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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.

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Understanding the origin of intermediate magmas that commonly erupt from subduction zone volcanoes is important to better constraining the mechanisms of continental crust formation. We performed a detailed mineralogical and petrological study of the eruptive products from April 2015 eruption of Calbuco volcano, Chile, a three-phase sub-Plinian eruption that produced pyroclastic deposits of andesitic composition. The eruptive products comprise a glass phase and a high but variable proportion of minerals dominated by plagioclase, clinopyroxene, and orthopyroxene, with minor olivine, amphibole, and magnetite. Plagioclase is very strongly zoned with highly anorthitic cores surrounded by more albitic rims, and no intermediate compositions between them. Based on thermodynamic calculations and published experimental data, we estimate that the anorthitic cores crystallized from a basaltic andesite melt containing 3.5–4.5 wt.% H2O. The bulk-rock major and trace element variability at Calbuco is best explained by the accumulation of minerals (72% plagioclase, 28% pyroxene) in a dacitic melt. These minerals most likely formed in the crystal mush zone of the magma chamber, at 200–300 MPa (8–11 km depth) according to pyroxene and amphibole compositions. A few weeks to months before the eruption, the crystal mush was disaggregated, perhaps due to magmatic underplating, and a crystal-bearing dacitic melt migrated into a sub-surface storage region where the albitic plagioclase rims crystallized. The eruption was probably internally triggered by over-pressurization in the shallow magma chamber.

a b s t r a c t a r t i c l e i n f oMagmatic rocks containing economic concentrations of iron, titanium, vanadium and phosphorous are commonly associated with massif-type anorthosites and related rocks. This rock association is part of the anorthositemangerite-charnockite-(rapakivi-)granite suites that are restricted to the Proterozoic. Understanding the geochemistry and emplacement mechanisms of ilmenite, magnetite and apatite ore deposits is crucial for exploration, efficient mining operations and ore processing. This review discusses the controlling factors on the grade of an ore, its mineralogy, and its major and trace element distribution. We present petrogenetic models of currently mined deposits (Lac Tio, Tellnes, Damiao) and discuss the characteristics of minor ore bodies from anorthosite provinces worldwide (Grenville, North China Craton, East European Craton, Rogaland, Laramie). Models of formation of anorthosite and related rocks are presented, as well as the nature of the possible parental magmas of the suite. A mineralogical classification of Fe-Ti ores is proposed: (1) Gabbro-noritic ilmenite ore ± apatite ± magnetite; (2) Ti-magnetite-dominated ore; (3) Nelsonite (Fe-Ti oxides + apatite); and (4) Rutile-ilmenite ore. The stability of ilmenite and magnetite is then critically reviewed and the influence of various factors, particularly oxygen fugacity and crystallization pressure, is examined. We discuss liquidus compositions of Fe-Ti oxides and the behavior of important trace elements such as Cr and V, both of which are sensitive to fO 2 variations. Postcumulus evolution of both oxides can occur due to re-equilibration with trapped liquid, re-equilibration with ferromagnesian silicates, exsolution, oxidation, reaction between ilmenite and magnetite, and metamorphic overprinting. These various processes are described and their effects on the oxide geochemistry are emphasized. Several potential ore-forming processes have been invoked and can explain the formation of huge concentration of ilmenite, ±magnetite, ±apatite. Fractional crystallization can be combined with crystal sorting and plagioclase buoyancy to produce relative enrichment of dense ore minerals. Silicate liquid immiscibility can segregate conjugate Si-rich and Fe-rich melts, the latter being enriched in Fe-Ti-P. Magma mixing can produce hybrid magmas located in a single-phase field of the phase diagram and precipitate a pure ilmenite cumulate. Alternative processes are also described, such as ejection of Fe-Ti-enriched residual melts by filter-pressing and compaction, solidstate remobilization of ilmenite in veins, and hydrothermal transport of Fe and Ti from the host anorthosite followed by concentration in veins and lenticular ore bodies. The magnetic properties of Fe-Ti ore deposits present contrasting signatures, depending on whether the natural remanent magnetization is dominated by hemo-ilmenite or multi-domain magnetite. Micro-and macro-scale deformation features of ore rocks are intimately correlated with magma emplacement, and...

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