Résumé
[Display omitted]
•Many dated phases are xenocrysts, giving only maximum ages of intrusion.•Most examples of KKAC magmas intruded coevally between 60 and 88 Ma, peaking at 81 Ma.•Their timing and location link the KKAC magmas to the South Atlantic’s opening.
Kimberlitic, kamafugitic and alkaline-carbonatitic (KKAC) rocks have been proposed to be directly related to supercontinent cycles. However, the hybrid nature of KKAC magmas complicates the geochronological studies required to understand their mutual relationships. Robust evaluation of their history depends on appropriate choice of dating techniques and careful attention to the nature of the dated material. Extensive KKAC outcrops in the magmatic province on the south-western margin of the São Francisco Craton (SW-SFC) are heavily weathered and most existing ages are based on residual perovskite, phlogopite, apatite and zircon. A multi-methodological appraisal (in situ U-Pb on perovskite and zircon, in situ Rb-Sr on phlogopite; major and trace elements) shows that both primary minerals and entrained xenocrysts (perovskite, phlogopite, zircon) can be found within the same pipe. Most xenocrysts can be recognised from microstructural features, chemical/isotopic differences and/or the petrological incompatibility of a phase (e.g., zircon) that cannot be cognate in the KKAC melts. Xenocrysts of phlogopite and a few apatites suggest that the KKAC magmatism may have begun as early as ca 110 Ma but is cryptic, i.e. poorly expressed in surface eruptions. When the (maximum) ages derived from xenocrysts are filtered out, accepted intrusion ages group almost entirely in the period of 76–88 Ma, with mean ages of 80, 79 and 83 Ma for kimberlites, kamafugites and alkaline-carbonatite rocks, respectively. This timing, and the location of the KKAC eruptions 400–900 km from the continental shelf margin, are consistent with a geodynamic model in which far-field mantle melting and magmatism is driven by the migration of convective instabilities from the margin of an opening ocean, beneath the adjacent craton roots. The refined geochronological framework thus suggests that the KKAC magmatism in the SW-SFC is best interpreted as a far-field effect of the South Atlantic opening (ca 127 Ma).