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

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 Tangua. Zircon U-Pb Laser Ablation ages show that Soarinho is younger than Rio Bonito and Tangua. Ages obtained from the monzonite (60 & PLUSMN; 2 Ma), alkali feldspar syenite (58 & PLUSMN; 2 Ma), and quartz syenite (58 & PLUSMN; 2 Ma) are indistinguishable within error. One nepheline syenite sample from Tangua gives an age equal to those of Rio Bonito already published (65 & PLUSMN; 1 Ma). Petrographic and geochemical data show a clear contrast among Soarinho, Rio Bonito, and Tangua. 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.

Automated summary

The Soarinho Alkaline Intrusion in southeastern Brazil is part of the Serra do Mar Igneous Province and consists of various rock types. It is younger than its neighboring intrusions, Rio Bonito and Tangua. Geochemical and geochronological analyses reveal similarities and differences between the three intrusions, suggesting a potential enriched mantle source for Soarinho.