南澜沧江带北段上二叠统陆缘弧火山岩的厘定

DETERMINATION OF EPICONTINENTAL ARC VOLCANIC ROCKOF LATE PERMIAN IN NORTH PART OF SOUTH LANCANGJIANG BELT,SOUTHWESTERN CHINA

从岩石组合、岩相学特征、岩石化学、稀土元素、微量元素、大地构造环境等诸多方面分析云县-临沧花岗岩东侧的小定西、崴里等地分布的上二叠统P2火山岩具有陆缘弧火山岩特征。火山岩组合以玄武岩-安山岩-英安岩为主,极少量的粗安岩-粗面岩;火山岩系列以钙碱性系列为主,拉斑系列及碱性系列较少;火山岩的化学成分以A l2O3低T iO2为特征,K2O含量显示极强的极性;稀土模式为轻稀土富集右倾斜型;微量元素大阳离子元素富集,亏损T i,C r和部分亏损P,N b;岩石化学投图绝大部分落在岛弧火山岩区,与南澜沧江带南段及北澜沧江带陆缘弧火岩投点相一致。厘定了南澜沧江带北段上二叠统P2陆缘弧火山岩的存在。该陆缘弧火岩带与昌宁-孟连带的洋脊/洋岛型火山岩、蛇绿岩构成了成对分布的洋脊火山岩、蛇绿岩-弧岩浆岩带,指示澜沧江洋壳向东俯冲。该成果对探讨澜沧江带古特提斯的演化具有极重要的作用。

The late Permian volcanic rocks in Xiaodingxi,Weili and so on in the east of Yunxian-Lincang granite were characterized by epicontinental arc according to studies of rock association, petrography, petrochemistry, REE, trace elements and tectonic setting. Volcanic rocks include basalt ,andesite ,dacite and with minor trachyandesite and trachyte,which belong to series calc-alkaline series with minor tholeiitic and alkaline series. Composition of volcanic rocks is characterized by high in A1203 and low in TiO2,and K2O polarity is very obvious. Chondrite-normalized REE patterns are oblique to right side and enriched in LREE. Their trace elements showed enrichment in LIL ,depletion in Ti and Cr,and partially depleted in P and Nb. Petrochemistry study indicates they belong to island-arc volcanic rocks and consist with continental marginal arc volcanic rocks in the south part of south and north Lancangjiang belt. This epicontinental volcanic rock,in association with the Changning-Menglian ocean ridge/ocean island volcanic rocks,ophiolite,and indicates that Lancangjiang oceanic crust is characteristic of eastward subduction. The result in the paper was very important to the study of paleoTethyan evolution in the Lancangjiang belt.