Petrographic studies and mineralogical characterization of the Dellen impactites

1,2Satu Hietala,3Herbert Henkel,2Jüri Plado
Meteoritics & Planetary Science (in Press) Open Access Lik to Article []
1Geological Survey of Finland, Kuopio, Finland
2Department of Geology, University of Tartu, Tartu, Estonia
3Royal Institute of Technology, Stockholm, Sweden
Published by arrangement with John Wiley & Sons

The impact origin of the Early Cretaceous (140.82 ± 0.51 Ma) 20-km diameter Dellen structure was proven in the late 60s based on the discovery of planar deformation features (PDFs) in quartz grains. Although decades have passed, impactites found from the crater have not received much attention. Thus, this study provides a detailed petrological and mineralogical description of impactites from Dellen. Impactites were classified based on mineralogical observations using the latest recommendations of nomenclature. The studied samples include impact melt rocks (clast rich, clast poor, and clast free), suevitic impact breccias, shocked and unshocked granite, and a shatter cone. Altogether, 16 samples with different lithologies were studied using a polarization microscope. Selected samples were studied with an energy dispersive spectroscopy detector attached to the scanning electron microscopy. PDFs were indexed using a four-axis universal stage from seven samples. Selected samples for PDF studies consisted of clast-rich impact melt rocks (DEL10, DEL13, D99), suevitic impact breccias (DEL14, DEL16, DEL24), and shocked granite target rock (DEL17). A total of 197 PDF sets in 113 quartz grains were studied, and 186 sets resulted in rational crystallographic orientations. Common orientations include π{101̅2}, ω{101̅3}, z{101̅1}, ξ{112̅2}, and {101̅4}. In suevitic impact breccias and impact melt rocks, ballen silica and plagioclase with checkerboard texture were abundant. The petrographic results in Dellen impactites indicate a range of shock pressures from at least 2 to over 60 GPa, based on diagnostic shock metamorphic features in minerals and the occurrence of impact melt rock.


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