U PB AND SM/ND GEOCHRONOLOGICAL STUDIES OF THE EASTERN BORBOREMA PROVINCE, NORTHEASTERN BRAZIL - INITIAL CONCLUSIONS/

The Borborema Province of NE Brasil comprises the central part of a wi de Pan-African-Brasiliano orogenetic belt that formed as a consequence of late Neoproterozoic convergence and collision of the Sb Luis-West Africa craton and the Sao Francisco-Congo-Kasai cratons. New Sm/Nd and U/Pb results from the eastern part of this province help to define th e basic internal architecture and pre-collisional history of this prov ince, with particular emphasis on delineating older cratonic terranes, their fragmentation during the Mesoproterozoic, and their assembly in to West Gondwana during the Pan African-Brasiliano orogeny at ca 600 M a. The region can be divided into three major geotectonic domains: a) Rio Piranhas-Caldas Brandao massif, with overlying Paleoproterozoic to Neoproterozoic supracrustal rocks, north of the Patos Lineament; b) t he Archean to Paleoproterozoic Sao Francisco craton (SFC) to the south ; and c) a complex domain of Paleoproterozoic to Archean basement bloc ks with several intervening Mesoproterozoic to Neoproterozoic fold bel ts in the center (south of Patos Lineament and north of SFC). The nort hern and central domains comprise the Borborema Province. Archean base ment gneiss and Transamazonian granulite of northern SFC are exposed i n the southern part of the central domain, underlying southern parts o f the Sergipano fold belt. Basement in the Rio Piranhas massif appears to consist mostly of Transamazonian (2.1 to 2.2 Ga) gneissic rocks; N d model ages (T-DM) Of ca. 2.6 Ga for 2.15 Ga gneisses indicate a subs tantial Archean component in the protoliths to these gneisses. The Cal das Brandao massif to the east yields both Transamazonian and Archean U/Pb zircon and Nd (T-DM) ages, indicating a complex architecture. Met asedimentary rocks of the Jucurutu Formation yield detrital zircons wi th original crystallization ages as young as 1.8 Ga, indicating that t hese rocks may be late Paleoproterozoic and correlate with other ca. 1 .8 Ga cratonic supracrustal rocks in Brazil such as the Roraima Group and Espinhaco Group. Most metavolcanic and pre-Brasiliano granitic uni ts of the Sergipano (SDS), Pajeu-Paraiba (SPP), Riacho Pontal (SRP), a nd Pianco-Alto Brigida (SPAE) fold belts in the central domain formed similar to 1.0 +/- 0.1 Ga, based on U/Pb ages of zircons. Nd model age s (TDM) for these same rocks, as well as Brasiliano granites intruded into them and large parts of the Pernambuco-Alagoas massif, are common ly 1.3-1.7 Ga, indicating that rocks of the fold belts were not wholly derived hom either older (> 2.1 Ga) or juvenile (ca 1.0 Ga) crust, bu t include mixtures of both components. A simple interpretation of Bras iliano granite genesis and the Nd data implies that there is no Transa mazonian or Archean basement underlying large parts of these fold belt s or of the Pernambuco-Alagoas massif. An exception is a belt of syeni tic Brasiliano plutons (Syenitoid Line) and host gneisses between SPAE and SPP that clearly has a Transamazonian (or older) source. In addit ion, there are several smaller blocks of Archean to Transamazonian gne iss that can be defined within and among these fold belts. These block s do not appear to constitute a continuous basement complex, but appea r to be isolated older crustal fragments. Our data support a model in which ca. 1.0 Ga rifting was an important tectonic and crust-forming e vent along the northern edge of the Sb Francisco craton. Our data also show that significant parts of the Borborema Province are not remobil ized Transamazonian to Archean crust, but that Mesoproterozoic crust i s a major feature of the Province. There are several small remnants of older crust within the area dominated by Mesoproterozoic crust, sugge sting that the rifting event created several small continental fragmen ts that were later incorporated into the Brasiliano collisional orogen . We cannot at present determine if the Rio Piranhas-Caldas Brandao ma ssifs and the older crustal blocks of the central domain were original ly part of the Sao Francisco craton or whether some (or all) of them c ame from more exotic parts of the Proterozoic Earth. Finally, our data have not yet revealed any juvenile terranes of either Transamazonian or Brasiliano age.