Navegando por Orientadores "LAFON, Jean Michel"

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Assinatura isotópica de Pb em sedimentos da margem leste da Baía do Guajará (orla de Belém) e rio Maguari
(Universidade Federal do Pará, 2020-10-05) BARRA, Ingledir Suely Silva; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
Estuarine sediments provide a long-term record of the accumulation of trace metals from riverside, atmospheric and anthropogenic sources. The release of trace metals from point sources is largely controlled by the natural processes of physical and chemical weathering of rocks, in addition to anthropogenic disturbances that can occur on a large scale. In addition to diffuse anthropogenic sources, estuarine sediments can be enriched in trace metals by antifouling paints, with high contents of metals (Cu, Zn, Pb), which are applied to ship hulls and many submerged structures to prevent the growth of fouling organisms such as bacteria, macroalgae, mussels, bivalves, and invertebrates. Pb isotopic signatures are useful tools to investigate the sources and mobility of trace metals in estuarine systems and allow to distinguish between geogenic and anthropogenic source contributions. In recent years, several isotopic Pb studies for this purpose have been conducted in the hydrographic system of Belém. The aim of this work is the application of Pb isotope geochemistry in bottom sediments, suspended particulate matter (MPS), and fragments of vessels with antifouling paint of several shipyards in Guajará Bay and the Maguari River, in the metropolitan area of Belém-PA. It is intended to verify whether the paints used in the shipyards can be considered a point source of contamination by Pb. Bottom sediment samples were collected at 9 stations along the northern and southern waterfront of Belém, at the right margin of the Guajará Bay, and 3 on the Maguari River near shipyards, 6 samples of suspended particulate matter and 4 samples of vessel hull fragments. Samples of bottom sediments at the mouth of the Una Channel (3 samples) and Paracui River (1 sample) were also collected. Finally, a sample of bottom sediments was also sampled on Barra island, in the middle of Guajará Bay, as a reference to the geogenic background. Chemical analyzes of bioavailable Pb were carried out on the bottom sediment samples by inductively coupled plasma optical emission spectrometry (ICP-OES) and Pb isotopic analyzes on all samples by inductively coupled plasma mass spectrometry with magnetic sector and multiple collectors (ICP-MS). In the bottom sediment samples from the shipyards of the Belém waterfront and the Maguari River, important variations of Pb concentrations of the bioavailable fraction occur, from a value similar to the reference point (11 mg kg-1) up to contents of 25 mg kg-1. These variations suggest incipient processes of anthropic action. Even so, the levels are within the range of concentrations found so far and do not show evidence of a significant contribution of Pb linked to the proximity of the shipyards. The similar ranges of bioavailable Pb concentration between the various sectors studied (northern and southern sectors of the waterfront Belém and Maguari River) show that there is no clear pattern of distribution of Pb levels in the sectors studied. These concentrations were shown to be systematically lower than the reference values TEL (35 mg kg-1) and PEL (91.3 mg kg-1), thus indicating that Pb is not causing harmful effects to biota in these areas of the Guajará Bay system. In the northern sector of the waterfront Belém and along the Maguari River, the variations in 206Pb/207Pb isotopic ratios are within the range of the Barra island reference point and the values considered geogenic in previous works (1.189 – 1.197). Only the point identified as “vessel graveyard” presented a lower ratio (206Pb/207Pb= 1.183) and may indicate an increase in Pb anthropogenic contribution due to the deterioration of abandoned vessels and the working shipyard. In the southern sector of the Belém shore, practically all the sediments from the shipyards and the mouth of the Una canal presented lower 206Pb/207Pb isotopic ratios (1.163 and 1.178), indicating an anthropogenic contribution. This diffuse anthropogenic contribution is apparently related to the Una channel and also to the greater concentration of shipyards and other possible sources of metals. In addition, the comparison of the Pb isotopic composition of the sediments at the mouth of the drainage channels in the northern and southern sectors of the Belém waterfront suggests the existence of a relationship between anthropogenic contribution and the size of the drainage basins, population density, and urban activities. No correlation was found between the 206Pb/207Pb isotopic ratios of the bottom sediments and corresponding MPS, however, all points fit a trend line, confirming the mixture between two different sources of Pb for both sediments and suspended material. The isotopic signatures of the hull + paint fragments are positioned along the same trend line established between a geogenic end-member and an anthropogenic end-member (aerosols), for the hydrographic system of Belém, making it impossible to identify a specific anthropogenic source of Pb from the shipyards. The comparison of these signatures of the fragments with the bottom sediments of the shipyards indicates that this possible anthropogenic source enters as one of the subordinate components of the diffuse anthropogenic contribution of Guajará Bay. Finally, based on the data from this study and the works already developed in the hydrographic system of Belém, it is proposed to extend the range of 206Pb/207Pb ratio of the geogenic Pb to 1.189 – 1.204 for the hydrographic system of Belém, as a whole.
Acesso aberto (Open Access)
Caracterização da assinatura isotópica de Pb atual e da concentração de metais pesados em sedimentos de fundo da foz do rio Guamá e da Baía do Guajará (Belém - Pará)
(Universidade Federal do Pará, 2007-11-09) NASCIMENTO, Suziane Magalhães do; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The city of Belem launches a significant amount of domestic pollutants in natura and industrial effluents in the Guama river and the Guajara bay. Previous studies suggest that the bottom sediments deposited in the edges of the river and the bay have its chemical composition influenced by these anthropogenic inputs. The aim of this work is to characterize the Pb isotopic signature, associated to the study of the distribution of the concentration of Pb and other heavy metals (Cu, Cr and Ni) in bottom sediments from the hydrographic system of Belem (Guama river and Guajara bay), in order to evaluate the regional natural variations and to evidence possible anthropogenic contributions from the industrial and urban activities of the city of Belem. Thirty three georeferenced samples of bottom sediments from the Guama river and from the Guajara bay were collected, using a Petersen grab, with capacity to sample the first 10 cm of muddy sediments. Fourteen samples came from the left edge of the Guama river, including the Cumbu and Great islands, until it flow into the bay and nine samples from the right edge, at the city riverside. Nine samples of bottom sediments were also collected from the Guajara bay, beside eight samples from the islands (Jaguar, Jutuba, Jararaca and Mirin) and one from the riverside of Belem for the determination of heavy metal contents and Pb isotopic composition. At the laboratory, samples were dried in an oven (50°C), disaggregated in a agate mortar and sieved down to 63μm, for the separation of the fine fraction (silt + clay). The mineralogical composition of the fine fraction was determined by X-rays diffraction. The total and partial heavy metals concentrations were determined, respectively, by ICP-MS at commercial laboratory (Acme Ltda), and by ICP-OES at the Toxicological Laboratory of the Instituto Evandro Chagas, Belem. The determination of the Pb isotopic compositions was carried out on samples previously leached using HNO3 at the Para-Iso Laboratory of the Instituto de Geociencas - UFPA, and using a mixture HNO3 + HCl at the Geochronological Laboratory of the Centro de Pesquisas Geocronologicas - USP. The Pb separation and purification were carried out by ionic exchange chromatography. The Pb isotopic compositions have been determined by TIMS on a monocollector mass spectrometer VG Isomass 54E with, at the laboratory of UFPA and on a multicollector mass spectrometer VG354 at the laboratory of the USP. The results indicate that the mineralogical composition of the fine fraction of the sediments includes smectite, illite and Kaolinite. Chemical analyses of heavy metals furnished the following total and partial contents for the samples from the Guama river right edge: PbTotal (11-23 mg.Kg-1); PbPartial (9-18 mg.Kg-1); 10 CrTotal (26-69 mg.kg-1); CrPartial (11-23 mg.kg-1); NiTotal (7-29 mg.Kg-1); NiPartial (6-26 mg.Kg-1); CuTotal (7-23 mg.Kg-1); CuPartial (6-17 mg.Kg-1). For the samples from the left edge of the river, the following total and partial concentrations were obtained: PbTotal (16-20 mg.Kg-1), Pbpartial (13–19 mg.Kg-1); CrTotal (34-56 mg.kg-1), Crpartial (16-26 mg.kg-1); NiTotal (12-21 mg.Kg-1), Nipartial (12-16 mg.Kg-1), Cutotal (9-14 mg.Kg-1) and Cupartial (7-12 mg.Kg-1). The sediments from both edges of the Guama river similarly display a slight decrease of heavy metal contents downstream, in the direction of the bay. However, the sediments at south of the left edge islands display an increase of contents in the same direction. Variation in the dynamic of aquatic system of the Guama river by combined effect of currents and tides would probably account for such an opposite behavior. For the sediments from the right edge, significant variations of the metals contents were locally identified and are considered to be due to the confluence of the Aura tributary, which provokes local disturbs in the hydrodynamic of Guama river as well as a redistribution of the metals along the right edge of the river at a local scale. Physical-chemical conditions of the Aura water are probably modified by the effects of the Belem waste deposit, located on the edge of the river, by increasing of the metals solubility that may account for lower heavy metal contents of the bottom sediments from the Aura river up to the confluence with the Guama river. Consequently, the Aura river will not provide any significant input of heavy metals in the Guama river. Correlation diagrams display a good correlation between heavy metals and the organic matter and excellent correlations with Al, Mg and the Fe (r = 0.95-0.99) for the samples of the right edge of the Guama river, indicating that the heavy metals are associated to the organic matter, adsorbed by the clay-minerals (smectite, illite and Kaolinite) and, probably, associated to iron oxy-hydroxides, though they have not identified. Samples of the left edge did not provide any correlation of Pb and Cr with organic matter (r = 0.24 and 0.40), however excellent correlations were identified with Al and Fe (r = 0.94-0.97). Cu and Ni are poorly correlated with organic matter (r = 0.59 and 0.72) while the correlations with Al, Mg and Fe are good (r = 0.74-0.89). Therefore, clayminerals and, possibly, iron oxy-hydroxides prevail for heavy metals contents. The sediments of both edges of the Guama river have, respectively, total concentrations of Pb (23 and 20 mg Kg-1) and Cu (23 and 14 mg Kg-1) lower than the 35 mg Kg-1 (Pb) e 36 mg Kg-1 (Cr) values of TEL (Threshold effect level), established by the NOAA-EPA as a guide of quality of sediments and that defines the level below of which adverse biological effects will rarely occur. Then, sediments of the Guama river, even so they are in contact with domestic pollutants, do not represent any 11 risk, at the moment, for the aquatic ecosystem, at least regarding to Pb and Cu. The total concentrations of Cr (69 and 53 mg Kg-1) and Ni (26 and 18 mg kg-1), for the sediments of the right and left edges, respectively, are higher than or equal to the respective TEL values of 37 and 18 mg kg-1 for these metals and may constitute a risk for aquatic organisms of the Guama river. However, such Cr and Ni concentrations come probably from natural sources without any significant anthropic influences. The homogeneous isotopic signature of the sediments of the Guama river, together with the low Pb concentrations indicate a geogenic origin for this metal and allow to establish a value for the regional "background" of Pb concentration around 18-23 mg Kg-1. They also allow considering an isotopic signature of 1.200-1.194 for the 206Pb/207Pb ratios of Pb from natural sources, reinforcing the values previously proposed by Moura et al. (2004). The lower contents of Pb found in some samples from the right edge of the Guama river (11-13 mg Kg-1), associated to slightly anthropogenic isotopic signature (1.186 < 206Pb/207Pb < 1.193) are interpreted as a result of the influence of the Aura tributary on the sediments of the Guama river. The significant increase of Pb contents in the stream sediments of the Guajara bay is an indicator of possible future biological risks in the hydrographic system of Belem. The well established negative correlation between Pb concentrations (28-46 mg Kg-1) and 206Pb/207Pb isotopic ratios (1.172 < 206Pb/207Pb < 1.188), discards the possibility that the increase of Pb contents in the Guajara bay to be of geogenic origin and that the difference in concentration with the Guama river to be resulted only of distinct hydrodynamic processes.
Acesso aberto (Open Access)
Caracterização litoquímica e geocronologia Rb-Sr das rochas granitóides e ortognaisses da região de Santa Quitéria-Sobral, NW do Ceará
(Universidade Federal do Pará, 1992-12-21) TAVARES JÚNIOR, Stelio Soares.; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
In the SSW of the Sobral-Pedro II Lineament between the cities of Sobral, Forquilha and Santa Quitéria, in the northwest of the State of Ceará, there is a variety of isotropic and foliated granitic bodies, with a predominance of the former. The isotropic granites constitute batholithic masses and bodies of smaller dimensions, intrusive in the rocks of the gneissic-migmatitic complexes, in the set of supracrustal rocks, including those of the Ubajara and Jaibaras groups. The Forquilha orthogneisses, of tonalitic composition with subordinate trondhjemitic and granodiorite terms, constitute one of the individual units within the gneiss-migmatitic complexes. These rocks provided a total rock Rb-Sr age of 1981 ± 45 Ma, which is interpreted as a minimum age for gneissification processes. The similarity of this result with the ages obtained in the region of Granja evidences a significant performance of the Transamazonian Event in the NW of Ceará. The initial isotopic ratio (0.70138 ± 13) of these rocks suggests an evolution from a mantle source or from short-residence crustal protoliths. The Brasiliano granites have a predominantly monzogranitic modal composition, ranging from microclimate granites to granodiorites. They are bodies of alkaline composition, per to metaluminous, with geochemical characteristics that reveal a strong similarity with type A granites, which may represent alkaline granites of intraplate or post-tectonic tectonic environment. These characteristics, including the behavior of REEs, are also similar to those of the Mucambo and Meruoca granites. The Rb-Sr geochronological studies carried out on these granites provided an age of 524 ± 12 Ma and 482 ± 8 Ma for the Pajé and Serra da Barriga granites, respectively. As for the Morrinho Granite, conventional ages between 480 and 510 Ma were obtained. Adding these results to the existing data for the Meruoca and Mucambo granites, an epoch of intense granite genesis is evident in the entire NW region of Ceará, from the late Proterozoic to the Eo-Paleozoic. Foliated granites occur as elongated bodies in the gneissic terrains of the High Grade Belt of Cariré, in agreement with the regional structure. The geochemical characteristics obtained in these bodies show a lot of similarity with those of the isotropic granites set. However, the Rb-Sr age of 475 ± 15 Ma obtained in these granites is smaller than that of the isotropic granites and certainly corresponds to a rejuvenated age. This rejuvenation is probably related to the effects of Graben Jaibaras reactivations. It was not possible to determine the true age of the crystallization of these bodies, as well as the deformational processes that affect them, in a situation very similar to that found for the Chaval Granitoid. The arrangement of the initial ratios of the foliated granites (0.70521 ± 56) and the Pajé Granite (0.70488 ± 71) in relation to the evolution of the Rb/Sr ratios of the Forquilha orthogneisses is compatible with a derivation of the granites from these gneisses. This is not valid for Serra da Barriga Granite, which has a high initial ratio (0.70963 ± 160).
Acesso aberto (Open Access)
Contribuição à evolução geológica da parte oriental da Província Amazônia Central através da geocronologia Rb-Sr da Província Mineral de Carajás e região de São Félix do Xingu - PA
(Universidade Federal do Pará, 1992-09-21) PEREIRA, Ediléa Dutra; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
Acesso aberto (Open Access)
Distribuição de metais pesados e isótopos de Pb em sedimentos de fundo do rio Murucupi-Barcarena-Pará
(Universidade Federal do Pará, 2013-08-29) OLIVEIRA, Diomar Cavalcante; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The preoccupation with environmental contamination by trace metals by urban and industrial activities has led to studies with the purpose of measuring the impacts and incorporation of these substances on aquatic biota. This study aimed to identify by determining the levels of trace metals and Pb isotopic signatures a possible involvement of effluents coming from the urban areas and industrial wastes as sources of pollution of the Murucupi river, Barcarena region, Pará. It is intended to point out the potential of Pb isotopic signatures to detect future impacts of anthropogenic contamination with trace metals. In addition, reference values of natural metal concentrations and Pb isotopic composition were identified for this sector of the Pará river estuary system values. Eighteen samples were collected from the bottom sediment surface along rivers Murucupi (8 samples), Arrozal Channel (6 samples) and Pará river (4 samples) and two drill cores in the Murucupi river. Surface samples and sediment cores were collected with Van Veen dredge type and Russian Peat Borer, respectively. A granulometric analysis was performed with laser granulometer to quantify the percentage of sand, silt and clay. The mineralogical composition was determined by X-ray diffraction. The partial and total concentrations of the metals and Lead isotopic compositions were determined by ICP-MS. The organic matter contents were determined by volumetric redox by the Walkley Black method. Granulometric analyses showed that there was a predominance of silt over sand and clay. Mineralogical analyses performed on the total sample and in the clay fraction showed the presence of quartz, albite muscovite and clay minerals smectite, illite and kaolinite. In all three drainage study, the results indicate that the levels of Pb, Cu, Zn and Ni did not show significant differences and when exist, they may be related to natural variations in the sediment. The correlation matrices indicate that the contents of trace metals are not controlled by the organic matter and that these metals are predominantly associated with structures of clay minerals in Pará river and Arrozal Channel sediments and oxy-hydroxides of Fe and Mn in Murucupi river sediments, respectively. High correlations between the trace metals indicate that the concentrations of metals in the sediments are governed by similar chemical processes in the three drainages, excepting for Pb in the Murucupi river, which is not correlated with others metals and may have been introduced into the environment by different processes. Most of the enrichment factors, using the Pará river sediments as reference value, showed values lower than 1, indicating no evidence of anthropic influence on the trace metals concentrations. Sediments from Arrozal Channel displayed contents of trace metals slightly lower than those of Murucupi river. However, the similarity of their EF values point to natural geochemical variations. The levels of Pb, Cr, Cu, Zn and Ni were lower than TEL reference value for all the trace metals studied. Therefore the sediments of the Murucupi river and Arrozal Channel, although in contact with domestic sewage, do not represent, at the moment, no risk to aquatic organisms. The bottom sediments of Pará river exhibit a homogeneous isotopic signature (206Pb/207Pbmean = 1.204 ± 0.001), considered as geogenic, establishing a background value of 206Pb/207Pb isotopic ratio for the area. Lower values found in the Murucupi river (206Pb/207Pbmean = 1.186 ± 0.003) and Arrozal Channel (206Pb/207Pbmean = 1.193 ± 0.002) are interpreted as reflecting the influence of effluents from the urban areas (Vila dos Cabanos and Laranjal) and from Barcarena river, respectively. Through 206Pb/207Pb vs. 206Pb/204Pb and 208Pb/206Pb vs. 206Pb/207Pbdiagrams, it is possible to discard a contribution of red mud tailings as a source of pollution of the Murucupi river. The decrease in 206Pb/207Pb isotopic ratios from bottom to top in sediments of the Murucupi river corroborate the existence of a recent Pb anthropogenic contribution to the sediments in this river.
Acesso aberto (Open Access)
Distribuição de metais pesados e isótopos de Pb em sedimentos do rio Amapari, setor de Pedra Branca do Amapari – Porto Grande, Amapá
(Universidade Federal do Pará, 2011-03-18) SILVA, Danúbia Tavares da; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
This work consists of a geochemical study of heavy metals and Pb isotopic signature of sediments from the hydrographic basin of the Amapari river, which is the main affluent of the right edge of the Araguari river, in the central region of Amapá State. The study is more precisely located in a part of the river, close to the Serra do Navio area, between the cities of Pedra Branca do Amapari and Porto Grande. The region hosts the larger mining district of Amapá, including two important gold and iron mines at the Serra do Navio. The aim of this work is to investigate the distribution of heavy metals and other metals like Al, Fe, Th and U, together with the determination of Pb isotopic composition of recent sediments that are sediments deposited at the superior edge of the river during inundation events (overbank sediments), sampled along the Amapari river and tributaries, in order to distinguish the natural contributions from the main geological units and a possible contribution of mining activities. The sampling strategy depended on two factors: (1) the occurrence of adequate outcrops and (2) the accessibility. The sediments, collected at 25 sampling points in November, 2007, include a set of samples from the Amapari river and three sets of samples collected in tributary of the right edge (Cachorrinho river and Cupixi river) and left edge (Cupixizinho river). In all the samples, granulometric, mineralogical, chemical and isotopic analyses were performed. The granulometric analyses were effectuated in order to determine the proportions of fine material (silt + clay) and clay. The mineralogical composition was determined by Xrays Diffractometry using a X-rays diffractometer with cobalt anode tube. The determination of major end trace elements contents has been carried out at a commercial laboratory (ACME Analytic Laboratory) by ICP-EOS and ICP–MS after four acids dissolution (HF + HCl + HClO4 + HNO3) in 8 whole samples and 22 samples of the fine fraction of the sediments. The elements of major interest were: Al, Fe, Pb, Mn, Cu, Zn, Ni, As, Cd, Cr, Th e U. For the isotopic analyses, the samples suffered total digestion with HF and HNO3+HCl (20 samples) and acid leaching with HNO3 (5 samples). The Pb separation and purification were achieved by ionic exchange chromatography (1x8 DOWEX resin). The Pb isotopic compositions were determined in static mode with a multi-collector thermal ionisation mass spectrometer Finnigan MAT262 at the Pará-Iso Laboratory of the IG-UFPA. The geochemical results indicate that all heavy metals behaved similarly and are enriched in the same samples, indicating that the sediments suffer the same processes of enrichment for all investigated heavy metals, excepting cadmium that displayed a different trend. No relationship between granulometry and heavy metals content was pointed out. Thus, the proportion of clays presents in the sediments does not appear to have been preponderant for heavy metals concentrations, at the contrary of that commonly occur in bottom sediments. Not significant variation of mineralogical composition has been observed between the fine fraction and the fraction superior to 62μm that account for the similarity of the chemical compositions between those fractions. In both granulometric fractions, the same main paragenesis composed by quartz, kaolinite, gibbsite and muscovite was identified. Other minerals were recognized in some of the samples (anatase, rutile, microline, etc.), which probably belong to the primary paragenesis. The presence of these minerals does not have influence on the heavy metals concentrations. The heavy metals contents furnished a better correlation with Fe than with Al contents although Fe-minerals were not been identified in the samples. The Pb isotopic compositions displayed significant variations and linear rend that permitted to construct isochrons in the 207Pb/204Pb vs. 206Pb/204Pb diagram, which defined Paleoproterozoic ages around 2,0 Ga, (Amapari river: 1964 ± 88 Ma, mswd = 1,6), indicating that the sediments came essentially from the Paleoproterozoic supracrustal units (metassedimentary and mafic rocks) and associated granitoids. Despite its extensive exposition in the area, the Archean basement does not significantly contribute as a source of the sediments. The concentrations of metals display strong variations according to the geographical location but they are randomly distributed along the Amapari river, indicating that homogenization does not occurred and that the sediments reflect the difference in the source rocks. Therefore the processes which account for that geochemical distribution are probably naturals as no was evidenced no correlation has been evidenced between metal contents and sample locations in relation the mining areas. The comparison between metals contents of the Amapari river and tributaries and those determinate by others authors in bottom sediments of a creek close to the Fe and Au mining plants of the Serra do Navio – Pedra Branca do Amapari confirms that hypothesis.
Acesso aberto (Open Access)
Distribuição e registro histórico de metais pesados e assinaturas isotópicas de PB em testemunhos de sedimentos de fundo da Baia do Guajará, Belém-PA
(Universidade Federal do Pará, 2011-08-29) CARVALHO, Jully Hellen dos Santos; CORRÊA, José Augusto Martins; http://lattes.cnpq.br/6527800269860568; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
tuaries constitute the final destination of a significant part of dissolved and particulate materials that are brought by the rivers toward the ocean. These estuaries also act as the final destination for domestic and industrial effluent, which are most of time discarded in natura. A variety of factors and sources together with the complexity of hydrodynamics and physicochemical conditions of this kind of environment makes the study of metals pollution of estuaries a challenge. The analysis of Pb isotopes is a valuable tool to differentiate anthropogenic from geogenic sources, since the lead dispersed in the environment keeps the isotopic characteristics of the source from which it was derived, once the isotopic compositions of Pb are not affected by physical or chemical processes. The hydrographic system of Belem (PA) belongs to the Guajara estuary. In the eastern margin, is located the Belém city. The west margin is an islander set composed 39 islands. About 30 km of natural drainages divide the city, forming channels that drain into, above all, the Guajara Bay which constitutes the main water reservoir receiving pollutants produced by domestic and economic activities of the city. However, the distribution of pollutants released daily in the bay is difficult to be evaluated, due to the hydrodynamic complexity of the estuary. In this context, the purpose of this study was to combine the determination of metal concentrations with Pb isotope signature to establish a historical record and to distinguish the sources of Pb, whether natural or anthropogenic, in sediments on the west edge of Guajará Bay. Samples were collected using a Russian Peat Borer. A set of seven cores of about 50 cm length was collected and subsequently sliced from 10 to 10 cm, totalizing 31 samples. According to the speed of deposition of approximately 0.7 cm/year, previously determined for the sediments of Guajara bay with geochronology 210Pb, cores collected correspond to a historical record of approximately 70 years. The study included grain size study by centrifugation method, mineralogical identification by X-ray diffraction, geochemical analysis to determine the percentage of organic matter and heavy metal contents by ICP-MS spectrometry and isotopic mass spectrometer thermoionization (TIMS). The grain size and mineralogical study of core in bottom sediments from the west edge of the Guajará bay showed a changing of hydrodynamic regime, which became more energetic, throughout the bay over time. This statement was based on the characterization of sediments, which show predominance of silt and sand fraction with a reduction of the sand fraction with the depth and texture that ranged from sandy to sandy-silty. The mineralogical study no showed change in mineralogical composition of the clays, and kaolinite, illite and smectite are claysminerals predominant. Over time the concentrations of metals vary little, however, have detected a slight tendency to decline in more recent times, probably linked to changes in the hydrodynamic regime. Moreover, there is no evidence in the core of a significant a significant increase in metal concentrations (mean 25.3 to 29.1 mg.Kg-1) with the population increase over the last 70 years. These results indicate that the background of Pb in the bay is significantly higher than in rio Guama (18 mg.Kg-1). The isotopic composition of source 206Pb/207Pb geogenic over the cores ranged between 1.186 and 1.199. The average isotopic signature of 1.193 ± 0.0035 was established as the value of the background to the Guajará bay, this value lower than the average value of 1,196 - 1.20 previously determined for the isotope ratio 206Pb/ 207Pb of Pb geogenic in Belém region.The differences in content and isotopic composition of Pb between this study carried out by Saraiva (2007) and Nascimento (2007) on the west edge of the bay, were explained by the difference of the technique used for sampling. This difference indicated that, despite the population growth accelerated during the last decades the contribution of heavy metals by anthropic action is recent, intensifying over the last 7 years.
Acesso aberto (Open Access)
Evolução crustal do setor Sul da Província Borborema: Domínio Macururé, sistema Orogênico Sergipano, nordeste do Brasil
(Universidade Federal do Pará, 2023-09-29) PEREIRA, Fábio dos Santos; ROSA, Maria de Lourdes da Silva; http://lattes.cnpq.br/9814085660417748; https://orcid.org/0000-0002-5099-829X; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645; https://orcid.org/0000-0003-0854-3053
The Sergipano Orogenic System (SOS), located in the southern Borborema Province, is part of a large Brasiliano/Pan-African orogen structured during the Gondwana supercontinent assembly, which spread out to central Africa up to Cameroon. Six domains compose the SOS, namely Estancia, Vaza Barris, Macurure, Maranco, Poco Redondo, and Caninde. The Macurure Domain stretches 350 km from the Sergipe coast to Macurure City, Bahia state. It consists of a succession of quartzites overlaying by a thick package of phyllites, schists, metarhytmites, and metagraywacke, with intercalations of amphibolite and calc-silicate rocks. The rocks were metamorphosed under greenschist-amphibolite facies and intruded by numerous late Cryogenian-Ediacaran granites and mafic plutons. In this work, new geochemical, U-Pb geochronological (zircon and titanite), and isotopic (whole-rock Rb-Sr, Sm-Nd, and Pb-Pb and in situ zircon Lu-Hf) results from the main units composing the Macurure Domain allowed to refine the local stratigraphy and improve the comprehension about sources and processes involved in the generation of the magmatic rocks. U-Pb isotope analyses on detrital zircon from quartzites yielded two main peaks in the ranges of 1980-1950 Ma and 1000-910 Ma, consistent with the Rhyacian Orogeny (2.2-1.9 Ga) e Cariris Velhos event (1.0-0.9 Ga). Paleoproterozoic dates overlap those observed in the Provincia Borborema basement, represented in the study area by the Jirau do Ponciano Dome and Arapiraca Complex. Negative to slightly positive εHf(t) values between -15.6 and +0.5 and Hf-TDMC model ages from 2.5 to 3.5 Ga characterize this population, suggesting extensive reworking of the Archean crust during the Rhyacian-Orosirian periods. Early Neoproteozoic zircon grains display sub- to superchondritic Hf isotope composition (-12.3 to +7.7) and Hf-TDMC model ages from 2.5 to 1.3 Ga, implying the addition of juvenile material and reworking of preexistent crusts during the Cariris Velhos event. Less abundant Mesoproterozoic (1120-1040 Ma) and late Neoproterozoic (880-740 Ma) zircon populations suggest that the sediment deposition took place before the Brasiliano Orogeny. The groups of magmatic rocks were individualized based on field, petrographic, and geochemical features. (i) The oldest plutonic rocks in the area are diorites and gabbros, with minor cumulate hornblendites. They generally show evidence of solid-state deformation and well-developed tectonic foliation, defined by the plagioclase, hornblende, and biotite orientations. These features point to emplacement in a pre- to early-collisional stage between 643 and 628 Ma. Geochemical data reveal a magnesian nature and affinities with the high-K calc-alkaline and shoshonite magmatic series. REE and multielement spectra exhibit enrichment in light REE and large ion lithophile elements, with deep troughs at Ti-Nb-Ta, typically interpreted as a subduction setting signature. Whole-rock isotope data indicate an evolved signature with subchondritic Nd (εNd(t) = -2.0 a -5.2) e radiogenic Sr (87Sr/86Sr(t) = 0.708-0.710), and Pb (206Pb/204Pb = 18.50-19.18; 207Pb/204Pb = 15.69-15.77; 208Pb/204Pb = 38.54-40.04), indicating derivation from an enriched mantle source. High 87Sr/86Sr(t) and Rb/Sr ratios, together with low Sr/Th and Ba/Rb ratios, suggest that the enrichment episode occurred in response to the introduction of sedimentary components in the mantle via subduction processes, leading to the formation of phlogopite as the main metasomatic phase. Hf-TDMC model ages between 2.47 and 2.09 Ga suggest that the enrichment of the lithospheric mantle beneath the southern Borborema Province occurred during the accretionary events of the Rhyacian Orogeny. Incorporating sediments by the mantle peridotites may have led to increasing Rb/Sr, (U-Th)/Pb coupled with decreasing Sm/Nd and Lu/Hf ratios, which resulted in the time-integrated crustal signature observed in the mafic rocks. (ii) Biotite- and muscovite-bearing leucocratic granodiorites, monzogranites, and syenogranites outcrop as stocks and sheets. They show magmatic foliation marked by the orientation of micas and surmicaceous enclaves, parallel to the schistosity of country rocks, indicating synchronous emplacement to the collisional event between 630 and 624 Ma. Leucogranites are metaluminous to strongly peraluminous, high-K calc-alkaline, and magnesian to ferroan. εNd(t) values and Nd-TDM model ages overlap those of their host5 rocks, suggesting derivation from mostly sedimentary protholiths. (iii) The younger group consists of monzonites, quartz-monzonites, granodiorites, and granites, with crystallization ages from 625 to 603 Ma. They are predominantly isotropic and truncate the regional foliation of the country rocks, indicating a late emplacement relative to the collisional event. The metaluminous and magnesian signatures, with high-K calc-alkaline and shoshonite filiations, resemble the composition of experimental liquids obtained by partial melting of basalt protoliths moderately enriched in incompatible elements. Lu-Hf isotope data provided subchondritic εHf(t) values between -8.3 and 4.0 with Hf-TDMC model ages from 1.77 to 2.03, indicating the reworking of ancient continental crust, possibly related to the Cariris Velhos event. The geochemical and isotopic data of this study, together with those available in the literature allows us to infer that the Neoproterozoic geodynamic evolution of the SOS along the western Gondwana margin can be explained by a lithospheric extension of the Borborema Province basement, followed by basin inversion, and continental collision.
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Geocronologia 207Pb/206Pb, Sm-Nd, U-Th-Pb E 40Ar-39Ar do segmento sudeste do Escudo das Guianas: evolução crustal e termocronologia do evento transamazônico
(Universidade Federal do Pará, 2006-07-06) ROSA-COSTA, Lúcia Travassos da; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The southeastern portion of the Guiana Shield is part of a large Paleoproterozoic orogenic belt, with evolution related to the Transamazonian Orogenic Cycle (2.26 – 1.95 Ga). In this area, previous works defined distinct tectonic domains, named Jari, Carecuru and Paru, which present outstanding differences in terms of age, lithological content, structural pattern and geophysical signature. The Jari Domain is constituted of a granulite-gneiss-migmatite basement assemblage derived from Archean protoliths, and the Carecuru Domain is composed mainly of calc-alkaline rocks and metavolcano-sedimentary sequences, developed during the Transamazonian Event. The Paru Domain is an oval-shaped granulitic nucleous, located within the Carecuru Domain, formed by granulitic gneisses with Archean precursors and Paleoproterozoic charnockitic plutons. In this study, distinct geochonological methods were employed in rocks from the distinct domains, in order to define their tectonic meaning and crustal evolution processes during Archean and Paleoproterozoic times. Pb-evaporation on zircon and Sm-Nd on whole rock dating were provided on magmatic and metamorphic units from the Jari Domain, defining its long-lived evolution, marked by several stages of crustal accretion and crustal reworking. Magmatic activity occurred mainly at the Meso-Neoarchean transition (2.80-2.79 Ga) and during the Neoarchean (2.66-2.60 Ga). The main period of crust formation occurred during a protracted episode at the end of Paleoarchean and along the whole Mesoarchean (3.26-2.83 Ga). Conversely, crustal reworking processes have dominated in Neoarchean times. During the Transamazonian Event, the main geodynamic processes were related to reworking of older Archean crust, with minor juvenile accretion at about 2.3 Ga, during an early orogenic phase. Transamazonian magmatism consisted of syn- to late-orogenic granitic pulses, which were dated between 2.22 and 2.03 Ga. Most of the εNd values and TDM model ages (2.52-2.45 Ga) indicate an origin of the Paleoproterozoic granites by mixing of juvenile Paleoproterozoic magmas with Archean components. The new geochronological results, added to data from previous studies, revealed that the Jari Domain represents the southwestern part of the most expressive Archean continental landmass of the Guiana Shield, here defined and named Amapá Block. The recognition of an extended Archean block precludes previous statements that the Archean in the southeast of the Guiana Shield, was restricted to isolated remnants or inliers within Paleoproterozoic terrains. In the Carecuru Domain the widespread calc-alkaline magmatism occurred at 2.19-2.18 Ga and at 2.15-2.14 Ga, and granitic magmatism was dated at 2.10 Ga. Crustal accretion was recognized at about 2.28 Ga, in agreement with the predominantly Rhyacian crust-forming pattern of the Guiana Shield. Nevertheless, TDM model ages (2.50-2.38 Ga), preferentially interpreted as mixed ages, and εNd < 0, point to some participation of Archean components in the source of the Paleoproterozoic rocks. The lithological association and the available isotopic data registered in the Carecuru Domain, suggests a geodynamic evolution model based on the development of a magmatic arc system during the Transamazonian Orogenic Cycle, which was accreted to the southwest border of the Archean Amapá Block. In the Paru Domain, Neoarchean magmatism at about 2.60 Ga was produced by reworking of Mesoarchean crust, as registered in the Amapá Block. Crustal accretion events and calc-alkaline magmatism were recognized at 2.32 Ga and at 2.15 Ga, respectively, as well as charnockitic magmatism at 2.07 Ga. U-Th-Pb chemical ages in monazites from high-grade rocks of the southwestern part of Amapá Block, dated two main tectono-thermal events. The first one was revealed by the monazite ages of about 2.09 Ga and marks the age of the granulite-facies metamorphism. These data, added to petro-structural information, indicate that the granulite-facies metamorphism was contemporaneous to the development of a thrusting system associated to the collisional stage of the Transamazonian Orogeny. The later event was testified by monazite ages at about 2.06 Ga and 2.04 Ga, and is consistent with a late-orogenic stage marked by granitic emplacement and coeval migmatization of the Archean basement along strike-slip zones. Finally, 40Ar/39Ar geochronological study on amphibole and biotite from representative units of the Amapá Block and of the Carecuru Domain delineated contrasting cooling and exhumation stories. In the former amphibole vary from 2.13 to 2.09 Ga, and biotite ages range mainly between 2.10 and 2.05 Ga. In the later, amphibole and biotite ages are between 2.16 and 2.06 Ga, and 1.97 and 1.85 Ga, respectively. In the Amapá Block, fast cooling rates around 67 °C/m.y. and 40 °C/m.y indicate a tectonically controlled exhumation, related to collisional stages of the Transamazonian Event. Conversely, in the Carecuru Domain, regional cooling rates in the order of 3-2.3 °C/m.y. suggest slow cooling and gradual uplift, which is consistent with the magmatic arc model, where continental growth results mainly from lateral magmatic accretion, precluding significant tectonic crustal thickening.
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Geocronologia das rochas ígneas e mineralizações auríferas associadas da porção Centro Sul do Estado de Tocantins (Região de Porto Nacional – Monte do Carmo)
(Universidade Federal do Pará, 1994-05-10) BARRADAS, João Augusto da Silva; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The studied area is located along the BR-010 Highway (Belém-Brasília) between the cities of Açailândia (MA) and Ligação do Pará (PA), in the southern part of the Bauxiti-bearing Province of Paragominas and has an extension of about 115 km. In it, two surfaces were distinguished, each one supported by a specific alteritic-sedimentary sequence. The highest surface corresponds to the tops of vast residual plateaus from 360 to 220 m in altitude and presents a slight slope to the north. The other surface, embedded in the first, culminates between 210 m, to the south, and 150 m, to the north, and shows a slight swell. The old lateritic-bauxitic cover supports the higher surface is constituted by a saprolitic horizon originating from the alteration of clayey or archosean sandstones and claystones from the “Itapecurú Deposits” of Cretaceous. On the saprolite rests a powerful armor, essentially ferruginous in the extreme south, which, towards the north, is progressively enriched in gibbsite. The first signs of bauxitic levels, still diffuse, are found about 40 km north of Açailândia. This trend is confirmed to the north with the presence of increasingly clear and consistent bauxitic horizons, above and at the base of the ferruginous horizon. Superimposed on this cover and in extremely sharp contact, there is a thick clay capping, the Belterra Clay. The lateritic-bauxitic cover developed in three phases. During the first phase, the ferruginous armor was formed through the ferruginization of the Itapecurú sediments, during the Lower Tertiary. Afterwards, the armor was affected by partial degradation and physical reworking of its products. Remains of older alterites may have also been involved in this process that resulted in deposition of nodular to pseudopisolithic gravel and sandy clay. The third phase, essentially chemical, consisted of the generation of bauxite in areas that presented more favorable climatic and geomorphological conditions. This event probably occurred at the end of the Eocene. The exclusively ferruginous lateritic armor that supports the lower surface has as substrate a reddish clayey sandstone containing small lateritic fragments and quartz pebbles. The cuirass itself has a massive, columnar or nodular structure, and the degraded stone-layer can be found in several places. Its cap is formed by a yellowish clayey-sandy material, rich in quartz grains and disseminated letheritic fragments. This laterite probably formed in the late Pliocene – early Pleistocene. Its substrate can be correlated with the Barreiras Group, while its capping would correspond to the unit called Post-Barreiras further north, in the Ipixuna-Aurora region. The stone-layers quite frequent in the area are located in the same stratigraphic position as the most recent lateritic armor and resulted from in situ degradation of this armor, without significant physical rework. However, locally, colluvial deposits of the glacis type or pediments composed of fragments of ancient armor were identified. The following stages characterized the evolution of the two alteritic-sedimentary sequences: 1- formation of a ferruginous armor from the Itapecurú Deposits; 2- partial degradation of the armor; 3- Bauxitization in the Paleogene; 4- deposition of sediments that later change to Belterra clay; 5- major erosive phase and individualization of vast plateaus; 6- filling of the valleys and lowered areas separating the plateaus by sandy-clay sediments correlated with the Barreiras Group and degradation products of the old armor; 7- ferruginization of the sediments from step 6; 8- more or less accentuated degradation of the ferruginous crust from stage 7, giving rise to stone-layers; 9- deposition of clayey-sandy sediments correlated with the Post-Barreiras on the armoring of stage 7 and the stone-layers; 10- dissection and establishment of the current drainage network. Climatic variations and periodic tectonic (epirogenetic) reactivations throughout the Cenozoic were the main factors responsible for this evolution.
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Geocronologia Pb-Pb e Pb-Sr das rochas graníticas da região de Redenção, sudeste do estado do Pará
(Universidade Federal do Pará, 1996-01-05) BARBOSA, Alayde Alfaia; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
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Geocronologia Pb-Pb em zircão e Sm-Nd rocha total da porção centro-norte do Estado do Amapá-Brasil: implicações para a evolução geodinâmica do setor oriental do Escudo das Guianas
(Universidade Federal do Pará, 2002-09-13) AVELAR, Valter Gama de; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The Guyana Shield is an extensive Paleoproterozoic domain whose main evolution is related to the Transamazonian orogenic event (2.2-1.9 Ga). However, registrations of on Archean history were obtained in metamorphic and igneous rocks of the Imataca Complex in Venezuela (>3.0 G a). The R b-Sr and S m-Nd ages, obtained for g ranulitic and o rthogneissic r ocks o f t he central area of the Amapá State (2.45 Ga and 3.0 Ga), are other evidences of the presence of Archean relics in that shield. The eastern Guyana Shield includes the Amapá State, in Brazil and French Guyana. This portion of the shield belongs to the Maroni-Itacaiúnas Province, considered a Paleoproterozoic mobile belt added to an Archean block (Central Amazonian Province), between 2.20 and 1.95 Ga. Recent works provide a model of the Transamazonian geodynamical evolution between 2.20 and 2.08 Ga for this part of the Guyana Shield. A first period is related to early- to middle-Transamazonian crustal growthing by magmatic accretion (2.20-2.13 Ga) and a second one consists of crustal recycling (2.10-2.08 Ga). The main geological units found in Amapá consist of Achean tonalitic orthogneisses, migmatites and granulites (3.1-2.6 Ga), Paleoproterozoic greenstones belts (2.26 Ga) and, predominantly, Transamazonian granitoids and orthogneisses, of calc-alkaline to syenogranitic composition. In the northern area, an age of 2.15 Ga was defined for a tonalite, while in the central region, migmatitic rocks are associated to a potassic magmatism which happened at 2.06 Ga. Felsic (1.76 Ga) and alkaline (1.68 Ga) post-Transamazonian intrusions have also been recognized in Amapá. In this work a set of 41 isotopic data was obtained by Pb-Pb on zircon (18) and Sm-Nd on whole rocks (23) methods for 25 samples of orthogneiss rocks, metassedimentary rocks and granitoids from central and north Amapá. These data permitted to bring new chronological references for some key units of Amapá and to establish a chronology of the thermo-tectonic events during the Transamazonian orogeny. The data also allowed to investigate the nature and extension of reworked Archean crust and newly accreted Paleoproterozoic crust in that part of the shield. In central Amapá, in the vicinity of Tartarugal Grande city, zircon crystals of felsic granulites yielded a Pb-Pb age around 2.6 Ga. Still in that area, Pb-Pb zircon age of 2053 ± 1 Ma was obtained for a charnockitic pluton. In the surroundings of Cupixi village, zircon crystals from a tonalitic gneiss defined an age of 2849 + 6 Ma, while ages ranging from 2.13 to 2.07 Ga was defined by the zircons of an associated granitic mobilized. Zircon crystals from a monzogranite gave a crystallization age of 2055 ± 6 Ma and ages up to 2.56 Ga for an inherited component. The Nd T(DM) ages for ali these rocks ranged between 2.70 Ga and 3.29 Ga. In northern Amapá, severa' syenogranites provided crystallization ages of 2107 + 2 Ma, 2098 ± 2 Ma and 2087 ± 3 Ma. However, for one syenogranite and an alkali-feldspar gravite the Pb-Pb zircon ages defined an interval of 2.13-2.05 Ga and 2.10-1.95 Ga, respectively. The latter grafite also presented zircons with an inherited component of 2.60-2.54 Ga. Zircons from a diorite, defined a Pb-Pb crystallization age of 2181 ± 2 Ma. The Nd T(DM) model ages for that group of rocks spread in the interval of 2.75 Ga to 2.18 Ga. At the border area with French Guyana, along the Oyapock river zircons of a syenogranite and of a gabbroic intrusion yielded crystallization ages of 2096 ± 2 Ma and 2099 ± 1 Ma, respectively. Pb-Pb data on zircons from a quartzite, associate to the Paramacá Group, gave ages between 3.19-2.77 Ga, for the sources of the sediments. Two main magmatic episodes were identified by the Pb-Pb zircon data. A calk-alkaline one (dioritic and tonalitic), early- to middle-Transamazonian between 2.18-2.14 Ga, is associated to magmatic accretion. Another alkaline-potassic magmatic episode, among 2.11-2.09 Ga, which prevails in northern Amapá, is characterized by transcurrent tectonics and crustal anatetic processes. The emplacement of a charnockitic pluton at 2.05 Ga, in the central Amapá, suggests a late-Transamazonian age for the high-grade metamorphism identified, in this same area, in granolithic rocks with Archean protolith (2.6 Ga). This high-grade event is related to the late-Transamazonian (2.07-2.06 Ga) UHT (ultra high temperature) event identified in Surinam. The post-orogenic regional cooling was registered by the K-Ar, Ar-Ar and Rb-Sr methods on minerais between 2.05-1.80 Ga. In central and northern Amapá, the Nd T(DM) model ages and Pb-Pb zircon ages indicate a main period of mantle-crust differentiation during Middle-archean, among 3.0-2.9 Ga, with possible relics of crust of up to 3.29 Ga. Two magmatic episodes were recognized, one at around 2.85-2.79 Ga, defined by the tonalitic gneisses of Cupixi, and the other at around 2.62-2.58 Ga, constituted by the igneous precursors of the Tartaruga) Grande granulites. These results confirm the presence of preserved Archean nuclei, with similar age to those of the Archean crust of the Carajás Province. However, for the latter area there is not an outstanding registration of a Neoarchean episode, among 2.62-2.58 Ga, suggesting that the Carajás Province behaved as a stabilized area, while the Archean crustal segment of the southeast of the Guyana Shield was reactivated at the end of Neoarchean. In the northern Amapá and at the border with French Guyana witness of an Archean crust are only registered in detrital zircons (3.19-2.77 Ga) of metassediments and as inherited zircons in Pelaoproterozoic granitoids and orthogneisses (2.6 Ga to 2.9 Ga). The Nd T(DM) rnodel ages among 2.75-2.40 Ga of the Paloproterozoic rocks (2.18-2.05 Ga), indicate a mixture between a revvorked Archean crust and a Paleoproterozoic juvenile crust in the source of these rocks. The Pb-Pb data and Sm-Nd ages obtained in this work coníĩrm a Transamazonian evolution for the Central and northern Amapá, similar to that of the French Guyana, in the period between 2.20-2.08 Ga. However, the geological evolution of Amapá differs from the evolution of French Guyana by the presence of reworked Archean crust and by the existence of a late- Transamazonian high-grade magmatic-metamorphic event. Three domains were recognized in southeast Guyana Shield. A northemmost domain, in French Guyana, displays simatic juvenile characteristics. The southemmost domain, in central Amapá, possesses ensialic characteristics, being fonned by midle- to neoarchean nuclei, reworked during Transamazonian orogeny. A transitional domain between those two domains has been identified in the north portion of Amapá. In French Guyana the limit between the transitional and simatic domains is probably WNW-ESE oriented, while the limit between the transitional and the Archean reworked domain is located nearby the at north of granolithic complex of the Tartarugal Grande region.
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Geocronologia Pb-Pb por evaporação em monocristal de zircão do magmatismo da região de Tucumã, SE do estado do Pará, Amazônia Oriental
(Universidade Federal do Pará, 1996-10-16) AVELAR, Valter Gama de; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The region of Tucumã, in the southeast of the State of Pará (Eastern Amazon-Brazil), constitutes a continuity to the southwest of the Mineral Province of Carajás-PMC. The Archean compartmentalization of this region is represented by a preserved granite-greenstone terrain (TGG), including Archean granitoids (Granodiorite Rio Maria) and supracrustal sequences (Tucumã Group), and by the Itacaiúnas Shear Belt (CCI) that includes granodiorite and tonalitics (Xingu Complex), granulites (Pium Complex), and stratoid granitoids (Plaquê Suite). In the Proterozoic, the region is marked by the intrusion of several anorogenic bodies (Seringa and Velho Guilherme Granites), which cut through all Archean units. The geochronological results, presented here for Archean and Proterozoic rocks from the Tucumã region, were obtained through the Pb/Pb method by evaporation in zircon microcrystals separated from saprolite and fresh rocks. The zircons from a granodiorite orthogneiss from the Xingu Complex provided a Pb/Pb age of 2972 ± 16 Ma (2σ), while the zircons from the Granodiorite Rio Maria presented an age of 2850 ± 17 Ma (2σ). The zircons of the Plaque Suite Stratoid Granitoid defined a Pb/Pb age of 2729 ± 29 Ma (2σ). Among the anorogenic granite bodies, only the zircons from the Granite Seringa allowed the determination of Pb/Pb defined around 1893 ± 15 Ma (2σ). The zircons of Granito Velho Guilherme did not reveal sufficient amounts of Pb to allow the calculation of the age of this rock. The Pb/Pb ages by evaporation obtained in this work are similar to the ages given by the U/Pb method in zircons of correlated rocks in the PMC, particularly in the Rio Maria region, and are interpreted in terms of crystallization ages. The age obtained for the Plaque Suite Stratoid Granitoid rock (2972 ± 29 Ma) should be understood as a minimum age for crystallization of this rock, given the absence of other U/Pb geochronological information in this type of rocks throughout the PMC. The Rb/Sr ages between 2.57 Ga and 2.74 Ga, previously obtained respectively for the gneiss rocks (Xingu Complex) and for the Archean granites of the TGG, as well as the Rb/Sr age of 1.75 Ga obtained for the Granite Syringe, should be considered as rejuvenated ages. The geochronological data presented here indicate the existence of at least 4 distinct magmatic events for the Tucumã region, confirming that this portion of the PMC had a similar evolution to the granite-greenstone terrains of the Rio Maria region, where the same events were recognized. The Pb/Pb age by evaporation obtained for the granodiorite orthogneiss of the Xingu Complex at 2972 ± 16 Ma, represents the oldest data found so far for the arquane rocks of Tucumã. The Pb/Pb age around 2850 ± 17 Ma of the Rio Maria Granodiorite confirms the age of the TTG magmatism associated with the greenstone-belts occurring at about 2.85-2.87 Ga, in the south-central portion of the PMC. The 2729 ± 29 Ma age of the Plaque Suite Stratoid Granitoid provides an age for the structuring of the Itacaiúnas shear belt. The age of 1893 ± 15 Ma obtained for the Seringa Granite confirms that the formation of anorogenic granites in the PMC occurred in a short period of time during the Lower Proterozoic (1.87-1.89 Ga). The similarities of the 207Pb/206Pb ages provided by the zircons of saprolitic material and fresh rock from the same rocks demonstrated the effectiveness of the use of saprolite for the geochronological study of Pb/Pb in zircons. Finally, the Pb/Pb ages in zircons of Archean and Proterozoic rocks from the Tucumã region obtained in this work demonstrate the potential of the method for determining the age of crystallization of ancient rocks in the Amazon region.
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Geocronologia U-Pb e geoquímica isotópica Nd-Hf dos granitoides transamazônicos do domínio Lourenço, sudeste do Escudo das Guianas.
(Universidade Federal do Pará, 2019-05-16) VIANNA, Sâmia Queiroz; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The eastern part of the Guyana Shield, northern portion of the Amazonian Craton, is located in the Maroni-Itacaiúnas Province (PMI), defined as a paleoproterozoic mobile belt added to an Archaean block during the Transamazonian event (2.26-1.95 Ga). In the state of Amapá, Northern Brazil, the Paleoproterozoic Lourenço Domain consists mainly of Rhyacian gneisses, granitoids and greenstone sequences, with some fragments of Archean rocks in its southernmost portion. To the south, the Amapá Block is defined as a continental landmass formed by Neo-Mesoarchean units (2.85-2.60 Ga) strongly reworked during the Transamazonian orogenic cycle, and by Rhyacian granitoids and supracrustal sequences. The region of Tartarugalzinho, in central-eastern sector of the state of Amapá, is located at the transition between the Paleoproterozoic Lourenço Domain and the Archean Amapá Block. Several granitoid suites were identified in this region (Flexal Intrusive Suite, Papa-Vento Tonalite and Vila Bom Jesus Granite). We performed in situ U-Pb and Lu-Hf isotope analyses on zircon byplasma induced mass spectrometry with laser ablation (LA-ICP-MS), Sm-Nd wholerock by thermal ionization mass spectrometry (TIMS), together with petrography and major and trace element geochemistry, in order to constrain the evolutionary stages of the Transamazonian orogenesis, in which these granitoids are inserted. In addition, the study aimed to contribute to insert the southern portion of the Lourenço Domain within a geodynamic models for the Southeastern Guiana Shield (SGS) and to investigate the processes of crustal growth and reworking during the Rhyacian. The U-Pb geochronology furnished cristalization ages of 2176 ± 9 Ma, 2176 ± 5 Ma e 2166 ± 15 Ma for the Flexal Intrusive Suite, 2131 ± 11 Ma for the Papa-Vento Tonalite and 2085 ± 16 Ma and 2078 ± 8 Ma for the Vila Bom Jesus Granite, that achieved the identification of the two magmatic episodes (~2.18-2.13 and ~2.08 Ga) during he Transamazonian orogeny for this sector of the Lourenço Domain. The Archean Nd-TDM (2.87-2.63 Ga) and Hf-TDM C (3.63-2.79) model ages and the negative values of εNd(t) (-2.74 to -5.43) and εHf(t) (-1.40 to -15.65) indicate the mixture of juvenile Rhyacian material with an Archean crustal component in the source of these magmas. Also, the combination of the petrographic, geochronological geochemical and isotope data allows us to recognize two distinct events. The first magmatic episode displays a calc-alkaline to high-K calcalkaline signature and peraluminous character indicating that it formed during subduction stages in a magmatic arc environment. The second magmatic episode exhibits a calc-alkaline to high-K calc-alkaline signature, metaluminous character and affinity with syn- to post-collision granites. It is still possible to correlate these magmatic events to the two main magmatic episodes that marked the Transamazonian evolution throughout the southeastern Guiana Shield; The geochronological and geochemical data suggest that this sector of the Transamazonian orogeny was built up by accretion of volcanic arcs with subsequent arc-continent collision, however the existence of a continental magmatic arc at the margin of the Archean block cannot be still totally descarded.
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Geologia, geoquímica isotópica e aspectos metalogenéticos das rochas ígneas e mineralizações auríferas associadas - Região de Monte do Carmo - TO
(Universidade Federal do Pará, 1996-12-27) SACHETT, Cleris Regina; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The region of Monte do Carmo is located in the middle east portion of the Tocantins state and comprises in its geologic view, rocks from the Lajeado Granite, acid volcanics, metasediments of the Natividade Group and sediments of Phanerozoic age. It consists yet of quartz veins gold bearing mineralizations with several associated sulfides cross-cutting the Lajeado Granite, the volcanics and the metasediments of the Natividade Group. Acid volcanic rocks represented by rhyolite, dacites, rhyodacites and acid pyroclastics occur in the northern part of the Monte do Carmo town. This unity was analised by the Pb-Pb method with zircons vaporization, from which was obtained an age of 2138±11 Ma (2a), also interpreted as the crystallization epoch. The Lajeado Granite, which readly outcrops in the south region of Monte do Carmo, consists of granites, granophyres, as well as amphibole bearing gravite and subordinated leucogranite. Zircons from this body were analysed through the Pb-Pb method and provided an age of 2025±26 Ma (2a). This result was interpreted as the crystallization epoch of the Lajeado Granite inasmuch as the analised samples carne from non-deformed portions of the grafite. These two unities are seccionated by metric shear zones, commonly orientated in the N-S direction, with auriferous quartz veins and sulfide mineralizations. Markable mineralogical and textural transformations of the minerais occur in the wall rock. Samples from these phylonites were analysed through the Rb-Sr method and provided an age of 565±43 Ma (2a). On the basis of regional geologic data, which show the existence of magmatism and tectonism during this period, as well as an intense transformation on the analised rocks, this result was interpretated as the epoch of rocks transformation. Thus, the epochs of the shear zones development and related mineralizations were determined. Within the associated sulfide mineralizations are galena and pyrite. These sulfides were analised through the Pb-Pb method in minerais and yielded the isotopic compositions compatible with an Upper Proterozoic evolution from sources of the Lower Proterozoic. The microthermometric analyses of the quartz veins related inclusions revealed the existence of two kinds of fluids. An aqueous-carbonic fluid with low salinity was trapped in the earliest stages of formation of the veins in a temperature between 250 and 350°C and pressure of 2-3 Kb. With the evolvement of the shear zones, the fluids became more aqueous and more saline (22% weight NaC1), with the existence of the Na and Ca cations that were trapped in the temperatures between 150 and 200°C. In the Morro Lajeado arca localized 10 Km from the Porto Nacional town, small tonalitic bodies occur intrusives in the volcano-sedimentary sequence. Zircon from these granitoids also underwent Pb-Pb analyses, from which was obtained an age of 2069 ± 76 Ma (2a). These analyses however did not yield a well defined plateau, and some ages were much superiors than the average age 2538 ± 207 Ma (2a), what leads to consider this result as a minimum crystallization age to the Granitoid Torres.
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Geoquímica e geocronologia U-Pb em zircão e Sm-Nd em rocha total do magmatismo tardi-transamazônico da região de Calçoene, norte do Amapá, sudeste do Escudo das Guianas
(Universidade Federal do Pará, 2018-05-03) SILVA, Lilian Paula Almeida da; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The region of Calçoene (Paleoproterozoic Lourenço Domain), northern part of Amapá, Brazil, belongs to the Maroni-Itacaiunas Province, southeast of the Guiana Shield, which represents an extensive Paleoproterozoic orogenic belt developed during the Transamazonian orogeny (2,26-1,95 Ga). The Lourenço domain consists mainly of metavulcanosedimentary sequences, gneissic complexes, several calc-alkaline granitoid suites related to magmatic arc context (2,26 to 2,09 Ga) and granitic plutons, including charnockites that represent syncollisional to late-orogenic stages (2.1 to 1.99 Ga). The present study is focused on the Cunani Granite (~2.10 Ga), the main unit of the Calçoene area. Subordinately, the Cricou Suite (2.11-2.09 Ga), adjacent to the Cunani Granite, was studied at the comparison level. In spite of the recent progress in the regional geological knowledge conducted by the CPRM, geochemical and geochronological data are still scarce and make it difficult to establish a reliable chronology and geodynamic context of this magmatism, as well as to the evaluate the processes of juvenile accretion and crustal reworking during the Transamazonian orogeny. In order to better characterize the late-Transamazonian magmatism and to evaluate the influence of Archaean crustal material on the Rhyacian magmatism in the Lourenço Domain, petrographic study, geochemistry, LA-ICP-MS U-Pb zircon geochronology and Nd-Sr isotopic study were performed and provided important contributions about the geological evolution of this portion of the Guiana Shield. The petrographic study allowed to characterize the Cunani Granite as an unit constituted by dominant biotite monzogranites and biotite sienogranites, subordinate hornblende-biotite tonalites and biotite granodiorites, which contains enclaves of granulites (quartz-diorites with orthopyroxene), and hornblende metatonalites. Rocks of enderbitic composition were also found in this unit. The Cricou Suite in the study area is constituted by biotite monzogranites, and subordinate enderbite was also identified. The geochemical characteristics of these rocks are consistent with both a magmatic arc and syn- to post-collisional context. Most samples displayed enrichment in incompatible elements with high contents of Large Ion Lithophile elements (LILE) like Ba and K, while some High Field Strengh elements (HFSE) such as Th, La, Ce and Nd also show relatively high values. Significant negative anomaly of Nb and P are observed in the rocks of the Cunani Granite and its enclaves. The two samples of the Cricou Suite present Nb positive and negative anomaly, respectively. The Rare Earth Elements diagram showed enrichment of the light elements in relation to the heavy elements in the two units. Only the Cricou Suite shows accentuated positive anomaly of Eu. Overall, the geochemical features are interpreted as of post-collisional magmatism related to tectonic accretion of magmatic arc to the continent. The LA-ICP-MS U-Pb dating of zircon from lithotypes of the Cunani Granite furnished ages of 2097±17 Ma (upper intercept) for a biotite sienogranite (DAC-08-06), 2017±73 Ma (upper intercept) and 1990±16 Ma (concordant age) for another biotite sienogranite (LKV-06-03) and 2019±53 Ma (upper intercept) and 1995±37 Ma (concordant age) for a sample of biotite monzogranite (DAC-08-09a). These ages confirm a late-Transamazonian age (Neorhyacian) for this unit and suggest that the Cunani Granite may encompass different magmatic pulses. The granulitic enclaves of the Cunani Granite (DAC-08-07b) furnished an LA-ICP-MS U-Pb zircon age of 2112 ± 10 Ma and may represent deeper crustal-level rocks that correspond in depth to the migmatization event that affected the rocks of arcs around 2.11-2.10 Ga during the collisional stage. The age around 2.0 Ga obtained for the biotite monzogranite (DAC-08-09a) at the same outcrop than the hornblende metatonalite (DAC-08-09b) previously dated at 2151 ± 2 Ma (TIMS Pb evaporation in zircon) allows to reinforce that the latter corresponds to enclaves of rocks from the Mesorhyacian magmatic Arcs within the Cunani Granite. U-Pb dating by LA-ICP-MS of zircons from a biotite monzogranite (DAC-08-11) of the Cricou Suite provided an age of 1839 ± 62 Ma considered as statistically unreliable. However a resetting of the U-Pb zircon system by younger events cannot be ruled out. The Paleoproterozoic crystallization ages obtained in the analyzed samples and the Archean NdTDM model ages between 3.17 and 2.51 Ga, together with negative values of εNd[2.08 Ga] between -8.67 and -0.72, besides inherited zircons with ages of 3056±63 Ma and 2654±43 Ma identified in a biotite sienogranite, indicate the contribution of Meso-Neoarchean crustal sources during the formation of the Cunani Granite. The Sr-TUR model ages ranged from 2.52 to 2.29 Ga, also indicating Siderian-Neoarchean signature for the source of these granitoids, compatible with tectonic accretion of the magmatic arc with continental landmass.
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Geoquímica e geocronologia U-Pb, Sm-Nd e Rb-Sr dos granitoides orosirianos do domínio Erepecuru-Trombetas, Província Amazônia Central, Noroeste do Pará
(Universidade Federal do Pará, 2015-08-10) LEAL, Rafael Estumano; COSTA, Lúcia Travassos da Rosa; http://lattes.cnpq.br/3352314623448523; https://orcid.org/0000-0002-2666-9824; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The granitic rocks of the northwestern Pará State are part of two large volcano- plutonic associations that marked the central region of the Amazonian Craton during Orosirian. The area of this study is located in the southwest portion of the Erepecuru- Trombetas Domain, south of the Guyana Shield. The oldest association recently mapped and dated at about 2.0-1.95 Ga, is formed by volcanic rock of Igarapé Paboca formation and granitoids of Caxipacoré Suite. The youngest association (1.90-1.86 Ga) is related to the Água Branca and Mapuera suites and the effusive and pyroclastic rocks of Iricoumé Group. Petrographic studies in the Caxipacoré, Água Branca and Mapuera granitoids allowed the definition of five granitoids varieties (quartz-monzonite, monzonite, monzogranites, syenogranites and ácali-feldspar granites) with variable content of hornblende and biotite, the main mafic phases. In general, they are isotropic rocks, leucocratic, inequigranular, medium- to coarse-grained with hypidiomorphic granular, rapakivi, granophyric and micro-porphyritic textures. The geochemical characterization allowed the distinction of two rock groups with different signatures. The first group formed by Caxipacoré and Água Branca suites, shows high-K to shoshonitic, magnesian, metaluminous to peraluminous calc-alkaline trends. LILE- enrichment (e.g. K, Rb, Ba and Sr), strong negative Nb anomalies, moderate heavy REE- fractionation and weak Eu anomalies are common characteristics of these granites. The second lithological group formed by Mapuera Suite, comprises acid rocks (71.29
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Geoquímica e Geocronologia U-Pb-Hf do Magmatismo Meso- Neoarqueano da Borda Norte do Bloco Amapá, Sudeste do Escudo das Guianas
(Universidade Federal do Pará, 2021-12-06) GONÇALVES, Renato Cantão; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The southeastern portion of the Guiana Shield (SEG), in the northern region of the Amazonian Craton, is defined as a broad Paleoproterozoic orogenic belt added to an Archean block strongly reworked during the Transamazonian Cycle (2.26–1.95 Ga). This Archean block in the central portion of the state of Amapá, called the Amapá Block, is made up of a mesoneoarchean granulite-gneiss-migmatite association (~2.85 Ga and ~2.70-2.60 Ga) and by Rhyacian granitoids and metavolcanosedimentary sequences. The Amapá Block is delimited to the south and north by the Carecuru and Lourenço Rhyacian domains with Archean crustal relics. The Lourenço Domain is formed by Rhyacian granitoids, gneisses and metavolcanosedimentary sequences formed in magmatic arcs (2.20-2.12 Ga), followed by syncollisional to late orogenic magmatism (~2.11-2.07 Ga) and late granulitic metamorphism (~2.06-2.04 Ga). On the boundary between the Amapá Block and Lourenço Domain, Mesoarchean units (Gnaisse Porfírio - 3.19 Ga and Complexo Tumucumaque - 2.85 Ga) and Neoarchean units (Complex Guianense - ~2.65 Ga and Metagranitoid Pedra do Meio - 2.59 Ga) occur. Near Vila Bom Jesus village, (municipality of Tartarugalzinho - AP), at the transition between Archean and Rhyacian domains, orthogneisses and metagranitoids were dated by the U-Pb method in zircon by ICP-MS mass spectrometry and laser ablation (LA-ICP- MS) to investigate the geographic extension of the Porfírio Gneiss and Pedra do Meio Metagranitoid. Additionally, petrographic data, geochemical analyzes in whole rock and Lu-Hf isotopic geochemistry in zircon by LA-ICP-MS were used, together with previous data from orthogneisses and granitoids of the northern portion of the Amapá Block, to investigate the geodynamic context of formation of these granitoids and the episodes of generation and reworking of the continental crust during the Archean in this portion of the SEG. U-Pb dating of a biotite granodiorite orthogneiss provided a 207Pb/206Pb crystallization age of 2846 ± 36 Ma (MSWD = 1.3) for the orthogneiss magmatic precursor, indicating a Mesoarchean magmatic episode. Three samples of orthogneiss and metagranitoids provided 207Pb/206Pb crystallization ages of 2654 ± 12 Ma (MSWD = 1.4), 2618 ± 31 Ma (MSWD = 1.15) and 2618 ± 22 Ma (MSWD = 0.71) respectively, indicating a protracted Neoarchean magmatic episode. The U-Pb dating of a biotite granodiorite orthogneiss with crystallization age 207Pb/206Pb of 2096 ± 24 Ma (MSWD = 0.75) indicates the presence of imbricated Paleoproterozoic rocks in the Archean units of the investigated sector. These results lead to reconsider the configuration of the Archean units on the north edge of the Amapá Block. The Porfírio Gneiss and the Tumucumaque Complex must constitute only enclaves or xenoliths of the Mesoarchean basement in Neoarchean orthogneisses and granitoids. The Pedra do Meio Metagranitoid represents intrusive charnockitic plutons in the Guianense Complex, which is the dominant unit in the investigated sector. The geochemical data showed that the Mesoarchean samples have signature of biotite granites of crustal derivation. Neoarchean magmatism is also dominated by crustalderived biotite granites, but also includes granitoids with sanukitoids and high-pressure TTG affinities, in addition to hybrid granites. The geochemical signatures of biotite granites, the subchondrite values of ƐHf(t) (-11.3 < ƐHf(t) < -0.4) with Hf-TDM model ages between 3.9 and 3.2 Ga, and the presence of inherited zircons in most Neoarchean samples, with Mesoarchean (3.0, 2.89, 2.84 Ga) and Neoarchean (2.77, 2.74, 2.72 Ga) ages, indicate that this protracted Neoarchean magmatic episode reworked older units from the basement of the Amapá Block (Meso-Paleoarchean), in a collisional context with no evidence of crustal growth during the Neoarchean. However, it is still open which continental masses collided to form this portion of the Amapá Block, considering that the other Archean domains of the Amazonian (Carajás Province; Imataca Complex) and West African (Leo-Man Domain; Reguibat Shield) present a different geological history from the Amapá Block during the Neoarchean.
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Geoquímica elemental e isotópica Pb-Sr-Nd dos sedimentos de fundo do sistema estuarino de Belém e do litoral paraense
(Universidade Federal do Pará, 2016-12-19) OLIVEIRA, Elma Costa; CORRÊA, José Augusto Martins; http://lattes.cnpq.br/6527800269860568; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
In recent years, urban growth has occurred in a disorga¬nized manner in the metropolitan area of Belém, State of Pará, and this is reflected directly in the quality of water and sediments of the Guajará Bay, the main hydrological element of the estuarine system of Belém. This study aimed to conduct an elemental and isotopic geochemical research on bottom sediments from the western margin of Guajará Bay and Carnapijó river to understand the geographical and historical variations of heavy metals (Cu, Cr, Ni, Pb and Zn) contents and isotopic signature of Pb, and to evaluate the gradual environmental degradation in the hydrographic system of Belém. In addition, the provenance of the bottom sediments in three areas of the Amazon Coastal Zone (mouth of the Amazon River, Marajoara Gulf and northeastern coast of Pará) was investigated by elemental and Sr-Nd-Pb isotopic geochemistry. Sediments from the western margin of the Guajará Bay and from the Carnapijó River display mineralogical homogeneity and their sand to silty sand textural composition reflects very high hydrodynamic conditions. 210Pb dating furnished a rate around 0.7 cm.year-1 for the sedimentation at the western margin of Guajará bay. The trace metal contents in the fine fraction indicate that there are no significant anthropogenic contribution for Cu, Cr, Ni and Zn in the sediments from these sectors of the hydrographic system of Belém. However, the Pb contents suggest a weak anthropic action for this metal. The concentrations of Cu, Cr, Ni, Pb and Zn in the exchangeable fraction below the Threshold Effect Level (TEL) indicate that the metals do not offer risk to the local biota. Comparison between the studied sediments and those of the riverside of Belém point to a higher contribution in the latter from domestic sewage and industrial waste for Pb and Ni, followed by Cr and virtually nonexistent for Cu and Zn. The study indicates that the sediments of the Carnapijó River are suitable for use as background of the estuarine system of Belém in future geochemical studies. Isotopic signatures of sediments from the western margin of Guajará Bay confirm an anthropogenic contribu¬tion of Pb throughout the entire bay. The Pb accumulation process has become more efficient over the last 10 years, and this can be attributed to the rapid population growth of Belém city. Sediments in Carnapijó River are not affected by human activities, and the average concentration values (Pb = 19.6 ± 3.7 mg kg-1) and isotopic signatures (206Pb/207Pb = 1.196 ± 0.004) confirm the background Pb values previously proposed for the river system in the Belém region. The isotopic signatures of suspended matter on the eastern (206Pb/207Pb = 1.188) and western (206Pb/207Pb = 1.174) margins of Guajará Bay show that suspended matter is an efficient Pb trans¬portation mechanism of domestic and industrial wastewater from Belém to the western margin of the bay due to tidal effects at the confluence with Guamá River. The Pb isotopic compositions along the sediment cores show a decrease of the 206Pb/207Pb ratio to a value of 1,180 in the shallower 20cm of the cores from the western margin of Guajará Bay, not observed in the cores of Carnapijó River. This decrease indicates a probable anthropogenic contribution over the past 15 years in the western margin of the bay. The provenance of the bottom sediments in three areas of the Amazon Coastal Zone (mouth of the Amazon River, Golfão Marajoara and northeastern coast of Pará) was investigated by elemental and Sr-Nd-Pb isotopic geochemistry. The geochemical signatures of the bottom sediments from the three sectors indicate a provenance from felsic geological units of the upper continental crust. Sediments of the Southern Channel of the Amazon River, north of the Marajó island (mouth of the Amazon River) had lower contents of Na2O and K2O and a higher CIA, indicating a higher degree of weathering. The distinct isotopic signatures of Sr, Pb and, principally, Nd of bottom sediments of the three studied sectors indicate different sources in nature and age. The sediments of the mouth of the Amazon River derived predominantly from the Andean and sub-Andean regions, as already shown in previous works. The more radiogenic Sr signatures and more negative ƐNd values of the sediments from Guajará Bay and Carnapijó and Guamá rivers, in Belém area, Marajoara Gulf (0,7267< 87Sr/86Sr <0,7316; -17,97< ƐNd < -13,58) and from the estuaries of Caeté and Maracanã rivers, northeastern coast of Pará (0,7220< 87Sr/86Sr < 0,7264; -24,05< ƐNd<-17,58) indicate a greater contribution of Precambrian rocks in their sources. Nd-TDM model ages (1.62-1.99 Ga) of the sediments from the Golfão Marajoara suggest a predominant contribution from the metasedimentary units of the Araguaia Belt and a subordinated contribution from the magmatic and metamorphic basement of Tocantins Province. Nd-TDM model ages (1.70-2.83 Ga) of sediment in the estuaries of Maracanã and Caeté rivers portray a strong contribution of Precambrian basement rocks (Craton Fragments of San Luis and Gurupi Belt) that outcrop in coastal region of northeastern Pará.
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Geoquímica isotópica Sr e geocronologia Pb-Pb da capa carbonática neoproterozoica do Grupo Araras, Tangará da Serra, MT
(Universidade Federal do Pará, 2012-04-24) ROMERO, John Alexander Sandoval; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The Neoproterozoic cap carbonates have been the subject of countless paleoenvironmental and chemicalstratigraphy studies in almost all Cratons around the world. Those were deposited after global glacial events deposits, are composed of dolostones and limestones, overlying glacial diamictites with specific features as stromatolits, tubes like structures, megaripples and crystal fans of calcite. In Brazil this ocucurrence, principal objetive of this work is exposed in Mirassol d ́Oeste and Tangará da Serra towns at Southeast of Mato Grosso State, Southeastern of Amazon Craton, recovering glacial diamictites with age of ≈ 630 Ma, The cap carbonate is part of base of Araras Group compoused by dolostone of Mirassol d ́Oeste and limestones of base of the Guia Formation. The main goal of this work is consolidate an Early Ediacaran age for the cap carbonate of the Tangará da Serra area by Pb-Pb geochronology and bring new Sr isotopic data to understand the paleoemviromental conditions as well as the increasing Sr marine evolution curve of during Neoproterozoic. Were collected 36 samples from top of Mirassol d ́Oeste (13 samples) and base of Guia Formation (23 samples), for petrography, X-ray difractometry, X- fluorescence as well as Sr isotopic analyses and Pb-Pb dating, using mass spectrometer TIMS and ICP-MS. The petrography allow recognize diagenetic features as stylolits, dolomite grains recristalized, spatic dolomite, iron oxides and autigenic quartz in dolostones of Mirassol d ́Oeste Formation. In Guia Formation, were observed partial substitution from crystal fans of calcite to spatic dolomite, stilolits as well as microfractures filled by calcite and secondary dolomite. The X-difractometry complemented the determination of mineralogical assemblage about carbonates, identificating the presence of secondary dolomite as well as quartz and feldspar in limestones of Guia Formation. The X-ray fluorescence allows determinate the teors of Fe, Mn, Ca and Sr, which were used to help the interpretation of Sr isotopic signatures. The Sr isotopic signature in 5 samples in dolostones of Mirassol d ́Oeste Formation and 6 limestones samples of base of Guia Formation, were determinated by sequential leaching with acetic acid. For dolostones the 87 Sr/ 86 Sr ratios ranging 0.708 to 0.709. For limestones the 87 Sr/ 86 Sr ratios ranging 0.707093 to 0.707289, after eliminate the leaching by probably terrigenous contribution for 87 Sr/ 86 Sr ratios until 0.7114. The lowest 87 Sr/ 86 Sr ratios were found in samples with lowest Mn/Sr ratios (0.31-0.45) and Fe/Sr (6.03-9.8). The differences in analytical procedure used to obtain the 87 Sr/ 86 Sr ratios, allow to explain the differences a out the isotopic Sr signatures in limestones from Tangará da Serra área (sequential leaching) with which, more radiogenic from limestones from Mirassol d ́Oeste área (total dissolution), previously published. The 87 Sr/ 86 Sr ratios ranging 0,70709 – 0,70729 in limestones from Tangará da Serra area, are positioned above from Sr marine curve in late Neoproterozoic, in transition from Cryogenian to Ediacaran, before the abrupt rise in early Ediacaran. Six dolostones samples and 17 limestones samples were analyzed, for determination of isotopic Pb composition, the samples were leached with HCl. The isotopic Pb signatures are homogeneous in dolostones (19.05< 206 Pb/ 204 Pb < 19.50; 15.69 < 207 Pb/ 204 Pb < 16.89) and were not used for elaborate the Pb-Pb isochron. From 17 limestones samples just 15 provided and age of 622 ± 33 Ma (2σ), despite de limited isotopic variations of the isotopic composition (18.77 < 206 Pb/ 204 Pb < 31.18; 15.71 < 207 Pb/ 204 Pb < 16.46). This result reinforces the Ediacaran early age to cap carbonate of Araras Group and check their association with the events after the last Criogenian glaciation at South of Amazon Craton.