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太湖冲击坑溅射物的发现及其意义

王鹤年,谢志东*,钱汉东   

  1. 南京大学 内生金属矿床成矿机制研究国家重点实验室,地球科学与工程学院,南京 210093
  • 收稿日期:2009-12-20 修回日期:2009-12-20 出版日期:2009-12-20 发布日期:2009-12-20

Discovery of Impact Ejecta from Taihu Lake Impact Crater

WANG He-nian, XIE Zhi-dong*, QIAN Han-dong   

  1. State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering,Nanjing University, Nanjing 210093, China
  • Received:2009-12-20 Revised:2009-12-20 Online:2009-12-20 Published:2009-12-20

摘要: 太湖冲击成因说由来已久,但始终未成定论。近几年在太湖及周边湖泊的淤泥层中发现了许多形态各异的奇石,经多种方法测试研究,确定为太湖冲击坑的溅射物。根据成份,溅射物分为两大类。一类富铁质,以菱铁矿及其胶结的碎屑为主,包括微小球粒、棍状及各种形态的块状和片状体 ;另一类贫铁质,以方解石及其胶结的长英质碎屑为主,碎屑为锐角状的石英晶屑及少量粘土和长英质岩屑。溅射物的大小从厘米级块体到毫米级球粒,再至微米级尘粉都有。溅射物外形多具有旋转扭曲形态及熔壳特征,显示了熔融、塑性- 半塑性特征。这些特征显示其成因经历了冲击震碎、熔融、挖掘抛射、空中飞行,最后溅落在冲击坑及其周围。溅射物的成分反映了太湖靶岩基岩岩性特征。太湖冲击坑溅射物的发现是继太湖诸岛石英砂岩中石英晶体的冲击变质微结构发现之后,又一重大突破。综合其它特征,可以确定太湖为一冲击坑。

Abstract: The hypothesis of impact origin of the Taihu Lake was proposed about twenty years ago, but never confirmed.Recently some weird-shaped concretions were found embedded in the mud layer in the vicinity of the Taihu Lake, which might be impact ejecta from the Taihu impact crater. These new samples comprise two categories. One is rich in iron with fine siderite as concretion matrix, including abundant small spheroid concretions, stick-shaped concretions, and irregular shaped concretions.The other one is poor in iron with calcite as concretion matrix, including twisted-shaped concretions. The fragments embedded in matrix mainly consist of angular-sharp-edged quartz grains, and minor fragments of clay and plagioclase minerals. The sizes of the ejected materials range from centimeters of massive rocks to millimeters even to micrometers of the spheroids. Most hand samples show abundant semi-plastic features, such as twisted shape and molten crust, which might be caused by their flying in the air in the molten or semi-molten status. All these features suggest that these new samples once experienced a series of the impact cratering processes, such as the shock-induced fragmentation, shock-induced melting, ejection into the air, flying in the air, and falling in and around the impact crater. The components of the ejected materials are consistent with the target rocks in the Taihu Lake area. The confirmation of impact ejecta is another important discovery after the discovery of the shock-induced microfeatures in deformed quartz in the sandstone of islands in the Taihu Lake. The discovery of the ejection materials, combining with other features, confirmed the impact origin of the Taihu Lake.