Evolução estrutural e idades U-Pb das unidades litoestratigráficas da Nappe de Passos e do Grupo Canastra, Sudoeste de Minas Gerais
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Data
2018-10-01
Autores
Silva, Alex Joaquim Choupina Andrade [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
A Nappe de Passos é uma importante unidade tectono-estratigráfica da porção meridional da Faixa Brasília, com forma alongada na direção ESE. Está relacionada aos processos de convergência tectônica das paleoplacas Paranapanema e São Francisco que, ao final do Neoproterozoico, culminaram na construção do Supercontinente Gondwana. O presente trabalho enfoca a porção noroeste da Nappe de Passos, entre os estados de Minas Gerais e São Paulo, utilizando-se o mapeamento geológico, para discutir o complexo padrão estrutural da área, aliado a estudos de proveniência dos metassedimentos dessa unidade e de compartimentos adjacentes, por meio de datação U-Pb em zircão detríticos. A Nappe de Passos corresponde, na região, o Domínio Interno e cavalga o Domínio Externo constituído por rochas metassedimentares do Grupo Canastra. Na porção central da área a superfície de cavalgamento desenha uma estrutura braquiantiforma em cujo núcleo ocorrem rochas dos grupos Canastra e Bambuí. Os planos de estratificação dos metassedimentos são paralelos a subparalelos à superfície de cavalgamento. A braquiantiforma apresenta a foliação principal em posição plano axial, definindo um domínio com orientação preferencial NW, que se estende por toda a porção oeste da área, onde o sistema de dobras têm continuidade, desenhadas pelas unidades litoestratigráficas da Nappe de Passos, com comprimento de onda quilométrico com dobras parasíticas de diversas escalas. Na porção nordeste da área, rochas do Grupo Canastra definem um domínio onde a foliação principal tem direção WNW com mergulhos íngremes para SW. Essa foliação é plano axial de dobras apertadas, com eixos com caimentos suaves dominantemente para NW, mas também para SE. Os indicadores cinemáticos evidenciam regime tectônico compatível com rampa lateral sinistral. Essa foliação é afetada por dobras com planos axiais com mergulhos suaves para SSW, em geral com foliação plano axial, paralelos à superfície de cavalgamento da NP, indicando a sobreposição da foliação relacionada ao transporte da NP nas estruturas da rampa lateral. Deformações tardias de caráter dúcti-rúptil afetam as foliações tanto do domínio NW quando do WNW, gerando dobras abertas a suaves com planos axiais íngremes, por vezes associadas a falhas. Essas últimas apresentam dimensões quilométricas, como a Falha de Cássia de direção NW na porção sul da área que a norte se ramifica nas falhas do Funil e Peixoto, que têm orientação aproximadamente NS. Duas lentes de gnaisses, que com as idades U-Pb obtidas no presente trabalho puderam ser confirmados como ortognaisses pertencentes ao embasamento relacionado ao Cráton do São Francisco, ocorrem ao longo de falhas, como lascas tectônicas embutidas em meio às rochas da Nappe de Passos e do Grupo Canastra. Análises combinadas U-Pb e Lu-Hf em zircão pelo método Laser Ablation Split Stream (LASS) forneceram idades de cristalização em ca. 2,10 Ga (Riaciano) com valores de "Hf(T) entre +10,0 a -1,8 e -0,2 a -13,0, respectivamente, para os ortognaisses alojados no Grupo Canastra e na Nappe de Passos. Datações U-Pb em grãos detríticos de zircão obtidos por LA-ICP-MS e LA-Q-ICP-MS permitiram determinar limites deposicionais e indicar possíveis fontes dos sedimentos do Grupo Canastra, Nappe de Passos e Grupo Bambuí. Grãos de zircão do Grupo Canastra apresentaram idades entre 1,30 e 3,41 Ga, com pico principal em torno de 2,14 Ga (Riaciano), seguido por fontes secundárias de 2,70 Ga (Neoarqueano). A idade máxima de sedimentação em 1301 ± 22 Ma é dada pelo zircão mais novo. As rochas da Nappe de Passos forneceram idades de zircões com populações meso- a neoarqueanos (3,08-2,50 Ga), Riaciano-Orosiriano (2,29-1,9 Ga, com importante pico em ca. 2,13 Ga), e, em menor proporção, Estateriano-Calimiano (1,78-1,61 Ga). O grão mais jovem forneceu a idade 753 Ma (2,65% discordante), sendo interpretada como a idade máxima de deposição para as unidades inferiores da Nappe de Passos, cerca de 150 Ma mais jovem do que estava estabelecido pelos dados existentes até então. A fonte dominantemente riaciana para os sedimentos da Nappe de Passos e do Grupo Canastra aponta forte contribuição do Cráton São Francisco, possivelmente relacionada aos granitóides juvenis do Cinturão Mineiro. As idades em torno de 1,70 Ga podem ser atribuídas a de diques máficos equivalentes aos denominados Pará de Minas, que cortam a porção sul do Cráton São Francisco. O metassiltito associado às rochas do Grupo Canastra na estrutura braquiantiforma, apresenta idades com picos de concentração em 586 Ma, 1157 Ma, predominante 2190 Ma, seguido de menores 2701 Ma e 3301 Ma. A idade máxima para a deposição dada pelo grão concordante mais jovem é 541 ± 14 Ma. Idades no limite Ediacarano/Cambriano (541 Ma) tem sido registradas para o Grupo Bambuí, por isso o metassiltito é atribuído a essa unidade, interpretado como depósito de foreland fortemente envolvido no processo de alojamento da NP que, por consequência, tem que ser Ediacarano ou mais jovem.
The Passos Nappe is an important tectono-stratigraphic unit of the southern sector of Brasília Belt, exhibits an ESE-elongated structural. It is related to the tectonic convergence processes of the Paranapanema and São Francisco paleoplates that collided during the assembly of Gondwana supercontinent by the end of the Neoproterozoic. The present work focuses the northwestern sector of Passos Nappe, between the states of Minas Gerais and São Paulo, using geological mapping, to discuss the structural pattern of the area, together with provenance studies carried out on metasediments of Passos Nappe and adjacent compartments, by U-Pb dating on detrital zircon. The Passos Nappe is represented by Internal Domain and is thrusted over the External Domain. This is constituted predominantly by metassedimentary rocks of Canastra Group. In the central portion of the area the thrust surface delineates a brachiantiformal structure, cored by rocks of the Canastra and Bambuí groups. The plane of stratification of the metasedimentary rocks are parallel to subparallel to the the thrust surface. The brachiantiforme displays axial plane foliation, affecting the rocks of the Passos Nappe and defining a domain with preferential orientation NW. The foliation extends on westerly of the area, where the system of folds has continuity, delineated by the lithostratigraphic units of Passos Nappe. These exposures show kilometer wavelength folding, with parasitic folds of different scales. In the northeast sector of the area the Canastra Group rocks define a domain where the main foliation has WNW direction with steep dips towards the SW. This foliation is axial plane of tight folds, with slightly plunging axes to the NW and SE. The kinematic indicators show tectonic regime compatible with sinistral lateral ramp. This foliation is affected by folds with axial planes of dip gently to SSW, parallel to the thrust surface of the Passos Nappe, indicating superposition structures developed in Passos Nappe under the External Domain. Late deformations of ductile-brittle behaviour affect the foliations of both NW and WNW domains, generating gentle to open folds with steep axial planes, sometimes associated with faults. These last have kilometric dimensions, such as the Cassia Fault of NW direction in the southern sector of the area. To the north the Cássia Fault branches in the Funil and Peixoto faults, oriented approximately NS. U–Pb ages of two lenses of gneisses recognized in this work were confirmed as orthogneisses belonging to the basement of the São Francisco Craton. The orthogneisses occur along faults, such as tectonic slices hosted in the rocks of Passos Nappe and Canastra Group. Combined analyzes of U-Pb and Lu-Hf in zircon by the Laser Ablation Split Stream method (LASS) provided ca. 2.10 Ga (Rhyacian) with values of "Hf(t) between +10.0 to -1.8 and -0.2 to -13.0 respectively, for orthogneisses housed in the Canastra Group and Nappe de Passos. U-Pb data in detrital zircons grains obtained by LA-ICP-MS and LA-Q-ICP-MS allowed to determine depositional limits and indicate possible sources of sediments of the Canastra Group, Passos Nappe and Bambuí Group. Zircon grains of the Canastra Group yielded ages between 1.30 and 3.41 Ga, with main peak around 2.14 Ga (Rhyacian), followed by secondary sources of 2.70 Ga (Neoarchean). The maximum sedimentation age at 1301 ± 22 Ma is given by the younger zircon. The rocks of Passos Nappe yielded ages with meso- to neoarchean populations (3.08-2.50 Ga), Rhyacian-Orosirian (2.29-1.9 Ga, with an important peak at ca. 2,13 Ga ), and, to a lesser extent, Statherian -Calymmian (1,78-1,61 Ga). The youngest grain yielded the age of 753 Ma (2.65 % discordant), interpreted as the maximum depositional age for metasediments of the lower portion of Passos Nappe, about 150 Ma younger than was established by the then existing ones. The dominant Rhyacian source for the sediments of the Passos Nappe and the Canastra Group shows a strong contribution of the São Francisco Craton, possibly related to the juvenile granitoids of the Mineiro Belt. The ages around 1.70 Ga can be attributed to the swarm of mafic dikes equivalent to the so-called Pará de Minas, which cut the southern portion of the São Francisco Craton. The metasiltite associated with the rocks of the Canastra Group in the brachiantiformal structure, show ages with concentration peaks at 586 Ma, 1157 Ma, main peaks at 2190 Ma and minor peaks at 2701 Ma and 3301 Ma. The youngest zircon age (541 ± 14 Ma) is interpreted as the maximum depositional age of the metasiltite. Ages at the Ediacaran–Cambrian limit (541 Ma) have been registered for the Bambuí Group, therefore, the metasiltite is attributed to this unit, interpreted as a deposit of foreland strongly involved in the process of housing the NP which, therefore, has to be Ediacaran or younger.
The Passos Nappe is an important tectono-stratigraphic unit of the southern sector of Brasília Belt, exhibits an ESE-elongated structural. It is related to the tectonic convergence processes of the Paranapanema and São Francisco paleoplates that collided during the assembly of Gondwana supercontinent by the end of the Neoproterozoic. The present work focuses the northwestern sector of Passos Nappe, between the states of Minas Gerais and São Paulo, using geological mapping, to discuss the structural pattern of the area, together with provenance studies carried out on metasediments of Passos Nappe and adjacent compartments, by U-Pb dating on detrital zircon. The Passos Nappe is represented by Internal Domain and is thrusted over the External Domain. This is constituted predominantly by metassedimentary rocks of Canastra Group. In the central portion of the area the thrust surface delineates a brachiantiformal structure, cored by rocks of the Canastra and Bambuí groups. The plane of stratification of the metasedimentary rocks are parallel to subparallel to the the thrust surface. The brachiantiforme displays axial plane foliation, affecting the rocks of the Passos Nappe and defining a domain with preferential orientation NW. The foliation extends on westerly of the area, where the system of folds has continuity, delineated by the lithostratigraphic units of Passos Nappe. These exposures show kilometer wavelength folding, with parasitic folds of different scales. In the northeast sector of the area the Canastra Group rocks define a domain where the main foliation has WNW direction with steep dips towards the SW. This foliation is axial plane of tight folds, with slightly plunging axes to the NW and SE. The kinematic indicators show tectonic regime compatible with sinistral lateral ramp. This foliation is affected by folds with axial planes of dip gently to SSW, parallel to the thrust surface of the Passos Nappe, indicating superposition structures developed in Passos Nappe under the External Domain. Late deformations of ductile-brittle behaviour affect the foliations of both NW and WNW domains, generating gentle to open folds with steep axial planes, sometimes associated with faults. These last have kilometric dimensions, such as the Cassia Fault of NW direction in the southern sector of the area. To the north the Cássia Fault branches in the Funil and Peixoto faults, oriented approximately NS. U–Pb ages of two lenses of gneisses recognized in this work were confirmed as orthogneisses belonging to the basement of the São Francisco Craton. The orthogneisses occur along faults, such as tectonic slices hosted in the rocks of Passos Nappe and Canastra Group. Combined analyzes of U-Pb and Lu-Hf in zircon by the Laser Ablation Split Stream method (LASS) provided ca. 2.10 Ga (Rhyacian) with values of "Hf(t) between +10.0 to -1.8 and -0.2 to -13.0 respectively, for orthogneisses housed in the Canastra Group and Nappe de Passos. U-Pb data in detrital zircons grains obtained by LA-ICP-MS and LA-Q-ICP-MS allowed to determine depositional limits and indicate possible sources of sediments of the Canastra Group, Passos Nappe and Bambuí Group. Zircon grains of the Canastra Group yielded ages between 1.30 and 3.41 Ga, with main peak around 2.14 Ga (Rhyacian), followed by secondary sources of 2.70 Ga (Neoarchean). The maximum sedimentation age at 1301 ± 22 Ma is given by the younger zircon. The rocks of Passos Nappe yielded ages with meso- to neoarchean populations (3.08-2.50 Ga), Rhyacian-Orosirian (2.29-1.9 Ga, with an important peak at ca. 2,13 Ga ), and, to a lesser extent, Statherian -Calymmian (1,78-1,61 Ga). The youngest grain yielded the age of 753 Ma (2.65 % discordant), interpreted as the maximum depositional age for metasediments of the lower portion of Passos Nappe, about 150 Ma younger than was established by the then existing ones. The dominant Rhyacian source for the sediments of the Passos Nappe and the Canastra Group shows a strong contribution of the São Francisco Craton, possibly related to the juvenile granitoids of the Mineiro Belt. The ages around 1.70 Ga can be attributed to the swarm of mafic dikes equivalent to the so-called Pará de Minas, which cut the southern portion of the São Francisco Craton. The metasiltite associated with the rocks of the Canastra Group in the brachiantiformal structure, show ages with concentration peaks at 586 Ma, 1157 Ma, main peaks at 2190 Ma and minor peaks at 2701 Ma and 3301 Ma. The youngest zircon age (541 ± 14 Ma) is interpreted as the maximum depositional age of the metasiltite. Ages at the Ediacaran–Cambrian limit (541 Ma) have been registered for the Bambuí Group, therefore, the metasiltite is attributed to this unit, interpreted as a deposit of foreland strongly involved in the process of housing the NP which, therefore, has to be Ediacaran or younger.
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Palavras-chave
Proveniência sedimentar, Idade U-Pb em zircão detrítico, Nappe de Passos, Sedimentary provenance, U–Pb detrital zircon ages, Passos Nappe