Abstract
The Colônia impact crater, developed on crystalline basement rocks, offers an excellent example of one of the most unique features of the impact process: the effects of shock waves on textural and mineralogical changes of the target rock. The impact melt-bearing impactites were derived essentially from the igneous and metamorphic rocks, including granite, mica schist, granitic gneiss, and quartzite. Investigations using optical microscopy indicate that the effect of shock waves on those lithologies caused a wide variety of deformation features and generation of new materials. The most common features include fluidal textures, unusual rearrangement patterns between grains, recrystallization, decomposition and precipitation of new phases, agglutination of glassy and crystalline spherules, and the mobilized melt formed different types of impact melt particles. These transformations cover processes that may involve a new grain growing at the expense of parental grains of the same species, or crystallization of different mineral types from component-providing grains until a complete textural and compositional change of the target rocks occurs. Small-scale structures in deformed rocks are particularly interesting for exploring elastic-plastic deformation, phase transformations, and generation of impact melt products.
Highlights
The formation of hypervelocity impact craters on the Earth’s surface is a geologic process that occurs in a highly dynamic physical environment and releases a very large amount of energy
Whether formed by an asteroid or comet, the kinetic energy of the bolide is transformed at the impact site into high-pressure shock waves that propagate through the target rocks with a velocity much greater than that of sound (Grieve, 1987; Melosh, 1989)
Many details of such energy transfer processes are still not well understood, a clear observation in experimental studies and geological records suggests that the impactor and target rocks are pulverized and ejected into a fireball cloud containing a mixture of vapor, melt products, shocked and unshocked rocks debris, and breccias (Osinski & Pierazzo, 2012)
Summary
The formation of hypervelocity impact craters on the Earth’s surface is a geologic process that occurs in a highly dynamic physical environment and releases a very large amount of energy. (suevite), melt rock, lithic breccia dikes, intrusive melt dikes, and monomictly brecciated basement (French, 1998) In such lithologies, it is common to observe a mesostasis constituted by the material that was molten and mixed with lithic clasts of the target rock (Grieve et al, 1977; Grieve & Therriault, 2012). The stratigraphic position of this deposit, between the underlying crystalline basement and overlying crater fill sediments, provides an exceptional opportunity to investigate in detail the processes that govern impact melt generation, as well as the distribution of impact melt rocks and impact glasses, in small structures (diameter less than 4 km) developed on crystalline basement targets that have been subjected to intense weathering and erosion
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
