云南墨江县早石炭世五素火山岩的地球化学特征及其地质意义

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。

GEOCHEMISTRY OF THE EARLY CARBONIFEROUS WUSU VOLCANIC,MOJIANG COUNTY,YUNNAN, AND ITS TECTONIC SIGNIFICANCE

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 La_N/Yb_N=1.9-2.4,Yb_N=11-23.The rhyolites are higher in REE than the basalts,but with La_N/Yb_N=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.