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

Knesel, Kurt M., Souza, Zorano S., Vasconcelos, Paulo M., Cohen, Benjamin E. and Silveira, Francisco V. (2011) Young volcanism in the Borborema Province, NE Brazil, shows no evidence for a trace of the Fernando de Noronha plume on the continent. Earth and Planetary Science Letters, 302 1-2: 38-50. doi:10.1016/j.epsl.2010.11.036


Author Knesel, Kurt M.
Souza, Zorano S.
Vasconcelos, Paulo M.
Cohen, Benjamin E.
Silveira, Francisco V.
Title Young volcanism in the Borborema Province, NE Brazil, shows no evidence for a trace of the Fernando de Noronha plume on the continent
Journal name Earth and Planetary Science Letters   Check publisher's open access policy
ISSN 0012-821X
1385-013X
Publication date 2011-02-01
Year available 2010
Sub-type Article (original research)
DOI 10.1016/j.epsl.2010.11.036
Volume 302
Issue 1-2
Start page 38
End page 50
Total pages 13
Editor R. W. Carlson
Place of publication Amsterdam, Netherlands
Publisher Elsevier
Collection year 2012
Language eng
Formatted 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.
Keyword Ar-40/Ar-39 geochronology
Edge-driven convection
Hotspot
Intraplate volcanism
Mantle plume
Tertiary basaltic rocks,
U-series isotope
NortheasternBrazil.
Hot-spot
Mantle plume
Peridotite nodules
Igneous activity
Rio-grande
SE Brazil
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ
Additional Notes Available online 22 December 2010.

Document type: Journal Article
Sub-type: Article (original research)
Collections: School of Earth Sciences Publications
Official 2012 Collection
 
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Created: Thu, 13 Oct 2011, 03:50:46 EST by Dr Benjamin Cohen on behalf of School of Earth Sciences