Zircon Pb-Pb Geochronology of the Caruaru Area, Northeastern Brazil: Temporal Constraints on the Proterozoic Evolution of Borborema Province

Abstract

The Caruaru-Arcoverde batholith, with an outcrop area of ~2000 km2, is one of the largest Brasiliano plutons in Borborema Province, northeastern Brazil. The batholith intrudes orthogneisses and supracrustal rocks characterized by flat-lying foliation, and is bounded in the south by a synmagmatic, transcurrent shear zone, interpreted by some as a terrane boundary. In order to constrain the emplacement age of the batholith and the Proterozoic history of the eastern portion of Borborema Province, single zircon grains from seven samples from the Caruaru-Arcoverde batholith and nearby country rocks were analyzed by the Pb-evaporation technique. Zircons from orthogneisses immediately north of the Caruaru-Arcoverde batholith yielded ages of 2098 ± 15 Ma and 2072 ± 3 Ma. One zircon sample from an orthogneiss located south of the batholith gave an age of 2075 ± 7 Ma. Combined with existing zircon U-Pb ages in orthogneisses and Nd model ages of plutons and country rocks in eastern Borborema Province, these data indicate important crustal growth during the Middle Paleoproterozoic (2.2-2.0 Ga). In contrast, a sample from another orthogneiss southeast of the Caruaru-Arcoverde batholith gave an age of 629 ± 9 Ma. Because the flat-lying foliation in different country rocks was formed under the same high-amphibolite-facies conditions, this age suggests that the regional fabric developed during the Late Neoproterozoic. Two samples from the main phase of the Caruaru-Arcoverde batholith yielded mean ages of 591 ± 5 Ma and 587 ± 5 Ma, whereas a sample from a late facies gave an mean age of 583 ± 5 Ma. The oldest age of 591 Ma places an upper bound on the beginning of strike-slip motion, because field evidence indicates that the shear zone developed after emplacement of the Caruaru-Arcoverde batholith. Correlations of the Caruaru area with other sectors of Borborema Province and the Cameroon and Nigerian provinces of western Africa suggest that these areas: (1) were assembled during the Transamazonian/Eburnean orogeny; and (2) formed a coherent unit during the Late Neoproterozoic Brasiliano/Pan-African orogeny, when they were deformed first by a tectonic event, possibly resulting from thrust motion, and then subjected to a strike-slip regime.