Detrital zircon age spectra of the Yungou Formation and its constrain to the related block affinity, western Yunnan

Abstract

滇西允沟岩组为一套低绿片岩相的泥质浅变质岩，岩性以云母石英片岩、云母石英千枚岩、云母片岩、细晶灰岩为主，局部夹硅质岩，一直以来被认为是前寒武纪变质基底的重要组成部分。其原岩以碎屑岩为主，含有部分灰岩、白云岩，是一套形成于特提斯洋被动大陆边缘的半深海-深海沉积物，因此，是研究特提斯洋构造演化的重要窗口。但是，对允沟岩组的形成时代至今仍存在较大争议，为此本文选取允沟岩组中的片岩进行了碎屑锆石U-Pb精确定年。分析结果表明，允沟岩组原岩沉积时代为新元古代晚期-寒武纪期间（551~491Ma），主要由新太古代、新元古代及少量古元古代碎屑物质组成。其中，大量太古宙晚期碎屑锆石表明其源区有太古宙基底的存在，而1749Ma这组碎屑锆石可能与哥伦比亚超大陆聚散有关，956Ma和848Ma这两组锆石记录的事件可能是对罗迪尼亚超大陆三阶段裂离事件的前两期的响应。此外，还含有少量泛非运动信息，但泛非运动对其源区的影响极其有限。结合前人研究成果和本文碎屑锆石年龄谱，显示允沟岩组形成于原特提斯洋的被动陆缘，与印度板块、南羌塘地块具有显著的亲缘性。;The Yungou Formation in western Yunnan Province, a suit of metapelites of lower greenschist facies, consists dominantly of mica quartz schist, mica quartz phyllite, mica schist and fine crystalline limestone, with some siliceous rocks. It has always been considered as an important part of the Precambrian metamorphic basement. The protoliths are mainly clastic rocks with some limestone and dolomite, belonging to a set of semi-deep sea sediments formed in the passive continental margin of the Tethys Ocean. So the Yungou Formation witnessed the origin, development and extinction of the Tethys Ocean, and played a great role in understanding the tectonic evolution of the Tethys Ocean. However, its forming age is still under controversial. In this paper, the zircon U-Pb dating of schists from the Yungou Formation, southwestern Yunnan Province, has been carried out. The analysis results show that the depositional age of the protolith of the Yungou Formation is in Late Neoproterozoic-Cambrian (551~491Ma), and mainly composed of Neoarchean, Neoproterozoic and some Paleoproterozoic detrital rocks. The discovery of massive Neoarchean detrital zircons indicates the existence of the Archean basement in the source area. The 1749Ma zircons may be related to the Columbia Supercontinent from assembly to breakup. The zircons of 956Ma and 848Ma may be a response to the former two stages of the three-stage-rifting evolution of the Rodinia Supercontinent. In addition, the dating result also revealed the existence of pan-African tectono-thermal event, but its impact on the source region of the Yungou Formation is extremely limited. Combined with previous study, the U-Pb age spectra of detrital zircons shows that the Yungou Formation developed on the passive continental margin of the Proto-Tethys Ocean and when it deposited this block has the good affinity of the Indian and South Qiangtang blocks.