LATE CRETACEOUS RIFT-RELATED UPWELLING AND MELTING OF THE TRINDADE STARTING MANTLE PLUME HEAD BENEATH WESTERN BRAZIL

High mantle potential temperatures and local extension, associated wit h the Late-Cretaceous impact of the Trindade mantle plume, produced su bstantial widespread and voluminous magmatism around the northern half of the Parana sedimentary basin. Our previous studies have shown that , above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume c aused large-scale melting of the more fusible parts of the subcontinen tal lithospheric mantle beneath the margin of the Sao Francisco craton and the surrounding Brasilia mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Br azil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser Ar-40/Ar-39 age determinations indicate that rift-related basaltic magmas of the Poxo reu Igneous Province were emplaced at similar to 84 Ma. Our detailed g eochemical study of the mafic magmas shows that the parental melts und erwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to ha ve assimilated upper crust whereas others (nepheline- and hypersthene- normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite- normalised La/Nb = 1) and isotopic ratios (Sr-87/Sr-86 = 0.704 and Nd- 143/Nd-144 = 0.51274) of the Hy-normative basalts resemble those of oc eanic islands (GIB). We therefore propose that these ''OIB-like'' magm as were predominantly derived from convecting-mantle-source melts (i.e . Trindade mantle plume). Inverse modelling of rare-earth element (REE ) abundances suggests that the initial melts were predominantly genera ted within the depth range of similar to 80-100 km, in mantle with a p otential temperature of similar to 1500 degrees C.