Progressive metamorphism and classification of shocked and brecciated crystalline rocks at impact craters

Abstract

The principle of progressive shock metamorphism in nonporous silicate rocks is discussed on the basis of petrographic observations and experimental data. The p-T conditions and the nature of the basic types of shock effects observed in rock-forming minerals are considered as far as they are indicative of the degree of shock metamorphism of the source rock. Particular shock effects that are related to the main regimes of the Hugoniot curves of quartz and feldspar, as well as the shock-melting and vaporization behavior of the whole rock, characterize and define six stages or zones of increasing shock metamorphism. Each stage or zone represents a certain range of peak pressure and temperature. Rocks of stage O are shocked to shock states below the Hugoniot elastic limit of quartz and feldspar that are irregularly fractured. Shock stage I is characterized by diaplectic quartz and feldspar that are released from shock states in the ‘two-phase regime’ of the Hugoniot. Diaplectic glass of quartz and/or feldspar composition occurs within stage II, which is related to the lowest part of the ‘high-pressure-phase regime’ of the Hugoniot. Stage III is characterized by selective melting of feldspar minerals at sufficiently high postshock temperature. Postshock temperatures exceeding the liquidus of the whole rock produce rock glasses on pressure and temperature release (stage IV). Stage V is represented by condensation products of shockvaporized rock material. On the basis of experimental data of several authors, the proposed stages of metamorphism are tentatively correlated with a pressure-temperature scale. It is concluded that the proposed classification is generally valid for shock processes in nonporous rocks and can well be applied to the recognition and distinction of the different kinds of impact breccias of terrestrial or extraterrestrial origin.