勉略带关帝坪角闪岩岩石地球化学特征及LA-ICP-MS锆石U-Pb定年

Geochemistry and LA-ICP-MS Zircon U-Pb Dating of the Guandiping Amphibolite in the Mian-Lue Structure Zone

关帝坪蛇绿岩位于勉县关帝坪地区,具典型的堆晶结构,是勉略蛇绿岩带的重要组成部分。对其中的角闪岩进行了岩石地球化学和LA-ICP-MS锆石U-Pb测年分析。结果表明角闪岩具高MgO、CaO,低K2O的特征,为钙碱性系列,TiO2含量与活动大陆边缘及岛弧区火山岩相似,富集大离子亲石元素Ba、Th、U,显著亏损Nb、Ta,略微富集轻稀土元素,轻重稀土元素分馏不明显,具有岛弧玄武岩的地球化学特征,与勉略带印支期花岗岩可能具有一定的同源性,可能是印支期勉略洋盆向北俯冲的产物,并受到了大陆壳的混染。LA-ICP-MS锆石U-Pb测年结果,表明角闪岩的成岩年龄为(222.5±2.1)Ma,表明其形成于晚三叠世。该年龄略早于研究区花岗岩的成岩年龄,二者的成对性关系说明在勉略古洋壳向北的俯冲一直持续到晚三叠世岛弧火山岩的形成或者更晚时期,之后才进入了碰撞造山阶段。

Located in Mianxian County and with a typical heap crystal structure,the Guandipin ophiolite is an important part of the Mian-lue ophiolite structure zone.In this study,rock geochemistry and LA-ICP-MS zircon U-Pb dating are applied to analyze the intrusive environment and period of the amphibolite.The result shows that the amphibolite belongs to Calc-alkaline series with typically high content of MgO and CaO but low K2O.The TiO2content is similar to that of active continental margin and island arc volcanic rocks.Besides,it is rich in LILE such as Ba,Th,U and LREE,while low in HFSE of Nb and Ta.Moreover,the fractionation of light and heavy REE is not obvious.These features indicate that it is probably the product of the northward subduction of the Mian-lüe oceanic basin under island arc environment.Its geochemistry characteristics are similar to the Indosinian granites,suggesting that they may derive from thesame source,and are contaminated by the continental crust.LA-ICP-MS zircon U-Pb dating result shows that the diagenetic age of the amphibolite is 222.5±2.1Ma,which indicates that it was formed in the Late Triassic Epoch.This age is a bit earlier than the granites in the study area,implying that the ancient Mian-lüe was formed in the Late Triassic Epoch.This age is a bit earlier than the granites in the study area,implying that the ancient Mian-lüe oceanic crust continued northward subduction until the forming of island arc volcanic rocks in Late Triassic or later,and then entered the collision orogenic stage.