Isotopic clues tracking the open-system evolution of the Ponte Nova mafic-ultramafic alkaline massif, SE Brazil: The contribution of Pb isotopes

Author(s)

Azzone, Rogerio Guitarrari ^[1] ; Chmyz, Luanna ^[1] ; Guarino, Vincenza ^[2] ; Alves, Adriana ^[1] ; Gomes, Celso de Barros ^[1] ; Ruberti, Excelso ^[1]

Affiliation

^[1] Univ Sao Paulo, Inst Geociencias, Rua Lago 562, BR-05508080 Sao Paulo - Brazil

^[2] Univ Napoli Federico II, Dipartimento Sci Terra Ambiente & Risorse, Complesso Univ Monte St Angelo, Ed L, Via Cintia 26, I-80126 Naples - Italy

• Document type: Journal article
• Source: CHEMIE DER ERDE-GEOCHEMISTRY; v. 80, n. 4 DEC 2020.
• Web of Science Citations: 0

Abstract

The Pb isotope ratios of olivine- and nepheline-normative rocks from the Ponte Nova mafic-ultramafic alkaline massif (SE Brazil) corroborate evidence that the assimilation of crustal components played an important role in this small magma chamber. New whole-rock Pb-206/Pb-204(i) (17.655-18.362) and Pb-208/Pb-204(i) (38.029-38.591) ratios correlate negatively with Nd-143/Nd-144(i) (0.512216-0.512555) and positively with Sr-87/Sr-88(i) (0.70432-0.70641). The whole-rock ratios of Pb-207/Pb-204(i) (15.494-15.556) are not clearly correlated with other isotopic pairs. Most of Ponte Nova samples straddle the curve for mu = similar to 8.7, but the majority plot to the right of the geochron, indicating U enrichment. The least contaminated samples plot within the Tristan-type isotopic composition and close to the Trindade isotopic field and might have formed through mixing between OIB and EM-I components. Mixing model isotope curves correlating Sr-Nd-Pb isotope ratios, considering the most pristine compositions (central plug) and the main host rocks of this area, indicate low crustal contributions for some intrusions and high contributions for others. More pronounced contributions from local metagranitic host rocks are associated with intrusions showing high amounts of crustal components. The wide interval of calculated Nd depleted-mantle model ages (T-DM: 703-1217 Ma) for Ponte Nova rocks indicates that crustal contamination and clinopyroxene accumulation can strongly modify these values versus those from liquid compositions (nearby alkaline dikes). Therefore, rocks affected by crustal contamination and clinopyroxene accumulation should not be considered for periods of enrichment/metasomatism of the mantle source. (AU)

• 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: 17/03768-8 - Xenocryst and Antecryst compositional influence in basic to ultrabasic alkaline magmas
• Grantee: Rogério Guitarrari Azzone
• Support Opportunities: Regular Research Grants
• FAPESP's process: 12/06082-6 - The Paraná-Etendeka magmatic province in Brazil: temporal and petrologic relationships between the tholeiitic and alkaline magmatism and geodynamic implications
• Grantee: Excelso Ruberti
• Support Opportunities: Research Projects - Thematic Grants