Extent of the São Francisco Craton, South America: a mantle perspective

Abstract

Although cratons owe their longevity to thick, depleted and consequently buoyant sub-continental lithospheric mantle, their extents are commonly indicated by the known distribution of crustal rocks. However, the limits of a mapped cratonic area often do not match the seismically imaged thick sub-continental lithospheric mantle roots. New mantle xenolith petrology and geochemistry, combined with continental scale geophysical data, confirm the interpretation that the São Francisco Craton extends beyond the extent of surface exposures of Archean rocks. Mantle xenoliths from outside the typically recognised São Francisco Craton boundaries from the Limeira-1 and Redondão kimberlite diatremes have refractory olivine [Mg# ≤ 93, with Mg# = 100× molar Mg/(Mg + Fe); Limeira], high-Cr pyrope garnets (Cr₂O₃ ≤ 7 wt% from Redondão), low bulk Al₂O₃ < 1 wt%, and ¹⁸⁷Os/¹⁸⁸Os_i = 0.11202–0.11916 (Limeira) and 0.10964–0.11576 (Redondão) that equate to Mesoproterozoic minimum T_RD ages. Geothermobarometry indicates that the lithospheric thickness from which the Redondão xenolith suite was exhumed, extended to > 150 km at the time of eruption, similar to the lithosphere thickness estimated from garnet pyroxenites from the ‘on-craton’ Braúna field. The occurrence of thick, refractory and ancient lithosphere extending outside the known surface extent of the São Francisco Craton suggests that this cratonic nucleus is significantly larger than recognised at the surface. Based on the evidence reported in this study coupled with existing literature, we propose that the current limits of the São Francisco Craton nucleus should be revised to consider deep lithosphere to the north and southwest edges, despite these areas having younger rocks at the surface.