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 similar to 2 Ma after the volcanic peak of the Parana-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 similar to 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. (AU)