Two generations of mafic dyke swarms in the Southeastern Brazilian coast: reactivation of structural lineaments during the gravitational collapse of the Araçuaí-Ribeira Orogen (500 Ma) and West Gondwana breakup (140 Ma)

• Published in: Precambrian research Vol. 340; p. 105344
• Main Authors: Santiago, Raíssa, Caxito, Fabrício de Andrade, Neves, Mirna Aparecida, Dantas, Elton Luiz, Medeiros Júnior, Edgar Batista de, Queiroga, Gláucia Nascimento
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-05-2020
• Subjects: Alkaline and tholeiitic magmatism; Dyke swarm; Gravitational collapse of orogenic crust; Isotope geochemistry; Paleocontinent rifting and breakup; Reactivation of structures; Alkaline and tholeiitic magmatism; Reactivation of structures; Dyke swarm; Gravitational collapse of orogenic crust; Isotope geochemistry; Paleocontinent rifting and breakup
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2019.105344

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•Two distinct groups of mafic dykes emplaced in same NW-SE structures in SE Brazil.•Ca. 140 Ma tholeiitic dykes related to West Gondwana breakup.•Ca. 500 Ma alkaline dykes related to gravitational collapse of the Araçuaí Orogen.•They are distinguished by petrographic, geochemical and isotopic signatures.•Reactivation of structures as magma conduits in distinct tectonic settings and ages.
Mafic dyke swarms emplaced in regional NW-SE trending structures crosscutting the Precambrian basement in the Southeastern Brazilian coast are commonly interpreted as associated to West Gondwana breakup. Recently, however, U-Pb dating suggested that at least some of those dykes were emplaced earlier, at ca. 500–490 Ma, during the gravitational collapse of the Araçuaí Orogen. In order to clarify this issue, we studied dykes from the southern Espírito Santo State. Our results indicate that dykes emplaced in the same regional lineaments can be separated into two distinct groups, according to petrographic, geochemical, isotopic and geochronological characteristics. Group 1 is tholeiitic, with 87Sr/86Sr(i) of 0.7041–0.7065, εNd(t) of −3.4 to −5.5 and TDM ages between 0.8 and 1.5 Ga. Zircon crystals from a dyke of this group yielded the first robust lower Cretaceous U-Pb age for mafic dykes of SE Brazil with a Concordia age of 141.9 ± 1.9 Ma and εHf(t) in a range of −5.5 to −7.9. Group 2 is alkaline, shows higher 87Sr/86Sr(i) of 0.7064–0.7088, evolved εNd(t) < −12 and older TDM ages of 1.7–1.9 Ga. The youngest zircon crystals from a dyke of this group yielded a Concordia age of 504.7 ± 6.9 Ma and εHf(t) of −18.9. Our results suggest that the regional NW-SE trending lineaments of the Brazilian coast were active in at least three episodes, serving as conduits for mafic magmatism of distinct sources and tectonic settings: First, in the Cambrian, during the Araçuaí-Ribeira Orogen collapse, when they served as conduits for both the post-tectonic G5 Supersuite (530–490 Ma; composed of granitic and mafic plutons) and the alkaline dykes of Group 2; second, during the lower Cretaceous breakup of West Gondwana, when they served as conduits for the tholeiitic dykes of Group 1, synchronous to syn-rift evolution of the Phanerozoic Brazilian coast basins; and third, during the Cenozoic, when they were reactivated as normal brittle faults.