Abstract Oncoid nucleation and growth have been attributed to microorganism activity, commonly cyanobacteria, resulting in the trapping of organic, bioclastic or lithoclastic nuclei by concentric laminations. However, the factors controlling these processes are not well understood, especially regarding freshwater precipitation systems. Freshwater oncoids from alluvial terraces underlying paludal tufas in Quaternary carbonate deposits in Central-West Brazil (Bodoquena downstream plains) were submitted to petrological studies by optical microscope, x-ray diffraction and fluorescence, scanning electron microscopy and C&O stable isotopes analyses. Biogenic structures where well-preserved cyanobacteria EPS sheath structures suggest in vivo precipitation by CO2 concentrating mechanisms, whereas smooth rhomb calcite crystals likely indicate post mortem precipitation. The four identified morphologic features of spherical and subspherical oncoid types suggest growing in a shallow water body, with constant laminar flow and the absence of predators. The chemical and isotopic composition of the oncoids may represent nucleation and growth under relatively milder climatic conditions than today. This oncoid nucleation system may bring valuable information about climatic changes in Central-West Brazil (Tropical South Hemisphere) at the end of the Pleistocene and the beginning of the Holocene.

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.