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

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 (9.5–14.7wt.% 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±0.3Ma. Our age for Cabelo de Negro (7.9±0.3Ma) is roughly 20Ma 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 5Ma. 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 600km 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.