滇东南八布蛇绿岩地球化学特征及构造背景

八布蛇绿岩出露在北西向文山-麻栗坡和富宁走滑断裂之间的断块内,主要由蛇纹岩、辉长岩、玄武岩三个单元组成,彼此间均以断裂相接.地球化学研究表明,蛇纹岩SiO2、Al2O3、TiO2、MgO含量变化较大,依据主、微量元素组成可以分为两类,第一类球粒陨石标准化稀土元素分配模式整体较为平坦,推测原岩为镁铁质堆晶岩;另一类呈U型配分,推断原岩是方辉橄榄岩.玄武岩属拉斑系列,主量元素具有富MgO、TiO2,低Al2O3、K2O、P2O5,并且Na2O﹥K2O等特征,类似于MORB型玄武岩;REE配分模式也显示出N-MORB型玄武岩的特征;同时,微量元素普遍具有LILE、Th富集, Nb、Ta、Zr、Hf等元素具明显亏损特征,又暗示源区曾遭受不同程度的俯冲带流体交代作用.综合分析认为,八布蛇绿岩形成于弧后盆地环境,其形成可能是古太平洋从SE至NW向华南大陆俯冲的产物.

GEOCHEMISTRY AND TECTONIC SETTING OF THE BABU OPHIOLITE,SOUTHEAST YUNNAN

The Babu ophiolite in Malipo County,southeastern Yunnan,outcrops in a fault-block between the NW-striking Wenshan-Malipo fault and the Funing strike-slip fault.It is composed of serpentites,gabbros,diabases and basalts.Different units in this block contact each other by faults.The serpentites show great variations in SiO2,Al2O3,TiO2 and MgO contents,and the serpentites can be classified as two groups.The first group shows the flat-type REE distribution patterns,similar to mafic accumulative rocks.The second group shows the U-type REE distribution patterns,similar to harzburgites.Meanwhile,the composition of basalts from the Babu ophiolite are characterized by high MgO and TiO2 contents,low Al2O3,K2O,P2O5 contents,and Na2OK2O.These basalt rocks have the flat REE patterns and are depleted in LREE,indicating that they are similar to the Normal Mid-Ocean Ridge Basalts(N-MORB).Nevertheless,the samples are enriched in the large-ion lithophile(LIL) elements and depleted in Nb and Ta in the trace spider diagram,suggesting that the source of magma was influenced by input of the components from the subduction zone.From all above,it is reasonable to suggest that the Babu ophiolite was formed in a back-arc basin setting,and might be orogenic products when the paleo-Pacific ocean plate subducted beneath the South China Continent plate in the SE-NW direction.