Archean supracrustal rocks that occur as rafts within tonalite–trondhjemite–granodiorite (TTG) gneisses can reach ultra-high temperature (UHT) conditions. Their metamorphic paths and geodynamic regimes are debated. Herein, we present detailed analyses of UHT pelitic migmatites (JD1730 and JD1733) from the East Hebei, North China Craton, using petrology, phase equilibria modelling, and zircon dating, to recover the metamorphic history. Sample JD1730 is a sillimanite–orthopyroxene–cordierite migmatite that contains the sillimanite + orthopyroxene (+cordierite) symplectites, which are interpreted to be the pseudomorphs after sapphirine. JD1733 is a corundum-bearing garnet–cordierite–sillimanite migmatite that contains the corundum + spinel ± ilmenite/rutile aggregates, representing a stable mineral association of low pressures in local Si-unsaturated domains. The two rocks record consistent counterclockwise paths involving pre-peak up- processes to the UHT peak conditions followed by cooling. The pre-peak up- processes occurred at 5–7 kbar and > 970 °C constrained from the textural relations involving (i) spinel enveloped by sillimanite and cordierite and mantled by the Sil + Opx symplectites in JD1730, and (ii) the inclusion variation in garnet of JD1733 with the Spl + Crn (±Ilm) aggregates embayed by sillimanite in the core and mantle, and worm-like quartz in the rim. The UHT peak conditions can be constrained to be 7–8 kbar, 980–1060 °C based on the stability of the inferred peak mineral assemblages in pseudosections added with the plots of the re-integrated anorthite contents from perthite. The post-peak cooling processes to 5–8 kbar / 820–850 °C are marked by the growth of biotite in leucosomes of both samples. Zircon dating and geothermometry results suggest that the UHT metamorphism might have persisted for more than 20 Myr from ∼ 2.52 Ga to ∼ 2.50 Ga and the post-UHT cooling continued another 20 Myr or even 40 Myr. Considering the geological features in the terrane, a vertical sagduction regime was preferred for the Neoarchean tectonic evolution of the East Hebei terrane.

中文翻译：

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逆时针P-T路径的太古代超高温变质作用：来自华北克拉通冀东泥质麻粒岩的见解


太古代表壳岩石以英闪长岩-长斜长岩-花岗闪长岩（TTG）片麻岩中的筏形式出现，可以达到超高温（UHT）条件。它们的变质路径和地球动力学机制存在争议。在此，我们利用岩石学、相平衡模型和锆石测年技术，对华北克拉通河北东部的超高温泥质混合岩（JD1730和JD1733）进行了详细分析，以恢复变质历史。样品 JD1730 是硅线石 - 斜方辉石 - 堇青石混合岩，含有硅线石 + 斜方辉石（+堇青石）辛晶石，被解释为蓝宝石之后的假晶。 JD1733 是一种含刚玉的石榴石-堇青石-硅线石混合岩，含有刚玉 + 尖晶石 ± 钛铁矿/金红石聚集体，代表局部硅不饱和区域中低压的稳定矿物组合。这两块岩石记录了一致的逆时针路径，涉及峰值前的上升过程到超高温峰值条件，然后冷却。峰前上升过程发生在 5–7 kbar 和 > 970 °C，受结构关系的限制，涉及 (i) 尖晶石被硅线石和堇青石包覆，并被 JD1730 中的 Sil + Opx 辛辉石覆盖，以及 (ii) 包裹体JD1733 石榴石的变异，其核心和地幔中含有硅线石，边缘含有蠕虫状石英。根据伪剖面中推断的峰值矿物组合的稳定性以及钙长石重新积分的钙长石含量图，UHT 峰值条件可以限制为 7-8 kbar、980-1060 °C。峰后冷却至 5–8 kbar / 820–850 °C 的特点是两个样品的无色体中黑云母的生长。 锆石测年和地温测量结果表明，UHT 变质作用可能持续了超过 20 Myr，从 ∼ 2.52 Ga 到 ∼ 2.50 Ga，而 UHT 后的冷却又持续了 20 Myr 甚至 40 Myr。考虑到地体的地质特征，冀东地体新太古代构造演化倾向于垂直凹陷形态。