云南东川望厂组熔结凝灰岩锆石SHRIMP U-Pb年龄及其意义

SHRIMP U-Pb Age and Its Significance of Zircons in Welded Tuff of Wangchang Formation in Dongchuan Area, Yunnan Province,SW China

本文报道了云南东川地区望厂组中熔结凝灰岩锆石的SHRIMP U-Pb年龄。其中自形结构、环带清晰锆石的不一致曲线与谐和线上交点的207Pb/206Pb年龄分别为2317±13Ma(n=13,MSWD=3.3),13个点的加权平均年龄为2299±14Ma(n=13,MSWD=5.9),表明望厂组地层的沉积时代为古元古代早—中期。东川地区下四组地层与滇中昆阳群(黄草岭组—美党组)和四川会理群(淌塘组—天宝山组)的沉积时代相差近1.0Ga。为了区别于昆阳群和会理群,建议对本区下四组地层(洒海沟组、望厂组、菜园湾组、平顶山组)独立建群,即汤丹群。汤丹群是目前为止康滇地区已发现的最古老的地层建造之一,经历过古元古代中期的挤压过程(东川运动)和古元古代末期的拉张过程,可能是相对于扬子古陆核独立存在、并构成早—中元古宙Columbia超级大陆的一个小的古老陆块。

The SHRIMP U-Pb ages for zircons in welded tuff from Wangchang Formation in Dongchuang Area, Yunan Province, are reported in this paper,and the 207pb/206pb age of the point of intersection between discordant and concordia curves for zircons with automorphic and zonal tecture in P13 are 2317 ＋ 13 Ma（ n = 13, MSWD = 3. 3） ,and weighted average age is 2299 ± 14 Ma （ n = 13, MSWD = 5.9） , implying that the sedimentary age of Wangchang Formation is early to mid-paleoproterozoic. The sedimentary age of underlying four formations in Dongchuang Area almost are ten billion years older than the Kunyang Group in central Yunnan and the Huili Group in south Sichuan Province. To distinguish the underlying four formation （Sahaigou, Wangchang, Caiyuanwang and Pingdingshan Formations） from the Kunyang and the Huili Group, and the independent the Tangdan Group which is one of the oldest gourp in Kangdian Area so far is advised to be established. The Tangdan Group experienced convergence process （Dongchuan Movement） during early Paleoproterozoic and divergence process during late Paleoproterzoic, and may be a small independent block that constitute part of Columbia Supercontinent to ancient Yangtze Nucleus.