Abstract Metabasites and paragneisses from the high-grade Ediacaran-Cambrian Cabo Frio Tectonic Domain, SE Brazilian coast, are investigated with field, geochronological (U-Pb combined with Lu-Hf), isotopic (Sm-Nd) and geochemical analyses. These units, tectonically interleaved with a Paleoproterozoic basement, record the pre-collisional setting within the eastern Ribeira Orogen. The four analyzed metabasite bodies indicate tholeiitic basalts/gabbros with OIB affinity as protoliths tending to high-Mg terms, also corroborated by their higher LILE, LREE and HFSE contents, in comparison to the E-MORB and N-MORB basalts. The Nb and Ta (Nb/Yb – alkaline proxy) and Ti/Yb (plume-melting proxy) resemble present-day OIBs values. One metabasite provided mainly metamorphic zircon crystals (n = 20) with a U-Pb age of 530 Ma. Only three crystals show oscillatory zoning, revealing ages of 1890 Ma, 676 Ma and 580 Ma. The Paleoproterozoic zircon, interpreted as a xenocryst, and the Epsilon Nd values (+3 and −6) for whole rock indicate crustal contamination. U-Pb data from detrital zircon of two paragneiss layers, structurally below and above the metabasite, constrain maximum depositional ages of 626 Ma and 549 Ma, respectively. The older has a major 750-620 Ma zircon source, similar to the age of Cryogenian magmatic units in the African terranes. The sediments of this layer are here interpreted as deposited in a distal continental passive margin, coeval with the OIB magmatism (metabasites) crystallized near or at the Ocean-Continent Transitional zone. The upper paragneiss layer shows a major juvenile Ediacaran detrital zircon source between 620 and 550 Ma, with mainly positive Epsilon Hf values (0 to +13). It also presents disrupted blocks of the metabasites, forming a sedimentary breccia. This latter evidence, in addition to its medium P-high T metamorphic paragenesis, supports the hypothesis of an accretionary prism system. The metabasites are here interpreted as ophiolite relics from an Ediacaran Ocean that predated the Gondwana amalgamation. The subsequent ca. 550-530 Ma continental collision preserved this tectonic sliver between the Paleoproterozoic continental crust, below, and the Ediacaran forearc sedimentary sequence, above (Palmital Succession). The formed Cambrian suture was later reactivated as one of the Cretaceous rift branches that evolved to the South Atlantic.

中文翻译：

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巴西东南海岸的埃迪卡拉纪蛇绿岩遗迹：来自变碱性矿物和副片麻岩的野外、地球化学和地质年代学证据

摘要 通过野外、年代学（U-Pb 结合 Lu-Hf）、同位素（Sm-Nd）和地球化学分析，研究了来自巴西东南海岸高品位埃迪卡拉纪-寒武纪 Cabo Frio 构造域的变基石和副片麻岩。这些单元在构造上与古元古代基底交错，记录了里贝拉造山带东部的碰撞前环境。四个分析的变碱性玄武岩体表明具有 OIB 亲和力的拉斑玄武岩/辉长岩作为倾向于高镁项的原岩，与 E-MORB 和 N-MORB 玄武岩相比，它们的 LILE、LREE 和 HFSE 含量也证实了这一点。Nb 和 Ta（Nb/Yb – 碱性代理）和 Ti/Yb（羽状熔化代理）类似于当今的 OIB 值。一种变碱石主要提供了 U-Pb 年龄为 530 Ma 的变质锆石晶体 (n = 20)。只有三个晶体显示出振荡分区，揭示了 1890 Ma、676 Ma 和 580 Ma 的年龄。古元古代锆石，被解释为异晶，整个岩石的 Epsilon Nd 值（+3 和 -6）表明地壳污染。来自两个副片麻岩层碎屑锆石的 U-Pb 数据，在结构上位于变贝岩下方和上方，分别限制了 626 Ma 和 549 Ma 的最大沉积年龄。较老的有一个主要的 750-620 Ma 锆石源，类似于非洲地体中低温纪岩浆单元的年龄。该层的沉积物在这里被解释为沉积在远端大陆被动边缘，与在海洋-大陆过渡带附近或在此结晶的 OIB 岩浆作用（变基岩）同时期。上副片麻岩层显示了 620 至 550 Ma 之间的主要幼年埃迪卡拉纪碎屑锆石源，主要具有正 Epsilon Hf 值（0 到 +13）。它还呈现了变基岩的破碎块，形成了沉积角砾岩。后一个证据，除了其中 P-高 T 变质共生外，还支持增生棱柱系统的假设。这些变碱金属在这里被解释为来自埃迪卡拉纪海洋的蛇绿岩遗迹，该遗迹早于冈瓦纳河合并。随后的约。550-530 Ma 大陆碰撞保留了位于下方的古元古代大陆地壳和上方的埃迪卡拉纪弧前沉积层序（棕榈层序）之间的构造条带。形成的寒武纪缝合线后来被重新激活，成为演化到南大西洋的白垩纪裂谷分支之一。后一个证据，除了其中 P-高 T 变质共生外，还支持增生棱柱系统的假设。这些变碱金属在这里被解释为来自埃迪卡拉纪海洋的蛇绿岩遗迹，该遗迹早于冈瓦纳河合并。随后的约。550-530 Ma 大陆碰撞保留了位于下方的古元古代大陆地壳和上方的埃迪卡拉纪弧前沉积层序（棕榈层序）之间的构造条带。形成的寒武纪缝合线后来被重新激活，成为演化到南大西洋的白垩纪裂谷分支之一。后一个证据，除了其中 P-高 T 变质共生外，还支持增生棱柱系统的假设。这些变碱金属在这里被解释为来自埃迪卡拉纪海洋的蛇绿岩遗迹，该遗迹早于冈瓦纳河合并。随后的约。550-530 Ma 大陆碰撞保留了位于下方的古元古代大陆地壳和上方的埃迪卡拉纪弧前沉积层序（棕榈层序）之间的构造条带。形成的寒武纪缝合线后来被重新激活，成为演化到南大西洋的白垩纪裂谷分支之一。随后的约。550-530 Ma 大陆碰撞保留了位于下方的古元古代大陆地壳和上方的埃迪卡拉纪弧前沉积层序（棕榈层序）之间的构造条带。形成的寒武纪缝合线后来被重新激活，成为演化到南大西洋的白垩纪裂谷分支之一。随后的约。550-530 Ma 大陆碰撞保留了位于下方的古元古代大陆地壳和上方的埃迪卡拉纪弧前沉积层序（棕榈层序）之间的构造条带。形成的寒武纪缝合线后来被重新激活，成为演化到南大西洋的白垩纪裂谷分支之一。