The relationship between 1.88 Ga old magmatism and the Baltic-Bothnian shear zone in northern Sweden

Abstract

New geochemical data and U-Pb ages for Proterozoic mafic and felsic dykes and their granitic host rocks in the Bl~isberget Massif, proximal to the Baltic-Bothnian shear zone, indicate that these rocks are part of a terrane distinct from Svecofennian terranes in Central and Southern Sweden. The dykes comprise a NNE-trending swarm which can be traced over a strike length of about 20 km. Most of the mafic dykes are internally deformed and have systematic internal structures showing a sinistral horizontal component and an eastern-side-up sense of movement. This sinistral shearing is also observed within the nearby Baltic-Bothnian shear zone.The mafic dykes have a uniformly basaltic composition. High content of elements such as Ti and Zr illustrate the typical 'within-plate' character. The felsic dykes are chemically homogeneous and have a Sit 2 content of about 77 wt%. High content of Y, Yb and Nb confirms the 'within-plate' character of the dyke swarm.Zircons extracted from one of the felsic dykes indicate a U-Pb age of c. 1.88 Ga. Approximately the same age has been obtained from the host rock granite and from a nearby occurrence of a similar late-orogenic granite. Since the dykes cross-cut most deformation and migmatite structures and most likely represent the waning stages of orogenic deformation in the area, it is evident that the timing of the orogenic events in the area is different and older than the Svecofennian development in central and southern Sweden.A genetic relationship is suggested between the magmatism and the development of the Baltic-Bothnian shear zone.

Multistage growth and reworking of the Palaeoproterozoic crust in the Bergslagen area, southern Sweden: evidence from U–Pb geochronology

“…A record of Early Svecofennian metamorphism (> 1.86 Ga) has been proposed to be present in the Skellefte district and neighbouring areas of northern Sweden (e.g. Wikström, Skiöld &Öhlander, 1996;Lundström, Persson & Bergström, 1999;Rutland, 2001;Rutland et al 2001). The latter authors advocate episodes of deformation prior to 1.9 Ga, but this has been strongly contested by Weihed (2003) and Weihed et al (2002).…”

A new tectonic model for the Palaeoproterozoic Kautokeino Greenstone Belt, northern Norway, based on high-resolution airborne magnetic data and field structural analysis and implications for mineral potential

“…that the orogen-parallel ductile shear zones that have been studied in some detail in Norway (e.g., Marker, 1988;Armitage & Bergh, 2005;Myhre et al, 2013;Angvik, 2014;Henderson, et al, in prep. ), Sweden (Wikström et al, 1996;Bergman et al, 2006;Luth et al, 2014) and Finland (Ward et al, 1989;Saalmann et al, 2009). These ductile shear zones, like other Palaeoproterozoic shear zones around the world (e.g., Zhao et al, 2002;Goodenough et al, 2013;MacDonald & Goodenough, 2013), acted as sites of enhanced fluid flow and, as a result, are endowed with rich mineralisations, particularly gold (e.g., Koistinen, 1981;Ward et al, 1989;Sandstad & Nilsson, 1998;Airo & Kurimo, 1999;Ihlen & Furuhaug, 2000;Eilu et al, 2003;Weihed et al, 2005;Bark & Weihed, 2007;Viola et al, 2008b;Saalmann et al, 2009;Torgersen et al, 2013;Angvik, 2014;Henderson et al, in prep.…”

Crustal reflectivity near the Archaean-Proterozoic boundary in northern Sweden and implications for the tectonic evolution of the area

“…The contacts between the Archaean rocks and surrounding rocks are either tectonic or bounded by intrusions of the 1.88 Ga plutonic rocks. Published ages for the Archaean rocks found so far in the area are 2.6–2.7 Ga (Lundqvist et al 1996, 2000; Wikström et al 1996b).…”