Abstract Basaltic underplating revealed by lower crustal xenoliths is usually considered to be manifested by younger zircon ages than those of the pre-existing crust and/or Sr–Nd isotopic heterogeneity resulting from mixing between mantle-derived basaltic melts and crust. The Hannuoba mafic lower crustal xenoliths have long been regarded as a typical example of Mesozoic underplating owing to the presence of 80–160 Ma zircons and evolved Sr–Nd isotopic compositions. However, our integrated study shows that most Mesozoic zircons in the Hannuoba mafic xenoliths precipitated from partial melts derived from the ancient lower crust. Although a few young zircons may record Mesozoic underplating, none of the xenoliths are products of Mesozoic underplating. It provides the first direct evidence that some zircons in lower crustal xenoliths could be exotic. The contrasting O–Hf isotopic compositions of Mesozoic zircons from the Hannuoba lower crustal xenoliths allow us to distinguish zircons that were grown from different hydrous melts from those that represent recrystallized pre-existing zircons. This has major implications for geological interpretation of the age diversity commonly observed in deep-seated xenoliths worldwide. Furthermore, at Hannuoba both the ∼1.8 Ga zircon ages from the granulite terrain and most of the 1.8–1.9 Ga zircon ages from a previously reported banded granulite xenolith also reflect metamorphism rather than underplating. It demonstrates that high-grade metamorphism or partial melting of Archean rocks can result in zircons with significantly younger U–Pb and THfDM ages. We infer that some granulite xenoliths previously regarded as products of Paleoproterozoic basaltic underplating from other regions (e.g., the Wyoming craton and the Siberian craton) may actually be remnants or derivatives of the pre-existing Archean lower crust. The large range in Sr–Nd isotopic compositions for various Hannuoba lower crustal xenoliths is unlikely to have resulted from mixing between basaltic melts and crust but was rather inherited from the ancient lower crust. Most of the Hannuoba mafic xenoliths can be best explained as residues left after partial melting of the late Archean lower crust that may be represented by the granulite terrain to produce the voluminous 125–143 Ma intermediate–felsic magmatic rocks. Therefore, young zircon ages and heterogeneous Sr–Nd isotopic compositions are insufficient criteria to infer recent underplating. Combined with literature data, it shows that Archean mafic granulite xenoliths are widespread globally, thus arguing against previous suggestion that mafic granulites from the lower crust of most Archean cratons might have formed from post-Archean basaltic underplating. It is implied that the role of basaltic underplating in the evolution of the lower crust in many regions may need to be re-evaluated.

中文翻译：

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玄武岩底板在下大陆地壳演化中的作用

摘要 下地壳捕虏体所揭示的玄武岩底镀通常被认为表现为比原有地壳更年轻的锆石年龄和/或地幔玄武岩熔体与地壳混合导致的Sr-Nd同位素非均质性。由于存在 80-160 Ma 锆石和演化的 Sr-Nd 同位素组成，汉诺坝基性下地壳捕虏体一直被认为是中生代底镀的典型例子。然而，我们的综合研究表明，汉诺坝镁铁质包体中的大多数中生代锆石是从古代下地壳的部分熔体中沉淀出来的。虽然一些年轻的锆石可能记录了中生代底镀，但没有一个捕虏体是中生代底镀的产物。它提供了第一个直接证据，证明下地壳捕虏体中的一些锆石可能是奇异的。来自汉诺坝下地壳捕虏体的中生代锆石的 O-Hf 同位素组成对比鲜明，使我们能够将来自不同含水熔体的锆石与代表再结晶的预先存在的锆石的锆石区分开来。这对世界范围内深部捕虏体中普遍观察到的年龄多样性的地质解释具有重大意义。此外，在汉诺坝，来自麻粒岩地形的~1.8 Ga 锆石年龄和来自先前报道的带状麻粒岩包体的大部分 1.8-1.9 Ga 锆石年龄也反映了变质作用而不是底镀。这表明太古宙岩石的高等级变质作用或部分熔融可导致锆石的 U-Pb 和 THfDM 年龄显着年轻。我们推断，一些先前被认为是来自其他地区（例如怀俄明克拉通和西伯利亚克拉通）的古元古代玄武岩底板的产物的麻粒岩捕虏体实际上可能是先前存在的太古代下地壳的残余物或衍生物。各种汉诺坝下地壳捕虏体的 Sr-Nd 同位素组成差异很大，不太可能是玄武岩熔体与地壳混合的结果，而是从古代下地壳继承而来。大多数汉诺坝镁铁质捕虏体可以最好地解释为晚太古代下地壳部分熔融后留下的残余物，可能以麻粒岩地形为代表，产生大量的 125-143 Ma 中长英质岩浆岩。所以，年轻的锆石年龄和异质 Sr-Nd 同位素组成不足以推断最近的底镀。结合文献资料，表明太古代镁铁质麻粒岩包体在全球范围内广泛分布，从而与先前关于大多数太古代克拉通下地壳的镁铁质麻粒岩可能是由后太古代玄武质底板形成的说法相悖。这意味着玄武岩底盘在许多地区的下地壳演化中的作用可能需要重新评估。