Shrimp and conventional U-Pb age, Sm-Nd isotopic characteristics and tectonic significance of the K-rich Itapuranga suite in Goiás, Central Brazil

The Itapuranga alkali granite and Uruana quartz syenite are large K-rich EW-elongated intrusions, in the central part of the Neoproterozoic Brasília Belt, central Brazil. They are associated with Pireneus lineaments, which cut the regional NNW-SSE structures of the southern part of the belt. SHRIMP and conventional U-Pb data for the Itapuranga and Uruana intrusions indicate crystallization ages of 624 ± 10 Ma and 618 ± 4 Ma, respectively. Three zircon cores from the Itapuranga granite yielded U-Pb ages between 1.79 and 1.49 Ga. Sm-Nd TDM ages for both intrusions are 1.44 Ga and eNd(T) values are -5.1 and -5.7, suggesting the input of material derived from older (Paleo- to Mesoproterozoic) sialic crust in the origin of the parental magmas. Magma mixing structures indicate co-existence of mafic and felsic end-members. The felsic end-member of the intrusions is dominantly represented by crust-derived melts, formed in response to the invasion of Paleo/Mesoproterozoic sialic crust by alkali-rich mafic magmas at ca. 620 Ma. These intrusions are roughly contemporaneous with, or perhaps slightly younger than, the peak of regional metamorphism in the southern Brasília Belt. Their emplacement along the Pireneus lineament suggest a syn-tectonic origin for them, most probably in transtensional settings along these faults

Shrimp U-Pb age and Sr-Nd isotopes of the Morro do Baú mafic intrusion: implications for the evolution of the Arenópolis volcano-sedimentary sequence, Goiás Magmatic Arc

The Arenópolis volcano-sedimentary sequence is located in the southern part of the Goiás Magmatic Arc and includes a ca. 900 Ma calc-alkaline arc sequence made of volcanic rocks ranging in composition from basalts to rhyolites, metamorphosed under greenschist to amphibolite facies. Small calc-alkaline gabbro to granite sub-volcanic bodies are also recognized. The Morro do Baú intrusion is the largest of these intrusions, and is made of gabbros and diorites. Zircon grains separated from one gabbro sample and analyzed by SHRIMP I yielded the mean 206Pb/238U age of 890 +/– 8 Ma, indicating that the intrusion is roughly coeval or only slightly younger than the Arenópolis volcanics. Contrary to the metavolcanics, which are juvenile, the Nd isotopic composition of the Morro do Baú gabbro indicates strong contamination with archean sialic material (TDM of 2.8 Ga and EpsilonNd(T) of –9.7), represented in the area by an allochthonous sliver of archean/paleoproterozoic gneisses (Ribeirão gneiss) which are the country-rocks for the gabbro/dioritic intrusion. The emplacement age of ca. 890 Ma represents a minimum age limit for the tectonic accretion of the gneiss sliver to the younger rocks of the Arenópolis sequence. The data suggest that this happened early in the evolution of the Goiás Magmatic Arc, between ca. 920 and 890 Ma

Aeromagnetometry and aerogammaspectrometry integrated with U-Pb zircon geochronology of northern Bossoroca ophiolite, Brasiliano Orogen

Age delimitation integrated with aeromagnetometric and aerogammaspectrometric survey advances the understanding of ophiolite evolution in the Brasiliano Orogen. We focused on the Bossoroca ophiolite, because oceanic crustal and mantle rocks contain zircon in metasomatic chloritite. A metadiorite and a metavolcanoclastic rock were also studied to delimit relationship between ophiolite and island-arc infrastructure and superstructure. Zircon crystals were dated by laser ablation inductively coupled plasma emission spectroscopy. Ages of zircon from Campestre metavolcanoclastic rock are 920-840 (peak 842) Ma, Bossoroca chloritite 900-800 (peak 868 Ma) and Capivaras metadiorite 698 Ma. Ages 920-800 Ma correspond to processes in the oceanic crust, whereas 698 Ma was a late magmatic intrusion (Capivaras metadiorite) into the island-arc infrastructure. Aeromagnetometric and aerogammaspectrometric data delimit the occurrence and structure of the ophiolite. These are major multiproxy markers of geotectonic processes early in the Brasiliano Orogen

Shrimp and conventional U-Pb age, Sm-Nd isotopic characteristics and tectonic significance of the K-rich Itapuranga suite in Goiás, Central Brazil

The Itapuranga alkali granite and Uruana quartz syenite are large K-rich EW-elongated intrusions, in the central part of the Neoproterozoic Brasília Belt, central Brazil. They are associated with Pireneus lineaments, which cut the regional NNW-SSE structures of the southern part of the belt. SHRIMP and conventional U-Pb data for the Itapuranga and Uruana intrusions indicate crystallization ages of 624 ± 10 Ma and 618 ± 4 Ma, respectively. Three zircon cores from the Itapuranga granite yielded U-Pb ages between 1.79 and 1.49 Ga. Sm-Nd TDM ages for both intrusions are 1.44 Ga and eNd(T) values are -5.1 and -5.7, suggesting the input of material derived from older (Paleo- to Mesoproterozoic) sialic crust in the origin of the parental magmas. Magma mixing structures indicate co-existence of mafic and felsic end-members. The felsic end-member of the intrusions is dominantly represented by crust-derived melts, formed in response to the invasion of Paleo/Mesoproterozoic sialic crust by alkali-rich mafic magmas at ca. 620 Ma. These intrusions are roughly contemporaneous with, or perhaps slightly younger than, the peak of regional metamorphism in the southern Brasília Belt. Their emplacement along the Pireneus lineament suggest a syn-tectonic origin for them, most probably in transtensional settings along these faults

U-Pb SHRIMP and Sm-Nd geochronology of the Silvânia Volcanics and Jurubatuba Granite : juvenile paleoproterozoic crust in the basement of the neoproterozoic Brasília belt, Goiás, central Brazil

U-Pb SHRIMP and Sm-Nd isotopic ages were determined for felsic metavolcanic rocks from the Silvânia Sequence and Jurubatuba Granite in the central part of the Brasília Belt. Zircon grains from a metavolcanic sample yielded 2115 ± 23 Ma and from the granite yielded 2089 ± 14 Ma, interpreted as crystallization ages of these rocks. Six metavolcanic samples of the Silvânia Sequence yielded a six-point whole-rock Sm-Nd isochron indicating a crystallization age of 2262 ± 110 Ma and positive eNd(T) = +3.0 interpreted as a juvenile magmatic event. Nd isotopic analyses on samples from the Jurubatuba Granite have Paleoproterozoic TDM model ages between 2.30 and 2.42 Ga and eNd(T) values vary between -0.22 and -0.58. The oldest TDM value refers to a sedimentary xenolith in the granite. These results suggest crystallization ages of Silvânia volcanics and Jurubatuba Granite are the first evidence of a ca. 2.14-2.08 juvenile magmatic event in the basement of the central part of the Brasília Belt that implies the presence of arc/suture hidden in reworked basement of the Brasília Belt

Química mineral e estimativas de pressão e temperatura em rochas metassedimentares do grupo Araxá na região de Morrinhos, sul do estado de Goiás

In the Morrinhos region, Goiás state, Brazil, the rocks of the Araxá Group have mineral assemblages related to the metamorphic peak which occurred in this region and is typical of the amphibolite facies (kyanite zone). These rocks are associated with the initial stage of the main ductile deformation that occurred in this area. Thermobarometric calculations, including the associations, garnet + plagioclase + biotite + muscovite; garnet + plagioclase + biotite + muscovite + kyanite; garnet + plagioclase + biotite + muscovite + amphibole, with quartz in excess, indicate that the metamorphic peak occurred at temperatures of approximately 610°C and pressures in the order of 10 kbar. The P-T path is probably clockwise and is in agreement with the tectonic conditions observed in collisional settings.Na região de Morrinhos, estado de Goiás, as rochas do Grupo Araxá apresentam associações minerais, relacionadas ao ápice metamórfico, típicas da fácies anfibolito (zona da cianita) e são correlacionadas a estágio anterior ou inicial da principal fase deformacional de caráter dúctil (Dn) observada regionalmente. Cálculos termobarométricos envolvendo granada + plagioclásio + biotita + muscovita, granada + plagioclásio + biotita + muscovita + cianita, granada + plagioclásio + biotita + muscovita + anfibólio, com quartzo em excesso, indicam que o pico metamórfico ocorreu à temperatura na ordem de ~610°C e pressões de ~10 kbar; com trajetória P-T provavelmente horária, compatível com as condições tectônicas observadas em ambientes colisionais

Mecanismos de deformação e propriedades físicas anisotrópicas em granulitos

Orientador: Prof. Dr. Leonardo Evangelista LagoeiroCoorientadora: Profa. Dra. Paola Ferreira BarbosaDissertação (mestrado) - Universidade Federal do Paraná, Setor de Ciências da Terra, Programa de Pós-Graduação em Geologia. Defesa : Curitiba, 20/02/2018Inclui referências: p.72-79Resumo: O trabalho apresenta dados microestruturais e texturais que foram coletados a fim de investigar os mecanismos de deformação ativos em plagioclásios e piroxênios e compreender a influência desses na propagação de ondas sísmica. A técnica de difração de elétrons retro-espalhados, associado à microscopia eletrônica de varredura foi utilizada a fim de avaliar condições deformacionais por orientação cristalográfica prefencial (CPO), parâmetros dos grãos e misorientatios. Os dados foram processados a partir da ferramenta MTEX, que permitiu também o cálclulo das propiedades físicas anisotrópicas pelo método Voith-Reuss-Hill. As amostras pertencem ao Complexo Barro Alto, um cinturão granulítico composto de gabro-noritos deformados em temperature entre 750 e 900°C. O complexo posiciona-se dentro da Província Estrutural Tocantins, desenvolvida entre os crátons da Amazônico São Francisco e Paranapanema durante o Ciclo Orogênico Brasileiro (Neoproterozóico). As amostras são compostas por plagioclásio + diópsido + enstatia + opacos (ilmenita) ± hornblenda ± olivina (fosterita)± biotita em diferentes intensidades deformacionais, separadas em dois domínios. O primeiro contém quatro amostras que são formadas por uma matriz fina (0.03 a 0.08 mm) intensamente recristlizada que envolve porfiroclástos de plagioclásio, orto e clinopiroxênios. Os porfiroclastos de plagioclásicos apresentam extinção ondulante, maclas arqueadas e estrutura do núcleo-manto, enquanto piroxênios exibem extinção ondulante. Outro domínio é composto por um agregado de granulação média (± 0.5 mm), que apresentam forma granular e junções tríplices a 120°, onde plagioclásios e piroxênios com similar extinção ondulante. Em ambos os domínios ocorre a separação entre bandas máficas e félsicas. Em termos texturais, há considerável variações entre tais domínios. Enquanto para anortita os pólos de {100}, {010} e {001} são distribuídos aleatoriamente em relação ao plano XZ na porção mais fina, percebe-se a orientação de {010} próximo a direção Z e [100] próximo a X com aumento do tamanho de grão, caracterizando o sistema de deslizamento [100]{010} neste domínio. Essa variação é acompanhada pelo aumento do J index (2.5891 para 15.3275). Para o diopsídio, tal variação não é tão perceptível, predominado uma fraca textura em ambos os domínios. Sugere-se portantdo que a deformação é controlado for fluxo de deslocações em agregados mais grosseiros e a progressão da deformação e intensa recristalização resulta em grãos finos o suficiente para ativação do deslizamento ao longo da borda de grãos. Os perfis sísmicos derivados de CPO foram plotados em relação a velocidade de propagação de ondas P (Vp), anisotropia de onda S (AvS) e direção de polarização de S1 (VS1). Para o domínio mais fino, percebe-se Vp máxima paralela ou subparalela à [010], com pequena variação do valor máximo além de Avs extremamente baixo (3 %), sugerindo que mecanismos sensíveis ao tramanho do grão fazem com que o meio tenha um comportamento próximo do isotrópico. A fim de avaliar a influência de cada fase, três perfis com semelhante tamanho de grão e diferentes proporções de anortita/diopsidios foram analisados e, por essa análise, concluiu-se que o aumento do teor de diopsidio influencia diretamente a Vp. Palavras-chave: mecanismos de deformação, plagioclásio, piroxênio.Abstract: In this work, we used microstructural and textural analyses to evaluate aspects related to plastic deformation in plagioclase and pyroxenes and the relationship between textures and seismic properties. The EBSD technique was employed to analyze the crystallographic preferred orientation (CPO) of feldspar in high-temperature mylonites from the Barro Alto complex.The Barro Alto complex is a continental-scale feature exposed in the Brasília Belt, which belongs to the Tocantins Structural Province in Central Brazil. This complex was formed by a mafic-ultramafic layered intrusion mylonitized and metamorphosed under granulite facies conditions. The samples are composed of porphyroclasts of plagioclase and diopside embedded in a fine matrix of plagioclase, clinopyroxene, orthopyroxene and, less commonly, amphibole and biotite. The plagioclase porphyroclasts exhibit undulose extinction and core-mantle structures. The samples were separated in two domainal microstructures: (1) large porphyroclasts (0.5 to 2 mm) surrounded by a very fine-grained mylonitic matrix (0.03 to 0.08 mm); (2) plagioclase with grain size ranging from 0.5 to 1 mm. Both domainal types also show a compositional layering of mafic and felsic components. In the first domain, CPO from the fine matrix plagioclase poles figures shows the planes (100),(010),(001) randomly distributes, with a low concentration in {010} parallel to Z direction. The weak texture is confirmed also for a low J index (2.4). In the second domain, coarse plagiclase exhibits maximum of poles to {100} parallel to the X direction (stretching lineation) and poles to {010} parallel to the Z direction (pole to foliation). The J index is high (15.33). The microstructure and texture analyses suggest that different deformation mechanisms were active in each group. The weak texture observed in the finer-grained domain may be a result of grain boundary sliding processes. On the other hand, in coarser-grained domains dislocation creep may dominate, which led to the development of a stronger texture. In these domains the prime deformation mechanism is dislocation creep controlled by the [100](010) slip system, and secondarily by the [100](001) slip system in plagioclase. These results are consistent with high-grade deformation conditions of the Barro Alto Complex. The seismic anisotropy patterns for the fine-grained domains present low value of P-wave velocity (Vp), being the fast velocity direction perpendicular to the foliation, while the S wave anisotropy is extremely low (3%). The mineral assembly and the deformation controlled by diffusion probably contribute to decrease the anisotropic behavior of these rocks, creating patterns similar to those found in an isotropic media. In the coarser-grained domains, the P-wave velocity is also low, but the strongest texture are directally reflected by the S-wave anisotropy (6%) and S1 polarization. The Vp distribution is dependet on proportion plagioclase/diopside in matrix and it is controlled by the b-axis of both minerals. We conclude that differences in grain size suggests a strong partitioning of deformation between grain size sensitive processes in fine-grained aggregates, and dislocation creep in large grains aggregates and these differences are also perceived in seismc behavior of the aggregate. Key words: deformation mechanism, plagioclase, diopside