Abstract

This study explores the impact of open-system processes on the evolution of the most-contaminated intrusions within the potassic Ponte Nova alkaline mafic-ultramafic massif (PNAM) located in southeastern Brazil. We target cumulate and inequigranular to porphyritic lithologies that host Ba-rich minerals (alkali feldspar and biotite) and exhibit petrographic evidence of the digestion of crustal xenoliths. We interpret chemical patterns in crystals to be indicative of a two-stage process: (1) assimilation-fractional crystallization (AFC), which signatures dominate the main stage of evolution, while (2) the second stage of evolution records a combination of AFC and magmatic recharge. Sharp zonation between cores and rims of alkali feldspar crystals and disequilibrium textures in their respective cores reinforce an abrupt chemical change of the system during crystallization. Modeling results using the Magma Chamber Simulator suggest that the presence of Ba-rich minerals in PNAM, particularly Ba-rich feldspars, arises from extensive fractional crystallization of enriched primitive alkaline melts coupled with increasing SiO2 attributed to crustal assimilation. Compositional patterns of Ba and Sr in plagioclase align with simulations using log|${\mathrm{D}}_{\mathrm{Ba}\ \mathrm{or}\ \mathrm{Sr}}^{\mathrm{pl}/\mathrm{melt}}$| as a function of 1/T, rather than constant coefficient values. The extent of variability in trace elements and isotopes of crystals may be related to disequilibrium crystallization and chemical heterogeneities in the magma chamber.

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