西藏马乡地区典中组年龄厘定及其构造背景

Age and Tectonic Setting of Dianzhong Formation in the Maxiang Area,Tibet,China

分布在冈底斯带南部典中组火山岩的喷发时代到底是晚白垩世还是古新世一直存在争论。典中组与其下伏设兴组之间角度不整合是冈底斯地区最重要的不整合面之一,可能代表新特提斯洋俯冲的开始,其年龄跨度的确定具有重要意义。在西藏马乡地区系统采集了典中组火山岩样品和在不整合面附近的设兴组中的安山岩样品,进行LA-ICP-MS锆石U-Pb测年,得出典中组火山岩喷发时限在69.1～64.3Ma之间,跨越K/T界限,而典中组与设兴组间的角度不整合年龄限制在87.7～69.1Ma之间。岩石地球化学分析表明,典中组火山岩属于钙碱性—高钾钙碱性系列,富集大离子亲石元素Rb、Ba、K、Th、U、Pb,具有明显的Pb、K峰和Ti谷,轻稀土元素富集,具有弱Eu负异常。典中组火山岩应属于俯冲造山带陆缘弧—岛弧型火山岩。

Whether located on south of the Gangdese belt Dianzhong Formation eruption age is Late Cretaceous or Paleocene has still being debated.And the angular unconformity between Dianzhong Formation and Shexing Formation is one of the most important unconformity in Gangdese area,may represent the beginning of the Neo-Tethys subduction,therefore,age span determination is significant.In this study,we systematicly collected volcanic rock samples of Dianzhong Formation and andesite samples found in the Shexing Formation adjacent to the unconformity in Maxiang,Tibet,for LA-ICP-MS zircon U-Pb dating come to Dianzhong Formation volcanic rocks eruption time limit between 69.1-64.3 Ma,across K/T boundaries,and the age of angular unconformity between Dianzhong Formation and Shexing Formation is during the period of 87.7-69.1 Ma.The lithogeochemical characteristics indicate volcanic rocks are calc-alkaline-kcalc-alkaline series,enriched in large ion lithophile elements of Rb,Ba,K,Th,U and with Pb,obvious Pb,K peak and Ti valley,LREE enrichment,with weak negative Eu anomalies.Dianzhong Formation volcanic rocks should belong to continental margin magmatic arc-island arc volcanic rocks in a tectonic environment of subduction.