SUEVITE BRECCIA OF THE RIES IMPACT CRATER, GERMANY - PETROGRAPHY, CHEMISTRY AND SHOCK METAMORPHISM OF CRYSTALLINE ROCK

Clasts of deep-seated crystalline basement rocks in suevites of the Ri es crater, Germany, were catalogued lithologically and classified with regard to their degree of shock metamorphism. The sample suite consis ted of 806 clasts from 10 outcrops in fallout suevites and 447 clasts from drill cores encountering crater suevite in the crater interior. T hese clasts can be grouped into seven types of metamorphic and nine ty pes of igneous rocks. One hundred forty-three clasts, representing the se lithologies, were analyzed for major element bulk composition. The fallout suevite contains on average 4 vol% of crystalline basement cla sts, 0.4 vol% of sedimentary rocks, 16 vol% Of glass bodies (some of t hem aerodynamically shaped), and 79 vol% of groundmass. On average, 52 % of all crystalline clasts are from metamorphic sources and 42% are o f igneous origin. Using the shock classification of Stoffler (1974), 8 % of all crystalline clasts appear unshocked (< 10 Gpa), and 34, 30 an d 27% of clasts are shocked to stages I (10-35 Gpa), II (35-45 GPa) an d III (45-60 GPa), respectively. The bulk composition of suevite glass es is consistent with the modal proportions of crystalline rock types observed in the clast populations. This indicates that the glasses ori ginate by shock-fusion of a similarly composed basement. The crater su evite contains the same crystalline rock types that occur in the fallo ut suevites. The bore hole ''Nordlingen 1973'' yields an average of 62 vol% metamorphic and 38 vol% igneous rocks. The crater suevite differ s from fallout suevites by a higher clast/glass ratio, by preponderanc e (65-95%) of clasts shocked to stage I only, and by the absence of ae rodynamically shaped glass bodies. The source of crystalline clasts an d melt particles of suevites is a volume of rocks, located deep in the crystalline basement, to which the projectile transmitted most of its energy so that only rocks of the basement were shocked by pressures e xceeding 10 GPa (deep-burst impact model). Fallout suevites were eject ed, propelled by an expanding plume of vaporized rock, and withdrew pr eferentially 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.