Abstract Petrological and geochemical studies have revealed the contribution of garnet pyroxenites in basalt petrogenesis. However, whether primary mantle melts are produced with such signature or acquired it subsequently remains somewhat controversial. We here integrate new major-, trace-element and Sr-Nd-Hf isotopic compositions of garnet pyroxenite xenoliths in Holocene alkali basalts from Lakes Bullenmerri and Gnotuk, Southeastern Australia, to relate their petrogenesis to mantle-wedge melt circulation and subsequent lithospheric evolution. Results show that the clinopyroxenites have lower MgO and Cr2O3 contents than the associated websterites, and range in compositions from depleted LREE patterns and highly radiogenic Nd and Hf isotopic signatures in relatively low-MgO samples (Type 1), to enriched REE patterns with negative HFSE anomalies, unradiogenic Nd and Hf isotopes, and extremely radiogenic Sr-isotopic ratios in samples with higher MgO (Type 2). Such compositional variabilities suggest that these pyroxenites represent segregates from melts derived from a recycled oceanic lithosphere with a potential contribution from pelagic sediments. Variable LREE contents and isotopic compositions between those of Type 1 and 2 clinopyroxenites are observed in amphibole-bearing samples (Type 3), which are interpreted as Type 1-like protoliths metasomatized by the basaltic and carbonatitic melts, possibly parental to Type 2 clinopyroxenites. The lithosphere beneath Southeastern Australia thus has received variable melt contributions from a heterogeneous mantle-wedge source, which notably includes a subducted oceanic slab package that has retained its integrity during subduction. On this basis, we suggest that the compositional heterogeneity and temporal evolution of the subsequent Southeastern Australian basaltic magmatism were probably affected by the presence of pyroxenite fragments in the basalt source and formed by the tectonic reactivation of this lithosphere during Cenozoic rifting. This interpretation is notably consistent with a trend of Nd-Pb isotopes towards EMII in Older Volcanic Provinces (OVP basalts) and limited Sr-Nd-Pb isotopic variations towards HIMU in the Newer Volcanic Provinces (NVP basalts, including the host lavas), which also exhibit low SiO2, high FeO and high CaO/Al2O3 commonly interpreted as due to pyroxenite contributions. Therefore, the identification of a subduction signature in these rift-related lavas attests to a “lithospheric memory” of earlier subduction episodes (as documented by the xenoliths), rather than a reflection of contemporaneous subduction tectonics.

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来自裂谷相关玄武岩的地幔辉石岩包体俯冲的岩石圈记忆

摘要 岩石学和地球化学研究揭示了石榴石辉石在玄武岩成因中的贡献。然而，原始地幔熔体是产生这样的特征还是后来获得的，仍然有些争议。我们在这里整合了来自澳大利亚东南部布伦梅里湖和格诺图克湖的全新世碱性玄武岩中石榴石辉石捕体的新的主要、微量元素和 Sr-Nd-Hf 同位素组成，将它们的成因与地幔楔熔体循环和随后的岩石圈演化联系起来。结异常，非放射性 Nd 和 Hf 同位素，以及具有较高 MgO（类型 2）的样品中具有极高放射性的 Sr 同位素比率。这种成分变化表明，这些辉石岩代表了来自循环海洋岩石圈的熔体的分离物，可能来自远洋沉积物。在含角闪石的样品（类型 3）中观察到 1 型和 2 型单斜辉石岩之间的 LREE 含量和同位素组成不同，这些样品被解释为由玄武质和碳酸岩熔体交代的类 1 型原岩，可能是 2 型单斜辉石岩的母体。因此，澳大利亚东南部下方的岩石圈收到了来自异质地幔楔源的可变熔体贡献，其中特别包括在俯冲过程中保持其完整性的俯冲大洋板块包。在此基础上，我们认为随后澳大利亚东南部玄武质岩浆作用的成分非均质性和时间演化可能受到玄武岩源中辉石岩碎片的影响，并由新生代裂谷期间该岩石圈的构造再激活形成。这种解释与较老火山省（OVP 玄武岩）中 Nd-Pb 同位素向 EMII 的趋势和较新火山省（NVP 玄武岩，包括寄主熔岩）中有限的 Sr-Nd-Pb 同位素向 HIMU 变化趋势一致，还表现出低 SiO2、高 FeO 和高 CaO/Al2O3，通常被解释为由于辉石的贡献。所以，