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Geochemical evidence for the Trindade hotspot trace: Columbia seamount ankaramite |
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| Authors: | R V Fodor B B Hanan |
| Institution: | a Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, P.O.Box 8208, Raleigh, NC 27695, USA b Department of Geological Sciences, San Diego State University, San Diego, CA 92182, USA |
| 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 SiO2-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 SiO2-undersaturated Al-rich clinopyroxene (8–13 wt.% Al2O3) 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. |
| Keywords: | Trindade hotspot Brazil magmatism Columbia seamount Ankaramite Pb isotopes |
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