太湖冲击坑:沉积岩中冲击变质现象新证据

doi:10.13745/j.esf.sf.2024.8.30

摘要/Abstract

摘要：

总结了15年环太湖及内部岛屿的研究的成果。太湖具有震裂锥、冲击变质角砾岩、石英岩的冲击变质现象(PDFs,PFs)和冲击熔融作用等冲击坑诊断性特征。构成冲击坑理想剖面的气化熔融带—冲击变质带—冲击变质角砾岩和裂隙带等要素完备,确证了太湖冲击坑的存在。依据冲击坑诊断性特征,编制了太湖冲击变质现象分布示意图。首次将太湖冲击坑从前人的卫星图像推测落实到了太湖湖区,为太湖冲击坑深入研究打下了坚实基础。根据太湖冲击变质现象的分布,可以将太湖湖区划分为两个冲击变质现象集中区和一个冲击溅射物分布带,称为两区一带。两区一带都在湖区的内部和边缘,集中分布于太湖的中部和南部,表明冲击坑应与太湖的形成、演化和发展有关。太湖冲击坑产于陆台边缘褶皱带,靶岩为沉积石灰岩和孔隙石英砂岩地区,为一冲击坑新类型,具有许多新的冲击变质现象和特征。文中提出了震裂锥的多样性、新特征和新类型;揭示了孔隙石英砂岩中冲击变质PDFs的新特征,粒间剪切摩擦—局部熔融形成的硅酸玻璃和固化熔融包裹体等新现象,丰富和发展了沉积岩中冲击变质现象的新特征及有关理论。

关键词: 太湖, 冲击坑, 震裂锥, 冲击变质作用, 冲击熔融作用, 靶岩

Abstract:

This paper summarizes the results of fifteen years of research on the area surrounding Taihu Lake and its inner islands. Taihu Lake exhibits diagnostic features of an impact crater, including shatter cones, impact-metamorphosed breccias, and evidence of impact metamorphism in quartzite (PDFs and PFs) and impact melting. The ideal profile of an impact crater, comprising a vaporized melt zone, an impact metamorphic zone, impact-metamorphosed breccias, and a fracture zone, is fully present, confirming the existence of the Taihu impact crater. A schematic map of the distribution of impact metamorphism in Taihu has been compile based on these diagnostic features. For the first time, the Taihu impact crater, previously speculated from satellite images, has been confirmed in the Taihu Lake area, laying a solid foundation for further research. The distribution of impact metamorphism in Taihu can be divided into two concentrated zones and one ejecta distribution zone. These zones are located in the interior and edges of the lake area, primarily concentrated in the central and southern parts of Taihu Lake, indicating a relationship between the impact crater and the formation, evolution, and development of Taihu Lake. The Taihu impact crater, located in a fold belt at the margin of a continental platform with target rocks of sedimentary limestone and porous quartz sandstone, represents a new type of impact crater with many novel impact metamorphic phenomena and characteristics. The paper introduces the diversity, new features, and new types of shatter cones; reveals new characteristics of impact metamorphic PDFs in porous quartz sandstone; and describes new phenomena such as silica glass formed by intergranular shear friction-local melting and solidified melt inclusions. These findings enrich and advance the understanding of new characteristics and theories related to impact metamorphism in sedimentary rocks.

Key words: Taihu Lake, impact crater, shatter cones, impact metamorphism, impact melting, target rock

中图分类号:

P588.3

王鹤年, 陈旸, 盛雪芬, 钱汉东. 太湖冲击坑:沉积岩中冲击变质现象新证据[J]. 地学前缘, 2024, 31(6): 320-330.

WANG Henian, CHEN Yang, SHENG Xuefeng, QIAN Handong. Taihu crater: New evidence of impact metamorphism in sedimentary rocks, Taihu Lake, Jiangsu, China[J]. Earth Science Frontiers, 2024, 31(6): 320-330.

图/表 13

图1 太湖遥感图像

Fig.1 Remote sensing image of Taihu Lake

图2 太湖穹窿构造图(据文献[10]) 1 —背斜;2—向斜;3—逆冲断层;4—推覆体;5—侵入体;6—上志留统与上泥盆统交界线;7—太湖岸线。

Fig.2 Structural map of Taihu Lake dome. Adapted from [10].

图3 太湖冲击变质现象分布示意图

Fig.3 Schematic diagram of the distribution of impact-metamorphism effects in Taihu Lake area

图4 石英砂岩小型震裂锥

Fig.4 A small shatter cone in quartzite

图5 西山震裂锥的特征 a—成簇群产出的厘米级微型震裂锥。b—中型震裂锥,锥顶向下辐射的锥纹具有分叉的特征。c—震裂锥天然剖面,显示锥体内部结构与构造特征。锥体内部具有次级锥和锥纹,并有熔蚀的裂隙和孔隙发育。d—西山震裂锥新类型——锥体表面具有网状结构。

Fig.5 Characteristics of shatter cones in Xishan. (a) A cm-size micro shatter cone was produced by clusters. (b) A medium-size shatter cone with conical striations radiating from an apex of cone, has a bifurcation feature. (c) A natural section of the Xishan shatter cone shows the internal structural features of the cone. Secondary litter cones and striations occur inside of the cone, with fracture and porosity development. (d) A new type of shatter cone in Xishan displays mesh structure on the surface of the cone.

图6 石灰岩靶岩中震裂锥具标准“马尾丝”特征

Fig.6 Shatter cones in limestone target rocks with standard “horse-tail” striations features

图7 石灰岩中冲击熔融现象,方解石斑块具击变双晶

Fig.7 Impact melting in limestone, residual clasts with impact induced calcite twinning

图8 成群出现的靶岩石灰岩中的震裂锥 a—中—小型震裂锥,具两种不同类型的锥纹;b—较大型震裂锥,中间部分由上至下辐射状分叉锥纹十分明显。

Fig.8 Shatter cones in target rock limestone appearing in groups. (a) Medium-small shatter cones with two different cone types.(b) Larger shatter cones with radiating bifurcated cones from top to bottom in the middle section.

图9 锥状构造立体图 A—破裂锥;B—锥状岩块;C—扇形裂隙锥;D—冲击变质石英砂岩角砾熔岩。

Fig.9 Conical structures. A—fractured cone; B—conical rock block; C—fan-shaped fracture cone; D—impact metamorphic quartzitic breccia lava.

图10 石英砂岩中独立石英晶体内可见细而密集的3组PDFs 其左下方另一石英晶体内也有2~3组PDFs。在与上一晶粒交界处,有一再生边,其中PDFs消失。图中粒间多亮黄色熔融玻璃。

Fig.10 PDFs in quartz sandstone. Three groups of fine, dense PDFs can be seen in the monocrystalline quartz grains in quartz sandstone; 2—3 groups of PDFs also appear in another quartz crystal at the lower left. PDFs disappear at the regenerative edge at the junction with the upper grain. In the picture, many bright yellow molten glass can be seen between the grains.

图11 独立石英晶体中,具1~2组(中上)密集的PDFs成组出现,沿PDFs有密集的微小黑点分布

Fig.11 PDFs in monocrystalline quartz grains. One to two groups of dense PDFs appear in the middle and upper parts of quartz crystal, with tiny black dots densely distributed along the PDFs.

图12 剪切交叉微裂隙将石英颗粒剪碎成微细颗粒

Fig.12 Cross-shear microfissures shear quartz crystal into fine particles

图13 冲击变质石英砂岩中,广泛分布剪切摩擦局部熔融形成的亮黄色玻璃(硅酸玻璃)及气—固矿物包裹体

Fig.13 Impact metamorphism in quartz sandstone. Bright yellow glass (silicate glass) and gas-solid mineral inclusions formed by shear friction melting are widely distributed in impact metamorphic quartz sandstone.

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