Petrogenetic insights into the volcanic evolution of the Brazilian Equatorial Margin - Sr-Nd-Pb modeling of magmatism in the Fernando de Noronha Archipelago and seamounts from the North Brazilian Ridge

The alkaline magmatism found in the Brazilian Equatorial Margin (BEM) in the Southwest Atlantic Ocean, such as that observed in Fernando de Noronha Archipelago (FNA) and in the North Brazilian Ridge (NBR) comprises three segments composed of several seamounts such as Ceará, São Luís and Maranhão. This magmatism poses intriguing questions about its origin and relationships with the offshore and onshore alkaline magmatism in northeast Brazil. The BEM is characterized by post-rift magmatic episodes that formed seamounts, guyots, and volcanic lineaments. Two prominent structures are the Fernando de Noronha Ridge (FNR), a W-E lineament about 550 km long comprising the FNA, Rocas Atoll and seamounts (e.g. Cabugi, Sirius, Fernando de Almeida) and the NBR, consisting of closely spaced seamounts roughly parallel to the continental margin. These volcanic structures have an intrinsic relationship to the Fernando de Noronha Fracture zone, related to the reactivation of extensional faults, which are associated with transpression  and the elevation of thick sedimentary cover by up to 100 meters, indicating recent tectonism in the region. The FNA exhibits a broad spectrum of alkaline magmatism, ranging from foiditic/basanitic to phonolitic/trachytic compositions and alkali-basalts and pyroclastic deposits, while the NBR seamounts are mugearites, alkali-basalts and basanites. The presence of glomerophyric textures and embayed porphyritic clinopyroxenes with compositional zoning indicates that the FNA and NBR magmatism resulted from multistage crystallization processes due to repeated injections of hot new magmas. Skeletal crystals of olivine and clinopyroxene have been identified and suggest rapid ascent of magmas promoting supercooling of the minerals. These petrographic interpretations are supported by a multi-approach thermobarometer study using clinopyroxene, amphibole, and zircon for FNA, revealing different stages for crystallization, with pressure and temperature ranging from 320 to 1,300 MPa and 750 to 1,240 ºC, respectively. For NBR, pyroxene, olivine and plagioclase reveal minimum and maximum temperatures of 926 to 1,292 ºC and pressures ranging from 70 to 3,600 MPa. The chemical modeling from whole-rock geochemistry suggests a low degree of partial melting (around 4% or less) of an enriched source with variable proportions from spinel and garnet-lherzolite melting zones. The FNA and the NBR present ⁸⁷Sr/⁸⁶Sr isotope values of ca. 0.705523, ¹⁴³Nd/¹⁴⁴Nd of ca. 0.51232 and Pb of ca. ²⁰⁶Pb/²⁰⁴Pb = 19.27, ²⁰⁷Pb/²⁰⁴Pb = 15.7 and ²⁰⁸Pb/²⁰⁴Pb = 39.2. Ɛ_Nd ranges from + 0.2 to + 4.4 in the FNA from - 4.8 to + 3.1 in the NBR. Modeling a 2D mixture line for these radiogenic isotopes, a dominant involvement of a depleted asthenosphere-type mantle source (DMM) with a minor input of two enriched components (EMI and young HIMU or FOZO) is noticed. The young HIMU (or FOZO) mantle component was invoked due to the radiogenic Pb isotopic compositions, which can be explained by the recycling of subducted dehydrated oceanic crust. Moreover, the EMI signatures are also observed in the Vitória-Trindade Ridge and in the Abrolhos Magmatic Province. This isotopic signature in the South Atlantic is related to the Brazilian Neoproterozoic continental lithosphere which was delaminated during the Gondwana Breakup, which generated a contaminated zone in the South Atlantic asthenosphere.