Lee, M.

Lindgren, P.

Sjoqvist, A.S.L.

Sturkell, E.F.F.

English

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Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk       Sjoqvist, A.S.L., Lindgren, P., Sturkell, E.F.F., and Lee, M. (2011) Distribution and Origin of Mafic Impact Ejecta from the Lockne Crater, Sweden. In: 74th Annual Meteoritical Society Meeting, August 8–12, 2011, London, UK.       0B0Bhttp://eprints.gla.ac.uk/59763/     Deposited on: 6 February 2012   DISTRIBUTION AND ORIGIN OF MAFIC IMPACT EJECTA FROM THE LOCKNE CRATER, SWEDEN A.S.L. Sjöqvist1, P. Lindgren2, E.F.F. Sturkell1, and M.R. Lee2. 1Department of Earth Sciences, University of Gothenburg 2School of Geographical and Earth Sciences, University of Glas-gow, E-mail: axel@student.gu.se  Introduction: The Lockne crater formed ca. 455 million years ago in a target composed of a crystalline Proterozoic basement, overlain by early Palaeozoic sedimentary rocks, and seawater. The inner crater has a diameter of ca. 7 km, with an up to 2.5 km wide brim of ejected, crushed crystalline basement spread around it [1]. Large ejecta blocks within this rim range from a few me-ters to ca. 100 m in diameter are found as far 15 km from the edge of the crater. The largest ejecta blocks have created smaller, secondary craters around Lockne with diameters of 100-200 m (i.e. Målingen, Karsåtjärnen, and Kloxåsen) [2]. The field distri-bution of the Kloxåsen ejecta blocks is unknown but determining their relation to the rest of the impact structure is important for a fuller understanding of the formation of Lockne and other simi-larly sized craters in mixed (crystalline and sedimentary) targets. Here, we have mapped the distribution of ejecta around Kloxåsen. As the chemical and mineralogical composition of the Kloxåsen ejecta blocks is unknown we have also carried out a mineralogical and chemical analysis of this material to establish the source rock of the mafic ejecta blocks, to characterise the ex-tent of hydrothermal alteration, and to determine the levels of shock in the ejecta. Results and discussion: Mapping of the Kloxåsen area showed that the mafic ejecta breccia is much more widely distributed than previously thought, although the granitic breccia dominates. Op-tical petrography, SEM-EDS, and Raman spectroscopy revealed extensive and curious hydrothermal alteration in the mafic clasts. Olivine has been completely replaced by serpentine and chlorite that in turn are wholly or partly replaced by calcite (and translu-cent sphalerite). This strongly depletes the clasts in Fe and Mg, and enriches them in Ca. Titanaugite has been replaced by quartz, chlorite, calcite, and anatase, although unaffected titanaugite grains still occur. Some biotites appear “split open” and are re-placed by quartz and anatase, but no kinkbanding has been ob-served. Titanaugite affected by mechanical twinning suggests shock pressure of at least 5 GPa [3]. Fluid-inclusion analysis in calcite revealed that the hydrothermal system in Kloxåsen is the same as in the rest of the crater [2]. Sector-zoned titanaugite oikocrysts, which are only known to occur in CSDG (Central Scandinavian Dolerite Group) rocks [4], and whole-rock geo-chemistry conducted in this study, strongly suggest the Åsby dolerite as a protolith for the observed mafic impact ejecta at Kloxåsen. Acknowledgements: The Royal Astronomical Society is thanked for funding this project. References: [1] Lindström M. et al. 2005. Impact Tectonics, Berlin, Springer, 357-388 [2] Sturkell E.F.F. 1998. Geologische Rundschau 87: 253-267 [3] Melosh H.J. 1989. Impact cratering: A geologic process. New York, NY: Oxford University Press [4] Claeson D.T. et al. 2007. Journal of Petrology 48: 711-728 74th Annual Meteoritical Society Meeting (2011) 5373.pdf

Distribution and Origin of Mafic Impact Ejecta from the Lockne Crater, Sweden

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Distribution and Origin of Mafic Impact Ejecta from the Lockne Crater, Sweden

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