GEOCHEMICAL CHARACTERISTICS AND ORIGIN OF THE JACUPIRANGA CARBONATITES, BRAZIL

New major-, trace-element and Sr, Nd and Pb isotope data are presented for carbonatites and pyroxenites from the 130-Ma-old Jacupiranga comp lex in southern Brazil. Measured Sr-87/Sr-86 ranges from 0.7047 to 0.7 055, Nd-143/Nd-144 from 0.51251 to 0.51264 and Pb-206/Pb-204 from 17.0 to 18.2. Most samples have high measured Th/U(5-17), and high Pb-208/ Pb-204 ratios (37.9-40.2) relative to Pb-206/Pb-204, with time-integra ted Th/U>4. The pyroxenites have higher initial Pb-206/Pb-204 (17.63-1 7.67) and lower initial Sr-87/Sr-86(similar to 0.7047) than the carbon atites (17.05-17.47 and 0.7049-0.7054, respectively). There is a broad negative Pb-Sr isotope array between the carbonatites and the pyroxen ites which precludes simple binary mixing, because of their very diffe rent Sr/Pb ratios. The isotope differences also appear to preclude mod els in which the carbonatites segregated by liquid immiscibility from magmas similar to those from which the pyroxenites crystallised, and m odels in which the isotope arrays are the product of crustal contamina tion processes. Rather, the initial Sr, Nd and Pb isotope ratios in th e Jacupiranga complex, and the negative correlation of Pb and Sr isoto pes are similar to those in the Parana high-Ti basalts, and the oceani c basalts of the Walvis Ridge and Tristan da Cunha. Thus, the initial isotope ratios of the Jacupiranga rocks are considered to have been in herited from the mantle source regions associated with incipient magma tism of the Tristan da Cunha hotspot and the opening of the South Atla ntic. Finally, the observed variations are used to evaluate suggestion s that certain element ratios in upper-mantle rocks, such as high Ca/A l and La/Yb and low Ti/Eu, are features of carbonatite metasomatism. M antle xenoliths altered by carbonatite metasomatism exhibit elevated S r contents and low Rb/Sr ratios, and most carbonatites have high U/Pb, low Rb/Sr and Sm/Nd. Thus, infiltration and/or metasomatism by carbon atitic melts is one process which may have been responsible for the ne gative array of U/Pb and Rb/Sr inferred for the source of certain ocea nic basalts.