Navegando por Autor "AVELAR, Valter Gama de"

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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 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.