Rb-Sr dating of strain-induced mineral growth in two ductile shear zones in the Western Gneiss Region of Nord-Trøndelag, Central Norway.

Abstract

In the Bjugn district of the northern part of the Western Gneiss Region,Nord-Trøndelag, a basement gneiss - cover nappe boundary is marked by a thickzone of ductile shearing, in which a layer-parallel mylonitic fabric withrelated new mineral growth overprints and retrogresses a previous fabricassociated with a granulite(?) facies mineral assemblage. Related minor shearbelts contain abundant new minerals and vein systems, including pegmatites,belived to represent strain-induced products formed at the time of the shearingmovements. Central parts of two large muscovite books' from such a pegmatite yieldedRb-Sr, early to Middle Devonian ages of 389 \u00B1 6 and 386 \u00B1 5 Ma, thought torelate to post-shearing uplift and cooling. East of the Verran Fault, a major shear zone also contained syn-shearingpegmatites from which a large muscovite book yielded a Rb-Sr age of 4214 \u00B1 6 Mainterpreted as indicating the time of the pegmatite formation during Scandiannappe movements. This shear zone has been subsequently reactivated by latershearing associated with regional retrogression. The fabrics of the major and minor shear zones in Bjugn, and of the lateshearing east of the Verran fault, are characterised by NE-SW trendingstreching lineation. Kinematic markers indicate a sense of layer-parallel,subhorizontal overthrusting movements not towards the southeast, but towardsthe southwest, parallel to the strike trend of the orogen. This suggests thatthis particular region may have had a different tectonic history to that ofadjacent parts of the central Scandinavian Caledonides.