The origin of diamonds from Serra do Espinhaço in Diamantina region (State of Minas Gerais) and in Chapada Diamantina, Lençóis region (State of Bahia) remains uncertain, even taking into account the ample research carried out during the last decades. The lack of typical satellite minerals in both districts makes a kimberlitic source for these diamonds uncertain. In mid 18th century the occurrence of a metamorphosed igneous rock composed of martite, sericite and tourmaline was described in Diamantina region and named hematitic phyllite, considered by some researchers as a possible diamond source. Similar rocks were found in Lençóis and examined petrographically and their heavy mineral concentration was investigated by means of scanning electron microscopy (SEM). Petrographic analyses indicated an igneous origin for these rocks and SEM analyses showed the discovery of microdiamonds. Geochronological studies using the Ar/Ar technique in muscovites yielded minimum ages of 1515 ± 3 Ma, which may correlate with 1710 ± 12 Ma from U-Pb method in igneous zircons from the hematitic phyllites. Both rock types also have the same mineral and chemical composition which leads to the conclusion that the intrusive rocks were protolith of the hematitic phyllites. This fi rst discovery of microdiamonds in intrusive rocks opens the possibility of new investigation models for diamond mineralization in Brazilian Proterozoic terrains.
GLACIOCONTINENTAL FÁCIES (OUTWASH PLAIN) IN THE MACAÚBAS MEGASEQUENCE (SERRA DA ÁGUA FRIA/MG) MEGASEQUENCE AND ITS TECTONIC CONTEXT This paper presents the results of integrated studies on sedimentary fácies, depositional architecture, sedimentary petrology and paleogeography, carried out in sandstones associated with the Jequitaí diamictites. These rocks belong to the base of the Macaúbas Megasequence and crop out in the Serra da Água Fria, in the central-north part of Minas Gerais State. The data allow to interpret the studied deposits as a product of a proglacial outwash plain sustem, which has evolved in the NNE direction. These paleocurrent data, analysed together with data from the western border of the Serra da Agua Fria, indicate the existence of a sindepositional structural high in the Serra da Água Fria area, which, under the action of glaciers and seasonal cycles of melt-water, had supplied sediments (most probably from a source área in the Espinhaço rocks) towards a structural low located to the west. This paleogeography is related to the Early Neoproterozoic rift extensional tectonics responsible for the development of the Macaúbas/Salinas basin. Although many authors had stated that the Jequitaí glaciation was of a continental type with eastward moving glaciers, we interpret that this glaciation as a rift-shoulder related, altitude model. The paleocurrent data achieved in the Serra da Água Fria área show sediment dispersion towards the interior of the São Francisco Craton and from this area towards a NNE direction.
TRIÂNGULO MINEIRO The paper characterizes the depositional model and the diagenetic evolution of the Uberaba Formation, the lowermost unit of Bauru Group, Upper Cretaceous of the Paraná Basin. The Uberaba Formation is geographically restricted to the Triângulo Mineiro region and consists of medium-grained sandstones, conglomerates, pebbly sandstones and siltstones. Its framework consists of quartz, feldspars, rock fragments (basalt, quartzites, schists, sandstones) and sandstone intraclasts. These rocks can be classified as lithic sandstones, sublithic sandstones and feldspathic lithic sandstones. The presence of thick conglomeratic sequences-consisting of sandstone intraclastsnear Serra do Veríssimo, is interpreted as due to tectonic activity in this area during the deposition of the Uberaba Formation. The diagenetic evolution may be divided into eodiagenetic and mesodiagenetic stages. The eodiagenesis is represented by the following aspects: a) presence of large volumes of volcanic dust due to the volcanic activity in the Alto Paranaiba region, which were mechanically infiltrated in the intergranular pores of rocks; b) autigenesis of zeolite and silica; c) autigenesis of iron oxides, dolomites, calcretes, silcretes, attapulgite and sepiolite. The mesodiagenesis is marked by formation of calcite and barite, the former being the most important phase in volume. The textural relationships as well as the reddish colors of cathodoluminescence due to the high Mn content suggest the mesodiagenetic origin of calcite. The sedimentary environments characterized by braided river complex controlled the development of the autigenic minerals, during the eodiagenesis. The high permeability of these rocks is related to the textural heterogeneity, which allowed the influx of meteoric water. The framework constituents of these rocks were the main source of material for the diagenetic phases.
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