No abstract

No abstract

Sm-Nd whole-rock and U-Pb zircon geochronological studies of the basement gneisses of Ceará (NW Borborema Province, NE Brazil) have identified two major pulses of Paleoproterozoic crustal growth within the state; the first between ca. 2.35 to 2.30 Ga, and the second from ca. 2.19 to 2.05 Ga. The former was characterized exclusively by juvenile growth and accretion, whereas the latter involved the amalgamation of new juvenile crustal material, reworked or enriched crust, and Archean crustal fragments. It was during this second event, also known as the Transamazonian orogeny (ca. 2.2 to 2.0 Ga), that Ceará's fundamental crustal framework was assembled. Isotopic and geologic data indicate that this assembly involved the fusion of three distinct crustal blocks; the Northwest Ceará (AKA Médio Coreaú) Domain (NCD), the Central Ceará Domain (CCD), and the Rio Grande do Norte Domain (RND). This network of crustal blocks was subsequently affected by an episode of Late Paleoproterozoic intracratonic rifting at around 1.8 Ga. The presence of similar rift sequences of this age in other parts of South America and Africa lend support to the existence of the Paleoproterozoic supercontinental mass Atlantica. Based on the available data, this supercontinent appears to have included the Paleoproterozoic (and older) basement of the Brasiliano/Pan-African provinces in Brazil and West Africa as well as the cratons adjacent to them.Keywords: Borborema Province, Paleoproterozoic crustal growth, U-Pb zircon and Sm-Nd whole-rock geochronology (Almeida et al. 1981). Although the BP formed at the end of the Precambrian during the assembly of West Gondwana, its crustal framework is made up primarily of older Mesoproterozoic to Archean crustal fragments and domains . Understanding the growth and evolution of these constituent basement blocks is important for reconstructing the precollisional history of the BP and that of West Gondwana. As Ceará encompasses a significant portion of the BP, accurate constraints on the growth and evolution its basement complex are key for unraveling the early growth of the BP.Pioneering Rb-Sr and K-Ar geochronological studies in Ceará State and the surrounding region, e.g., Brito ), Brito Neves et al. (1978, Torquato et al. (1986), andPessoa et al. (1986), provided clues to the antiquity of lithologic units in the region, but the data lacked the necessary accuracy and precision to resolve age differences between different rock units and crustal domains. Furthermore, the susceptibility of the Rb-Sr and K-Ar systems to metamorphic resetting left some doubt as to whether the data represented original or reset ages, e.g. Caby and Arthaud (1986). In recent years, however, U-Pb zircon and Sm-Nd whole-rock studies, e.g., Van Schmus et al. (1995, 1997, Dantas et al. (1995, 1998) and Fetter et al. (1995, 1997, have provided precise and accurate constraints on the formation ages and crustal histories of different blocks within the BP.In this paper, we report Sm-Nd whole-rock and U-Pb zircon geochronological da...

The Borborema and Benin-Nigeria provinces of NE Brazil and NW Africa, respectively, are key areas in the amalgamation of West Gondwana by continental collision during the Brasiliano/Pan-African orogenies. Both are underlain by complex basement: Nigeria has c. 3.05 Ga Archaean crust but no known Palaeoproterozoic rocks .2.0 Ga; in NE Brazil, 2.6-3.5 Ga Archaean rocks form small cores within Palaeoproterozoic gneiss terrains affected by plutonism at c. 2.17 Ga. Both regions exhibit Late Palaeoproterozoic (c. 1.8 Ga) rift-related magmatism and metasedimentary sequences overlying the basement. The Seridó Group of NE Brazil (,0.65 Ga) is similar to the Igarra Sequence in SW Nigeria. The Ceará Group, which may date back to c. 0.85 Ga, is a passive margin deposit on crust thinned during initiation of an oceanic domain. In both provinces, basement and sedimentary cover were involved in tangential tectonics that resulted in crust-thickening by nappe-stacking associated with closure of this ocean. Frontal collision between c. 0.66 and 0.60 Ga later evolved to an oblique collision, generating northsouth continental strike-slip shear zones at c. 0.59 Ga. In NE Brazil, the main Pan-African suture is probably buried beneath the Parnaíba Basin. The Transbrasiliano Lineament, interpreted as the prolongation of the Kandi-4850 Lineament in Hoggar, may represent a cryptic suture. Opening of the Atlantic Ocean in the Mesozoic led to the break-up of Pangaea (and West Gondwana) and the consequent individualization of the South America and Africa continents, each one containing part of the Brasiliano/Pan-African fold belt. The Borborema Province (Almeida et al. 1981) in northeastern Brazil (Fig. 1) was built during the Brasiliano/Pan-African orogeny, as the result of convergence and final collision of the São LuisWest Africa and São Francisco -Congo cratons, in the context of West Gondwana amalgamation.The present structural framework of the province (Fig. 2) dates from the end of the Brasiliano/ Pan-African orogeny, forming a mosaic of independent domains juxtaposed along large crustal-scale shear zones (Vauchez et al. 1995) in a continental-scale collage (Van Schmus et al. 1998).Since the first attempts to reconstruct West Gondwana, the similarity of geological features between NE Brazil and NW Africa were used to argue for the juxtaposition of the two continents. However, there is still much uncertainty, and correlation between the two domains remains imprecise . The northern part of the Borborema Province, north of the Patos Lineament, and the Benin -Nigeria province are the key for correlation between the continents (Fig. 2). Integration of recent geological and geochronological data allows re-evaluation of the geological framework of the two provinces and provides new insights for the correlation between the two provinces.

No abstract