Young volcanism in the Borborema Province, NE Brazil, shows no evidence for a trace of the Fernando de Noronha plume on the continent
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
References (71)
- et al.
Evidence of magmatic activity related to Middle Jurassic and Lower Cretaceous rifting from northeastern Brazil (Ceará–Mirim): K/Ar age, palaeomagnetism, petrology and Sr–Nd isotope characteristics
Chem. Geol.
(1992) - et al.
Upper mantle material in the Brazilian shield
Earth Sci. Rev.
(1981) - et al.
Partial melting and upwelling rates beneath the Azores from a U-series isotope perspective
Earth Planet. Sci. Lett.
(2005) - et al.
Plume-lithosphere interactions in the ocean basins: constraints from the source mineralogy
Earth Planet. Sci. Lett.
(1997) - et al.
Evidence of mantle metasomatism and heterogeneity from peridotite inclusions of northeastern Brazil and Paraguay
Earth Planet. Sci. Lett.
(1986) - et al.
Three distinct types of hotspots in the Earth's mantle
Earth Planet. Sci. Lett.
(2003) - et al.
Age-probability spectra for examination of single-crystal 40Ar/39Ar dating results: examples from Olorgesailie, southern Kenya Rift
Quat. Int.
(1992) - et al.
U-series isotope and geodynamic constraints on mantle melting processes beneath the Newer Volcanic Province in South Australia
Earth Planet. Sci. Lett.
(2007) - et al.
Interactions of 3D mantle flow and continental lithosphere near passive margins
Tectonophysics
(2010) - et al.
Interpretation of discordant 40Ar/39Ar age-spectra of Mesozoic tholeiites from Antarctica
Geochim. Cosmochim. Acta
(1977)
Isotopic and trace-element indications of lithospheric and astenospheric components in Tertiary alkalic basalts, northeastern Brazil
Lithos
Petrology of spinel peridotite xenoliths from northeastern Brazil: lithosphere with a high geothermal gradient imparted by the Fernando de Noronha plume
J. S. Am. Earth Sci.
Isotopic geochemistry of Fernando de Noronha
Earth Planet. Sci. Lett.
Lithosphere–asthenosphere interaction and the origin of Cretaceous tholeiitic magmatism in Northeastern Brazil: Sr–Nd–Pb isotopic evidence
Lithos
Edge-driven convection
Earth Planet. Sci. Lett.
A redetermination of the isotopic abundance of atmospheric Ar
Geochim. Cosmochim. Acta
The composition of the Earth
Chem. Geol.
Mesozoic and Cenozoic igneous activity and its tectonic control in northeastern Brazil
J. S. Am. Earth Sci.
Hotspot tracks and the early rifting of the Atlantic
Tectonophysics
The effect of the Fernando de Noronha plume on the mantle lithosphere in north-eastern Brazil
Lithos
Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology
Earth Planet. Sci. Lett.
Magmatism and rifting in Western and Central Africa, from Late Jurassic to recent times
Tectonophysics
Magmatismo pós-Paleozóico no nordeste oriental do Brasil
Rev. Bras. Geocienc.
Scoring hotspots: the plume and plate paradigms
Petrology of kamafugites and kimberlites from the Alto Paranaiba Alkaline Province, Minas Gerais, Brazil
Contrib. Mineral. Petrol.
40Ar/39Ar dating of volcanic rocks from Fernando de Noronha hot spot. 8th Annual V. M. Goldschmidt Conference
Geochim. Cosmochim. Acta
Chemical and Os isotopic study of Cretaceous potassic rocks from southern Brazil
Contrib. Mineral. Petrol.
Ativação tectônica, fluzo geotérmico e sismicidade no nordeste oriental Brasileiro
Rev. Bras. Geociênc.
Thermal gradients in the continental crust
Idade do vulcanismo do Oceano Atlantico Sul
Bol. IAG USP
Crustal evolution of the South American platform
Potassium-argon age and paleomagnetism of diabase dikes in Liberia: initiation of Central Atlantic rifting
Geol. Soc. Am. Bull.
Single crystal 40Ar/39Ar dating of the Olorgesailie Formation, Southern Kenya Rift
J. Geophys. Res.
Zoned clinopyroxenes in alkalic basalt: clues to fractionation and magma-mixing histories for seemingly primitive magmas
Chem. Erde
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