滇中易门地区新元古代澄江组凝灰岩锆石U-Pb年龄及其地质意义

Tuffs from the Neoproterozoic Chengjiang Formation in the Yimen region, central Yunnan: Zircon U-Pb dating and its geological implications

关键词:  澄江组  凝灰岩  锆石U-Pb年龄  新元古代  易门地区  滇中

Key words:  Chengjiang Formation  tuff  zircon U-Pb dating  Neoproterozoic  Yimen region  central Yunnan

摘要:

扬子西缘广泛出露的新元古代澄江组是一套与晋宁运动密切相关的沉积岩夹火山岩系，其形成时限对华南新元古代区域地层格架、地层划分对比及古大陆再造具有重要意义。出露于滇中易门地区的新元古代澄江组底部发育有厚数米的凝灰岩夹层，本次采集凝灰岩进行LA-ICP-MS锆石U-Pb定年研究，结果表明，锆石²⁰⁶Pb/²³⁸U年龄加权平均值为812±5.5Ma （n=17，MSWD=0.46），可以代表滇中澄江组的底界年龄，从而进一步确定澄江组底界年龄为812Ma左右。此外，凝灰岩中捕获的锆石年龄信息指示扬子西缘可能存在2.5Ga、1.8～1.6Ga、1.5Ga、1.3Ga及1.0Ga等数期重要的区域构造-热事件。

Abstract:

The widespread Chengjiang Formation strata on the western margin of the Yangtze platform consist dominantly of sedimentary rocks intercalated with the volcanic rock series associated with the Jinning Movement. The timing of the formation of the sedimentary rocks is important to the establishment of the Neoproterozoic regional stratigraphic framework, stratigraphic division and correlation and palaeogeographic reconstruction. In this study, the tuffs with several meters of thickness are found in the basal part of the Neoproterozoic Chengjiang Formation in the Yimen region, central Yunnan. The LA ICP-MS zircon U-Pb dating of the tuff samples shows that the ²⁰⁶Pb/²³⁸U weighted average age of zircons equals to 812±5.5 Ma (n=17 MSWD=0.46), and thus the basal boundary of the Chengjiang Formation may be dated at about 812 Ma. The additional age of 723±11.6 Ma represents the age of the top boundary of the Chengjiang Formation. The above mentioned ages indicate that the volcanic rocks in the Chengjiang Formation may be traced back to the late Neopreterozoic. The age data for the zircons trapped in the tuffs may also imply that several phases of important regional tectonic-thermal events once took place at 2.5 Ga, 1.8-1.6 Ga and 1.0 Ga.