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

• © 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.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences (hsv//eng)

Nyckelord

hydrothermal alteration

Lockne impact structure

Mafic impact ejecta

shock metamorphism

Mafic impact ejecta

The changing Earth

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