ISOTOPIC AND TRACE-ELEMENT INDICATIONS OF LITHOSPHERIC AND ASTHENOSPHERIC COMPONENTS IN TERTIARY ALKALIC BASALTS, NORTHEASTERN BRAZIL

Plate reconstructions at 30 Ma place northeastern Brazil over the Fern ando de Noronha hotspot, presently similar to 250 km offshore northeas tern Brazil. Tertiary basaltic centers in northeastern Brazil may ther efore be products of the Fernando de Noronha hotspot. We examined alka lic basalt from seven similar to 30-13 Ma old centers in Rio Grande do Norte and Pernambuco states to assess this possible link. Composition s are primitive, where MgO concentrations range from 12.5 to 17 wt.%, and SiO2, from similar to 41 to 48 wt.%. Trace-element abundances and Sr. Nd, and Pb isotopic compositions compare well with those of ocean island basalt: Sr-87/Sr-86 = similar to 0.7038-0.7051, Nd-143/Nd-144 = similar to 0.51266-0.51280, and Pb-206/Pb-204 = similar to 18.52-19.3 5. There are correlations among SiO2-undersaturation, incompatible-ele ment abundances, relative percentages 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 ( similar to 0.6-0.86) vs. Ba/Nb (6.6-10.6), where lower La/Nb samples r epresent larger percentages of melting of a source comparatively enric hed in radiogenic Sr. We attribute these compositional relationships t o the lavas representing mixing of melts mainly from asthenosphere of largely HIMU plus DM characterization, probably the Fernando de Noronh a plume, with melts from subcontinental lithosphere that was isotopica lly closer to EM1. Isotopic and trace-element compositions of the nort heastern Brazil basalts are generally similar to those of Fernando de Noronha lavas (12-2 Ma), and some minor trace-element differences obse rved (e.g., more Zr, Nb, and less Th compared to northeastern Brazil b asalts) are probably due to heterogeneity within the asthenospheric pl ume and to melt contributions from delaminated subcontinental lithosph ere that may underlie Fernando de Noronha. (C) 1998 Elsevier Science B .V. All rights reserved.