摘要
大规模的则弄群火山岩呈带状近东西向展布于西藏冈底斯带中北部地区。对出露状况较好的措勤地区则弄群火山岩进行了锆石 U-Pb 定年,以建立其年代学格架便于区域对比。措勤地区则弄群火山岩多数锆石具有生长振荡环带,部分锆石显示核边结构。在措勤达雄北西部上覆于中二叠统下拉组灰岩的1件则弄群英安岩样品的 LA-ICP-MS 锆石 U-Pb 年龄为130±1Ma,1件则弄群流纹岩样品的 SHRIMP 锆石 U-Pb 年龄为129±3Ma;在措勤达雄北东部与下白垩统多尼组碎屑岩呈断层接触的1件则弄群英安岩样品中的锆石具有两组 LA-ICP-MS 锆石 U-Pb 年龄,最年轻的一组(包括增生边)为121±1Ma,较老的一组(包括核部)为131±1Ma;在措勤南西部1件则弄群流纹岩样品的 LA-ICP-MS 锆石 U-Pb 年龄为111±1Ma,与措勤地区花岗岩类的侵位时代相当。这些高质量的锆石 U-Pb 年代学数据表明,措勤地区的则弄群火山作用很可能开始于约130Ma,停息于约110Ma,持续时间约20Ma。区域对比表明,东西延伸约1000km 的则弄群火山作用同时发生在约130Ma。区域上目前的年代学数据结合同时代岩浆作用的分布特征表明,冈底斯带中北部地区早白垩世岩浆作用不太可能由新特提斯洋壳向北的低角度或平板俯冲产生。
Extensive Zenong Group volcanic rocks are zonedly E-W distributed in the middle-northern Gangdese, Tibet. Well- exposed Zenong Group volcanic rocks in Coqen, western Gangdese are selected for zircon U-Pb dating by using the SHRIMP II system at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing and the LA-ICP-MS system at the China University of Geosciences, Wuhan to construct the geochronological framework of the Zenong Group volcanic activity for regional comparison. Most zircons separated from the Zenong Group volcanic rocks exhibit striped absorption and obvious oscillatory zoning, and a few ones show core-rim structure. One dacite sample and one rhyolite sample from the Zenong Group volcanic rocks overlying the Middle Permian Xiala Formation limestone in northwestern Daxiong, Coqen yield LA-ICP-MS zircon U-Pb age of 130 ± 1Ma and SHRIMP zircon U-Pb age of 129 ± 3Ma, respectively. One dacite sample from the Zenong Group volcanic rocks underlying the clastic rocks of Lower Cretaceous Duoni Formation by fault contact in northeastern Daxiong, Coqen yields younger age cluster of 121 ± 1Ma （include growth rim） and older age cluster of 131 ± 1Ma （include core）. One rhyolite sample from southwestern Coqen yields LA-ICP-MS U-Pb age of 111± 1Ma, coeval with the emplacement age of granitoids in Coqen area. High-quality zircon U-Pb ages reported in this study indicate that the Zenong Group volcanism began at ca. 130Ma, and ceased at ca. llOMa with a volcanic duration of ca. 20Ma. Regional comparison indicates that the Zenong Group volcanism extending for ca. 1000km E-W is synchronously commenced at ca. 130Ma. Present geochronological age dates combined with the distribution of contemporaneous magmatism in a regional context suggest that the Early Cretaceous magmatism in the middle-northern Gangdese cannot be generated by the low-angle or flat-slab subduction of Neo-Tethyan oceanic crust.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2008年第3期401-412,共12页
Acta Petrologica Sinica
基金
国家重点基础研究发展规划项目(编号:2002CB412603)
国家973项目(2006CB701402)
国土资源部百名优秀青年科技人才计划项目
中国地质调查局青藏高原基础地质综合研究项目
中国科学院同位素年代学和地球化学重点实验室开放课题联合资助
