西南三江临沧地体澜沧岩群火山岩成因与构造意义

Genesis and Tectonic Significance of Volcanic Rocksfrom the Lancang Group in the Lincang Terrane,Sanjiang Region,Southwest China

临沧地体是重建西南三江原特提斯构造演化的关键,但其构造属性存在争议,作为地体基底的澜沧岩群是解决该问题的关键。本文选择临沧地体澜沧岩群正层型剖面为研究对象,对其中惠民岩组的变中基性火山岩进行锆石U-Pb年代学和全岩元素地球化学组成分析。2件钠长绿泥片岩的岩浆锆石^(206)Pb/^(238)U加权平均年龄为(457±2)Ma和(455±2)Ma,指示晚奥陶世岩浆作用。岩石地球化学特征显示,22件绿泥石片岩的原岩为低K拉斑玄武岩系列,6件钠长绿泥片岩的原岩为高K钙碱-钾玄岩系列的玄武安山岩,两者都表现出大离子亲石元素的强烈富集和高场强元素的亏损,为典型弧火山岩。上述证据指示惠民岩浆弧形成于奥陶纪—志留纪期间原特提斯洋向思茅地块的俯冲消减。综合分析认为,临沧地体可能不是一个具有前寒武纪基底的独立地块,而是代表了原—古特提斯洋在思茅地块大陆边缘俯冲形成的陆缘弧地体。

Studying the Lincang terrane is essential to understand the tectonic evolution of the Proto-Tethys in the Sanjiang region of Southwest China.However,there have been ongoing debates about its tectonic characte-ristics.This study focuses on the Lancang Group,which serves as the basement of the terrane.Our study presents zircon U-Pb chronology and whole-rock elemental composition of altered metavolcanic rocks in the area.The zircon U-Pb dating results indicate that these rocks formed during the Late Ordovician period,specifically at(457±2)Ma and(455±2)Ma.The protoliths of the 22 chlorite schists are from the low-potassium tholeiite series,while the protoliths of the six albite chlorite schists are basaltic-andesites from the high-potassium calc-alkaline-potassium basalt series.These rocks share similar characteristics with typical arc volcanic features,such as strong enrichment of light rare earth elements(LREEs),large-ion lithophile elements(LILEs),and negative anomalies of high field strength elements(HFSE).This evidence supports the idea that the Huimin magmatism was formed through the subduction of the Proto-Tethys Ocean to the Simao Block during the Ordovician-Silurian period.Therefore,we propose that the Lincang terrane may not be a distinct block with Precambrian basement,but rather a continental arc that formed through the subduction of the Proto-and Paleo-Tethys at the margin of the Simao Block.