The role of basaltic underplating in the evolution of the lower continental crust
The role of basaltic underplating in the evolution of the lower continental crust
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DOI：
10.1016/j.gca.2020.02.002
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发表时间：
2020-04
影响因子：
5
通讯作者：
Jun-Cheng Hu;Neng Jiang;Jing-hui Guo;Wenbo Fan;Danqing Liu
中科院分区：
文献类型：
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作者：
Jun-Cheng Hu;Neng Jiang;Jing-hui Guo;Wenbo Fan;Danqing Liu
下地壳捕虏体所揭示的玄武岩底侵作用通常被认为是由比先存地壳的锆石年龄更年轻的锆石年龄和/或幔源玄武岩熔体与地壳混合所导致的Sr-Nd同位素不均匀性所表现的。汉诺坝镁铁质下地壳捕虏体中含有80-160 Ma的锆石和演化的Sr-Nd同位素组成，一直被认为是中生代底侵作用的典型代表。然而，综合研究表明，汉诺坝镁铁质捕虏体中的绝大多数锆石来自于下地壳的部分熔融产物。虽然一些年轻的锆石可能记录中生代底侵作用，但没有捕虏体是中生代底侵作用的产物。它提供了第一个直接证据，证明下地壳捕虏体中的某些锆石可能是外来的。对比的O-Hf同位素组成的中生代锆石从汉诺坝下地壳捕虏体，使我们能够区分锆石从不同的含水熔体中生长的那些代表重结晶的预先存在的锆石。这对全球深源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，汉诺坝麻粒岩地体的1.8Ga锆石年龄和以前报道的条带状麻粒岩捕虏体的1.8- 1.9Ga锆石年龄也反映了变质作用，而不是底侵作用。这表明太古代岩石的高级变质作用或部分熔融作用可导致锆石U-Pb和THfDM年龄显著年轻。我们推断，一些以前被认为是古元古代玄武质底侵作用产物的麻粒岩捕虏体来自其他地区（例如，怀俄明州克拉通和西伯利亚克拉通）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。汉诺坝下地壳捕虏体Sr-Nd同位素组成的大范围变化不太可能是玄武质熔体与地壳混合的结果，而是继承了古老的下地壳。汉诺坝镁铁质捕虏体大多为太古代晚期下地壳部分熔融后的残留物，以麻粒岩地体为代表，产生了125-143 Ma的中长英质岩浆岩。因此，年轻的锆石年龄和不均匀的Sr-Nd同位素组成不足以推断现代底侵作用。结合文献资料，表明太古宙基性麻粒岩捕虏体在全球范围内广泛分布，从而反驳了以往认为大多数太古宙下地壳的基性麻粒岩可能是由后太古宙玄武岩底侵作用形成的观点。这意味着，玄武底侵作用在许多地区的下地壳演化中的作用可能需要重新评估。
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 THfDMages. 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.