Geochemistry and Geochronology (U-Pb and Lu-Hf) of the Soarinho Alkaline Massif (Brazil): Implications on Mantle versus Crustal Signature of Syenitic Magma

Abstract

The Soarinho Alkaline Intrusion, southeastern Brazil, makes up part of the Serra do Mar Igneous Province, and it is composed of alkali feldspar syenite, alkali feldspar trachyte, quartz syenite, and monzonite. Geochemical and geochronological analyses of the Soarinho were compared with data from its neighbors, Rio Bonito and Tanguá. Zircon U-Pb Laser Ablation ages show that Soarinho is younger than Rio Bonito and Tanguá. Ages obtained from the monzonite (60 ± 2 Ma), alkali feldspar syenite (58 ± 2 Ma), and quartz syenite (58 ± 2 Ma) are indistinguishable within error. One nepheline syenite sample from Tanguá gives an age equal to those of Rio Bonito already published (65 ± 1 Ma). Petrographic and geochemical data show a clear contrast among Soarinho, Rio Bonito, and Tanguá. Lu-Hf isotopic data for the three alkaline intrusions (all negative) point to some similarities in their sources. The data suggest an enriched mantle source for Soarinho; however, an older continental crust contribution cannot be discarded.

Geochemical and isotopic constraints on the petrogenesis of the Cretaceous alkaline Morro Redondo intrusive complex, Rio de Janeiro, Brazil: implications for alkaline magma genesis

“…What is currently being discussed is what type of mantle is involved; for example, Ulbrich et al (2003), Riccomini et al (2005), and Guarino et al (2021) consider that the bodies of the PCCFA originated from the partial melting of the lithospheric mantle, while Gordon et al (2023) consider a strong association with the enriched mantle. What has not been considered is the significant participation of components of the lower crust as a source of magmatism, which was also observed by Silva et al (2023) in the Soarinho complex. Therefore, it can be suggested that, at least for the Morro Redondo complex, the genesis of magmatism involved mantle origin and significant crustal contribution.…”