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Shock metamorphism and hydrothermal alteration of mafic impact ejecta from the Lockne impact structure, Sweden

Journal article
Authors Axel S.L. Sjöqvist
Paula Lindgren
Erik Sturkell
Johan Hogmalm
Curt Broman
Magnus Ivarsson
Martin R. Lee
Published in GFF
Volume 139
Issue 2
Pages 119-128
ISSN 1103-5897
Publication year 2017
Published at Department of Earth Sciences
Pages 119-128
Language en
Links dx.doi.org/10.1080/11035897.2016.12...
Keywords hydrothermal alteration, Lockne impact structure, Mafic impact ejecta, shock metamorphism
Subject categories Earth and Related Environmental Sciences

Abstract

© 2016 Geologiska FöreningenThe local geology at Kloxåsen is characterised by ejecta deposits from the 458 Ma Lockne marine impact. The Kloxåsen ejecta are located on a Caledonian parautochthonous unit, approximately 7 km from the centre of the 7.5-km-wide Lockne crater structure. The ejecta were deposited on the seafloor and were covered with seawater immediately after the impact event. Of special interest is a mafic impact breccia within the ejecta, which before the impact was Åsby dolerite that belongs to the Jämtland suite of the 1.25 Ga Central Scandinavian Dolerite Group. The mafic impact breccia occurs mainly as a coherent thin domain within a larger block of granitic breccia, which we interpret as a result of the in situ brecciation of a dolerite sill within granitic bedrock. Shock pressure in the doleritic breccia was low, in the order of 0.4 GPa, constrained by the presence of mechanically twinned clinopyroxene. Low shock pressure and brecciation corresponds well to the spall zone of an impact crater, where ejecta originate from. Whereas spalled ejecta can also show signs of having been exposed to high shock pressures, including shocked quartz, evidence for this was not found in the Kloxåsen ejecta. The breccia has been hydrothermally altered, but the ejecta are too far removed from the crater to have been affected by hydrothermal circulation in relation to Lockne’s impact event. Fluid inclusion analyses suggest that most of the alteration happened later, during the Caledonian orogeny. Geochemical analyses reflect observed mineral alterations well, such as serpentinisation of olivine.

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