GEOCHEMISTRY OF THE EARLY CARBONIFEROUS WUSU VOLCANIC,MOJIANG COUNTY,YUNNAN, AND ITS TECTONIC SIGNIFICANCE
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摘要: 五素火山岩位于哀牢山蛇绿岩带西侧,大地构造位置属于思茅地块。火山岩整合产于早石炭世地层中,厚约800m,具双峰式特征。野外见4层流纹岩和英安岩,且均与玄武岩呈互层产出。与N-MORB比较,五素玄武岩的Ti含量偏高,Ti/V、Zr/Y比值和LREE丰度较高,具有E-MORB的特征,说明五素玄武岩的原始岩浆可能类似OIB的特征,指示其产于板块拉张环境而非岛弧环境。五素玄武岩的Th>Ta,暗示玄武岩经历了陆壳混染作用。综合野外地质和地球化学证据说明,五素双峰式火山岩应当属于大陆溢流玄武岩(CFB),产于板块被动陆缘拉张减薄环境。Abstract: The Wusu volcanics,west of the Ailaoshan Suture Zone,are located in the Simao Massif.They are composed of basalt,pillow lava,dacite,rhyolite and tuff,about 800m thick,occurring in the early Carboniferous sequence.As compared to N-MORB,the Wusu basalt has a higher content of TiO2 and REE,and higher Ti/V and Zr/Y ratios characteristic of E-MORB.The basalts are LREE-enriched,with LaN/YbN=1.9-2.4,YbN=11-23.The rhyolites are higher in REE than the basalts,but with LaN/YbN=2.0-2.8.This shows that both of them may have the same origin,i.e. the bi-modal type.Such composition of the Wusu basalt indicates an extension environment rather than an island arc environment.The higher Th/Ta ratios (1.4-4.0) may imply a hibridization with the continental crust during its rise to the surface.
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Key words:
- Yunnan /
- Wusu /
- volcanics /
- geochemistry /
- tectonic setting
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