A precursory study of magnetic fabrics in the transition zone between magmatic and metamorphic textures in the Askersund granitoid, Sweden

Abstract

The Askersund granitoid (age 1845–1850 Ma) is the oldest member of the Transscandinavian Igneous Belt which forms a major structure in the Scandinavian peninsula. The external, metamorphic and gneissic parts of this older generation have the same age as the more central and isotropic parts. A petrographical investigation of samples collected in two traverses from the macroscopically isotropic part to the gneissic part of the intrusion has been complemented with studies of the anisotropy of the magnetic susceptibility of the rocks. The mean degree of anisotropy (P_J ) is different from site to site and increases from the macroscopically isotropic part to the gneissic part. The shape of the susceptibility ellipsoid is slightly oblate in the former part and prolate in the latter. Under the microscope this change can be correlated with a change in textures indicating deformation in a partly crystallized magma to more solid state deformation structures. A major conclusion of this investigation with regard to the geological development of the area, is that the emplacement of the granite magma took place contemporaneously with regional deformation that continued after the intrusion.

Wikström, A., Sjöberg, B. & Kapicka, A., 1997: A precursory study of magnetic fabrics in the transition zone between magmatic and metamorphic textures in the Askersund granitoid, Sweden. GFF, Vol. 119 (Pt. 4, December), pp. 285–290. Stockholm. ISSN 1103–5897.

Keywords:

• Anisotropy
• magnetic susceptibility
• granite‐gneiss transition
• Transscandinavian Igneous Batholith