[目的]火地垭群是扬子陆块西北缘前寒武纪重要的地层单元,同时也是中国重要的石墨矿含矿层位,但其沉积时代、物源及形成的构造背景长期存在争论.[方法]对旺苍地区火地垭群上两组的绢云千枚岩进行了LA-ICP-MS锆石U-Pb定年研究.[结果与结论]获得碎屑锆石的年龄范围为832～988 Ma,集中分布于832～843 Ma、855～883 Ma、895～936 Ma 和952～973 Ma 四个区间,最大沉积年龄为837.6±6.0 Ma(MSWD=0.60,n=5).结合已有研究成果,将旺苍地区火地垭群的沉积时代限定为910～835 Ma,而非前人长期认为的中元古代;物源主要来自周围的岩体,主体方向为东(南)和西北;综合沉积时代、物源、碎屑锆石年龄及区域地质特征,旺苍地区火地垭群上两组形成于接收双向物源的弧后盆地,是Rodinia超大陆聚合晚期在扬子陆块西北缘的响应.
Depositional Age,Provenance and Tectonic Significance of the Huodiya Group in the Wangcang Area,Northwestern Margin of the Yangtze Block:Constraints from detral zircon U-Pb geochronology
[Objective]The Huodiya Group is an important Precambrian stratigraphic unit in the northwestern margin of the Yangtze Block and an important graphite-bearing stratum in China.However,its depositional age,provenance,and tectonic attribution have long been debated.[Methods]In this study,laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)zircon U-Pb dating was conducted on the sericite phyllite from the Shangliang Formation of the Huodiya Group,Wangcang area.[Results and Conclusions]The results show that maximum depositional age of the sample is 837.6±6.0 Ma(MSWD=0.60,n=5).Combined with existing data,the depositional age of Huodiya Group in the Wangcang area is constrained between ca.910 Ma and ca.835 Ma,rather than previously-considered Mesoproterozoic strata.The age populations of detrital zircons are concentrated in four peaks of 832-843 Ma,855-883 Ma,895-936 Ma,and 952-973 Ma,and the detrital provenance is mainly from the southeast and northwest magmatic rocks.A synthesis of depositional age,provenance and regional geological back-ground indicates that the Shangliang Formation of the Huodiya Group in the Wangcang area were likely deposited in a back-arc basin receiving provenance from both sides,which is a response to the late-stage convergence of the Rodinia supercontinent in the northwestern margin of the Yangtze Block.
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