Abstract-The well-preserved Kardla impact crater, on Hiiumaa Island, Estonia, is a 4 Ian diameter structure formed in a shallow Ordovician sea -455 Ma ago into a target composed of thin (-150 m) unconsolid ated sedimen tary layer above a crystalline basement composed of migmatite granites, amphibolites and gne isses. The fractured and crushed amphibolites in the crater area are strongly altered and repl aced with secondary chloritic minerals. The most intensive chloritization is found in permeable bre ccias and heavil y shattered basement around and above the central uplift. Alteration is believ ed to have resulted from convective flow of hydrothermal fluids through the central areas of the crater. Chlo ritic min eral associations suggest formation temperatures of 100-300°C, in agreement with the most frequ ent quartz fluid inclusion homogenization temperatures of ISO-300°C in allochthonous bre ccia. The rath er low salinity of fluids in Kardla crater «13 wt% NaCl eq ) suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the crater rim walls rais ed abo ve sea level after the impact, or by invasion of sea water through the disturbed sedimentary cover and fra ctur ed crys talline bas ement. The well-developed hydrothermal system in Kardl a crater show s that the thermal history of the shock-heated and uplifted rocks in the central crater area, rather than coo ling of impact melt or suevite sheets, controlled the distribution and intensity of the imp act-induced hydrothermal processes.

Abstract-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 Kenrichment 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 HMXmixing, 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.

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