中天山早奥陶世弧岩浆岩的确定——来自桑树园子一带花岗质片麻岩年代学和地球化学证据

对新疆中天山东段桑树园子一带侵入中元古界星星峡群中的花岗质片麻岩进行LA-ICP-MS锆石U-Pb定年和地球化学研究.LA-ICP-MS锆石U-Pb定年结果表明,花岗质片麻岩原岩形成时代为(475.7±1.6) Ma,属早奥陶世岩浆活动产物,同时锆石25个测点中未发现有任何前寒武纪的残存锆石或时代信息.结合造岩矿物组合特征,认为其原岩为中-酸性侵入岩.岩石地球化学分析表明,花岗质片麻岩SiO2含量为62.45％～66.52％,K2O/Na2O变化于1.74～1.99,A/CNK值介于0.94～0.95,具高钾和弱过铝质的特征.稀土分布曲线呈右倾轻稀土富集型,弱的负铕异常,δEu值为0.69～0.76.岩石富集大离子亲石元素Rb,Sr,Ba,K等,亏损Nb和Ta等高场强元素及Ti和P,具Ⅰ型花岗岩特征.微量元素构造环境判别图解表明,花岗质片麻岩原岩可能形成于与俯冲有关的大陆边缘弧构造环境.结合岩石的锆石LA-ICP-MS定年结果和区域地质背景,认为桑树园子花岗质片麻岩原岩可能是以干沟蛇绿岩为代表的中天山北缘古大洋向南俯冲的弧岩浆岩记录.

Early Ordovician Arc Magmatite in Central Tianshan:Evidences of Geochronology and Geochemical on Granitic Gnesiss from Sangshuyuanzi Area

LA-ICP-MS zircon U-Pb dating and geochemical composition analysis have been applied to the granitic gneiss that intruded into Xingxingxia group from Sangshuyuanzi in the eastern segment of the Central Tianshan.LA-ICP MS Zircon U-Pb dating of granitic gneiss gives concordant ages of(453.1±6.5) Ma,indicating that they are the products of magmatic activities of Early Ordovician.The results of 22 measuring points of zircons of the granitic gneiss show that there are no remnant of Late Precambrian zircons or age information.Combining with the feature of rock minerals,we suggest that the original rocks are intermediate-acid intrusive rocks.Geochemical analyses show that SiO2=62.45%-66.52%,K2O/Na2O=1.74-1.99 and A/CNK =0.94-0.95,which indicates that the rocks are high K and peraluminous.The chondrite-nomalized REE pattern shows the enrichment of LREE,weak negative Eu anomalies,and Eu at 0.69-0.76.The granitic gneisses are rich in large ion lithophile elements(LILE),such as Rb,Sr,Ba,K,whereas the high field strength elements(HFSE:Nb and Ta) as well as Ti and P content are depleted.Trace element discrimination indicate that the granitic gneiss have a feature of I-type granite.Analysis of tectonic setting suggests that the granitic gneiss formed in an active continental margin arc which is subduction-related.In combination with the result of LA-ICP MS zircon U-Pb dating and regional geological setting,we suggest that the original rocks of the granitic gneiss from Sangshuyuanzi is likely the record of the arc magmatic rocks in the southward subduction of the ancient ocean of the northern margin of the Central Tianshan which is represented by Gangou ophiolite.