Ophiolitic metamafic remnants of Palaeoproterozoic oceanic crust from the Cláudio shear zone, Southern São Francisco Craton (Brazil)

Early evidence of plate tectonics as a global phenomenon can be provided by Palaeoproterozoic ophiolites. Metamafic rocks preserved within the Southern São Francisco Craton (Brazil) along the Cláudio shear zone (suture zone) are regarded as part of the dismembered Cláudio meta-ophiolite. Here, we investigate the petrology and tectono-magmatic setting of these metamafites using petrography, whole-rock geochemistry, and mineral chemistry. Geochronological data and geodynamic implications were explored using Sm-Nd and U-Pb methods. We found that their protoliths (gabbro and basalt) are tholeiitic and sub-alkaline with an E-MORB signature, suggesting that they were remnants of oceanic crust. Metagabbro preserves igneous relics and prograde assemblages from very low- to medium-grade metamorphism. Metabasalts showed an assemblage of amphibolite facies. The 2276 Ma whole-rock Sm-Nd T_DM model age and the 2050 Ma U-Pb age of metamorphic zircons indicate a Palaeoproterozoic evolution of the palaeo-oceanic basin. It was closed owing to NW-dipping subduction, followed by continental collision involving the Divinópolis and Campo Belo/Bonfim metamorphic complexes. Our results support previous studies that suggest a possible connection between São Francisco-Congo and North China/Yangtze cratons during the formation of the Columbia (Nuna) supercontinent. Furthermore, this study provides additional evidence supporting that modern-style plate tectonics operated during the Palaeoproterozoic.

古元古代蛇绿岩（Palaeoproterozoic ophiolites）可为板块构造作为全球性现象的早期识别提供关键证据。巴西南圣弗朗西斯科克拉通（Southern São Francisco Craton）沿克劳迪奥剪切带（Cláudio shear zone，即缝合带）产出的变质镁铁质岩，属于解体后的克劳迪奥变质蛇绿岩的组成部分。本文通过岩相学、全岩地球化学及矿物化学分析，对该套变质镁铁质岩的岩石学特征与构造-岩浆背景开展研究，并利用Sm-Nd与U-Pb测年方法，探讨了其年代学数据与地球动力学意义。研究发现，其原岩（辉长岩与玄武岩）属于拉斑系列与亚碱性系列，且具有E-MORB（富集型洋中脊玄武岩）的地球化学特征，表明它们属于洋壳残余。变质辉长岩保留了从极低级到中级变质作用形成的火成残留体与进变质矿物组合；而变质玄武岩则呈现角闪岩相的矿物组合。全岩Sm-Nd T_DM模式年龄为2276 Ma，变质锆石的U-Pb年龄为2050 Ma，指示该古洋盆经历了古元古代演化。该古洋盆因北西向俯冲作用而闭合，随后发生了涉及迪维诺波利斯（Divinópolis）与坎普贝卢/邦芬变质杂岩的大陆碰撞事件。本研究结果支持前人研究结论，即在哥伦比亚（努纳）超大陆（Columbia (Nuna) supercontinent）形成时期，圣弗朗西斯科-刚果克拉通（São Francisco-Congo Craton）与华北/扬子克拉通（North China/Yangtze cratons）之间可能存在构造联系。此外，本研究还为现代型板块构造在古元古代时期便已运作提供了新的佐证。