HETEROGENEOUS ENRICHED MANTLE MATERIALS AND DUPAL-TYPE MAGMATISM ALONG THE SW MARGIN OF THE SAO-FRANCISCO CRATON, BRAZIL

The nature and restricted range of Dupal-type Sr, Nd and Pb isotopic c ompositions of Cretaceous kimberlites, tuffaceous diatremes of kamafug itic affinity and carbonatite complexes which intrude the southwestern Sao Francisco craton margin in Brazil, indicate that these magmas eit her interacted extensively with, or were derived from, a light rare ea rth element (LREE) enriched homogeneous lithospheric mantle source iso topically similar to the ''enriched mantle I'' (EMI) component. The sh allow-derived alkalic rocks contain a greater proportion of this EMI-l ike component, whereas the lower time-averaged Rb/Sr, Nd/Sm and Pb/U r atios of the kimberlites compared to the other rock types suggest mixi ng of the EMI-like mantle material with variable amounts of mantle wit h a high U-238/Pb-204 (HIMU-like) component. Systematic variations in rock types and geochemistry on a regional scale are believed to be ind icative of vertical geochemical heterogeneities which are translated i nto lateral heterogeneities by different depths of melting. It is prop osed that HIMU- and EMI-like signatures in particular, are concentrate d in laterally extensive but vertically distinctive portions of the ma ntle beneath the Sao Francisco craton. The EMI-type signatures appear to be restricted to shallow-derived volcanism, whereas the HIMU-type s ignatures may originate from a source that started melting deeper in t he mantle. The Nd signatures of the EMI-type volcanics follow the evol ution path defined by the NeoProterozoic crustal sequences which overl ie and flank the craton margin. This suggests that the source of the E MI-type mantle signatures might be related to the tectono-thermal proc esses which led to the formation and evolution of such crustal sequenc es. The isotopic similarity of the sources of the studied rocks and of the high-Ti basalts of the northern Parana basin to those of some Oce an Island Basalts with Dupal signatures in the South Atlantic (viz. in Walvis Ridge) is ascribed to processes by which continental lithosphe re became firstly delaminated, and then contaminated a zone of South A tlantic asthenosphere from which hotspot islands have been erupting.