April 2025 LIP of the Month

The Equatorial Atlantic Magmatic Province (EQUAMP)

Antomat A. Macêdo Filho^1,6, Alisson L. Oliveira^1,2, Maria Helena B.M. Hollanda¹, Alana R. Dantas¹, Mark D. Schmitz², Emmanuel D. Ngonge¹, Alanny C. Melo³, David de Castro³, Carlos J. Archanjo¹, Elisabeta Erba⁴, Francisco A. Negri⁵, Carlos Ávila¹, Stephen Fraser⁶,James L. Crowley², Ravi Sampaio¹

¹Instituto de Geociências, Universidade de São Paulo, São Paulo, Brazil; antomat@alumni.usp.br

²Department of Geosciences, Boise State University, Boise, USA.

³Universiade Federal do Rio Grande do Norte, Natal, RN, Brasil

⁴Department of Earth Sciences Ardito Desio, University of Milan, Milan, Italy

⁵Instituto Geológico, Secretaria do Meio Ambiente do Estado de São Paulo, São Paulo, SP, Brazil

⁶BRC, Sustainable Minerals Institute, The University of Queensland, Queensland, Australia

Extracted and modified from:

1. Introduction

The EQUAMP is a plumbing-system type LIP in NE Brazil (Hollanda et al., 2019), with sill complexes and giant dike swarms exposed near the shoreline of the Equatorial Atlantic Ocean. The EQUAMP is intrusive in the Paleozoic Parnaíba Basin and Precambrian basement of the Borborema Province (Fig. 1), sills occur along the eastern side of the basinand dikes were emplaced immediately east along several swarms (Macêdo Filho et al., 2019; Macêdo Filho and Hollanda, 2022; Oliveira et al., 2022).

The primary feature of the EQUAMP is the Rio Ceará-Mirim (RCM) dike swarm, an approximately 1,000 km-long arcuate structure that runs parallel to the current E- (Ngonge et al., 2016) and NE-trending Atlantic margins (Macêdo Filho and Hollanda, 2022). Additionally, there are at least two other subswarms (Canindé and Riacho do Cordeiro), each 250-300 km long (Dantas et al., 2024; Macêdo Filho and Hollanda, 2022), situated parallel to the Atlantic coastline, located north and southeast of the RCM (Macêdo Filho et al., 2023a). The province was initially constrained by ⁴⁰Ar/³⁹Ar geochronology, dating between 133 and 126 Ma (Macêdo Filho et al., 2023a for a review).

The EQUAMP has been associated with the initial phases of the South Atlantic Rift System formation in the Equatorial regions of Brazil and Africa. This connection highlights a similarity between the EQUAMP and the Paraná-Etendeka Magmatic Province (PEMP; Peate et al., 1992). The PEMP is dated, also by ⁴⁰Ar/³⁹Ar geochronology, between 135 and 132 Ma (Gomes and Vasconcelos, 2021), which is either synchronous with or slightly older than the EQUAMP. Together, the two constitute one of the largest Mesozoic LIPs (>10 × 10⁶ km²) in the world (Macêdo Filho et al., 2023a).

Here, we present a geochemical and geochronological overview of the main EQUAMP elements. The geochemical data and recently published U-Pb (CA-ID-TIMS) ages allow for a robust compositional and temporal correlation with the PEMP, suggesting a possible genetic link between these LIPs. Here, we also provide brief considerations on the geodynamic implications of the EQUAMP, as well as its possible role in the Valanginian climate disturbances (Weissert event; Erba et al., 2004; Cavalheiro et al., 2021).

2. Magma Types

The EQUAMP intrusions are composed of tholeiitic diabases exhibiting both high- and low-TiO₂ compositions (Fig. 2). The high-Ti suite (TiO₂ > 2.0 wt%) is also enriched in incompatible elements (e.g., Sr > 400 ppm) and represents the dominant magma type within the EQUAMP, being prevalent in the Rio Ceará-Mirim and Canindé swarms, as well as in the Sardinha sills. Trachyandesites and trachytes with lower TiO₂ (< 2.5 wt%; Mg < 3 wt%) are interpreted as fractionated magmas from the high-Ti group (Macêdo Filho and Hollanda, 2022; Macêdo Filho et al., 2023b). Low-Ti tholeiites (TiO₂ < 2.0 wt%; Sr < 400 ppm) have also been identified within the Rio Ceará-Mirim swarm (Ngonge et al., 2016; Macêdo Filho and Hollanda, 2022), and constitute the exclusive magma type in the southern portion of the province, represented by the Riacho do Cordeiro swarm (Macêdo Filho et al., 2023a, b, c; Dantas et al., 2024). A more restricted type of olivine tholeiite occurs along the E–W branch of the RCM (Ngonge et al., 2016).

Previous geochemical and isotopic studies indicate enriched mantle sources (EMI-like) for the high-Ti magmatism that dominates the EQUAMP. These compositions are analogous to those found in the South Atlantic (Gough-type). Petrogenetic models for high-Ti tholeiites consider an asthenospheric source mixed with the subcontinental lithospheric mantle (SCLM) of West Gondwana or even the involvement of a plume with an EMI-Gough-type composition. On the other hand, low-Ti tholeiites would be derived from upper mantle domains, while low-Ti olivine tholeiites display trace element spectra similar to E-MORBs (Macêdo Filho and Hollanda, 2022).

3. High-Resolution Geochronology

Oliveira et al. (2025) collected eight samples from the three core elements representing the EQUAMP in an attempt to date the main features of this LIP (Fig. 1, 3). High-Ti basalts from both the E-trending (CM10) and NE-trending branches of the RCM swarm (DCE46 and DCE49), and one from the WNW-trending Canindé swarm (DCE66), produced CA-ID-TIMS weighted mean ages clustering within ~110 kyr, ranging from a maximum of 133.384 ± 0.026 [0.07/0.16] Ma to a minimum of 133.270 ± 0.041 [0.08/0.16] Ma. Two sill samples, the high-Ti basaltic andesite (BP245) and the andesite (BP255), were also dated. Sample BP255 yielded an age of 133.343 ± 0.017 [0.07/0.16] Ma, similar to those of the dikes, while sample BP245 provided two age clusters at 133.363 ± 0.029 [0.07/0.16] Ma and 133.071 ± 0.031 [0.07/0.16] Ma. Finally, the low-Ti dikes DBA03 and DPE06 from the NE-trending Riacho do Cordeiro swarm provided the oldest U-Pb ages of 133.805 ± 0.021 [0.07/0.16] Ma and 133.762 ± 0.050 [0.08/0.17] Ma, respectively. A subset of dikes emplaced along the same structural trend as the NE–SW branch of the RCM has been dated at 201.579 ± 0.057 Ma (DCE68; Fig. 1) and is therefore attributed to the Central Atlantic Magmatic Province (Oliveira et al., 2023).

4. South Atlantic Correlations

According to Macêdo Filho et al. (2023a), the high-Ti magmas of the EQUAMP are correlatable with the high-Ti types of the PEMP. These relationships are well illustrated by comparing samples from the Rio Ceará-Mirim dikes with those from the Transminas dikes in the northern PEMP, which display identical isotopic signatures. Such correlations can also be extended to other dike swarms, such as Serra do Mar, Resende–Ilha Grande, Ponta Grossa, and Florianópolis, as well as to the Pitanga-type (and Urubici-type) flood basalts and sills. Regarding the low-Ti types, there is a strong similarity between samples from the Rio Ceará-Mirim and Riacho do Cordeiro dikes.

The Riacho do Cordeiro dikes, located in the southern portion of the LIP, appear to be part of a large magma plumbing system that developed along the Atlantic margin, with the Vitória dikes being the closest equivalents within the PEMP. The compositions of the Riacho do Cordeiro dikes are correlatable with the Gramado type, whereas the Vitória dikes are more akin to the Esmeralda type, although there is significant overlap between these two types in both swarms.All these characteristics reinforce the correlation between the major South Atlantic LIPs, suggesting a large-scale tectonic-magmatic event linked to the South Atlantic Rift System (Macêdo Filho et al., 2023a; Dantas et al., 2024).

5. Geodynamics

If we consider a model associated with extensional tectonics during South Atlantic rifting, low-Ti magmas could have been generated by decompression melting of the upwelling asthenosphere (Fig. 4). These MORB-like magmas may have subsequently assimilated portions of the overlying lithosphere (both mantle and crust), producing (older) low-Ti tholeiites with trace element signatures that closely resemble those of arc-related magmas, along with pronounced isotopic heterogeneity. The generation of (younger) high-Ti tholeiitic magmas, in contrast, would require a more significant contribution from the subcontinental lithospheric mantle (Macêdo Filho and Hollanda, 2022).

In an active rifting scenario, plume upwelling would trigger decompression melting of the depleted upper mantle, producing MORB-like magmas that could assimilate lithospheric components to form low-Ti tholeiites. In this case, the plume would have a limited geochemical role, acting primarily as a thermomechanical driver — or, at most, playing a minor role in enriching (E-)MORBs. Within this framework, an OIB (EMI) plume impinging at the base of the continental lithosphere could partially melt the lithospheric mantle, leading to the generation of high-Ti tholeiitic magmas (Fig. 4).

6. Role In The Weissert Event

The beginning of the Weissert Event (133.9-132.6 Ma) marks a period of sea surface temperature (SST) warming (Cavalheiro et al., 2021; Ogg, 2020), which aligns with the early episode of magmatism of the EQUAMP and slightly postdates the emplacement of PEMP mafic dikes by approximately 0.8 Ma (Fig. 5). This warming period may be related to greenhouse effects following the mafic extrusive lava pile buildup of the PEMP around 134.8 Ma. The peak of the carbon isotope excursion (CIE) was positioned at approximately 133.3 Ma in the Late Valanginian, synchronous with the main magmatic activity of the EQUAMP, about 1.5 Ma after the initial dated PEMP extrusion. From the warmest peak to the end of the CIE there is a drop in global temperatures and pCO₂, where basalt weathering may have played a crucial role in atmospheric CO₂ capture, especially considering that these LIPs were emplaced in tropical to subtropical regions (da Conceição et al., 2015).

The emplacement of EQUAMP dikes, despite accounting for significant volumes of magma, was not able to maintain global pCO₂ levels. However, they strongly correlate to the maximum warming in the early part of the Weissert Event, which marks the Weissert Event temperature peak. Progressive weathering of PEMP basalts since the onset of the CIE may have caused CO₂ drawdown and the major temperature shift that ultimately cooled global temperatures. Late PEMP silicic volcanism at 132.6 Ma (Rocha et al., 2020) correlates with the relative warming shortly postponing the Weissert Event termination. However, ongoing massive weathering may have triggered another cooling episode culminating at the Valanginian/Hauterivian boundary (Gradstein and Ogg, 2020). The general hypothesis regarding the involvement of South Atlantic-related igneous provinces in the Weissert Event is illustrated in Fig. 5.

References