Publication details
The origin of extremely magnetic granitoids from the Brno batholith: evidence of metasomatic replacement of paramagnetic to ferromagnetic phases influencing geomagnetic properties of the entire Slavkov terrane
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| Year of publication | 2018 |
| Type | Appeared in Conference without Proceedings |
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| Description | The bulk magnetic susceptibility of granitoids is controlled by the presence of pure magnetite (TiO2 < 0.145 wt. %) which originated along with diamond-shaped titanite as a result of the reaction between early magmatic Fe-Ti oxides, annite and anorthite during late-magmatic stages under relatively oxidizing conditions. The late oxidation of granitoids has led to releasing of Fe2+ from mafic silicates and subsequent formation of pure magnetite with increased contents within tonalites due to different whole-rock chemistry (SiO2 58–65 wt. %, FeO 3.98–7.19 %, MgO 1.46–3.52 %, CaO 3.7–6.05 %, TiO2 0.63–0.89 %) and degree of fractionation (Rb/Sr 0.036–0.048, K/Ba 19.434–22.43) reflecting distinct chemical composition of Fe-rich mafic silicates (chamosite ~ 24 wt. % FeO) compared to more felsic granodiorites (SiO2 ~ 67.3 wt. %, FeO 2.86–3.04 %, MgO 1.08–1.28 %, CaO 1.5–1.86 %, TiO2 0.38–0.5 %) exhibiting relatively higher degree of fractionation (Rb/Sr 0.085–0.233, K/Ba 30.23–51.91) which typically contain Mg-clinochlore depleted in Fe2+ (~ 20 wt. % FeO). The substantial source of Fe2+ for production of pure magnetite in an evolved and crystallized magmatic system in oxidizing regime appears to be biotite which consumes O2 as was experimentally proven: Annite + 1/2O2 = Magnetite + K-feldspar + H2O (Wones and Eugster 1965). The metasomatic mineralogical alteration, well described on examined granitoid varieties of the Eastern complex of the Brno batholith, apparently had a tremendous impact on the geomagnetic behaviour of entire eastern part of the Brunovistulicum (Slavkov terrane) forming widespread regional magnetic anomalies in the basement of Neogene sediments of Western Carpathians. |