Paleomagnetism of Paleoproterozoic mafic dyke swarm from the Uauل region, northeastern Sمo Francisco Craton, Brazil: tectonic implications

This paper presents paleomagnetic analyses of 146 samples collected from 45 mafic dykes (sites) and 18 samples from basement rocks in contact with dykes. Two groups of dykes are recognized: the first (metamorphic dykes-MD) is composed of amphibolites with distinct schistosity and the second group is formed of two main rock types; a BD (basic dykes) type which retains its original texture and mineralogy and a MBD (metamorphic basic dykes) type where the clinopyroxene is almost completely substituted by amphibole. The MBD dykes occur near shear zones which are associated with the metamorphism of these rocks (ages of 2003 ± 86 Ma and 1975 ± 2 Ma). Most K-Ar and 40Ar39Ar mineral age determinations on the analysed dykes and basement rocks yielded dates in the interval 1.9-2.14 Ga. RbSr mineral isochrons on dykes from the BD type suggest two different generations of emplacement, at 2.38 Ga (minimum age) and 1.98 Ga. After AF and thermal demagnetization, a characteristic direction of magnetization (Dm = 17.1 °; Im = 69.8 °; N = 31; K = 40.6; α95 = 4.1 °) could be isolated for 29 metamorphosed and unmetamorphosed dykes, and basement rocks from two localities, which yielded a paleomagnetic pole situated at 331.4 °E; 23.8 °N (K = 16.8; α95 = 6.5 °). The paleomagnetic, petrographic, and geochronological data are interpreted to indicate that this characteristic component of magnetization is secondary. This component may have been acquired during the uplift and cooling of the Transamazonian cycle, at about 1.98-1.9 Ga ago. Tectonically, our data when compared with paleomagnetic poles of similar age from the Kalahari Craton are consistent with a model in which the São Francisco/Congo and Kalahari Cratons were part of a single stable continental mass in the late Paleoproterozoic.