Abstract The Mesoarchean Akia Terrane in West Greenland contains a detailed magmatic and metamorphic mineral growth record from 3.2 Ga to at least c. 2.5 Ga. This time span makes this region an important case study in the quest to track secular changes in geodynamic style which may ultimately inform on the development of plate tectonics as a globally linked system of lateral rigid plate motions. The common accessory mineral titanite has recently become recognised as a powerful high temperature geochronometer whose chemistry may chart the thermal conditions of its growth. Furthermore, titanite offers the potential to record the time-temperature history of mafic lithologies, which may lack zircon. Although titanite suffers from higher levels of common Pb than many other U Pb chronometers, we show how measurement of 207Pb/206Pb in texturally coeval biotite may assist in the characterization of the appropriate common Pb composition in titanite. Titanite extracted from two samples of mafic gneisses from the Akia Terrane both yield U Pb ages of c. 2.54 Ga. Although coeval, their chemistry implies growth under two distinctly different processes. In one case, the titanite has elevated total REE, high Th/U and grew from an in-situ partial melt, consistent with an identical date to granite dyke zircon. In contrast, the second titanite sample contains greater common Pb, lower total REE, lower Th/U, and grew from dominantly hydrothermal fluids. Zr-in-titanite thermometry for partial melt-derived titanite, with activities constrained by phase equilibrium modelling, indicates maxima of c. 690 °C. Elsewhere in the Akia Terrane, coeval metamorphism linked to growth of hydrothermal titanite is estimated at temperatures of c. 670 °C. These new results when coupled with existing findings indicate punctuated, repeated metamorphic events in the Akia Terrane, in which high temperature conditions (re)occurred at least three times between 3.0 and 2.5 Ga, but crucially changed in style across a c. 3.0 Ga change point. We interpret this change in metamorphism as reflecting a fundamental shift in geodynamic style in West Greenland at 3.0 Ga, consistent with other estimates for the onset of widespread plate tectonic-type processes.

中文翻译：

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与相平衡模型相关的泰坦岩年代学限制了西格陵兰 Akia Terrane 的构造热事件

摘要 西格陵兰岛中太古代 Akia Terrane 包含详细的岩浆和变质矿物生长记录，从 3.2 Ga 到至少 c。2.5 Ga。这个时间跨度使该地区成为追踪地球动力学样式长期变化的重要案例研究，这可能最终为板块构造作为横向刚性板块运动的全球链接系统的发展提供信息。常见的副矿物钛钛矿最近被认为是一种强大的高温地质计时仪，其化学成分可以描绘出其生长的热条件。此外，钛铁矿提供了记录可能缺乏锆石的镁铁质岩性的时间-温度历史的潜力。尽管与许多其他 U Pb 天文钟相比，钛铁矿的常见 Pb 含量更高，我们展示了在结构上同时期的黑云母中 207Pb/206Pb 的测量如何有助于表征钛铁矿中适当的常见 Pb 成分。从 Akia Terrane 的两个镁铁质片麻岩样品中提取的钛铁矿都产生了 c 的 U Pb 年龄。2.54 Ga。虽然是同代的，但它们的化学性质意味着在两种截然不同的过程中生长。在一种情况下，钛铁矿的总稀土元素含量升高，Th/U 高，并且是从原位部分熔体中生长出来的，与花岗岩脉锆石的年代一致。相比之下，第二个钛钛矿样品含有更多的普通 Pb、更低的总 REE、更低的 Th/U，并且主要来自热液流体。Zr-in-titanite 温度测定法用于部分熔体衍生的钛铁矿，其活性受相平衡模型的限制，表明 c 的最大值。690℃。在 Akia Terrane 的其他地方，在 c 的温度下估计与热液钛铁矿生长有关的同期变质作用。670℃。这些新结果与现有发现相结合，表明 Akia Terrane 中发生了间断、重复的变质事件，其中高温条件在 3.0 到 2.5 Ga 之间（重新）发生了至少 3 次，但在 c. 期间发生了重大的风格变化。3.0 Ga 变点。我们将变质作用的这种变化解释为反映了西格陵兰岛 3.0 Ga 地球动力学风格的根本转变，这与对广泛板块构造型过程开始的其他估计一致。其中高温条件（重新）在 3.0 到 2.5 Ga 之间至少发生了 3 次，但在整个 c. 3.0 Ga 变点。我们将变质作用的这种变化解释为反映了西格陵兰岛 3.0 Ga 地球动力学风格的根本转变，这与对广泛板块构造型过程开始的其他估计一致。其中高温条件（重新）在 3.0 到 2.5 Ga 之间至少发生了 3 次，但在 c. 3.0 Ga 变点。我们将变质作用的这种变化解释为反映了西格陵兰岛 3.0 Ga 地球动力学风格的根本转变，这与对广泛板块构造型过程开始的其他估计一致。