Abstract
The Columbia seamount ∼825 km offshore from Brazil at ∼20°S lies on the east–west ‘trace’ of the Trindade hotspot. Continental and oceanic magmatism believed to have originated with this hotspot is alkalic and SiO 2-undersaturated, and dates from ∼85 Ma in southern Brazil to <3 Ma on the islands of Trindade and Martin Vaz ∼1100 km offshore. An ankaramite (clinopyroxene ∼16 vol%) dredged from Columbia seamount (est. 10 Ma) conforms to this geochemistry with SiO 2-undersaturated Al-rich clinopyroxene (8–13 wt.% Al 2O 3) and rhönite. Clinopyroxene isotopic compositions are 87Sr/ 86Sr=0.703900, 143Nd/ 144Nd=0.512786, 206Pb/ 204Pb=19.190, 207Pb/ 204Pb=15.045, and 208Pb/ 204Pb=39.242 — resembling those for Trindade, except for slightly higher 207Pb/ 204Pb. The isotopic composition and abundance ratios among weathering-resistant Nb, La, and Yb suggest that Columbia seamount magmatism represents the present-day Trindade plume, but ∼10 million years earlier and perhaps when the plume manifested a signature of ‘contamination’ from subducted sediments. The Columbia seamount analyses provide the first quantitative assessment for the Trindade hotspot trace existing between the Brazil margin and Trindade, strengthening the case for a continuum of magmatism extending from the ∼85 Ma Brazilian igneous provinces of Poxoréu and Alto Paranaiba.
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