Abstract Arclogites, i.e., lower crustal garnet-pyroxenite cumulates, are suggested to play an important role in controlling magma differentiation in modern continental arcs. Until now, arclogite-related magmatism has only been described from the Phanerozoic Era. The Svecofennian orogen in the central Fennoscandian Shield hosts a rare association of 1.86 Ga igneous rocks geochemically distinct from the surrounding and much more abundant 1.90–1.87 Ga subduction-related calc-alkaline magmatism. The 1.86 Ga magmatic rocks are divided into three groups: 1) high-Nb gabbros (HNB) which are enriched in Fe2O3T, TiO2, P2O5, F, LILE, and HFSE (especially Nb; 18.9–44 ppm), show positive initial eNd value, and near-chondritic but variable initial zircon eHf values; 2) high-Mg gabbros (HMG) which are characterised by high MgO, CaO, Cr and Ni contents, slight enrichment in LILE, positive eNd, and positive but variable zircon eHf values; 3) adakite-like rocks showing high Al2O3 and Na2O contents, slight enrichment in LILE, relative depletion in some HFSE, positive eNd value, and chondritic to negative zircon eHf values. The three groups yield zircon U-Pb ages of ~1.86 Ga and exhibit undeformed textures in contrast to the surrounding supracrustal rocks metamorphosed at ~1.88 Ga. The ages and compositions are clearly different from the adjacent 1.90–1.87 Ga arc-related igneous rocks suggesting a distinct origin. Despite similar ages and close spatial relationship, separate sources are required for each of the different 1.86 Ga rock groups. Trace element modelling of partial melting suggests that arclogites, with compositions similar to pyroxenite xenoliths found in the kimberlite pipes of eastern Finland, are the source for the HNB rocks. In contrast, subduction-modified mantle peridotite is the source for the HMG rocks, and a mafic lower crustal source is suggested for the adakite-like rocks. The following geodynamic model is suggested: (rutile-bearing) arclogite formation at 1.90–1.87 Ga followed by arclogite delamination and partial melting during extension of the thickened Svecofennian crust at 1.86 Ga.

中文翻译：

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追踪古元古代的弧长岩——芬诺斯坎迪亚地盾中部从 1.88 Ga 钙碱性到 1.86 Ga 高铌和埃达克岩岩浆作用的转变

摘要 弧长岩，即下地壳石榴石-辉石堆积体，被认为在控制现代大陆弧岩浆分异中起重要作用。迄今为止，与弧长岩有关的岩浆作用仅在显生宙开始被描述过。Fennoscandian 地盾中部的 Svecofennian 造山带拥有罕见的 1.86 Ga 火成岩组合，其地球化学特征与周围环境不同，并且具有更丰富的 1.90-1.87 Ga 俯冲相关钙碱性岩浆作用。1.86 Ga 岩浆岩分为三组：1) 富含 Fe2O3T、TiO2、P2O5、F、LILE 和 HFSE（特别是 Nb；18.9-44 ppm）的高 Nb 辉长岩（HNB），显示出正的初始 eNd值和接近球粒陨石但可变的初始锆石 eHf 值；2) 高镁辉长岩 (HMG)，其特点是 MgO、CaO、Cr 和 Ni 含量高，LILE 中的轻微富集、正 eNd 和正但可变的锆石 eHf 值；3) 埃达克岩显示出高 Al2O3 和 Na2O 含量，LILE 轻微富集，一些 HFSE 相对贫化，eNd 值为正，球粒状到负锆石 eHf 值。这三组的锆石 U-Pb 年龄为~1.86 Ga，与周围在~1.88 Ga 变质的上壳岩形成对比，显示出未变形的结构。年龄和成分明显不同于相邻的 1.90-1.87 Ga 弧相关火成岩一个独特的起源。尽管年龄相似且空间关系密切，但每个不同的 1.86 Ga 岩石群都需要不同的来源。部分熔化的微量元素模型表明，弧长岩、成分类似于芬兰东部金伯利岩管中发现的辉石岩捕虏体，是 HNB 岩石的来源。相比之下，俯冲改造的地幔橄榄岩是 HMG 岩石的来源，而类埃达克岩则建议为基性下地壳来源。建议采用以下地球动力学模型：（含金红石）弧长岩在 1.90-1.87 Ga 处形成，随后在 1.86 Ga 处加厚的 Svecofennian 地壳伸展过程中发生弧长岩分层和部分熔融。