Geology, petrography, shock petrography, and geochemistry of impactites and target rocks from the Kärdla crater, Estonia

The Kärdla crater is a 4 km-wide impact structure of Late Ordovician age located on Hiiumaa Island, Estonia. The 455 Ma-old buried crater was formed in shallow seawater in Precambrian crystalline target rocks that were covered with sedimentary rocks. Basement and breccia samples from 13 drill cores were studied mineralogically, petrographically, and geochemically. Geochemical analyses of major and trace elements were performed on 90 samples from allochthonous breccias, sub-crater and surrounding basement rocks. The breccia units do not include any melt rocks or suevites. The remarkably poorly mixed sedimentary and crystalline rocks were deposited separately within the allochthonous breccia suites of the crater. The most intensely shockmetamorphosed allochthonous granitoid crystalline-derived breccia layers contain planar deformation features (PDFs) in quartz, indicating shock pressures of 20–35 GPa. An apparent K-enrichment and Ca-Na-depletion of feldspar- and hornblende-bearing rocks in the allochthonous breccia units and sub-crater basement is interpreted to be the result of early stage alteration in an impact-induced hydrothermal system. The chemical composition of the breccias shows no definite sign of an extraterrestrial contamination. By modeling of the different breccia units with HMX-mixing, the indigenous component was determined. From the abundances of the siderophile elements (Cr, Co, Ni, Ir, and Au) in the breccia samples, no unambiguous evidence for the incorporation of a meteoritic component above about 0.1 wt% chondrite-equivalent was found.