Navegando por Assunto "Escudo das Guianas"

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Análise de feições geológicas na região de Tartarugalzinho, Amapá, através de métodos geofísicos
(Universidade Federal do Pará, 2009) MARTINS, Saulo Siqueira; PINHEIRO, Roberto Vizeu Lima; http://lattes.cnpq.br/3251836412904734; LUIZ, José Gouvêa; http://lattes.cnpq.br/8676530374310847
Two geophysical methods were applied to detect geologic structures on the oriental edge of the Guianas Shield. The gravimetric method was applied mainly to find faulting on the basement, associated to shear belts that have orientation NW-SE. It was found faulting on the basement as well as an indication that the crust suffered a thinning in some region of the edge. The GPR electromagnetic method was applied to find geologic discontinuities on the sediments and to check if they are reflex of structural faults on the basement and/or reactivation of some of them. The GPR showed places where discontinues and some paleostructures can be observed. The study was also applied to understand how the basement discontinuities can influence on the formation of Tertiary sedimentary basins and on the formation of shallow paleostructures.
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Evolução geológica da porção centro-sul do Escudo Guianas com base no estudo geoquímico, geocronológico (evaporação de Pb e U-Pb ID-TIMS em zircão) e isótopo (Nd-Pb) dos granitóides paleoproterozóicos do sudeste de Roraima, Brasil
(Universidade Federal do Pará, 2006-11-17) ALMEIDA, Marcelo Esteves; MACAMBIRA, Moacir José Buenano; http://lattes.cnpq.br/8489178778254136
This study focuses the granitoids of center-southern portion of Guyana Shield, southeastern Roraima, Brazil. The region is characterized by two tectono-stratigraphic domains, named as Central Guyana (GCD) and Uatumã-Anauá (UAD) and located probably in the limits of geochronological provinces (e.g. Ventuari-Tapajós or Tapajós-Parima, Central Amazonian and Maroni-Itacaiúnas or Transamazon). The aim this doctoral thesis is to provide new petrological and lithostratigraphical constraints on the granitoids and contribute to a better understanding of the origin and geodynamic evolution of Guyana Shield. The GCD is only locally studied near to the UAD boundary, and new geological data and two single zircon Pb-evaporation ages in mylonitic biotite granodiorite (1.89 Ga) and foliated hastingsite-biotite granite (1.72 Ga) are presented. These ages of the protholiths contrast with the lithostratigraphic picture in the other areas of CGD (1.96-1.93 Ga). Regional mapping, petrography, geochemistry and zircon geochronology carried out in the UAD have showed widespread paleoproterozoic calc-alkaline granitic magmatism. These granitoids are distributed into several magmatic associations with different paleoproterozoic (1.97-1.89 Ga) ages, structural and geochemical affinities. Detailed mapping, petrographic and geochronological studies have distinguished two main subdomains in UAD. In the northern UAD, the high-K calc-alkaline Martins Pereira (1.97 Ga) and Serra Dourada S-type granites (1.96 Ga) are affected by NE-SW and E-W ductile dextral shear-zones, showing coexistence of magmatic and deformational fabrics related to heterogeneous deformation. Inliers of basement (2.03 Ga) crop out to northeastern part of this area, and are formed by metavolcano-sedimentary sequence (Cauarane Group) and TTG-like calc-alkaline association (Anauá Complex). Xenoliths of meta-diorites (Anauá Complex) and paragneisses (Cauarane Group) reinforce the intrusive character of Martins Pereira Granite. On the other hand, xenoliths of Martins Pereira and biotitebearing enclaves are founded in the younger, undeformed, and SiO2-rich Igarapé Azul Granite (1.89 Ga). This last and the high-K calc-alkaline Caroebe Granite (1.90-1.89 Ga, Água Branca Suite), including coeval volcanic rocks (1.89 Ga, Jatapu volcanics) and charnockitoids (1.89 Ga, e.g. Santa Maria Enderbite), crop out in the southern UAD. This subdomain is characterized only by local and slight NE-SW ductile-brittle dextral shear zones. A-type granites such as Moderna (ca. 1.81 Ga) and Mapuera (ca. 1.87 Ga) granites, cross cut both areas of UAD. Furthermore, the geological mapping also identified three main types of metalotects in this region. Gold mineralization is observed in Martins Pereira-Serra Dourada granitoids (northern UAD), alluvial columbite-tantalite is related to Igarapé Azul granitoids (southern UAD), and amethyst is associated to pegmatites from Moderna A-type granites. The Nd-Pb isotope data suggest that all granitoids of UAD are generated by reworking of older and pre-existence crustal sources (sialic Rhyacian-Archean and/or juvenile Transamazonian origin) and mantle input is not problably a viable model. Although the dominant process may be one subduction in the early stage of NUAD evolution, post-colisional magmatism may be a significant process in the production of new continental crust in the southern UAD. It is possible that, following oceanic closure in the Anauá arc system (2.03 Ga) and subsequent collisional orogeny (1.97-1.94 Ga?), underplated mantle melts (basalt liquids) were trapped below preexisting lower crustal rocks of various compositions (e.g. granulites, metatonalites, amphibolites). The basalt liquids and subsequently melted lower crust could produced the immense volumes of granite (and volcanics) observed at 1.90-1.87 Ga. This geological picture is similar to the Tapajós Domain (TD) in the southern Amazonian Craton and suggest that both belongs to the same province (Ventuari-Tapajós or Tapajós-Parima). Nevertheless, the scarcity of S-type granites and high-grade metamorphic rocks show that the collisional stage is not so evident in TD.
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Fácies deposicionais, estratigrafia e aspectos estruturais da cobertura sedimentar paleoproterozoica na serra do Tepequém, Escudo das Guianas, Estado de Roraima
(Universidade Federal do Pará, 2010-12-09) FERNANDES FILHO, Lucindo Antunes; TRUCKENBRODT, Werner Hermann Walter; http://lattes.cnpq.br/5463384509941553; 5463384509941553
In the northern South America, occurs the largest continuous exposure of Paleoproterozoic sedimentary deposits related to the Roraima Supergroup, forming the Pacaraima Block with 73,000 km2. This siliciclastic succession of more than 2 km thick is inserted in the Guyana Shield, northern Amazon Craton, extending from in the border of Brazil, Venezuela, Guyana and Suriname. Near of the Pacaraima Block occur isolated mountains with little-known siliciclastic successions of the faciologic and stratigraphic point view, which does not allow a better correlation with the Roraima Supergroup and, consequently, hinders the paleogeographic and paleoenvironmental reconstitution of the Paleoproterozoic in this part of Amazonia. Outcrop-based stratigraphic and facies studies in the Serra do Tepequém and Uiramutã region, State of Roraima, northern Brazil, allowed redefined and redescribed the lower unit of Roraima Supergroup as Arai Group. The Arai Group of approximately 400 m thick overlies acid volcanic rocks of the Surumu Group, is covered by siliciclastic deposits of the Suapi Group and has been subdivided into two formations. The lower one, similar in both areas, generally consists of polymictic conglomerates, cross-bedded sandstones with microplacers of hematite, and subordinate mudstones interpreted as braided stream deposits. In contrast, the upper formation, in the Uiramutã region, is composed of silty fine-grained sandstones with medium-scale trough cross bedding and convolute lamination deposited in a more distal braided stream environment. This unit, in the Serra do Tepequém region, consists of fine to medium-grained cross-bedded sandstones with mudstones, sandstone/mudstone rhythmites and subordinate conglomerates and breccias interpreted as coastal, tide-influenced deposits. The top of the Arai Group is marked by an expressive unconformity covered with diamond-bearing conglomerates and pebbly sandstones of the basal Suapi Group, interpreted as braided stream deposits. This study confirms the previous interpretation of a big braided stream system migrating to southwest in the central part of the Guyana Shield. In addition it could be shown that the fluvial system in its distal part (Serra do Tepequém) was influenced by tidal processes. The stratigraphy of the Arai Group has established the basis for a regional correlation included deposits of isolated occurrences in the Guyana Shield and points to the presence of a large intracratonic Paleoproterozoic basin connected to the open sea. The structural framework of Serra do Tepequém indicates that major sinistral oblique, normal and reverse NE-SW fault zones bound domains whose bedding dips mainly towards SE and NW. Regional scale forced folds are represented by kilometer scale kink bands and chevron folds compatible with upper-to-middle crustal level. These findings differ from previous regional models based on folding under ductile conditions related to collisional tectonics and evidence the importance of Guiana Shield early basement structures, reactivated probably during the K'Mudku event (~ 1.2 Ga).
Acesso aberto (Open Access)
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.
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)
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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Geometria, cinemática e modelamento tectônico das rochas do Grupo Vila Nova, região de Pedra Branca do Amapari – AP.
(Universidade Federal do Pará, 2009-05-25) CAVALCANTE, Geane Carolina Gonçalves; PINHEIRO, Roberto Vizeu Lima; http://lattes.cnpq.br/3251836412904734
The studied area is located at Pedra Branca do Amaparí region, center south Amapá state, at about 190 km from Macapá. The rocks exposed in this region comprise Palaeoproterozoic granites (Lafon et al., 2008) and greenstone belts related to Vila Nova Group. The Vila Nova Group represents a Palaeoproterozoic greenstone belt metamorphosed and deformed under greenschist to amphibolite facies conditions. This Group belongs to the Maroni-Itacaiúnas Province setting, in the eastern boundary of the Guianas Shield. The data obtained in this research point to the presence of ductile and brittle fabrics elements such as spaced and continous foliations, flexural, tight, open parasitic and isoclinal folds, structural and mineral lineation, kink bands and oblique to normal faults. Foliations are the main structures and display anastomotic patterns with a N-S and NE-SW preferred orientation. They have shallow to steep dips mainly to NW and SE. Flexural folds occur in the banded iron formations and amphibolitic rocks in centimeter to meter scale, with shallow to moderate plunging axes (12º to 50º) to NW, NE and SW. Stretching lineation is defined by elongation of quartz grains and shows shallow to steep plunges generally to NW. Boudins are also defined by quartz grains and normally occur between banded iron formation foliation planes, where they configure sigmoidal features. The data obtained suggest that the rocks composing the Vila Nova Group were deformed in consequence of diapirism of adjacent granite plutons, associated with partitioned transpression in the post-diapirical state when occurred compartimentation between simple shear and pure shear components. Transpression shows a sinistral component of simple shear and a E-W shortening pure shear component.
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Paleoambiente da Formação Prosperança, embasamento neoproterozóico da Bacia do Amazonas
(2011-03) BARBOSA, Roberto César de Mendonça; NOGUEIRA, Afonso César Rodrigues
The Neoproterozoic Prosperança Formation represents a sedimentary cover of southern Guyana Shield and is poorly exposed when contrasted with Paleozoic record of Amazonas and Solimões basins. Conglomerates, arkosic sandstones and mudstones are the characteristics deposits filling basement grabens. This unit is overlain unconformably by neoproterozoic carbonate rocks of Acarí Formation (observed only in cores). These formations represent the sedimentary basement of oil producing Paleozoic basins of the Amazon region. The precise characterization and paleoenvironmental reconstruction of Prosperança Formation are important for their distinction from Paleozoic units. Stratigraphic analysis was carried out at lower Negro River, State of Amazonas. Four facies associations were interpreted as products of a fluvial-deltaic system: prodelta/lacustrine, delta front, foreshore/shoreface and distal braided plain. Mudstones distributed along several kilometers suggest a sedimentary basin probably of lacustrine/restricted sea origins. Complex structured deltaic lobes were fed by braided distributaries that migrated mainly to SE. Sandstones generated under oscillatory/combined flux are conformable with shoreline deposits. Planar to trough cross-bedded sandstones are related to migration of subaqueous dunes, associated to fluvial-braided processes. Lenses of conglomerate, planar to trough cross-bedded sandstones, possibly of paleozoic age, which overlies unconformably the Prosperança Formation migrated to NW as product of proximal braided plain system and is in contrast to the S-directed cross-stratification of the Prosperança sandstones.