Mafic Dykes: Petrogenesis and Tectonic Inferences

The main mafic dyke swarms of the São Francisco craton range in age from Late Archean to the Early Neoproterozoic, and record extensional and transpression tectonic events. The Uauá and Lavras regions include norite (2.73 and 2.66 Ga) and tholeiite swarms (2.62 and 1.97 Ga) respectively. Other swarms are made up of tholeiites: Paraopeba (2.10 Ga), Pará de Minas (1.71 Ga), Curaçá and Chapada Diamantina (1.5 Ga), Diamantina (0.93 Ga) and Salvador-Olivença (0.92 Ga). Geochemical and Sr–Nd isotopic data indicate variable scales of mantle heterogeneity, caused by recurrent metasomatic effects since Archean times. The main processes are attributed to the action of slab fluids from recycling of oceanic crust and overlying sediments on the sub-lithospheric mantle in continental settings, and by upwelling of OIB-like material. These processes are consistent with the protracted geologic history of the country rocks, given by episodic juvenile accretion and successive continental amalgamations from Archean to ca. 2.0 Ga. The adopted mixing geochemical model permits to evaluate the variation range of N-MORB melts, OIB and slab fluids and melts, which are considered to represent the main components of the parental mantle composition of the studied dykes.