滇西南西盟群帕可组沉积时代厘定及构造意义:锆石U-Pb年代学及Lu-Hf同位素证据

Depositional Age of the Pake Formation of Ximeng Group and Its Tectonic Implications:Constraints from Zircon U-Pb Geochronology and Lu-Hf Isotopes

研究滇西南地区出露的西盟群碎屑岩沉积时限及其古地理属性对了解冈瓦纳大陆东缘的构造演化及其古地理重建具有重要意义。本文对西盟群帕可组3个变质砂岩样品中的碎屑锆石进行了 LA-ICP-MS U-Pb测年和原位 Lu-Hf同位素组成分析，获得样品的最小年龄峰值分别为568Ma、541Ma和560Ma，εHf（t）值为-24.0～26.3。结合前人对帕可组内片麻状花岗岩的研究成果，推测西盟群帕可组沉积于早寒武世—中奥陶世（541～460Ma），而非以往所认为的前寒武纪地层。Lu-Hf同位素组成则表明西盟群帕可组物源为古老地壳物质的再循环并混入新生地壳组分，结合其他区域地质资料推断西盟群物源类似东印度板块，与东冈瓦纳大陆具有亲缘性。

Depositional age,provenance and paleogeography characteristics of the Ximeng metasedimentary sequence is key for better understanding the tectonic evolution and paleogeographic reconstruction of East Gondwana Continent.This paper presented the zircon U-Pb geochronology and Lu-Hf isotope for three metamorphic sandstone samples from the Pake Formation of Ximeng Group in SW Yunnan.The group has been traditionally considered as the Precambrian metamorphic basement of the Shan-Thai Block.The detrital zircons from three samples yielded the youngest age peaks at 568Ma, 541Ma and 560Ma,respectively,and the εHf（t）values change from -24.0 to ＋26.3.In combination with other data,it is inferred that the Pake Formation deposited at 541-460Ma,equivalent to Late Cambrian to early-middle Ordovician pe-riod,different from previous proposed Precambrian strata.Their εHf（t）values indicate that their source rocks might origi-nate from the mixing of recycled ancient continental material with juvenile crustal component.Our results,along with other geological data,suggest that the source rocks of the Ximeng Group have an affinity of Tethys-Himalaya and from East India of East Gondwana.