The characterization of Mg-clays in rock samples (well P1) from the Barra Velha Formation (Early Cretaceous) allowed the establishment of mineral assemblages on the basis of their kerolite and Mg-smectite (stevensite and saponite) content. Kerolite-rich assemblages (A and B) rarely contain saponite. Assemblage B is composed of kerolite-stevensite mixed layers, while assemblage A consists of more than 95% kerolite. Mg-smectite-rich assemblages (C and CB) are made up of both Mg-smectites. The predominance of stevensite in the lower interval of the stratigraphic succession suggests evaporative conditions, higher salinity and pH, which would favor its authigenesis by neoformation. In the upper portion, the occurrence of thick kerolite-rich intervals suggests regular water inputs, contributing with a decreasing in salinity and pH, favoring the neoformation of kerolite and later kerolite-stevensite mixed layering. The saponite would be the result of the transformation from Al-smectite into Mg-smectite in a Mg2+ rich medium. The results indicate that lake hydrochemical processes would have allowed the establishment of a basic depositional sequence, from base to top, as follows: (i) initial lake expansion stage marked by the occurrence of saponite, (ii) later kerolite neoformation, (iii) formation of kerolite-stevensite mixed layer with increasing salinity, and (iv) neoformation of stevensite, marking a final stage of maximum salinity (evaporation) and alkalinity of the lake.

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Shales are composed of minerals and organic matter, whose individual properties are essential to determining the rock’s macroscopical deformation and strength. Scanning electron microscopy combined with electron energy dispersive spectroscopy (EDS) has been extensively used to evaluate composition, while peak-force atomic force microscopy (AFM) has been used on the determination of elastic modulus with nanometric resolution. Still, there is a need for tools to conduct an in-depth study of the minerals’ tribomechanical properties. Atomic force microscopy is a tool that can contribute to these studies, as it can simultaneously measure the tribomechanical properties and identify the phases. In this work, we propose using atomic force microscopy and energy dispersive spectroscopy to identify the shale components and to measure the in situ tribomechanical properties from the different phases. Friction images between the atomic force microscopy tip and the surface were acquired as a function of load. Minerals and organic matter were later identified by colocalized energy dispersive spectroscopy mapping. Then, the frictional characteristics of the major shale constituents were obtained by adjusting the Derjaguin-Muller-Toporov model to the selected components. Moreover, the identification of the different phases was performed. The results show that friction at the nanometer scale was observed to be higher for organic matter than for any other shale constituent, while shear strength was observed to be higher for quartz and lower for organic matter. These characteristics were used to differentiate shale constituents. It is shown that a careful comparison of friction can be used to differentiate the sulfite pyrite, tectosilicates (quartz, andesine, and albite), phyllosilicate biotite, and organic matter. The presented methodology gives novel information on friction properties in the nanoscale that are comparable to available centimetric characterization techniques contributing to the understanding of rock strength.

região de Colatina, Estado do Espírito Santo, foi objeto de mapeamento geológico e detalhado estudo petrográfico e geoquímico visando reconhecer e caracterizar paragnaisses migmatíticos do Complexo Nova Venécia e granitóides sincolisionais do tipo S – Granitóide Colatina, que ocorrem no núcleo metamórfico-anatético do Orógeno Araçuaí. O Complexo Nova Venécia, é representado por paragnaisses migmatíticos, cujas relações de campo, dados de mineralogia, petrografia, geoquímica e geocronologia indicam que estas rochas foram, originalmente, sedimentos peraluminosos de composição grauvaquiana, depositados em possível bacia de retroarco. O Granitóide Colatina varia de monzogranitos a tonalitos, representando uma fusão anatética do tipo-S, ocorrida na fase sincolisional do Orógeno Araçuaí, mostrando uma persistente foliação concordante com a foliação regional. As relações do Granitóide Colatina com as rochas do Complexo Nova Venécia evidenciam que se trata de uma fusão autóctone.Palavras chave: Orógeno Araçuaí, núcleo metamórfico, granitóides. ABSTRACTGEOLOGY, PETROLOGY AND GEOTECTONIC CONTEXT OF GNEISSES AND GRANITOIDS OF THE COLATINA REGION, ESPIRITO SANTO, BRAZIL. The Colatina region, Espírito Santo state, Brazil, was the subject of detailed geological mapping and geochemical and petrographic study aiming to recognize and characterize the Nova Venécia Complex: migmatitic paragneisses, and S-type syncolisional granitoids – the Colatina Granitoid – occurring in the metamorphic core of the Araçuaí orogen. The Nova Venécia Complex is represented by migmatitic paragneiss, whose field relations and mineralogical, petrographical, geochemical and geochronological data indicate that these rocks were originally peraluminous sediments with wacke affinities, probably deposited in a retroarc basin. The Colatina Granitoids varies from monzogranites to tonalites, representing a S-type anatetic melt occurred during the syn-collisional event of the Araçuaí orogen, showing a persistent foliation concordant with the regional foliation. An authoctonous melt is interpreted from the relations between the Colatina Granitoids and the rocks from the Nova Venécia Complex.Keywords: Araçuaí orogen, metamorphic core, granitoids.