首页> 中文期刊>大地构造与成矿学 >安徽洪镇地区董岭群碎屑锆石U-Pb年代学及其地质意义

安徽洪镇地区董岭群碎屑锆石U-Pb年代学及其地质意义

     

摘要

The Dongling Group,exposed in the Hongzhen area,Anhui province,was previously considered to be the Paleoproterozoic basement.It has been extensively studied because it was believed to be the core of the Hongzhen metamorphic core complex and the metamorphic crystalline basement of South China as well.To accurately interpret the Precambrian tectonic evolution of South China and the formation mechanism of the Hongzhen "metamorphic core complex",detrital zircons from the gneiss,the main part of the Dongling Group,were analyzed and three age groups of 760-830 Ma,1930-2060 Ma and 2360-2530 Ma were obtained.The age distribution of the zircons is similar to those of the Yangtze Craton as whole,indicating that the detrital material of the Dongling Group was derived from adjacent basement of the Yangtze Carton.The youngest detrital zircon age (768±16 Ma) of the Dongling Group and the 635 Ma lower boundary age of overlying Sinian System suggest that the Dongling Group should be deposited in Neoproterozoic rather than Paleoproterozoic.Thus,the Dongling Group is not the crystalline basement of South China.We thus suggest that there is no crystalline basement has yet been discovered in the southeast of the Yangtze Craton.In addition,the strata in the Hongzhen area is continuous so long as the Dongling Group is Neoproterozoic.Therefore,the viewpoints concerning the Hongzhen metamorphic core complex proposed previously need to be re-evaluated.%安徽洪镇地区的董岭群为一套以二长片麻岩为主的中深变质岩系,长期以来被认为是长江中下游地区古元古代的变质结晶基底,因作为洪镇变质核杂岩的“变质核”而受到广泛关注.为了能够更好地揭示华南前寒武纪的大地构造演化和洪镇“变质核杂岩”的成因,本文对洪镇地区董岭群主体的二长片麻岩进行了碎屑锆石LA-ICP-MS U-Pb定年,获得了760~830Ma、1930~2060Ma和2360~2530Ma 3个主要年龄区间.这一年龄谱与扬子克拉通前寒武纪的碎屑锆石年龄分布特征类似,暗示董岭群的物质源于其附近克拉通物质的剥蚀及搬运;其中最年轻的锆石年龄为768±16 Ma,这一年龄限定了董岭群地层时代下限,结合其上覆盖层为震旦系(底界小于635 Ma),将董岭群的地层时代限定于新元古代,这一结论否定了其为古元古代古老变质结晶基底的说法.综合近年来的研究,扬子板块西缘及东南缘目前并未发现太古宙-古元古代的结晶基底.另外,由于董岭群地层时代的重新厘定,得知这一地区并不存在大规模地层缺失和大规模剥离断层,因此前人提出的洪镇“变质核杂岩”的构造模型值得商榷.

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