Secondary fabrics revealed by remanence anisotropy: methodological study and examples from plutonic rocks

Abstract

Summary The fabric of fine-grained magnetite is isolated by partial remanence anisotropy (pAAR) measurements that combine cycles of anhysteretic remanence acquisition and alternating frequency (AF) tumbling demagnetization of the low-coercivity fraction. Since remanence intensities of the most coercive, fine-grained magnetite are very weak (rarely exceeding 10 per cent of the measured remanence in the studied samples), a methodological study is presented that defines optimal measurement schemes and examines the role of gyroremanence and background remanences due to hard remanence carriers. This procedure is applied to some ‘reddened’ granites from Tana (Corsica) and Wichita (Oklahoma) and to the Tellnes ilmenite-rich norite (Norway). The pAAR fabrics for fine-grained magnetites are compared to the fabrics of the coarse-grained, primary magnetite grains, as shown by both the anisotropy of magnetic susceptibility (AMS) and the remanence anisotropy of the low-coercivity fraction. In Tellnes, fine-grained magnetite is believed to be of secondary origin; this is strongly suggested by the correlation between the pAAR fabrics and the orientations of sets of microfractures decorated by oxides. In the ‘reddened’ granites, although no direct correlation between pAAR fabrics and microstructures could be evidenced, the comparison between unaltered specimens and hydrothermally altered ones reveals changes in magnetic properties and fabrics that are ascribable to this secondary geological process.