Granulite xenoliths, sourced from the lower cratonic crust and brought to the Earth's surface within volcanic rocks, offer unique insights into the formation and destruction of cratons. Age determinations are essential for constructing the thermal history of cratons. However, accurate dating is complicated by the xenoliths' extended exposure to high temperatures and their complex thermal histories. Specifically, the effects of extended lower crust residency and entrainment heating on coupled Sm–Nd and Lu–Hf systematics remain to be explored. Six xenoliths hosted in the Mesozoic dioritic porphyry from the eastern North China Craton were sampled from center to edge using micro-drilling/sawing techniques. From each xenolith, two to four concentric zones were successfully extracted for coupled Lu–Hf and Sm–Nd geochronological analysis. These xenoliths predominantly exhibit a granoblastic texture, with minerals that are mostly unzoned. Textural and chemical analyses of minerals indicate a decompression process following the granulite-facies metamorphism, with subsequent multiple reheating events. Thermobarometric calculations suggest granulite-facies equilibration at temperatures of ∼750 °C and pressures of 1.2–2.0 GPa. Consistent Lu–Hf and U–Pb ages of 1.7 Ga, obtained from garnet, zircon, and titanite in chemical and textural equilibrium, are attributed to granulite-facies metamorphism. The Paleoproterozoic Lu–Hf ages and the Neoproterozoic Sm–Nd dates are indistinguishable across all concentric zones within each xenolith, whereas Sm–Nd ages are consistently ∼1.0 Ga younger than the corresponding Lu–Hf ages. This discrepancy suggests a complex history for this section of the lower cratonic crust, marked by a granulite-facies metamorphic event around 1.7 Ga, followed by prolonged cooling. A significant heating event during the Tonian, likely due to regional mantle plume activity, is inferred to have partially or completely reset the Sm–Nd isochrons. This thermal event and subsequent thermal relaxation on the samples at different residence depths thereby decoupled the Sm–Nd system from the Lu–Hf system, signifying a thermal pulse at the southern margin of North China Craton. The Mesozoic entrainment heating event appears to have had minimal impact on the Lu–Hf/Sm–Nd systematics of garnet and U–Pb of zircon/titanite. Our data highlight the potential of coupled microsampling Lu–Hf and Sm–Nd geochronology on individual xenoliths to reveal tectonic episodes within the lower crust that might be undetected by accessory mineral U–Pb geochronology.

中文翻译：

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单个捕虏体微采样石榴石年代学揭示华北克拉通热演化


粒岩捕虏体源自克拉通下地壳并在火山岩中带到地球表面，为克拉通的形成和破坏提供了独特的见解。年龄测定对于构建克拉通的热历史至关重要。然而，由于捕虏体长期暴露在高温下及其复杂的热历史，准确的年代测定变得复杂。具体而言，延长的下地壳驻留时间和夹带加热对 Sm-Nd 和 Lu-Hf 耦合系统的影响仍有待探索。使用微钻/锯技术从中心到边缘对华北克拉通东部中生代闪长斑岩中的六块捕虏体进行了采样。从每个捕虏体中，成功提取了两到四个同心带，用于耦合 Lu-Hf 和 Sm-Nd 地质年代学分析。这些捕虏体主要表现出花岗母质结构，其中矿物大多未分带。矿物的结构和化学分析表明，麻粒岩相变质作用之后存在减压过程，随后发生多次再加热事件。温压计算表明，麻粒岩相在 750 °C 的温度和 1.2–2.0 GPa 的压力下达到平衡。从化学和结构平衡的石榴石、锆石和钛矿中获得的一致的 1.7 Ga 的 Lu-Hf 和 U-Pb 年龄归因于麻粒岩相变质作用。每个捕虏体内所有同心带的古元古代 Lu-Hf 年龄和新元古代 Sm-Nd 年龄无法区分，而 Sm-Nd 年龄始终比相应的 Lu-Hf 年龄小 ∼1.0 Ga。这种差异表明下克拉通地壳的这一部分经历了复杂的历史，以 1 左右的麻粒岩相变质事件为标志。7 Ga，然后长时间冷却。托尼阶期间的一次重大加热事件，可能是由于区域地幔柱活动所致，被推断部分或完全重置了 Sm-Nd 等时线。这一热事件以及随后在不同停留深度的样品上的热弛豫从而使Sm-Nd系统与Lu-Hf系统脱钩，这标志着华北克拉通南缘的热脉冲。中生代夹带加热事件似乎对石榴石和锆石/钛矿的 U-Pb 的 Lu-Hf/Sm-Nd 系统学影响很小。我们的数据强调了对单个捕虏体进行耦合微采样 Lu-Hf 和 Sm-Nd 地质年代学的潜力，以揭示下地壳内的构造事件，而辅助矿物 U-Pb 地质年代学可能无法检测到这些构造事件。