O. Bolle et al., Magnetic fabric and deformation in charnockitic igneous rocks of the Bjerkreim-Sokndal layered intrusion (Rogaland, Southwest Norway), J STRUC GEO, 22(5), 2000, pp. 647-667
The Bjerkreim-Sokndal intrusion (BKSK) belongs to the Proterozoic Rogaland
igneous complex of Southwest Norway. The BKSK displays an anorthosite to ma
ngerite 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 mi
croscope examination of the samples collected at 148 AMS stations, and cons
iderations based on mineral intrinsic magnetic susceptibilities reveal magn
etic mineralogy and anisotropy controlled by magnetite. Macro- to micro-sca
le petrofabrics and magnetic fabrics indicate that the acidic rocks, which
intruded after deposition of the underlying cumulate series, were also affe
cted 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 ano
rthosite plutons. This model is mainly supported by the structural pattern
in the BKSK, which reveals a convergent how towards a central trough where
lineations are sub-vertical. (C) 2000 Elsevier Science Ltd. All rights rese
rved.