Evidence for silicate liquid immiscibility in recharging, alkali-rich tholeiitic systems: The role of unmixing in the petrogenesis of intermediate, layered plutonic bodies and bimodal volcanic suites

Abstract

The role of silicate liquid immiscibility in the magmatic evolution of basalts and accounting for the scarcity of intermediate compositions on Earth's surface (Daly Gap) remains a matter of debate. Here we report the first finding of natural, immiscible Fe- and Si-rich silicate melts entrapped as apatite-hosted silicate Melt Inclusions (MIs) in the sub-volcanic Limeira Intrusion, a basic-intermediate layered occurrence associated with the Paraná Magmatic Province (PMP), southeastern Brazil. At least three nested magma pulses constitute this igneous body, in which compositional and textural features of the youngest one indicate the effective redistribution of a Si-rich liquid to the top and the sinking of a paired Fe-Ti-P-rich one to the bottom. Compared to other well-known natural immiscible liquids through multivariate statistics (Principal Component Analysis), MIs and whole-rock compositions of the Limeira Intrusion strongly support the role of silicate liquid immiscibility in the evolution of this intrusion. The incomplete unmixing or the mixing between varying proportions of the immiscible liquids containing early-formed crystals within a mush is the best mechanism explaining the origin of intermediate compositions in this intrusion, possibly induced by multiple magma batches. Based on whole-rock compositions and thermodynamic models, when the liquid line of descent reaches relatively high amounts of alkalis (Na2O + K2O ≥ 5–6 wt%) and the SiO2 (55–57 wt%), residual liquids can unmix to give rise to a paired Fe-rich and Si-rich immiscible liquids if the temperature is kept at relatively high values during prolonged periods. To test this hypothesis, experimental studies were conducted on two natural basic and intermediate samples, representing the first basaltic pulse that initiated the intrusion and the third/last basaltic-andesitic batch, respectively. Experiments were conducted under atmospheric conditions and oxygen fugacities close to the Fayalite-Magnetite-Quartz (FMQ) buffer, and immiscible liquids were produced only in isothermal conditions (ca. 1010 °C) starting with the intermediate sample. Our findings suggest that silicate liquid immiscibility can be significant in making intermediate compositions in this recharging basaltic-fed system, owing to the incomplete/inefficient unmixing of paired immiscible liquids and heat maintenance in the magma reservoir. Additionally, our interpretations can be extended to the whole PMP and contribute to our understanding of the Daly Gap for the high-Ti, alkali-rich tholeiitic compositions, suggesting large-scale magmatic differentiation due to liquid immiscibility along the tholeiitic liquid line of descent.