Fingerprints of Assimilation and Recharge Processes in a Small Shallow Magma Chamber: Detailed Mineral Elemental and Sr Isotopic Signatures from the Ponte Nova Alkaline Massif, SE Brazil.

This article investigates the magmatic evolution of the most-contaminated intrusions within the potassic Ponte Nova alkaline mafic-ultramafic massif (PNAM) in southeastern Brazil, focusing on open-system processes such as assimilation-fractional crystallization (AFC) and magma recharge. Using petrographic observations, major and trace element analyses, Sr isotope ratios of feldspars, and thermodynamic modeling with the Magma Chamber Simulator (MCS), the study identifies a two-stage magmatic evolution: an initial AFC-dominated stage during plagioclase crystallization followed by mafic magma recharge during alkali feldspar rim formation. The presence of Ba-rich minerals is attributed primarily to fractional crystallization of an enriched primitive alkaline melt rather than crustal assimilation, while isotopic and elemental heterogeneities in crystals reflect disequilibrium crystallization and incomplete magma mixing. The findings highlight the complexity of shallow magma chamber processes and the limitations of current modeling in fully capturing dynamic natural magmatic events.