Neoformation of magnetite during selective metasomatism controlling large-scale positive magnetic anomalies within the Brunovistulian unit (Bohemian Massif)

An airborne magnetometer survey together with field measurements of magnetic susceptibility have distinguished two main granitic suites within the eastern zone of the Brno batholith (the exposed part of the Brunovistulicum). Highly magnetic biotite-amphibole tonalites, forming widespread positive magnetic anomalies (up to 300 nT), have susceptibility (χ) regularly above 5 × 10 −3 . Less magnetic biotite granites display lower χ values on the order of 1 × 10 −4 . The metaluminous tonalites significantly differ in whole-rock chemistry (enriched in FeO, MgO, and depleted in SiO 2 ) and degree of fractionation (decreased Rb/Sr and K/Ba ratios) reflecting the distinct microchemistry of Fe-rich mafic silicates (chamosite ~24 wt% FeO). Conversely, more felsic metaluminous to peraluminous biotite granites exhibiting a higher degree of fractionation characteristically contain mafic silicates depleted in Fe 2+ (Mg-clinochlore ~20 wt% FeO). The geochemistry of granitoids and chemical composition of mafic phases suggest that increased contents of pure magnetite (TiO 2 <0.15 wt%) along with diamond-shaped titanite originated as a result of the reaction between primary magmatic Fe-Ti oxides, annite and anorthite during late oxidation of granitic rocks. The subsolidus oxidation of granitoid has led to releasing of Fe 2+ from mafic silicates representing a substantial source of Fe 2+ for the formation of pure magnetite in an evolved and crystallized magmatic system under relatively oxidative conditions. The Curie temperature (585–601 °C) and the Verwey transition (-160 °C) demonstrate the absence of titanomagnetite in both types and the overall magnetic properties of granitoids dominated by pure magnetite slightly affected by oxidation. Nevertheless, the more extensive magnetite hematitization in biotite granites indicates a higher degree of oxidation caused by dissociation of a greater amount of water phase in the post-magmatic stage of evolution of a more fractionated felsic melt. The metasomatic transformation of paramagnetic (Fe-rich biotite and amphibole) to ferromagnetic (pure magnetite) phases, well described on granitoid varieties of the Brno batholith, seems to be responsible for the extraordinarily geomagnetic behavior of the entire eastern part of the Brunovistulicum (Slavkov terrane) mostly covered by Paleozoic sediments.