Plume-related Paranà-Etendeka igneous province: An evolution from plateau to continental rifting and breakup

A critical review of the available multidisciplinary data on the Paranà-Etendeka Province allows for the reconciliation of the controversial aspects of its origin in a coherent tectonomagmatic scenario, in which continental flood basalt (CFB) magmatism evolved from the Paranà plateau s.s. (Stage 1) to progressive continental rifting in Etendeka (Stage 2), and then opened to the South Atlantic at the same latitude; the CFB magmatism is triggered by the prolonged impingement of the proto Tristan plume on the western Gondwana lithosphere. The provinciality of the CFB is evidenced by the incompatible element distribution and Sr-Nd-Pb isotopic data, where the Paranà and Etendeka magmas are more akin to lithospheric and asthenospheric (plume-related) components, respectively. Stage 1 consisted of the rapid outpouring of the Paranà Plateau s.s. CFB (135–134 Ma, ~ 800,000 km3, and eruption rate ~0.8 km3/a) that was zonally arranged with prevailing high-TiO2 (HT) basalts in the central-northern part and low-TiO2 (LT) suites at the southern periphery. The differentiated nature (MgO = 8–4%) of these plateau magmas suggests the variable extent of fractional crystallisation during rise through a relatively thick lithosphere. Petrological modelling, isotopic signatures (87Sr/86Sr 0.70483–0.70620, and εNd(t) from −1.27 to −5.78), and incompatible element distributions approaching the EM1 mantle component suggest that the HT and LT Paranà basalts may be derived from mantle sources located in the lower lithosphere at P 3–4 GPa and a potential temperature (Tp) of 1500–1550 °C. The high Tp recorded (and the relative Texcess 250–300 °C thermal anomaly) can be attained after several million years of lithospheric heating, beginning from the first plume impact, which is represented by precursor alkaline events (145–138 Ma) at the westernmost border of the Paranà plateau. The subsequent rifting (Stage 2, mostly 134–128 Ma) developed SE of the Paranà plateau in relation to the NW drifting of the Gondwana plate over the rising plume and was accompanied by progressive lithospheric arching, thinning, and rifting, which culminated in continental breakup close to the Etendeka border. The concomitant and exclusive appearance, in both the HT and LT suites of this region, of the hottest and deepest (Tp up to ~1590 °C and P up to ~5 GPa) high-MgO sub-lithospheric magmas (87Sr/86Sr 0.70319–0.70533 and εNd(t) from 9.08 to 0.53) is consistent with their generation from the axial zone of the upwelling plume. The basic–acidic bimodal character of CFB magmatism, since the beginning of this stage, must be related to the intensive block faulting of the rifted margins that favoured magma trapping and crystal fractionation to trachydacite and dacite-rhyolite differentiates from the respective HT and LT basalts. Owing to the higher SiO2 and viscosity, the prevailing dacite-rhyolite magmas were more prone to pond in crustal magma chambers, where they experienced assimilation fractional crystallisation (87Sr/86Sr 0.71466–0.72558 and εNd(t) from −5.50 to −9.31) before erupting as extensive rheo-ignimbrites. During the final rifting stage, CFB activity continued until 128 Ma (with sporadic episodes until 122 Ma), and the plume dynamic support gradually vanished beneath the two conjugate South American/South African continental margins, up to the opening of the South Atlantic and hot-spot volcanism of the Rio Grande and Walvis Ridges.