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1.
自80年代初新突变学说兴起以来,陨击坑及冲击变质作用研究,已成为国际地学界的重大课题之一.本文归纳了国内外的大量资料,对岩矿识别标志进行简介和评述.主要涉及标志有:1.陨石碎块;2.玻陨石和微玻陨石;3.击变者和熔石玻璃;4.击变矿物;5.击变玻璃;6.冲击变形组构;7.冲击变质石英的光学常数变化;8.冲击微页理:9.冲击变质石英的结构变化和变形;10.陨击角砾岩;11.震裂锥;12.瞬变岩. 相似文献
2.
太湖现代沉积物中发育大量各种形态的菱铁矿结核和褐铁矿结核。前者的矿物组成以菱铁矿为主,后者以针铁矿及纤铁矿为主,结核中混有陆源碎屑、黏土和大量各种水生植物碎片、细胞、花粉、植物蛋白石、少量动物介壳碎片以及大量细菌,表明铁质结核的形成与生物有关。菱铁矿结核形成于太湖现代沉积泥层较下部的封闭还原环境中,不同形状的菱铁矿结核的14C测年证明其形成于不同年代。褐铁矿结核形成于沉积泥层上部的氧化环境中,细菌起了关键作用。二者均为沉积成岩作用的产物。 太湖的形成机制一直存在广泛争议,其中“陨石撞击说”最受关注。王鹤年等（2009）把前述的菱铁矿结核、褐铁矿结核、甚至黄土层中的钙结核当成“冲击坑溅射物”,并将之作为太湖形成于陨石撞击的确凿证据。陨石撞击地球是一种近乎瞬间的冲击变质作用,其相应的特征产物有:柯石英、斯石英,矿物中的变形页理和撞击玻璃（撞击岩）。迄今为止,在太湖及周边地区还没有发现石英的高压矿物相。观察到的石英砂岩中的石英变形纹是较低应力作用的产物,在沉积岩中常见,与冲击变质作用无关。太湖湖底平坦,水深不超过3 m,沉积层仅厚2 m,与陨石撞击造成的陨石坑地形特征不符。从沉积学的角度来看太湖形成的“洪水淹没说”更有说服力,不仅符合沉积物特征和沉积物年龄,也有考古学遗迹和史料的佐证。 相似文献
3.
针对太湖发现由菱铁矿组成的“铁质溅射物”和“贫铁溅射物”是太湖形成于陨石冲击的观点,本文对这些实为铁
质结核样品进行了详细的矿物学、地球化学和年代学测试。数据表明,这些样品是在太湖现代沉积淤泥层中形成的菱铁矿
和褐铁矿结核。结核中含有大量水生植物碎片、植物蛋白石、细菌和少量动物壳体碎片。菱铁矿高温灼烧后全部变成赤铁
矿,说明太湖地区不存在菱铁矿熔融状的高温相。菱铁矿14C测年结果表明各个样品形成于年代,不是瞬间形成的。δ13C 数
据指示为淡水无机碳来源,并非湖区碳酸盐岩直接来源。所谓的“贫铁溅射物”实际是黄土层的成岩钙结核。这些证据表明,
太湖发现的这些结核是沉积成岩期的产物,并非冲击高温溅射熔融形成的溅射物。 相似文献
质结核样品进行了详细的矿物学、地球化学和年代学测试。数据表明,这些样品是在太湖现代沉积淤泥层中形成的菱铁矿
和褐铁矿结核。结核中含有大量水生植物碎片、植物蛋白石、细菌和少量动物壳体碎片。菱铁矿高温灼烧后全部变成赤铁
矿,说明太湖地区不存在菱铁矿熔融状的高温相。菱铁矿14C测年结果表明各个样品形成于年代,不是瞬间形成的。δ13C 数
据指示为淡水无机碳来源,并非湖区碳酸盐岩直接来源。所谓的“贫铁溅射物”实际是黄土层的成岩钙结核。这些证据表明,
太湖发现的这些结核是沉积成岩期的产物,并非冲击高温溅射熔融形成的溅射物。 相似文献
4.
海南岛白沙陨击坑是一个直径约３．５ｋｍ的环形镶边坳陷，组成陨击坑边缘的环形山连续性好，并具二元结构；下部是层理清晰的下白垩统紫红色砂岩，其中长石、石英等粒状矿物普遍受冲击破碎，发育有冲击微页理和击变玻璃，云母呈膝折状变形；上部是冲击角砾岩块杂乱堆垒成的溅射覆盖层，冲击角砾岩因冲击熔融结晶而貌似凝灰岩，但其中矿物成分十分复杂，含有镁橄榄石、镍纹石以及高密度石英等，岩石化学计算结果说明它是由砂岩变质而成的，与火成岩无关。坑内保留有回落角砾岩，常见到沿裂缝贯入的脉状角砾岩。在陨击坑内找到了重３．７５ｋｇ的石陨石碎块，其中含碱硅镁石、陨铁大隅石、四方镍纹石、陨硫钙石和陨硫铁等陨石标型矿物，但不具球粒结构，ＣａＯ含量为９．１９％，属富钙的无球粒陨石，认为是白沙陨石坑的成坑陨石。在陨击坑中找到富钙无球粒陨石，为陨击坑提供了最直接可靠的证据，也为石陨石撞击成坑提供了实例。 相似文献
5.
太湖冲击坑溅射物的发现及其意义 总被引：3，自引：3，他引：0
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。 相似文献
6.
本文的主要目的是回顾太湖湖盆成因假说,并重点介绍太湖湖盆冲击成因假说的研究历史,首先对各类太湖湖盆非冲击类成因假说做简单的介绍和评述,这些假说包括潟湖说、构造说、堰塞说、火山爆喷说及综合成因说,然后重点回顾太湖湖盆冲击成因假说的演变过程.太湖湖盆冲击成因假说的第一阶段在20 世纪 90 年代初,根据太湖地区砂岩中存在大量石英变形纹现象提出冲击假说,因认知不足,渐渐势微;第二阶段是 21 世纪初,太湖湖底淤泥层赋存大量奇特富铁质结核的新发现再次复兴了冲击假说,但太湖冲击坑确定的结论并不成熟,证据并不充分,遭到质疑;第三阶段自2012 年开始,认识到工作中的失误和冲击假说的难点,再次聚焦富铁质结核的矿物学成因及所处淤泥层位的沉积学特征研究,以此为基础提出了太湖空爆冲击成因假说.最新研究结果显示太湖存在一层含富铁质结核的泥质层;富铁质结核的形态和内部特征有可能提供太湖湖盆成因的相关信息.富铁质结核可能是湖盆形成初期时大水面下淤泥空隙沉积结核物,也可能是空爆气柱合成产物和回落物.太湖湖盆空爆成因假说研究具有非常重要的科学意义,值得深入研究. 相似文献
7.
文章主要通过电子探针、扫描电镜、激光拉曼光谱、透射电镜等微区微分析技术研究GRV 022115球粒陨石的基础矿物学特征和冲击变质矿物学特征，探讨陨石冲击熔融脉的形成机制和界定其母体的冲击条件。陨石主岩主要由橄榄石、辉石、熔长石、铁镍金属和硫化物等矿物组成。根据主岩的硅酸盐矿物学特征，确定GRV 022115是风化程度较低（W1） 的L6型普通球粒陨石，与前期分类结果一致。根据熔融脉内含有大量林伍德石的现象，修正GRV 022115陨石的冲击级别为S6，比原定的S5高一个级别。GRV 022115球粒陨石中有多条冲击熔融脉，熔融脉由基质和主岩碎块包裹体两类岩相组组成。熔融脉基质的主要组成是微米级粒状镁铁榴石与纳米级的含铁方镁石，是在平衡冲击压力下结晶的产物。冲击熔融脉主岩碎块包裹体中的橄榄石、低钙辉石、长石碎块已部分或全部转为相对应的高压相。橄榄石相变为林伍德石；个别低钙辉石相变为钙钛矿结构布里奇曼石微晶的集合体；长石主要相变为熔长石与玲根石。几乎所有的主岩碎块都有高温熔融的圆滑边界。熔融脉内外同类矿物的主量和微量元素具有一定的差异性，该差异性可以反映高温高压下混溶作用和扩散作用的影响。结合陨石冲击熔融脉形成机制和结晶模型，根据熔脉基质中镁铁榴石+方镁石矿物组合及静态高温高压实验相图，界定该陨石经受的冲击压力为23~27 GPa。 相似文献
8.
撞击角砾岩是陨石撞击过程形成的特有岩石种类,是研究撞击成坑过程、陨石坑定年、矿物岩石冲击变质的理想对象。岫岩陨石坑是一个直径1800m的简单陨石坑,坑内有大量松散堆积的撞击角砾岩。本研究通过光学显微镜、费氏台、电子探针、X射线荧光光谱仪、电感耦合等离子质谱仪等分析测试手段,主要研究了岫岩陨石坑撞击角砾岩的岩相学和冲击变质特征,并在此基础上讨论了撞击角砾岩的形成过程和陨石坑的形貌特征。岫岩陨石坑内产出有三种撞击角砾岩,分别是来自上部的玄武质角砾岩和复成分岩屑角砾岩,以及底部的含熔体角砾岩。组成玄武质角砾岩和复成分岩屑角砾岩的碎屑受到的冲击程度较低,仅有少量石英发育面状变形页理,指示不超过20GPa的冲击压力。而组成含熔体角砾岩的碎屑受到了很强的冲击,发育了熔融硅酸盐玻璃、石英面状变形页理、柯石英、二氧化硅玻璃、击变长石玻璃、莱氏石等冲击变质特征,指示的峰值压力超过50GPa。本研究证实了含熔体角砾岩通常产出在简单陨石坑底部,由瞬间坑的坑缘和坑壁垮塌的岩石碎屑与坑底的冲击熔体混合形成。岫岩坑的真实深度是495m,真实深度与直径的比值为0.275,符合简单陨石坑的尺寸特征。陨石坑内的撞击角砾岩中心厚度为188m,与直径之比为0.104,略低于其它简单坑,可能是受丘陵地貌影响导致改造阶段垮塌到坑内的岩石角砾偏少。 相似文献
9.
太湖冲击成因假说作为众多太湖成因说之一,于20 多年前被提出至2009 年的再次复兴,颇受争议。本文从冲击的关
键证据之一石英变形特征入手,以新的实证研究重新审视前人的工作,并对太湖冲击假说展开探讨。主要通过光学显微镜
观察太湖地区泥盆系砂岩中石英受应力而致的不均匀消光、碎裂、变形纹等变形特征;利用费氏台测量石英变形纹的定向
特征,研究薄片中大量的人字形微裂隙现象;同时对比分析了九江地区同类砂岩及太湖地区菱铁矿结核体中石英碎屑的变
形特征。研究显示太湖地区砂岩中发育的石英变形纹并非典型的击变页理纹（PDF）, 石英变形特征主要为较低应力下的变
形特征,其成因具有多解性。因此太湖冲击成因假说以及太湖的形成机制值得多角度进一步研究。 相似文献
10.
海南白沙陨坑冲击变质岩岩石：地球化学特征初步研究 总被引：1，自引：0，他引：1
海南白沙陨石冲击坑是中国境内发现的第—个保存良好、可供直观的陨坑.经野外考察和室内分析鉴定工作,进一步对白沙陨坑内击变岩的岩石一地球化学特征进行了研究,证明堆积在底盘原岩之上的貌似“火成岩”的块状、角砾岩岩石,是长石石英砂岩受冲击变质而成的击变岩.现简介如下. 相似文献
11.
陨石撞击构造作用的研究现状与前景 总被引：1，自引：0，他引：1
当陨石撞击地球表面时, 它所引起的瞬时(1秒至1微秒)动态高压(数十至数百个吉帕)和高温(大于1500℃), 必然导致陨石撞击坑(astroblems)的生成并使周围岩石发生变质作用, 这便是撞击变质作用(impact metamorphism)。目前关于撞击构造(impact tectonics)作用研究的主要进展涵盖如下四个方面:1.撞击坑的构造形貌及形成机制; 2.撞击变质作用及撞击变质岩; 3.撞击作用的数值模拟和实验研究; 4.撞击作用的经济地质意义。陨击作用研究不仅具有学术意义, 还具有实际的经济地质意义。南非的Vredefort-Witwatersrand是最老的撞击构造, 它以丰富的金铀矿床而闻名; 加拿大的肖德贝里(Sudbury)以同撞击作用期的大型铜镍矿床著称; 如今北美、中美不少地区正在探索与陨击期后构造有关的油气储集。此外, 许多陨击变质岩可以供作建筑材料。不少撞击坑已经辟为旅游胜地, 如德国的Nordlingen Ries撞击坑。我国撞击构造的研究已有不少进展。20世纪90年代初发现了海南白沙陨击坑, 此外内蒙古的多伦、江苏的太湖和辽宁的岫岩罗圈里等地也发现有陨击构造的证迹, 不过, 都尚待进一步的工作予以证实。全球撞击构造研究经验可以借鉴的是, 在长期构造稳定区较易于发现陨击坑; 不少撞击构造被隐伏在第四纪或更老的沉积物之下, 所以采用人造卫星发回资料解译地球表面的照片, 加上地球物理勘探, 可以有效地确定研究靶区, 在此基础上再加强岩石学、构造学研究予以确认。 相似文献
12.
K. R. Subrahmanya K. N. Prakash Narasimha 《Journal of the Geological Society of India》2017,90(4):387-395
The region to the east of Palghat gap is of low elevation and nearly circular in shape. It forms a part of the Kaveri river basin. The predominantly gneissic terrain is surrounded by the charnockitic hill ranges, prominent among which are Nilgiris and Biligirirangan to the north; and Anaimalai and Kodaikanal to the south. The charnockite massifs have a steep slope facing the circular feature and a gentler slope in the opposite direction. Fractures/faults/shear zones are noticed in many parts. The Bouguer anomaly in the gneissic terrain is elliptical in shape and positive, relative to the surrounding elevated region. The magnetic contours are also elliptical and the magnetic basement is deeper by about one km compared to regions in the periphery. The shallow seismic velocity picture from Chennimalai to Palani indicates a graben structure. The velocity structure also depicts a 4–5 km Moho upwarp near Chennimalai. Junction between the gneissic and charnockitic terrain and even beyond, is marked by the presence of pseudotachylites and breccia. Field and petrographic studies indicate presence of suevite, Planar Deformation Features (PDF), Planar Fractures, diaplectic glass of quartz and plagioclase and spherical inclusion in suevite. These evidences taken together point to an extra-terrestrial impact which created a crater of approximately 120 km in diameter. Several lines of indirect evidences point to Neoproterozoic age for the impact. Kaveri Crater is the fourth largest crater on the surface of the Earth; other larger craters being Vredefort (160 km), Chicxulub (150 km) and Sudbury (130 km). 相似文献
13.
F. A. Macdonald K. Mitchell A. J. Stewart 《Australian Journal of Earth Sciences》2013,60(4-5):631-640
The Amelia Creek impact structure is located in Australia's Northern Territory in folded Palaeoproterozoic strata of the Davenport Ranges (20°51'S, 134°53′E). An impact origin is confirmed by presence of unequivocal shatter cones with apices that point upwards, and by planar microstructures in quartz grains from target sandstones of the Hatches Creek Group. Aeromagnetic, advanced spaceborne thermal emission and reflection radiometer (ASTER), and X-band synthetic aperture radar (X-SAR) images show an area of anomalous deformation in which smooth regional trends are disrupted by arcuate features at a 10 km radius to the north and south of the shock-metamorphosed rocks. However, no arcuate forms are apparent to the east and west of these shocked rocks, and instead, large south-southwest-trending faults are present about 6 km away on both sides. Despite pervasive shatter coning, typical of the central region of complex impact structures, no structural uplift is apparent, but instead the shocked rocks lie at the southern toe of a north-northeast-trending syncline. These shatter cones overprint and post-date the Palaeoproterozoic regional deformation, and thus, the impact structure has not been refolded and its abnormal form is likely due to pre-existing structure in the target rocks and/or an oblique impact. Small pockets of undeformed Late Neoproterozoic and Middle Cambrian strata are exposed in palaeovalleys in the central region of the structure, constraining the time of the impact to the Proterozoic. 相似文献
14.
Remote sensing and GIS techniques play a substantial role for the identification of possible terrestrial impact structures, for mapping target-rock lithologies and deciphering the structural style of known craters. In this case study the lithological and structural characteristics of the highly eroded Proterozoic Strangways impact crater in the Northern Territory have been analysed on the basis of Landsat Enhanced Thematic Mapper satellite imagery, topographical data and airborne geophysical data. Regarding Landsat data, the calculation of basic statistical parameters and the optimum index factor has been found useful for a pre-selection of informative band combinations. By means of the analysis of multisensoral data, the distribution of crystalline basement rocks, siliciclastic target rocks of the Roper Group as well as post-impact deposits and deeper seated Proterozoic dykes can be detected. The original crater dimensions of the Strangways structure are carefully estimated at 26?–?29 km by combining the remote sensing data with the distribution of shatter cones localised in the field. The remote sensing/GIS approach of a geological interpretation based on multisensoral sources and combined fieldwork data can be successfully applied to other impact structures on earth, as well. 相似文献
15.
In the geological sciences it has only recently been recognized how important the process of impact cratering is on a planetary scale, where it is commonly the most important surface-modifying process. On the Moon and other planetary bodies that lack an appreciable atmosphere, meteorite impact craters are well preserved, and they can commonly be recognized from morphological characteristics, but on Earth complications arise as a consequence of the weathering, obliteration, deformation, or burial of impact craters and the projectiles that formed them. These problems made it necessary to develop diagnostic criteria for the identification and confirmation of impact structures on Earth. Diagnostic evidence for impact events is often present in the target rocks that were affected by the impact. The conditions of impact produce an unusual group of melted, shocked, and brecciated rocks, some of which fill the resulting crater, and others which are transported, in some cases to considerable distances from the source crater. Only the presence of diagnostic shock-metamorphic effects and, in some cases, the discovery of meteorites, or traces thereof, is generally accepted as unambiguous evidence for an impact origin. Shock deformation can be expressed in macroscopic form (shatter cones) or in microscopic forms (e.g., distinctive planar deformation features [PDFs] in quartz). In nature, shock-metamorphic effects are uniquely characteristic of shock levels associated with hypervelocity impact. The same two criteria (shock-metamorphic effects or traces of the impacting meteorite) apply to distal impact ejecta layers, and their presence confirms that materials found in such layers originated in an impact event at a possibly still unknown location. As of 2009 about 175 impact structures have been identified on Earth based on these criteria. A wide variety of shock-metamorphic effects has been identified, with the best diagnostic indicators for shock metamorphism being features that can be studied easily by using the polarizing microscope. These include specific planar microdeformation features (planar fractures [PFs], PDFs), isotropization (e.g., formation of diaplectic glasses), and phase changes (high pressure phases; melting). The present review provides a detailed discussion of shock effects and geochemical tracers that can be used for the unambiguous identification of impact structures, as well as an overview of doubtful criteria or ambiguous lines of evidence that have erroneously been applied in the past. 相似文献
16.
Philippe Paillou Ake Rosenqvist Jean-Marie Malezieux Bruno Reynard Tom Farr Essam Heggy 《Comptes Rendus Geoscience》2003,335(15):1059-1069
Using orbital imaging radar, we detected a double circular structure, located in the southeastern part of the Libyan Desert, which is partially hidden under sandy sediments. Fieldwork confirmed it to be an unknown double impact crater, each crater having a diameter of about 10 km, younger than 140 Ma. Sampling on the site enabled the observation of quantities of shatter cone structures and impact breccias containing planar fractures. To cite this article: P. Paillou et al., C. R. Geoscience 335 (2003). 相似文献
17.
Goat Paddock in northern Western Australia is a ～5 km-diameter impact crater of Eocene age excavated in gently dipping Proterozoic sandstones. Roughly radial gorges formed by post-impact erosion provide cross-sectional views of the wall and rim zone. The predominant structural theme is one of synclinal rim folding with broad zones in which bedrock strata were deformed by impact to steep, vertical and overturned attitudes. Impact breccia is found craterward of deformed bedrock, on top of it, and downdropped into fault troughs roughly concentric to the crater. The bedrock?–?breccia contact is sharp in some places and gradational in others. In at least one section, the entire mass of upturned bedrock and breccia was displaced radially over essentially undisturbed bedrock, as indicated by slickensides on the horizontal contact. Talus deposits are similar to breccia, but show rough size sorting and clast orientation that dips steeply craterward, indicating that the talus formed as slides down the oversteepened crater wall immediately after crater formation. Shatter cones in some clasts indicate that allogenic material is incorporated in these deposits. Suevite, characterised by ropy flow textures, and by microclasts of quartz with planar deformation features, planar fractures, and of vesiculated silica glass, was found overlying deformed bedrock at a point where the surface of the bedrock forms a nearly horizontal bench midway up the crater wall. The crater was at least partially filled by later sediments, represented by bedded conglomerate close to the crater wall grading inward to sand, silt and mudstone recovered by drillholes on the crater floor. Some of the talus and conglomerate occupy re-entrants in the crater walls, suggesting an original scalloped outline to the crater. Two drillholes, one central and one halfway to the wall, both reached brecciated sandstone after penetrating 210 m of lake sediments. Goat Paddock has a flat floor with no indication of a central uplift and a depth/diameter ratio of ～0.073. This crater form, coupled with the modification of the crater walls by slumping and the scalloped outline of the crater rim suggests that Goat Paddock bridges the two traditional classes of impact crater: simple and complex. 相似文献