Jonas Kaempf, Tim E. Johnson, Chris Clark, Julian Alfing, Michael Brown, Pierre Lanari, Kai Rankenburg
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引用次数: 0
摘要
地球上已知最古老的演化(长石)岩石(约 4.03 Ga)位于加拿大西北部的阿卡斯塔片麻岩群(AGC),是揭示哈代和早阿基坦时期地壳生长和分化地质过程的重要基石。虽然对这些黑云母片麻岩中岩浆作用、变质作用和变形作用的时间进行了广泛的研究,但对岩石所记录的变质压力-温度(P-T)条件却知之甚少。在这里,我们利用相平衡模型,结合对两块含石榴石的黑云母片麻岩进行的原位石榴石lu-hf地质年代和微量元素分析,来解读AGC的变质历史。观察到的矿物组合与T=725-780°C和P=4.5-6.2千巴的峰值变质条件以及少量熔体(<7 vol.%)的产生相一致。石榴石地质年代学将变质作用的年龄推定为3.3-3.2 Ga,这与之前在AGC发生古新世晚期构造变质作用的证据一致。随后在约 1.9 Ga 的石榴石同位素扰动被解释为对应于在古近纪沃普梅造山运动期间石榴石重吸收/重结晶对原生 Lu-Hf 系统学的改变,从而导致这两个年龄成分之间的显著差异。我们的研究补充了已发表的少量年龄大于 2.8 Ga 的变质岩的 P-T 数据,并表明 AGC 地区的黑云母片麻岩在古新世晚期记录了约 140°C/kbar 的高表观热梯度。这个热梯度是有限数据集中最高的,但与其他古新世-中新世地壳岩石记录的高T/P比(77.5°C/kbar)数据大致相似。
Paleoarchean metamorphism in the Acasta Gneiss Complex: Constraints from phase equilibrium modelling and in situ garnet Lu–Hf geochronology
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).
期刊介绍:
The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.