From extension to shortening: Dating the onset of the Brasiliano Orogeny in eastern Borborema Province (NE Brazil) |
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| Institution: | 1. Departamento de Geologia, Universidade Federal de Pernambuco, 50740-530 Recife, Brazil;2. Géoscience Montpellier, Université de Montpellier II, Montpellier, France;3. Pós-graduação em Geociências, Universidade Federal de Pernambuco, 50740-530 Recife, Brazil;1. Instituto de Ciência e Tecnologia, Universidade Federal de Alfenas, Poços de Caldas, MG, Brasil;2. Instituto de Geociências, Universidade Estadual de Campinas, SP, Brasil;3. Instituto de Geociências, Universidade de Brasília, DF, Brasil;1. Centro de Pesquisas Manoel Teixeira da Costa, Instituto de Geociências, Universidade Federal de Minas Gerais (CPMTC-IGC-UFMG), Campus Pampulha, Av. Antônio Carlos 6627, CEP 31270-901, Belo Horizonte, MG, Brazil;2. Instituto de Geociências, Universidade de Brasília, Campus Universitário, Asa Norte, CEP 70910-900, Brasília, DF, Brazil;3. GEOTOP, Université du Québec à Montréal, P.O. Box 8888, Station Centre Ville, Montreal, Quebec H3C 3P8, Canada;4. NEG-LABISE, Department of Geology, Universidade Federal de Pernambuco, Recife, PE 50740-530, Brazil;1. Departamento de Geologia, Programa de Pós-Graduação em Geodinâmica e Geofísica, Universidade Federal do Rio Grande do Norte, Natal, RN, Brazil;2. CPRM – Serviço Geológico do Brasil, Núcleo de Apoio de Natal (NANA/CPRM), Caixa Postal, CEP 59.078-970 Natal, RN, Brazil;1. Departamento de Geologia, Univ. Federal do Ceará, CE, Brazil;2. Instituto de Geociências, Universidade de Brasília, DF, Brazil;3. Instituto de Geociências, Universidade Estadual de Campinas, SP, Brazil;4. Géosciences, Université des Sciences et Techniques du Languedoc, Montpellier, France;5. Research School of Earth Science, Australian National University, Canberra, ACT, Australia |
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| Abstract: | In pre-drift reconstructions, the central and southern parts of the Borborema Province, northeastern Brazil, belong to a large Brasiliano-Pan-African orogenic realm situated to the north of the São Francisco-Congo Craton. In order to better understand the timing and geodynamic setting under which this orogenic system developed, a structural, geochemical and geochronological study was conducted across the east Pernambuco shear zone (EPSZ) system, which separates the Pernambuco-Alagoas Domain (PEAL) from the Central Domain. A sample of the Pinhões orthogneiss (GE-1), in the Central Domain, one sample of a syenitic orthogneiss (CA-34) wrapped by the EPSZ, and one sample of orthogneiss named Altinho (CA-40), in the northern portion of the PEAL, were dated by LA-ICP-MS. The Pinhões orthogneiss yielded an age of 869 ± 9 Ma, interpreted as the emplacement age of the protolith during a late Tonian magmatic episode. Samples CA-40 and CA-34 yielded 206Pb/238U weighted mean ages of 652 ± 6 Ma and 636 ± 3 Ma, respectively, which are interpreted as dating emplacement and crystallization of the magmatic protoliths. However, it is also possible that these rocks were formed during the same magmatic event in view of the identical ages of 646 ± 13 Ma and 646 ± 11 Ma, respectively, given by the less precise upper intercept of the discordia lines. The metaluminous and magnesian nature of the Altinho orthogneiss is akin to the calc-alkalic suite. However, some samples plot in the intraplate field in tectonic discrimination diagrams and the Nd TDM model age of 1.36 Ga is unlike that of juvenile magmas in convergent settings. The Altinho orthogneiss is quite similar in terms of trace elements geochemistry to the syenitic orthogneiss, which has a clearer intraplate affinity, and the dated samples have identical initial Sr isotope ratios (0.7047). Therefore, emplacement in an extensional setting is preferred over a convergent one. Two samples of paragneisses (SB-1 and BB-9) from the PEAL were also dated. The ages of the youngest zircon grains in sample BB-9 (655–642 Ma) overlap the crystallization age of the Altinho orthogneiss, implying that sedimentation is younger than or, at best, synchronous with magmatism. The age of low Th/U grains in samples CA-34 (615 ± 8 Ma) and SB-1 (587 ± 12 Ma) are related to a subsequent metamorphic overprint, which is loosely constrained between 580 and 620 Ma. These observations, combined with evidence provided by previous studies, suggest that the change from an extensional to a contracional setting occurred at ca. 640–630 Ma. In contrast with most collisional orogens, where a long period of oceanic subduction precedes collision, the inferred tectonic evolution suggests that the Brasiliano Orogeny resulted from inversion of continental and/or proto oceanic rifts. |
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| Keywords: | Brasiliano-Pan-African Orogeny LA-ICP-MS Geochemistry Provenance |
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