Petrography and geochemistry of the Pedra Dourada Granulite, southeastern Minas Gerais, Brazil

Abstract The Pedra Dourada Granulite (PDG) occurs at the southeastern portion of the Araçuaí Belt, north of the town of Ponte Nova. It comprises bodies up to 45 km2 inserted into amphibolite-facies gneisses of the Mantiqueira Complex. Motivated by the discrepancy of metamorphic grade with surrounding rocks, this paper presents results of the petrographic and geochemical investigation of the PDG. The unit is comprised of meta-igneous and metasedimentary rocks. Meta-igneous rocks dominate and include felsic granulites (biotite ± garnet-bearing and orthopyroxene-bearing) and subordinate mafic granulites. Metasedimentary rocks are aluminous granulites with Al-rich mineral assemblages (garnet, sillimanite, spinel). Geochemical data show that most of felsic protoliths are peraluminous rocks including granites, granodiorites and diorites of calc-alkaline character, chemically similar to granitoids of convergent tectonic settings. Mafic protoliths are metaluminous rocks comprised of gabbros and subordinate diorites of tholeiitic affinity, compositionally similar to plate margin basalts. Aluminous protoliths may be peraluminous pelitic rocks and wackes, analogous to sediments from convergent environments. The mineral assemblages indicate that these rocks were metamorphosed under medium-P granulite-facies conditions. Coronitic garnet textures suggest a near-isobaric cooling (IBC-path) after metamorphic peak.

摘要 黄金粒麻岩（Pedra Dourada Granulite, PDG）产出于阿拉瓜伊造山带（Araçuaí Belt）东南部、蓬图诺瓦镇北侧。该岩体以面积可达45 km²的岩块形式赋存于曼蒂凯拉杂岩（Mantiqueira Complex）的角闪岩相片麻岩中。鉴于其变质级别与围岩存在显著差异，本文开展了PDG的岩石学与地球化学研究，并报道相关成果。该岩石单元由变火成岩与变沉积岩构成：变火成岩占主导，包括长英质麻粒岩（含黑云母±石榴子石、含斜方辉石）及少量基性麻粒岩；变沉积岩则为具备富铝矿物组合（石榴子石、矽线石、尖晶石）的铝质麻粒岩。地球化学数据显示，多数长英质原岩为钙碱性系列的过铝质岩石，涵盖花岗岩、花岗闪长岩与闪长岩，其地球化学特征与汇聚构造环境下的花岗岩类高度相似。基性原岩为准铝质岩石，由具拉斑玄武质亲缘性的辉长岩及少量闪长岩构成，成分与板块边缘玄武岩相近。铝质原岩可能为过铝质泥质岩与杂砂岩，类似汇聚环境中的沉积物。矿物组合表明，此类岩石形成于中压麻粒岩相变质条件。冠状石榴子石结构指示其经历了变质峰期后的近等压冷却（IBC-path）过程。