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首页> 外文期刊>Contributions to Mineralogy and Petrology >Stabilization and reactivation of cratonic lithosphere from the lower crustal record in the western Canadian shield
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Stabilization and reactivation of cratonic lithosphere from the lower crustal record in the western Canadian shield

机译：来自加拿大西部盾构下地壳记录的克拉通岩石圈的稳定和活化

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New U–Pb geochronology for an extensive exposure of high-pressure granulites in the East Lake Athabasca region of the western Canadian shield is consistent with a history characterized by 2.55 Ga stabilization of cratonic lithosphere, 650 million years of lower crustal residence and cratonic stability, and 1.9 Ga reactivation of the craton during lithospheric attenuation and asthenospheric upwelling. High precision single-grain and fragment zircon data define distinctive discordia arrays between 2.55 and 1.9 Ga. U–Pb ion microprobe spot analyses yield a similar range of U–Pb dates with no obvious correlation between date and cathodoluminescence zonation. We attribute the complex U–Pb zircon systematics to growth of the primary populations during a 2.55 Ga high-pressure granulite facies event (~1.3 GPa, 850°C) recorded by the dominant mineral assemblage of the mafic granulite gneisses, with subsequent zircon recrystallization and minor secondary zircon growth during a second high-pressure granulite facies event (1.0 GPa, ~800°C) at 1.9 Ga. The occurrence of two discrete granulite facies metamorphic events in the lower crust, separated by an interval of 650 million years that included isobaric cooling for at least some of this time, suggests that the rocks resided at lower crustal depths until 1.9 Ga. We infer that this phase of lower crustal residence and little tectonic activity is coincident with an extended period of cratonic stability. Detailed structural and thermochronological datasets indicate that multistage unroofing of the lower crustal rocks occurred in the following 200 million years. Extended lower crustal residence would logically be the history inferred for lower crust in most cratonic regions, but the unusual aspect of the history in the East Lake Athabasca region is the subsequent lithospheric reactivation that initiated transport of the lower crust to the surface. We suggest that a weakened strength profile related to the 1.9 Ga heating left the lithosphere susceptible to far-field tectonic stresses from bounding orogens that drove the lower crustal exhumation. An ultimate return to cratonic stability is responsible for the preservation of this extensive lower crustal exposure since 1.7 Ga.
机译：新的U–Pb地球年代学用于在加拿大西部盾构的东湖阿萨巴斯卡地区广泛暴露高压花岗石，这与克拉通岩石圈2.55 Ga稳定，6.5亿年低地壳驻留和克拉通稳定性的历史相吻合，在岩石圈衰减和软流圈上升期间，克拉通的1.9 Ga活化。高精度单晶锆石和碎片锆石数据在2.55和1.9 Ga之间定义了独特的Discordia阵列。U–Pb离子微探针斑点分析产生的U–Pb日期范围相似，并且日期和阴极发光分区之间无明显关联。我们将复杂的U–Pb锆石系统化归因于镁铁质花岗质片麻岩的主要矿物组合记录的2.55 Ga高压花岗石相事件（〜1.3 GPa，850°C）期间主要种群的增长，随后锆石重结晶在1.9 Ga下发生的第二次高压花岗石相事件（1.0 GPa，〜800°C）中发生了少量的次生锆石生长。下地壳中发生了两个离散的花岗石相变质事件，相隔时间为6.5亿年包括等压冷却至少一段时间，这表明这些岩石一直处于较低的地壳深度，直到1.9 Ga。我们推断，这个较低的地壳驻留阶段和少量构造活动与克拉通稳定期的延长相吻合。详细的结构和热年代学数据表明，下地壳岩石的多阶段解顶发生在随后的2亿年中。从逻辑上讲，扩大下地壳的居住时间可以推断是大多数克拉通地区下地壳的历史，但是东湖阿萨巴斯卡地区历史的不寻常方面是随后的岩石圈活化作用，从而引发了下地壳向地表的运输。我们认为，与1.9 Ga加热有关的强度分布减弱，使岩石圈容易受到来自远造山带的约束，这些造山带驱使下地壳发掘。自1.7 Ga以来，这种广泛的较低的地壳暴露得以保持，最终恢复了克拉通的稳定性。

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