Petrogenesis of the Early Paleogene North Island Syenite Complex, Seychelles

The Early Paleogene (63.65 ± 0.52 Ma, 63.11 ± 0.45 Ma) North Island syenitic complex of the Seychelles microcontinent is composed principally of diorite (SiO₂ ≈ 57 wt%), syenite (SiO₂ = 61–65 wt%), and microsyenite (SiO₂ = ∼70 wt%). The rocks are metaluminous, ferroan, and alkalic, and are compositionally similar to the A₁-type granitoids. The trace element compositions of the syenitic rocks show minor spatial variability between the eastern (Congoment, Bernica) and western portions (Grand’Anse, Mt. Des Cèdres) of the island. The whole rock Sr-Nd (⁸⁷Sr/⁸⁶Sr_i = 0.704095–0.707533; _Nd(t) = +1.2–+1.9) and zircon Hf (_Hf(t) = +2.1–+8.4) isotopes are indicative of a juvenile magma source. The low Th/Nb_PM (0.3–1.5) and high Nb/U (30.9–109) ratios do not indicate a crustal origin of the rocks nor do they suggest crustal contamination was significant. Hydrous fractional crystallization modeling shows that a mafic alkaline parental magma can yield residual liquid compositions similar to the diorites and syenites under reducing conditions (FMQ = −1) at a pressure of 0.3 GPa. However, feldspar accumulation likely occurred as some rocks have elevated Eu/Eu* (>1.1) values. The emplacement of the North Island complex is contemporaneous with the eruption of the Deccan Traps and rifting of the Seychelles microcontinent from India. Rifting and magmatism was likely related to the passage of the Indian plate over the Réunion hotspot. The modeling results of the study demonstrate that crystallization pressure has an influence on whether basalt-derived A-type granitoids will evolve to metaluminous or peralkaline compositions.