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

Research areas:
Year:
2000
Authors:
Journal:
JOURNAL OF STRUCTURAL GEOLOGY
Volume:
22
Number:
5
Pages:
647-667
Month:
May
ISSN:
0191-8141
BibTex:
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 how towards a central trough where lineations are
sub-vertical. (C) 2000 Elsevier Science Ltd. All rights reserved.