Wall rock assimilation in carbonatite magmas: Textural, mineral and whole-rock geochemical signatures in the Jacupiranga complex, Brazil

Abstract

Carbonatite magmas, with their unique low SiO₂ and high volatile compositions, can be extremely reactive with silicate country rocks, driving large chemical exchanges through processes such as magmatic assimilation and fenitization. In this study, we investigate the assimilation process between Jacupiranga carbonatite and clinopyroxenite xenoliths, focusing on the textural, mineralogical, and whole-rock geochemical features of the resulting reaction rocks. These data are compared with those from fluid-derived phlogopitites from the Alto Paranaíba Igneous Province, to evaluate the processes behind the reaction between carbonatite magma and ultramafic rocks. We report the first trace element data on phlogopites from Brazilian carbonatites and associated reaction rocks. Textural, mineralogical and whole-rock geochemical features can be interpreted as the effect of carbonatite magma assimilation of ultramafic rocks, as seen in the Jacupiranga complex. Thermal and chemical gradients promote unidirectional solidification, allowing the comb-layering formation. Xenolith boundaries act as orbicule cores, favoring nucleation and the crystallization of the reaction assemblage. Phlogopites from Jacupiranga reaction rocks exhibit a wide range of estimated temperatures (550 up to >1000 °C), although diffusive re-equilibration during magmatic and post-magmatic cooling cannot be ruled out. Reaction phlogopites are enriched in Ni and Co near the clinopyroxenite, with depletion of these elements towards the carbonatite front. Conversely, reaction phlogopites near the carbonatite are enriched in Ba, highlighting the effect of the carbonatite melt in the generation of the reaction rocks, which gradually decreases towards the clinopyroxenite contact. The reaction rocks have whole-rock major and trace element contents intermediate between those of the clinopyroxenites and the carbonatite front. Partition coefficients between calcite and apatite in the reaction rocks have flat, somewhat U-shaped patterns for REE, typical signatures of primary igneous calcite and consistent with an origin by magmatic assimilation. This study shows that several imprints of assimilation of ultramafic wall rocks by carbonatite magmas are preserved in the resulting reaction rocks, allowing such a mechanism to be traced by using textural, mineralogical, and whole-rock geochemical evidence.