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.

中文翻译：

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控制Brunovistulian单元（波希米亚地块）内大规模正磁异常的选择性交代过程中磁铁矿的新形成

机载磁力计勘测与磁化率的现场测量一起区分了布尔诺基岩东部区域（Brunovistulicum 的暴露部分）内的两个主要花岗岩组。高磁性黑云母-角闪石闪石，形成广泛的正磁异常（高达 300 nT），磁化率 (χ) 有规律地高于 5 × 10 -3。磁性较小的黑云母花岗岩显示出较低的 χ 值，大约为 1 × 10 -4。金属铝质透辉石在全岩化学（富含 FeO、MgO 和贫 SiO 2）和分馏度（降低 Rb / Sr 和 K / Ba 比率）方面存在显着差异，反映了富铁镁铁质硅酸盐（菱镁矿）的独特微化学~ 24 wt% FeO)。反过来，更多 长英质金属铝至过铝质黑云母花岗岩具有更高的分馏度，其特征是含有贫化 Fe 2+ 的镁铁质硅酸盐（Mg-斜绿石 ~ 20 wt% FeO）。花岗岩的地球化学和镁铁质相的化学成分表明纯磁铁矿（TiO 2 <0.15 wt%）以及菱形钛钛矿的含量增加是由于原生岩浆铁钛氧化物、安妮石和钙长石在花岗岩的晚期氧化。花岗岩的亚固相氧化导致 Fe 2+ 从代表 Fe 2+ 的重要来源的镁铁质硅酸盐中释放，用于在相对氧化条件下在演化和结晶的岩浆系统中形成纯磁铁矿。居里温度 (585-601 °C) 和 Verwey 转变 (-160 °C) 表明两种类型中都不存在钛磁铁矿，并且花岗岩的整体磁性以纯磁铁矿为主，受氧化影响较小。然而，黑云母花岗岩中更广泛的磁铁矿赤铁矿化表明，在更分馏的长英质熔体演化的后岩浆阶段，由于大量水相的解离导致更高程度的氧化。顺磁性（富含铁的黑云母和角闪石）到铁磁性（纯磁铁矿）相的交代转化，在布尔诺基岩的花岗岩变种上得到了很好的描述，似乎是造成整个布鲁诺维斯图利克（斯拉夫科夫）东部异常地磁行为的原因地体）主要被古生代沉积物覆盖。