Uranium-lead and samarium-neodymium geochronological constraints on the crustal framework and geologic history of Ceara State, northwest Borborema Province, northeast Brazil: Implications for the assembly of Gondwana.

摘要

A record of Archean to Early Paleozoic magmatic and tectonic events in Ceara State is determined from field and isotopic data that offer important insight into the crustal and tectonic evolution of the northwest part of the Borborema Province as well as assembly of West Gondwana at the end of the Precambrian. Ceara's basic crustal framework was established during the ca. 2.1 Ga Transamazonian orogeny. The fragments of Archean crust in Ceara range in age from ca. 2.68 Ga to around 2.86 Ga and appear to be pieces of older evolved cratonic masses. Paleoproterozoic gneisses, which make up most of Ceara's basement, fall within two distinct age ranges; 2.35 to 2.30 Ga and 2.20 to 2.10 Ga.; Subsequent to ca. 2.1 Ga Transamazonian orogenesis, this Archean-Paleoproterozoic basement complex was affected by several tectonic and magmatic events. The ca. 1.79 Ga Oros - Jaguaribe belt in southeastern Ceara records a phase of intracratonic rifting which has been postulated to have been the result of either relaxation of the Transamazonian orogen or heat buildup under a large continental mass. Although other examples of 1.8 Ga rifting are not found within the Borborema Province, several similar sequences of this age are present in other parts of Brazil and Africa, suggesting that much of Brazil and West Africa were a coherent mass at that time. Another rifting event also occurred in Ceara ca. 775 Ma, an age corresponding to rifting and ocean basin formation in West Africa. The direct evidence for ocean basin opening and closure seen in Africa, however, has not been found in NE Brazil.; Subduction associated with ocean basin closure generated a complex of calc-alkaline granitoids of continental-arc affinity in Ceara starting at around 622 Ma. Based upon U-Pb ages from several igneous bodies, magmatism and tangential deformation associated with Gondwana convergence occurred through 614 Ma. Between 614 and 591 Ma, however, transcurrent deformation and tectonics became dominant, resulting in the formation of extensive shear zones throughout the state. In zones of transtension, granitoids and other igneous bodies were intruded, and rapid exhumation and cooling of the orogen occurred in areas of transpression. By about 557 Ma, metamorphic minerals such as sphene and garnet in the northwest part of the state had cooled to their blocking temperatures. The absolute end of the Brasiliano orogeny in NW Ceara is constrained by a U-Pb zircon age of 532 Ma on the Mucambo granite, an intrusion associated with post-tectonic extension along the Sobral shear zone. (Abstract shortened by UMI.)