滇西南大红山群变火山-沉积岩地球化学属性、年代格架及其构造意义

Geochemistry,chronological framework and tectonic implications of metamorphic volcanic-sedimentary rocks from the Dahongshan Group in southwestern Yunnan Province

云南新平地区大红山群出露于扬子地块西南缘,主要由低绿片岩相-角闪岩相变质的火山-沉积岩组成。大红山群的岩石成因、年代格架及其形成的构造背景缺乏系统研究,制约了地质学家们全面认识和理解扬子西南缘<~1.75 Ga的构造演化历史。本文以大红山群底部老厂河组变沉积岩及其内部变火山岩夹层为重点研究对象,开展岩相学、全岩地球化学和锆石U-Pb定年等综合研究。岩石地球化学研究结果表明,变沉积岩的化学成分与大陆上地壳沉积物成分接近,原岩为成熟度较高的泥岩/页岩,未经历沉积再循环,形成于被动大陆边缘的构造背景;变火山岩原岩化学成分相当于钙碱性过铝质A型流纹岩,形成于造山后的大陆裂谷拉张环境。锆石U-Pb定年结果显示,老厂河组变沉积岩的碎屑锆石记录了2.3~2.2 Ga和1.9~1.75 Ga两个主年龄峰以及2.7~2.6 Ga次年龄峰。结合前人研究结果,表明大红山群物源主要来源于扬子地块西南缘的太古宙-古元古代基底岩石。变火山岩样品的岩浆锆石核部记录了1713~1711 Ma的年龄,应代表老厂河组原岩的形成时代,锆石的变质增生边限定峰期变质时代为约843 Ma。综合前人研究结果表明,大红山群普遍经历了849~837 Ma的新元古代变质事件。综上所述,扬子地块西南缘的大红山群完好记录了与Columbia超大陆裂解有关的非造山岩浆活动,新元古代变质事件可能与Rodinia超大陆裂解和聚合过程密切相关。

The Dahongshan Group in Xinping County,Yunnan Province,is located in the southwestern Yangtze Block.It mainly consists of volcanic-sedimentary rocks with lower greenschist-amphibolite facies.The lack of systematic research on the petrology,chronological framework and tectonic setting of the Dahongshan Group has hampered a comprehensive understanding of the tectonic evolution history in the southwestern Yangtze Block at<~1.75 Ga.In this paper,meta-sedimentary rocks and intercalated meta-volcanic rocks from the Laochanghe Formation in the bottom of the Dahongshan Group were selected for an integrated study including petrographic observation,whole-rock geochemistry and zircon U-Pb dating.The geochemical data reveal that the meta-sedimentary rocks are chemically close to that of the upper crust sediments.The meta-sedimentary rocks are derived from mudstone/shale that was formed in the tectonic setting of the passive continental margins with high maturity and low degree of sedimentary recirculation.Chemical compositions of the meta-volcanic protolith equates to a calc-alkaline peraluminous A-type rhyolite which formed in the post-orogenic continental rift environment.Zircon U-Pb age data show that the detrital zircons from meta-sedimentary rocks of the Laochanghe Formation document two main age peaks of 2.3~2.2 Ga and 1.9~1.75 Ga and a secondary age peak of 2.7~2.6 Ga.Combined with the previous studies,we proposed that the sources of the Dahongshan Group mainly from the Paleoproterozoic and Archean basement rocks in the southwestern Yangtze Block.The zircon cores of the meta-volcanic rocks define the protolith formation age of the Laochanghe Formation as 1713~1711 Ma.Zircon rims of the meta-volcanic rocks constrain the timing of peak metamorphism at 843±5 Ma.Combined with the previous and studies,we conclude that the Dahongshan Group underwent the Neoproterozoic metamorphic event of 849~837 Ma.In summary,the Dahongshan Group well preserved an anorogenic magmatism is related to Columbia supercontinent Cracking event in the southwestern Yangtze Block.The Neoproterozoic metamorphic event may be closely related to the assemblage and break-up of Rodinia supercontinent.