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

Author(s)

Chmyz, Luanna ; Azzone, Rogerio Guitarrari ; Ruberti, Excelso ; Guarino, Vincenza

• Document type: Journal article
• Source: CHEMIE DER ERDE-GEOCHEMISTRY; v. 85, n. 1, p. 16-pg., 2025-04-25.

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 Parana & iacute;ba 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 degrees 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. (AU)

• FAPESP's process: 23/12323-0 - Revisiting the ages of the Poços de Caldas Alkaline Massif: 40Ar/39Ar dating of key samples to solve a conundrum.
• Grantee: Luanna Chmyz
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 18/15731-4 - Elements mobility during fenitization processes: the carbonatite metasomatism in the Alto Paranaíba and Ponta Grossa Arch alkaline provinces
• Grantee: Luanna Chmyz
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 19/22084-8 - The Paraná Magmatic Province: petrogenesis, chronology and environmental impact of Cretaceous tholeiitic, alkaline and silicic magmatism in the Brazilian Platform
• Grantee: Valdecir de Assis Janasi
• Support Opportunities: Research Projects - Thematic Grants
• FAPESP's process: 23/11675-0 - Basic-ultrabasic magmatic bodies of SiO2-undersaturated affinity from the Brazilian platform: study of sources and open-system evolution processes in complex plumbing systems using microanalytical approach
• Grantee: Rogério Guitarrari Azzone
• Support Opportunities: Regular Research Grants