INVESTIGATION OF MAJOR ROCK-UNIT INTERNAL STRUCTURE

USING LOW-FIELD ANISOTROPY OF MAGNETIC SUSCEPTIBILITY

Olivier Bolle, Jean-Marc Lambert and Hervé Diot

The low-field anisotropy of magnetic susceptibility (AMS) technique is a powerful tool, which allows rapid and easy fabric characterisation of weakly anisotropic rocks. It also gives access to several parameters, as e.g. the bulk magnetic susceptibility, which can be very useful in the rock mineralogical characterisation.

This technique has been applied to the Bjerkreim-Sokndal intrusion, as part of a structural study focusing on the massive acidic upper part of this intrusion and on the stratigraphically highest units of the underlying cumulate series. Systematic microscope examination of the AMS samples 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 and the cumulate series have been affected by a gravity-induced subsidence, probably coeval with the diapiric emplacement final stage of the neighbouring anorthosite plutons. This model is mainly supported by the structural pattern, which reveals a convergent flow towards a central trough where lineations are sub-vertical.

In the prolongation of these successful results obtained on the Bjerkreim-Sokndal intrusion, another structural study using the AMS is in progress on the Tellnes ore deposit. Preliminary results of this study indicate that magnetic fabric and petrofabric have similar orientation and result from (sub-)magmatic flow, during the emplacement of the orebody in a dextral-opening weakness zone developed in the Åna-Sira anorthosite. The mean direction of this flow is given by the average magnetic lineation measured inside the orebody (160° SE 20°). The bulk susceptibility magnitude values indicate a dominant ferrimagnetic behaviour and cover a large range, from 3.1 to 84.3 mSI, in probable relation with variation in the magnetite content of the Ti-ore.

The AMS technique can be used to investigate both the internal structure and mineralogy of other magmatic units of the Rogaland anorthosite province, especially ilmenite deposits. It is also worth mentioning that an AMS study of biotite-hornblende post-collisional granitoid plutons associated with the Mandal-Ustaoset Line (MUL), several tens of km to the east of the Rogaland anorthosite province, is presently in progress. Even if these granitoids and the anorthosites are not spatially related, both emplaced at the same time period and their genesis and emplacement appear to have been controlled by major lithospheric weakness zones. The structural study of the MUL granitoids could therefore bring important additional constraints on the understanding of the genesis and the emplacement mechanisms of the Rogaland anorthosite province and associated ore deposits.