Zalužany - a circular structure in the Czech Republic accompanied by glass of granodiorite composition

Abstract

The Hejný Pond, 500 × 400 m, southeast of the Zalužany village in central Bohemia, attracted attention as a possible impact structure. The pond has a notable bilateral symmetry with an axis oriented NW–SE, indicating a significant subhorizontal vector in the overall deformation pattern. A negative gravity anomaly elongated in the same direction (600 × 300 m) with the amplitude of –0.35 mGal was disclosed by detailed gravity survey. The incomplete ring of elevations suggests slightly uplifted basement segments with a diameter of 1 km. Search for glass and other possible indicators of shock metamorphism resulted in a find of glass fragments 22 to 37 mm across. The glass has composition comparable to the local Kozárovice hornblende–biotite granodiorite of the Central Bohemian Plutonic Complex. Unusual features include locally high contamination by iron (up to 10–13 wt. % FeOt) and abundant cristobalite. The absence of planar deformation features in quartz and feldspars together with the absence of platinum-group elements and Ni enrichment in local glass may be considered as obstacles in defining Zalužany as an impact structure. Although glass fragments are rare at the locality, and the considerable Quaternary erosion, the occurrence of rock inclusions in two of the fragments makes the Zalužany structure a promising object for future study.