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<p>The crystallization conditions of orthopyroxene and fayalite-bearing granitoids remain critical for understanding the evolution of charnockitic magmas. This study integrates petrography, mineral chemistry, thermobarometry, and thermodynamic modeling to evaluate the P-T-<i>f</i>O<sub>2</sub>-H<sub>2</sub>O conditions during the crystallization of two contrasting Rhyacian igneous charnockites: the Maravilha (2.09 Ga) and Serra Azul (2.06 Ga) plutons, located in the Bacajá Domain, SE Amazonian Craton. The Maravilha Charnockite comprises (i) fayalite + quartz and (ii) pyroxene-bearing granites, whereas Serra Azul is dominated by pyroxene-bearing tonalites and granodiorites. Fayalite-bearing rocks evolved under reduced conditions (FMQ ± 0.5), with initial H<sub>2</sub>O content of ≤ 3 wt%, while the pyroxene-bearing association evolved under more oxidizing conditions (NNO ± 0.5), with ~ 4 wt% initial H<sub>2</sub>O. Both associations were emplaced at ~ 0.2–0.5 GPa. Serra Azul Charnockite crystallized under oxidizing conditions (NNO ± 0.7 to NNO + 2), with initial H₂O of ~ 2–3 wt% and pressures between 0.3 and 0.6 GPa. Thermodynamic modeling indicates that fayalite and orthopyroxene can crystallize in moderately hydrous melts, ranging from 2.3 to 6 wt% H₂O for fayalite, and ~ 5–6 wt% for orthopyroxene. Fayalite is restricted to reduced conditions and low pressures (≤ 0.3 GPa), whereas orthopyroxene is stable from reduced to moderately oxidizing conditions and over a wider pressure range. These findings highlight that crystallization of fayalite and orthopyroxene is not restricted to low-H<sub>2</sub>O conditions, but instead reflects a complex interaction among H<sub>2</sub>O content, melt pressure, redox state, and melt composition.</p>

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Water, redox, and compositional controls on the crystallization of Rhyacian metaluminous igneous charnockites, Bacajá Domain, SE Amazonian Craton: insights from mineral chemistry and thermodynamic modeling

  • 期刊论文

  • 发表日期：

    2025年11月26日

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  • Arthur Santos da Silva Neri,
  • Roberto Dall’Agnol,
  • Gilmara Regina Lima Feio,
  • Caio José Mesquita Soares,
  • Ingrid Roberta Viana da Cunha,
  • José de Arimatéia Costa de Almeida

摘要

The crystallization conditions of orthopyroxene and fayalite-bearing granitoids remain critical for understanding the evolution of charnockitic magmas. This study integrates petrography, mineral chemistry, thermobarometry, and thermodynamic modeling to evaluate the P-T-fO2-H2O conditions during the crystallization of two contrasting Rhyacian igneous charnockites: the Maravilha (2.09 Ga) and Serra Azul (2.06 Ga) plutons, located in the Bacajá Domain, SE Amazonian Craton. The Maravilha Charnockite comprises (i) fayalite + quartz and (ii) pyroxene-bearing granites, whereas Serra Azul is dominated by pyroxene-bearing tonalites and granodiorites. Fayalite-bearing rocks evolved under reduced conditions (FMQ ± 0.5), with initial H2O content of ≤ 3 wt%, while the pyroxene-bearing association evolved under more oxidizing conditions (NNO ± 0.5), with ~ 4 wt% initial H2O. Both associations were emplaced at ~ 0.2–0.5 GPa. Serra Azul Charnockite crystallized under oxidizing conditions (NNO ± 0.7 to NNO + 2), with initial H₂O of ~ 2–3 wt% and pressures between 0.3 and 0.6 GPa. Thermodynamic modeling indicates that fayalite and orthopyroxene can crystallize in moderately hydrous melts, ranging from 2.3 to 6 wt% H₂O for fayalite, and ~ 5–6 wt% for orthopyroxene. Fayalite is restricted to reduced conditions and low pressures (≤ 0.3 GPa), whereas orthopyroxene is stable from reduced to moderately oxidizing conditions and over a wider pressure range. These findings highlight that crystallization of fayalite and orthopyroxene is not restricted to low-H2O conditions, but instead reflects a complex interaction among H2O content, melt pressure, redox state, and melt composition.