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



Year of publication 2018
Type Appeared in Conference without Proceedings
MU Faculty or unit

Faculty of Science

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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.

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