Low-temperature metamorphism in the Capiru Formation, Morro Grande Synform, Southern Ribeira Belt

ABSTRACT: The Capiru Formation consists of a low-grade meta-sedimentary succession of slates, phyllites, quartzites and marbles, disposed in blocks delimited by thrust and strike-slip faults in the Curitiba Terrane, Southern Ribeira Belt, Southern Brazil. The metamorphic and deformation records are heterogeneous, with deformed zones tectonically interbedded by domains with preserved sedimentary features. The present work aims to understand the development of mineral paragenesis based on geochemical and petrographic analysis. We selected samples of metasandstones, slates, phyllites and rhythmic phyllites, with different metamorphic and deformation records to be analysed. We selected samples with different metamorphic records to be analysed. The petrographic characterization of foliations was made through field studies supported by microtectonic analysis. The mineralogical and chemical compositions were determined by X-ray diffractometry and X-ray fluorescence, respectively. The results show a preserved sedimentary bedding (S0), defined by sedimentary structures with top-and-bottom indicators. The S1 foliation is related to a thrust tectonics, and it is characterized by a continuous slaty cleavage. The S1 slaty cleavage is crenulated and cut by a millimetric S2 cleavage. The mineral assemblage is composed by quartz, sericite, magnetite/goethite and carbonaceous material. The metamorphism was developed under low-temperature conditions (between 250-350ºC) and low pressure gradients (2.5-4.5 kbar). The geotectonic environment is considered as a thrust-and-fold-belt system, with structures developed in superior crust levels.

摘要：卡皮鲁组（Capiru Formation）由一套低品级变沉积岩系组成，包含板岩、千枚岩、石英岩及大理岩，该岩系以岩块形式产出，被巴西南部南里贝拉造山带库里巴纳地体（Curitiba Terrane）内的逆冲断层与走滑断层所围限。其变质与变形记录具有非均质性，构造上互层的变形带与保留沉积特征的区域共存。本研究旨在基于地球化学与岩石学分析，阐明矿物共生组合的演化过程。研究选取了变砂岩、板岩、千枚岩及韵律千枚岩样品，这些样品具备不同的变质与变形记录以供分析。通过野外研究结合微构造分析，完成了面理的岩石学表征；矿物组成与化学成分分别通过X射线衍射（X-ray diffractometry）与X射线荧光光谱（X-ray fluorescence）测定。研究结果显示，区内保留有沉积层理（S0），该层理由具备顶底指示构造的沉积结构所界定。S1面理与逆冲构造相关，表现为连续的板状劈理，该劈理具褶皱纹构造，并被毫米级S2劈理切割。矿物组合由石英、绢云母、磁铁矿/针铁矿及碳质物质组成。变质作用形成于250~350℃的低温条件与2.5~4.5 kbar的低压梯度环境中。研究认为其大地构造环境为逆冲褶皱带体系，相关构造形成于上地壳深度层次。