Navegando por Assunto "Magmatismo"

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Análise palaeoambiental e caracterização dos Folhelhos Negros da Formação Barreirinha utilizando análises Multiproxy
(Universidade Federal do Pará, 2025-06-04) CARVALHO, Wivian Maria Rodrigues; BRITO, Ailton da Silva; http://lattes.cnpq.br/9873489431846769; HTTPS://ORCID.ORG/0000-0001-9224-5563; SOARES, Joelson Lima; http://lattes.cnpq.br/1345968080357131
The initial sedimentation phase of the Barreirinha Formation was associated with a rapid relative sea-level rise during a significant marine transgression event that flooded the Amazon Basin. These organic-rich shales outcrop along a narrow yet extensive belt located on the southern margin of the Amazon Basin. Few studies have specifically addressed the potential paleoenvironmental variations linked to the deposition of these shales. This is mainly due to the relative lithological uniformity of these rocks—composed predominantly of fine-grained sediments—and their economic relevance, which has directed most research toward the maturation of organic matter. To investigate the paleoenvironmental variations during the deposition of these fine sediments—focusing on sedimentary dynamics, the origin, and provenance of the organic matter—a multiproxy approach was applied, combining various quantitative, semi-quantitative, and qualitative techniques. The analyzed stratigraphic succession is mainly composed of gray to black shales, exhibiting facies variations related to coarse terrigenous input and episodes of bioturbation. These features suggest a deep, distal, anoxic marine depositional environment, with no evidence of carbonate sedimentation, typical of the Abacaxis Member of the Barreirinha Formation. Mineralogical cluster analyses indicate a dominance of kaolinite, characterizing the Kaolinite Facies, with subordinate quartz, sulfates, and sulfides in the lower portions. The base of the succession includes massive fine-grained sandstones with cross-bedding, correlated with the Ereré Formation, interpreted as deltaic to inner shelf deposits. The transition to laminated shales interbedded with sandstones and siltstones marks the onset of the Devonian (Frasnian) transgression, with substantial continental input evidenced by heavy minerals, pyritized plant remains, and tasmanites. The presence of dumpstones suggests glacial influence and ice-rafted debris deposition. Upper levels show more homogeneous shales, enriched in organic matter, lacking bioturbation and detrital minerals, indicating maximum anoxia during the peak of the transgressive event in the Amazon Basin. Diagenetically, the shales underwent compaction, fracturing, mineral substitution, oxidation, and intense pyritization, mainly as framboidal pyrite—typical of reducing marine environments. The mineralogy is dominated by kaolinite and quartz, with accessory minerals indicating alteration processes and possible Jurassic-Triassic igneous intrusions (Penatecaua magmatism), which contributed to increasing the thermal maturity of the kerogen. Rock-Eval pyrolysis and biomarker analyses reveal Type II-III kerogen with gas-generating potential, ranging from immature to post-mature depending on proximity to igneous intrusions. These findings reflect a transgressive system strongly influenced by environmental controls and regional thermal input.
Acesso aberto (Open Access)
Estudo tectono-sedimentar da Bacia de Jaibaras, na região entre as cidades de Pacujá e Jaibaras, noroeste do estado do Ceará
(Universidade Federal do Pará, 1996-04-22) QUADROS, Marcos Luiz do Espírito Santo; ABREU, Francisco de Assis Matos de; http://lattes.cnpq.br/9626349043103626
Studies carried out in southwestern portion of the Jaibaras Basin in the area of the 300 km², situated between the Pacujá and Jaibaras towns, northwest region of Ceará State, including geological mapping in 1:25,000 scale, faciologic, petrographic and structural analysis of the Pacujá and Aprazível formations, allowed a better visualization of spatial distribution of these units, as well as the characterization of their depositional environment, structural pattern and, at last to take considerations about the tectono-sedimentary evolution of the Jaibaras Basin. The Pacujá Formation is characterized by a folded and faulted volcanosedimentary sequence without fossils, made of rhythmic interlayers of fine arkosian sandstones to siltistones with pelites, occurring in tabular decimeter thick beds, laterally continuous, showing abrupt base and upward gradation to siltistones. The sandstones could be massive or stratified, showing planar lamination, micro-hummocky cross lamination, climbing wave -ripple cross lamination, locally parting lineation and convolute lamination. On the top of the sandstone beds occur symetrical and assymetrical wavy-ripples. Pelites show planar lamination and mudcracks. Interlayered with Pacujá Formation sediments occur volcanic and subvolcanic rocks (basalts, andesites, dacites and ryolites), at the form sills, dikes and flows, associated with volcaniclastic rocks, included on the Parapuí Suite. The environment of the Pacujá Formation was characterized as lacustrine with volcanism associated, subjected to storm-wave action, proved by the occurrence of microhummocky cross lamination.The rhythmics interlayering of sandstones and pelites characterizes tempestites cycles, usually incomplete. However, marine environment must be assoc iated, but unfortunately the field datas is insufficient to define this environments. In the Jaibaras Basin setting, Pacujá Formation represents the first deposicional pulse that extend from Neoproterozoic era to Cambrian period. This sedimentation occurred in an area wider than Jaibaras Basin out of their present boundaries. The Pacujá Formation shows a complex fold pattern, that resulted from superimposed folding, with geometric shapes similar to type 1 interference pattern - “domes and basins”, and kinks folds. This folding could be related to transpression in ductile -brittle regime, linked to the northest-southwest sinistral strike -slip system in the Eopaleozoic era, that caused a weak inversion of the Jaibaras Basin. The Aprazível Formation comprises a thin sedimentary sequence, faulted and tilted to southeastern on the whole, unconformably covering the Pacujá Formation. It is made of polymitic conglomerates, with clast-supported and matrix -supported framework, massive or stratified, with volcanic, gneiss, granite, rocks calcissilicatic, quartz, amphibolite, rhyolite, marble, mylonite, siltistone and sandstone clasts, varying in size from granules to boulders. The matrix is coarse to very coarse sandy arkosean, locally microconglomeratic. To a lesser proportion, occurs medium to very coarse grained arkosean sandstones, locally stratified, and intercalations of laterally continuos beds of arkosean sandstones and laminated pelites with mudcracks in surfaces of the beds. These sandstones show planar lamination, climbing-ripple cross lamination, and locally trough cross-bedding and convolute lamination. In the surface of sandstone beds, there are, occasionally, symetrical and assymetrical wave-ripples. The depositional environment of the Aprazível Formation was characterized as alluvial fan/plain, dominated by debris-flows and stream-flows, prograding distally over small lacustrine bodies. The tilting of the Aprazível Formation beds to southeastern, is a results from rotation of blocks, due to an extensional axis in the northwest-southeast direction, acting in Ordovician period, that controlled the deposition of Aprazível Formation in the area between Sobral-Pedro II and Café -Ipueiras shear zones. The Aprazível Formation represents the second and last depositional pulse that occurred in the Jaibaras Basin in the Ordovician period, in a more restrict depositional area, controled by Sobral-Pedro II e Café-Ipueiras shear zones. Its deposition occurred in a time interval between the post-Pacujá sequence and Meruoca Suite Granites, and pre-Serra Grande Group sequence of the Parnaiba Basin.
Acesso aberto (Open Access)
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.
Acesso aberto (Open Access)
Geologia e petrologia dos enxames de diques máficos da região de Santa Maria das Barreiras-Conceição do Araguaia: evidências de eventos distintos de magmatismo intracontinental no Centro- Norte do Brasil.
(Universidade Federal do Pará, 2019-10-28) CRUZ, Danilo José do Nascimento; GORAYEB, Paulo Sérgio de Sousa; http://lattes.cnpq.br/4309934026092502
N-S and NNW-SEE-trending subparallel mafic dike swarms are intruded into metasedimentary rocks of the Tocantins Group, Araguaia Belt, central-north Brazil. They are under-examined and there is little to no information about their origin and mantellic sources and uncertainty about their ages. Representative mafic dikes from the Santa Maria das Barreiras-Conceição do Araguaia region, at the boundary between the states of Pará and Tocantins, were studied in order to address these problems. It was possible to separate the dikes into two groups: one consisting of diabases affected by the Neoproterozoic regional metamorphism of the Araguaia Belt with varied degrees of transformations and mineral deformation; and the other consisting of unmetamorphosed and undeformed diabases and leucodiabases. The studied dikes are compositionally classified as subalkaline basalts with tholeiitic affinity. However, metadiabases present an arc-like geochemical signature characterized by a pronounced Nb-Ta negative anomaly, whereas leucodiabases and diabases lack a negative Nb-Ta anomaly and show a LREE-enriched pattern, which resembles the signatures of plume-generated basaltic rocks. Both group of dikes were interpreted to be originated in an intracontinental setting with the aid of Ti–V, Zr–Zr/Y and Zr–Ti discrimination diagrams. There is evidence of important contribution of enriched (EN) mantle components in the source of metadiabases and significant contribution of primitive mantle (PM) to the source of both leucodiabases and diabases. We suggested that the metadiabases represent the exposed plumbing system of arc-like intracontinental basalts which precede the regional Neoproterozoic metamorphism of the area and the leucodiabases and diabases represent the exposed conduits of intracontinental basalts whose magmatism succeed the metamorphic event. The rocks from the older event share several similarities with Neoproterozoic mafic rocks from the eastern domain of the Araguaia Belt and nearby Tonian rocks of the 1100 Ma Rincón del Tigre-Huanchaca LIP event, while the rocks from the newer event are remarkably similar to nearby CAMP basalts and diabase dikes.
Acesso aberto (Open Access)
Magmatismo basáltico na sucessão sedimentar do Grupo Tucuruí - Cinturão Araguaia, Nordeste do Pará
(Universidade Federal do Pará, 2012-04-04) DUTRA, Alessandra de Cássia dos Santos; GORAYEB, Paulo Sérgio de Sousa; http://lattes.cnpq.br/4309934026092502
The main objective of this work was the study of magmatic and sedimentary environments associated to the rocks of the Neoproterozoic Tucuruí Group, and discusses their relation in the evolution of the Araguaia Belt. This unit is exposed in the region of Tucuruí, northeast state of Pará, northern Brazil, in transition zone between the Amazonian craton and Araguaia belt and is described as a volcano-sedimentary succession composed by basaltic volcanic deposits including occasional intrusions of diabase and basalt breccia intercalated with amalgamated arkosic arenite and siltstones interpreted as coastal deposits with layers oriented in the NNE-SSW dipping down to SE. The Tucuruí overthrusting fault affected this succession toward west, causing fracturing and fluid percolation. The siliciclastic deposits are organized and thickening sucession that include two facies associations: 1) Shoreface formed by arkosic with low angle cross-stratification; and 2) Storm influenced shoreface the comprise the arkosic sandstones with parallel lamination , siltstone with lamination parallel, laminated siltstone truncated by wave and siltstone with hummocky cross-stratification. These associations suggest processes for easy transport and sedimentation connected to a shallow marine environment, following the foreshore-shoreface zones influenced by storms. The petrographic analysis of arkosic arenite and siltstones reveals compositional and textural immaturity of these rocks, suggesting proximity of source area composed of igneous mafic to intermediate igneous or metamorphic rocks. The sedimentary sucession was submitted to eodiagenetic environment and mesodiagenetic conditions. The volcanic deposits are composed of mesocratics and hipocrystalline rocks, whose mineralogy consists of labradorite (An60), augite, Fe and Ti oxides and sulfides and apatite accessory. The basalt flows are aphanitic composed of amygdaloidal basalts, located on the bottom and top of them and massive basalts, located in the central portions of the basaltic flows. In type amygdaloidal, the amygdales are filled by chlorite, quartz, zeolite and green cryptocrystalline material and are embedded in the matrix or intergranular intersertal, the latter being the main plots of textural massive basalts. The diabases of the sills are phaneritic and have coarse grained, intergranular texture and micrographic intergrowth. The basaltic breccias, occur between basaltic flows and / or in contact with the sedimentary sequence and is composed of fragments of basalt showing flow structures involved in matrix grain size of sediment silt, locally intercalated with irregular concentrations of zeolites and epidote, indicating thus the concomitant processes of sedimentation and volcanism. The geochemical analysis carried out on 14 samples of volcanic rocks, using inductively coupled plasma spectrometry with mass spectrometry (ICP-MS) reveals concentrations of SiO2 ranging from 46-48% and alkalis (Na2O + K2O) between 2-4% predominantly. The concentrations of CaO, and TiO2 Fe2O3 are high and ranges from 8-128%, 12-16%, 2-3% and respectively, while the concentration of MgO is moderate and will vary between 5-7%. As for the trace elements have values of Cu, Cr and Co in moderate concentration between 102-216ppm, 160-560 ppm, 44-52 ppm, since the values of elements such as Ba, Rb and Sr vary between 115-350 ppm, 5 -34 ppm and 145-424 ppm, respectively, and ratios (La/Yb)n between 2-5. This framework is consistent with compositional affinity tholeiitic basalts.The behavior of trace elements is typical of continental basalt provinces, where the rare earth elements show weak fractionation. Anomalies are observed, which were negative in the case of Eu and Sr, indicating fractionation of plagioclase and positive in the case of Ti, indicating a high similarity with suites magmatic source, the reasons La/Yb and La/Nb, both greater than 1, allow us the derivation of enriched mantle sources, with a partial melting of subcontinental lithosphere mantle. Thus, can summarize that the Tucuruí group represents the portion preserved of coastal segment influenced by storm in a rift or foreland basin, with contribution for igneous or metamorphic rocks as source area marked for high topography and by physical weathering dominant that achieved during its formation by effusive volcanism with geochemical signatures of tholeiitic affinity and continental during the final stages of the geological evolution of the Araguaia Belt. Moreover, the fracturing and percolation of hydrothermal fluids from veins observed in clusters recorded rocky GT, coupled with the discrete mineralogical transformation that occurs in the basalts are understood to influence the later stages of deformation and regional metamorphism of the geological evolution of the Araguaia Belt.
Acesso aberto (Open Access)
Magmatismo bimodal da área de Tucumã, Província Carajás: geocronologia U-Pb, classificação e processos
(Universidade Federal do Pará, 2015-08-06) SILVA, Fernando Fernandes da; OLIVEIRA, Davis Carvalho de; http://lattes.cnpq.br/0294264745783506
In the geological context of the Carajás Province, dikes are an important mechanism for magma transport and represent the beginning of a rifting, which allow the emplacement through the crust significant amounts of magma. The geological mapping of the Tucumã area has enabled the identification of dike swarms, which are intruded in an Archean basement. The disposition of these dikes is consistent with the regional, NW-SE trending, and can reach up to 20 kilometric lengths. They were individualized in three main groups: (i) felsic dikes (70% of the dikes), composed exclusively of pink to dark purple porphyritic rhyolites with euhedral phenocrysts of quartz, K-feldspar and plagioclase immersed in a felsitic afiric matrix; (ii) mafic dikes, with restrict occurrence, composed primarily of basaltic andesites and subordinately by basalts, with a mineralogical assembly represented by plagioclase, cline- and orthopyroxene and olivine, forming an ofitic texture; and (iii) intermediate rocks, represented by gray to greenish gray andesites and dacites. Dacitic rocks are found outcropping associated to felsic dikes, showing different degrees of hybridization or mixing between mafic and felsic magmas. This is evidenced by large presence of mafic enclaves in the felsic dikes and the frequent presence of embayment textures. SHRIMP U-Pb zircon dating of felsic dikes yielded an age of 1888 ± 3.3Ma, which is interpreted as crystallization age. The felsic dikes are peraluminous to slightly metaluminous mainly due to the fractionation of K-feldspar and plagioclase with minor contribution of amphibole. They are characterized as akin to A2, ferrous and reduced granites. The intermediate and mafic dikes belong to tholeiitic series and are exclusively metaluminous with large fractionation of amphibole and plagioclase. It is noted a decrease in the amounts of CaO, FeOt, MgO, TiO2, Sr, Cr and P in the mafic rocks in the direction to the rhyolitic composition, which are more enriched in silica, while K2O/Na2O, Al2O3 and Na2O, Rb, Ba and Y increase in the same sense. In rhyolitic varieties, the REE patterns are characterized by LREE enrichment in relation to HREE (high ratios La/Yb), and a significant Eu anomaly. A similar behavior is also seen in the intermediate rocks. On the other hand, the mafic rocks presents flatter REE patterns (low ratio La/Yb) and little or no Eu anomaly. Geochemical modeling and the fractional crystallization vectors showed that the mafic rocks evolved by crystallization of pyroxene and plagioclase, while the K-feldspar and biotite are the fractionating phases in felsic magma. For a discussion about the origin of the intermediate rocks, it was utilized a simple binary mixture model, in which the rhyolites and basaltic andesite are the primary components. From this model was shown that by mixing of 60% of rhyolite and 40% of basaltic andesite melts is possible to generate the dacite composition, while the andesite liquid could be x originated by mixing of 60% and 40% of basaltic andesite and rhyolite melts, respectively. The model proposed suggested that mixing of basaltic and andesitic magmas occurred during the ascent and storage in the crust, where the andesitic dikes are probably generated by a more homogeneous mixture at high depths in the continental crust (mixing), while the dacite dikes can be generated in the upper crust at a lower temperature, thus providing a less efficient mixing process (migling). The petrographic, geochemical, and geochronological affinities observed between the felsic dikes studied and the A-type granites of the Rio Maria and São Felix do Xingu regions, demonstrate that the bimodal magmatism of the Tucumã area is a clear evidence that the Paleoproterozoic magmatism of the Carajás Province has been formed by processes involving thermal perturbations in the upper mantle, mafic underplating, and associated extension or transtension of the crust.
Acesso aberto (Open Access)
O Magmatismo granítico e os seus efeitos na região de Xambioá/Araguanã, To
(Universidade Federal do Pará, 2009-05-22) POINSIGNON, Janaina Reis; KOTSCHOUBEY, Basile; http://lattes.cnpq.br/0096549701457340
The Xambioá region is located in the eastern part of Arguaia belt, in the northern portion of the domain of Estrondo Group (northwest of the State of Tocantins). The granites of Ramal do Lontra and Serra da Ametista, intrusive in the rocks of Estrondo Group, were until recently, the only felsic bodies known in the region. More recent geological observations, during detailed geological mapping, identified several intrusive granitic bodies, generally small, as well as in many sectors, unequivocal signs of albitization, greisenization and caulinization in both granitic bodies and host rocks. Quartz vein systems, host of hyaline quartz and eventually amethyst are commonly associated with granitic intrusions and/or with altered host rocks. The felsic magmatism expressed by the emplacement of alkali-granites (Ramal do Lontra and Serra da Ametista granites and occurrences of Fazenda Novo Horizonte, Fazenda Bela Vista, Fazenda Belém, Morro das Antenas and adjacency) and of albite-granites (Araguaci and sector of Pedra Preta granites). Fine to medium-grained alkali-granites, are usually deformed. These bodies have been considered late-tectonic and correspond most likely to apical zones of larger granitic intrusions, not yet exposed. Fine to medium-grained albite-granites consist essentially of albite, like euhedral crystals, and quartz do not show deformation. It has been interpreted as products of extreme granitic magma differentiation, which originated the alkali-granite. The postmagmatic context had a complex development, resulting in the formation of quartz vein systems and marked alteration of both the intrusive garnitic rocks and host rocks of Estrondo Group formations and the Archean basement. Based on fluid inclusions studies of vein quartz, two types of fluids have been recognized: aqueous- carbonic and aqueous. The first, with high salinity (38 to 53% eq of NaCl) and temperature (340 to 500°C) of metamorphic origin with important magmatic contribution, may have caused the formation of the early milky quartz, and probably the metasomatic alterations at high temperature, namely the albitization and greisenization of both granitic intrusions and host rocks. The second type, of moderate to low temperature and salinity (to 120 from 200°C and 1 to 18% eq of NaCl), is interpreted as having a magmatic origin and after mixing with meteoric waters caused its temperature and salinity decrease. It is possible that the drops in temperature and salinity may have been, in part, resulted from the natural cooling of hydrothermal process. These fluids were responsible for the formation of hyaline quartz, in distensive context of decreasing pressure. They also caused the alteration of igneous rocks and host rocks at low temperature, essentially kaolinization. The albitization was the earliest metasomatic alteration. At high temperature and alkaline conditions, it affected both alkali-granites and albite-granites, and the nearby host formations. Albitite with riebeckite and aegirine resulted also from this process. Strong signs of albitization were also detected in the Estrondo Group rocks and Complexo Colméia, regardless of any magmatic influence, what suggests the possibility of this process not be local but regional and related to the regional metamorphism or dynamometamorphism that acted during the structural evolution of Araguaia belt. The greisenization, even more local, succeeds to albitization, at high temperature but in acidic conditions. It is restricted to immediately host rocks of the intrusive granitic bodies and probably to their marginal portions. At last, at lower temperatures and acidic conditions, kaolinization occurred and their effects have been observed only in the host schists, although this alteration may have also affected the intrusive bodies. The total destruction of all primary minerals but not quartz, resulted in the formation of pure kaolin consisting of kaolinite of high crystallinity.The geochemical data reveal coherence between several processes, like the concentration of trace elements (Th, U, Hf, Nb, Zr, V and W) very common in granitic environment.The results obtained in this study suggest that magmatic/ hydrothermal manifestations have been more intense that believed before, being recommended the continuation of investigations, in more meridian zones of the Estrondo Group domain.