滇西三江地区西盟群的再认识——来自构造变形特征及锆石U-Pb年代学的证据

New understanding on Ximeng Group in Three River area,western Yunnan: Evidence from structural characterization and zircon U-Pb dating

位于保山-掸泰地块东缘的西盟群,长期以来被认为是该地块在我国境内唯一出露的"前寒武纪"基底变质岩系,但其缺乏可靠的年代学数据和化石支持。本文基于详细地野外考察和显微构造分析认为,西盟群变质岩系可解体为花岗质糜棱岩和浅变质岩。前者主要出露于西盟穹形隆起的核部,遭受了强烈的韧性剪切变形;后者分布于隆起的边缘及东西两侧,归属为帕克组、王雅-允沟组。通过变形岩石的石英EBSD组构分析,揭示出位于西盟隆起核部的岩石经历了中等温度条件(550~650℃,角闪岩相)下的变形作用,并叠加了较低温度条件(400~550℃,绿片岩相)下的变形作用;而靠近西盟隆起边部的岩石仅经历了较低温度条件(400~550℃,绿片岩相)下的变形作用。本文还对西盟群变质岩中的3个花岗质糜棱岩样品进行了LA-ICP-MS锆石U-Pb测年,得到的锆石U-Pb年龄分别为455±3Ma、456±3Ma和454±3Ma。结合前人获得的年代学数据和化石资料,认为西盟群变质岩系的原岩主要由两个不同时期的物质组成,即中-晚奥陶世(~460Ma)侵入的花岗岩及寒武纪的碳酸盐岩和碎屑岩夹少量基性火山岩等;前者遭受强烈变质变形作用的改造,形成花岗质糜棱岩;后者遭受低级变质作用形成千枚岩、片岩、大理岩等浅变质岩。进而表明,前人将西盟群作为保山地块的"前寒武系"结晶基底变质岩系是不合理的。

Located in the eastern margin of Baoshan-Shan Thai block,the Ximeng Group was long regarded as the only Precambrian metamorphic basement rocks exposed in China of this block.Without scientific and reliable data of geochronology and paleontology,the age of the metamorphic rocks is still controversial.Based on detailed macro-and micro-structural observations,the metamorphic rocks of Ximeng Group which shows domelike uplift can be divided into granitic mylonites and low grade metamorphic rocks.The former are distributed in core of the uplift and have experienced intensively ductile deformation.The latter are distributed in the rim and have the similar features of Pake Formation and Wangya-Yungou Formation.According to EBSD fabric analysis of quartz in the deformed rocks of Ximeng Group,it can reveal that the rocks closed to the core have experienced an early stage of deformation at intermediate temperatures（550-650℃,corresponding to amphibolite facies）and a late deformation at low temperature（400-550℃,greenschist facies）,while the rocks near the rim of uplift have been deformed at low temperature（400-550℃）.In this paper,three samples of granitic mylonites were chosen for LA-ICP-MS zircon U-Pb dating,giving ages of 455±3Ma,456±3Ma and 454±3Ma respectively.Combined with previous research results of geochronology and paleontology,it indicates that the protolith of the Ximeng metamorphic rocks may be composed of two parts.The first part is the Ordovician granite（-460Ma）and the other mainly comprises Cambrian carbonate,clastic rocks and minor basic volcanic rock.The former is highly deformed into mylonites during the ductile shearing,and the latter is transformed into phylite,schist and marble due to low grade metamorphism at low temperature（400-550℃,greenschist facies）.It also can be concluded that Ximeng Group should not be regarded as the Precambrian metamorphic basement rocks of Baoshan block.