四川锦屏山地区碱性杂岩锆石U-Pb年代学、地球化学特征及其地质意义

Zircon U-Pb Chronology,Petrogeochemistry and Their Geological Implication for Alkaline Complex in the Jinpingshan region

四川锦屏山地区碱性杂岩位于扬子地台西南缘,其岩体的主要岩石类型为霓石英碱正长岩、英碱正长岩、含萤石霓石钠闪-钠铁闪石碱性花岗岩、霓石(碱性)花岗斑岩、碱性伟晶岩脉。为确定该碱性杂岩的形成时代、成因及地质意义,对其进行了岩相学、LA-ICP-MS锆石U-Pb年龄及地球化学研究。结果表明,碱性杂岩中的锆石为岩浆成因,锆石206Pb/238U加权平均年龄为27.06±0.51Ma,侵入时代属于渐新世。该岩石具有富碱,高钾,贫镁、钙的岩石化学特征,属于碱性岩石系列。其稀土元素总量极高,轻、重稀土元素分馏明显,Eu异常不显著。微量元素相对富集Th、Zr、La、Nd等大离子亲石元素,亏损Nb、P、Ti等高场强元素的特征。综合以上数据、结合区域构造演化背景,说明该碱性杂岩为扬子地块插入到龙门山地壳的过程中,俯冲陆壳发生变质、脱水后形成的产物。

Alkaline complex in the Jinpingshan region,Sichuan consists of aegirine quartz alkali syenite,caustic syenite,alkaline granite,aegirine(alkaline)granite porphyry and alkaline pegmatite dyke.The zircon 206Pb/238U weighted mean ages are 27.06±0.51Ma(Oligocene).The rock is characterized by enrichment in alkaline,high K,and depletion in Mg and Ca with very highΣREE,obvious fractionation of light and heavy REE and not significant Eu anomaly,relative enrichment in large ion lithophile elements such as Th,Zr,La,Nd and depletion in high field strength element such as Nb,P and Ti.These indicate that the Jinping alkaline complex is the product of metamorphism and dehydration of subduction continental crust during the insertion of the Yangtze block into Longmenshan crust.