GEOCHEMISTRY OF K-ENRICHED IMPACTITES BASED ON DRILLINGS INTO THE KÄRDLA CRATER, ESTONIA

The study is based on drill cores into the allochthonous breccias and sub-crater crystalline basement in the centre of the 4 km Ordovician shelf-targeted Kärdla crater (1), as well as cores from the rim wall and distal ejecta layer. The breccias were studied by optical and scanning electron microscopy, microprobe, and XRD1.

The granite of within-crater allochthonous breccia units is replaced with K-enriched, Na- and Ca-poor metasomatic K-granite. Amphibolite is replaced by orthoclase-chloritic metasomatic rock. These, high-temperature, impact-induced metasomatic processes are supposed as the reason of K-enrichment and Na-Ca depletion.

The magnetic fraction of impactites includes specific aggregates, so-called estolites2, composed of a large variety of agglutinated nano-to micrometer-sized chips and rounded grains. They are very variable in their composition. Remarkable are alloys of Cr-Fe-C and Fe-W or of Fe-Pb-Sn-Sb-(Cu).

The amount of estolites in impactites up to 10-4 % is insufficient to have effect on the total chemical composition of them. PGEs are concentrated in estolites, but not particularly enriched in Kärdla impactites. However, Cr is concentrated in both whole rocks and estolites (2). The REE spectrum of estolites is similar to that of surrounding crustal crystalline rocks. This leads to the suggestion that the major part of the estolite composition in Kärdla is derived from the target source.

References 1 Puura V. and Suuroja K. (1992) Ordovician impact crater at Kärdla, Hiiumaa Island, Estonia. Tectonophysics 216, 143-156. 2 Puura V. et al. (2001) Estolites - Magnetic metalliferous particles from suevites from craters of Estonia, Fennoscandia, and Africa. Met. & Plan. Sci. 36, A169-A170.