前寒武纪突发事件与小行星/彗星碰撞的联系——地球科学中灾变观的重现

包括地外事件在内的地球中阵发性的意义重新引起了１８世纪均变论与灾变论之间的论争。看来有些前寒武纪构造热事件可能关系到和代表着大直径（Ｄｐ１０ｋｍ）抛射物大型撞击的最终效果。特别是，影响到构造上活动、地热上活跃的即硅镁壳下厚度小于２０ｋｍ的地区，可望引发区域性及全球性的亚岩石圈扰动、岩浆突发作用和大岩浆省的形成。对地壳活动带大型撞击的结果可能难以与完全由内因触发的事件相区别，因为①由撞击反应的地幔绝热熔融形成的、热传导能够广泛的再结晶和有关的变形覆盖初始冲击变质效应；②隆起地区的剥蚀、埋没和俯冲可能使初始撞击效应广泛消失。因此最终的，包括滑塌堆积、混杂堆积、浊积沉积、微波陨石（ｍｉｃｒｏｔｅｘｔｉｔｅ）及微晶柱（ｍｉｃｒｏｃｒｙｓｔｉｔｅ）（气化小行星和标准物质的球粒冷凝液）与地球化学异常一起提供了揭示前寒武纪撞击史的钥匙。主要撞击事件撞击期的论据可从保存的撞击标志、岩墙群、火山岩序列及碱性侵入岩的同位素年龄值分布直方图的热峰值的比较中获取。文章对前寒武纪地壳记录中这些事件标志的确认有两个前途：①用西澳具有保存良好的太古宇至新元古界（２１５～１Ｇａ）的约２０００ｋｍ２的Ｐｉｌｂａｒａ克拉通地区为例进行区域研?

ASTEROID/COMET IMPACT CONNECTIONS OF PRECAMBRIAN EPISODES: RE EMERGENCE OF THE CATASTROPHIC PARADIGM IN EARTH SCIENCE

The significance of episodicity in Earth history, including the role of extraterrestrial bombardment events, reinvokes the controversy between uniformitarianism and catastrophism of the Eighteenth century. It is suggested that some peak Precambrian tectonic thermal episodes may be genetically related to and represent the distal consequences of mega impact by large diameter ( D p10 km) projectiles. In particular, mega impacts affecting tectonically mobile and geothermally active crustal tracts, i.e. simatic crustal domains overlying relatively thin (<20 km) lithosphere, are expected to have triggered regional to global sub lithospheric perturbations, magmatic episodes and formation of large igneous provinces. The consequences of mega impacts on mobile crustal domains may be difficult to discriminate from purely endogenically triggered events due to (1) heat transfer from impact rebounded adiabatically melting mantle is capable of overprinting proximal shock metamorphic effects through extensive recrystallisation and related deformation; and (2) widespread elimination of the proximal impact effects by erosion of elevated terrains, burial and subduction. For these reasons, distal impact deposits including olistostromes, diamictites, turbidity currents, microtektites and microkrystites (spherulitic condensates of vaporised asteroid and target materials) accompanied by geochemical anomalies furnish the key for unravelling the Precambrian impact history. Clues for major impact events and bombardment periods are provided by comparisons between the isotopic ages of preserved impact signatures, dyke swarms, volcanic sequences and alkaline intrusions, and thermal peaks on isotopic age distribution histograms. In the present paper the identification of the signatures of such events in the Precambrian crustal records is attempted in two ways: (1) a regional approach, using the Pilbara Craton, Western Australia a well preserved Archaean to upper Proterozoic (3.5 1.1 Ga) c.200 000 km 2 large crustal province as an example, and (2) tests of possible correlations between impact, tectonic and magmatic events in Precambrian shields world wide.