The 3.4-3.5 Ga São José do Campestre massif, NE Brazil

Abstract

The oldest fragment of continental crust recognized in South America occurs as an isolated Archean enclave in northeastern Brazil's Borborema Province, ca. 600Ma Brasiliano-Pan African orogenic belt. This Archean fragment, the São José do Campestre massif, is surrounded by large tracts of 2.2-2.0Ga Paleoproterozoic gneisses and is located more than 600-1500km from the much larger assemblages of Archean rocks found in the São Fransciso and Amazonian cratons, located to the south and west, respectively. Geochronological studies of the São José do Campestre massif show that its oldest rocks contain zircons with U-Pb ages up to 3.5Ga and Sm-Nd TDM model ages of more than 3.7Ga, indicating that they represent "reworked" crust. This older "nucleus" is flanked by both reworked and juvenile 3.25 and 3.18Ga rocks which are intruded by both 3.00 and 2.69Ga plutonic bodies. The protracted evolution the São José do Campestre massif is consistent with that of a larger continental mass as opposed to a small crustal fragment that grew in isolation. As such, the São José do Campestre massif is interpreted as representing a detached piece of an evolved craton that became entrained with younger rocks during a subsequent Paleoproterozoic accretionary-orogenic event. This hypothesis is bolstered by the presence of Paleoproterozoic gneisses that envelop the São José do Campestre massif, as well as the existence of ca. 2.0Ga metamorphic zircon and monazite within its rocks. The occurrence of several different Archean "cratonic basement inliers" within the greater Paleoproterozoic crustal framework of the Borborema Province suggests that cratonic slices spalled off one or more larger Archean masses prior to the ca. 2.2-2.0Ga Paleoproterozoic orogenic collage. A important challenge is to link these older fragments to their parent cratons. Although results are not unique, the pattern of ages and isotopic signatures observed in the São José do Campestre massif is similar to that seen in parts of the São Francisco Craton, and it is possible that the São José do Campestre massif is a fragment of an Archean continental fragment formed during an episode of continental breakup prior to 2200Ma. Show more