Multi-system geochronology of North Dabie eclogite: Ineffective garnet `shielding' on rutile inclusions under multi-thermal conditions
Multi-system geochronology of North Dabie eclogite: Ineffective garnet `shielding' on rutile inclusions under multi-thermal conditions
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北大别榴辉岩多系统年代学：多热条件下石榴石对金红石包裹体的无效“屏蔽”
DOI：
10.1016/j.lithos.2020.105573
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发表时间：
2020
期刊：
影响因子：
3.5
通讯作者：
Liu Yu
中科院分区：
文献类型：
--
作者：
Zhou Tian;Li Qiuli;Klemd Reiner;Shi Yonghong;Tang Xu;Li Chaofeng;Liu Yu
研究表明，在随后的热事件中，某些寄主矿物可以强有力地屏蔽矿物包裹体，防止岩石年代学信息的丢失。石榴石中的金红石包裹体可以在广泛的变质条件下结晶，因此金红石有望成为限制变质P-T路径不同阶段的有用地时计。因此，确定被石榴石屏蔽的金红石包裹体在后来的逆行冷却或随后的新的热事件中是否保留了原始的结晶年龄信息是很重要的。研究了华北大别带百张崖榴辉岩中石榴石对金红石包裹体的“屏蔽作用”，该榴辉岩经历了多期热演化过程。二次离子质谱（SIMS）对继承的锆石核域进行单键dpb定年，确定了榴辉岩原岩的新元古代形成年龄（~770 Ma）。利用变质锆石的单键pb定年和互补热电离质谱（TIMS）对石榴石的单键nd等时线定年揭示了三叠纪榴辉岩相变质年龄（245-210 Ma）。由基体金红石颗粒和金红石包裹体获得的早白垩世SIMS单键pb年龄约为130 Ma，表明自结晶以来存在明显的同位素变化。这表明屏蔽是无效的，因为来自金红石包裹体的usingsingle bond dpb年龄虽然被石榴石密封包裹，但受到随后的早白垩世构造-热事件的影响。透射电镜（TEM）分析显示，赤铁矿微石沿石榴石-金红石边界发育，可能是金红石释放的放射性成因铅的潜在储集层。这项研究表明，沿着矿物边界新形成的矿物似乎对理解屏蔽效应至关重要。
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) Usingle bondPb dating of inherited zircon core domains constrains a Neoproterozoic formation age (~770 Ma) of the eclogite protolith. SIMS Usingle bondPb dating of metamorphic zircon and complementary thermal-ionization mass spectrometry (TIMS) Smsingle bondNd isochron dating of garnet revealed Triassic eclogite-facies metamorphic ages (245–210 Ma). An Early Cretaceous SIMS Usingle bondPb 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 Usingle bondPb 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.