Cenozoic volcanism in Northeast Brazil and its links to the Cameroon Line
Guimarães, André Rezende
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Cenozoic volcanism in Northeast Brazil occurs in three distinct regions: the Macau-Queimadas volcanic lineament (MQVL) and the Mecejana volcanic field, both in the Borborema Province, and the Fernando de Noronha archipelago. They are the only significant occurrences of Cenozoic magmatism along the South American passive margin and their location matches the position of the Cameroon Line in Africa, to which they were adjacent before the opening of the South Atlantic. The Cameroon Line is one of the few and the largest occurrence of Cenozoic volcanism along the African Atlantic passive margin. Volcanic activity in the two conjugate margins postdate continental separation by 10s of millions of years. Here, the Brazilian occurrences are compared with one another and with the Cameroon Line using sixteen new 40Ar/39Ar ages obtained for Borborema, which for the first time include Mecejana, major and trace element data and 87Sr/86Sr and 140Nd/139Nd isotopic ratios. Activity in Mecejana dates to the late Eocene to early Oligocene (35.03 ± 0.28 to 30.80 ± 0.22 Ma). Volcanism in the MQVL is synchronous with Mecejana but also continues intermittently to the Miocene (12.39 ± 0.34 Ma), with a peak in the late Oligocene to the early Miocene marked by activity in the local basins. Comparisons with other published ages show that volcanism in the MQVL also continues into the late Miocene and is synchronous with activity in Fernando de Noronha. There are no age progression patterns in the region, and a deep mantle plume is not a suitable model to explain local magmatism. Geochemical results show a compositional continuum in Northeast Brazil, with variability controlled by depth and degrees of partial melting of a stratified sublithospheric source that includes both spinel and garnet lherzolite and extends across both continental and oceanic lithospheres. Northeast Brazil volcanic occurrences result from the same magmatic processes and should be treated as a single volcanic province. They are also synchronous and geochemically virtually identical to the Cameroon Line, indicating a continental lithospheric control in their genesis. We propose a model that relies on edge-driven convection and the rheological characteristics of the lithospheric thermal boundary layer (TBL) to explain magmatism in both conjugate margins, where edge-driven convection triggered by the local continental lithospheric morphology affects the continental TBL and cause it to flow laterally towards the oceanic sector. The intermediate rheology of the TBL explains the mainly asthenospheric characteristics of the magmatic source in both conjugate margins, but with the presence of subtle geochemical enrichment that must be caused by relatively recent enrichment events. The model finds strong support in the distribution of volcanism along the Atlantic passive margins, including seamounts and other volcanic provinces that also straddle continental and oceanic lithospheres such as the Canaries and the adjacent Moroccan volcanism, and the New England seamount chain.