Abstract It has been shown that certain host minerals can robustly shield mineral inclusions and prevent loss of petrochronological information during subsequent thermal events. Rutile inclusions in garnet may crystallize under a wide range of metamorphic conditions and thus rutile is expected to be a useful geochronometer for constraining different stages of a metamorphic P-T path. Hence, it is important to ascertain whether rutile inclusions shielded by garnet retain the original crystallization age information despite later retrograde cooling or subsequent new thermal events. In this study, we test the “shielding effect” of garnet on rutile inclusions from the Baizhangya eclogite in the North Dabie Zone (China), which underwent a multi-stage thermal evolution. Secondary ion mass spectrometry (SIMS) U Pb dating of inherited zircon core domains constrains a Neoproterozoic formation age (~770 Ma) of the eclogite protolith. SIMS U Pb dating of metamorphic zircon and complementary thermal-ionization mass spectrometry (TIMS) Sm Nd isochron dating of garnet revealed Triassic eclogite-facies metamorphic ages (245–210 Ma). An Early Cretaceous SIMS U Pb age of ca. 130 Ma, yielded by matrix rutile grains and rutile inclusions, suggests significant isotopic alteration since crystallization. This indicates that shielding was ineffective, since the U Pb ages derived from the rutile inclusions, although hermetically armored by garnet, were influenced by a subsequent Early Cretaceous tectono-thermal event. Further transmission electron microscope (TEM) analysis revealed the presence of hematite microlites along the garnet-rutile boundaries, which may have acted as a potential reservoir to store the expelled radiogenic Pb from rutile. This study suggests that newly formed minerals located along mineral boundaries seem to be crucial for understanding the shielding effect.

中文翻译：

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北大别榴辉岩的多系统年代学：多热条件下石榴石对金红石包裹体的无效“屏蔽”

摘要 已经表明，某些寄主矿物可以强有力地屏蔽矿物包裹体，并防止在随后的热事件中丢失岩石年代学信息。石榴石中的金红石包裹体可能会在各种变质条件下结晶，因此金红石有望成为一种有用的地质年代计，用于约束变质 PT 路径的不同阶段。因此，重要的是要确定被石榴石屏蔽的金红石包裹体是否保留了原始的结晶年龄信息，尽管后来发生了逆行冷却或随后的新热事件。在这项研究中，我们测试了石榴石对经历多阶段热演化的北大别地区百丈崖榴辉岩的金红石包裹体的“屏蔽效应”。继承的锆石核心域的二次离子质谱 (SIMS) U Pb 定年限制了榴辉岩原岩的新元古代形成年龄 (~770 Ma)。变质锆石的 SIMS U Pb 定年和石榴石的互补热电离质谱 (TIMS) Sm Nd 等时线定年揭示了三叠纪榴辉岩相变质年龄（245-210 Ma）。早白垩世 SIMS U Pb 年龄约。由基体金红石晶粒和金红石包裹体产生的 130 Ma 表明自结晶以来发生了显着的同位素变化。这表明屏蔽是无效的，因为来自金红石包裹体的 U Pb 年龄虽然被石榴石密封，但受到了随后的早白垩世构造热事件的影响。进一步的透射电子显微镜 (TEM) 分析显示，沿石榴石-金红石边界存在赤铁矿微晶石，这可能是储存从金红石中排出的放射性铅的潜在储库。这项研究表明，位于矿物边界的新形成的矿物似乎对于理解屏蔽效应至关重要。