Abstract The recent paper by Chaves and Porcher published in Gondwana Research (2020, v.87, 224–237) asserts the presence of 2.2 Ga eclogite facies rocks in the Itaguara Sequence, Sao Francisco craton (SFC), Brazil. They present images in which the diagnostic high-pressure minerals either occur on cracks (rutile), or as fragmentary matrix grains with unclear mineral relationships (omphacite and phengite). Here I discuss the sample petrology in order to identify the equilibrium assemblage, and utilise a P-T phase modelling approach, integrating the major mineral compositions, to show that these rocks are not retrogressed eclogites. The modelling presented here indicates that peak conditions were likely in the amphibolite facies, reaching maximum pressures of no more than 8 kbar and peak temperatures of ca. 700 °C (at 5 kbar). This result is consistent with that found by Massonne (2020, JSAMES, v.99, 102495) who suggested that mica schists from Itaguara reached maximum pressures of 13–14 kbar, and possibly lower than 10 kbar at 600 °C (i.e. within the medium to high-pressure amphibolite facies). This work shows that declarations of eclogite facies conditions, particularly from Paleoproterozoic terrains, should be accompanied by carefully evaluated petrography and thermodynamic investigations based on minerals (or mineral replacement textures) that indicate equilibrium relationships.

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评论“来自圣弗朗西斯科/刚果克拉通的古元古代Itaguara后榴辉岩的岩石学、地球化学和钐钕系统学：现代板块构造最古老的记录之一”

摘要 Chaves 和 Porcher 最近在 Gondwana Research (2020, v.87, 224–237) 上发表的论文断言，巴西圣弗朗西斯科克拉通 (SFC) 的 Itaguara 层序中存在 2.2 Ga 榴辉岩相岩石。他们呈现的图像显示诊断性高压矿物要么出现在裂缝（金红石）上，要么作为具有不清楚矿物关系的碎片基质颗粒（绿辉石和菱镁矿）。在这里，我讨论样品岩石学以识别平衡组合，并利用 PT 相建模方法，整合主要矿物成分，以表明这些岩石不是退化榴辉岩。此处提供的模型表明，角闪岩相中可能存在峰值条件，达到不超过 8 kbar 的最大压力和约 100 ℃ 的峰值温度。700 °C (5 kbar)。这一结果与 Massonne (2020, JSAMES, v.99, 102495) 发现的一致，他认为来自 Itaguara 的云母片岩达到了 13-14 kbar 的最大压力，并且在 600 °C 时可能低于 10 kbar（即在中高压角闪岩相）。这项工作表明，榴辉岩相条件的声明，特别是来自古元古代地形，应该伴随着基于指示平衡关系的矿物（或矿物替代结构）的仔细评估的岩石学和热力学调查。