite;③The Hongtoushan period volcanic rocks consisting of alkalic rhyolite. The petrology, REE, incompatible elements and Nd, Sr, Pb isotopes characteristics ( 143 Nd/ 144 Nd, 87 Sr/ 86 Sr and 206 Pb/ 204 Pb vary within 0 512567～0 512642, 0 705198～0 716742 and 17 434～17 934 respectively) reveal that the magma source had significantly experienced ocean crust subduction and mantle crust mixing, that the volcanic rocks of the three periods had the common magma source and close genetic evolution relation, and that the crystallization differentiation had controlled the evolution process of magma. The original alkalic series of basalic magma evolutes into trachybasalt through crystallization differentiation of olivine and pyroxene, and the trachybasalt is transformed into alkalic rhyolite with remarkable Eu, Sr, Ba, Ti, P nagetive abnormalities through strong crystallization differentiation of plagioclase and also pyroxene Ti Fe oxides and apatite, all together leading to the loss of neutral (SiO 2=57%～63%) andesite and trachyandesite in the volcanic rocks in this area and appearance of bimodal volcanic rock combination feature.