Tracking the Transition From a Gondwana LIP to the South Atlantic Ocean With Geochronological and Geochemical Indicators.

Evidence of early oceanization during transition from large igneous provinces (LIPs) to new ocean crust is mostly missing in the geological record. Mafic dykes geochemically akin to mid‐ocean ridge basalts (MORBs) occur in late stages of LIPs, but their tectonic role during margin evolution remains poorly constrained. Here we use whole‐rock and high‐resolution clinopyroxene chemistry of two distinct types of mafic dykes from the southeastern Brazilian margin to show that a MORB‐like plumbing system became magmatically active in the central South Atlantic ∼2 Ma after the volcanic peak of the Paraná‐Etendeka Magmatic Province. The LIP basaltic dyke compositions resulted from the mixing between a highly metasomatized lithospheric mantle comprising enriched DMM and Archean contributions. The lack of elemental oscillatory zoning in clinopyroxene crystals suggests that they were produced by a single magma pulse, consistent with the strong magma fluxes in LIPs. In contrast, the MORB‐like dykes, which exhibit strong similarities with South Atlantic MORBs, are interpreted to represent early stages of oceanization deriving from the mixing of enriched asthenospheric melts either with ancient SCLM contributions or small crustal assimilation. The MORB‐like intrusions ascended and cooled rapidly along faults and were fed by numerous magma injections that suggest episodic decompression of the asthenospheric source ∼20 Ma before the birth of central South Atlantic. The combination of subduction‐influenced and MORB‐like dykes is recurrent in ancient and modern rifts globally, suggesting that thinned lithospheric blocks affected by ancient subduction events may become more susceptible to early oceanization in rifted margins. Plain Language Summary: The processes that promote initial formation of new ocean floor, whenever a continent breaks, are still poorly described. In the South Atlantic Ocean case, for example, magnetic registers of oceanic rocks indicate that seafloor boomed ∼20 Ma after the ancient Gondwana continent broke up, creating distinct South American and African plates. However, we know very little about the preliminary steps of "oceanization" of the continental margins during the transition period. In this research, we studied two groups of magmatic rocks located in the Brazilian southeast margin. One of them is geochemically similar to the abundant volcanism that affected Gondwana prior to breakup; the other is akin to South Atlantic seafloor. The later was formed only ∼2 Ma after initial stages of Gondwana breakup. Geochemical signatures and mineralogical textures allowed us to describe how the two types of magmas were formed and how they ascended through the crust. The results and comparisons suggest that ∼20 Ma before the South Atlantic seafloor developed there were localized areas of the continental margins affected by early "oceanization," where ocean floor‐like rocks started forming along weakened continental structures. Such areas were probably more susceptible to breakup because of their more complex previous geological histories. Key Points: Oceanization of the South Atlantic started as early as ∼132 Ma through lithospheric thinning and focused mid‐ocean ridge basalt (MORB)‐like intrusive magmatismMagma dynamics of Paraná‐Etendeka and MORB dykes in the Brazilian margin suggest an intermittent shift toward a passive regime at ∼132 MaThinned lithospheric blocks affected by ancient subduction potentially allow early asthenospheric magma ascent and crustal oceanization [ABSTRACT FROM AUTHOR]