Resumo em Inglês:

Abstract The Paraná deposit, located at Southwestern Rio Grande do Norte state, in Brazil, is one of the few emerald deposits found at Borborema Province. The mineralization occurs in phlogopite schists and actinolite-phlogopite schists associated with pegmatites and albitites within the Portalegre Shear Zone. Unlike other well-known Brazilian emerald deposits, the mineralogy of Paraná emeralds has remained poorly investigated for the last 40 years. In this study, we conducted mineralogical characterization of theses emeralds through gemological testing, mineral chemistry, absorption and reflectance spectroscopy, and thermal analysis. The Paraná emeralds are bluish-green colored, characterized by high refractive index, several two-phase fluid inclusions and mica is the main mineral inclusion. Electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometry analyses detected the presence of Fe2+ (0.43-1.94 wt.% FeO) and Cr3+ (0.04-0.14 wt.% Cr2O3) as the main chromophores replacing octahedral Al3+ in the crystal structure. In addition, substantial amounts of MgO (0.40-2.72 wt.%), Na2O (0.50-1.81 wt.%), and Cs2O (0.07-0.44 wt.%) were also identified. The main causes for its coloration were attributed to Cr3+ absorption features in visible spectral range, which were corroborated by absorption and reflectance spectra. The presence of types I and II H2O at channel-sites was recorded in Fourier-transform infrared spectra and demonstrated by dehydration processes observed in different thermal and thermogravimetric analyses.

Resumo em Inglês:

Abstract We present a combined analysis of airborne geophysical data (magnetics and gamma-ray spectrometry) and field aspects of Itapetim lode gold district region, aiming to identify structurally deformed and hydrothermally altered zones, which are suggestive of gold mineralized sectors. The main geophysical lineaments are oriented in the NE-SW and E-W directions and display a clear correlation with the major gold-bearing structures in the region. In the magnetic maps, the key pathfinder for gold occurrences is their association with strongly magnetized areas positioned along the Itapetim Shear Zone and outer contour of Teixeira Batholith. In the field, such structures are materialized in protomylonitic and mylonitic rocks that present evidence of ductile and brittle deformation, whereas kinematic criteria are suggestive of a transpressional tectonic regime. In addition, the main radiometric pattern is characterized by K enrichment, which is correlated with a hydrothermal mineral assemblage dominated by quartz, potassium feldspar, and tourmaline. Based on the obtained data integration, we produced an integrative map and located ten new target areas of possible gold mineralization, which is correlative to other well-known lode deposits in Northeast Brazil and Africa.

Resumo em Inglês:

Abstract The Morro do Coco granite (MC) is a post-collisional intrusive body that was emplaced at Ribeira Belt, in the Cambro-Ordovician, succeeding the final stages of the Gondwana supercontinent amalgamation. This unit requires more studies, contrasting with similar best investigated occurrences of the central Ribeira Belt. This work presents a detailed investigation of such unit using petrography, Inductively Coupled Plasma (ICP), X-Ray Fluorescence (XRF) whole-rock geochemistry, U-Pb (SHRIMP) geochronology and Sm-Nd isotopes. The rock is a nearly undeformed syenogranite, geochemically classified as a high-K calc-alkaline magnesian, with metaluminous to slightly peraluminous affinity. Strongly light rare earth elements (LREE)-enriched patterns with significant negative Eu anomalies, typical of post-collisional granites, are identified. Similarly to other post-collisional granites of the central Ribeira Belt, U-Pb zircon data provide crystallization age of 496 ± 3 Ma . Sm-Nd isotopic data suggest crustal magma source as indicated by εNd(496) of -9, 143Nd/144Nd ratios between 0.511747 and 0.511752 and Mesoproterozoic TDM ages of 1.3 Ga. Based on its geochemical characteristics, morphology and geochronology, this unit is considered here as a member of Nova Friburgo Suite. Geochronological data defined three pulses of magmatism in this part of the orogen during the Cambro-Ordovician (515, 490 and 460 Ma).

Resumo em Inglês:

ABSTRACT Rio Itanguá Batholith (RIB) is a plutonic body located in the Northern border of Guanhães block, one of the various basement blocks reworked within the Araçuaí orogen, Eastern Brazil. Although its existence is known since the beginning of the 20th century, its tectonic history remains poorly understood. The RIB was mapped at 1:100,000 scale in the 1990s, before geophysical and satellite data became available. This work aims to re-evaluate the cartography of the RIB, as an initial step towards understanding its genesis and evolution. In order to do so, field and petrographic data, together with airborne gamma-ray spectrometry and magnetometry data, were integrated, resulting in a geologic map that shows different features from the previous maps produced in the area, such as the batholith division into two different petrographic and geophysical units; the presence of a mafic dike intruding the batholith rocks and the reduction in size of the batholith area. The magnetometric and gamma-spectrometric data proved to be important tools for geological mapping, and the substantial data obtained in the analysis of this tiny area points to the importance of re-evaluating older geological maps, taking geophysical data into account.

Resumo em Inglês:

Abstract A paleoenvironmental reconstruction was performed in a Riparian Forest near Campinas to improve knowledge of paleoclimate and paleoenvironment in the State of São Paulo, Brazil. A sediment core of 182 cm depth was collected in a swamp located within a Cerrado/Seasonal Semi-deciduous ecotone forest. The chronological frame is given by eight radiocarbon dating methods. Pollen and stable isotope analyses (δ 13C and δ 15N) were performed all along the core. Modern pollen rain is based on five surface samples collected along the Riparian Forest. Results show a sequence of changes in vegetation and climate between 25 and 13 cal kyr before present (BP), and from 4 cal kyr BP to the present time, with a hiatus between 11 and 4 kyr cal BP. Drier climatic conditions characterized the late Pleistocene and early Holocene, although they had moisture peaks able to maintain an open forest. The Riparian Forest became fully installed from 4 cal kyr BP onward. Our results are in agreement with other regional studies and contribute to build a regional frame for past climatic conditions at the latitude of São Paulo.

Resumo em Inglês:

Abstract The Precambrian basement of Northeastern Brazil is the product of Rhyacian block convergence. The main deformation and intrusion of crustal granitoids occurred between 2.15 and 1.8 Ga ago. This large area has been subjected to long-lasting and rather uniform stresses during the Cambrian period. Gold provinces in Bahia represent metallogenetic products of distinct tectonothermal events. Gold mineralization took place during the Paleoproterozoic collision and the Cambrian convergence, respectively, accompanied by heat flow, crustal deformation, and granite intrusion. The tectonic framework of the region was reconstructed at ca. 700 Ma ago, considering the hypothesis of wrench-fault tectonics. The new hypothesis provides geological evidence and metallogenic constraints that make further investigation necessary, with reference to well-established São Francisco Craton concept and its peripheral fold belts.

Resumo em Inglês:

ABSTRACT In the Southeastern Quadrilátero Ferrífero, a package of metapelitic rocks previously attributed to the Archean Rio das Velhas Supergroup crops out in Piranga locality. This study presents the mineral chemistry and U-Pb-Hf zircon geochronology on foliated staurolite-garnet mica schists. Garnet and staurolite index minerals are syn- to post-kinematic towards the main schistosity. Garnet porphyroblasts display well-developed compositional zoning of Mg-Fe-Mn-Ca, with increase of almandine and pyrope and decrease of spessartine towards the rim, implying in prograde metamorphic pattern. Estimates of P-T values for the metamorphic peak resulted in temperatures between 630 to 650ºC and pressure around 7 kbar. Pseudosections show well-defined stability fields in amphibolite facies, with a metamorphic path displaying progressive increase in P-T conditions. Maximum depositional age of 1,875 ± 51 Ma is established for the Piranga mica schists pointing to a depositional history that is younger than those previously described. Metamorphic Cambrian ages characterize the strong influence of deformational processes related to the final stages of Brasiliano Orogeny in the Southeastern Quadrilátero Ferrífero.

Resumo em Inglês:

Abstract The Iriri-Xingu domain, located in the central part of the Amazonian craton, consists of extensive occurrences of Paleoproterozoic volcanic rocks and granites with published ages of ca. 1990 to 1840 Ma, which show a strong crustal contribution for their magmas. Exposures of basement rocks are small and rare. Samples from two areas were dated in this work by U-Pb SHRIMP in zircon. In the northern Maribel area, a high-grade pelitic paragneiss presented an age of 2160 ± 8 Ma and the leucosome of a migmatitic orthogneiss, probably a diatexite, crystallized at 2149 ± 20 Ma. These Rhyacian ages suggest that it is part of the Bacajá domain, related to the Trans-Amazonian cycle. The Morro Grande area occurs in the central part of Iriri-Xingu domain, where a high-grade muscovite gneiss with a protolith of 2120-2180 Ma is dated at 1982 ± 7 Ma, and the leucosome of a migmatitic orthogneiss is crystallized at 1979 ± 8 Ma. These are the first records of high-grade metamorphism at ca. 1980 Ma in the Central Brazil shield and may be related to the generation of this Orosirian felsic magmatism, which is widespread throughout the nearby Ventuari-Tapajós province.

Resumo em Inglês:

Abstract New petrography, geochemistry and Sm-Nd and Sr data from the orthogranulites of the Juiz de Fora complex in southern Rio de Janeiro State and compiled information provide insights on the petrogenetic and tectonic evolution. The complex comprises several geochemical groups including mafic orthogranulites (three tholeiitic and one alkaline) and felsic orthogranulites (three calc-alkaline and one tholeiitic/low K calc-alkaline). New geochemical and isotope data, combined with available U-Pb data suggest a long evolutionary history from the Paleoproterozoic to the Neoproterozoic. The oldest magmatic episode produced juvenile to slightly contaminated arc-type Rhyacian rocks, as well as granitic rocks related to collision or post-collision episodes. Altogether these rocks integrate part of a dismembered Rhyacian orogen within the Ribeira belt. Few TDM Nd model ages yielded 2.75 to 2.58 Ga, suggesting minor Archean contribution for magma genesis. Some of the basic granulites’ bodies yield Meso- to Neoproterozoic TDM Nd model ages, which may refer to an extensional magmatism. Orthogranulites present granulite facies paragenesis, related to the youngest tectonic episode in the Juiz de Fora Complex (Brasiliano Orogeny). The new data are potentially important for Paleoproterozoic reconstruction models, due to the predominantly juvenile character of the Juiz de Fora complex, as similarly worldwide.

Resumo em Inglês:

Abstract Sedimentologic and stratigraphic studies of paleo-deserts from the Brazilian Cretaceous are of great potential to understand the paleoclimate and paleogeography of Gondwana during its final rifting stage. Herein, we describe and discuss two depositional units characterized by two distinct eolian systems that are bounded by a long-lived unconformity. The Lower Unit (Barremian/Aptian) encompasses a wet eolian system composed of dune, interdunes, and ephemeral alluvial deposits. A continuous paleosol horizon in the upper part of the Lower Unit records dune stabilization and end of eolian accumulation, in a period of climate amelioration possibly in the late Aptian. The stratigraphic gap proposed for the unconformity (Cenomanian to Coniacian) coincides with the Cretaceous Thermal Maximum. The Upper Unit (Santonian?/Campanian) comprises dune fields of a dry eolian system capped by the Mata da Corda volcanic rocks. Cross stratification dip directions from both eolian systems shows transport towards SSW. Paleo-winds coming from the northeast quadrant reveal that the continental breakup and drifting had little influence on the surface winds in this Gondwana sector. The Upper Cretaceous paleo-winds are coherent with global paleo-circulation models, based on a high-pressure cell over the South Atlantic proto-ocean, favoring desertification in the inner portion of the southwestern Gondwana.