Magnetic fabrics in the Holum granite (Vest-Agder, southernmost Norway): implications for the late evolution of the Sveconorwegian (Grenvillian) orogen of SW Scandinavia

The Holum granite (Vest-Agder, southernmost Norway) has been considered to be the oldest (980 Ma) undeformed (“post-tectonic”) pluton of a late-Sveconorwegian granitoid series that form two belts in southern Norway and SW Sweden. An integrated structural study of this granite pluton, based on a survey of its anisotropy of low-field magnetic susceptibility (AMS), complemented by microstructural observations, optical microscopy and rock magnetism investigations (including measurements of partial anhysteretic remanent magnetization and its anisotropy, pAARM), is presented here. The magnetic susceptibility and the AMS of the Holum granite are carried by large magnetite grains. Coaxiality between the magnetite-controlled AMS fabric and the rock fabric is verified, which allows the use of the former as a proxy for the latter. Microstructural evidence of a deformation continuum in the granite from magmatic to solid state, and homogeneity and continuity of the AMS fabrics across the pluton demonstrate that the Holum granite is not post-tectonic, but that it was emplaced in a tectonic strain field. A model of emplacement and deformation during the last Sveconorwegian regional folding phase evidenced in the country rocks of the pluton is proposed.