Young volcanism in the Borborema Province, NE Brazil, shows no evidence for a trace of the Fernando de Noronha plume on the continent

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Abstract

We present 40Ar/39Ar ages for four volcanic bodies from a group of volumetrically minor alkaline plugs, necks, and dikes in northeastern Brazil, previously ascribed to passage over the purported Fernando de Noronha plume. The rocks are relatively primitive (9.5–14.7 wt.% MgO), typically nepheline-normative basanites with ocean island basalt (OIB)-like trace-element compositions similar to alkalic basalts from the Fernando de Noronha archipelago. Highly fractionated REE coupled with relative depletions of K and Rb indicates that the silica undersaturated magmas were generated by small degrees of melting in the presence of residual garnet and a hydrous metasomatic phase. Three of the four units (Caracarazinho, Cabugizinho da Arara and Serra Preta de Bodó) were heretofore undated. The fourth body (Cabelo de Negro) was included to facilitate comparison with published K–Ar dates. 40Ar/39Ar age determinations by the laser incremental-heating method on duplicate grains of groundmass reveal the youngest continental volcanism in Brazil, with emplacement ages between 8.9 ± 0.5 and 7.1 ± 0.3 Ma. Our age for Cabelo de Negro (7.9 ± 0.3 Ma) is roughly 20 Ma younger than the published K–Ar date for this plug. The reproducibility of our duplicate analyses and the consistency of the plateau, ideogram and isochron ages for this sample attest to the reliability of the new 40Ar/39Ar results. Our geochronological results show that volcanic activity on the continent did not shut down prior to the onset of volcanism on the island of Fernando de Noronha. Both areas were active contemporaneously for at least 5 Ma. We argue that the extended duration, small volume and lack of a clear age progression suggest that this example of alkaline intraplate volcanism is more likely the surface manifestation of the upwelling flow seen in an edge-driven convection mode, rather than tracking passage over a deep-seated mantle plume. This hypothesis is supported by xenolith thermobarometery, heat-flow data and seismic tomography, which collectively provide evidence for a mild thermal anomaly (upwelling) in the upper mantle under the eastern-most equatorial Brazil that extends into the Atlantic and appears to travel with the plate. Small-scale convection is also consistent with the seismic evidence for a downwelling to about 600 km depth in the mantle beneath the cratonic margin of eastern Brazil. Given the inherently sluggish upwelling associated with such small-scale convection, it appears that the entrainment of metasomatically enriched lithospheric mantle is likely vital to melt generation. If so, the spatial distribution of edge-driven magmatism may provide a tool for mapping fertility of the subcontinental lithospheric mantle.

Research Highlights

►40Ar/39Ar geochronology reveals the youngest volcanism in mainland Brazil. ►Volcanism is a result of edge-driven convection, rather than a deep-seated mantle plume. ►Distribution of edge-driven magmatism maps fertility of the subcontinental mantle.

Introduction

Northeastern Brazil has been a site of recurrent magmatism over the past 200 Ma, with marked events inferred from K–Ar geochronology at about 210–180, 140–120 and 90 Ma, with a less-well defined episode since 50 Ma (Mizusaki et al., 2002, and references therein). The Mesozoic record is dominated by relatively voluminous tholeiitic extrusive and intrusive rocks and has been linked to mantle-plume activity (St. Helena hotspot) and continental rifting associated with the opening of the central and equatorial Atlantic (Bellieni et al., 1992, Fodor et al., 1990, Hollanda et al., 2006, Mizusaki et al., 2002, Wilson and Guiraud, 1992). In contrast, Cenozoic activity is restricted to small-volume alkaline magmatism, but has nonetheless been interpreted to represent the surface track of the purported Fernando de Noronha plume (Almeida et al., 1988, Fodor et al., 1998, Mizusaki et al., 2002, Rivalenti et al., 2000), which according to plate-tectonic reconstructions was near the margin of northeastern Brazil around 30 Ma (e.g., Morgan, 1983, O'Connor and Duncan, 1990). At face value, mantle xenoliths in alkalic basalts, both on the island of Fernando de Noronha and on the mainland, appear to support this link, providing evidence of a comparatively high mantle geotherm at the time of magmatism (Fodor et al., 2002, Princivalle et al., 1994, Rivalenti et al., 2000). However, the distribution of volcanism along two nearly orthogonal lineaments in northeastern Brazil (Fig. 1) is not easily reconciled with a simple plume-fed hotspot model.

Sleep (2003) has also recently questioned the conventional plume origin of the Fernando de Noronha and Martin Vaz tracks, drawing attention to their parallel alignment with oceanic fracture zones and the discordance of these approximately E–W volcanic lineaments with other tracks having apparent southeast-striking orientations (e.g., Rio Grande Rise; Fig. 1). Such observations are not new and other authors (e.g., Almeida et al., 1988) have previously proposed a genetic link between the Fernando de Noronha Archipelago and the oceanic fracture zones. On the basis of scaling and numerical analysis of the physics of ponded plume material, Sleep (2003) advocates a flow-line hotspot model (e.g., Morgan, 1978, Thompson and Gibbson, 1991) in which lateral flow from a single modern plume conduit, proposed to currently lie under the Paraná basin in southeastern Brazil, has fed the mainland, Fernando, and Martin Vaz volcanism.

Alternatively, the volcanism may not be directly related to plumes (Anderson, 2005). Small-scale convective flow driven by thermal contrasts associated with steps in lithospheric thickness, such as a cratonic or continent-ocean boundary, represents an alternative mechanism for some intraplate volcanism (e.g., King and Anderson, 1998, Vogt, 1991). Both Fernando and Trindade (Martin Vaz) are geometrically favorable candidates for such edge-driven convection (King, 2007, King and Ritsema, 2000).

A difficulty in evaluating any model for the origin of Cenozoic magmatism in equatorial Brazil is the lack of reliable geochronological data for these occurrences (see Section 2 later). In order to address this problem, we have initiated a campaign to map and date intrusive and extrusive igneous bodies regionally distributed in northeastern Brazil, also known as the Borborema Province. As a pilot study, we focus here on three bodies in the Rio Grande do Norte state (i.e., the Cabugizinho da Arara, Caracarazinho, and Serra Preta de Bodó volcanic centers) for which emplacement ages were previously unknown, but have been inferred to be between roughly 30 and 13 Ma (Fodor et al., 1998, Rivalenti et al., 2000) on the basis of geographic, geologic, and petrographic similarities with other Rio Grande do Norte basaltic centers dated by the K–Ar method (Sial et al., 1981). We also include in this study one plug (Cabelo de Negro) previously dated at 27.7 ± 0.6 Ma by Sial et al. (1981). Our 40Ar/39Ar results show that all four of these bodies are younger than previously assumed or determined by K–Ar dating, with emplacement ages between 8.9 ± 0.5 and 7.1 ± 0.3 Ma. Thus, mainland northeastern Brazil has experienced a protracted Cenozoic magmatic history that extends well into the Late Miocene, and we contend that the case for a simple plume origin requires re-evaluation.

Section snippets

Background and previous geochronology of Cenozoic basalts in northeastern Brazil

The Cenozoic volcanic suite in northeastern Brazil is primarily composed of plugs, necks, minor flows and dikes of alkalic basalt (Sial, 1976) that form a N–S lineament in central Rio Grande do Norte and Paraíba states (Fig. 1). Most of the basalts cut Precambrian metasedimentary basement reworked during the Late Proterozoic to Early Paleozoic Brasiliano–Pan-African orogenic event, although those along the continental margin in the Potiguar basin pierce mid-Cretaceous sedimentary rocks. Many of

Field relations, petrography and petrochemistry

The volcanic bodies studied here occur in a geographically restricted area of about 30 × 40 km in the central-northern portion of Rio Grande do Norte state (Fig. 1). They intrude Precambrian gneissic basement, and they are aligned along the presently seismically active NE–SW Touros–João Câmara fault (i.e., Cabugizinho and Serra Preta de Bodó) and the NW–SE Afonso Bezerra fault (i.e., Cabelo de Negro and Caracarazinho). Two of these bodies, the Cabugizinho and Caracarazinho necks, produce

Methods

Groundmass from Cabugizinho da Arara (VD-4B and VD-5A), Caracarazinho (VD-8), Serra Preta de Bodó (SP-7) and Cabelo de Negro (CN-11) were sampled for 40Ar/39Ar dating. After crushing and washing in distilled water and absolute ethanol, 5 to 10 grains, ca. 2 mm in size and suitably devoid of weathering or alteration zones, were selected by picking under a binocular microscope. The groundmass grains were loaded into a 21-pit aluminium disk along with the neutron fluence monitor Fish Canyon

Discussion and interpretation of geochronological results

In all but one case, the isochron ages agree with the plateau, ideogram and integrated ages for each sample, and 40Ar/36Ar intercepts are similar to present-day atmosphere (Table 2). The majority of the individual groundmass grains from our samples thus do not appear to contain significant excess 40Ar. The exception is sample SP-7 from Serra Preta de Bodó, which yielded a 40Ar/36Ar intercept of 319 ± 6 Ma, suggestive of some extraneous Ar (Fig. 6P). The saddle-shaped spectra (Fig. 6M) and the

Geodynamic implications

Our 40Ar/39Ar results demonstrate that volcanic activity in the Borborema Province continued well into the Late Miocene (ca. 9–7 Ma), overlapping temporally with volcanism on the island of Fernando de Noronha (Fig. 1). Although the inception of volcanic activity at Fernando is poorly constrained (see Section 2), available geochronological data (Buikin et al., 2008, Cordani, 1970, Kogarko et al., 2007; this study) imply that both areas were active contemporaneously for a minimum of 5 Ma and

Conclusions

Our 40Ar/39Ar geochronological results expand the lower age range for volcanic activity in mainland northeastern Brazil to roughly 7 Ma and show that this activity did not shut down prior to the inception of volcanism offshore on the island of Fernando de Noronha. These results also extend a growing body of evidence indicating that small-volume alkaline volcanic activity occurred over much of the Cenozoic in equatorial Brazil. The extended duration and lack of a systematic age progression make

Acknowledgments

ZSS acknowledges the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Brazil) for a post-doctorate scholarship (grant 201050/04-8) in the School of Earth Sciences of the University of Queensland. Useful reviews by Sally Gibson and an anonymous reviewer, along with the helpful editorial comments of Richard Carlson, were much appreciated. CTPetro/FINEP/Petrobras are acknowledged for providing most of the necessary funds for developing this research. Construction of the 40Ar/39

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