Multiple processes of geochemical evolution for the alkaline rocks of Rio Bonito intrusive complex, Rio de Janeiro State, Brazil: ⁴⁰Ar/³⁹Ar and U-Pb ages and Lu-Hf isotopes on zircon and constraints on crustal signature

This article presents geochemical characteristics of the alkaline rocks of Rio Bonito intrusive complex, State of Rio de Janeiro, Brazil, which is constituted mainly by nepheline syenite. The fractional crystallisation of this magma decreases K₂O/(Na₂O + K₂O) and increases (Na + K)/Al. The TiO₂, Fe₂O₃*, MgO, CaO, and P₂O₅ contents indicate fractionation of titanite, ilmenite, and clinopyroxene or amphibole. The total rare earth elements (REEs) are high, and the REE pattern is linear with negative gradient. The nepheline syenite aplite has low REEs, concave REE pattern, and positive Eu anomaly. The ultrabasic and basic mela-nepheline syenite samples have total REEs and light REEs higher than the felsic alkaline rocks. Therefore, the nepheline syenite magma is not derived directly from the alkaline ultrabasic magma. Laser-spot step-heating ⁴⁰Ar/³⁹Ar ages for biotite and amphibole are 65.03 ± 0.70 and 65.03 ± 0.46. U-Pb ages LA-ICP-MS for two samples are 65.49 ± 0.30 and 65.18 ± 0.30. Values of ε_Hf are negative for both samples, indicating an important crustal component in the evolution of Rio Bonito. (author)