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 佩德拉杜拉达粒岩（Pedra Dourada Granulite, PDG）出露于阿拉苏阿伊造山带（Araçuaí Belt）东南部，蓬蒂诺瓦（Ponte Nova）镇北侧。该岩体以面积可达45 km²的岩席形式赋存于曼蒂凯拉杂岩（Mantiqueira Complex）的角闪岩相片麻岩中。鉴于其变质级与周围岩石存在显著差异，本文开展了PDG的岩石学与地球化学研究并报道相关结果。该岩系由变火成岩与变沉积岩组成，其中变火成岩占主导，包括长英质粒岩（含黑云母±石榴子石、含斜方辉石）及少量镁铁质粒岩；变沉积岩则为具富铝矿物组合（石榴子石、红柱石、尖晶石）的富铝粒岩。地球化学数据显示，多数长英质原岩为钙碱性特征的过铝质岩石，包括花岗岩、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相似。镁铁质原岩为准铝质岩石，由拉斑质系列的辉长岩及少量闪长岩组成，成分与板块边缘玄武岩一致。富铝原岩可能为过铝质泥质岩与杂砂岩，类似汇聚环境下的沉积物。矿物组合指示这些岩石形成于中压粒岩相变质条件。冠状结构石榴子石的结构特征表明，其在变质峰期后经历了近等压冷却（IBC路径）过程。