Abstract-The distribution and petrography of surficial suevite breccias of the Ries impact crater in Southern Germany are reviewed, and the morphology, petrography and chemical composition of impact glasses in suevite breccias and their postdepositional devitrification is synthesized. Origin and thermal history of suevite breccia and suevite glasses are inferred from these data and from recent results of cooling and crystallization experiments with suevite glass melts under controlled conditions. In a montmorillonitic groundmass, the suevite breccia contains pieces of glass, up to some decimeters in size, and crystalline rock clasts of all stages of shock metamorphism. The glass particles originated in impact melt of basement gneisses and cooled by adiabatic pressure release from -80 GPa to atmospheric pressure during ejection from the crater. They were deposited on the ground together with the other suevite components at a temperature of~750 DC. Fractured glass pieces in the breccia show that during deposition of the suevite the temperature was below the temperature at which undercooled melt transforms to rigid glass. The suevite cooled after deposition mainly by convection of heat by emanating gases and vapors. In chilled layers at the base and at the top of suevite deposits, the glasses are preserved in vitreous state. Between these zones, the glasses were devitrified, yet crystallization of pyroxene, plagioclase and magnetite took place below the glass-transformation temperature. Annealing experiments show that this unusual devitrification below the transformation temperature can be explained by the impact origin of suevite glasses. Due to rapid adiabatic cooling on decompression, the glasses were oversaturated with water and internally strained. Under these conditions, devitrification, especially the formation of plagioclase, was possible at temperatures below the transformation range. The origin from adiabatically cooled impact melt of deep-seated rocks distinguishes water-bearing suevite glasses from the Ries-derived, water-free moldavite tektites, which are interpreted as condensates of vaporized, surficial sediments (Engelhardt et ai., 1987).
Abstract— Clasts of deep‐seated crystalline basement rocks in suevites of the Ries crater, Germany, were catalogued lithologically and classified with regard to their degree of shock metamorphism. The sample suite consisted of 806 clasts from 10 outcrops in fallout suevites and 447 clasts from drill cores encountering crater suevite in the crater interior. These clasts can be grouped into seven types of metamorphic and nine types of igneous rocks. One hundred forty‐three clasts, representing these lithologies, were analyzed for major element bulk composition. The fallout suevite contains on average 4 vol% of crystalline basement clasts, 0.4 vol% of sedimentary rocks, 16 vol% of glass bodies (some of them aerodynamically shaped), and 79 vol% of groundmass. On average, 52% of all crystalline clasts are from metamorphic sources and 42% are of igneous origin. Using the shock classification of Stöffler (1974), 8% of all crystalline clasts appear unshocked (<10 Gpa), and 34, 30 and 27% of clasts are shocked to stages I (10–35 Gpa), II (35–45 GPa) and III (45–60 GPa), respectively. The bulk composition of suevite glasses is consistent with the modal proportions of crystalline rock types observed in the clast populations. This indicates that the glasses originate by shock‐fusion of a similarly composed basement. The crater suevite contains the same crystalline rock types that occur in the fallout suevites. The bore hole “Nördlingen 1973” yields an average of 62 vol% metamorphic and 38 vol% igneous rocks. The crater suevite differs from fallout suevites by a higher clast/glass ratio, by preponderance (65–95%) of clasts shocked to stage I only, and by the absence of aerodynamically shaped glass bodies. The source of crystalline clasts and melt particles of suevites is a volume of rocks, located deep in the crystalline basement, to which the projectile transmittted most of its energy so that only rocks of the basement were shocked by pressures exceeding 10 GPa (deep‐burst impact model). Fallout suevites were ejected, propelled by an expanding plume of vaporized rock, and withdrew preferentially from this volume melt and highly shocked clasts, leaving in the transient cavity the crater suevite with more clasts of modest shock levels and less melt.
ZusammenfassungSuevite sind Impaktbreccien, die in einer montmorillonitischen Matrix geschockte und ungeschockte Mineral-und Gesteinsfragmente aus dem kristallinen Untergrund, Glas-Einschlfisse und eine geringe Menge von sediment~iren Fragmenten enthalten. Es werden Daten fiber die modale Zusammensetzung der Fall-out-Suevite (abgelagert an isolierten Punkten rings um den Krater) und des Krater-Suevits (der eine Schicht unter den nach-Riesischen Seesedimenten im Krater bildet) mitgeteilt. Fall-out-Suevite enthalten aerodynamisch geformte Bomben, die im Krater-Suevit fehlen. Unter Berficksichtigung der feineren Glaskomponenten -nicht nur der groBen Bomben und Fragmente -erh~ilt man ffir den Glasgehalt der Fall-out-Suevite 47 vol%, fiir den des Krater-Suevits 29 vol%. Die kristatlinen Einschtfisse in den Sueviten geh6ren zu den magmatischen und metamorphen Gesteinstypen, die den Untergrund des Riesgebietes aufbauen. Dieser besteht aus einer h6he-ren Lage magmatischer Gesteine (haupts/ichtich Granite) und einer tieferen Serie von Gneisen und Amphiboliten. Auf Grund einer Aufsammlung von 1 200 Kristallineinschliissen aus 13 Suevit-Vorkommen wurde die H~iufigkeit der einzelnen Gesteinstypen in den Sueviteu bestimmt. Die Suevite enthalten im Mittel 46 % magmatische und 54 % metamorphe Gesteinseinschlfisse. Im Gegensatz dazu bestehen die nicht oder nur wenig geschockten kristallinen Ejekta um den Ries-Krater aus 82 % magmatischen und 18 V0 metamorphen Gesteinen.Die mittleren chemischen Zusammensetzungen der kristallinen Gesteinstypen wurden auf Grund yon 138 chemischen Analysen bestimmt. Ein Vergleich mit der Zusammensetzung der Suevit-Glaser ergibt, dab diese durch die Aufschmelzung der Gneise gebildet wurden, welche in einem tieferen Niveau des kristallinen Untergrundes lagen.Die AbstractSuevites are impact breccias with a montmorillonitic matrix that contains shocked and unshocked mineral and rock fragments from the crystalline basement, glass inclusions and a small amount of sedimentary clasts. Data are given of the modal composition of fall-out suevites (deposited at isolated points around the crater) and crater suevite (forming a layer below post-impact lake sediments in the crater cavity). Fall-out suevites contain aerodynamically shaped bombs which are absent in crater suevite. Taking into account not only large glass fragments and bombs, but also the finer fractions, the glass content of fall-out and crater suevites amounts to 47 and 29 vol%, respectively. Crystalline clasts in suevites consist of all igneous and metamorphic rock types that constitute the local basement which consists of an upper layer of igneous rocks (mainly granites) and a lower series of gneisses and amphibolite. Based on a collection of 1 200 clasts from 13 suevite occurrences the average crystalline clast population of suevites was determined. Suevites contain on the average 46 % igneous and 54 % metamorphic clasts. In constrast, weakly shocked and unshocked crystalline ejecta of the Ries structure consist of 82 % igneous and 18 % metamor...
— The central uplift of the 40‐km wide Araguainha impact structure, Brazil, consists of a ring, about 8 km in diameter, of up to 150‐m high blocks of Devonian Furnas sandstone, which surround a central depression of elliptical shape (4.5 × 3.0 km). The depression is occupied by a pre‐Devonian alkali‐feldspar granite, shocked by pressures of 20–25 GPa and permeated by cataclastic shear zones and dikes of shocked granitic material. The granite is flanked and partly covered by several impact breccias: (1) Impact breccia with melt matrix overlies the granite in places and forms hills, bordering the granitic center in the S and SW. It is chemically identical with the granite and consists of thermally altered granitic clasts in a matrix of sanidine, quartz, biotite, muscovite, chlorite and riebeckite. (2) Polymict breccias form hills which border the central depression in the N and NW. Components are unshocked and shocked sediments, shock‐melted sandstone, shocked granite and shock melt rocks in irregular masses and individual bodies, embedded in a fine‐grained matrix. 40Ar/39Ar analyses show that the melt rocks solidified 246 Ma ago, indicating that the impact occurred at near the Permian‐Triassic boundary, possibly when the area was covered by a shallow sea. The present chemistry and petrography of the melt rocks suggest that by reacting with seawater granitic impact melt was depleted of K and Rb and enriched in Na, and that later diagenetic processes produced replacement of feldspar by quartz and deposition of hematite. (3) Monomict breccias, consisting of unshocked, shocked and shock‐fused quartz sandstones, form hills which surround the central depression in the SE and S. The Araguainha structure is an eroded complex crater, produced by an impact, 246 Ma ago. The depth of excavation was about 2.4 km, comprising Permian, Permo‐Carboniferous and Devonian sediments and the granitic basement. The diameter of the transient crater was about 24 km. Erosion and weathering have removed most of the original crater fill and ejecta deposits, with the exception of remnants, preserved in the central uplift.
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