Structural, geophysical and petrological investigations of the Sarasåsen intrusion and host rocks, Sveconorwegian orogen, southern Norway

Abstract

The Kongsberg terrane consists of amphibolite to granulite-facies ortho- and para-gneisses formed during one or more Mesoproterozoic orogenies, in which the Sveconorwegian orogeny is the most important. The possible existence of a suture zone within the Sveconorwegian belt has been a long-lasting and still ongoing discussion. Here a possible candidate of such a suture zone, the ultramafic Sarasåsen intrusion in the Kongsberg terrane, is investigated by combining field mapping, structural analysis, geochemical and geophysical studies. The study documents that the Sarasåsen intrusion is not affected by high-grade metamorphism and that it post-dates the development of regional tectonic foliation. It is thus interpreted to be a late- to post-Sveconorwegian intrusive event and not part of a suture zone-related ophiolite complex. Field mapping reveal the presence of a major shear zone along the shores of Lake Væleren, named the Væleren Shear Zone (VSZ). The VSZ consists of a steeply dipping sheared foliation and mylonites with south-plunging steep stretching lineations, that are associated with sinistral kinematics. The shear zone has later been affected by large scale steeply plunging isoclinal folding. The mylonitic fabric is cut at a high angle by a lamprophyric dyke that has been dated to 1032 Ma. This age represents a minimum age for the development of the VSZ.