Suevite breccia of the Ries impact crater, Germany: Petrography, chemistry and shock metamorphism of crystalline rock clasts

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.