华夏地块寻乌组碎屑锆石U-Pb年代学及其地质意义

Detrital Zircon U-Pb Geochronology of the Xunwu Formation in Cathaysia Block and Its Geological Significance

对赣南地区寻乌组变沉积岩进行了碎屑锆石U-Pb年代学研究,获得有效年龄208组,下、中、上段变沉积岩中最年轻碎屑锆石的加权平均年龄分别为602 Ma、641 Ma和570 Ma,与地层层序基本吻合。结合侵位其中的岐山岩体结晶时代为445 Ma,将寻乌组变沉积岩成岩时代限定为570~445 Ma。寻乌组中段底部与顶部均见有灰白色薄层状硅质板岩,上段底部见有灰白色薄层状硅质岩夹层,可与震旦纪老虎塘组硅质岩、硅质板岩进行良好的对比,明显区别于寒武纪牛角河组灰黑色高炭质板岩(石煤层)、灰黑色含炭硅质岩,因此推断寻乌组属于震旦纪地层,而不是前人认为的古元古代或南华纪。寻乌组变沉积岩碎屑锆石以存在丰富的Grenville期年龄信息为特征。寻乌组下、中、上段变沉积岩的物源均具有部分来自泛非时期的构造热事件的锆石记录,而且上段构造热运动记录更多,表明同时期泛非时期岩浆活动非常强烈,进一步表明华夏与冈瓦纳陆块有一定的亲缘性。

The detrital zircon U-Pb geochronology of the metasedimentary rocks of the Xunwu Formation in southern Jiangxi is studied in this paper,and 208 groups of effective ages are obtained.The weighted average ages of the youngest detrital zircons in the lower,middle,and upper members of meta-sedimentary rocks are 602 Ma,641 Ma and 570 Ma,respectively,which are basically consistent with the stratigraphic sequence.Combined with the crystallization age of the Qishan pluton for 445 Ma,the diagenetic age of the meta-sedimentary rocks of the Xunwu Formation is limited between 570 and 445 Ma.Gray-white thin-bedded siliceous slate is seen at the bottom and top of the middle section of the Xunwu Formation,and off-white thin-bedded siliceous rock interlayers are seen at the bottom of the upper section,which can be well matched with the SinianLaohutang Formation siliceous rock and siliceous slate.The comparison is obviously different from the Cambrian Niujiaohe Formation gray-black high carbonaceous slate(stone coal seam)and gray-black carbonaceous siliceous rock.Therefore,the Xunwu Formation meta-sedimentary rock belongs to the Sinian strata,not the Paleoproterozoic or Nanhua Period.The detrital zircons of the meta-sedimentary rocks of the Xunwu Formation are characterized by abundant Grenville age information.The provenance of the meta-sedimentary rocks of the lower,middle and upper members of the Xunwu Formation all have zircon records from the tectonic-thermal movement in the Pan-African period,and there are more tectonic-thermal movement records in the upper member,indicating that the strong volcanic activity existed during the Pan-African period,and that the Cathaysia and Gondwana blocks have a certain affinity.