Magnetic fabric and deformation in charnockitic igneous rocks of the Bjerkreim–Sokndal layered intrusion (Rogaland, Southwest Norway)

Abstract

The Bjerkreim–Sokndal intrusion (BKSK) belongs to the Proterozoic Rogaland igneous complex of Southwest Norway. The BKSK displays an anorthosite to mangerite cumulate series, folded into a syncline, in the core of which crop out massive acidic rocks of quartz mangerite to charnockite composition. We present a study which focuses on the acidic rocks, in which we use the low-field anisotropy of magnetic susceptibility (AMS) technique. Systematic microscope examination of the samples collected at 148 AMS stations, and considerations based on mineral intrinsic magnetic susceptibilities reveal magnetic mineralogy and anisotropy controlled by magnetite. Macro- to micro-scale petrofabrics and magnetic fabrics indicate that the acidic rocks, which intruded after deposition of the underlying cumulate series, were also affected by the syn- to post-magmatic folding event which gave rise to the BKSK syncline. In absence of any recognized plate-scale regional stress field, it is suggested that this folding results from a gravity-induced subsidence linked to the final stage of diapiric emplacement of huge neighbouring anorthosite plutons. This model is mainly supported by the structural pattern in the BKSK, which reveals a convergent flow towards a central trough where lineations are sub-vertical.