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SUMMARY Exsolved iron oxides in silicate minerals can be nearly ideal palaeomagnetic recorders, due to their single-domain-like behaviour and the protection from chemical alteration by their surrounding silicate host. Because their geometry is crystallographically controlled by the host silicate, these exsolutions possess a shape preferred orientation that is ultimately controlled by the mineral fabric of the silicates. This leads to potentially significant anisotropic acquisition of remanence, which necessitates correction to make accurate interpretations in palaeodirectional and palaeointensity studies. Here, we investigate the magnetic shape anisotropy carried by magnetite exsolutions in pyroxene single crystals, and in pyroxene-bearing rocks based on torque measurements and rotational hysteresis data. Image analysis is used to characterize the orientation distribution of oxides, from which the observed anisotropy can be modelled. Both the high-field torque signal and corresponding models contain components of higher order, which cannot be accurately described by second-order tensors usually used to describe magnetic fabrics. Conversely, low-field anisotropy data do not show this complexity and can be adequately described with second-order tensors. Hence, magnetic anisotropy of silicate-hosted exsolutions is field-dependent and this should be taken into account when interpreting isolated ferromagnetic fabrics, and in anisotropy corrections. 

Abstract. Numerous studies have revealed genetic similarities between Tethyanophiolites and oceanic “proto-arc” sequences formed above nascent subductionzones. The Semail ophiolite (Oman–U.A.E.) in particular can be viewed as ananalogue for this proto-arc crust. Though proto-arc magmatism and themechanisms of subduction initiation are of great interest, insight isdifficult to gain from drilling and limited surface outcrops in marinesettings. In contrast, the 3–5 km thick upper-crustal succession of theSemail ophiolite, which is exposed in an oblique cross section, presents anopportunity to assess the architecture and volumes of different volcanicrocks that form during the proto-arc stage. To determine the distribution ofthe volcanic rocks and to aid exploration for the volcanogenic massivesulfide (VMS) deposits that they host, we have remapped the volcanic unitsof the Semail ophiolite by integrating new field observations, geochemicalanalyses, and geophysical interpretations with pre-existing geological maps.By linking the major-element compositions of the volcanic units to rockmagnetic properties, we were able to use aeromagnetic data to infer theextension of each outcropping unit below sedimentary cover, resulting ina new map showing 2100 km2 of upper-crustal bedrock. Whereas earlier maps distinguished two main volcanostratigraphic units, wehave distinguished four, recording the progression from early spreading-axisbasalts (Geotimes), through axial to off-axial depleted basalts (Lasail), topost-axial tholeiites (Tholeiitic Alley), and finally boninites (BoniniticAlley). Geotimes (“Phase 1”) axial dykes and lavas make up ∼55 vol % of the Semail upper crust, whereas post-axial (“Phase 2”) lavasconstitute the remaining ∼45 vol % and ubiquitously coverthe underlying axial crust. Highly depleted boninitic members of the Lasailunit locally occur within and directly atop the axial sequence, marking anearlier onset of boninitic magmatism than previously known for theophiolite. The vast majority of the Semail boninites, however, belong to theBoninitic Alley unit and occur as discontinuous accumulations up to 2 kmthick at the top of the ophiolite sequence and constitute ∼15 vol % of the upper crust. The new map provides a basis for targetedexploration of the gold-bearing VMS deposits hosted by these boninites. Thethickest boninite accumulations occur in the Fizh block, where magma ascentoccurred along crustal-scale faults that are connected to shear zones in theunderlying mantle rocks, which in turn are associated with economicchromitite deposits. Locating major boninite feeder zones may thus be anindirect means to explore for chromitites in the underlying mantle.