First petrologic data for Vitória Seamount, Vitória-Trindade Ridge, South Atlantic: a contribution to the Trindade mantle plume evolution

Highlights

• We detail new petrographic, whole rock and mineral chemistry from Vitória Seamount.

• Isotopic analyzes show radiogenic ⁸⁷Sr/⁸⁶Sr and unradiogenic ¹⁴³Nd/¹⁴⁴Nd values.

• The whole rock chemical signature of Vitória Seamount is typical of primitive melts.

• Its genesis points to a low partial melting proportion in garnet-lherzolite stability field.

Abstract

The Vitória Seamount (VTS), distant ~300 km from the Brazilian coastline at latitude 20°S, is the second closest offshore volcanic complex of the Vitória-Trindade Ridge (VTR) which corresponds to a ca. 1200 km long ridge of seamounts and islands composed of SiO₂-undersaturated magmatic rocks commonly considered to be the volcanic track of the Trindade mantle plume in the South American Plate. Based on the first sample dredged from Vitória Seamount, new petrographic and electron microprobe analyses from its rock show an alkaline basalt with pseudotrachytic texture consisting of bytownite and salite phenocrysts, labradorite microliths, anhedral titanomagnetite, and a yellowish green pseudomorphic phase composed of MgO–Al₂O₃–SiO₂–FeO. The fine-grained groundmass is mainly composed of strongly oriented lath-shaped labradorite microliths, opaque minerals, and vesicles filled by a yellowish green pseudomorphic phase. Whole-rock analyses of the Vitória Seamount rock reveal its SiO₂ undersaturation (SiO₂ ca. 40 wt.%; normative nepheline = 13.8), enrichment in Cr, Co, Ni, V and Sc, along with depletion in Zr, La and Nd contents compared to the other seamounts of the VTR. VTS show a strong enrichment in light-REE (La/Sm_N ca. 2.68) compared to heavy-REE (La/Yb_N = 20.79). Major and trace element evidence indicate that the melting of an enriched mantle source to generate the Vitória Seamount magma occurred dominantly in the garnet stability field. Trace element composition of VTS is consistent with ≤ 3% partial melting of the mantle source. Neodymium and Sr isotopic data suggest that the mantle source of the Vitória Seamount had been variably metasomatized by melts derived from enriched mantle component, which may have developed approximately 600 Ma, reconciling with the Brasiliano Orogeny, according to Nd age model. Modeling of the Nd–Sr isotope systematics points out that the primary melt was formed from an asthenospheric mantle (DMM – Depleted MORB [Mid-Ocean Ridge Basalts] Mantle) that underwent mixing with a continentally derived material (represented by EMI [Enriched Mantle I] component). This process can be explained by the mixing of melts from these mantle components during magma genesis.

Introduction

The Vitória Seamount (ca. 4700 km³) located offshore Brazil at ca.20°S, south of the Besnard Bank, is the second closest offshore volcanic edifice of the Vitória-Trindade Ridge (VTR), which some authors interpreted as the Trindade Plume volcanic trail on the South American Plate (Fig. 1) (see references for details - Gibson et al., 1999; Fodor and Hanan, 2000; Siebel et al., 2000, Santos, 2013; Bongiolo et al., 2015; Pires et al., 2016; Santos, 2016; Santos et al., 2018a, 2018b). Marques et al. (1999) also brought up the hypotheses that the Trindade Island's extrusive materials could come from stratified magma chambers that might be periodically replenished with ultrabasic magmas in the late stages of magmatic activity. The VTR is believed to be associated with the Vitória-Trindade Fracture Zone, that acted as a conduit for this enriched mantle-derived magmatism (Veloso and Machado, 1986; Szatmari and Mohriak, 1995; Conceição et al., 1996; Ferrari and Riccomini, 1999; Almeida, 2006; Alves et al., 2006).

The VTR is a west-east-trending alkaline igneous province that extends from the Brazilian eastern shelf to the deep-water portion of the southern Atlantic Ocean, towards the Trindade Archipelago located ca. 1200 km away from the coastline. This aseismic ridge is composed of several alkaline seamounts, banks, guyots, and islands. VTR's morphology studies date back to the 1950s and during the period 1972–78 an agreement between several Brazilian institutions gave rise to the REMAC project, which is a global reconnaissance of the Brazilian Continental Margin project. Hereafter, LEPLAC Program (Brazilian Continental Shelf Survey Program – 1987–2020) was started and carried out several surveys along the Brazilian margin, especially in the last decade, when additional multibeam bathymetric data were acquired in the VTR region, so that all banks and seamounts could be better described.

The most expressive submerged volcanic edifices (Fig. 2) correspond to the Besnard Bank (55 m), southeast of the Abrolhos Volcanic Complex (AVC), the Vitória Seamount (52 m), the Congress Bank (63 m), the Champlain Seamount (62 m), the Jaseur Seamount (54 m), the Montague Seamount (57 m), the Colúmbia Bank (60 m), the Davis Bank (61 m), the Asmus Bank, the Dogaressa Bank (54 m), the Colúmbia Seamount (96 m), the Motoki Hill, and the Palma Seamount, as well as the Trindade Island and the Martin Vaz Archipelago, which represent the easternmost and emerged segment of the ridge (Almeida, 2006; Santos et al., 2018a, 2018b). The magmatic rocks of this aseismic ridge have typical oceanic island basalts (OIBs) geochemical signatures, since they are characterized by the occurrence of alkaline rocks enriched in titanium (mean TiO₂ = 4.1 wt.%) and other incompatible lithophile trace elements (e.g., enrichment in light rare earth elements [LREE]). Some geochemical and isotopic studies (e.g., Marques et al., 1999; Siebel et al., 2000; Santos, 2013, 2016) carried out at the VTR indicate that these rocks were derived from magmas that originated from an asthenosphere-like source (DMM) metasomatized by recycled component (represented by EMI).

This work presents the first petrographic, mineralogical, geochemical, and isotopic data of the Vitória Seamount (VTS) magmatic rock, since it is the first sample dredged from this seamount. The goal of this study is to further characterize the mantle source(s) and the processes involved in the genesis of the Vitória Seamount using the first data of this magmatism, as well as comparing these data with the other VTR magmatic rocks and Abrolhos Volcanic Complex.