The Significance of Magnetic Fabric in Layered Mafic-Ultramafic Intrusions

Abstract

Anisotropy of magnetic susceptibility (AMS) has been recognised as a well-established fabric analysis tool for intrusive igneous rocks since the 1990s. The AMS technique provides directional information for magnetic foliation and magnetic lineation fabric components of the AMS ellipsoid, potentially coupled with a quantification of the overall fabric strength and geometry. The magnetic susceptibility (and therefore the AMS) of igneous rocks is often dominated by ferromagnetic mineral phases such as magnetite or low-Ti titanomagnetite, even where present in very minor amounts (e.g., ~ 0.1 vol.%). Fe-bearing silicates exhibit subordinate paramagnetic behaviour but are volumetrically much more important constituents of igneous rocks than Fe-Ti oxides, so may also contribute considerably to the AMS.A significant application of AMS is in the characterisation, constraint and quantification of very weak or subtle mineral fabrics related to flow or tectonic deformation. In particular, studies of magnetic fabrics in sheet intrusions and in granite plutons have enormously enhanced our understanding of the magma flow regimes and emplacement kinematics in these settings. Studies of AMS in layered maficultramafic intrusions have been comparatively sparse. This is despite the fact that magnetic fabrics from layered cumulates may provide information on a range of magma chamber processes, from initial magma emplacement to cumulate textural evolution and solidification, if accompanied by careful petrographic documentation and an understanding of the 'magnetic mineralogy' of the rock. A wide array of rock magnetic and complementary quantitative fabric analysis techniques can be employed to support an AMS dataset in this regard. With studies of layered mafic-ultramafic intrusions currently proceeding at unprecedented (micro-)scales of textural and geochemical detail, AMS offers petrologists a unique approach to investigating the microstructure of cumulates and the textural complexity they exhibit.

Linking In Situ Crystallization and Magma Replenishment via Sill Intrusion in the Rum Western Layered Intrusion, NW Scotland

“…We suggest that the porphyritic peridotite layers such as that illustrated in Figure 4j are harrisite sills that underwent shearing within the crystal mush, causing fragmentation of the large skeletal crystals within the hot tears and shear zones (see Hepworth et al, 2017), and producing the strong foliations that characterise both the megacrysts and groundmass olivine. O'Driscoll et al (2015) showed that the porphyritic peridotites also exhibit a well-developed olivine lineation, supporting the idea that a component of simple shear was involved in fabric development. The consistency of thickness of the porphyritic peridotite layers might mean that this process is only important where the sill is thin, where magma-static pressure is overcome by shear stress, where the inflation rate (or flux rate) in the sill is too low.…”

“…Unlike the fine-grained olivine crystals, the megacrysts contain abundant crystallographically-constrained skeletal magnetite inclusions (Fig. 10a, b) (see also O'Driscoll et al 2015). In the porphyritic peridotite, these inclusions only occur in the olivine megacrysts (i.e., they are absent in the fine-grained olivine in the granular-textured peridotite).…”

Layered Intrusions: From Petrological Paradigms to Precious Metal Repositories

“…The application of other quantitative textural techniques to layered intrusions is also flourishing. Crystal size distributions and methods of quantifying crystal alignment such as electron backscatter diffraction (EBSD) and anisotropy of magnetic susceptibility (AMS; O'Driscoll et al 2015) are examples of such investigative tools. The power of these techniques, especially when combined, as well as their potential to future research is illustrated in the article by Cheadle and Gee (this issue).…”

Magnetic fabrics in the Bjerkreim Sokndal Layered Intrusion, Rogaland, southern Norway: Mineral sources and geological significance

“…Charlier et al, 2015). Nevertheless, magnetic fabric studies in mafic -ultramafic intrusions are relatively rare (O'Driscoll et al, 2015). A possible reason for this is that interpreting magnetic fabrics in mafic rocks can be complicated by the presence of oriented ferromagnetic inclusions in mafic silicates .…”