Plate reconstructions at 30 Ma place northeastern Brazil over the Fernando de Noronha hotspot, presently ~ 250 km offshore northeastern Brazil. Tertiary basaltic centers in northeastern Brazil may therefore be products of the Fernando de Noronha hotspot. We examined alkalic basalt from seven ~ 30-13 Ma old centers in Rio Grande do Norte and Pernambuco states to assess this possible link. Compositions are primitive, where MgO concentractions range from 12.5 to 17 wt.%, and SiO2 from ~ 41 to 48 wt.%. Trace-element abundances and Sr, Nd, and Pb isotopic compositions compare well with those of ocean island basalt: 87Sr/86Sr = ~ 0.7038-0.7051, 143Nd/144Nd = ~ 0.51266-0.51280, and 206Pb/204Pb = ~ 18.52-19.35. There are correlations among SiO2-undersaturation, incompatible-element abundances, relative % of partial melting (based on La/Yb and La/Y ratios), and the degree of isotopic 'enrichment' inherited from mantle sources. there is also a negative correlation for La/Nb (~ 0.6-0.86) vs. Ba/Nb (6.6-10.6), where lower La/Nb samples represent larger % of melting of a source comparatively enriched in radiogenic Sr. We attribute these compositional relationships to the lavas representing mixing of melts mainly from asthenosphere of largely HIMU plus DM characterization, probably the Fernando de Noronha plume, with melts from subcontinental lithosphere that was isotopically closer to EM1. Isotopic and trace-element compositons of the northeastern Brazil basalts are generally similar to those of Fernando de Noronha lavas (12-2 Ma), and some minor trace-element differences observed (eg, more Zr, Nb, and less Th compared to northeastern Brazil basalts) are probably due to heterogeneity within the asthenospheric plume and to melt contributions from delaminated subcontinental lithosphere that may underlie Fernando de Noronha.
Bibliographical noteCited By :18
Export Date: 3 November 2016