Varieties of Schmelzelithologien in the impactites of the Steinheim Basin (SW-Germany) | Varietäten von schmelzelithologien in den impaktiten des steinheimer beckens (SW-Deutschland)

The 3.8 km diameter wide Steinheim Basin, located in SW Germany, was formed 14.8 Ma ago, most likely simultaneously with the Ries crater, and is one of the best preserved terrestrial impact craters. It lies within a sequence of flat-lying sediments composed of continental Middle Triassic and marine Jurassic formations, topped by Palaeogene and Neogene units. The impact breccias formed during this event occur in a thickness of almost 50 m and mainly consist of Middle to Upper Jurassic limestones and sandstones. In the present study we investigated monomineralic calcite globules as inclusions within silicate melt particles as well as carbonate crack fillings detected in the crater floor limestone. Carbonate-silicate liquid immiscibility provides evidence of a melt origin for the examined calcite globules. The carbonate crack fillings show a dolomitic composition with MgO up to 19.7 wt.-% and are characterised by relics of fluidal textures, "schlieren", and indicators of partial melting of enclosed clasts and the melt-surrounding crater floor limestone. Based on these observations, the observed carbonate crack fillings can be interpreted as dolomite melt assemblages. © 2013 E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany.