This study presents mineralogical characterization of opaque assemblages from I- and S-type granites from the Araçuaí orogen, southeastern Brazil that belong respectively, to the pre- and syn-collisional stages of the orogeny. Although these granites are geochemically well-characterized, with a robust geochemical, isotopic and geochronological database, their opaque minerals have not yet been investigated, and they provide important information about the oxygen fugacity and temperature conditions of their magmas. I-type granites (G1 Supersuite) consist of biotite hornblende granites and their opaque assemblage is ilmenite + pyrite + pyrrhotite ± magnetite ± Fe-Ti oxides ± chalcopyrite. S-type rocks (G2 Supersuite) are biotite muscovite sillimanite granites with ilmenite + graphite + pyrrhotite + pyrite as opaques. Our results combined with literature data show that ranges for oxygen fugacity (fO2) are: I-type granitoids containing magnetite and free of pyrite and phyrrhotite likely crystallized under fO2 between 10−15 bars and 10–8.5 bars, whereas magnetite free rocks containing pyrite and pyrrhotite should have crystallized with fO2 higher than 10−18 bars and lower than 10−15 bars. Regarding S-type granites, they must have crystallized under fO2 lower than 10−18 bars.

中文翻译：

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对比来自巴西东南部 Araçuaí 造山带的 I 型和 S 型花岗岩的氧逸度：一种基于不透明矿物组合的方法

本研究展示了来自巴西东南部 Araçuaí 造山带的 I 型和 S 型花岗岩的不透明组合的矿物学特征，它们分别属于造山运动的前碰撞和同碰撞阶段。尽管这些花岗岩具有良好的地球化学特征，拥有强大的地球化学、同位素和地质年代数据库，但它们的不透明矿物尚未得到研究，它们提供了有关其岩浆氧逸度和温度条件的重要信息。I 型花岗岩（G1 Supersuite）由黑云母角闪石花岗岩组成，它们的不透明组合是钛铁矿 + 黄铁矿 + 磁黄铁矿 ± 磁铁矿 ± Fe-Ti 氧化物 ± 黄铜矿。S 型岩石 (G2 Supersuite) 是黑云母白云母硅线石花岗岩，其中不透明的钛铁矿 + 石墨 + 磁黄铁矿 + 黄铁矿。我们的结果结合文献数据表明，氧逸度 (fO2) 的范围是： I 型花岗岩类含有磁铁矿且不含黄铁矿和绿黄铁矿，可能在 fO2 下结晶在 10-15 bar 和 10-8.5 bar 之间，而不含磁铁矿的岩石含有黄铁矿并且磁黄铁矿应该在 fO2 高于 10-18 巴和低于 10-15 巴的情况下结晶。对于 S 型花岗岩，它们必须在低于 10-18 巴的 fO2 下结晶。