Extremely magnetic granitoids from the Eastern part of the Brno batholith: influence of late-magmatic alteration in oxidizing regime

• Authors: KUBEŠ Martin; LEICHMANN Jaromír
• Year of publication: 2018
• Type: Appeared in Conference without Proceedings
• Description: Within Eastern Granitoid Complex were distinguished two petrophysically different varieties of plutonic rocks: (1) extremely magnetic amphibole-biotite tonalites (? ~ 1 x 10-2 SI) exhibiting lower contents of radioactive elements (2) biotite granodiorites with lower susceptibility ca. 1 x 10-4 SI and relatively higher natural radioactivity. The bulk magnetic susceptibility of granitoids is controlled by the presence of pure magnetite with low TiO2 content (< 0,145 wt. %) and other trace components, such as V2O3, Al2O3, Cr2O3, SiO2 (< 1 wt. %). Additionally, the results of thermal analysis confirmed the absence of earlier magmatic titanomagnetite and measured values of Curie temperature (585 – 601 oC) and the Verwey transition (approximately – 160 oC) demonstrated the occurrence of pure to slightly oxidized pure multi-domain magnetite closely associated with mafic silicates. The increased contents of pure magnetite along with diamond-shaped titanite originated as a result of the reaction between early magmatic titanomagnetite, annite and anorthite in metaluminous tonalites due to different whole-rock chemistry (SiO2 58–65 wt. %, FeO 3,98–7,19 wt %, MgO 1,46–3,52 wt %, CaO 3,7–6,05 wt. %, TiO2 0,63–0,89 wt. %) reflecting distinct chemical composition of Fe-rich mafic silicates (chamosite ~ 24 wt. % FeO) which represented the substantial source of Fe2+ for the formation of pure magnetite in an evolved and crystallized magmatic system under relatively oxidative conditions. In contrast with more felsic metaluminous to peraluminous biotite granodiorites (SiO2 ~ 67,3 wt. %, FeO 2,86–3,04 wt. %, MgO 1,08–1,28 wt. %, CaO 1,5–1,86 wt. %, TiO2 0,38–0,5 wt. %) which characteristically contain mafic silicates depleted in Fe2+ (Mg-clinochlore ~ 20 wt. % FeO).