The oldest known evolved (felsic) rocks on Earth (c. 4.03 Ga) are found in the Acasta Gneiss Complex (AGC) in north-western Canada and represent a fundamental keystone in unravelling the geological processes governing crustal growth and differentiation during the Hadean and early Archean. Although the timing of multiple episodes of magmatism, metamorphism and deformation in these tonalitic gneisses has been investigated extensively, the metamorphic pressure–temperature (P–T) conditions recorded by the rocks are poorly constrained. Here, we use phase equilibrium modelling coupled with in situ garnet Lu–Hf geochronology and trace element analysis for two garnet-bearing tonalitic gneisses to decipher the metamorphic history of the AGC. The observed mineral assemblages are consistent with peak metamorphic conditions of T = 725–780°C and P = 4.5–6.2 kbar and the generation of a small amount of melt (<7 vol.%). Garnet geochronology constrains the age of metamorphism to 3.3–3.2 Ga, consistent with previous evidence for a late Paleoarchean tectono-metamorphic event in the AGC. Subsequent isotopic disturbance of garnet at c. 1.9 Ga is interpreted to correspond to a modification of the primary Lu–Hf systematics in response to garnet resorption/recrystallization during the Paleoproterozoic Wopmay orogeny, resulting in significant scatter between these two age components. Our study adds to the small number of published P–T data for metamorphic rocks older than 2.8 Ga and shows that tonalitic gneisses in the AGC record a high apparent thermal gradient of ~140°C/kbar in the late Paleoarchean. This thermal gradient is the highest among the limited dataset, but is broadly similar to data from other Paleoarchean-Mesoarchean crustal rocks in recording high T/P ratios (>77.5°C/kbar).

中文翻译：

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阿卡斯塔片麻岩杂岩中的古太古代变质作用：来自相平衡模型和原位石榴石 Lu-Hf 地质年代学的约束

地球上已知最古老的演化（长英质）岩石（约4.03 Ga）发现于加拿大西北部的阿卡斯塔片麻岩杂岩体（AGC）中，它代表着揭示冥古宙和冥古宙期间控制地壳生长和分异的地质过程的基本基石。太古宙早期。尽管对这些英闪长片麻岩中多次岩浆作用、变质作用和变形的时间进行了广泛的研究，但岩石记录的变质压力-温度（ P-T ）条件受到的限制很少。在这里，我们使用相平衡模型，结合原位石榴石 Lu-Hf 地质年代学和微量元素分析，对两块含石榴石英闪长片麻岩进行破译，以破译 AGC 的变质历史。观察到的矿物组合与T  = 725–780°C 和P  = 4.5–6.2 kbar 的峰值变质条件以及少量熔体的生成 (<7 vol.%)一致。石榴石地质年代学将变质作用年龄限制在 3.3–3.2 Ga，这与 AGC 中古太古代晚期构造变质事件的先前证据一致。随后石榴石c处的同位素扰动。1.9 Ga被解释为对应于古元古代Wopmay造山运动期间石榴石再吸收/重结晶对原始Lu-Hf系统学的修改，导致这两个年龄成分之间存在显着的分散。我们的研究补充了少数已发表的2.8 Ga 以上变质岩的P-T数据，并表明 AGC 中的英云闪长质片麻岩在古太古代晚期记录了约 140°C/kbar 的高表观温度梯度。该热梯度在有限的数据集中是最高的，但在记录高T/P比 (>77.5°C/kbar)方面与其他古太古代-中太古代地壳岩石的数据大致相似。