台湾东澳片岩锆石U-Pb年龄及地质意义

摘要

台湾东澳片岩位于台湾东部,属太鲁阁带的北东部分,由绿泥绿帘片岩、绿泥钠长片岩、云母石英片岩、石英片岩夹含石墨云母片岩组成.在野外地质调查的基础上,结合岩石学、岩石主微量元素分析研究结果表明,原岩为玄武岩、副长岩或碱性玄武岩及陆源碎屑岩,岩石主要为碱性系列,具低Ba、Sr和高Pb,稀土总量普遍较低,具有轻稀土富集、铕无亏损的稀土分布模式,与陆源裂陷的火山(岩浆)-沉积作用特征类似;锆石可分为自形晶或尖棱角状、次棱角状及圆状三类;LA-ICP-MS锆石U-Pb年龄具多个峰值,其中206pb/238U有4个,最小值为(95±3)Ma,为自形晶或尖棱角状锆石,代表了晚白垩世火山作用的时代;其余3个分别为(118±2)Ma(早白垩世)、(160±5)Ma(晚侏罗世)、(250±3)Ma(早三叠世),为次棱角状和少量自形晶锆石,代表了相应时代源岩的岩浆年龄;207pb/206Pb表面年龄峰值2个,分别为(1838±41)Ma和(2404±27)Ma(古元古代),为磨圆状的变质锆石或岩浆锆石,代表了古元古代变质基底的年龄.东澳片岩形成时代为晚白垩世,属欧亚陆块边缘构造环境下火山-沉积作用的产物.%Located in the eastern part of Taiwan,the Tongao schist belongs to the northeast part of Tailuko belt and is composed of chlorite epidote schist,chlorite albite schist,mica quartz schist,quartz schist,and mica schist intercalated with graphite mica schist layer.Based on geological survey in combination with petrologic as well as major and trace element analysis,the authors hold that the original rocks were basalt,foidite or alkaline basalts and terrigenous clastic rocks.The rocks are mainly of alkaline series characterized by low Ba,high Sr and Pb,generally low total rare earth,and REE distribution patterns with features of LREE enrichment and no loss of europium.These features are similar to features of terrigenous rifting (magmatic) volcano-sedimentary rock combination.Zircons can be divided into three types,i.e.,euhedral or sharply angular,subangular,and rounded ones.LA-ICP-MS U-Pb zircon ages have multiple peaks,in which there are four 206Pb/238U ratios;the minimum value is (95 ± 3)Ma,representing euhedral zircons or sharply angular zircons and indicating late Cretaceous volcanic activity;the remaining three values are 118±2 Ma (Early Cretaceous),160±5 Ma (Late Jurassic),and (250±3) Ma,Early Triassic),representing subangular and small euhedral zircons and indicating the magma age of corresponding source rock;there are two 207Pb/206pb surface peak ages,i.e.,1838±41Ma and 2404±27Ma (Paleoproterozoic),representing rounded metamorphic zircons or magmatic zircon and indicating the age of Paleoproterozoic metamorphic basement.The Tongao schist was formed during the late Cretaceous in the Eurasian continental margin tectonic environment,being a product of volcanic-sedimentary activity.